ECMAScript® 2021 Language Specification

)

states that the nonterminal WhileStatement represents the token while, followed by a left parenthesis token, followed by an Expression, followed by a right parenthesis token, followed by a Statement. The occurrences of Expression and Statement are themselves nonterminals. As another example, the syntactic definition:

states that an ArgumentList may represent either a single AssignmentExpression or an ArgumentList, followed by a comma, followed by an AssignmentExpression. This definition of ArgumentList is recursive, that is, it is defined in terms of itself. The result is that an ArgumentList may contain any positive number of arguments, separated by commas, where each argument expression is an AssignmentExpression. Such recursive definitions of nonterminals are common.

The subscripted suffix “_opt”, which may appear after a terminal or nonterminal, indicates an optional symbol. The alternative containing the optional symbol actually specifies two right-hand sides, one that omits the optional element and one that includes it. This means that:

opt

is a convenient abbreviation for:

and that:

for

(

opt

;

opt

)

is a convenient abbreviation for:

for

(

;

opt

)

for

(

;

opt

)

which in turn is an abbreviation for:

for

(

;

)

for

(

;

)

for

(

;

)

for

(

;

)

so, in this example, the nonterminal IterationStatement actually has four alternative right-hand sides.

A production may be parameterized by a subscripted annotation of the form “_[parameters]”, which may appear as a suffix to the nonterminal symbol defined by the production. “_parameters” may be either a single name or a comma separated list of names. A parameterized production is shorthand for a set of productions defining all combinations of the parameter names, preceded by an underscore, appended to the parameterized nonterminal symbol. This means that:

[Return]

is a convenient abbreviation for:

and that:

[Return, In]

is an abbreviation for:

StatementList_Return_In

Multiple parameters produce a combinatory number of productions, not all of which are necessarily referenced in a complete grammar.

References to nonterminals on the right-hand side of a production can also be parameterized. For example:

[+In]

is equivalent to saying:

ExpressionStatement_In

and:

[~In]

is equivalent to:

A nonterminal reference may have both a parameter list and an “_opt” suffix. For example:

[+In]

opt

is an abbreviation for:

Initializer_In

Prefixing a parameter name with “_?” on a right-hand side nonterminal reference makes that parameter value dependent upon the occurrence of the parameter name on the reference to the current production's left-hand side symbol. For example:

[In]

[?In]

is an abbreviation for:

VariableDeclaration_In

Initializer_In

If a right-hand side alternative is prefixed with “[+parameter]” that alternative is only available if the named parameter was used in referencing the production's nonterminal symbol. If a right-hand side alternative is prefixed with “[~parameter]” that alternative is only available if the named parameter was not used in referencing the production's nonterminal symbol. This means that:

[Return]

[+Return]

is an abbreviation for:

and that:

[Return]

[~Return]

is an abbreviation for:

When the words “one of” follow the colon(s) in a grammar definition, they signify that each of the terminal symbols on the following line or lines is an alternative definition. For example, the lexical grammar for ECMAScript contains the production:

one of

which is merely a convenient abbreviation for:

If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the production's right-hand side contains no terminals or nonterminals.

If the phrase “[lookahead ∉ set]” appears in the right-hand side of a production, it indicates that the production may not be used if the immediately following input token sequence is a member of the given set. The set can be written as a comma separated list of one or two element terminal sequences enclosed in curly brackets. For convenience, the set can also be written as a nonterminal, in which case it represents the set of all terminals to which that nonterminal could expand. If the set consists of a single terminal the phrase “[lookahead ≠ terminal]” may be used.

For example, given the definitions:

one of

the definition:

[lookahead ∉ { 1, 3, 5, 7, 9 }]

[lookahead ∉ DecimalDigit]

matches either the letter n followed by one or more decimal digits the first of which is even, or a decimal digit not followed by another decimal digit.

Similarly, if the phrase “[lookahead ∈ set]” appears in the right-hand side of a production, it indicates that the production may only be used if the immediately following input token sequence is a member of the given set. If the set consists of a single terminal the phrase “[lookahead = terminal]” may be used.

If the phrase “[no LineTerminator here]” appears in the right-hand side of a production of the syntactic grammar, it indicates that the production is a restricted production: it may not be used if a LineTerminator occurs in the input stream at the indicated position. For example, the production:

throw

[no LineTerminator here]

;

indicates that the production may not be used if a LineTerminator occurs in the script between the throw token and the Expression.

Unless the presence of a LineTerminator is forbidden by a restricted production, any number of occurrences of LineTerminator may appear between any two consecutive tokens in the stream of input elements without affecting the syntactic acceptability of the script.

When an alternative in a production of the lexical grammar or the numeric string grammar appears to be a multi-code point token, it represents the sequence of code points that would make up such a token.

The right-hand side of a production may specify that certain expansions are not permitted by using the phrase “but not” and then indicating the expansions to be excluded. For example, the production:

but not ReservedWord

means that the nonterminal Identifier may be replaced by any sequence of code points that could replace IdentifierName provided that the same sequence of code points could not replace ReservedWord.

Finally, a few nonterminal symbols are described by a descriptive phrase in sans-serif type in cases where it would be impractical to list all the alternatives:

any Unicode code point

5.2 Algorithm Conventions

The specification often uses a numbered list to specify steps in an algorithm. These algorithms are used to precisely specify the required semantics of ECMAScript language constructs. The algorithms are not intended to imply the use of any specific implementation technique. In practice, there may be more efficient algorithms available to implement a given feature.

Algorithms may be explicitly parameterized, in which case the names and usage of the parameters must be provided as part of the algorithm's definition.

Algorithm steps may be subdivided into sequential substeps. Substeps are indented and may themselves be further divided into indented substeps. Outline numbering conventions are used to identify substeps with the first level of substeps labelled with lower case alphabetic characters and the second level of substeps labelled with lower case roman numerals. If more than three levels are required these rules repeat with the fourth level using numeric labels. For example:

Top-level step
1. Substep.
2. Substep.
  1. Subsubstep.
    1. Subsubsubstep
      1. Subsubsubsubstep
        Subsubsubsubsubstep

A step or substep may be written as an “if” predicate that conditions its substeps. In this case, the substeps are only applied if the predicate is true. If a step or substep begins with the word “else”, it is a predicate that is the negation of the preceding “if” predicate step at the same level.

A step may specify the iterative application of its substeps.

A step that begins with “Assert:” asserts an invariant condition of its algorithm. Such assertions are used to make explicit algorithmic invariants that would otherwise be implicit. Such assertions add no additional semantic requirements and hence need not be checked by an implementation. They are used simply to clarify algorithms.

Algorithm steps may declare named aliases for any value using the form “Let x be someValue”. These aliases are reference-like in that both x and someValue refer to the same underlying data and modifications to either are visible to both. Algorithm steps that want to avoid this reference-like behaviour should explicitly make a copy of the right-hand side: “Let x be a copy of someValue” creates a shallow copy of someValue.

Once declared, an alias may be referenced in any subsequent steps and must not be referenced from steps prior to the alias's declaration. Aliases may be modified using the form “Set x to someOtherValue”.

5.2.1 Abstract Operations

In order to facilitate their use in multiple parts of this specification, some algorithms, called abstract operations, are named and written in parameterized functional form so that they may be referenced by name from within other algorithms. Abstract operations are typically referenced using a functional application style such as OperationName(arg1, arg2). Some abstract operations are treated as polymorphically dispatched methods of class-like specification abstractions. Such method-like abstract operations are typically referenced using a method application style such as someValue.OperationName(arg1, arg2).

5.2.2 Syntax-Directed Operations

A syntax-directed operation is a named operation whose definition consists of algorithms, each of which is associated with one or more productions from one of the ECMAScript grammars. A production that has multiple alternative definitions will typically have a distinct algorithm for each alternative. When an algorithm is associated with a grammar production, it may reference the terminal and nonterminal symbols of the production alternative as if they were parameters of the algorithm. When used in this manner, nonterminal symbols refer to the actual alternative definition that is matched when parsing the source text. The source text matched by a grammar production is the portion of the source text that starts at the beginning of the first terminal that participated in the match and ends at the end of the last terminal that participated in the match.

When an algorithm is associated with a production alternative, the alternative is typically shown without any “[ ]” grammar annotations. Such annotations should only affect the syntactic recognition of the alternative and have no effect on the associated semantics for the alternative.

Syntax-directed operations are invoked with a parse node and, optionally, other parameters by using the conventions on steps 1, 3, and 4 in the following algorithm:

Let status be SyntaxDirectedOperation of SomeNonTerminal.
Let someParseNode be the parse of some source text.
Perform SyntaxDirectedOperation of someParseNode.
Perform SyntaxDirectedOperation of someParseNode passing "value" as the argument.

Unless explicitly specified otherwise, all chain productions have an implicit definition for every operation that might be applied to that production's left-hand side nonterminal. The implicit definition simply reapplies the same operation with the same parameters, if any, to the chain production's sole right-hand side nonterminal and then returns the result. For example, assume that some algorithm has a step of the form: “Return the result of evaluating Block” and that there is a production:

{

}

but the Evaluation operation does not associate an algorithm with that production. In that case, the Evaluation operation implicitly includes an association of the form:

Runtime Semantics: Evaluation

{

}

Return the result of evaluating StatementList.

5.2.3 Runtime Semantics

Algorithms which specify semantics that must be called at runtime are called runtime semantics. Runtime semantics are defined by abstract operations or syntax-directed operations. Such algorithms always return a completion record.

5.2.3.1 Implicit Completion Values

The algorithms of this specification often implicitly return Completion Records whose [[Type]] is normal. Unless it is otherwise obvious from the context, an algorithm statement that returns a value that is not a Completion Record, such as:

Return "Infinity".

means the same thing as:

Return NormalCompletion("Infinity").

However, if the value expression of a “return” statement is a Completion Record construction literal, the resulting Completion Record is returned. If the value expression is a call to an abstract operation, the “return” statement simply returns the Completion Record produced by the abstract operation.

The abstract operation Completion(completionRecord) is used to emphasize that a previously computed Completion Record is being returned. The Completion abstract operation takes a single argument, completionRecord, and performs the following steps:

Assert: completionRecord is a Completion Record.
Return completionRecord as the Completion Record of this abstract operation.

A “return” statement without a value in an algorithm step means the same thing as:

Return NormalCompletion(undefined).

Any reference to a Completion Record value that is in a context that does not explicitly require a complete Completion Record value is equivalent to an explicit reference to the [[Value]] field of the Completion Record value unless the Completion Record is an abrupt completion.

5.2.3.2 Throw an Exception

Algorithms steps that say to throw an exception, such as

Throw a TypeError exception.

mean the same things as:

Return ThrowCompletion(a newly created TypeError object).

5.2.3.3 ReturnIfAbrupt

Algorithms steps that say or are otherwise equivalent to:

ReturnIfAbrupt(argument).

mean the same thing as:

If argument is an abrupt completion, return argument.
Else if argument is a Completion Record, set argument to argument.[[Value]].

Algorithms steps that say or are otherwise equivalent to:

ReturnIfAbrupt(AbstractOperation()).

mean the same thing as:

Let hygienicTemp be AbstractOperation().
If hygienicTemp is an abrupt completion, return hygienicTemp.
Else if hygienicTemp is a Completion Record, set hygienicTemp to hygienicTemp.[[Value]].

Where hygienicTemp is ephemeral and visible only in the steps pertaining to ReturnIfAbrupt.

Algorithms steps that say or are otherwise equivalent to:

Let result be AbstractOperation(ReturnIfAbrupt(argument)).

mean the same thing as:

If argument is an abrupt completion, return argument.
If argument is a Completion Record, set argument to argument.[[Value]].
Let result be AbstractOperation(argument).

5.2.3.4 ReturnIfAbrupt Shorthands

Invocations of abstract operations and syntax-directed operations that are prefixed by ? indicate that ReturnIfAbrupt should be applied to the resulting Completion Record. For example, the step:

? OperationName().

is equivalent to the following step:

ReturnIfAbrupt(OperationName()).

Similarly, for method application style, the step:

? someValue.OperationName().

is equivalent to:

ReturnIfAbrupt(someValue.OperationName()).

Similarly, prefix ! is used to indicate that the following invocation of an abstract or syntax-directed operation will never return an abrupt completion and that the resulting Completion Record's [[Value]] field should be used in place of the return value of the operation. For example, the step:

Let val be ! OperationName().

is equivalent to the following steps:

Let val be OperationName().
Assert: val is never an abrupt completion.
If val is a Completion Record, set val to val.[[Value]].

Syntax-directed operations for runtime semantics make use of this shorthand by placing ! or ? before the invocation of the operation:

Perform ! SyntaxDirectedOperation of NonTerminal.

5.2.4 Static Semantics

Context-free grammars are not sufficiently powerful to express all the rules that define whether a stream of input elements form a valid ECMAScript Script or Module that may be evaluated. In some situations additional rules are needed that may be expressed using either ECMAScript algorithm conventions or prose requirements. Such rules are always associated with a production of a grammar and are called the static semantics of the production.

Static Semantic Rules have names and typically are defined using an algorithm. Named Static Semantic Rules are associated with grammar productions and a production that has multiple alternative definitions will typically have for each alternative a distinct algorithm for each applicable named static semantic rule.

Unless otherwise specified every grammar production alternative in this specification implicitly has a definition for a static semantic rule named Contains which takes an argument named symbol whose value is a terminal or nonterminal of the grammar that includes the associated production. The default definition of Contains is:

For each child node child of this Parse Node, do
1. If child is an instance of symbol, return true.
2. If child is an instance of a nonterminal, then
  1. Let contained be the result of child Contains symbol.
  2. If contained is true, return true.
Return false.

The above definition is explicitly over-ridden for specific productions.

A special kind of static semantic rule is an Early Error Rule. Early error rules define early error conditions (see clause 16) that are associated with specific grammar productions. Evaluation of most early error rules are not explicitly invoked within the algorithms of this specification. A conforming implementation must, prior to the first evaluation of a Script or Module, validate all of the early error rules of the productions used to parse that Script or Module. If any of the early error rules are violated the Script or Module is invalid and cannot be evaluated.

5.2.5 Mathematical Operations

This specification makes reference to two kinds of numeric values:

Number: IEEE 754-2019 double-precision floating point values, used as the default numeric type.
Mathematical value: Arbitrary real numbers, used for specific situations.

In the language of this specification, numerical values and operations (including addition, subtraction, negation, multiplication, division, and comparison) are distinguished among different numeric kinds using subscripts. The subscript _𝔽 refers to Numbers, and the subscript _ℝ refers to mathematical values. A subscript is used following each numeric value and operation.

For brevity, the _𝔽 subscript can be omitted on Number values—a numeric value with no subscript is interpreted to be a Number. An operation with no subscript is interpreted to be a Number operation, unless one of the parameters has a particular subscript, in which case the operation adopts that subscript. For example, 1_ℝ + 2_ℝ = 3_ℝ is a statement about mathematical values, and 1 + 2 = 3 is a statement about Numbers.

In general, when this specification refers to a numerical value, such as in the phrase, "the length of y" or "the integer represented by the four hexadecimal digits ...", without explicitly specifying a numeric kind, the phrase refers to a Number. Phrases which refer to a mathematical value are explicitly annotated as such; for example, "the mathematical value of the number of code points in ...".

It is not defined to mix Numbers and mathematical values in either arithmetic or comparison operations, and any such undefined operation would be an editorial error in this specification text.

The Number value 0, alternatively written 0_𝔽, is defined as the double-precision floating point positive zero value. In certain contexts, it may also be written as +0 for clarity.

This specification denotes most numeric values in base 10; it also uses numeric values of the form 0x followed by digits 0-9 or A-F as base-16 values.

In certain contexts, an operation is specified which is generic between Numbers and mathematical values. In these cases, the subscript can be a variable; t is often used for this purpose, for example 5_t × 10_t = 50_t for any t ranging over ℝ and 𝔽, since the values involved are within the range where the semantics coincide.

Conversions between mathematical values and numbers are never implicit, and always explicit in this document. A conversion from a mathematical value to a Number is denoted as "the Number value for x", and is defined in 6.1.6.1. A conversion from a Number to a mathematical value is denoted as "the mathematical value of x", or ℝ(x). Note that the mathematical value of non-finite values is not defined, and the mathematical value of +0 and -0 is the mathematical value 0_ℝ.

When the term integer is used in this specification, it refers to a Number value whose mathematical value is in the set of integers, unless otherwise stated: when the term mathematical integer is used in this specification, it refers to a mathematical value which is in the set of integers. As shorthand, integer_t can be used to refer to either of the two, as determined by t.

The mathematical function abs_t(x) produces the absolute value of x, which is -_tx if x <_t 0_t and otherwise is x itself.

The mathematical function min_t(x1, x2, … , xN) produces the mathematically smallest of x1 through xN. The mathematical function max_t(x1, x2, ..., xN) produces the mathematically largest of x1 through xN. The domain and range of these mathematical functions include +∞ and -∞.

The notation “x modulo_t y” (y must be finite and nonzero) computes a value k of the same sign as y (or zero) such that abs_t(k) <_t abs_t(y) and x-_tk = q ×_t y for some integer_t q.

The mathematical function floor_t(x) produces the largest integer_t (closest to positive infinity) that is not larger than x.

Note

floor_t(x) = x -_t (x modulo_t 1_t).

5.2.6 Value Notation

In this specification, ECMAScript language values are displayed in bold. Examples include null, true, or "hello". These are distinguished from longer ECMAScript code sequences such as Function.prototype.apply or let n = 42;.

Values which are internal to the specification and not directly observable from ECMAScript code are indicated with a sans-serif typeface. For instance, a Completion Record's [[Type]] field takes on values like normal, return, or throw.

6 ECMAScript Data Types and Values

Algorithms within this specification manipulate values each of which has an associated type. The possible value types are exactly those defined in this clause. Types are further subclassified into ECMAScript language types and specification types.

Within this specification, the notation “Type(x)” is used as shorthand for “the type of x” where “type” refers to the ECMAScript language and specification types defined in this clause. When the term “empty” is used as if it was naming a value, it is equivalent to saying “no value of any type”.

6.1 ECMAScript Language Types

An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript programmer using the ECMAScript language. The ECMAScript language types are Undefined, Null, Boolean, String, Symbol, Number, BigInt, and Object. An ECMAScript language value is a value that is characterized by an ECMAScript language type.

6.1.1 The Undefined Type

The Undefined type has exactly one value, called undefined. Any variable that has not been assigned a value has the value undefined.

6.1.2 The Null Type

The Null type has exactly one value, called null.

6.1.3 The Boolean Type

The Boolean type represents a logical entity having two values, called true and false.

6.1.4 The String Type

The String type is the set of all ordered sequences of zero or more 16-bit unsigned integer values (“elements”) up to a maximum length of 2⁵³ - 1 elements. The String type is generally used to represent textual data in a running ECMAScript program, in which case each element in the String is treated as a UTF-16 code unit value. Each element is regarded as occupying a position within the sequence. These positions are indexed with nonnegative integers. The first element (if any) is at index 0, the next element (if any) at index 1, and so on. The length of a String is the number of elements (i.e., 16-bit values) within it. The empty String has length zero and therefore contains no elements.

ECMAScript operations that do not interpret String contents apply no further semantics. Operations that do interpret String values treat each element as a single UTF-16 code unit. However, ECMAScript does not restrict the value of or relationships between these code units, so operations that further interpret String contents as sequences of Unicode code points encoded in UTF-16 must account for ill-formed subsequences. Such operations apply special treatment to every code unit with a numeric value in the inclusive range 0xD800 to 0xDBFF (defined by the Unicode Standard as a leading surrogate, or more formally as a high-surrogate code unit) and every code unit with a numeric value in the inclusive range 0xDC00 to 0xDFFF (defined as a trailing surrogate, or more formally as a low-surrogate code unit) using the following rules:

A code unit that is not a leading surrogate and not a trailing surrogate is interpreted as a code point with the same value.
A sequence of two code units, where the first code unit c1 is a leading surrogate and the second code unit c2 a trailing surrogate, is a surrogate pair and is interpreted as a code point with the value (c1 - 0xD800) × 0x400 + (c2 - 0xDC00) + 0x10000. (See 10.1.3)
A code unit that is a leading surrogate or trailing surrogate, but is not part of a surrogate pair, is interpreted as a code point with the same value.

The function String.prototype.normalize (see 21.1.3.13) can be used to explicitly normalize a String value. String.prototype.localeCompare (see 21.1.3.10) internally normalizes String values, but no other operations implicitly normalize the strings upon which they operate. Only operations that are explicitly specified to be language or locale sensitive produce language-sensitive results.

Note

The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. If ECMAScript source text is in Normalized Form C, string literals are guaranteed to also be normalized, as long as they do not contain any Unicode escape sequences.

In this specification, the phrase "the string-concatenation of A, B, ..." (where each argument is a String value, a code unit, or a sequence of code units) denotes the String value whose sequence of code units is the concatenation of the code units (in order) of each of the arguments (in order).

6.1.5 The Symbol Type

The Symbol type is the set of all non-String values that may be used as the key of an Object property (6.1.7).

Each possible Symbol value is unique and immutable.

Each Symbol value immutably holds an associated value called [[Description]] that is either undefined or a String value.

6.1.5.1 Well-Known Symbols

Well-known symbols are built-in Symbol values that are explicitly referenced by algorithms of this specification. They are typically used as the keys of properties whose values serve as extension points of a specification algorithm. Unless otherwise specified, well-known symbols values are shared by all realms (8.2).

Within this specification a well-known symbol is referred to by using a notation of the form @@name, where “name” is one of the values listed in Table 1.

Table 1: Well-known Symbols

Specification Name	[[Description]]	Value and Purpose
@@asyncIterator	"Symbol.asyncIterator"	A method that returns the default AsyncIterator for an object. Called by the semantics of the `for`-`await`-`of` statement.
@@hasInstance	"Symbol.hasInstance"	A method that determines if a constructor object recognizes an object as one of the constructor's instances. Called by the semantics of the `instanceof` operator.
@@isConcatSpreadable	"Symbol.isConcatSpreadable"	A Boolean valued property that if true indicates that an object should be flattened to its array elements by `Array.prototype.concat`.
@@iterator	"Symbol.iterator"	A method that returns the default Iterator for an object. Called by the semantics of the for-of statement.
@@match	"Symbol.match"	A regular expression method that matches the regular expression against a string. Called by the `String.prototype.match` method.
@@matchAll	"Symbol.matchAll"	A regular expression method that returns an iterator, that yields matches of the regular expression against a string. Called by the `String.prototype.matchAll` method.
@@replace	"Symbol.replace"	A regular expression method that replaces matched substrings of a string. Called by the `String.prototype.replace` method.
@@search	"Symbol.search"	A regular expression method that returns the index within a string that matches the regular expression. Called by the `String.prototype.search` method.
@@species	"Symbol.species"	A function valued property that is the constructor function that is used to create derived objects.
@@split	"Symbol.split"	A regular expression method that splits a string at the indices that match the regular expression. Called by the `String.prototype.split` method.
@@toPrimitive	"Symbol.toPrimitive"	A method that converts an object to a corresponding primitive value. Called by the ToPrimitive abstract operation.
@@toStringTag	"Symbol.toStringTag"	A String valued property that is used in the creation of the default string description of an object. Accessed by the built-in method `Object.prototype.toString`.
@@unscopables	"Symbol.unscopables"	An object valued property whose own and inherited property names are property names that are excluded from the `with` environment bindings of the associated object.

6.1.6 Numeric Types

ECMAScript has two built-in numeric types: Number and BigInt. In this specification, every numeric type T contains a multiplicative identity value denoted T::unit. The specification types also have the following abstract operations, likewise denoted T::op for a given operation with specification name op. All argument types are T. The "Result" column shows the return type, along with an indication if it is possible for some invocations of the operation to return an abrupt completion.

Table 2: Numeric Type Operations

Invocation Synopsis	Example source	Invoked by the Evaluation semantics of ...	Result
`T`::unaryMinus(x)	`-x`	Unary `-` Operator	`T`
`T`::bitwiseNOT(x)	`~x`	Bitwise NOT Operator ( `~` )	`T`
`T`::exponentiate(x, y)	`x ** y`	Exponentiation Operator and Math.pow ( `base`, `exponent` )	`T`, may throw RangeError
`T`::multiply(x, y)	`x * y`	Multiplicative Operators	`T`
`T`::divide(x, y)	`x / y`	Multiplicative Operators	`T`, may throw RangeError
`T`::remainder(x, y)	`x % y`	Multiplicative Operators	`T`, may throw RangeError
`T`::add(x, y)	`x ++` `++ x` `x + y`	Postfix Increment Operator, Prefix Increment Operator, and The Addition Operator ( `+` )	`T`
`T`::subtract(x, y)	`x --` `-- x` `x - y`	Postfix Decrement Operator, Prefix Decrement Operator, and The Subtraction Operator ( `-` )	`T`
`T`::leftShift(x, y)	`x << y`	The Left Shift Operator ( `<<` )	`T`
`T`::signedRightShift(x, y)	`x >> y`	The Signed Right Shift Operator ( `>>` )	`T`
`T`::unsignedRightShift(x, y)	`x >>> y`	The Unsigned Right Shift Operator ( `>>>` )	`T`, may throw TypeError
`T`::lessThan(x, y)	`x < y` `x > y` `x <= y` `x >= y`	Relational Operators, via Abstract Relational Comparison	Boolean or undefined (for unordered inputs)
`T`::equal(x, y)	`x == y` `x != y` `x === y` `x !== y`	Equality Operators, via Strict Equality Comparison	Boolean
`T`::sameValue(x, y)		Object internal methods, via SameValue ( `x`, `y` ), to test exact value equality	Boolean
`T`::sameValueZero(x, y)		Array, Map, and Set methods, via SameValueZero ( `x`, `y` ), to test value equality ignoring differences among members of the zero cohort (e.g., -0 and +0)	Boolean
`T`::bitwiseAND(x, y)	`x & y`	Binary Bitwise Operators	`T`
`T`::bitwiseXOR(x, y)	`x ^ y`	Binary Bitwise Operators	`T`
`T`::bitwiseOR(x, y)	`x \| y`	Binary Bitwise Operators	`T`
`T`::toString(x)	`String(x)`	Many expressions and built-in functions, via ToString ( `argument` )	String

The T::unit value and T::op operations are not a part of the ECMAScript language; they are defined here solely to aid the specification of the semantics of the ECMAScript language. Other abstract operations are defined throughout this specification.

Because the numeric types are in general not convertible without loss of precision or truncation, the ECMAScript language provides no implicit conversion among these types. Programmers must explicitly call Number and BigInt functions to convert among types when calling a function which requires another type.

Note

The first and subsequent editions of ECMAScript have provided, for certain operators, implicit numeric conversions that could lose precision or truncate. These legacy implicit conversions are maintained for backward compatibility, but not provided for BigInt in order to minimize opportunity for programmer error, and to leave open the option of generalized value types in a future edition.

6.1.6.1 The Number Type

The Number type has exactly 18437736874454810627_ℝ (that is, 2_ℝ^64_ℝ - 2_ℝ^53_ℝ + 3_ℝ) values, representing the double-precision 64-bit format IEEE 754-2019 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9007199254740990_ℝ (that is, 2_ℝ^53_ℝ - 2_ℝ) distinct “Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a single special NaN value. (Note that the NaN value is produced by the program expression NaN.) In some implementations, external code might be able to detect a difference between various Not-a-Number values, but such behaviour is implementation-dependent; to ECMAScript code, all NaN values are indistinguishable from each other.

Note

The bit pattern that might be observed in an ArrayBuffer (see 24.1) or a SharedArrayBuffer (see 24.2) after a Number value has been stored into it is not necessarily the same as the internal representation of that Number value used by the ECMAScript implementation.

There are two other special values, called positive Infinity and negative Infinity. For brevity, these values are also referred to for expository purposes by the symbols +∞ and -∞, respectively. (Note that these two infinite Number values are produced by the program expressions +Infinity (or simply Infinity) and -Infinity.)

The other 18437736874454810624_ℝ (that is, 2_ℝ^64_ℝ - 2_ℝ^53_ℝ) values are called the finite numbers. Half of these are positive numbers and half are negative numbers; for every finite positive Number value there is a corresponding negative value having the same magnitude.

Note that there is both a positive zero and a negative zero. For brevity, these values are also referred to for expository purposes by the symbols +0 and -0, respectively. (Note that these two different zero Number values are produced by the program expressions +0 (or simply 0) and -0.)

The 18437736874454810622_ℝ (that is, 2_ℝ^64_ℝ - 2_ℝ^53_ℝ - 2_ℝ) finite nonzero values are of two kinds:

18428729675200069632_ℝ (that is, 2_ℝ^64_ℝ - 2_ℝ^54_ℝ) of them are normalized, having the form

s \times m \times 2 e

where s is +1_ℝ or -1_ℝ, m is a positive mathematical integer less than 2_ℝ^53_ℝ but not less than 2_ℝ^52_ℝ, and e is a mathematical integer ranging from -1074_ℝ to 971_ℝ, inclusive.

The remaining 9007199254740990_ℝ (that is, 2_ℝ^53_ℝ - 2_ℝ) values are denormalized, having the form

s \times m \times 2 e

where s is +1_ℝ or -1_ℝ, m is a positive mathematical integer less than 2_ℝ^52_ℝ, and e is -1074_ℝ.

Note that all the positive and negative mathematical integers whose magnitude is no greater than 2⁵³ are representable in the Number type (indeed, the mathematical integer 0 has two representations, +0 and -0).

A finite number has an odd significand if it is nonzero and the mathematical integer m used to express it (in one of the two forms shown above) is odd. Otherwise, it has an even significand.

In this specification, the phrase “the Number value for x” where x represents an exact real mathematical quantity (which might even be an irrational number such as π) means a Number value chosen in the following manner. Consider the set of all finite values of the Number type, with -0 removed and with two additional values added to it that are not representable in the Number type, namely 2_ℝ^1024_ℝ (which is +1_ℝ × 2_ℝ^53_ℝ × 2_ℝ^971_ℝ) and -2_ℝ^1024_ℝ (which is -1_ℝ × 2_ℝ^53_ℝ × 2_ℝ^971_ℝ). Choose the member of this set that is closest in value to x. If two values of the set are equally close, then the one with an even significand is chosen; for this purpose, the two extra values 2_ℝ^1024_ℝ and -2_ℝ^1024_ℝ are considered to have even significands. Finally, if 2_ℝ^1024_ℝ was chosen, replace it with +∞; if -2_ℝ^1024_ℝ was chosen, replace it with -∞; if +0 was chosen, replace it with -0 if and only if x is less than zero; any other chosen value is used unchanged. The result is the Number value for x. (This procedure corresponds exactly to the behaviour of the IEEE 754-2019 roundTiesToEven mode.)

Some ECMAScript operators deal only with integers in specific ranges such as -2³¹ through 2³¹ - 1, inclusive, or in the range 0 through 2¹⁶ - 1, inclusive. These operators accept any value of the Number type but first convert each such value to an integer value in the expected range. See the descriptions of the numeric conversion operations in 7.1.

The Number::unit value is 1.

6.1.6.1.1 Number::unaryMinus ( `x` )

If x is NaN, return NaN.
Return the result of negating x; that is, compute a Number with the same magnitude but opposite sign.

6.1.6.1.2 Number::bitwiseNOT ( `x` )

Let oldValue be ! ToInt32(x).
Return the result of applying bitwise complement to oldValue. The result is a signed 32-bit integer.

6.1.6.1.3 Number::exponentiate ( `base`, `exponent` )

Returns an implementation-dependent approximation of the result of raising base to the power exponent.

If exponent is NaN, the result is NaN.
If exponent is +0, the result is 1, even if base is NaN.
If exponent is -0, the result is 1, even if base is NaN.
If base is NaN and exponent is nonzero, the result is NaN.
If abs(base) > 1 and exponent is +∞, the result is +∞.
If abs(base) > 1 and exponent is -∞, the result is +0.
If abs(base) is 1 and exponent is +∞, the result is NaN.
If abs(base) is 1 and exponent is -∞, the result is NaN.
If abs(base) < 1 and exponent is +∞, the result is +0.
If abs(base) < 1 and exponent is -∞, the result is +∞.
If base is +∞ and exponent > 0, the result is +∞.
If base is +∞ and exponent < 0, the result is +0.
If base is -∞ and exponent > 0 and exponent is an odd integer, the result is -∞.
If base is -∞ and exponent > 0 and exponent is not an odd integer, the result is +∞.
If base is -∞ and exponent < 0 and exponent is an odd integer, the result is -0.
If base is -∞ and exponent < 0 and exponent is not an odd integer, the result is +0.
If base is +0 and exponent > 0, the result is +0.
If base is +0 and exponent < 0, the result is +∞.
If base is -0 and exponent > 0 and exponent is an odd integer, the result is -0.
If base is -0 and exponent > 0 and exponent is not an odd integer, the result is +0.
If base is -0 and exponent < 0 and exponent is an odd integer, the result is -∞.
If base is -0 and exponent < 0 and exponent is not an odd integer, the result is +∞.
If base < 0 and base is finite and exponent is finite and exponent is not an integer, the result is NaN.

Note

The result of base ** exponent when base is 1 or -1 and exponent is +Infinity or -Infinity differs from IEEE 754-2019. The first edition of ECMAScript specified a result of NaN for this operation, whereas later versions of IEEE 754-2019 specified 1. The historical ECMAScript behaviour is preserved for compatibility reasons.

6.1.6.1.4 Number::multiply ( `x`, `y` )

The * MultiplicativeOperator performs multiplication, producing the product of x and y. Multiplication is commutative. Multiplication is not always associative in ECMAScript, because of finite precision.

The result of a floating-point multiplication is governed by the rules of IEEE 754-2019 binary double-precision arithmetic:

If either operand is NaN, the result is NaN.
The sign of the result is positive if both operands have the same sign, negative if the operands have different signs.
Multiplication of an infinity by a zero results in NaN.
Multiplication of an infinity by an infinity results in an infinity. The sign is determined by the rule already stated above.
Multiplication of an infinity by a finite nonzero value results in a signed infinity. The sign is determined by the rule already stated above.
In the remaining cases, where neither an infinity nor NaN is involved, the product is computed and rounded to the nearest representable value using IEEE 754-2019 roundTiesToEven mode. If the magnitude is too large to represent, the result is then an infinity of appropriate sign. If the magnitude is too small to represent, the result is then a zero of appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754-2019.

6.1.6.1.5 Number::divide ( `x`, `y` )

The / MultiplicativeOperator performs division, producing the quotient of x and y. x is the dividend and y is the divisor. ECMAScript does not perform integer division. The operands and result of all division operations are double-precision floating-point numbers. The result of division is determined by the specification of IEEE 754-2019 arithmetic:

If either operand is NaN, the result is NaN.
The sign of the result is positive if both operands have the same sign, negative if the operands have different signs.
Division of an infinity by an infinity results in NaN.
Division of an infinity by a zero results in an infinity. The sign is determined by the rule already stated above.
Division of an infinity by a nonzero finite value results in a signed infinity. The sign is determined by the rule already stated above.
Division of a finite value by an infinity results in zero. The sign is determined by the rule already stated above.
Division of a zero by a zero results in NaN; division of zero by any other finite value results in zero, with the sign determined by the rule already stated above.
Division of a nonzero finite value by a zero results in a signed infinity. The sign is determined by the rule already stated above.
In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the quotient is computed and rounded to the nearest representable value using IEEE 754-2019 roundTiesToEven mode. If the magnitude is too large to represent, the operation overflows; the result is then an infinity of appropriate sign. If the magnitude is too small to represent, the operation underflows and the result is a zero of the appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754-2019.

6.1.6.1.6 Number::remainder ( `n`, `d` )

The % MultiplicativeOperator yields the remainder of its operands from an implied division; n is the dividend and d is the divisor.

Note

In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts floating-point operands.

The result of a floating-point remainder operation as computed by the % operator is not the same as the “remainder” operation defined by IEEE 754-2019. The IEEE 754-2019 “remainder” operation computes the remainder from a rounding division, not a truncating division, and so its behaviour is not analogous to that of the usual integer remainder operator. Instead the ECMAScript language defines % on floating-point operations to behave in a manner analogous to that of the Java integer remainder operator; this may be compared with the C library function fmod.

The result of an ECMAScript floating-point remainder operation is determined by the rules of IEEE arithmetic:

If either operand is NaN, the result is NaN.
The sign of the result equals the sign of the dividend.
If the dividend is an infinity, or the divisor is a zero, or both, the result is NaN.
If the dividend is finite and the divisor is an infinity, the result equals the dividend.
If the dividend is a zero and the divisor is nonzero and finite, the result is the same as the dividend.
In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the floating-point remainder r from a dividend n and a divisor d is defined by the mathematical relation r = n - (d × q) where q is an integer that is negative only if n/d is negative and positive only if n/d is positive, and whose magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient of n and d. r is computed and rounded to the nearest representable value using IEEE 754-2019 roundTiesToEven mode.

6.1.6.1.7 Number::add ( `x`, `y` )

The + operator performs addition when applied to x and y, producing the sum of the operands.

Addition is a commutative operation, but not always associative.

The result of an addition is determined using the rules of IEEE 754-2019 binary double-precision arithmetic:

If either operand is NaN, the result is NaN.
The sum of two infinities of opposite sign is NaN.
The sum of two infinities of the same sign is the infinity of that sign.
The sum of an infinity and a finite value is equal to the infinite operand.
The sum of two negative zeroes is -0. The sum of two positive zeroes, or of two zeroes of opposite sign, is +0.
The sum of a zero and a nonzero finite value is equal to the nonzero operand.
The sum of two nonzero finite values of the same magnitude and opposite sign is +0.
In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, and the operands have the same sign or have different magnitudes, the sum is computed and rounded to the nearest representable value using IEEE 754-2019 roundTiesToEven mode. If the magnitude is too large to represent, the operation overflows and the result is then an infinity of appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754-2019.

6.1.6.1.8 Number::subtract ( `x`, `y` )

The - operator performs subtraction when applied to two operands of numeric type, producing the difference of its operands; x is the minuend and y is the subtrahend. It is always the case that x - y produces the same result as x + (-y).

The result of - operator is then x + (-y).

6.1.6.1.9 Number::leftShift ( `x`, `y` )

Let lnum be ! ToInt32(x).
Let rnum be ! ToUint32(y).
Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that is, compute rnum & 0x1F.
Return the result of left shifting lnum by shiftCount bits. The result is a signed 32-bit integer.

6.1.6.1.10 Number::signedRightShift ( `x`, `y` )

Let lnum be ! ToInt32(x).
Let rnum be ! ToUint32(y).
Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that is, compute rnum & 0x1F.
Return the result of performing a sign-extending right shift of lnum by shiftCount bits. The most significant bit is propagated. The result is a signed 32-bit integer.

6.1.6.1.11 Number::unsignedRightShift ( `x`, `y` )

Let lnum be ! ToUint32(x).
Let rnum be ! ToUint32(y).
Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that is, compute rnum & 0x1F.
Return the result of performing a zero-filling right shift of lnum by shiftCount bits. Vacated bits are filled with zero. The result is an unsigned 32-bit integer.

6.1.6.1.12 Number::lessThan ( `x`, `y` )

If x is NaN, return undefined.
If y is NaN, return undefined.
If x and y are the same Number value, return false.
If x is +0 and y is -0, return false.
If x is -0 and y is +0, return false.
If x is +∞, return false.
If y is +∞, return true.
If y is -∞, return false.
If x is -∞, return true.
If the mathematical value of x is less than the mathematical value of y—note that these mathematical values are both finite and not both zero—return true. Otherwise, return false.

6.1.6.1.13 Number::equal ( `x`, `y` )

If x is NaN, return false.
If y is NaN, return false.
If x is the same Number value as y, return true.
If x is +0 and y is -0, return true.
If x is -0 and y is +0, return true.
Return false.

6.1.6.1.14 Number::sameValue ( `x`, `y` )

If x is NaN and y is NaN, return true.
If x is +0 and y is -0, return false.
If x is -0 and y is +0, return false.
If x is the same Number value as y, return true.
Return false.

6.1.6.1.15 Number::sameValueZero ( `x`, `y` )

If x is NaN and y is NaN, return true.
If x is +0 and y is -0, return true.
If x is -0 and y is +0, return true.
If x is the same Number value as y, return true.
Return false.

6.1.6.1.16 NumberBitwiseOp ( `op`, `x`, `y` )

The abstract operation NumberBitwiseOp takes arguments op, x, and y. It performs the following steps when called:

Let lnum be ! ToInt32(x).
Let rnum be ! ToInt32(y).
Return the result of applying the bitwise operator op to lnum and rnum. The result is a signed 32-bit integer.

6.1.6.1.17 Number::bitwiseAND ( `x`, `y` )

Return NumberBitwiseOp(&, x, y).

6.1.6.1.18 Number::bitwiseXOR ( `x`, `y` )

Return NumberBitwiseOp(^, x, y).

6.1.6.1.19 Number::bitwiseOR ( `x`, `y` )

Return NumberBitwiseOp(|, x, y).

6.1.6.1.20 Number::toString ( `x` )

The abstract operation Number::toString takes argument x (a Number). It converts x to String format. It performs the following steps when called:

If x is NaN, return the String "NaN".
If x is +0 or -0, return the String "0".
If x is less than zero, return the string-concatenation of "-" and ! Number::toString(-x).
If x is +∞, return the String "Infinity".
Otherwise, let n, k, and s be integers such that k ≥ 1, 10^{k - 1} ≤ s < 10^k, the Number value for ℝ(s) × 10_ℝ^{ℝ(n) - ℝ(k)} is x, and k is as small as possible. Note that k is the number of digits in the decimal representation of s, that s is not divisible by 10_ℝ, and that the least significant digit of s is not necessarily uniquely determined by these criteria.
If k ≤ n ≤ 21, return the string-concatenation of:
- the code units of the k digits of the decimal representation of s (in order, with no leading zeroes)
- n - k occurrences of the code unit 0x0030 (DIGIT ZERO)
If 0 < n ≤ 21, return the string-concatenation of:
- the code units of the most significant n digits of the decimal representation of s
- the code unit 0x002E (FULL STOP)
- the code units of the remaining k - n digits of the decimal representation of s
If -6 < n ≤ 0, return the string-concatenation of:
- the code unit 0x0030 (DIGIT ZERO)
- the code unit 0x002E (FULL STOP)
- -n occurrences of the code unit 0x0030 (DIGIT ZERO)
- the code units of the k digits of the decimal representation of s
Otherwise, if k = 1, return the string-concatenation of:
- the code unit of the single digit of s
- the code unit 0x0065 (LATIN SMALL LETTER E)
- the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS) according to whether n - 1 is positive or negative
- the code units of the decimal representation of the integer abs(n - 1) (with no leading zeroes)
Return the string-concatenation of:
- the code units of the most significant digit of the decimal representation of s
- the code unit 0x002E (FULL STOP)
- the code units of the remaining k - 1 digits of the decimal representation of s
- the code unit 0x0065 (LATIN SMALL LETTER E)
- the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS) according to whether n - 1 is positive or negative
- the code units of the decimal representation of the integer abs(n - 1) (with no leading zeroes)

Note 1

The following observations may be useful as guidelines for implementations, but are not part of the normative requirements of this Standard:

If x is any Number value other than -0, then ToNumber(ToString(x)) is exactly the same Number value as x.
The least significant digit of s is not always uniquely determined by the requirements listed in step 5.

Note 2

For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 5 be used as a guideline:

Otherwise, let n, k, and s be integers such that k ≥ 1, 10^{k - 1} ≤ s < 10^k, the Number value for ℝ(s) × 10_ℝ^{ℝ(n) - ℝ(k)} is x, and k is as small as possible. If there are multiple possibilities for s, choose the value of s for which ℝ(s) × 10_ℝ^{ℝ(n) - ℝ(k)} is closest in value to ℝ(x). If there are two such possible values of s, choose the one that is even. Note that k is the number of digits in the decimal representation of s and that s is not divisible by 10_ℝ.

Note 3

Implementers of ECMAScript may find useful the paper and code written by David M. Gay for binary-to-decimal conversion of floating-point numbers:

Gay, David M. Correctly Rounded Binary-Decimal and Decimal-Binary Conversions. Numerical Analysis, Manuscript 90-10. AT&T Bell Laboratories (Murray Hill, New Jersey). November 30, 1990. Available as
http://ampl.com/REFS/abstracts.html#rounding. Associated code available as
http://netlib.sandia.gov/fp/dtoa.c and as
http://netlib.sandia.gov/fp/g_fmt.c and may also be found at the various netlib mirror sites.

6.1.6.2 The BigInt Type

The BigInt type represents a mathematical integer value. The value may be any size and is not limited to a particular bit-width. Generally, where not otherwise noted, operations are designed to return exact mathematically-based answers. For binary operations, BigInts act as two's complement binary strings, with negative numbers treated as having bits set infinitely to the left.

The BigInt::unit value is 1n.

6.1.6.2.1 BigInt::unaryMinus ( `x` )

If x is 0n, return 0n.
Return the BigInt value that represents the mathematical value of negating x.

6.1.6.2.2 BigInt::bitwiseNOT ( `x` )

The abstract operation BigInt::bitwiseNOT takes argument x (a BigInt). It returns the one's complement of x; that is, -x - 1.

6.1.6.2.3 BigInt::exponentiate ( `base`, `exponent` )

If exponent < 0n, throw a RangeError exception.
If base is 0n and exponent is 0n, return 1n.
Return the BigInt value that represents the mathematical value of base raised to the power exponent.

6.1.6.2.4 BigInt::multiply ( `x`, `y` )

The abstract operation BigInt::multiply takes arguments x (a BigInt) and y (a BigInt). It returns the BigInt value that represents the result of multiplying x and y.

Note

Even if the result has a much larger bit width than the input, the exact mathematical answer is given.

6.1.6.2.5 BigInt::divide ( `x`, `y` )

If y is 0n, throw a RangeError exception.
Let quotient be the mathematical value of x divided by y.
Return the BigInt value that represents quotient rounded towards 0 to the next integral value.

6.1.6.2.6 BigInt::remainder ( `n`, `d` )

If d is 0n, throw a RangeError exception.
If n is 0n, return 0n.
Let r be the BigInt defined by the mathematical relation r = n - (d × q) where q is a BigInt that is negative only if n/d is negative and positive only if n/d is positive, and whose magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient of n and d.
Return r.

Note

The sign of the result equals the sign of the dividend.

6.1.6.2.7 BigInt::add ( `x`, `y` )

The abstract operation BigInt::add takes arguments x (a BigInt) and y (a BigInt). It returns the BigInt value that represents the sum of x and y.

6.1.6.2.8 BigInt::subtract ( `x`, `y` )

The abstract operation BigInt::subtract takes arguments x (a BigInt) and y (a BigInt). It returns the BigInt value that represents the difference x minus y.

6.1.6.2.9 BigInt::leftShift ( `x`, `y` )

The abstract operation BigInt::leftShift takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:

If y < 0n, then
1. Return the BigInt value that represents x ÷ 2^-y, rounding down to the nearest integer, including for negative numbers.
Return the BigInt value that represents x × 2^y.

Note

Semantics here should be equivalent to a bitwise shift, treating the BigInt as an infinite length string of binary two's complement digits.

6.1.6.2.10 BigInt::signedRightShift ( `x`, `y` )

The abstract operation BigInt::signedRightShift takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:

Return BigInt::leftShift(x, -y).

6.1.6.2.11 BigInt::unsignedRightShift ( `x`, `y` )

The abstract operation BigInt::unsignedRightShift takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:

Throw a TypeError exception.

6.1.6.2.12 BigInt::lessThan ( `x`, `y` )

The abstract operation BigInt::lessThan takes arguments x (a BigInt) and y (a BigInt). It returns true if x is less than y and false otherwise.

6.1.6.2.13 BigInt::equal ( `x`, `y` )

The abstract operation BigInt::equal takes arguments x (a BigInt) and y (a BigInt). It returns true if x and y have the same mathematical integer value and false otherwise.

6.1.6.2.14 BigInt::sameValue ( `x`, `y` )

The abstract operation BigInt::sameValue takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:

Return BigInt::equal(x, y).

6.1.6.2.15 BigInt::sameValueZero ( `x`, `y` )

The abstract operation BigInt::sameValueZero takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:

Return BigInt::equal(x, y).

6.1.6.2.16 BinaryAnd ( `x`, `y` )

The abstract operation BinaryAnd takes arguments x and y. It performs the following steps when called:

Assert: x is 0 or 1.
Assert: y is 0 or 1.
If x is 1 and y is 1, return 1.
Else, return 0.

6.1.6.2.17 BinaryOr ( `x`, `y` )

The abstract operation BinaryOr takes arguments x and y. It performs the following steps when called:

Assert: x is 0 or 1.
Assert: y is 0 or 1.
If x is 1 or y is 1, return 1.
Else, return 0.

6.1.6.2.18 BinaryXor ( `x`, `y` )

The abstract operation BinaryXor takes arguments x and y. It performs the following steps when called:

Assert: x is 0 or 1.
Assert: y is 0 or 1.
If x is 1 and y is 0, return 1.
Else if x is 0 and y is 1, return 1.
Else, return 0.

6.1.6.2.19 BigIntBitwiseOp ( `op`, `x`, `y` )

The abstract operation BigIntBitwiseOp takes arguments op, x, and y. It performs the following steps when called:

Assert: op is "&", "|", or "^".
Let result be 0n.
Let shift be 0.
Repeat, until (x = 0 or x = -1) and (y = 0 or y = -1),
1. Let xDigit be x modulo 2.
2. Let yDigit be y modulo 2.
3. If op is "&", set result to result + 2^shift × BinaryAnd(xDigit, yDigit).
4. Else if op is "|", set result to result + 2^shift × BinaryOr(xDigit, yDigit).
5. Else,
  1. Assert: op is "^".
  2. Set result to result + 2^shift × BinaryXor(xDigit, yDigit).
6. Set shift to shift + 1.
7. Set x to (x - xDigit) / 2.
8. Set y to (y - yDigit) / 2.
If op is "&", let tmp be BinaryAnd(x modulo 2, y modulo 2).
Else if op is "|", let tmp be BinaryOr(x modulo 2, y modulo 2).
Else,
1. Assert: op is "^".
2. Let tmp be BinaryXor(x modulo 2, y modulo 2).
If tmp ≠ 0, then
1. Set result to result - 2^shift.
2. NOTE: This extends the sign.
Return result.

6.1.6.2.20 BigInt::bitwiseAND ( `x`, `y` )

Return BigIntBitwiseOp("&", x, y).

6.1.6.2.21 BigInt::bitwiseXOR ( `x`, `y` )

Return BigIntBitwiseOp("^", x, y).

6.1.6.2.22 BigInt::bitwiseOR ( `x`, `y` )

Return BigIntBitwiseOp("|", x, y).

6.1.6.2.23 BigInt::toString ( `x` )

The abstract operation BigInt::toString takes argument x (a BigInt). It converts x to String format. It performs the following steps when called:

If x is less than zero, return the string-concatenation of the String "-" and ! BigInt::toString(-x).
Return the String value consisting of the code units of the digits of the decimal representation of x.

6.1.7 The Object Type

An Object is logically a collection of properties. Each property is either a data property, or an accessor property:

A data property associates a key value with an ECMAScript language value and a set of Boolean attributes.
An accessor property associates a key value with one or two accessor functions, and a set of Boolean attributes. The accessor functions are used to store or retrieve an ECMAScript language value that is associated with the property.

Properties are identified using key values. A property key value is either an ECMAScript String value or a Symbol value. All String and Symbol values, including the empty String, are valid as property keys. A property name is a property key that is a String value.

An integer index is a String-valued property key that is a canonical numeric String (see 7.1.21) and whose numeric value is either +0 or a positive integer ≤ 2⁵³ - 1. An array index is an integer index whose numeric value i is in the range +0 ≤ i < 2³² - 1.

Property keys are used to access properties and their values. There are two kinds of access for properties: get and set, corresponding to value retrieval and assignment, respectively. The properties accessible via get and set access includes both own properties that are a direct part of an object and inherited properties which are provided by another associated object via a property inheritance relationship. Inherited properties may be either own or inherited properties of the associated object. Each own property of an object must each have a key value that is distinct from the key values of the other own properties of that object.

All objects are logically collections of properties, but there are multiple forms of objects that differ in their semantics for accessing and manipulating their properties. Please see 6.1.7.2 for definitions of the multiple forms of objects.

6.1.7.1 Property Attributes

Attributes are used in this specification to define and explain the state of Object properties. A data property associates a key value with the attributes listed in Table 3.

Table 3: Attributes of a Data Property

Attribute Name	Value Domain	Description
[[Value]]	Any ECMAScript language type	The value retrieved by a get access of the property.
[[Writable]]	Boolean	If false, attempts by ECMAScript code to change the property's [[Value]] attribute using [[Set]] will not succeed.
[[Enumerable]]	Boolean	If true, the property will be enumerated by a for-in enumeration (see 13.7.5). Otherwise, the property is said to be non-enumerable.
[[Configurable]]	Boolean	If false, attempts to delete the property, change the property to be an accessor property, or change its attributes (other than [[Value]], or changing [[Writable]] to false) will fail.

An accessor property associates a key value with the attributes listed in Table 4.

Table 4: Attributes of an Accessor Property

Attribute Name	Value Domain	Description
[[Get]]	Object \| Undefined	If the value is an Object it must be a function object. The function's [[Call]] internal method (Table 7) is called with an empty arguments list to retrieve the property value each time a get access of the property is performed.
[[Set]]	Object \| Undefined	If the value is an Object it must be a function object. The function's [[Call]] internal method (Table 7) is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed. The effect of a property's [[Set]] internal method may, but is not required to, have an effect on the value returned by subsequent calls to the property's [[Get]] internal method.
[[Enumerable]]	Boolean	If true, the property is to be enumerated by a for-in enumeration (see 13.7.5). Otherwise, the property is said to be non-enumerable.
[[Configurable]]	Boolean	If false, attempts to delete the property, change the property to be a data property, or change its attributes will fail.

If the initial values of a property's attributes are not explicitly specified by this specification, the default value defined in Table 5 is used.

Table 5: Default Attribute Values

Attribute Name	Default Value
[[Value]]	undefined
[[Get]]	undefined
[[Set]]	undefined
[[Writable]]	false
[[Enumerable]]	false
[[Configurable]]	false

6.1.7.2 Object Internal Methods and Internal Slots

The actual semantics of objects, in ECMAScript, are specified via algorithms called internal methods. Each object in an ECMAScript engine is associated with a set of internal methods that defines its runtime behaviour. These internal methods are not part of the ECMAScript language. They are defined by this specification purely for expository purposes. However, each object within an implementation of ECMAScript must behave as specified by the internal methods associated with it. The exact manner in which this is accomplished is determined by the implementation.

Internal method names are polymorphic. This means that different object values may perform different algorithms when a common internal method name is invoked upon them. That actual object upon which an internal method is invoked is the “target” of the invocation. If, at runtime, the implementation of an algorithm attempts to use an internal method of an object that the object does not support, a TypeError exception is thrown.

Internal slots correspond to internal state that is associated with objects and used by various ECMAScript specification algorithms. Internal slots are not object properties and they are not inherited. Depending upon the specific internal slot specification, such state may consist of values of any ECMAScript language type or of specific ECMAScript specification type values. Unless explicitly specified otherwise, internal slots are allocated as part of the process of creating an object and may not be dynamically added to an object. Unless specified otherwise, the initial value of an internal slot is the value undefined. Various algorithms within this specification create objects that have internal slots. However, the ECMAScript language provides no direct way to associate internal slots with an object.

Internal methods and internal slots are identified within this specification using names enclosed in double square brackets [[ ]].

Table 6 summarizes the essential internal methods used by this specification that are applicable to all objects created or manipulated by ECMAScript code. Every object must have algorithms for all of the essential internal methods. However, all objects do not necessarily use the same algorithms for those methods.

An ordinary object is an object that satisfies all of the following criteria:

For the internal methods listed in Table 6, the object use those defined in 9.1.
If the object has a [[Call]] internal method, it uses the one defined in 9.2.1.
If the object has a [[Construct]] internal method, it uses the one defined in 9.2.2.

An exotic object is an object that is not an ordinary object.

This specification recognizes different kinds of exotic objects by those objects' internal methods. An object that is behaviourally equivalent to a particular kind of exotic object (such as an Array exotic object or a bound function exotic object), but does not have the same collection of internal methods specified for that kind, is not recognized as that kind of exotic object.

The “Signature” column of Table 6 and other similar tables describes the invocation pattern for each internal method. The invocation pattern always includes a parenthesized list of descriptive parameter names. If a parameter name is the same as an ECMAScript type name then the name describes the required type of the parameter value. If an internal method explicitly returns a value, its parameter list is followed by the symbol “→” and the type name of the returned value. The type names used in signatures refer to the types defined in clause 6 augmented by the following additional names. “any” means the value may be any ECMAScript language type.

In addition to its parameters, an internal method always has access to the object that is the target of the method invocation.

An internal method implicitly returns a Completion Record, either a normal completion that wraps a value of the return type shown in its invocation pattern, or a throw completion.

Table 6: Essential Internal Methods

Internal Method	Signature	Description
[[GetPrototypeOf]]	( ) → Object \| Null	Determine the object that provides inherited properties for this object. A null value indicates that there are no inherited properties.
[[SetPrototypeOf]]	(Object \| Null) → Boolean	Associate this object with another object that provides inherited properties. Passing null indicates that there are no inherited properties. Returns true indicating that the operation was completed successfully or false indicating that the operation was not successful.
[[IsExtensible]]	( ) → Boolean	Determine whether it is permitted to add additional properties to this object.
[[PreventExtensions]]	( ) → Boolean	Control whether new properties may be added to this object. Returns true if the operation was successful or false if the operation was unsuccessful.
[[GetOwnProperty]]	(`propertyKey`) → Undefined \| Property Descriptor	Return a Property Descriptor for the own property of this object whose key is `propertyKey`, or undefined if no such property exists.
[[DefineOwnProperty]]	(`propertyKey`, `PropertyDescriptor`) → Boolean	Create or alter the own property, whose key is `propertyKey`, to have the state described by `PropertyDescriptor`. Return true if that property was successfully created/updated or false if the property could not be created or updated.
[[HasProperty]]	(`propertyKey`) → Boolean	Return a Boolean value indicating whether this object already has either an own or inherited property whose key is `propertyKey`.
[[Get]]	(`propertyKey`, `Receiver`) → any	Return the value of the property whose key is `propertyKey` from this object. If any ECMAScript code must be executed to retrieve the property value, `Receiver` is used as the this value when evaluating the code.
[[Set]]	(`propertyKey`, `value`, `Receiver`) → Boolean	Set the value of the property whose key is `propertyKey` to `value`. If any ECMAScript code must be executed to set the property value, `Receiver` is used as the this value when evaluating the code. Returns true if the property value was set or false if it could not be set.
[[Delete]]	(`propertyKey`) → Boolean	Remove the own property whose key is `propertyKey` from this object. Return false if the property was not deleted and is still present. Return true if the property was deleted or is not present.
[[OwnPropertyKeys]]	( ) → List of propertyKey	Return a List whose elements are all of the own property keys for the object.

Table 7 summarizes additional essential internal methods that are supported by objects that may be called as functions. A function object is an object that supports the [[Call]] internal method. A constructor is an object that supports the [[Construct]] internal method. Every object that supports [[Construct]] must support [[Call]]; that is, every constructor must be a function object. Therefore, a constructor may also be referred to as a constructor function or constructor function object.

Table 7: Additional Essential Internal Methods of Function Objects

Internal Method	Signature	Description
[[Call]]	(any, a List of any) → any	Executes code associated with this object. Invoked via a function call expression. The arguments to the internal method are a this value and a list containing the arguments passed to the function by a call expression. Objects that implement this internal method are callable.
[[Construct]]	(a List of any, Object) → Object	Creates an object. Invoked via the `new` operator or a `super` call. The first argument to the internal method is a list containing the arguments of the constructor invocation or the `super` call. The second argument is the object to which the `new` operator was initially applied. Objects that implement this internal method are called constructors. A function object is not necessarily a constructor and such non-constructor function objects do not have a [[Construct]] internal method.

The semantics of the essential internal methods for ordinary objects and standard exotic objects are specified in clause 9. If any specified use of an internal method of an exotic object is not supported by an implementation, that usage must throw a TypeError exception when attempted.

6.1.7.3 Invariants of the Essential Internal Methods

The Internal Methods of Objects of an ECMAScript engine must conform to the list of invariants specified below. Ordinary ECMAScript Objects as well as all standard exotic objects in this specification maintain these invariants. ECMAScript Proxy objects maintain these invariants by means of runtime checks on the result of traps invoked on the [[ProxyHandler]] object.

Any implementation provided exotic objects must also maintain these invariants for those objects. Violation of these invariants may cause ECMAScript code to have unpredictable behaviour and create security issues. However, violation of these invariants must never compromise the memory safety of an implementation.

An implementation must not allow these invariants to be circumvented in any manner such as by providing alternative interfaces that implement the functionality of the essential internal methods without enforcing their invariants.

Definitions:

The target of an internal method is the object upon which the internal method is called.
A target is non-extensible if it has been observed to return false from its [[IsExtensible]] internal method, or true from its [[PreventExtensions]] internal method.
A non-existent property is a property that does not exist as an own property on a non-extensible target.
All references to SameValue are according to the definition of the SameValue algorithm.

Return value:

The value returned by any internal method must be a Completion Record with either:

[[Type]] = normal, [[Target]] = empty, and [[Value]] = a value of the "normal return type" shown below for that internal method, or
[[Type]] = throw, [[Target]] = empty, and [[Value]] = any ECMAScript language value.

Note 1

An internal method must not return a completion with [[Type]] = continue, break, or return.

[[GetPrototypeOf]] ( )

The normal return type is either Object or Null.
If target is non-extensible, and [[GetPrototypeOf]] returns a value V, then any future calls to [[GetPrototypeOf]] should return the SameValue as V.

Note 2

An object's prototype chain should have finite length (that is, starting from any object, recursively applying the [[GetPrototypeOf]] internal method to its result should eventually lead to the value null). However, this requirement is not enforceable as an object level invariant if the prototype chain includes any exotic objects that do not use the ordinary object definition of [[GetPrototypeOf]]. Such a circular prototype chain may result in infinite loops when accessing object properties.

[[SetPrototypeOf]] ( `V` )

The normal return type is Boolean.
If target is non-extensible, [[SetPrototypeOf]] must return false, unless V is the SameValue as the target's observed [[GetPrototypeOf]] value.

[[IsExtensible]] ( )

The normal return type is Boolean.
If [[IsExtensible]] returns false, all future calls to [[IsExtensible]] on the target must return false.

[[PreventExtensions]] ( )

The normal return type is Boolean.
If [[PreventExtensions]] returns true, all future calls to [[IsExtensible]] on the target must return false and the target is now considered non-extensible.

[[GetOwnProperty]] ( `P` )

The normal return type is either Property Descriptor or Undefined.
If the Type of the return value is Property Descriptor, the return value must be a complete property descriptor.
If P is described as a non-configurable, non-writable own data property, all future calls to [[GetOwnProperty]] ( P ) must return Property Descriptor whose [[Value]] is SameValue as P's [[Value]] attribute.
If P's attributes other than [[Writable]] may change over time or if the property might be deleted, then P's [[Configurable]] attribute must be true.
If the [[Writable]] attribute may change from false to true, then the [[Configurable]] attribute must be true.
If the target is non-extensible and P is non-existent, then all future calls to [[GetOwnProperty]] (P) on the target must describe P as non-existent (i.e. [[GetOwnProperty]] (P) must return undefined).

Note 3

As a consequence of the third invariant, if a property is described as a data property and it may return different values over time, then either or both of the [[Writable]] and [[Configurable]] attributes must be true even if no mechanism to change the value is exposed via the other essential internal methods.

[[DefineOwnProperty]] ( `P`, `Desc` )

The normal return type is Boolean.
[[DefineOwnProperty]] must return false if P has previously been observed as a non-configurable own property of the target, unless either:
1. P is a writable data property. A non-configurable writable data property can be changed into a non-configurable non-writable data property.
2. All attributes of Desc are the SameValue as P's attributes.
[[DefineOwnProperty]] (P, Desc) must return false if target is non-extensible and P is a non-existent own property. That is, a non-extensible target object cannot be extended with new properties.

[[HasProperty]] ( `P` )

The normal return type is Boolean.
If P was previously observed as a non-configurable own data or accessor property of the target, [[HasProperty]] must return true.

[[Get]] ( `P`, `Receiver` )

The normal return type is any ECMAScript language type.
If P was previously observed as a non-configurable, non-writable own data property of the target with value V, then [[Get]] must return the SameValue as V.
If P was previously observed as a non-configurable own accessor property of the target whose [[Get]] attribute is undefined, the [[Get]] operation must return undefined.

[[Set]] ( `P`, `V`, `Receiver` )

The normal return type is Boolean.
If P was previously observed as a non-configurable, non-writable own data property of the target, then [[Set]] must return false unless V is the SameValue as P's [[Value]] attribute.
If P was previously observed as a non-configurable own accessor property of the target whose [[Set]] attribute is undefined, the [[Set]] operation must return false.

[[Delete]] ( `P` )

The normal return type is Boolean.
If P was previously observed as a non-configurable own data or accessor property of the target, [[Delete]] must return false.

[[OwnPropertyKeys]] ( )

The normal return type is List.
The returned List must not contain any duplicate entries.
The Type of each element of the returned List is either String or Symbol.
The returned List must contain at least the keys of all non-configurable own properties that have previously been observed.
If the object is non-extensible, the returned List must contain only the keys of all own properties of the object that are observable using [[GetOwnProperty]].

[[Call]] ( )

The normal return type is any ECMAScript language type.

[[Construct]] ( )

The normal return type is Object.

6.1.7.4 Well-Known Intrinsic Objects

Well-known intrinsics are built-in objects that are explicitly referenced by the algorithms of this specification and which usually have realm-specific identities. Unless otherwise specified each intrinsic object actually corresponds to a set of similar objects, one per realm.

Within this specification a reference such as %name% means the intrinsic object, associated with the current realm, corresponding to the name. A reference such as %name.a.b% means, as if the "b" property of the "a" property of the intrinsic object %name% was accessed prior to any ECMAScript code being evaluated. Determination of the current realm and its intrinsics is described in 8.3. The well-known intrinsics are listed in Table 8.

Table 8: Well-Known Intrinsic Objects

Intrinsic Name	Global Name	ECMAScript Language Association
%Array%	`Array`	The Array constructor (22.1.1)
%ArrayBuffer%	`ArrayBuffer`	The ArrayBuffer constructor (24.1.2)
%ArrayBufferPrototype%	`ArrayBuffer.prototype`	The initial value of the "prototype" data property of %ArrayBuffer%; i.e., %ArrayBuffer.prototype%
%ArrayIteratorPrototype%		The prototype of Array iterator objects (22.1.5); i.e., %ArrayIterator.prototype%
%ArrayPrototype%	`Array.prototype`	The initial value of the "prototype" data property of %Array% (22.1.3); i.e., %Array.prototype%
%ArrayProto_entries%	`Array.prototype.entries`	The initial value of the "entries" data property of %Array.prototype% (22.1.3.4); i.e., %Array.prototype.entries%
%ArrayProto_forEach%	`Array.prototype.forEach`	The initial value of the "forEach" data property of %Array.prototype% (22.1.3.12); i.e., %Array.prototype.forEach%
%ArrayProto_keys%	`Array.prototype.keys`	The initial value of the "keys" data property of %Array.prototype% (22.1.3.16); i.e., %Array.prototype.keys%
%ArrayProto_values%	`Array.prototype.values`	The initial value of the "values" data property of %Array.prototype% (22.1.3.32); i.e., %Array.prototype.values%
%AsyncFromSyncIteratorPrototype%		The prototype of async-from-sync iterator objects (25.1.4)
%AsyncFunction%		The constructor of async function objects (25.7.1)
%AsyncFunctionPrototype%		The initial value of the "prototype" data property of %AsyncFunction%; i.e., %AsyncFunction.prototype%
%AsyncGenerator%		The initial value of the "prototype" property of %AsyncGeneratorFunction%; i.e., %AsyncGeneratorFunction.prototype%
%AsyncGeneratorFunction%		The constructor of async iterator objects (25.3.1)
%AsyncGeneratorPrototype%		The initial value of the "prototype" property of %AsyncGenerator%; i.e., %AsyncGenerator.prototype%
%AsyncIteratorPrototype%		An object that all standard built-in async iterator objects indirectly inherit from
%Atomics%	`Atomics`	The `Atomics` object (24.4)
%BigInt%	`BigInt`	The BigInt constructor (20.2.1)
%BigInt64Array%	`BigInt64Array`	The BigInt64Array constructor (22.2)
%BigUint64Array%	`BigUint64Array`	The BigUint64Array constructor (22.2)
%Boolean%	`Boolean`	The Boolean constructor (19.3.1)
%BooleanPrototype%	`Boolean.prototype`	The initial value of the "prototype" data property of %Boolean% (19.3.3); i.e., %Boolean.prototype%
%DataView%	`DataView`	The DataView constructor (24.3.2)
%DataViewPrototype%	`DataView.prototype`	The initial value of the "prototype" data property of %DataView%; i.e., %DataView.prototype%
%Date%	`Date`	The Date constructor (20.4.2)
%DatePrototype%	`Date.prototype`	The initial value of the "prototype" data property of %Date%.; i.e., %Date.prototype%
%decodeURI%	`decodeURI`	The `decodeURI` function (18.2.6.2)
%decodeURIComponent%	`decodeURIComponent`	The `decodeURIComponent` function (18.2.6.3)
%encodeURI%	`encodeURI`	The `encodeURI` function (18.2.6.4)
%encodeURIComponent%	`encodeURIComponent`	The `encodeURIComponent` function (18.2.6.5)
%Error%	`Error`	The Error constructor (19.5.1)
%ErrorPrototype%	`Error.prototype`	The initial value of the "prototype" data property of %Error%; i.e., %Error.prototype%
%eval%	`eval`	The `eval` function (18.2.1)
%EvalError%	`EvalError`	The EvalError constructor (19.5.5.1)
%EvalErrorPrototype%	`EvalError.prototype`	The initial value of the "prototype" data property of %EvalError%; i.e., %EvalError.prototype%
%Float32Array%	`Float32Array`	The Float32Array constructor (22.2)
%Float32ArrayPrototype%	`Float32Array.prototype`	The initial value of the "prototype" data property of %Float32Array%; i.e., %Float32Array.prototype%
%Float64Array%	`Float64Array`	The Float64Array constructor (22.2)
%Float64ArrayPrototype%	`Float64Array.prototype`	The initial value of the "prototype" data property of %Float64Array%; i.e., %Float64Array.prototype%
%ForInIteratorPrototype%		The prototype of For-In iterator objects (13.7.5.16)
%Function%	`Function`	The Function constructor (19.2.1)
%FunctionPrototype%	`Function.prototype`	The initial value of the "prototype" data property of %Function%; i.e., %Function.prototype%
%Generator%		The initial value of the "prototype" data property of %GeneratorFunction%
%GeneratorFunction%		The constructor of generator objects (25.2.1)
%GeneratorPrototype%		The initial value of the "prototype" data property of %Generator%; i.e., %Generator.prototype%
%Int8Array%	`Int8Array`	The Int8Array constructor (22.2)
%Int8ArrayPrototype%	`Int8Array.prototype`	The initial value of the "prototype" data property of %Int8Array%; i.e., %Int8Array.prototype%
%Int16Array%	`Int16Array`	The Int16Array constructor (22.2)
%Int16ArrayPrototype%	`Int16Array.prototype`	The initial value of the "prototype" data property of %Int16Array%; i.e., %Int16Array.prototype%
%Int32Array%	`Int32Array`	The Int32Array constructor (22.2)
%Int32ArrayPrototype%	`Int32Array.prototype`	The initial value of the "prototype" data property of %Int32Array%; i.e., %Int32Array.prototype%
%isFinite%	`isFinite`	The `isFinite` function (18.2.2)
%isNaN%	`isNaN`	The `isNaN` function (18.2.3)
%IteratorPrototype%		An object that all standard built-in iterator objects indirectly inherit from
%JSON%	`JSON`	The `JSON` object (24.5)
%JSONParse%	`JSON.parse`	The initial value of the "parse" data property of %JSON%; i.e., %JSON.parse%
%JSONStringify%	`JSON.stringify`	The initial value of the "stringify" data property of %JSON%; i.e., %JSON.stringify%
%Map%	`Map`	The Map constructor (23.1.1)
%MapIteratorPrototype%		The prototype of Map iterator objects (23.1.5)
%MapPrototype%	`Map.prototype`	The initial value of the "prototype" data property of %Map%; i.e., %Map.prototype%
%Math%	`Math`	The `Math` object (20.3)
%Number%	`Number`	The Number constructor (20.1.1)
%NumberPrototype%	`Number.prototype`	The initial value of the "prototype" data property of %Number%; i.e., %Number.prototype%
%Object%	`Object`	The Object constructor (19.1.1)
%ObjectPrototype%	`Object.prototype`	The initial value of the "prototype" data property of %Object% (19.1.3); i.e., %Object.prototype%
%ObjProto_toString%	`Object.prototype.toString`	The initial value of the "toString" data property of %Object.prototype% (19.1.3.6); i.e., %Object.prototype.toString%
%ObjProto_valueOf%	`Object.prototype.valueOf`	The initial value of the "valueOf" data property of %Object.prototype% (19.1.3.7); i.e., %Object.prototype.valueOf%
%parseFloat%	`parseFloat`	The `parseFloat` function (18.2.4)
%parseInt%	`parseInt`	The `parseInt` function (18.2.5)
%Promise%	`Promise`	The Promise constructor (25.6.3)
%PromisePrototype%	`Promise.prototype`	The initial value of the "prototype" data property of %Promise%; i.e., %Promise.prototype%
%PromiseProto_then%	`Promise.prototype.then`	The initial value of the "then" data property of %Promise.prototype% (25.6.5.4); i.e., %Promise.prototype.then%
%Promise_all%	`Promise.all`	The initial value of the "all" data property of %Promise% (25.6.4.1); i.e., %Promise.all%
%Promise_reject%	`Promise.reject`	The initial value of the "reject" data property of %Promise% (25.6.4.5); i.e., %Promise.reject%
%Promise_resolve%	`Promise.resolve`	The initial value of the "resolve" data property of %Promise% (25.6.4.6); i.e., %Promise.resolve%
%Proxy%	`Proxy`	The Proxy constructor (26.2.1)
%RangeError%	`RangeError`	The RangeError constructor (19.5.5.2)
%RangeErrorPrototype%	`RangeError.prototype`	The initial value of the "prototype" data property of %RangeError%; i.e., %RangeError.prototype%
%ReferenceError%	`ReferenceError`	The ReferenceError constructor (19.5.5.3)
%ReferenceErrorPrototype%	`ReferenceError.prototype`	The initial value of the "prototype" data property of %ReferenceError%; i.e., %ReferenceError.prototype%
%Reflect%	`Reflect`	The `Reflect` object (26.1)
%RegExp%	`RegExp`	The RegExp constructor (21.2.3)
%RegExpPrototype%	`RegExp.prototype`	The initial value of the "prototype" data property of %RegExp%; i.e., %RegExp.prototype%
%RegExpStringIteratorPrototype%		The prototype of RegExp String Iterator objects (21.2.7)
%Set%	`Set`	The Set constructor (23.2.1)
%SetIteratorPrototype%		The prototype of Set iterator objects (23.2.5)
%SetPrototype%	`Set.prototype`	The initial value of the "prototype" data property of %Set%; i.e., %Set.prototype%
%SharedArrayBuffer%	`SharedArrayBuffer`	The SharedArrayBuffer constructor (24.2.2)
%SharedArrayBufferPrototype%	`SharedArrayBuffer.prototype`	The initial value of the "prototype" data property of %SharedArrayBuffer%; i.e., %SharedArrayBuffer.prototype%
%String%	`String`	The String constructor (21.1.1)
%StringIteratorPrototype%		The prototype of String iterator objects (21.1.5)
%StringPrototype%	`String.prototype`	The initial value of the "prototype" data property of %String%; i.e., %String.prototype%
%Symbol%	`Symbol`	The Symbol constructor (19.4.1)
%SymbolPrototype%	`Symbol.prototype`	The initial value of the "prototype" data property of %Symbol% (19.4.3); i.e., %Symbol.prototype%
%SyntaxError%	`SyntaxError`	The SyntaxError constructor (19.5.5.4)
%SyntaxErrorPrototype%	`SyntaxError.prototype`	The initial value of the "prototype" data property of %SyntaxError%; i.e., %SyntaxError.prototype%
%ThrowTypeError%		A function object that unconditionally throws a new instance of %TypeError%
%TypedArray%		The super class of all typed Array constructors (22.2.1)
%TypedArrayPrototype%		The initial value of the "prototype" data property of %TypedArray%; i.e., %TypedArray.prototype%
%TypeError%	`TypeError`	The TypeError constructor (19.5.5.5)
%TypeErrorPrototype%	`TypeError.prototype`	The initial value of the "prototype" data property of %TypeError%; i.e., %TypeError.prototype%
%Uint8Array%	`Uint8Array`	The Uint8Array constructor (22.2)
%Uint8ArrayPrototype%	`Uint8Array.prototype`	The initial value of the "prototype" data property of %Uint8Array%; i.e., %Uint8Array.prototype%
%Uint8ClampedArray%	`Uint8ClampedArray`	The Uint8ClampedArray constructor (22.2)
%Uint8ClampedArrayPrototype%	`Uint8ClampedArray.prototype`	The initial value of the "prototype" data property of %Uint8ClampedArray%; i.e., %Uint8ClampedArray.prototype%
%Uint16Array%	`Uint16Array`	The Uint16Array constructor (22.2)
%Uint16ArrayPrototype%	`Uint16Array.prototype`	The initial value of the "prototype" data property of %Uint16Array%; i.e., %Uint16Array.prototype%
%Uint32Array%	`Uint32Array`	The Uint32Array constructor (22.2)
%Uint32ArrayPrototype%	`Uint32Array.prototype`	The initial value of the "prototype" data property of %Uint32Array%; i.e., %Uint32Array.prototype%
%URIError%	`URIError`	The URIError constructor (19.5.5.6)
%URIErrorPrototype%	`URIError.prototype`	The initial value of the "prototype" data property of %URIError%; i.e., %URIError.prototype%
%WeakMap%	`WeakMap`	The WeakMap constructor (23.3.1)
%WeakMapPrototype%	`WeakMap.prototype`	The initial value of the "prototype" data property of %WeakMap%; i.e., %WeakMap.prototype%
%WeakSet%	`WeakSet`	The WeakSet constructor (23.4.1)
%WeakSetPrototype%	`WeakSet.prototype`	The initial value of the "prototype" data property of %WeakSet%; i.e., %WeakSet.prototype%

6.2 ECMAScript Specification Types

A specification type corresponds to meta-values that are used within algorithms to describe the semantics of ECMAScript language constructs and ECMAScript language types. The specification types include Reference, List, Completion, Property Descriptor, Environment Record, Abstract Closure, and Data Block. Specification type values are specification artefacts that do not necessarily correspond to any specific entity within an ECMAScript implementation. Specification type values may be used to describe intermediate results of ECMAScript expression evaluation but such values cannot be stored as properties of objects or values of ECMAScript language variables.

6.2.1 The List and Record Specification Types

The List type is used to explain the evaluation of argument lists (see 12.3.8) in new expressions, in function calls, and in other algorithms where a simple ordered list of values is needed. Values of the List type are simply ordered sequences of list elements containing the individual values. These sequences may be of any length. The elements of a list may be randomly accessed using 0-origin indices. For notational convenience an array-like syntax can be used to access List elements. For example, arguments[2] is shorthand for saying the 3^rd element of the List arguments.

For notational convenience within this specification, a literal syntax can be used to express a new List value. For example, « 1, 2 » defines a List value that has two elements each of which is initialized to a specific value. A new empty List can be expressed as « ».

The Record type is used to describe data aggregations within the algorithms of this specification. A Record type value consists of one or more named fields. The value of each field is either an ECMAScript value or an abstract value represented by a name associated with the Record type. Field names are always enclosed in double brackets, for example [[Value]].

For notational convenience within this specification, an object literal-like syntax can be used to express a Record value. For example, { [[Field1]]: 42, [[Field2]]: false, [[Field3]]: empty } defines a Record value that has three fields, each of which is initialized to a specific value. Field name order is not significant. Any fields that are not explicitly listed are considered to be absent.

In specification text and algorithms, dot notation may be used to refer to a specific field of a Record value. For example, if R is the record shown in the previous paragraph then R.[[Field2]] is shorthand for “the field of R named [[Field2]]”.

Schema for commonly used Record field combinations may be named, and that name may be used as a prefix to a literal Record value to identify the specific kind of aggregations that is being described. For example: PropertyDescriptor { [[Value]]: 42, [[Writable]]: false, [[Configurable]]: true }.

6.2.2 The Set and Relation Specification Types

The Set type is used to explain a collection of unordered elements for use in the memory model. Values of the Set type are simple collections of elements, where no element appears more than once. Elements may be added to and removed from Sets. Sets may be unioned, intersected, or subtracted from each other.

The Relation type is used to explain constraints on Sets. Values of the Relation type are Sets of ordered pairs of values from its value domain. For example, a Relation on events is a set of ordered pairs of events. For a Relation R and two values a and b in the value domain of R, a R b is shorthand for saying the ordered pair (a, b) is a member of R. A Relation is least with respect to some conditions when it is the smallest Relation that satisfies those conditions.

A strict partial order is a Relation value R that satisfies the following.

For all a, b, and c in R's domain:
- It is not the case that a R a, and
- If a R b and b R c, then a R c.

Note 1

The two properties above are called, in order, irreflexivity and transitivity.

A strict total order is a Relation value R that satisfies the following.

For all a, b, and c in R's domain:
- a is identical to b or a R b or b R a, and
- It is not the case that a R a, and
- If a R b and b R c, then a R c.

Note 2

The three properties above are called, in order, totality, irreflexivity, and transitivity.

6.2.3 The Completion Record Specification Type

The Completion type is a Record used to explain the runtime propagation of values and control flow such as the behaviour of statements (break, continue, return and throw) that perform nonlocal transfers of control.

Values of the Completion type are Record values whose fields are defined as by Table 9. Such values are referred to as Completion Records.

Table 9: Completion Record Fields

Field Name	Value	Meaning
[[Type]]	One of normal, break, continue, return, or throw	The type of completion that occurred.
[[Value]]	any ECMAScript language value or empty	The value that was produced.
[[Target]]	any ECMAScript string or empty	The target label for directed control transfers.

The term “abrupt completion” refers to any completion with a [[Type]] value other than normal.

6.2.3.1 Await

Algorithm steps that say

Let completion be Await(value).

mean the same thing as:

Let asyncContext be the running execution context.
Let promise be ? PromiseResolve(%Promise%, value).
Let stepsFulfilled be the algorithm steps defined in Await Fulfilled Functions.
Let onFulfilled be ! CreateBuiltinFunction(stepsFulfilled, « [[AsyncContext]] »).
Set onFulfilled.[[AsyncContext]] to asyncContext.
Let stepsRejected be the algorithm steps defined in Await Rejected Functions.
Let onRejected be ! CreateBuiltinFunction(stepsRejected, « [[AsyncContext]] »).
Set onRejected.[[AsyncContext]] to asyncContext.
Perform ! PerformPromiseThen(promise, onFulfilled, onRejected).
Remove asyncContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
Set the code evaluation state of asyncContext such that when evaluation is resumed with a Completion completion, the following steps of the algorithm that invoked Await will be performed, with completion available.
Return.
NOTE: This returns to the evaluation of the operation that had most previously resumed evaluation of asyncContext.

where all aliases in the above steps, with the exception of completion, are ephemeral and visible only in the steps pertaining to Await.

Note

Await can be combined with the ? and ! prefixes, so that for example

Let result be ? Await(value).

means the same thing as:

Let result be Await(value).
ReturnIfAbrupt(result).

6.2.3.1.1 Await Fulfilled Functions

An Await fulfilled function is an anonymous built-in function that is used as part of the Await specification device to deliver the promise fulfillment value to the caller as a normal completion. Each Await fulfilled function has an [[AsyncContext]] internal slot.

When an Await fulfilled function is called with argument value, the following steps are taken:

Let F be the active function object.
Let asyncContext be F.[[AsyncContext]].
Let prevContext be the running execution context.
Suspend prevContext.
Push asyncContext onto the execution context stack; asyncContext is now the running execution context.
Resume the suspended evaluation of asyncContext using NormalCompletion(value) as the result of the operation that suspended it.
Assert: When we reach this step, asyncContext has already been removed from the execution context stack and prevContext is the currently running execution context.
Return undefined.

The "length" property of an Await fulfilled function is 1.

6.2.3.1.2 Await Rejected Functions

An Await rejected function is an anonymous built-in function that is used as part of the Await specification device to deliver the promise rejection reason to the caller as an abrupt throw completion. Each Await rejected function has an [[AsyncContext]] internal slot.

When an Await rejected function is called with argument reason, the following steps are taken:

Let F be the active function object.
Let asyncContext be F.[[AsyncContext]].
Let prevContext be the running execution context.
Suspend prevContext.
Push asyncContext onto the execution context stack; asyncContext is now the running execution context.
Resume the suspended evaluation of asyncContext using ThrowCompletion(reason) as the result of the operation that suspended it.
Assert: When we reach this step, asyncContext has already been removed from the execution context stack and prevContext is the currently running execution context.
Return undefined.

The "length" property of an Await rejected function is 1.

6.2.3.2 NormalCompletion

The abstract operation NormalCompletion with a single argument, such as:

Return NormalCompletion(argument).

Is a shorthand that is defined as follows:

Return Completion { [[Type]]: normal, [[Value]]: argument, [[Target]]: empty }.

6.2.3.3 ThrowCompletion

The abstract operation ThrowCompletion with a single argument, such as:

Return ThrowCompletion(argument).

Is a shorthand that is defined as follows:

Return Completion { [[Type]]: throw, [[Value]]: argument, [[Target]]: empty }.

6.2.3.4 UpdateEmpty ( `completionRecord`, `value` )

The abstract operation UpdateEmpty takes arguments completionRecord and value. It performs the following steps when called:

Assert: If completionRecord.[[Type]] is either return or throw, then completionRecord.[[Value]] is not empty.
If completionRecord.[[Value]] is not empty, return Completion(completionRecord).
Return Completion { [[Type]]: completionRecord.[[Type]], [[Value]]: value, [[Target]]: completionRecord.[[Target]] }.

6.2.4 The Reference Specification Type

Note

The Reference type is used to explain the behaviour of such operators as delete, typeof, the assignment operators, the super keyword and other language features. For example, the left-hand operand of an assignment is expected to produce a reference.

A Reference is a resolved name or property binding. A Reference consists of three components, the base value component, the referenced name component, and the Boolean-valued strict reference flag. The base value component is either undefined, an Object, a Boolean, a String, a Symbol, a Number, a BigInt, or an Environment Record. A base value component of undefined indicates that the Reference could not be resolved to a binding. The referenced name component is a String or Symbol value.

A Super Reference is a Reference that is used to represent a name binding that was expressed using the super keyword. A Super Reference has an additional thisValue component, and its base value component will never be an Environment Record.

The following abstract operations are used in this specification to operate on references:

6.2.4.1 GetBase ( `V` )

The abstract operation GetBase takes argument V. It performs the following steps when called:

Assert: Type(V) is Reference.
Return the base value component of V.

6.2.4.2 GetReferencedName ( `V` )

The abstract operation GetReferencedName takes argument V. It performs the following steps when called:

Assert: Type(V) is Reference.
Return the referenced name component of V.

6.2.4.3 IsStrictReference ( `V` )

The abstract operation IsStrictReference takes argument V. It performs the following steps when called:

Assert: Type(V) is Reference.
Return the strict reference flag of V.

6.2.4.4 HasPrimitiveBase ( `V` )

The abstract operation HasPrimitiveBase takes argument V. It performs the following steps when called:

Assert: Type(V) is Reference.
If Type(V's base value component) is Boolean, String, Symbol, BigInt, or Number, return true; otherwise return false.

6.2.4.5 IsPropertyReference ( `V` )

The abstract operation IsPropertyReference takes argument V. It performs the following steps when called:

Assert: Type(V) is Reference.
If either the base value component of V is an Object or HasPrimitiveBase(V) is true, return true; otherwise return false.

6.2.4.6 IsUnresolvableReference ( `V` )

The abstract operation IsUnresolvableReference takes argument V. It performs the following steps when called:

Assert: Type(V) is Reference.
If the base value component of V is undefined, return true; otherwise return false.

6.2.4.7 IsSuperReference ( `V` )

The abstract operation IsSuperReference takes argument V. It performs the following steps when called:

Assert: Type(V) is Reference.
If V has a thisValue component, return true; otherwise return false.

6.2.4.8 GetValue ( `V` )

The abstract operation GetValue takes argument V. It performs the following steps when called:

ReturnIfAbrupt(V).
If Type(V) is not Reference, return V.
Let base be GetBase(V).
If IsUnresolvableReference(V) is true, throw a ReferenceError exception.
If IsPropertyReference(V) is true, then
1. If HasPrimitiveBase(V) is true, then
  1. Assert: In this case, base will never be undefined or null.
  2. Set base to ! ToObject(base).
2. Return ? base.[[Get]](GetReferencedName(V), GetThisValue(V)).
Else,
1. Assert: base is an Environment Record.
2. Return ? base.GetBindingValue(GetReferencedName(V), IsStrictReference(V)) (see 8.1).

Note

The object that may be created in step 5.a.ii is not accessible outside of the above abstract operation and the ordinary object [[Get]] internal method. An implementation might choose to avoid the actual creation of the object.

6.2.4.9 PutValue ( `V`, `W` )

The abstract operation PutValue takes arguments V and W. It performs the following steps when called:

ReturnIfAbrupt(V).
ReturnIfAbrupt(W).
If Type(V) is not Reference, throw a ReferenceError exception.
Let base be GetBase(V).
If IsUnresolvableReference(V) is true, then
1. If IsStrictReference(V) is true, then
  1. Throw a ReferenceError exception.
2. Let globalObj be GetGlobalObject().
3. Return ? Set(globalObj, GetReferencedName(V), W, false).
Else if IsPropertyReference(V) is true, then
1. If HasPrimitiveBase(V) is true, then
  1. Assert: In this case, base will never be undefined or null.
  2. Set base to ! ToObject(base).
2. Let succeeded be ? base.[[Set]](GetReferencedName(V), W, GetThisValue(V)).
3. If succeeded is false and IsStrictReference(V) is true, throw a TypeError exception.
4. Return.
Else,
1. Assert: base is an Environment Record.
2. Return ? base.SetMutableBinding(GetReferencedName(V), W, IsStrictReference(V)) (see 8.1).

Note

The object that may be created in step 6.a.ii is not accessible outside of the above algorithm and the ordinary object [[Set]] internal method. An implementation might choose to avoid the actual creation of that object.

6.2.4.10 GetThisValue ( `V` )

The abstract operation GetThisValue takes argument V. It performs the following steps when called:

Assert: IsPropertyReference(V) is true.
If IsSuperReference(V) is true, then
1. Return the value of the thisValue component of the reference V.
Return GetBase(V).

6.2.4.11 InitializeReferencedBinding ( `V`, `W` )

The abstract operation InitializeReferencedBinding takes arguments V and W. It performs the following steps when called:

ReturnIfAbrupt(V).
ReturnIfAbrupt(W).
Assert: Type(V) is Reference.
Assert: IsUnresolvableReference(V) is false.
Let base be GetBase(V).
Assert: base is an Environment Record.
Return base.InitializeBinding(GetReferencedName(V), W).

6.2.5 The Property Descriptor Specification Type

The Property Descriptor type is used to explain the manipulation and reification of Object property attributes. Values of the Property Descriptor type are Records. Each field's name is an attribute name and its value is a corresponding attribute value as specified in 6.1.7.1. In addition, any field may be present or absent. The schema name used within this specification to tag literal descriptions of Property Descriptor records is “PropertyDescriptor”.

Property Descriptor values may be further classified as data Property Descriptors and accessor Property Descriptors based upon the existence or use of certain fields. A data Property Descriptor is one that includes any fields named either [[Value]] or [[Writable]]. An accessor Property Descriptor is one that includes any fields named either [[Get]] or [[Set]]. Any Property Descriptor may have fields named [[Enumerable]] and [[Configurable]]. A Property Descriptor value may not be both a data Property Descriptor and an accessor Property Descriptor; however, it may be neither. A generic Property Descriptor is a Property Descriptor value that is neither a data Property Descriptor nor an accessor Property Descriptor. A fully populated Property Descriptor is one that is either an accessor Property Descriptor or a data Property Descriptor and that has all of the fields that correspond to the property attributes defined in either Table 3 or Table 4.

The following abstract operations are used in this specification to operate upon Property Descriptor values:

6.2.5.1 IsAccessorDescriptor ( `Desc` )

The abstract operation IsAccessorDescriptor takes argument Desc (a Property Descriptor or undefined). It performs the following steps when called:

If Desc is undefined, return false.
If both Desc.[[Get]] and Desc.[[Set]] are absent, return false.
Return true.

6.2.5.2 IsDataDescriptor ( `Desc` )

The abstract operation IsDataDescriptor takes argument Desc (a Property Descriptor or undefined). It performs the following steps when called:

If Desc is undefined, return false.
If both Desc.[[Value]] and Desc.[[Writable]] are absent, return false.
Return true.

6.2.5.3 IsGenericDescriptor ( `Desc` )

The abstract operation IsGenericDescriptor takes argument Desc (a Property Descriptor or undefined). It performs the following steps when called:

If Desc is undefined, return false.
If IsAccessorDescriptor(Desc) and IsDataDescriptor(Desc) are both false, return true.
Return false.

6.2.5.4 FromPropertyDescriptor ( `Desc` )

The abstract operation FromPropertyDescriptor takes argument Desc (a Property Descriptor or undefined). It performs the following steps when called:

If Desc is undefined, return undefined.
Let obj be OrdinaryObjectCreate(%Object.prototype%).
Assert: obj is an extensible ordinary object with no own properties.
If Desc has a [[Value]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "value", Desc.[[Value]]).
If Desc has a [[Writable]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "writable", Desc.[[Writable]]).
If Desc has a [[Get]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "get", Desc.[[Get]]).
If Desc has a [[Set]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "set", Desc.[[Set]]).
If Desc has an [[Enumerable]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "enumerable", Desc.[[Enumerable]]).
If Desc has a [[Configurable]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "configurable", Desc.[[Configurable]]).
Return obj.

6.2.5.5 ToPropertyDescriptor ( `Obj` )

The abstract operation ToPropertyDescriptor takes argument Obj. It performs the following steps when called:

If Type(Obj) is not Object, throw a TypeError exception.
Let desc be a new Property Descriptor that initially has no fields.
Let hasEnumerable be ? HasProperty(Obj, "enumerable").
If hasEnumerable is true, then
1. Let enumerable be ! ToBoolean(? Get(Obj, "enumerable")).
2. Set desc.[[Enumerable]] to enumerable.
Let hasConfigurable be ? HasProperty(Obj, "configurable").
If hasConfigurable is true, then
1. Let configurable be ! ToBoolean(? Get(Obj, "configurable")).
2. Set desc.[[Configurable]] to configurable.
Let hasValue be ? HasProperty(Obj, "value").
If hasValue is true, then
1. Let value be ? Get(Obj, "value").
2. Set desc.[[Value]] to value.
Let hasWritable be ? HasProperty(Obj, "writable").
If hasWritable is true, then
1. Let writable be ! ToBoolean(? Get(Obj, "writable")).
2. Set desc.[[Writable]] to writable.
Let hasGet be ? HasProperty(Obj, "get").
If hasGet is true, then
1. Let getter be ? Get(Obj, "get").
2. If IsCallable(getter) is false and getter is not undefined, throw a TypeError exception.
3. Set desc.[[Get]] to getter.
Let hasSet be ? HasProperty(Obj, "set").
If hasSet is true, then
1. Let setter be ? Get(Obj, "set").
2. If IsCallable(setter) is false and setter is not undefined, throw a TypeError exception.
3. Set desc.[[Set]] to setter.
If desc.[[Get]] is present or desc.[[Set]] is present, then
1. If desc.[[Value]] is present or desc.[[Writable]] is present, throw a TypeError exception.
Return desc.

6.2.5.6 CompletePropertyDescriptor ( `Desc` )

The abstract operation CompletePropertyDescriptor takes argument Desc (a Property Descriptor). It performs the following steps when called:

Assert: Desc is a Property Descriptor.
Let like be the Record { [[Value]]: undefined, [[Writable]]: false, [[Get]]: undefined, [[Set]]: undefined, [[Enumerable]]: false, [[Configurable]]: false }.
If IsGenericDescriptor(Desc) is true or IsDataDescriptor(Desc) is true, then
1. If Desc does not have a [[Value]] field, set Desc.[[Value]] to like.[[Value]].
2. If Desc does not have a [[Writable]] field, set Desc.[[Writable]] to like.[[Writable]].
Else,
1. If Desc does not have a [[Get]] field, set Desc.[[Get]] to like.[[Get]].
2. If Desc does not have a [[Set]] field, set Desc.[[Set]] to like.[[Set]].
If Desc does not have an [[Enumerable]] field, set Desc.[[Enumerable]] to like.[[Enumerable]].
If Desc does not have a [[Configurable]] field, set Desc.[[Configurable]] to like.[[Configurable]].
Return Desc.

6.2.6 The Environment Record Specification Type

The Environment Record type is used to explain the behaviour of name resolution in nested functions and blocks. This type and the operations upon it are defined in 8.1.

6.2.7 The Abstract Closure Specification Type

The Abstract Closure specification type is used to refer to algorithm steps together with a collection of values. Abstract Closures are meta-values and are invoked using function application style such as closure(arg1, arg2). Like abstract operations, invocations perform the algorithm steps described by the Abstract Closure.

In algorithm steps that create an Abstract Closure, values are captured with the verb "capture" followed by a list of aliases. When an Abstract Closure is created, it captures the value that is associated with each alias at that time. In steps that specify the algorithm to be performed when an Abstract Closure is called, each captured value is referred to by the alias that was used to capture the value.

If an Abstract Closure returns a Completion Record, that Completion Record's [[Type]] must be either normal or throw.

Abstract Closures are created inline as part of other algorithms, shown in the following example.

Let addend be 41.
Let closure be a new Abstract Closure with parameters (x) that captures addend and performs the following steps when called:
1. Return x + addend.
Let val be closure(1).
Assert: val is 42.

6.2.8 Data Blocks

The Data Block specification type is used to describe a distinct and mutable sequence of byte-sized (8 bit) numeric values. A Data Block value is created with a fixed number of bytes that each have the initial value 0.

For notational convenience within this specification, an array-like syntax can be used to access the individual bytes of a Data Block value. This notation presents a Data Block value as a 0-origined integer-indexed sequence of bytes. For example, if db is a 5 byte Data Block value then db[2] can be used to access its 3^rd byte.

A data block that resides in memory that can be referenced from multiple agents concurrently is designated a Shared Data Block. A Shared Data Block has an identity (for the purposes of equality testing Shared Data Block values) that is address-free: it is tied not to the virtual addresses the block is mapped to in any process, but to the set of locations in memory that the block represents. Two data blocks are equal only if the sets of the locations they contain are equal; otherwise, they are not equal and the intersection of the sets of locations they contain is empty. Finally, Shared Data Blocks can be distinguished from Data Blocks.

The semantics of Shared Data Blocks is defined using Shared Data Block events by the memory model. Abstract operations below introduce Shared Data Block events and act as the interface between evaluation semantics and the event semantics of the memory model. The events form a candidate execution, on which the memory model acts as a filter. Please consult the memory model for full semantics.

Shared Data Block events are modeled by Records, defined in the memory model.

The following abstract operations are used in this specification to operate upon Data Block values:

6.2.8.1 CreateByteDataBlock ( `size` )

The abstract operation CreateByteDataBlock takes argument size (an integer). It performs the following steps when called:

Assert: size ≥ 0.
Let db be a new Data Block value consisting of size bytes. If it is impossible to create such a Data Block, throw a RangeError exception.
Set all of the bytes of db to 0.
Return db.

6.2.8.2 CreateSharedByteDataBlock ( `size` )

The abstract operation CreateSharedByteDataBlock takes argument size (an integer). It performs the following steps when called:

Assert: size ≥ 0.
Let db be a new Shared Data Block value consisting of size bytes. If it is impossible to create such a Shared Data Block, throw a RangeError exception.
Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
Let zero be « 0 ».
For each index i of db, do
1. Append WriteSharedMemory { [[Order]]: Init, [[NoTear]]: true, [[Block]]: db, [[ByteIndex]]: i, [[ElementSize]]: 1, [[Payload]]: zero } to eventList.
Return db.

6.2.8.3 CopyDataBlockBytes ( `toBlock`, `toIndex`, `fromBlock`, `fromIndex`, `count` )

The abstract operation CopyDataBlockBytes takes arguments toBlock, toIndex, fromBlock, fromIndex, and count. It performs the following steps when called:

Assert: fromBlock and toBlock are distinct Data Block or Shared Data Block values.
Assert: fromIndex, toIndex, and count are integer values ≥ 0.
Let fromSize be the number of bytes in fromBlock.
Assert: fromIndex + count ≤ fromSize.
Let toSize be the number of bytes in toBlock.
Assert: toIndex + count ≤ toSize.
Repeat, while count > 0
1. If fromBlock is a Shared Data Block, then
  1. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
  2. Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
  3. Let bytes be a List of length 1 that contains a nondeterministically chosen byte value.
  4. NOTE: In implementations, bytes is the result of a non-atomic read instruction on the underlying hardware. The nondeterminism is a semantic prescription of the memory model to describe observable behaviour of hardware with weak consistency.
  5. Let readEvent be ReadSharedMemory { [[Order]]: Unordered, [[NoTear]]: true, [[Block]]: fromBlock, [[ByteIndex]]: fromIndex, [[ElementSize]]: 1 }.
  6. Append readEvent to eventList.
  7. Append Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]: bytes } to execution.[[ChosenValues]].
  8. If toBlock is a Shared Data Block, then
    1. Append WriteSharedMemory { [[Order]]: Unordered, [[NoTear]]: true, [[Block]]: toBlock, [[ByteIndex]]: toIndex, [[ElementSize]]: 1, [[Payload]]: bytes } to eventList.
  9. Else,
    1. Set toBlock[toIndex] to bytes[0].
2. Else,
  1. Assert: toBlock is not a Shared Data Block.
  2. Set toBlock[toIndex] to fromBlock[fromIndex].
3. Set toIndex to toIndex + 1.
4. Set fromIndex to fromIndex + 1.
5. Set count to count - 1.
Return NormalCompletion(empty).

7 Abstract Operations

These operations are not a part of the ECMAScript language; they are defined here to solely to aid the specification of the semantics of the ECMAScript language. Other, more specialized abstract operations are defined throughout this specification.

7.1 Type Conversion

The ECMAScript language implicitly performs automatic type conversion as needed. To clarify the semantics of certain constructs it is useful to define a set of conversion abstract operations. The conversion abstract operations are polymorphic; they can accept a value of any ECMAScript language type. But no other specification types are used with these operations.

The BigInt type has no implicit conversions in the ECMAScript language; programmers must call BigInt explicitly to convert values from other types.

7.1.1 ToPrimitive ( `input` [ , `PreferredType` ] )

The abstract operation ToPrimitive takes argument input and optional argument PreferredType. It converts its input argument to a non-Object type. If an object is capable of converting to more than one primitive type, it may use the optional hint PreferredType to favour that type. It performs the following steps when called:

Assert: input is an ECMAScript language value.
If Type(input) is Object, then
1. If PreferredType is not present, let hint be "default".
2. Else if PreferredType is hint String, let hint be "string".
3. Else,
  1. Assert: PreferredType is hint Number.
  2. Let hint be "number".
4. Let exoticToPrim be ? GetMethod(input, @@toPrimitive).
5. If exoticToPrim is not undefined, then
  1. Let result be ? Call(exoticToPrim, input, « hint »).
  2. If Type(result) is not Object, return result.
  3. Throw a TypeError exception.
6. If hint is "default", set hint to "number".
7. Return ? OrdinaryToPrimitive(input, hint).
Return input.

Note

When ToPrimitive is called with no hint, then it generally behaves as if the hint were Number. However, objects may over-ride this behaviour by defining a @@toPrimitive method. Of the objects defined in this specification only Date objects (see 20.4.4.45) and Symbol objects (see 19.4.3.5) over-ride the default ToPrimitive behaviour. Date objects treat no hint as if the hint were String.

7.1.1.1 OrdinaryToPrimitive ( `O`, `hint` )

The abstract operation OrdinaryToPrimitive takes arguments O and hint. It performs the following steps when called:

Assert: Type(O) is Object.
Assert: Type(hint) is String and its value is either "string" or "number".
If hint is "string", then
1. Let methodNames be « "toString", "valueOf" ».
Else,
1. Let methodNames be « "valueOf", "toString" ».
For each name in methodNames in List order, do
1. Let method be ? Get(O, name).
2. If IsCallable(method) is true, then
  1. Let result be ? Call(method, O).
  2. If Type(result) is not Object, return result.
Throw a TypeError exception.

7.1.2 ToBoolean ( `argument` )

The abstract operation ToBoolean takes argument argument. It converts argument to a value of type Boolean according to Table 10:

Table 10: ToBoolean Conversions

Argument Type	Result
Undefined	Return false.
Null	Return false.
Boolean	Return `argument`.
Number	If `argument` is +0, -0, or NaN, return false; otherwise return true.
String	If `argument` is the empty String (its length is zero), return false; otherwise return true.
Symbol	Return true.
BigInt	If `argument` is 0n, return false; otherwise return true.
Object	Return true.

7.1.3 ToNumeric ( `value` )

The abstract operation ToNumeric takes argument value. It returns value converted to a numeric value of type Number or BigInt. It performs the following steps when called:

Let primValue be ? ToPrimitive(value, hint Number).
If Type(primValue) is BigInt, return primValue.
Return ? ToNumber(primValue).

7.1.4 ToNumber ( `argument` )

The abstract operation ToNumber takes argument argument. It converts argument to a value of type Number according to Table 11:

Table 11: ToNumber Conversions

Argument Type	Result
Undefined	Return NaN.
Null	Return +0.
Boolean	If `argument` is true, return 1. If `argument` is false, return +0.
Number	Return `argument` (no conversion).
String	See grammar and conversion algorithm below.
Symbol	Throw a TypeError exception.
BigInt	Throw a TypeError exception.
Object	Apply the following steps: Let `primValue` be ? ToPrimitive(`argument`, hint Number). Return ? ToNumber(`primValue`).

7.1.4.1 ToNumber Applied to the String Type

ToNumber applied to Strings applies the following grammar to the input String interpreted as a sequence of UTF-16 encoded code points (6.1.4). If the grammar cannot interpret the String as an expansion of StringNumericLiteral, then the result of ToNumber is NaN.

Note 1

The terminal symbols of this grammar are all composed of characters in the Unicode Basic Multilingual Plane (BMP). Therefore, the result of ToNumber will be NaN if the string contains any leading surrogate or trailing surrogate code units, whether paired or unpaired.

Syntax

:::

opt

opt

opt

:::

opt

:::

:::

StrUnsignedDecimalLiteral

StrDecimalLiteral

:::

:::

Infinity

opt

opt

opt

opt

All grammar symbols not explicitly defined above have the definitions used in the Lexical Grammar for numeric literals (11.8.3)

Note 2

Some differences should be noted between the syntax of a StringNumericLiteral and a NumericLiteral:

A StringNumericLiteral may include leading and/or trailing white space and/or line terminators.
A StringNumericLiteral that is decimal may have any number of leading 0 digits.
A StringNumericLiteral that is decimal may include a + or - to indicate its sign.
A StringNumericLiteral that is empty or contains only white space is converted to +0.
Infinity and -Infinity are recognized as a StringNumericLiteral but not as a NumericLiteral.
A StringNumericLiteral cannot include a BigIntLiteralSuffix.

7.1.4.1.1 Runtime Semantics: MV

The conversion of a String to a Number value is similar overall to the determination of the Number value for a numeric literal (see 11.8.3), but some of the details are different, so the process for converting a String numeric literal to a value of Number type is given here. This value is determined in two steps: first, a mathematical value (MV) is derived from the String numeric literal; second, this mathematical value is rounded as described below. The MV on any grammar symbol, not provided below, is the MV for that symbol defined in 11.8.3.1.

The MV of StringNumericLiteral:::[empty] is 0_ℝ.
The MV of StringNumericLiteral:::StrWhiteSpace is 0_ℝ.
The MV of StringNumericLiteral:::StrWhiteSpaceoptStrNumericLiteralStrWhiteSpaceopt is the MV of StrNumericLiteral, no matter whether white space is present or not.
The MV of StrDecimalLiteral:::-StrUnsignedDecimalLiteral is the negative of the MV of StrUnsignedDecimalLiteral. (Note that if the MV of StrUnsignedDecimalLiteral is 0, the negative of this MV is also 0. The rounding rule described below handles the conversion of this signless mathematical zero to a floating-point +0 or -0 as appropriate.)
The MV of StrUnsignedDecimalLiteral:::Infinity is 10_ℝ^10000_ℝ (a value so large that it will round to +∞).
The MV of StrUnsignedDecimalLiteral:::DecimalDigits.DecimalDigits is the MV of the first DecimalDigits plus (the MV of the second DecimalDigits times 10_ℝ^-_ℝn), where n is the mathematical value of the number of code points in the second DecimalDigits.
The MV of StrUnsignedDecimalLiteral:::DecimalDigits.ExponentPart is the MV of DecimalDigits times 10_ℝ^e, where e is the MV of ExponentPart.
The MV of StrUnsignedDecimalLiteral:::DecimalDigits.DecimalDigitsExponentPart is (the MV of the first DecimalDigits plus (the MV of the second DecimalDigits times 10_ℝ^-_ℝn)) times 10_ℝ^e, where n is the mathematical value of the number of code points in the second DecimalDigits and e is the MV of ExponentPart.
The MV of StrUnsignedDecimalLiteral:::.DecimalDigits is the MV of DecimalDigits times 10_ℝ^-_ℝn, where n is the mathematical value of the number of code points in DecimalDigits.
The MV of StrUnsignedDecimalLiteral:::.DecimalDigitsExponentPart is the MV of DecimalDigits times 10_ℝ^{e -_ℝ n}, where n is the mathematical value of the number of code points in DecimalDigits and e is the MV of ExponentPart.
The MV of StrUnsignedDecimalLiteral:::DecimalDigitsExponentPart is the MV of DecimalDigits times 10_ℝ^e, where e is the MV of ExponentPart.

Once the exact MV for a String numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is +0 unless the first non white space code point in the String numeric literal is -, in which case the rounded value is -0. Otherwise, the rounded value must be the Number value for the MV (in the sense defined in 6.1.6.1), unless the literal includes a StrUnsignedDecimalLiteral and the literal has more than 20 significant digits, in which case the Number value may be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit and then incrementing the literal at the 20th digit position. A digit is significant if it is not part of an ExponentPart and

it is not 0; or
there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.

7.1.5 ToInteger ( `argument` )

The abstract operation ToInteger takes argument argument. It converts argument to an integral Number value. It performs the following steps when called:

Let number be ? ToNumber(argument).
If number is NaN, +0, or -0, return +0.
If number is +∞ or -∞, return number.
Let integer be the Number value that is the same sign as number and whose magnitude is floor(abs(number)).
If integer is -0, return +0.
Return integer.

7.1.6 ToInt32 ( `argument` )

The abstract operation ToInt32 takes argument argument. It converts argument to one of 2³² integer values in the range -2³¹ through 2³¹ - 1, inclusive. It performs the following steps when called:

Let number be ? ToNumber(argument).
If number is NaN, +0, -0, +∞, or -∞, return +0.
Let int be the Number value that is the same sign as number and whose magnitude is floor(abs(number)).
Let int32bit be int modulo 2³².
If int32bit ≥ 2³¹, return int32bit - 2³²; otherwise return int32bit.

Note

Given the above definition of ToInt32:

The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
ToInt32(ToUint32(x)) is equal to ToInt32(x) for all values of x. (It is to preserve this latter property that +∞ and -∞ are mapped to +0.)
ToInt32 maps -0 to +0.

7.1.7 ToUint32 ( `argument` )

The abstract operation ToUint32 takes argument argument. It converts argument to one of 2³² integer values in the range 0 through 2³² - 1, inclusive. It performs the following steps when called:

Let number be ? ToNumber(argument).
If number is NaN, +0, -0, +∞, or -∞, return +0.
Let int be the Number value that is the same sign as number and whose magnitude is floor(abs(number)).
Let int32bit be int modulo 2³².
Return int32bit.

Note

Given the above definition of ToUint32:

Step 5 is the only difference between ToUint32 and ToInt32.
The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
ToUint32(ToInt32(x)) is equal to ToUint32(x) for all values of x. (It is to preserve this latter property that +∞ and -∞ are mapped to +0.)
ToUint32 maps -0 to +0.

7.1.8 ToInt16 ( `argument` )

The abstract operation ToInt16 takes argument argument. It converts argument to one of 2¹⁶ integer values in the range -32768 through 32767, inclusive. It performs the following steps when called:

Let number be ? ToNumber(argument).
If number is NaN, +0, -0, +∞, or -∞, return +0.
Let int be the Number value that is the same sign as number and whose magnitude is floor(abs(number)).
Let int16bit be int modulo 2¹⁶.
If int16bit ≥ 2¹⁵, return int16bit - 2¹⁶; otherwise return int16bit.

7.1.9 ToUint16 ( `argument` )

The abstract operation ToUint16 takes argument argument. It converts argument to one of 2¹⁶ integer values in the range 0 through 2¹⁶ - 1, inclusive. It performs the following steps when called:

Let number be ? ToNumber(argument).
If number is NaN, +0, -0, +∞, or -∞, return +0.
Let int be the Number value that is the same sign as number and whose magnitude is floor(abs(number)).
Let int16bit be int modulo 2¹⁶.
Return int16bit.

Note

Given the above definition of ToUint16:

The substitution of 2¹⁶ for 2³² in step 4 is the only difference between ToUint32 and ToUint16.
ToUint16 maps -0 to +0.

7.1.10 ToInt8 ( `argument` )

The abstract operation ToInt8 takes argument argument. It converts argument to one of 2⁸ integer values in the range -128 through 127, inclusive. It performs the following steps when called:

Let number be ? ToNumber(argument).
If number is NaN, +0, -0, +∞, or -∞, return +0.
Let int be the Number value that is the same sign as number and whose magnitude is floor(abs(number)).
Let int8bit be int modulo 2⁸.
If int8bit ≥ 2⁷, return int8bit - 2⁸; otherwise return int8bit.

7.1.11 ToUint8 ( `argument` )

The abstract operation ToUint8 takes argument argument. It converts argument to one of 2⁸ integer values in the range 0 through 255, inclusive. It performs the following steps when called:

Let number be ? ToNumber(argument).
If number is NaN, +0, -0, +∞, or -∞, return +0.
Let int be the Number value that is the same sign as number and whose magnitude is floor(abs(number)).
Let int8bit be int modulo 2⁸.
Return int8bit.

7.1.12 ToUint8Clamp ( `argument` )

The abstract operation ToUint8Clamp takes argument argument. It converts argument to one of 2⁸ integer values in the range 0 through 255, inclusive. It performs the following steps when called:

Let number be ? ToNumber(argument).
If number is NaN, return +0.
If number ≤ 0, return +0.
If number ≥ 255, return 255.
Let f be floor(number).
If f + 0.5 < number, return f + 1.
If number < f + 0.5, return f.
If f is odd, return f + 1.
Return f.

Note

Unlike the other ECMAScript integer conversion abstract operation, ToUint8Clamp rounds rather than truncates non-integer values and does not convert +∞ to 0. ToUint8Clamp does “round half to even” tie-breaking. This differs from Math.round which does “round half up” tie-breaking.

7.1.13 ToBigInt ( `argument` )

The abstract operation ToBigInt takes argument argument. It converts argument to a BigInt value, or throws if an implicit conversion from Number would be required. It performs the following steps when called:

Let prim be ? ToPrimitive(argument, hint Number).
Return the value that prim corresponds to in Table 12.

Table 12: BigInt Conversions

Argument Type	Result
Undefined	Throw a TypeError exception.
Null	Throw a TypeError exception.
Boolean	Return `1n` if `prim` is true and `0n` if `prim` is false.
BigInt	Return `prim`.
Number	Throw a TypeError exception.
String	Let `n` be ! StringToBigInt(`prim`). If `n` is NaN, throw a SyntaxError exception. Return `n`.
Symbol	Throw a TypeError exception.

7.1.14 StringToBigInt ( `argument` )

Apply the algorithm in 7.1.4.1 with the following changes:

Replace the StrUnsignedDecimalLiteral production with DecimalDigits to not allow Infinity, decimal points, or exponents.
If the MV is NaN, return NaN, otherwise return the BigInt which exactly corresponds to the MV, rather than rounding to a Number.

7.1.15 ToBigInt64 ( `argument` )

The abstract operation ToBigInt64 takes argument argument. It converts argument to one of 2⁶⁴ integer values in the range -2⁶³ through 2⁶³-1, inclusive. It performs the following steps when called:

Let n be ? ToBigInt(argument).
Let int64bit be n modulo 2⁶⁴.
If int64bit ≥ 2⁶³, return int64bit - 2⁶⁴; otherwise return int64bit.

7.1.16 ToBigUint64 ( `argument` )

The abstract operation ToBigUint64 takes argument argument. It converts argument to one of 2⁶⁴ integer values in the range 0 through 2⁶⁴-1, inclusive. It performs the following steps when called:

Let n be ? ToBigInt(argument).
Let int64bit be n modulo 2⁶⁴.
Return int64bit.

7.1.17 ToString ( `argument` )

The abstract operation ToString takes argument argument. It converts argument to a value of type String according to Table 13:

Table 13: ToString Conversions

Argument Type	Result
Undefined	Return "undefined".
Null	Return "null".
Boolean	If `argument` is true, return "true". If `argument` is false, return "false".
Number	Return ! Number::toString(`argument`).
String	Return `argument`.
Symbol	Throw a TypeError exception.
BigInt	Return ! BigInt::toString(`argument`).
Object	Apply the following steps: Let `primValue` be ? ToPrimitive(`argument`, hint String). Return ? ToString(`primValue`).

7.1.18 ToObject ( `argument` )

The abstract operation ToObject takes argument argument. It converts argument to a value of type Object according to Table 14:

Table 14: ToObject Conversions

Argument Type	Result
Undefined	Throw a TypeError exception.
Null	Throw a TypeError exception.
Boolean	Return a new Boolean object whose [[BooleanData]] internal slot is set to `argument`. See 19.3 for a description of Boolean objects.
Number	Return a new Number object whose [[NumberData]] internal slot is set to `argument`. See 20.1 for a description of Number objects.
String	Return a new String object whose [[StringData]] internal slot is set to `argument`. See 21.1 for a description of String objects.
Symbol	Return a new Symbol object whose [[SymbolData]] internal slot is set to `argument`. See 19.4 for a description of Symbol objects.
BigInt	Return a new BigInt object whose [[BigIntData]] internal slot is set to `argument`. See 20.2 for a description of BigInt objects.
Object	Return `argument`.

7.1.19 ToPropertyKey ( `argument` )

The abstract operation ToPropertyKey takes argument argument. It converts argument to a value that can be used as a property key. It performs the following steps when called:

Let key be ? ToPrimitive(argument, hint String).
If Type(key) is Symbol, then
1. Return key.
Return ! ToString(key).

7.1.20 ToLength ( `argument` )

The abstract operation ToLength takes argument argument. It converts argument to an integer suitable for use as the length of an array-like object. It performs the following steps when called:

Let len be ? ToInteger(argument).
If len ≤ +0, return +0.
Return min(len, 2⁵³ - 1).

7.1.21 CanonicalNumericIndexString ( `argument` )

The abstract operation CanonicalNumericIndexString takes argument argument. It returns argument converted to a Number value if it is a String representation of a Number that would be produced by ToString, or the string "-0". Otherwise, it returns undefined. It performs the following steps when called:

Assert: Type(argument) is String.
If argument is "-0", return -0.
Let n be ! ToNumber(argument).
If SameValue(! ToString(n), argument) is false, return undefined.
Return n.

A canonical numeric string is any String value for which the CanonicalNumericIndexString abstract operation does not return undefined.

7.1.22 ToIndex ( `value` )

The abstract operation ToIndex takes argument value. It returns value argument converted to a non-negative integer if it is a valid integer index value. It performs the following steps when called:

If value is undefined, then
1. Let index be 0.
Else,
1. Let integerIndex be ? ToInteger(value).
2. If integerIndex < 0, throw a RangeError exception.
3. Let index be ! ToLength(integerIndex).
4. If ! SameValue(integerIndex, index) is false, throw a RangeError exception.
Return index.

7.2 Testing and Comparison Operations

7.2.1 RequireObjectCoercible ( `argument` )

The abstract operation RequireObjectCoercible takes argument argument. It throws an error if argument is a value that cannot be converted to an Object using ToObject. It is defined by Table 15:

Table 15: RequireObjectCoercible Results

Argument Type	Result
Undefined	Throw a TypeError exception.
Null	Throw a TypeError exception.
Boolean	Return `argument`.
Number	Return `argument`.
String	Return `argument`.
Symbol	Return `argument`.
BigInt	Return `argument`.
Object	Return `argument`.

7.2.2 IsArray ( `argument` )

The abstract operation IsArray takes argument argument. It performs the following steps when called:

If Type(argument) is not Object, return false.
If argument is an Array exotic object, return true.
If argument is a Proxy exotic object, then
1. If argument.[[ProxyHandler]] is null, throw a TypeError exception.
2. Let target be argument.[[ProxyTarget]].
3. Return ? IsArray(target).
Return false.

7.2.3 IsCallable ( `argument` )

The abstract operation IsCallable takes argument argument (an ECMAScript language value). It determines if argument is a callable function with a [[Call]] internal method. It performs the following steps when called:

If Type(argument) is not Object, return false.
If argument has a [[Call]] internal method, return true.
Return false.

7.2.4 IsConstructor ( `argument` )

The abstract operation IsConstructor takes argument argument (an ECMAScript language value). It determines if argument is a function object with a [[Construct]] internal method. It performs the following steps when called:

If Type(argument) is not Object, return false.
If argument has a [[Construct]] internal method, return true.
Return false.

7.2.5 IsExtensible ( `O` )

The abstract operation IsExtensible takes argument O (an Object) and returns a completion record which, if its [[Type]] is normal, has a [[Value]] which is a Boolean. It is used to determine whether additional properties can be added to O. It performs the following steps when called:

Assert: Type(O) is Object.
Return ? O.[[IsExtensible]]().

7.2.6 IsInteger ( `argument` )

The abstract operation IsInteger takes argument argument. It determines if argument is a finite integer Number value. It performs the following steps when called:

If Type(argument) is not Number, return false.
If argument is NaN, +∞, or -∞, return false.
If floor(abs(argument)) ≠ abs(argument), return false.
Return true.

7.2.7 IsNonNegativeInteger ( `argument` )

The abstract operation IsNonNegativeInteger takes argument argument. It determines if argument is a non-negative integer Number value. It performs the following steps when called:

If ! IsInteger(argument) is true and argument ≥ 0, return true.
Otherwise, return false.

7.2.8 IsPropertyKey ( `argument` )

The abstract operation IsPropertyKey takes argument argument (an ECMAScript language value). It determines if argument is a value that may be used as a property key. It performs the following steps when called:

If Type(argument) is String, return true.
If Type(argument) is Symbol, return true.
Return false.

7.2.9 IsRegExp ( `argument` )

The abstract operation IsRegExp takes argument argument. It performs the following steps when called:

If Type(argument) is not Object, return false.
Let matcher be ? Get(argument, @@match).
If matcher is not undefined, return ! ToBoolean(matcher).
If argument has a [[RegExpMatcher]] internal slot, return true.
Return false.

7.2.10 IsStringPrefix ( `p`, `q` )

The abstract operation IsStringPrefix takes arguments p (a String) and q (a String). It determines if p is a prefix of q. It performs the following steps when called:

Assert: Type(p) is String.
Assert: Type(q) is String.
If q can be the string-concatenation of p and some other String r, return true. Otherwise, return false.

Note

Any String is a prefix of itself, because r may be the empty String.

7.2.11 SameValue ( `x`, `y` )

The abstract operation SameValue takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns a completion record whose [[Type]] is normal and whose [[Value]] is a Boolean. It performs the following steps when called:

If Type(x) is different from Type(y), return false.
If Type(x) is Number or BigInt, then
1. Return ! Type(x)::sameValue(x, y).
Return ! SameValueNonNumeric(x, y).

Note

This algorithm differs from the Strict Equality Comparison Algorithm in its treatment of signed zeroes and NaNs.

7.2.12 SameValueZero ( `x`, `y` )

The abstract operation SameValueZero takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns a completion record whose [[Type]] is normal and whose [[Value]] is a Boolean. It performs the following steps when called:

If Type(x) is different from Type(y), return false.
If Type(x) is Number or BigInt, then
1. Return ! Type(x)::sameValueZero(x, y).
Return ! SameValueNonNumeric(x, y).

Note

SameValueZero differs from SameValue only in its treatment of +0 and -0.

7.2.13 SameValueNonNumeric ( `x`, `y` )

The abstract operation SameValueNonNumeric takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns a completion record whose [[Type]] is normal and whose [[Value]] is a Boolean. It performs the following steps when called:

Assert: Type(x) is not Number or BigInt.
Assert: Type(x) is the same as Type(y).
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
If Type(x) is String, then
1. If x and y are exactly the same sequence of code units (same length and same code units at corresponding indices), return true; otherwise, return false.
If Type(x) is Boolean, then
1. If x and y are both true or both false, return true; otherwise, return false.
If Type(x) is Symbol, then
1. If x and y are both the same Symbol value, return true; otherwise, return false.
If x and y are the same Object value, return true. Otherwise, return false.

7.2.14 Abstract Relational Comparison

The comparison x < y, where x and y are values, produces true, false, or undefined (which indicates that at least one operand is NaN). In addition to x and y the algorithm takes a Boolean flag named LeftFirst as a parameter. The flag is used to control the order in which operations with potentially visible side-effects are performed upon x and y. It is necessary because ECMAScript specifies left to right evaluation of expressions. The default value of LeftFirst is true and indicates that the x parameter corresponds to an expression that occurs to the left of the y parameter's corresponding expression. If LeftFirst is false, the reverse is the case and operations must be performed upon y before x. Such a comparison is performed as follows:

If the LeftFirst flag is true, then
1. Let px be ? ToPrimitive(x, hint Number).
2. Let py be ? ToPrimitive(y, hint Number).
Else,
1. NOTE: The order of evaluation needs to be reversed to preserve left to right evaluation.
2. Let py be ? ToPrimitive(y, hint Number).
3. Let px be ? ToPrimitive(x, hint Number).
If Type(px) is String and Type(py) is String, then
1. If IsStringPrefix(py, px) is true, return false.
2. If IsStringPrefix(px, py) is true, return true.
3. Let k be the smallest nonnegative integer such that the code unit at index k within px is different from the code unit at index k within py. (There must be such a k, for neither String is a prefix of the other.)
4. Let m be the integer that is the numeric value of the code unit at index k within px.
5. Let n be the integer that is the numeric value of the code unit at index k within py.
6. If m < n, return true. Otherwise, return false.
Else,
1. If Type(px) is BigInt and Type(py) is String, then
  1. Let ny be ! StringToBigInt(py).
  2. If ny is NaN, return undefined.
  3. Return BigInt::lessThan(px, ny).
2. If Type(px) is String and Type(py) is BigInt, then
  1. Let nx be ! StringToBigInt(px).
  2. If nx is NaN, return undefined.
  3. Return BigInt::lessThan(nx, py).
3. NOTE: Because px and py are primitive values, evaluation order is not important.
4. Let nx be ? ToNumeric(px).
5. Let ny be ? ToNumeric(py).
6. If Type(nx) is the same as Type(ny), return Type(nx)::lessThan(nx, ny).
7. Assert: Type(nx) is BigInt and Type(ny) is Number, or Type(nx) is Number and Type(ny) is BigInt.
8. If nx or ny is NaN, return undefined.
9. If nx is -∞ or ny is +∞, return true.
10. If nx is +∞ or ny is -∞, return false.
11. If the mathematical value of nx is less than the mathematical value of ny, return true; otherwise return false.

Note 1

Step 3 differs from step 1.c in the algorithm that handles the addition operator + (12.15.5) by using the logical-and operation instead of the logical-or operation.

Note 2

The comparison of Strings uses a simple lexicographic ordering on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore String values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form. Also, note that for strings containing supplementary characters, lexicographic ordering on sequences of UTF-16 code unit values differs from that on sequences of code point values.

7.2.15 Abstract Equality Comparison

The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed as follows:

If Type(x) is the same as Type(y), then
1. Return the result of performing Strict Equality Comparison x === y.
If x is null and y is undefined, return true.
If x is undefined and y is null, return true.
If Type(x) is Number and Type(y) is String, return the result of the comparison x == ! ToNumber(y).
If Type(x) is String and Type(y) is Number, return the result of the comparison ! ToNumber(x) == y.
If Type(x) is BigInt and Type(y) is String, then
1. Let n be ! StringToBigInt(y).
2. If n is NaN, return false.
3. Return the result of the comparison x == n.
If Type(x) is String and Type(y) is BigInt, return the result of the comparison y == x.
If Type(x) is Boolean, return the result of the comparison ! ToNumber(x) == y.
If Type(y) is Boolean, return the result of the comparison x == ! ToNumber(y).
If Type(x) is either String, Number, BigInt, or Symbol and Type(y) is Object, return the result of the comparison x == ? ToPrimitive(y).
If Type(x) is Object and Type(y) is either String, Number, BigInt, or Symbol, return the result of the comparison ? ToPrimitive(x) == y.
If Type(x) is BigInt and Type(y) is Number, or if Type(x) is Number and Type(y) is BigInt, then
1. If x or y are any of NaN, +∞, or -∞, return false.
2. If the mathematical value of x is equal to the mathematical value of y, return true; otherwise return false.
Return false.

7.2.16 Strict Equality Comparison

The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:

If Type(x) is different from Type(y), return false.
If Type(x) is Number or BigInt, then
1. Return ! Type(x)::equal(x, y).
Return ! SameValueNonNumeric(x, y).

Note

This algorithm differs from the SameValue Algorithm in its treatment of signed zeroes and NaNs.

7.3 Operations on Objects

7.3.1 MakeBasicObject ( `internalSlotsList` )

The abstract operation MakeBasicObject takes argument internalSlotsList. It is the source of all ECMAScript objects that are created algorithmically, including both ordinary objects and exotic objects. It factors out common steps used in creating all objects, and centralizes object creation. It performs the following steps when called:

Assert: internalSlotsList is a List of internal slot names.
Let obj be a newly created object with an internal slot for each name in internalSlotsList.
Set obj's essential internal methods to the default ordinary object definitions specified in 9.1.
Assert: If the caller will not be overriding both obj's [[GetPrototypeOf]] and [[SetPrototypeOf]] essential internal methods, then internalSlotsList contains [[Prototype]].
Assert: If the caller will not be overriding all of obj's [[SetPrototypeOf]], [[IsExtensible]], and [[PreventExtensions]] essential internal methods, then internalSlotsList contains [[Extensible]].
If internalSlotsList contains [[Extensible]], then set obj.[[Extensible]] to true.
Return obj.

Note

Within this specification, exotic objects are created in abstract operations such as ArrayCreate and BoundFunctionCreate by first calling MakeBasicObject to obtain a basic, foundational object, and then overriding some or all of that object's internal methods. In order to encapsulate exotic object creation, the object's essential internal methods are never modified outside those operations.

7.3.2 Get ( `O`, `P` )

The abstract operation Get takes arguments O (an Object) and P (a property key). It is used to retrieve the value of a specific property of an object. It performs the following steps when called:

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Return ? O.[[Get]](P, O).

7.3.3 GetV ( `V`, `P` )

The abstract operation GetV takes arguments V (an ECMAScript language value) and P (a property key). It is used to retrieve the value of a specific property of an ECMAScript language value. If the value is not an object, the property lookup is performed using a wrapper object appropriate for the type of the value. It performs the following steps when called:

Assert: IsPropertyKey(P) is true.
Let O be ? ToObject(V).
Return ? O.[[Get]](P, V).

7.3.4 Set ( `O`, `P`, `V`, `Throw` )

The abstract operation Set takes arguments O (an Object), P (a property key), V (an ECMAScript language value), and Throw (a Boolean). It is used to set the value of a specific property of an object. V is the new value for the property. It performs the following steps when called:

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Assert: Type(Throw) is Boolean.
Let success be ? O.[[Set]](P, V, O).
If success is false and Throw is true, throw a TypeError exception.
Return success.

7.3.5 CreateDataProperty ( `O`, `P`, `V` )

The abstract operation CreateDataProperty takes arguments O (an Object), P (a property key), and V (an ECMAScript language value). It is used to create a new own property of an object. It performs the following steps when called:

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }.
Return ? O.[[DefineOwnProperty]](P, newDesc).

Note

This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false.

7.3.6 CreateMethodProperty ( `O`, `P`, `V` )

The abstract operation CreateMethodProperty takes arguments O (an Object), P (a property key), and V (an ECMAScript language value). It is used to create a new own property of an object. It performs the following steps when called:

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }.
Return ? O.[[DefineOwnProperty]](P, newDesc).

Note

This abstract operation creates a property whose attributes are set to the same defaults used for built-in methods and methods defined using class declaration syntax. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false.

7.3.7 CreateDataPropertyOrThrow ( `O`, `P`, `V` )

The abstract operation CreateDataPropertyOrThrow takes arguments O (an Object), P (a property key), and V (an ECMAScript language value). It is used to create a new own property of an object. It throws a TypeError exception if the requested property update cannot be performed. It performs the following steps when called:

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Let success be ? CreateDataProperty(O, P, V).
If success is false, throw a TypeError exception.
Return success.

Note

7.3.8 DefinePropertyOrThrow ( `O`, `P`, `desc` )

The abstract operation DefinePropertyOrThrow takes arguments O (an Object), P (a property key), and desc (a Property Descriptor). It is used to call the [[DefineOwnProperty]] internal method of an object in a manner that will throw a TypeError exception if the requested property update cannot be performed. It performs the following steps when called:

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Let success be ? O.[[DefineOwnProperty]](P, desc).
If success is false, throw a TypeError exception.
Return success.

7.3.9 DeletePropertyOrThrow ( `O`, `P` )

The abstract operation DeletePropertyOrThrow takes arguments O (an Object) and P (a property key). It is used to remove a specific own property of an object. It throws an exception if the property is not configurable. It performs the following steps when called:

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Let success be ? O.[[Delete]](P).
If success is false, throw a TypeError exception.
Return success.

7.3.10 GetMethod ( `V`, `P` )

The abstract operation GetMethod takes arguments V (an ECMAScript language value) and P (a property key). It is used to get the value of a specific property of an ECMAScript language value when the value of the property is expected to be a function. It performs the following steps when called:

Assert: IsPropertyKey(P) is true.
Let func be ? GetV(V, P).
If func is either undefined or null, return undefined.
If IsCallable(func) is false, throw a TypeError exception.
Return func.

7.3.11 HasProperty ( `O`, `P` )

The abstract operation HasProperty takes arguments O (an Object) and P (a property key) and returns a completion record which, if its [[Type]] is normal, has a [[Value]] which is a Boolean. It is used to determine whether an object has a property with the specified property key. The property may be either an own or inherited. It performs the following steps when called:

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Return ? O.[[HasProperty]](P).

7.3.12 HasOwnProperty ( `O`, `P` )

The abstract operation HasOwnProperty takes arguments O (an Object) and P (a property key) and returns a completion record which, if its [[Type]] is normal, has a [[Value]] which is a Boolean. It is used to determine whether an object has an own property with the specified property key. It performs the following steps when called:

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Let desc be ? O.[[GetOwnProperty]](P).
If desc is undefined, return false.
Return true.

7.3.13 Call ( `F`, `V` [ , `argumentsList` ] )

The abstract operation Call takes arguments F (an ECMAScript language value) and V (an ECMAScript language value) and optional argument argumentsList (a List of ECMAScript language values). It is used to call the [[Call]] internal method of a function object. F is the function object, V is an ECMAScript language value that is the this value of the [[Call]], and argumentsList is the value passed to the corresponding argument of the internal method. If argumentsList is not present, a new empty List is used as its value. It performs the following steps when called:

If argumentsList is not present, set argumentsList to a new empty List.
If IsCallable(F) is false, throw a TypeError exception.
Return ? F.[[Call]](V, argumentsList).

7.3.14 Construct ( `F` [ , `argumentsList` [ , `newTarget` ] ] )

The abstract operation Construct takes argument F (a function object) and optional arguments argumentsList and newTarget. It is used to call the [[Construct]] internal method of a function object. argumentsList and newTarget are the values to be passed as the corresponding arguments of the internal method. If argumentsList is not present, a new empty List is used as its value. If newTarget is not present, F is used as its value. It performs the following steps when called:

If newTarget is not present, set newTarget to F.
If argumentsList is not present, set argumentsList to a new empty List.
Assert: IsConstructor(F) is true.
Assert: IsConstructor(newTarget) is true.
Return ? F.[[Construct]](argumentsList, newTarget).

Note

If newTarget is not present, this operation is equivalent to: new F(...argumentsList)

7.3.15 SetIntegrityLevel ( `O`, `level` )

The abstract operation SetIntegrityLevel takes arguments O and level. It is used to fix the set of own properties of an object. It performs the following steps when called:

Assert: Type(O) is Object.
Assert: level is either sealed or frozen.
Let status be ? O.[[PreventExtensions]]().
If status is false, return false.
Let keys be ? O.[[OwnPropertyKeys]]().
If level is sealed, then
1. For each element k of keys, do
  1. Perform ? DefinePropertyOrThrow(O, k, PropertyDescriptor { [[Configurable]]: false }).
Else,
1. Assert: level is frozen.
2. For each element k of keys, do
  1. Let currentDesc be ? O.[[GetOwnProperty]](k).
  2. If currentDesc is not undefined, then
    1. If IsAccessorDescriptor(currentDesc) is true, then
      1. Let desc be the PropertyDescriptor { [[Configurable]]: false }.
    2. Else,
      1. Let desc be the PropertyDescriptor { [[Configurable]]: false, [[Writable]]: false }.
    3. Perform ? DefinePropertyOrThrow(O, k, desc).
Return true.

7.3.16 TestIntegrityLevel ( `O`, `level` )

The abstract operation TestIntegrityLevel takes arguments O and level. It is used to determine if the set of own properties of an object are fixed. It performs the following steps when called:

Assert: Type(O) is Object.
Assert: level is either sealed or frozen.
Let extensible be ? IsExtensible(O).
If extensible is true, return false.
NOTE: If the object is extensible, none of its properties are examined.
Let keys be ? O.[[OwnPropertyKeys]]().
For each element k of keys, do
1. Let currentDesc be ? O.[[GetOwnProperty]](k).
2. If currentDesc is not undefined, then
  1. If currentDesc.[[Configurable]] is true, return false.
  2. If level is frozen and IsDataDescriptor(currentDesc) is true, then
    1. If currentDesc.[[Writable]] is true, return false.
Return true.

7.3.17 CreateArrayFromList ( `elements` )

The abstract operation CreateArrayFromList takes argument elements (a List). It is used to create an Array object whose elements are provided by elements. It performs the following steps when called:

Assert: elements is a List whose elements are all ECMAScript language values.
Let array be ! ArrayCreate(0).
Let n be 0.
For each element e of elements, do
1. Perform ! CreateDataPropertyOrThrow(array, ! ToString(n), e).
2. Set n to n + 1.
Return array.

7.3.18 LengthOfArrayLike ( `obj` )

The abstract operation LengthOfArrayLike takes argument obj. It returns the value of the "length" property of an array-like object. It performs the following steps when called:

Assert: Type(obj) is Object.
Return ? ToLength(? Get(obj, "length")).

An array-like object is any object for which this operation returns an integer rather than an abrupt completion.

Note 1

Typically, an array-like object would also have some properties with integer index names. However, that is not a requirement of this definition.

Note 2

Array objects and String objects are examples of array-like objects.

7.3.19 CreateListFromArrayLike ( `obj` [ , `elementTypes` ] )

The abstract operation CreateListFromArrayLike takes argument obj and optional argument elementTypes (a List of names of ECMAScript Language Types). It is used to create a List value whose elements are provided by the indexed properties of obj. elementTypes contains the names of ECMAScript Language Types that are allowed for element values of the List that is created. It performs the following steps when called:

If elementTypes is not present, set elementTypes to « Undefined, Null, Boolean, String, Symbol, Number, BigInt, Object ».
If Type(obj) is not Object, throw a TypeError exception.
Let len be ? LengthOfArrayLike(obj).
Let list be a new empty List.
Let index be 0.
Repeat, while index < len
1. Let indexName be ! ToString(index).
2. Let next be ? Get(obj, indexName).
3. If Type(next) is not an element of elementTypes, throw a TypeError exception.
4. Append next as the last element of list.
5. Set index to index + 1.
Return list.

7.3.20 Invoke ( `V`, `P` [ , `argumentsList` ] )

The abstract operation Invoke takes arguments V (an ECMAScript language value) and P (a property key) and optional argument argumentsList (a List of ECMAScript language values). It is used to call a method property of an ECMAScript language value. V serves as both the lookup point for the property and the this value of the call. argumentsList is the list of arguments values passed to the method. If argumentsList is not present, a new empty List is used as its value. It performs the following steps when called:

Assert: IsPropertyKey(P) is true.
If argumentsList is not present, set argumentsList to a new empty List.
Let func be ? GetV(V, P).
Return ? Call(func, V, argumentsList).

7.3.21 OrdinaryHasInstance ( `C`, `O` )

The abstract operation OrdinaryHasInstance takes arguments C (an ECMAScript language value) and O. It implements the default algorithm for determining if O inherits from the instance object inheritance path provided by C. It performs the following steps when called:

If IsCallable(C) is false, return false.
If C has a [[BoundTargetFunction]] internal slot, then
1. Let BC be C.[[BoundTargetFunction]].
2. Return ? InstanceofOperator(O, BC).
If Type(O) is not Object, return false.
Let P be ? Get(C, "prototype").
If Type(P) is not Object, throw a TypeError exception.
Repeat,
1. Set O to ? O.[[GetPrototypeOf]]().
2. If O is null, return false.
3. If SameValue(P, O) is true, return true.

7.3.22 SpeciesConstructor ( `O`, `defaultConstructor` )

The abstract operation SpeciesConstructor takes arguments O (an Object) and defaultConstructor (a constructor). It is used to retrieve the constructor that should be used to create new objects that are derived from O. defaultConstructor is the constructor to use if a constructor @@species property cannot be found starting from O. It performs the following steps when called:

Assert: Type(O) is Object.
Let C be ? Get(O, "constructor").
If C is undefined, return defaultConstructor.
If Type(C) is not Object, throw a TypeError exception.
Let S be ? Get(C, @@species).
If S is either undefined or null, return defaultConstructor.
If IsConstructor(S) is true, return S.
Throw a TypeError exception.

7.3.23 EnumerableOwnPropertyNames ( `O`, `kind` )

The abstract operation EnumerableOwnPropertyNames takes arguments O (an Object) and kind (one of key, value, or key+value). It performs the following steps when called:

Assert: Type(O) is Object.
Let ownKeys be ? O.[[OwnPropertyKeys]]().
Let properties be a new empty List.
For each element key of ownKeys in List order, do
1. If Type(key) is String, then
  1. Let desc be ? O.[[GetOwnProperty]](key).
  2. If desc is not undefined and desc.[[Enumerable]] is true, then
    1. If kind is key, append key to properties.
    2. Else,
      1. Let value be ? Get(O, key).
      2. If kind is value, append value to properties.
      3. Else,
        Assert: kind is key+value.
        Let entry be ! CreateArrayFromList(« key, value »).
        Append entry to properties.
Return properties.

7.3.24 GetFunctionRealm ( `obj` )

The abstract operation GetFunctionRealm takes argument obj. It performs the following steps when called:

Assert: ! IsCallable(obj) is true.
If obj has a [[Realm]] internal slot, then
1. Return obj.[[Realm]].
If obj is a bound function exotic object, then
1. Let target be obj.[[BoundTargetFunction]].
2. Return ? GetFunctionRealm(target).
If obj is a Proxy exotic object, then
1. If obj.[[ProxyHandler]] is null, throw a TypeError exception.
2. Let proxyTarget be obj.[[ProxyTarget]].
3. Return ? GetFunctionRealm(proxyTarget).
Return the current Realm Record.

Note

Step 5 will only be reached if obj is a non-standard function exotic object that does not have a [[Realm]] internal slot.

7.3.25 CopyDataProperties ( `target`, `source`, `excludedItems` )

The abstract operation CopyDataProperties takes arguments target, source, and excludedItems. It performs the following steps when called:

Assert: Type(target) is Object.
Assert: excludedItems is a List of property keys.
If source is undefined or null, return target.
Let from be ! ToObject(source).
Let keys be ? from.[[OwnPropertyKeys]]().
For each element nextKey of keys in List order, do
1. Let excluded be false.
2. For each element e of excludedItems in List order, do
  1. If SameValue(e, nextKey) is true, then
    1. Set excluded to true.
3. If excluded is false, then
  1. Let desc be ? from.[[GetOwnProperty]](nextKey).
  2. If desc is not undefined and desc.[[Enumerable]] is true, then
    1. Let propValue be ? Get(from, nextKey).
    2. Perform ! CreateDataPropertyOrThrow(target, nextKey, propValue).
Return target.

Note

The target passed in here is always a newly created object which is not directly accessible in case of an error being thrown.

7.4 Operations on Iterator Objects

See Common Iteration Interfaces (25.1).

7.4.1 GetIterator ( `obj` [ , `hint` [ , `method` ] ] )

The abstract operation GetIterator takes argument obj and optional arguments hint and method. It performs the following steps when called:

If hint is not present, set hint to sync.
Assert: hint is either sync or async.
If method is not present, then
1. If hint is async, then
  1. Set method to ? GetMethod(obj, @@asyncIterator).
  2. If method is undefined, then
    1. Let syncMethod be ? GetMethod(obj, @@iterator).
    2. Let syncIteratorRecord be ? GetIterator(obj, sync, syncMethod).
    3. Return ! CreateAsyncFromSyncIterator(syncIteratorRecord).
2. Otherwise, set method to ? GetMethod(obj, @@iterator).
Let iterator be ? Call(method, obj).
If Type(iterator) is not Object, throw a TypeError exception.
Let nextMethod be ? GetV(iterator, "next").
Let iteratorRecord be the Record { [[Iterator]]: iterator, [[NextMethod]]: nextMethod, [[Done]]: false }.
Return iteratorRecord.

7.4.2 IteratorNext ( `iteratorRecord` [ , `value` ] )

The abstract operation IteratorNext takes argument iteratorRecord and optional argument value. It performs the following steps when called:

If value is not present, then
1. Let result be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]]).
Else,
1. Let result be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]], « value »).
If Type(result) is not Object, throw a TypeError exception.
Return result.

7.4.3 IteratorComplete ( `iterResult` )

The abstract operation IteratorComplete takes argument iterResult. It performs the following steps when called:

Assert: Type(iterResult) is Object.
Return ! ToBoolean(? Get(iterResult, "done")).

7.4.4 IteratorValue ( `iterResult` )

The abstract operation IteratorValue takes argument iterResult. It performs the following steps when called:

Assert: Type(iterResult) is Object.
Return ? Get(iterResult, "value").

7.4.5 IteratorStep ( `iteratorRecord` )

The abstract operation IteratorStep takes argument iteratorRecord. It requests the next value from iteratorRecord.[[Iterator]] by calling iteratorRecord.[[NextMethod]] and returns either false indicating that the iterator has reached its end or the IteratorResult object if a next value is available. It performs the following steps when called:

Let result be ? IteratorNext(iteratorRecord).
Let done be ? IteratorComplete(result).
If done is true, return false.
Return result.

7.4.6 IteratorClose ( `iteratorRecord`, `completion` )

The abstract operation IteratorClose takes arguments iteratorRecord and completion. It is used to notify an iterator that it should perform any actions it would normally perform when it has reached its completed state. It performs the following steps when called:

Assert: Type(iteratorRecord.[[Iterator]]) is Object.
Assert: completion is a Completion Record.
Let iterator be iteratorRecord.[[Iterator]].
Let innerResult be GetMethod(iterator, "return").
If innerResult.[[Type]] is normal, then
1. Let return be innerResult.[[Value]].
2. If return is undefined, return Completion(completion).
3. Set innerResult to Call(return, iterator).
If completion.[[Type]] is throw, return Completion(completion).
If innerResult.[[Type]] is throw, return Completion(innerResult).
If Type(innerResult.[[Value]]) is not Object, throw a TypeError exception.
Return Completion(completion).

7.4.7 AsyncIteratorClose ( `iteratorRecord`, `completion` )

The abstract operation AsyncIteratorClose takes arguments iteratorRecord and completion. It is used to notify an async iterator that it should perform any actions it would normally perform when it has reached its completed state. It performs the following steps when called:

Assert: Type(iteratorRecord.[[Iterator]]) is Object.
Assert: completion is a Completion Record.
Let iterator be iteratorRecord.[[Iterator]].
Let innerResult be GetMethod(iterator, "return").
If innerResult.[[Type]] is normal, then
1. Let return be innerResult.[[Value]].
2. If return is undefined, return Completion(completion).
3. Set innerResult to Call(return, iterator).
4. If innerResult.[[Type]] is normal, set innerResult to Await(innerResult.[[Value]]).
If completion.[[Type]] is throw, return Completion(completion).
If innerResult.[[Type]] is throw, return Completion(innerResult).
If Type(innerResult.[[Value]]) is not Object, throw a TypeError exception.
Return Completion(completion).

7.4.8 CreateIterResultObject ( `value`, `done` )

The abstract operation CreateIterResultObject takes arguments value and done. It creates an object that supports the IteratorResult interface. It performs the following steps when called:

Assert: Type(done) is Boolean.
Let obj be OrdinaryObjectCreate(%Object.prototype%).
Perform ! CreateDataPropertyOrThrow(obj, "value", value).
Perform ! CreateDataPropertyOrThrow(obj, "done", done).
Return obj.

7.4.9 CreateListIteratorRecord ( `list` )

The abstract operation CreateListIteratorRecord takes argument list. It creates an Iterator (25.1.1.2) object record whose next method returns the successive elements of list. It performs the following steps when called:

Let iterator be OrdinaryObjectCreate(%IteratorPrototype%, « [[IteratedList]], [[ListNextIndex]] »).
Set iterator.[[IteratedList]] to list.
Set iterator.[[ListNextIndex]] to 0.
Let steps be the algorithm steps defined in ListIteratorNext Functions.
Let next be ! CreateBuiltinFunction(steps, « »).
Return Record { [[Iterator]]: iterator, [[NextMethod]]: next, [[Done]]: false }.

Note

The list iterator object is never directly accessible to ECMAScript code.

7.4.9.1 ListIteratorNext Functions

A ListIteratorNext function is an anonymous built-in function. When called with no arguments, it performs the following steps:

Let O be the this value.
Assert: Type(O) is Object.
Assert: O has an [[IteratedList]] internal slot.
Let list be O.[[IteratedList]].
Let index be O.[[ListNextIndex]].
Let len be the number of elements of list.
If index ≥ len, then
1. Return CreateIterResultObject(undefined, true).
Set O.[[ListNextIndex]] to index + 1.
Return CreateIterResultObject(list[index], false).

The "length" property of a ListIteratorNext function is 0.

8 Executable Code and Execution Contexts

8.1 Environment Records

Environment Record is a specification type used to define the association of Identifiers to specific variables and functions, based upon the lexical nesting structure of ECMAScript code. Usually an Environment Record is associated with some specific syntactic structure of ECMAScript code such as a FunctionDeclaration, a BlockStatement, or a Catch clause of a TryStatement. Each time such code is evaluated, a new Environment Record is created to record the identifier bindings that are created by that code.

Every Environment Record has an [[OuterEnv]] field, which is either null or a reference to an outer Environment Record. This is used to model the logical nesting of Environment Record values. The outer reference of an (inner) Environment Record is a reference to the Environment Record that logically surrounds the inner Environment Record. An outer Environment Record may, of course, have its own outer Environment Record. An Environment Record may serve as the outer environment for multiple inner Environment Records. For example, if a FunctionDeclaration contains two nested FunctionDeclarations then the Environment Records of each of the nested functions will have as their outer Environment Record the Environment Record of the current evaluation of the surrounding function.

Environment Records are purely specification mechanisms and need not correspond to any specific artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to directly access or manipulate such values.

8.1.1 The Environment Record Type Hierarchy

Environment Records can be thought of as existing in a simple object-oriented hierarchy where Environment Record is an abstract class with three concrete subclasses: declarative Environment Record, object Environment Record, and global Environment Record. Function Environment Records and module Environment Records are subclasses of declarative Environment Record.

Environment Record (abstract)
- A declarative Environment Record is used to define the effect of ECMAScript language syntactic elements such as FunctionDeclarations, VariableDeclarations, and Catch clauses that directly associate identifier bindings with ECMAScript language values.
  - A function Environment Record corresponds to the invocation of an ECMAScript function object, and contains bindings for the top-level declarations within that function. It may establish a new this binding. It also captures the state necessary to support super method invocations.
  - A module Environment Record contains the bindings for the top-level declarations of a Module. It also contains the bindings that are explicitly imported by the Module. Its [[OuterEnv]] is a global Environment Record.
- An object Environment Record is used to define the effect of ECMAScript elements such as WithStatement that associate identifier bindings with the properties of some object.
- A global Environment Record is used for Script global declarations. It does not have an outer environment; its [[OuterEnv]] is null. It may be prepopulated with identifier bindings and it includes an associated global object whose properties provide some of the global environment's identifier bindings. As ECMAScript code is executed, additional properties may be added to the global object and the initial properties may be modified.

The Environment Record abstract class includes the abstract specification methods defined in Table 16. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.

Table 16: Abstract Methods of Environment Records

Method	Purpose
HasBinding(N)	Determine if an Environment Record has a binding for the String value `N`. Return true if it does and false if it does not.
CreateMutableBinding(N, D)	Create a new but uninitialized mutable binding in an Environment Record. The String value `N` is the text of the bound name. If the Boolean argument `D` is true the binding may be subsequently deleted.
CreateImmutableBinding(N, S)	Create a new but uninitialized immutable binding in an Environment Record. The String value `N` is the text of the bound name. If `S` is true then attempts to set it after it has been initialized will always throw an exception, regardless of the strict mode setting of operations that reference that binding.
InitializeBinding(N, V)	Set the value of an already existing but uninitialized binding in an Environment Record. The String value `N` is the text of the bound name. `V` is the value for the binding and is a value of any ECMAScript language type.
SetMutableBinding(N, V, S)	Set the value of an already existing mutable binding in an Environment Record. The String value `N` is the text of the bound name. `V` is the value for the binding and may be a value of any ECMAScript language type. `S` is a Boolean flag. If `S` is true and the binding cannot be set throw a TypeError exception.
GetBindingValue(N, S)	Returns the value of an already existing binding from an Environment Record. The String value `N` is the text of the bound name. `S` is used to identify references originating in strict mode code or that otherwise require strict mode reference semantics. If `S` is true and the binding does not exist throw a ReferenceError exception. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of `S`.
DeleteBinding(N)	Delete a binding from an Environment Record. The String value `N` is the text of the bound name. If a binding for `N` exists, remove the binding and return true. If the binding exists but cannot be removed return false. If the binding does not exist return true.
HasThisBinding()	Determine if an Environment Record establishes a `this` binding. Return true if it does and false if it does not.
HasSuperBinding()	Determine if an Environment Record establishes a `super` method binding. Return true if it does and false if it does not.
WithBaseObject()	If this Environment Record is associated with a `with` statement, return the with object. Otherwise, return undefined.

8.1.1.1 Declarative Environment Records

Each declarative Environment Record is associated with an ECMAScript program scope containing variable, constant, let, class, module, import, and/or function declarations. A declarative Environment Record binds the set of identifiers defined by the declarations contained within its scope.

The behaviour of the concrete specification methods for declarative Environment Records is defined by the following algorithms.

8.1.1.1.1 HasBinding ( `N` )

The concrete Environment Record method HasBinding for declarative Environment Records simply determines if the argument identifier is one of the identifiers bound by the record:

Let envRec be the declarative Environment Record for which the method was invoked.
If envRec has a binding for the name that is the value of N, return true.
Return false.

8.1.1.1.2 CreateMutableBinding ( `N`, `D` )

The concrete Environment Record method CreateMutableBinding for declarative Environment Records creates a new mutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If Boolean argument D has the value true the new binding is marked as being subject to deletion.

Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec does not already have a binding for N.
Create a mutable binding in envRec for N and record that it is uninitialized. If D is true, record that the newly created binding may be deleted by a subsequent DeleteBinding call.
Return NormalCompletion(empty).

8.1.1.1.3 CreateImmutableBinding ( `N`, `S` )

The concrete Environment Record method CreateImmutableBinding for declarative Environment Records creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If the Boolean argument S has the value true the new binding is marked as a strict binding.

Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec does not already have a binding for N.
Create an immutable binding in envRec for N and record that it is uninitialized. If S is true, record that the newly created binding is a strict binding.
Return NormalCompletion(empty).

8.1.1.1.4 InitializeBinding ( `N`, `V` )

The concrete Environment Record method InitializeBinding for declarative Environment Records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist.

Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec must have an uninitialized binding for N.
Set the bound value for N in envRec to V.
Record that the binding for N in envRec has been initialized.
Return NormalCompletion(empty).

8.1.1.1.5 SetMutableBinding ( `N`, `V`, `S` )

The concrete Environment Record method SetMutableBinding for declarative Environment Records attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. A binding for N normally already exists, but in rare cases it may not. If the binding is an immutable binding, a TypeError is thrown if S is true.

Let envRec be the declarative Environment Record for which the method was invoked.
If envRec does not have a binding for N, then
1. If S is true, throw a ReferenceError exception.
2. Perform envRec.CreateMutableBinding(N, true).
3. Perform envRec.InitializeBinding(N, V).
4. Return NormalCompletion(empty).
If the binding for N in envRec is a strict binding, set S to true.
If the binding for N in envRec has not yet been initialized, throw a ReferenceError exception.
Else if the binding for N in envRec is a mutable binding, change its bound value to V.
Else,
1. Assert: This is an attempt to change the value of an immutable binding.
2. If S is true, throw a TypeError exception.
Return NormalCompletion(empty).

Note

An example of ECMAScript code that results in a missing binding at step 2 is:

function f() { eval("var x; x = (delete x, 0);"); }

8.1.1.1.6 GetBindingValue ( `N`, `S` )

The concrete Environment Record method GetBindingValue for declarative Environment Records simply returns the value of its bound identifier whose name is the value of the argument N. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of S.

Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec has a binding for N.
If the binding for N in envRec is an uninitialized binding, throw a ReferenceError exception.
Return the value currently bound to N in envRec.

8.1.1.1.7 DeleteBinding ( `N` )

The concrete Environment Record method DeleteBinding for declarative Environment Records can only delete bindings that have been explicitly designated as being subject to deletion.

Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec has a binding for the name that is the value of N.
If the binding for N in envRec cannot be deleted, return false.
Remove the binding for N from envRec.
Return true.

8.1.1.1.8 HasThisBinding ( )

Regular declarative Environment Records do not provide a this binding.

Return false.

8.1.1.1.9 HasSuperBinding ( )

Regular declarative Environment Records do not provide a super binding.

Return false.

8.1.1.1.10 WithBaseObject ( )

Declarative Environment Records always return undefined as their WithBaseObject.

Return undefined.

8.1.1.2 Object Environment Records

Each object Environment Record is associated with an object called its binding object. An object Environment Record binds the set of string identifier names that directly correspond to the property names of its binding object. Property keys that are not strings in the form of an IdentifierName are not included in the set of bound identifiers. Both own and inherited properties are included in the set regardless of the setting of their [[Enumerable]] attribute. Because properties can be dynamically added and deleted from objects, the set of identifiers bound by an object Environment Record may potentially change as a side-effect of any operation that adds or deletes properties. Any bindings that are created as a result of such a side-effect are considered to be a mutable binding even if the Writable attribute of the corresponding property has the value false. Immutable bindings do not exist for object Environment Records.

Object Environment Records created for with statements (13.11) can provide their binding object as an implicit this value for use in function calls. The capability is controlled by a withEnvironment Boolean value that is associated with each object Environment Record. By default, the value of withEnvironment is false for any object Environment Record.

The behaviour of the concrete specification methods for object Environment Records is defined by the following algorithms.

8.1.1.2.1 HasBinding ( `N` )

The concrete Environment Record method HasBinding for object Environment Records determines if its associated binding object has a property whose name is the value of the argument N:

Let envRec be the object Environment Record for which the method was invoked.
Let bindings be the binding object for envRec.
Let foundBinding be ? HasProperty(bindings, N).
If foundBinding is false, return false.
If the withEnvironment flag of envRec is false, return true.
Let unscopables be ? Get(bindings, @@unscopables).
If Type(unscopables) is Object, then
1. Let blocked be ! ToBoolean(? Get(unscopables, N)).
2. If blocked is true, return false.
Return true.

8.1.1.2.2 CreateMutableBinding ( `N`, `D` )

The concrete Environment Record method CreateMutableBinding for object Environment Records creates in an Environment Record's associated binding object a property whose name is the String value and initializes it to the value undefined. If Boolean argument D has the value true the new property's [[Configurable]] attribute is set to true; otherwise it is set to false.

Let envRec be the object Environment Record for which the method was invoked.
Let bindings be the binding object for envRec.
Return ? DefinePropertyOrThrow(bindings, N, PropertyDescriptor { [[Value]]: undefined, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: D }).

Note

Normally envRec will not have a binding for N but if it does, the semantics of DefinePropertyOrThrow may result in an existing binding being replaced or shadowed or cause an abrupt completion to be returned.

8.1.1.2.3 CreateImmutableBinding ( `N`, `S` )

The concrete Environment Record method CreateImmutableBinding is never used within this specification in association with object Environment Records.

8.1.1.2.4 InitializeBinding ( `N`, `V` )

The concrete Environment Record method InitializeBinding for object Environment Records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist.

Let envRec be the object Environment Record for which the method was invoked.
Assert: envRec must have an uninitialized binding for N.
Record that the binding for N in envRec has been initialized.
Return ? envRec.SetMutableBinding(N, V, false).

Note

In this specification, all uses of CreateMutableBinding for object Environment Records are immediately followed by a call to InitializeBinding for the same name. Hence, implementations do not need to explicitly track the initialization state of individual object Environment Record bindings.

8.1.1.2.5 SetMutableBinding ( `N`, `V`, `S` )

The concrete Environment Record method SetMutableBinding for object Environment Records attempts to set the value of the Environment Record's associated binding object's property whose name is the value of the argument N to the value of argument V. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.

Let envRec be the object Environment Record for which the method was invoked.
Let bindings be the binding object for envRec.
Return ? Set(bindings, N, V, S).

8.1.1.2.6 GetBindingValue ( `N`, `S` )

The concrete Environment Record method GetBindingValue for object Environment Records returns the value of its associated binding object's property whose name is the String value of the argument identifier N. The property should already exist but if it does not the result depends upon the value of the S argument:

Let envRec be the object Environment Record for which the method was invoked.
Let bindings be the binding object for envRec.
Let value be ? HasProperty(bindings, N).
If value is false, then
1. If S is false, return the value undefined; otherwise throw a ReferenceError exception.
Return ? Get(bindings, N).

8.1.1.2.7 DeleteBinding ( `N` )

The concrete Environment Record method DeleteBinding for object Environment Records can only delete bindings that correspond to properties of the environment object whose [[Configurable]] attribute have the value true.

Let envRec be the object Environment Record for which the method was invoked.
Let bindings be the binding object for envRec.
Return ? bindings.[[Delete]](N).

8.1.1.2.8 HasThisBinding ( )

Regular object Environment Records do not provide a this binding.

Return false.

8.1.1.2.9 HasSuperBinding ( )

Regular object Environment Records do not provide a super binding.

Return false.

8.1.1.2.10 WithBaseObject ( )

Object Environment Records return undefined as their WithBaseObject unless their withEnvironment flag is true.

Let envRec be the object Environment Record for which the method was invoked.
If the withEnvironment flag of envRec is true, return the binding object for envRec.
Otherwise, return undefined.

8.1.1.3 Function Environment Records

A function Environment Record is a declarative Environment Record that is used to represent the top-level scope of a function and, if the function is not an ArrowFunction, provides a this binding. If a function is not an ArrowFunction function and references super, its function Environment Record also contains the state that is used to perform super method invocations from within the function.

Function Environment Records have the additional state fields listed in Table 17.

Table 17: Additional Fields of Function Environment Records

Field Name	Value	Meaning
[[ThisValue]]	Any	This is the this value used for this invocation of the function.
[[ThisBindingStatus]]	lexical \| initialized \| uninitialized	If the value is lexical, this is an ArrowFunction and does not have a local this value.
[[FunctionObject]]	Object	The function object whose invocation caused this Environment Record to be created.
[[HomeObject]]	Object \| undefined	If the associated function has `super` property accesses and is not an ArrowFunction, [[HomeObject]] is the object that the function is bound to as a method. The default value for [[HomeObject]] is undefined.
[[NewTarget]]	Object \| undefined	If this Environment Record was created by the [[Construct]] internal method, [[NewTarget]] is the value of the [[Construct]] `newTarget` parameter. Otherwise, its value is undefined.

Function Environment Records support all of the declarative Environment Record methods listed in Table 16 and share the same specifications for all of those methods except for HasThisBinding and HasSuperBinding. In addition, function Environment Records support the methods listed in Table 18:

Table 18: Additional Methods of Function Environment Records

Method	Purpose
BindThisValue(V)	Set the [[ThisValue]] and record that it has been initialized.
GetThisBinding()	Return the value of this Environment Record's `this` binding. Throws a ReferenceError if the `this` binding has not been initialized.
GetSuperBase()	Return the object that is the base for `super` property accesses bound in this Environment Record. The object is derived from this Environment Record's [[HomeObject]] field. The value undefined indicates that `super` property accesses will produce runtime errors.

The behaviour of the additional concrete specification methods for function Environment Records is defined by the following algorithms:

8.1.1.3.1 BindThisValue ( `V` )

Let envRec be the function Environment Record for which the method was invoked.
Assert: envRec.[[ThisBindingStatus]] is not lexical.
If envRec.[[ThisBindingStatus]] is initialized, throw a ReferenceError exception.
Set envRec.[[ThisValue]] to V.
Set envRec.[[ThisBindingStatus]] to initialized.
Return V.

8.1.1.3.2 HasThisBinding ( )

Let envRec be the function Environment Record for which the method was invoked.
If envRec.[[ThisBindingStatus]] is lexical, return false; otherwise, return true.

8.1.1.3.3 HasSuperBinding ( )

Let envRec be the function Environment Record for which the method was invoked.
If envRec.[[ThisBindingStatus]] is lexical, return false.
If envRec.[[HomeObject]] has the value undefined, return false; otherwise, return true.

8.1.1.3.4 GetThisBinding ( )

Let envRec be the function Environment Record for which the method was invoked.
Assert: envRec.[[ThisBindingStatus]] is not lexical.
If envRec.[[ThisBindingStatus]] is uninitialized, throw a ReferenceError exception.
Return envRec.[[ThisValue]].

8.1.1.3.5 GetSuperBase ( )

Let envRec be the function Environment Record for which the method was invoked.
Let home be envRec.[[HomeObject]].
If home has the value undefined, return undefined.
Assert: Type(home) is Object.
Return ? home.[[GetPrototypeOf]]().

8.1.1.4 Global Environment Records

A global Environment Record is used to represent the outer most scope that is shared by all of the ECMAScript Script elements that are processed in a common realm. A global Environment Record provides the bindings for built-in globals (clause 18), properties of the global object, and for all top-level declarations (13.2.8, 13.2.10) that occur within a Script.

A global Environment Record is logically a single record but it is specified as a composite encapsulating an object Environment Record and a declarative Environment Record. The object Environment Record has as its base object the global object of the associated Realm Record. This global object is the value returned by the global Environment Record's GetThisBinding concrete method. The object Environment Record component of a global Environment Record contains the bindings for all built-in globals (clause 18) and all bindings introduced by a FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, or VariableStatement contained in global code. The bindings for all other ECMAScript declarations in global code are contained in the declarative Environment Record component of the global Environment Record.

Properties may be created directly on a global object. Hence, the object Environment Record component of a global Environment Record may contain both bindings created explicitly by FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, or VariableDeclaration declarations and bindings created implicitly as properties of the global object. In order to identify which bindings were explicitly created using declarations, a global Environment Record maintains a list of the names bound using its CreateGlobalVarBinding and CreateGlobalFunctionBinding concrete methods.

Global Environment Records have the additional fields listed in Table 19 and the additional methods listed in Table 20.

Table 19: Additional Fields of Global Environment Records

Field Name	Value	Meaning
[[ObjectRecord]]	Object Environment Record	Binding object is the global object. It contains global built-in bindings as well as FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, and VariableDeclaration bindings in global code for the associated realm.
[[GlobalThisValue]]	Object	The value returned by `this` in global scope. Hosts may provide any ECMAScript Object value.
[[DeclarativeRecord]]	Declarative Environment Record	Contains bindings for all declarations in global code for the associated realm code except for FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, and VariableDeclaration `bindings`.
[[VarNames]]	List of String	The string names bound by FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, and VariableDeclaration declarations in global code for the associated realm.

Table 20: Additional Methods of Global Environment Records

Method	Purpose
GetThisBinding()	Return the value of this Environment Record's `this` binding.
HasVarDeclaration (N)	Determines if the argument identifier has a binding in this Environment Record that was created using a VariableDeclaration, FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, or AsyncGeneratorDeclaration.
HasLexicalDeclaration (N)	Determines if the argument identifier has a binding in this Environment Record that was created using a lexical declaration such as a LexicalDeclaration or a ClassDeclaration.
HasRestrictedGlobalProperty (N)	Determines if the argument is the name of a global object property that may not be shadowed by a global lexical binding.
CanDeclareGlobalVar (N)	Determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument `N`.
CanDeclareGlobalFunction (N)	Determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument `N`.
CreateGlobalVarBinding(N, D)	Used to create and initialize to undefined a global `var` binding in the [[ObjectRecord]] component of a global Environment Record. The binding will be a mutable binding. The corresponding global object property will have attribute values appropriate for a `var`. The String value `N` is the bound name. If `D` is true the binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows var declarations to receive special treatment.
CreateGlobalFunctionBinding(N, V, D)	Create and initialize a global `function` binding in the [[ObjectRecord]] component of a global Environment Record. The binding will be a mutable binding. The corresponding global object property will have attribute values appropriate for a `function`. The String value `N` is the bound name. `V` is the initialization value. If the Boolean argument `D` is true the binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows function declarations to receive special treatment.

The behaviour of the concrete specification methods for global Environment Records is defined by the following algorithms.

8.1.1.4.1 HasBinding ( `N` )

The concrete Environment Record method HasBinding for global Environment Records simply determines if the argument identifier is one of the identifiers bound by the record:

Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, return true.
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.HasBinding(N).

8.1.1.4.2 CreateMutableBinding ( `N`, `D` )

The concrete Environment Record method CreateMutableBinding for global Environment Records creates a new mutable binding for the name N that is uninitialized. The binding is created in the associated DeclarativeRecord. A binding for N must not already exist in the DeclarativeRecord. If Boolean argument D has the value true the new binding is marked as being subject to deletion.

Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, throw a TypeError exception.
Return DclRec.CreateMutableBinding(N, D).

8.1.1.4.3 CreateImmutableBinding ( `N`, `S` )

The concrete Environment Record method CreateImmutableBinding for global Environment Records creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If the Boolean argument S has the value true the new binding is marked as a strict binding.

Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, throw a TypeError exception.
Return DclRec.CreateImmutableBinding(N, S).

8.1.1.4.4 InitializeBinding ( `N`, `V` )

The concrete Environment Record method InitializeBinding for global Environment Records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist.

Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, then
1. Return DclRec.InitializeBinding(N, V).
Assert: If the binding exists, it must be in the object Environment Record.
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.InitializeBinding(N, V).

8.1.1.4.5 SetMutableBinding ( `N`, `V`, `S` )

The concrete Environment Record method SetMutableBinding for global Environment Records attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. If the binding is an immutable binding, a TypeError is thrown if S is true. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.

Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, then
1. Return DclRec.SetMutableBinding(N, V, S).
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.SetMutableBinding(N, V, S).

8.1.1.4.6 GetBindingValue ( `N`, `S` )

The concrete Environment Record method GetBindingValue for global Environment Records returns the value of its bound identifier whose name is the value of the argument N. If the binding is an uninitialized binding throw a ReferenceError exception. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.

Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, then
1. Return DclRec.GetBindingValue(N, S).
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.GetBindingValue(N, S).

8.1.1.4.7 DeleteBinding ( `N` )

The concrete Environment Record method DeleteBinding for global Environment Records can only delete bindings that have been explicitly designated as being subject to deletion.

Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, then
1. Return DclRec.DeleteBinding(N).
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
Let existingProp be ? HasOwnProperty(globalObject, N).
If existingProp is true, then
1. Let status be ? ObjRec.DeleteBinding(N).
2. If status is true, then
  1. Let varNames be envRec.[[VarNames]].
  2. If N is an element of varNames, remove that element from the varNames.
3. Return status.
Return true.

8.1.1.4.8 HasThisBinding ( )

Return true.

8.1.1.4.9 HasSuperBinding ( )

Return false.

8.1.1.4.10 WithBaseObject ( )

Global Environment Records always return undefined as their WithBaseObject.

Return undefined.

8.1.1.4.11 GetThisBinding ( )

Let envRec be the global Environment Record for which the method was invoked.
Return envRec.[[GlobalThisValue]].

8.1.1.4.12 HasVarDeclaration ( `N` )

The concrete Environment Record method HasVarDeclaration for global Environment Records determines if the argument identifier has a binding in this record that was created using a VariableStatement or a FunctionDeclaration:

Let envRec be the global Environment Record for which the method was invoked.
Let varDeclaredNames be envRec.[[VarNames]].
If varDeclaredNames contains N, return true.
Return false.

8.1.1.4.13 HasLexicalDeclaration ( `N` )

The concrete Environment Record method HasLexicalDeclaration for global Environment Records determines if the argument identifier has a binding in this record that was created using a lexical declaration such as a LexicalDeclaration or a ClassDeclaration:

Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
Return DclRec.HasBinding(N).

8.1.1.4.14 HasRestrictedGlobalProperty ( `N` )

The concrete Environment Record method HasRestrictedGlobalProperty for global Environment Records determines if the argument identifier is the name of a property of the global object that must not be shadowed by a global lexical binding:

Let envRec be the global Environment Record for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined, return false.
If existingProp.[[Configurable]] is true, return false.
Return true.

Note

Properties may exist upon a global object that were directly created rather than being declared using a var or function declaration. A global lexical binding may not be created that has the same name as a non-configurable property of the global object. The global property "undefined" is an example of such a property.

8.1.1.4.15 CanDeclareGlobalVar ( `N` )

The concrete Environment Record method CanDeclareGlobalVar for global Environment Records determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N. Redundant var declarations and var declarations for pre-existing global object properties are allowed.

Let envRec be the global Environment Record for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
Let hasProperty be ? HasOwnProperty(globalObject, N).
If hasProperty is true, return true.
Return ? IsExtensible(globalObject).

8.1.1.4.16 CanDeclareGlobalFunction ( `N` )

The concrete Environment Record method CanDeclareGlobalFunction for global Environment Records determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N.

Let envRec be the global Environment Record for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined, return ? IsExtensible(globalObject).
If existingProp.[[Configurable]] is true, return true.
If IsDataDescriptor(existingProp) is true and existingProp has attribute values { [[Writable]]: true, [[Enumerable]]: true }, return true.
Return false.

8.1.1.4.17 CreateGlobalVarBinding ( `N`, `D` )

The concrete Environment Record method CreateGlobalVarBinding for global Environment Records creates and initializes a mutable binding in the associated object Environment Record and records the bound name in the associated [[VarNames]] List. If a binding already exists, it is reused and assumed to be initialized.

Let envRec be the global Environment Record for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
Let hasProperty be ? HasOwnProperty(globalObject, N).
Let extensible be ? IsExtensible(globalObject).
If hasProperty is false and extensible is true, then
1. Perform ? ObjRec.CreateMutableBinding(N, D).
2. Perform ? ObjRec.InitializeBinding(N, undefined).
Let varDeclaredNames be envRec.[[VarNames]].
If varDeclaredNames does not contain N, then
1. Append N to varDeclaredNames.
Return NormalCompletion(empty).

8.1.1.4.18 CreateGlobalFunctionBinding ( `N`, `V`, `D` )

The concrete Environment Record method CreateGlobalFunctionBinding for global Environment Records creates and initializes a mutable binding in the associated object Environment Record and records the bound name in the associated [[VarNames]] List. If a binding already exists, it is replaced.

Let envRec be the global Environment Record for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined or existingProp.[[Configurable]] is true, then
1. Let desc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: D }.
Else,
1. Let desc be the PropertyDescriptor { [[Value]]: V }.
Perform ? DefinePropertyOrThrow(globalObject, N, desc).
Record that the binding for N in ObjRec has been initialized.
Perform ? Set(globalObject, N, V, false).
Let varDeclaredNames be envRec.[[VarNames]].
If varDeclaredNames does not contain N, then
1. Append N to varDeclaredNames.
Return NormalCompletion(empty).

Note

Global function declarations are always represented as own properties of the global object. If possible, an existing own property is reconfigured to have a standard set of attribute values. Steps 8-9 are equivalent to what calling the InitializeBinding concrete method would do and if globalObject is a Proxy will produce the same sequence of Proxy trap calls.

8.1.1.5 Module Environment Records

A module Environment Record is a declarative Environment Record that is used to represent the outer scope of an ECMAScript Module. In additional to normal mutable and immutable bindings, module Environment Records also provide immutable import bindings which are bindings that provide indirect access to a target binding that exists in another Environment Record.

Module Environment Records support all of the declarative Environment Record methods listed in Table 16 and share the same specifications for all of those methods except for GetBindingValue, DeleteBinding, HasThisBinding and GetThisBinding. In addition, module Environment Records support the methods listed in Table 21:

Table 21: Additional Methods of Module Environment Records

Method	Purpose
CreateImportBinding(N, M, N2)	Create an immutable indirect binding in a module Environment Record. The String value `N` is the text of the bound name. `M` is a Module Record, and `N2` is a binding that exists in `M`'s module Environment Record.
GetThisBinding()	Return the value of this Environment Record's `this` binding.

The behaviour of the additional concrete specification methods for module Environment Records are defined by the following algorithms:

8.1.1.5.1 GetBindingValue ( `N`, `S` )

The concrete Environment Record method GetBindingValue for module Environment Records returns the value of its bound identifier whose name is the value of the argument N. However, if the binding is an indirect binding the value of the target binding is returned. If the binding exists but is uninitialized a ReferenceError is thrown.

Assert: S is true.
Let envRec be the module Environment Record for which the method was invoked.
Assert: envRec has a binding for N.
If the binding for N is an indirect binding, then
1. Let M and N2 be the indirection values provided when this binding for N was created.
2. Let targetEnv be M.[[Environment]].
3. If targetEnv is undefined, throw a ReferenceError exception.
4. Return ? targetEnv.GetBindingValue(N2, true).
If the binding for N in envRec is an uninitialized binding, throw a ReferenceError exception.
Return the value currently bound to N in envRec.

Note

S will always be true because a Module is always strict mode code.

8.1.1.5.2 DeleteBinding ( `N` )

The concrete Environment Record method DeleteBinding for module Environment Records refuses to delete bindings.

Assert: This method is never invoked. See 12.5.3.1.

Note

Module Environment Records are only used within strict code and an early error rule prevents the delete operator, in strict code, from being applied to a Reference that would resolve to a module Environment Record binding. See 12.5.3.1.

8.1.1.5.3 HasThisBinding ( )

Module Environment Records provide a this binding.

Return true.

8.1.1.5.4 GetThisBinding ( )

Return undefined.

8.1.1.5.5 CreateImportBinding ( `N`, `M`, `N2` )

The concrete Environment Record method CreateImportBinding for module Environment Records creates a new initialized immutable indirect binding for the name N. A binding must not already exist in this Environment Record for N. M is a Module Record, and N2 is the name of a binding that exists in M's module Environment Record. Accesses to the value of the new binding will indirectly access the bound value of the target binding.

Let envRec be the module Environment Record for which the method was invoked.
Assert: envRec does not already have a binding for N.
Assert: M is a Module Record.
Assert: When M.[[Environment]] is instantiated it will have a direct binding for N2.
Create an immutable indirect binding in envRec for N that references M and N2 as its target binding and record that the binding is initialized.
Return NormalCompletion(empty).

8.1.2 Environment Record Operations

The following abstract operations are used in this specification to operate upon Environment Records:

8.1.2.1 GetIdentifierReference ( `env`, `name`, `strict` )

The abstract operation GetIdentifierReference takes arguments env (an Environment Record or null), name (a String), and strict (a Boolean). It performs the following steps when called:

If env is the value null, then
1. Return a value of type Reference whose base value component is undefined, whose referenced name component is name, and whose strict reference flag is strict.
Let exists be ? env.HasBinding(name).
If exists is true, then
1. Return a value of type Reference whose base value component is env, whose referenced name component is name, and whose strict reference flag is strict.
Else,
1. Let outer be env.[[OuterEnv]].
2. Return ? GetIdentifierReference(outer, name, strict).

8.1.2.2 NewDeclarativeEnvironment ( `E` )

The abstract operation NewDeclarativeEnvironment takes argument E (an Environment Record). It performs the following steps when called:

Let env be a new declarative Environment Record containing no bindings.
Set env.[[OuterEnv]] to E.
Return env.

8.1.2.3 NewObjectEnvironment ( `O`, `E` )

The abstract operation NewObjectEnvironment takes arguments O (an Object) and E (an Environment Record). It performs the following steps when called:

Let env be a new object Environment Record containing O as the binding object.
Set env.[[OuterEnv]] to E.
Return env.

8.1.2.4 NewFunctionEnvironment ( `F`, `newTarget` )

The abstract operation NewFunctionEnvironment takes arguments F and newTarget. It performs the following steps when called:

Assert: F is an ECMAScript function.
Assert: Type(newTarget) is Undefined or Object.
Let env be a new function Environment Record containing no bindings.
Set env.[[FunctionObject]] to F.
If F.[[ThisMode]] is lexical, set env.[[ThisBindingStatus]] to lexical.
Else, set env.[[ThisBindingStatus]] to uninitialized.
Let home be F.[[HomeObject]].
Set env.[[HomeObject]] to home.
Set env.[[NewTarget]] to newTarget.
Set env.[[OuterEnv]] to F.[[Environment]].
Return env.

8.1.2.5 NewGlobalEnvironment ( `G`, `thisValue` )

The abstract operation NewGlobalEnvironment takes arguments G and thisValue. It performs the following steps when called:

Let objRec be a new object Environment Record containing G as the binding object.
Let dclRec be a new declarative Environment Record containing no bindings.
Let env be a new global Environment Record.
Set env.[[ObjectRecord]] to objRec.
Set env.[[GlobalThisValue]] to thisValue.
Set env.[[DeclarativeRecord]] to dclRec.
Set env.[[VarNames]] to a new empty List.
Set env.[[OuterEnv]] to null.
Return env.

8.1.2.6 NewModuleEnvironment ( `E` )

The abstract operation NewModuleEnvironment takes argument E (an Environment Record). It performs the following steps when called:

Let env be a new module Environment Record containing no bindings.
Set env.[[OuterEnv]] to E.
Return env.

8.2 Realms

Before it is evaluated, all ECMAScript code must be associated with a realm. Conceptually, a realm consists of a set of intrinsic objects, an ECMAScript global environment, all of the ECMAScript code that is loaded within the scope of that global environment, and other associated state and resources.

A realm is represented in this specification as a Realm Record with the fields specified in Table 22:

Table 22: Realm Record Fields

Field Name	Value	Meaning
[[Intrinsics]]	Record whose field names are intrinsic keys and whose values are objects	The intrinsic values used by code associated with this realm
[[GlobalObject]]	Object	The global object for this realm
[[GlobalEnv]]	global Environment Record	The global environment for this realm
[[TemplateMap]]	A List of Record { [[Site]]: Parse Node, [[Array]]: Object }.	Template objects are canonicalized separately for each realm using its Realm Record's [[TemplateMap]]. Each [[Site]] value is a Parse Node that is a TemplateLiteral. The associated [[Array]] value is the corresponding template object that is passed to a tag function. Note Once a Parse Node becomes unreachable, the corresponding [[Array]] is also unreachable, and it would be unobservable if an implementation removed the pair from the [[TemplateMap]] list.
[[HostDefined]]	Any, default value is undefined.	Field reserved for use by host environments that need to associate additional information with a Realm Record.

8.2.1 CreateRealm ( )

The abstract operation CreateRealm takes no arguments. It performs the following steps when called:

Let realmRec be a new Realm Record.
Perform CreateIntrinsics(realmRec).
Set realmRec.[[GlobalObject]] to undefined.
Set realmRec.[[GlobalEnv]] to undefined.
Set realmRec.[[TemplateMap]] to a new empty List.
Return realmRec.

8.2.2 CreateIntrinsics ( `realmRec` )

The abstract operation CreateIntrinsics takes argument realmRec. It performs the following steps when called:

Let intrinsics be a new Record.
Set realmRec.[[Intrinsics]] to intrinsics.
Set fields of intrinsics with the values listed in Table 8. The field names are the names listed in column one of the table. The value of each field is a new object value fully and recursively populated with property values as defined by the specification of each object in clauses 18-26. All object property values are newly created object values. All values that are built-in function objects are created by performing CreateBuiltinFunction(<steps>, <slots>, realmRec, <prototype>) where <steps> is the definition of that function provided by this specification, <slots> is a list of the names, if any, of the function's specified internal slots, and <prototype> is the specified value of the function's [[Prototype]] internal slot. The creation of the intrinsics and their properties must be ordered to avoid any dependencies upon objects that have not yet been created.
Perform AddRestrictedFunctionProperties(intrinsics.[[%Function.prototype%]], realmRec).
Return intrinsics.

8.2.3 SetRealmGlobalObject ( `realmRec`, `globalObj`, `thisValue` )

The abstract operation SetRealmGlobalObject takes arguments realmRec, globalObj, and thisValue. It performs the following steps when called:

If globalObj is undefined, then
1. Let intrinsics be realmRec.[[Intrinsics]].
2. Set globalObj to OrdinaryObjectCreate(intrinsics.[[%Object.prototype%]]).
Assert: Type(globalObj) is Object.
If thisValue is undefined, set thisValue to globalObj.
Set realmRec.[[GlobalObject]] to globalObj.
Let newGlobalEnv be NewGlobalEnvironment(globalObj, thisValue).
Set realmRec.[[GlobalEnv]] to newGlobalEnv.
Return realmRec.

8.2.4 SetDefaultGlobalBindings ( `realmRec` )

The abstract operation SetDefaultGlobalBindings takes argument realmRec. It performs the following steps when called:

Let global be realmRec.[[GlobalObject]].
For each property of the Global Object specified in clause 18, do
1. Let name be the String value of the property name.
2. Let desc be the fully populated data property descriptor for the property, containing the specified attributes for the property. For properties listed in 18.2, 18.3, or 18.4 the value of the [[Value]] attribute is the corresponding intrinsic object from realmRec.
3. Perform ? DefinePropertyOrThrow(global, name, desc).
Return global.

8.3 Execution Contexts

An execution context is a specification device that is used to track the runtime evaluation of code by an ECMAScript implementation. At any point in time, there is at most one execution context per agent that is actually executing code. This is known as the agent's running execution context. All references to the running execution context in this specification denote the running execution context of the surrounding agent.

The execution context stack is used to track execution contexts. The running execution context is always the top element of this stack. A new execution context is created whenever control is transferred from the executable code associated with the currently running execution context to executable code that is not associated with that execution context. The newly created execution context is pushed onto the stack and becomes the running execution context.

An execution context contains whatever implementation specific state is necessary to track the execution progress of its associated code. Each execution context has at least the state components listed in Table 23.

Table 23: State Components for All Execution Contexts

Component	Purpose
code evaluation state	Any state needed to perform, suspend, and resume evaluation of the code associated with this execution context.
Function	If this execution context is evaluating the code of a function object, then the value of this component is that function object. If the context is evaluating the code of a Script or Module, the value is null.
Realm	The Realm Record from which associated code accesses ECMAScript resources.
ScriptOrModule	The Module Record or Script Record from which associated code originates. If there is no originating script or module, as is the case for the original execution context created in InitializeHostDefinedRealm, the value is null.

Evaluation of code by the running execution context may be suspended at various points defined within this specification. Once the running execution context has been suspended a different execution context may become the running execution context and commence evaluating its code. At some later time a suspended execution context may again become the running execution context and continue evaluating its code at the point where it had previously been suspended. Transition of the running execution context status among execution contexts usually occurs in stack-like last-in/first-out manner. However, some ECMAScript features require non-LIFO transitions of the running execution context.

The value of the Realm component of the running execution context is also called the current Realm Record. The value of the Function component of the running execution context is also called the active function object.

Execution contexts for ECMAScript code have the additional state components listed in Table 24.

Table 24: Additional State Components for ECMAScript Code Execution Contexts

Component	Purpose
LexicalEnvironment	Identifies the Environment Record used to resolve identifier references made by code within this execution context.
VariableEnvironment	Identifies the Environment Record that holds bindings created by VariableStatements within this execution context.

The LexicalEnvironment and VariableEnvironment components of an execution context are always Environment Records.

Execution contexts representing the evaluation of generator objects have the additional state components listed in Table 25.

Table 25: Additional State Components for Generator Execution Contexts

Component	Purpose
Generator	The generator object that this execution context is evaluating.

In most situations only the running execution context (the top of the execution context stack) is directly manipulated by algorithms within this specification. Hence when the terms “LexicalEnvironment”, and “VariableEnvironment” are used without qualification they are in reference to those components of the running execution context.

An execution context is purely a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation. It is impossible for ECMAScript code to directly access or observe an execution context.

8.3.1 GetActiveScriptOrModule ( )

The abstract operation GetActiveScriptOrModule takes no arguments. It is used to determine the running script or module, based on the running execution context. It performs the following steps when called:

If the execution context stack is empty, return null.
Let ec be the topmost execution context on the execution context stack whose ScriptOrModule component is not null.
If no such execution context exists, return null. Otherwise, return ec's ScriptOrModule.

8.3.2 ResolveBinding ( `name` [ , `env` ] )

The abstract operation ResolveBinding takes argument name (a String) and optional argument env (an Environment Record). It is used to determine the binding of name. env can be used to explicitly provide the Environment Record that is to be searched for the binding. It performs the following steps when called:

If env is not present or if env is undefined, then
1. Set env to the running execution context's LexicalEnvironment.
Assert: env is an Environment Record.
If the code matching the syntactic production that is being evaluated is contained in strict mode code, let strict be true; else let strict be false.
Return ? GetIdentifierReference(env, name, strict).

Note

The result of ResolveBinding is always a Reference value with its referenced name component equal to the name argument.

8.3.3 GetThisEnvironment ( )

The abstract operation GetThisEnvironment takes no arguments. It finds the Environment Record that currently supplies the binding of the keyword this. It performs the following steps when called:

Let env be the running execution context's LexicalEnvironment.
Repeat,
1. Let exists be env.HasThisBinding().
2. If exists is true, return env.
3. Let outer be env.[[OuterEnv]].
4. Assert: outer is not null.
5. Set env to outer.

Note

The loop in step 2 will always terminate because the list of environments always ends with the global environment which has a this binding.

8.3.4 ResolveThisBinding ( )

The abstract operation ResolveThisBinding takes no arguments. It determines the binding of the keyword this using the LexicalEnvironment of the running execution context. It performs the following steps when called:

Let envRec be GetThisEnvironment().
Return ? envRec.GetThisBinding().

8.3.5 GetNewTarget ( )

The abstract operation GetNewTarget takes no arguments. It determines the NewTarget value using the LexicalEnvironment of the running execution context. It performs the following steps when called:

Let envRec be GetThisEnvironment().
Assert: envRec has a [[NewTarget]] field.
Return envRec.[[NewTarget]].

8.3.6 GetGlobalObject ( )

The abstract operation GetGlobalObject takes no arguments. It returns the global object used by the currently running execution context. It performs the following steps when called:

Let currentRealm be the current Realm Record.
Return currentRealm.[[GlobalObject]].

8.4 Jobs and Host Operations to Enqueue Jobs

A Job is an Abstract Closure with no parameters that initiates an ECMAScript computation when no other ECMAScript computation is currently in progress.

Jobs are scheduled for execution by ECMAScript host environments. This specification describes the host hook HostEnqueuePromiseJob to schedule one kind of job; host environments may define additional abstract operations which schedule jobs. Such operations accept a Job Abstract Closure as the parameter and schedule it to be performed at some future time. Their implementations must conform to the following requirements:

At some future point in time, when there is no running execution context and the execution context stack is empty, the implementation must:
1. Push an execution context onto the execution context stack.
2. Perform any implementation-defined preparation steps.
3. Call the Abstract Closure.
4. Perform any implementation-defined cleanup steps.
5. Pop the previously-pushed execution context from the execution context stack.
Only one Job may be actively undergoing evaluation at any point in time.
Once evaluation of a Job starts, it must run to completion before evaluation of any other Job starts.
The Abstract Closure must return a normal completion, implementing its own handling of errors.

Note

Host environments are not required to treat Jobs uniformly with respect to scheduling. For example, web browsers and Node.js treat Promise-handling Jobs as a higher priority than other work; future features may add Jobs that are not treated at such a high priority.

8.4.1 HostEnqueuePromiseJob ( `job`, `realm` )

HostEnqueuePromiseJob is a host-defined abstract operation that schedules the Job Abstract Closure job to be performed, at some future time. The Abstract Closures used with this algorithm are intended to be related to the handling of Promises, or otherwise, to be scheduled with equal priority to Promise handling operations.

The realm parameter is passed through to hosts with no normative requirements; it is either null or a Realm.

Note

The realm for PromiseResolveThenableJobs is the result of calling GetFunctionRealm on the then function object. The realm for PromiseReactionJobs is the result of calling GetFunctionRealm on the handler if the handler is not undefined. Otherwise the realm is null. The WHATWG HTML specification, for example, uses realm to check for ability to run script and to prepare to run script.

The implementation of HostEnqueuePromiseJob must conform to the requirements in 8.4. Additionally, Jobs must be scheduled in FIFO order, with Jobs running in the same order as the HostEnqueuePromiseJob invocations which scheduled them.

8.5 InitializeHostDefinedRealm ( )

The abstract operation InitializeHostDefinedRealm takes no arguments. It performs the following steps when called:

Let realm be CreateRealm().
Let newContext be a new execution context.
Set the Function of newContext to null.
Set the Realm of newContext to realm.
Set the ScriptOrModule of newContext to null.
Push newContext onto the execution context stack; newContext is now the running execution context.
If the host requires use of an exotic object to serve as realm's global object, let global be such an object created in an implementation-defined manner. Otherwise, let global be undefined, indicating that an ordinary object should be created as the global object.
If the host requires that the this binding in realm's global scope return an object other than the global object, let thisValue be such an object created in an implementation-defined manner. Otherwise, let thisValue be undefined, indicating that realm's global this binding should be the global object.
Perform SetRealmGlobalObject(realm, global, thisValue).
Let globalObj be ? SetDefaultGlobalBindings(realm).
Create any implementation-defined global object properties on globalObj.
Return NormalCompletion(empty).

8.6 Agents

An agent comprises a set of ECMAScript execution contexts, an execution context stack, a running execution context, an Agent Record, and an executing thread. Except for the executing thread, the constituents of an agent belong exclusively to that agent.

An agent's executing thread executes a job on the agent's execution contexts independently of other agents, except that an executing thread may be used as the executing thread by multiple agents, provided none of the agents sharing the thread have an Agent Record whose [[CanBlock]] property is true.

Note 1

Some web browsers share a single executing thread across multiple unrelated tabs of a browser window, for example.

While an agent's executing thread executes jobs, the agent is the surrounding agent for the code in those jobs. The code uses the surrounding agent to access the specification level execution objects held within the agent: the running execution context, the execution context stack, and the Agent Record's fields.

Table 26: Agent Record Fields

Field Name	Value	Meaning
[[LittleEndian]]	Boolean	The default value computed for the isLittleEndian parameter when it is needed by the algorithms GetValueFromBuffer and SetValueInBuffer. The choice is implementation-dependent and should be the alternative that is most efficient for the implementation. Once the value has been observed it cannot change.
[[CanBlock]]	Boolean	Determines whether the agent can block or not.
[[Signifier]]	Any globally-unique value	Uniquely identifies the agent within its agent cluster.
[[IsLockFree1]]	Boolean	true if atomic operations on one-byte values are lock-free, false otherwise.
[[IsLockFree2]]	Boolean	true if atomic operations on two-byte values are lock-free, false otherwise.
[[IsLockFree8]]	Boolean	true if atomic operations on eight-byte values are lock-free, false otherwise.
[[CandidateExecution]]	A candidate execution Record	See the memory model.

Once the values of [[Signifier]], [[IsLockFree1]], and [[IsLockFree2]] have been observed by any agent in the agent cluster they cannot change.

Note 2

The values of [[IsLockFree1]] and [[IsLockFree2]] are not necessarily determined by the hardware, but may also reflect implementation choices that can vary over time and between ECMAScript implementations.

There is no [[IsLockFree4]] property: 4-byte atomic operations are always lock-free.

In practice, if an atomic operation is implemented with any type of lock the operation is not lock-free. Lock-free does not imply wait-free: there is no upper bound on how many machine steps may be required to complete a lock-free atomic operation.

That an atomic access of size n is lock-free does not imply anything about the (perceived) atomicity of non-atomic accesses of size n, specifically, non-atomic accesses may still be performed as a sequence of several separate memory accesses. See ReadSharedMemory and WriteSharedMemory for details.

Note 3

An agent is a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation.

8.6.1 AgentSignifier ( )

The abstract operation AgentSignifier takes no arguments. It performs the following steps when called:

Let AR be the Agent Record of the surrounding agent.
Return AR.[[Signifier]].

8.6.2 AgentCanSuspend ( )

The abstract operation AgentCanSuspend takes no arguments. It performs the following steps when called:

Let AR be the Agent Record of the surrounding agent.
Return AR.[[CanBlock]].

Note

In some environments it may not be reasonable for a given agent to suspend. For example, in a web browser environment, it may be reasonable to disallow suspending a document's main event handling thread, while still allowing workers' event handling threads to suspend.

8.7 Agent Clusters

An agent cluster is a maximal set of agents that can communicate by operating on shared memory.

Note 1

Programs within different agents may share memory by unspecified means. At a minimum, the backing memory for SharedArrayBuffer objects can be shared among the agents in the cluster.

There may be agents that can communicate by message passing that cannot share memory; they are never in the same agent cluster.

Every agent belongs to exactly one agent cluster.

Note 2

The agents in a cluster need not all be alive at some particular point in time. If agent A creates another agent B, after which A terminates and B creates agent C, the three agents are in the same cluster if A could share some memory with B and B could share some memory with C.

All agents within a cluster must have the same value for the [[LittleEndian]] property in their respective Agent Records.

Note 3

If different agents within an agent cluster have different values of [[LittleEndian]] it becomes hard to use shared memory for multi-byte data.

All agents within a cluster must have the same values for the [[IsLockFree1]] property in their respective Agent Records; similarly for the [[IsLockFree2]] property.

All agents within a cluster must have different values for the [[Signifier]] property in their respective Agent Records.

An embedding may deactivate (stop forward progress) or activate (resume forward progress) an agent without the agent's knowledge or cooperation. If the embedding does so, it must not leave some agents in the cluster active while other agents in the cluster are deactivated indefinitely.

Note 4

The purpose of the preceding restriction is to avoid a situation where an agent deadlocks or starves because another agent has been deactivated. For example, if an HTML shared worker that has a lifetime independent of documents in any windows were allowed to share memory with the dedicated worker of such an independent document, and the document and its dedicated worker were to be deactivated while the dedicated worker holds a lock (say, the document is pushed into its window's history), and the shared worker then tries to acquire the lock, then the shared worker will be blocked until the dedicated worker is activated again, if ever. Meanwhile other workers trying to access the shared worker from other windows will starve.

The implication of the restriction is that it will not be possible to share memory between agents that don't belong to the same suspend/wake collective within the embedding.

An embedding may terminate an agent without any of the agent's cluster's other agents' prior knowledge or cooperation. If an agent is terminated not by programmatic action of its own or of another agent in the cluster but by forces external to the cluster, then the embedding must choose one of two strategies: Either terminate all the agents in the cluster, or provide reliable APIs that allow the agents in the cluster to coordinate so that at least one remaining member of the cluster will be able to detect the termination, with the termination data containing enough information to identify the agent that was terminated.

Note 5

Examples of that type of termination are: operating systems or users terminating agents that are running in separate processes; the embedding itself terminating an agent that is running in-process with the other agents when per-agent resource accounting indicates that the agent is runaway.

Prior to any evaluation of any ECMAScript code by any agent in a cluster, the [[CandidateExecution]] field of the Agent Record for all agents in the cluster is set to the initial candidate execution. The initial candidate execution is an empty candidate execution whose [[EventsRecords]] field is a List containing, for each agent, an Agent Events Record whose [[AgentSignifier]] field is that agent's signifier, and whose [[EventList]] and [[AgentSynchronizesWith]] fields are empty Lists.

Note 6

All agents in an agent cluster share the same candidate execution in its Agent Record's [[CandidateExecution]] field. The candidate execution is a specification mechanism used by the memory model.

Note 7

An agent cluster is a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation.

8.8 Forward Progress

For an agent to make forward progress is for it to perform an evaluation step according to this specification.

An agent becomes blocked when its running execution context waits synchronously and indefinitely for an external event. Only agents whose Agent Record's [[CanBlock]] property is true can become blocked in this sense. An unblocked agent is one that is not blocked.

Implementations must ensure that:

every unblocked agent with a dedicated executing thread eventually makes forward progress
in a set of agents that share an executing thread, one agent eventually makes forward progress
an agent does not cause another agent to become blocked except via explicit APIs that provide blocking.

Note

This, along with the liveness guarantee in the memory model, ensures that all SeqCst writes eventually become observable to all agents.

9 Ordinary and Exotic Objects Behaviours

9.1 Ordinary Object Internal Methods and Internal Slots

All ordinary objects have an internal slot called [[Prototype]]. The value of this internal slot is either null or an object and is used for implementing inheritance. Data properties of the [[Prototype]] object are inherited (and visible as properties of the child object) for the purposes of get access, but not for set access. Accessor properties are inherited for both get access and set access.

Every ordinary object has a Boolean-valued [[Extensible]] internal slot which is used to fulfill the extensibility-related internal method invariants specified in 6.1.7.3. Namely, once the value of an object's [[Extensible]] internal slot has been set to false, it is no longer possible to add properties to the object, to modify the value of the object's [[Prototype]] internal slot, or to subsequently change the value of [[Extensible]] to true.

In the following algorithm descriptions, assume O is an ordinary object, P is a property key value, V is any ECMAScript language value, and Desc is a Property Descriptor record.

Each ordinary object internal method delegates to a similarly-named abstract operation. If such an abstract operation depends on another internal method, then the internal method is invoked on O rather than calling the similarly-named abstract operation directly. These semantics ensure that exotic objects have their overridden internal methods invoked when ordinary object internal methods are applied to them.

9.1.1 [[GetPrototypeOf]] ( )

When the [[GetPrototypeOf]] internal method of O is called, the following steps are taken:

Return ! OrdinaryGetPrototypeOf(O).

9.1.1.1 OrdinaryGetPrototypeOf ( `O` )

The abstract operation OrdinaryGetPrototypeOf takes argument O (an Object). It performs the following steps when called:

Return O.[[Prototype]].

9.1.2 [[SetPrototypeOf]] ( `V` )

When the [[SetPrototypeOf]] internal method of O is called with argument V, the following steps are taken:

Return ! OrdinarySetPrototypeOf(O, V).

9.1.2.1 OrdinarySetPrototypeOf ( `O`, `V` )

The abstract operation OrdinarySetPrototypeOf takes arguments O (an Object) and V (an ECMAScript language value). It performs the following steps when called:

Assert: Either Type(V) is Object or Type(V) is Null.
Let current be O.[[Prototype]].
If SameValue(V, current) is true, return true.
Let extensible be O.[[Extensible]].
If extensible is false, return false.
Let p be V.
Let done be false.
Repeat, while done is false,
1. If p is null, set done to true.
2. Else if SameValue(p, O) is true, return false.
3. Else,
  1. If p.[[GetPrototypeOf]] is not the ordinary object internal method defined in 9.1.1, set done to true.
  2. Else, set p to p.[[Prototype]].
Set O.[[Prototype]] to V.
Return true.

Note

The loop in step 8 guarantees that there will be no circularities in any prototype chain that only includes objects that use the ordinary object definitions for [[GetPrototypeOf]] and [[SetPrototypeOf]].

9.1.3 [[IsExtensible]] ( )

When the [[IsExtensible]] internal method of O is called, the following steps are taken:

Return ! OrdinaryIsExtensible(O).

9.1.3.1 OrdinaryIsExtensible ( `O` )

The abstract operation OrdinaryIsExtensible takes argument O (an Object). It performs the following steps when called:

Return O.[[Extensible]].

9.1.4 [[PreventExtensions]] ( )

When the [[PreventExtensions]] internal method of O is called, the following steps are taken:

Return ! OrdinaryPreventExtensions(O).

9.1.4.1 OrdinaryPreventExtensions ( `O` )

The abstract operation OrdinaryPreventExtensions takes argument O (an Object). It performs the following steps when called:

Set O.[[Extensible]] to false.
Return true.

9.1.5 [[GetOwnProperty]] ( `P` )

When the [[GetOwnProperty]] internal method of O is called with property key P, the following steps are taken:

Return ! OrdinaryGetOwnProperty(O, P).

9.1.5.1 OrdinaryGetOwnProperty ( `O`, `P` )

The abstract operation OrdinaryGetOwnProperty takes arguments O (an Object) and P (a property key). It performs the following steps when called:

Assert: IsPropertyKey(P) is true.
If O does not have an own property with key P, return undefined.
Let D be a newly created Property Descriptor with no fields.
Let X be O's own property whose key is P.
If X is a data property, then
1. Set D.[[Value]] to the value of X's [[Value]] attribute.
2. Set D.[[Writable]] to the value of X's [[Writable]] attribute.
Else,
1. Assert: X is an accessor property.
2. Set D.[[Get]] to the value of X's [[Get]] attribute.
3. Set D.[[Set]] to the value of X's [[Set]] attribute.
Set D.[[Enumerable]] to the value of X's [[Enumerable]] attribute.
Set D.[[Configurable]] to the value of X's [[Configurable]] attribute.
Return D.

9.1.6 [[DefineOwnProperty]] ( `P`, `Desc` )

When the [[DefineOwnProperty]] internal method of O is called with property key P and Property Descriptor Desc, the following steps are taken:

Return ? OrdinaryDefineOwnProperty(O, P, Desc).

9.1.6.1 OrdinaryDefineOwnProperty ( `O`, `P`, `Desc` )

The abstract operation OrdinaryDefineOwnProperty takes arguments O (an Object), P (a property key), and Desc (a Property Descriptor). It performs the following steps when called:

Let current be ? O.[[GetOwnProperty]](P).
Let extensible be ? IsExtensible(O).
Return ValidateAndApplyPropertyDescriptor(O, P, extensible, Desc, current).

9.1.6.2 IsCompatiblePropertyDescriptor ( `Extensible`, `Desc`, `Current` )

The abstract operation IsCompatiblePropertyDescriptor takes arguments Extensible (a Boolean), Desc (a Property Descriptor), and Current (a Property Descriptor). It performs the following steps when called:

Return ValidateAndApplyPropertyDescriptor(undefined, undefined, Extensible, Desc, Current).

9.1.6.3 ValidateAndApplyPropertyDescriptor ( `O`, `P`, `extensible`, `Desc`, `current` )

The abstract operation ValidateAndApplyPropertyDescriptor takes arguments O (an Object or undefined), P (a property key), extensible (a Boolean), Desc (a Property Descriptor), and current (a Property Descriptor). It performs the following steps when called:

Note

If undefined is passed as O, only validation is performed and no object updates are performed.

Assert: If O is not undefined, then IsPropertyKey(P) is true.
If current is undefined, then
1. If extensible is false, return false.
2. Assert: extensible is true.
3. If IsGenericDescriptor(Desc) is true or IsDataDescriptor(Desc) is true, then
  1. If O is not undefined, create an own data property named P of object O whose [[Value]], [[Writable]], [[Enumerable]], and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to its default value.
4. Else,
  1. Assert: ! IsAccessorDescriptor(Desc) is true.
  2. If O is not undefined, create an own accessor property named P of object O whose [[Get]], [[Set]], [[Enumerable]], and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to its default value.
5. Return true.
If every field in Desc is absent, return true.
If current.[[Configurable]] is false, then
1. If Desc.[[Configurable]] is present and its value is true, return false.
2. If Desc.[[Enumerable]] is present and ! SameValue(Desc.[[Enumerable]], current.[[Enumerable]]) is false, return false.
If ! IsGenericDescriptor(Desc) is true, then
1. NOTE: No further validation is required.
Else if ! SameValue(! IsDataDescriptor(current), ! IsDataDescriptor(Desc)) is false, then
1. If current.[[Configurable]] is false, return false.
2. If IsDataDescriptor(current) is true, then
  1. If O is not undefined, convert the property named P of object O from a data property to an accessor property. Preserve the existing values of the converted property's [[Configurable]] and [[Enumerable]] attributes and set the rest of the property's attributes to their default values.
3. Else,
  1. If O is not undefined, convert the property named P of object O from an accessor property to a data property. Preserve the existing values of the converted property's [[Configurable]] and [[Enumerable]] attributes and set the rest of the property's attributes to their default values.
Else if IsDataDescriptor(current) and IsDataDescriptor(Desc) are both true, then
1. If current.[[Configurable]] is false and current.[[Writable]] is false, then
  1. If Desc.[[Writable]] is present and Desc.[[Writable]] is true, return false.
  2. If Desc.[[Value]] is present and SameValue(Desc.[[Value]], current.[[Value]]) is false, return false.
  3. Return true.
Else,
1. Assert: ! IsAccessorDescriptor(current) and ! IsAccessorDescriptor(Desc) are both true.
2. If current.[[Configurable]] is false, then
  1. If Desc.[[Set]] is present and SameValue(Desc.[[Set]], current.[[Set]]) is false, return false.
  2. If Desc.[[Get]] is present and SameValue(Desc.[[Get]], current.[[Get]]) is false, return false.
  3. Return true.
If O is not undefined, then
1. For each field of Desc that is present, set the corresponding attribute of the property named P of object O to the value of the field.
Return true.

9.1.7 [[HasProperty]] ( `P` )

When the [[HasProperty]] internal method of O is called with property key P, the following steps are taken:

Return ? OrdinaryHasProperty(O, P).

9.1.7.1 OrdinaryHasProperty ( `O`, `P` )

The abstract operation OrdinaryHasProperty takes arguments O (an Object) and P (a property key). It performs the following steps when called:

Assert: IsPropertyKey(P) is true.
Let hasOwn be ? O.[[GetOwnProperty]](P).
If hasOwn is not undefined, return true.
Let parent be ? O.[[GetPrototypeOf]]().
If parent is not null, then
1. Return ? parent.[[HasProperty]](P).
Return false.

9.1.8 [[Get]] ( `P`, `Receiver` )

When the [[Get]] internal method of O is called with property key P and ECMAScript language value Receiver, the following steps are taken:

Return ? OrdinaryGet(O, P, Receiver).

9.1.8.1 OrdinaryGet ( `O`, `P`, `Receiver` )

The abstract operation OrdinaryGet takes arguments O (an Object), P (a property key), and Receiver (an ECMAScript language value). It performs the following steps when called:

Assert: IsPropertyKey(P) is true.
Let desc be ? O.[[GetOwnProperty]](P).
If desc is undefined, then
1. Let parent be ? O.[[GetPrototypeOf]]().
2. If parent is null, return undefined.
3. Return ? parent.[[Get]](P, Receiver).
If IsDataDescriptor(desc) is true, return desc.[[Value]].
Assert: IsAccessorDescriptor(desc) is true.
Let getter be desc.[[Get]].
If getter is undefined, return undefined.
Return ? Call(getter, Receiver).

9.1.9 [[Set]] ( `P`, `V`, `Receiver` )

When the [[Set]] internal method of O is called with property key P, value V, and ECMAScript language value Receiver, the following steps are taken:

Return ? OrdinarySet(O, P, V, Receiver).

9.1.9.1 OrdinarySet ( `O`, `P`, `V`, `Receiver` )

The abstract operation OrdinarySet takes arguments O (an Object), P (a property key), V (an ECMAScript language value), and Receiver (an ECMAScript language value). It performs the following steps when called:

Assert: IsPropertyKey(P) is true.
Let ownDesc be ? O.[[GetOwnProperty]](P).
Return OrdinarySetWithOwnDescriptor(O, P, V, Receiver, ownDesc).

9.1.9.2 OrdinarySetWithOwnDescriptor ( `O`, `P`, `V`, `Receiver`, `ownDesc` )

The abstract operation OrdinarySetWithOwnDescriptor takes arguments O (an Object), P (a property key), V (an ECMAScript language value), Receiver (an ECMAScript language value), and ownDesc (a Property Descriptor or undefined). It performs the following steps when called:

Assert: IsPropertyKey(P) is true.
If ownDesc is undefined, then
1. Let parent be ? O.[[GetPrototypeOf]]().
2. If parent is not null, then
  1. Return ? parent.[[Set]](P, V, Receiver).
3. Else,
  1. Set ownDesc to the PropertyDescriptor { [[Value]]: undefined, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }.
If IsDataDescriptor(ownDesc) is true, then
1. If ownDesc.[[Writable]] is false, return false.
2. If Type(Receiver) is not Object, return false.
3. Let existingDescriptor be ? Receiver.[[GetOwnProperty]](P).
4. If existingDescriptor is not undefined, then
  1. If IsAccessorDescriptor(existingDescriptor) is true, return false.
  2. If existingDescriptor.[[Writable]] is false, return false.
  3. Let valueDesc be the PropertyDescriptor { [[Value]]: V }.
  4. Return ? Receiver.[[DefineOwnProperty]](P, valueDesc).
5. Else,
  1. Assert: Receiver does not currently have a property P.
  2. Return ? CreateDataProperty(Receiver, P, V).
Assert: IsAccessorDescriptor(ownDesc) is true.
Let setter be ownDesc.[[Set]].
If setter is undefined, return false.
Perform ? Call(setter, Receiver, « V »).
Return true.

9.1.10 [[Delete]] ( `P` )

When the [[Delete]] internal method of O is called with property key P, the following steps are taken:

Return ? OrdinaryDelete(O, P).

9.1.10.1 OrdinaryDelete ( `O`, `P` )

The abstract operation OrdinaryDelete takes arguments O (an Object) and P (a property key). It performs the following steps when called:

Assert: IsPropertyKey(P) is true.
Let desc be ? O.[[GetOwnProperty]](P).
If desc is undefined, return true.
If desc.[[Configurable]] is true, then
1. Remove the own property with name P from O.
2. Return true.
Return false.

9.1.11 [[OwnPropertyKeys]] ( )

When the [[OwnPropertyKeys]] internal method of O is called, the following steps are taken:

Return ! OrdinaryOwnPropertyKeys(O).

9.1.11.1 OrdinaryOwnPropertyKeys ( `O` )

The abstract operation OrdinaryOwnPropertyKeys takes argument O (an Object). It performs the following steps when called:

Let keys be a new empty List.
For each own property key P of O such that P is an array index, in ascending numeric index order, do
1. Add P as the last element of keys.
For each own property key P of O such that Type(P) is String and P is not an array index, in ascending chronological order of property creation, do
1. Add P as the last element of keys.
For each own property key P of O such that Type(P) is Symbol, in ascending chronological order of property creation, do
1. Add P as the last element of keys.
Return keys.

9.1.12 OrdinaryObjectCreate ( `proto` [ , `additionalInternalSlotsList` ] )

The abstract operation OrdinaryObjectCreate takes argument proto (an Object or null) and optional argument additionalInternalSlotsList (a List of names of internal slots). It is used to specify the runtime creation of new ordinary objects. additionalInternalSlotsList contains the names of additional internal slots that must be defined as part of the object, beyond [[Prototype]] and [[Extensible]]. If additionalInternalSlotsList is not provided, a new empty List is used. It performs the following steps when called:

Let internalSlotsList be « [[Prototype]], [[Extensible]] ».
If additionalInternalSlotsList is present, append each of its elements to internalSlotsList.
Let O be ! MakeBasicObject(internalSlotsList).
Set O.[[Prototype]] to proto.
Return O.

Note

Although OrdinaryObjectCreate does little more than call MakeBasicObject, its use communicates the intention to create an ordinary object, and not an exotic one. Thus, within this specification, it is not called by any algorithm that subsequently modifies the internal methods of the object in ways that would make the result non-ordinary. Operations that create exotic objects invoke MakeBasicObject directly.

9.1.13 OrdinaryCreateFromConstructor ( `constructor`, `intrinsicDefaultProto` [ , `internalSlotsList` ] )

The abstract operation OrdinaryCreateFromConstructor takes arguments constructor and intrinsicDefaultProto and optional argument internalSlotsList (a List of names of internal slots). It creates an ordinary object whose [[Prototype]] value is retrieved from a constructor's "prototype" property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. internalSlotsList contains the names of additional internal slots that must be defined as part of the object. If internalSlotsList is not provided, a new empty List is used. It performs the following steps when called:

Assert: intrinsicDefaultProto is a String value that is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
Let proto be ? GetPrototypeFromConstructor(constructor, intrinsicDefaultProto).
Return OrdinaryObjectCreate(proto, internalSlotsList).

9.1.14 GetPrototypeFromConstructor ( `constructor`, `intrinsicDefaultProto` )

The abstract operation GetPrototypeFromConstructor takes arguments constructor and intrinsicDefaultProto. It determines the [[Prototype]] value that should be used to create an object corresponding to a specific constructor. The value is retrieved from the constructor's "prototype" property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. It performs the following steps when called:

Assert: intrinsicDefaultProto is a String value that is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
Assert: IsCallable(constructor) is true.
Let proto be ? Get(constructor, "prototype").
If Type(proto) is not Object, then
1. Let realm be ? GetFunctionRealm(constructor).
2. Set proto to realm's intrinsic object named intrinsicDefaultProto.
Return proto.

Note

If constructor does not supply a [[Prototype]] value, the default value that is used is obtained from the realm of the constructor function rather than from the running execution context.

9.1.15 RequireInternalSlot ( `O`, `internalSlot` )

The abstract operation RequireInternalSlot takes arguments O and internalSlot. It throws an exception unless O is an Object and has the given internal slot. It performs the following steps when called:

If Type(O) is not Object, throw a TypeError exception.
If O does not have an internalSlot internal slot, throw a TypeError exception.

9.2 ECMAScript Function Objects

ECMAScript function objects encapsulate parameterized ECMAScript code closed over a lexical environment and support the dynamic evaluation of that code. An ECMAScript function object is an ordinary object and has the same internal slots and the same internal methods as other ordinary objects. The code of an ECMAScript function object may be either strict mode code (10.2.1) or non-strict code. An ECMAScript function object whose code is strict mode code is called a strict function. One whose code is not strict mode code is called a non-strict function.

In addition to [[Extensible]] and [[Prototype]], ECMAScript function objects also have the internal slots listed in Table 27.

Table 27: Internal Slots of ECMAScript Function Objects

Internal Slot	Type	Description
[[Environment]]	Environment Record	The Environment Record that the function was closed over. Used as the outer environment when evaluating the code of the function.
[[FormalParameters]]	Parse Node	The root parse node of the source text that defines the function's formal parameter list.
[[ECMAScriptCode]]	Parse Node	The root parse node of the source text that defines the function's body.
[[ConstructorKind]]	base \| derived	Whether or not the function is a derived class constructor.
[[Realm]]	Realm Record	The realm in which the function was created and which provides any intrinsic objects that are accessed when evaluating the function.
[[ScriptOrModule]]	Script Record or Module Record	The script or module in which the function was created.
[[ThisMode]]	lexical \| strict \| global	Defines how `this` references are interpreted within the formal parameters and code body of the function. lexical means that `this` refers to the this value of a lexically enclosing function. strict means that the this value is used exactly as provided by an invocation of the function. global means that a this value of undefined is interpreted as a reference to the global object.
[[Strict]]	Boolean	true if this is a strict function, false if this is a non-strict function.
[[HomeObject]]	Object	If the function uses `super`, this is the object whose [[GetPrototypeOf]] provides the object where `super` property lookups begin.
[[SourceText]]	sequence of Unicode code points	The source text that defines the function.
[[IsClassConstructor]]	Boolean	Indicates whether the function is a class constructor. (If true, invoking the function's [[Call]] will immediately throw a TypeError exception.)

All ECMAScript function objects have the [[Call]] internal method defined here. ECMAScript functions that are also constructors in addition have the [[Construct]] internal method.

9.2.1 [[Call]] ( `thisArgument`, `argumentsList` )

The [[Call]] internal method for an ECMAScript function object F is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:

Assert: F is an ECMAScript function object.
If F.[[IsClassConstructor]] is true, throw a TypeError exception.
Let callerContext be the running execution context.
Let calleeContext be PrepareForOrdinaryCall(F, undefined).
Assert: calleeContext is now the running execution context.
Perform OrdinaryCallBindThis(F, calleeContext, thisArgument).
Let result be OrdinaryCallEvaluateBody(F, argumentsList).
Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
If result.[[Type]] is return, return NormalCompletion(result.[[Value]]).
ReturnIfAbrupt(result).
Return NormalCompletion(undefined).

Note

When calleeContext is removed from the execution context stack in step 8 it must not be destroyed if it is suspended and retained for later resumption by an accessible generator object.

9.2.1.1 PrepareForOrdinaryCall ( `F`, `newTarget` )

The abstract operation PrepareForOrdinaryCall takes arguments F (a function object) and newTarget (an ECMAScript language value). It performs the following steps when called:

Assert: Type(newTarget) is Undefined or Object.
Let callerContext be the running execution context.
Let calleeContext be a new ECMAScript code execution context.
Set the Function of calleeContext to F.
Let calleeRealm be F.[[Realm]].
Set the Realm of calleeContext to calleeRealm.
Set the ScriptOrModule of calleeContext to F.[[ScriptOrModule]].
Let localEnv be NewFunctionEnvironment(F, newTarget).
Set the LexicalEnvironment of calleeContext to localEnv.
Set the VariableEnvironment of calleeContext to localEnv.
If callerContext is not already suspended, suspend callerContext.
Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
NOTE: Any exception objects produced after this point are associated with calleeRealm.
Return calleeContext.

9.2.1.2 OrdinaryCallBindThis ( `F`, `calleeContext`, `thisArgument` )

The abstract operation OrdinaryCallBindThis takes arguments F (a function object), calleeContext (an execution context), and thisArgument (an ECMAScript language value). It performs the following steps when called:

Let thisMode be F.[[ThisMode]].
If thisMode is lexical, return NormalCompletion(undefined).
Let calleeRealm be F.[[Realm]].
Let localEnv be the LexicalEnvironment of calleeContext.
If thisMode is strict, let thisValue be thisArgument.
Else,
1. If thisArgument is undefined or null, then
  1. Let globalEnv be calleeRealm.[[GlobalEnv]].
  2. Assert: globalEnv is a global Environment Record.
  3. Let thisValue be globalEnv.[[GlobalThisValue]].
2. Else,
  1. Let thisValue be ! ToObject(thisArgument).
  2. NOTE: ToObject produces wrapper objects using calleeRealm.
Assert: localEnv is a function Environment Record.
Assert: The next step never returns an abrupt completion because localEnv.[[ThisBindingStatus]] is not initialized.
Return localEnv.BindThisValue(thisValue).

9.2.1.3 OrdinaryCallEvaluateBody ( `F`, `argumentsList` )

The abstract operation OrdinaryCallEvaluateBody takes arguments F (a function object) and argumentsList (a List). It performs the following steps when called:

Return the result of EvaluateBody of the parsed code that is F.[[ECMAScriptCode]] passing F and argumentsList as the arguments.

9.2.2 [[Construct]] ( `argumentsList`, `newTarget` )

The [[Construct]] internal method for an ECMAScript function object F is called with parameters argumentsList and newTarget. argumentsList is a possibly empty List of ECMAScript language values. The following steps are taken:

Assert: F is an ECMAScript function object.
Assert: Type(newTarget) is Object.
Let callerContext be the running execution context.
Let kind be F.[[ConstructorKind]].
If kind is base, then
1. Let thisArgument be ? OrdinaryCreateFromConstructor(newTarget, "%Object.prototype%").
Let calleeContext be PrepareForOrdinaryCall(F, newTarget).
Assert: calleeContext is now the running execution context.
If kind is base, perform OrdinaryCallBindThis(F, calleeContext, thisArgument).
Let constructorEnv be the LexicalEnvironment of calleeContext.
Let result be OrdinaryCallEvaluateBody(F, argumentsList).
Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
If result.[[Type]] is return, then
1. If Type(result.[[Value]]) is Object, return NormalCompletion(result.[[Value]]).
2. If kind is base, return NormalCompletion(thisArgument).
3. If result.[[Value]] is not undefined, throw a TypeError exception.
Else, ReturnIfAbrupt(result).
Return ? constructorEnv.GetThisBinding().

9.2.3 OrdinaryFunctionCreate ( `functionPrototype`, `sourceText`, `ParameterList`, `Body`, `thisMode`, `Scope` )

The abstract operation OrdinaryFunctionCreate takes arguments functionPrototype (an Object), sourceText (a sequence of Unicode code points), ParameterList (a Parse Node), Body (a Parse Node), thisMode (either lexical-this or non-lexical-this), and Scope (an Environment Record). sourceText is the source text of the syntactic definition of the function to be created. It performs the following steps when called:

Assert: Type(functionPrototype) is Object.
Let internalSlotsList be the internal slots listed in Table 27.
Let F be ! OrdinaryObjectCreate(functionPrototype, internalSlotsList).
Set F.[[Call]] to the definition specified in 9.2.1.
Set F.[[SourceText]] to sourceText.
Set F.[[FormalParameters]] to ParameterList.
Set F.[[ECMAScriptCode]] to Body.
If the source text matching Body is strict mode code, let Strict be true; else let Strict be false.
Set F.[[Strict]] to Strict.
If thisMode is lexical-this, set F.[[ThisMode]] to lexical.
Else if Strict is true, set F.[[ThisMode]] to strict.
Else, set F.[[ThisMode]] to global.
Set F.[[IsClassConstructor]] to false.
Set F.[[Environment]] to Scope.
Set F.[[ScriptOrModule]] to GetActiveScriptOrModule().
Set F.[[Realm]] to the current Realm Record.
Set F.[[HomeObject]] to undefined.
Let len be the ExpectedArgumentCount of ParameterList.
Perform ! SetFunctionLength(F, len).
Return F.

9.2.4 AddRestrictedFunctionProperties ( `F`, `realm` )

The abstract operation AddRestrictedFunctionProperties takes arguments F (a function object) and realm (a Realm Record). It performs the following steps when called:

Assert: realm.[[Intrinsics]].[[%ThrowTypeError%]] exists and has been initialized.
Let thrower be realm.[[Intrinsics]].[[%ThrowTypeError%]].
Perform ! DefinePropertyOrThrow(F, "caller", PropertyDescriptor { [[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: true }).
Return ! DefinePropertyOrThrow(F, "arguments", PropertyDescriptor { [[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: true }).

9.2.4.1 %ThrowTypeError% ( )

The %ThrowTypeError% intrinsic is an anonymous built-in function object that is defined once for each realm. When %ThrowTypeError% is called it performs the following steps:

Throw a TypeError exception.

The value of the [[Extensible]] internal slot of a %ThrowTypeError% function is false.

The "length" property of a %ThrowTypeError% function has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

The "name" property of a %ThrowTypeError% function has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

9.2.5 MakeConstructor ( `F` [ , `writablePrototype` [ , `prototype` ] ] )

The abstract operation MakeConstructor takes argument F (a function object) and optional arguments writablePrototype (a Boolean) and prototype (an Object). It converts F into a constructor. It performs the following steps when called:

Assert: F is an ECMAScript function object.
Assert: IsConstructor(F) is false.
Assert: F is an extensible object that does not have a "prototype" own property.
Set F.[[Construct]] to the definition specified in 9.2.2.
Set F.[[ConstructorKind]] to base.
If writablePrototype is not present, set writablePrototype to true.
If prototype is not present, then
1. Set prototype to OrdinaryObjectCreate(%Object.prototype%).
2. Perform ! DefinePropertyOrThrow(prototype, "constructor", PropertyDescriptor { [[Value]]: F, [[Writable]]: writablePrototype, [[Enumerable]]: false, [[Configurable]]: true }).
Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: writablePrototype, [[Enumerable]]: false, [[Configurable]]: false }).
Return NormalCompletion(undefined).

9.2.6 MakeClassConstructor ( `F` )

The abstract operation MakeClassConstructor takes argument F. It performs the following steps when called:

Assert: F is an ECMAScript function object.
Assert: F.[[IsClassConstructor]] is false.
Set F.[[IsClassConstructor]] to true.
Return NormalCompletion(undefined).

9.2.7 MakeMethod ( `F`, `homeObject` )

The abstract operation MakeMethod takes arguments F and homeObject. It configures F as a method. It performs the following steps when called:

Assert: F is an ECMAScript function object.
Assert: Type(homeObject) is Object.
Set F.[[HomeObject]] to homeObject.
Return NormalCompletion(undefined).

9.2.8 SetFunctionName ( `F`, `name` [ , `prefix` ] )

The abstract operation SetFunctionName takes arguments F (a function object) and name (a property key) and optional argument prefix (a String). It adds a "name" property to F. It performs the following steps when called:

Assert: F is an extensible object that does not have a "name" own property.
Assert: Type(name) is either Symbol or String.
Assert: If prefix is present, then Type(prefix) is String.
If Type(name) is Symbol, then
1. Let description be name's [[Description]] value.
2. If description is undefined, set name to the empty String.
3. Else, set name to the string-concatenation of "[", description, and "]".
If prefix is present, then
1. Set name to the string-concatenation of prefix, the code unit 0x0020 (SPACE), and name.
Return ! DefinePropertyOrThrow(F, "name", PropertyDescriptor { [[Value]]: name, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }).

9.2.9 SetFunctionLength ( `F`, `length` )

The abstract operation SetFunctionLength takes arguments F (a function object) and length (a Number). It adds a "length" property to F. It performs the following steps when called:

Assert: F is an extensible object that does not have a "length" own property.
Assert: Type(length) is Number.
Assert: ! IsNonNegativeInteger(length) is true.
Return ! DefinePropertyOrThrow(F, "length", PropertyDescriptor { [[Value]]: length, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }).

9.2.10 FunctionDeclarationInstantiation ( `func`, `argumentsList` )

Note 1

When an execution context is established for evaluating an ECMAScript function a new function Environment Record is created and bindings for each formal parameter are instantiated in that Environment Record. Each declaration in the function body is also instantiated. If the function's formal parameters do not include any default value initializers then the body declarations are instantiated in the same Environment Record as the parameters. If default value parameter initializers exist, a second Environment Record is created for the body declarations. Formal parameters and functions are initialized as part of FunctionDeclarationInstantiation. All other bindings are initialized during evaluation of the function body.

The abstract operation FunctionDeclarationInstantiation takes arguments func (a function object) and argumentsList. func is the function object for which the execution context is being established. It performs the following steps when called:

Let calleeContext be the running execution context.
Let code be func.[[ECMAScriptCode]].
Let strict be func.[[Strict]].
Let formals be func.[[FormalParameters]].
Let parameterNames be the BoundNames of formals.
If parameterNames has any duplicate entries, let hasDuplicates be true. Otherwise, let hasDuplicates be false.
Let simpleParameterList be IsSimpleParameterList of formals.
Let hasParameterExpressions be ContainsExpression of formals.
Let varNames be the VarDeclaredNames of code.
Let varDeclarations be the VarScopedDeclarations of code.
Let lexicalNames be the LexicallyDeclaredNames of code.
Let functionNames be a new empty List.
Let functionsToInitialize be a new empty List.
For each d in varDeclarations, in reverse list order, do
1. If d is neither a VariableDeclaration nor a ForBinding nor a BindingIdentifier, then
  1. Assert: d is either a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration.
  2. Let fn be the sole element of the BoundNames of d.
  3. If fn is not an element of functionNames, then
    1. Insert fn as the first element of functionNames.
    2. NOTE: If there are multiple function declarations for the same name, the last declaration is used.
    3. Insert d as the first element of functionsToInitialize.
Let argumentsObjectNeeded be true.
If func.[[ThisMode]] is lexical, then
1. NOTE: Arrow functions never have an arguments objects.
2. Set argumentsObjectNeeded to false.
Else if "arguments" is an element of parameterNames, then
1. Set argumentsObjectNeeded to false.
Else if hasParameterExpressions is false, then
1. If "arguments" is an element of functionNames or if "arguments" is an element of lexicalNames, then
  1. Set argumentsObjectNeeded to false.
If strict is true or if hasParameterExpressions is false, then
1. NOTE: Only a single Environment Record is needed for the parameters and top-level vars.
2. Let env be the LexicalEnvironment of calleeContext.
Else,
1. NOTE: A separate Environment Record is needed to ensure that bindings created by direct eval calls in the formal parameter list are outside the environment where parameters are declared.
2. Let calleeEnv be the LexicalEnvironment of calleeContext.
3. Let env be NewDeclarativeEnvironment(calleeEnv).
4. Assert: The VariableEnvironment of calleeContext is calleeEnv.
5. Set the LexicalEnvironment of calleeContext to env.
For each String paramName in parameterNames, do
1. Let alreadyDeclared be env.HasBinding(paramName).
2. NOTE: Early errors ensure that duplicate parameter names can only occur in non-strict functions that do not have parameter default values or rest parameters.
3. If alreadyDeclared is false, then
  1. Perform ! env.CreateMutableBinding(paramName, false).
  2. If hasDuplicates is true, then
    1. Perform ! env.InitializeBinding(paramName, undefined).
If argumentsObjectNeeded is true, then
1. If strict is true or if simpleParameterList is false, then
  1. Let ao be CreateUnmappedArgumentsObject(argumentsList).
2. Else,
  1. NOTE: A mapped argument object is only provided for non-strict functions that don't have a rest parameter, any parameter default value initializers, or any destructured parameters.
  2. Let ao be CreateMappedArgumentsObject(func, formals, argumentsList, env).
3. If strict is true, then
  1. Perform ! env.CreateImmutableBinding("arguments", false).
4. Else,
  1. Perform ! env.CreateMutableBinding("arguments", false).
5. Call env.InitializeBinding("arguments", ao).
6. Let parameterBindings be a new List of parameterNames with "arguments" appended.
Else,
1. Let parameterBindings be parameterNames.
Let iteratorRecord be CreateListIteratorRecord(argumentsList).
If hasDuplicates is true, then
1. Perform ? IteratorBindingInitialization for formals with iteratorRecord and undefined as arguments.
Else,
1. Perform ? IteratorBindingInitialization for formals with iteratorRecord and env as arguments.
If hasParameterExpressions is false, then
1. NOTE: Only a single Environment Record is needed for the parameters and top-level vars.
2. Let instantiatedVarNames be a copy of the List parameterBindings.
3. For each n in varNames, do
  1. If n is not an element of instantiatedVarNames, then
    1. Append n to instantiatedVarNames.
    2. Perform ! env.CreateMutableBinding(n, false).
    3. Call env.InitializeBinding(n, undefined).
4. Let varEnv be env.
Else,
1. NOTE: A separate Environment Record is needed to ensure that closures created by expressions in the formal parameter list do not have visibility of declarations in the function body.
2. Let varEnv be NewDeclarativeEnvironment(env).
3. Set the VariableEnvironment of calleeContext to varEnv.
4. Let instantiatedVarNames be a new empty List.
5. For each n in varNames, do
  1. If n is not an element of instantiatedVarNames, then
    1. Append n to instantiatedVarNames.
    2. Perform ! varEnv.CreateMutableBinding(n, false).
    3. If n is not an element of parameterBindings or if n is an element of functionNames, let initialValue be undefined.
    4. Else,
      1. Let initialValue be ! env.GetBindingValue(n, false).
    5. Call varEnv.InitializeBinding(n, initialValue).
    6. NOTE: A var with the same name as a formal parameter initially has the same value as the corresponding initialized parameter.
NOTE: Annex B.3.3.1 adds additional steps at this point.
If strict is false, then
1. Let lexEnv be NewDeclarativeEnvironment(varEnv).
2. NOTE: Non-strict functions use a separate Environment Record for top-level lexical declarations so that a direct eval can determine whether any var scoped declarations introduced by the eval code conflict with pre-existing top-level lexically scoped declarations. This is not needed for strict functions because a strict direct eval always places all declarations into a new Environment Record.
Else, let lexEnv be varEnv.
Set the LexicalEnvironment of calleeContext to lexEnv.
Let lexDeclarations be the LexicallyScopedDeclarations of code.
For each element d in lexDeclarations, do
1. NOTE: A lexically declared name cannot be the same as a function/generator declaration, formal parameter, or a var name. Lexically declared names are only instantiated here but not initialized.
2. For each element dn of the BoundNames of d, do
  1. If IsConstantDeclaration of d is true, then
    1. Perform ! lexEnv.CreateImmutableBinding(dn, true).
  2. Else,
    1. Perform ! lexEnv.CreateMutableBinding(dn, false).
For each Parse Node f in functionsToInitialize, do
1. Let fn be the sole element of the BoundNames of f.
2. Let fo be InstantiateFunctionObject of f with argument lexEnv.
3. Perform ! varEnv.SetMutableBinding(fn, fo, false).
Return NormalCompletion(empty).

Note 2

B.3.3 provides an extension to the above algorithm that is necessary for backwards compatibility with web browser implementations of ECMAScript that predate ECMAScript 2015.

Note 3

Parameter Initializers may contain direct eval expressions. Any top level declarations of such evals are only visible to the eval code (10.2). The creation of the environment for such declarations is described in 14.1.22.

9.3 Built-in Function Objects

The built-in function objects defined in this specification may be implemented as either ECMAScript function objects (9.2) whose behaviour is provided using ECMAScript code or as implementation provided function exotic objects whose behaviour is provided in some other manner. In either case, the effect of calling such functions must conform to their specifications. An implementation may also provide additional built-in function objects that are not defined in this specification.

If a built-in function object is implemented as an exotic object it must have the ordinary object behaviour specified in 9.1. All such function exotic objects also have [[Prototype]], [[Extensible]], [[Realm]], and [[ScriptOrModule]] internal slots.

Unless otherwise specified every built-in function object has the %Function.prototype% object as the initial value of its [[Prototype]] internal slot.

The behaviour specified for each built-in function via algorithm steps or other means is the specification of the function body behaviour for both [[Call]] and [[Construct]] invocations of the function. However, [[Construct]] invocation is not supported by all built-in functions. For each built-in function, when invoked with [[Call]], the [[Call]] thisArgument provides the this value, the [[Call]] argumentsList provides the named parameters, and the NewTarget value is undefined. When invoked with [[Construct]], the this value is uninitialized, the [[Construct]] argumentsList provides the named parameters, and the [[Construct]] newTarget parameter provides the NewTarget value. If the built-in function is implemented as an ECMAScript function object then this specified behaviour must be implemented by the ECMAScript code that is the body of the function. Built-in functions that are ECMAScript function objects must be strict functions. If a built-in constructor has any [[Call]] behaviour other than throwing a TypeError exception, an ECMAScript implementation of the function must be done in a manner that does not cause the function's [[IsClassConstructor]] internal slot to have the value true.

Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function. When a built-in constructor is called as part of a new expression the argumentsList parameter of the invoked [[Construct]] internal method provides the values for the built-in constructor's named parameters.

Built-in functions that are not constructors do not have a "prototype" property unless otherwise specified in the description of a particular function.

If a built-in function object is not implemented as an ECMAScript function it must provide [[Call]] and [[Construct]] internal methods that conform to the following definitions:

9.3.1 [[Call]] ( `thisArgument`, `argumentsList` )

The [[Call]] internal method for a built-in function object F is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:

Let callerContext be the running execution context.
If callerContext is not already suspended, suspend callerContext.
Let calleeContext be a new execution context.
Set the Function of calleeContext to F.
Let calleeRealm be F.[[Realm]].
Set the Realm of calleeContext to calleeRealm.
Set the ScriptOrModule of calleeContext to F.[[ScriptOrModule]].
Perform any necessary implementation-defined initialization of calleeContext.
Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
Let result be the Completion Record that is the result of evaluating F in a manner that conforms to the specification of F. thisArgument is the this value, argumentsList provides the named parameters, and the NewTarget value is undefined.
Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
Return result.

Note

When calleeContext is removed from the execution context stack it must not be destroyed if it has been suspended and retained by an accessible generator object for later resumption.

9.3.2 [[Construct]] ( `argumentsList`, `newTarget` )

The [[Construct]] internal method for built-in function object F is called with parameters argumentsList and newTarget. The steps performed are the same as [[Call]] (see 9.3.1) except that step 10 is replaced by:

Let result be the Completion Record that is the result of evaluating F in a manner that conforms to the specification of F. The this value is uninitialized, argumentsList provides the named parameters, and newTarget provides the NewTarget value.

9.3.3 CreateBuiltinFunction ( `steps`, `internalSlotsList` [ , `realm` [ , `prototype` ] ] )

The abstract operation CreateBuiltinFunction takes arguments steps and internalSlotsList (a List of names of internal slots) and optional arguments realm and prototype. internalSlotsList contains the names of additional internal slots that must be defined as part of the object. This operation creates a built-in function object. It performs the following steps when called:

Assert: steps is either a set of algorithm steps or other definition of a function's behaviour provided in this specification.
If realm is not present, set realm to the current Realm Record.
Assert: realm is a Realm Record.
If prototype is not present, set prototype to realm.[[Intrinsics]].[[%Function.prototype%]].
Let func be a new built-in function object that when called performs the action described by steps. The new function object has internal slots whose names are the elements of internalSlotsList.
Set func.[[Realm]] to realm.
Set func.[[Prototype]] to prototype.
Set func.[[Extensible]] to true.
Set func.[[ScriptOrModule]] to null.
Return func.

Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation.

9.4 Built-in Exotic Object Internal Methods and Slots

This specification defines several kinds of built-in exotic objects. These objects generally behave similar to ordinary objects except for a few specific situations. The following exotic objects use the ordinary object internal methods except where it is explicitly specified otherwise below:

9.4.1 Bound Function Exotic Objects

A bound function exotic object is an exotic object that wraps another function object. A bound function exotic object is callable (it has a [[Call]] internal method and may have a [[Construct]] internal method). Calling a bound function exotic object generally results in a call of its wrapped function.

An object is a bound function exotic object if its [[Call]] and (if applicable) [[Construct]] internal methods use the following implementations, and its other essential internal methods use the definitions found in 9.1. These methods are installed in BoundFunctionCreate.

Bound function exotic objects do not have the internal slots of ECMAScript function objects listed in Table 27. Instead they have the internal slots listed in Table 28, in addition to [[Prototype]] and [[Extensible]].

Table 28: Internal Slots of Bound Function Exotic Objects

Internal Slot	Type	Description
[[BoundTargetFunction]]	Callable Object	The wrapped function object.
[[BoundThis]]	Any	The value that is always passed as the this value when calling the wrapped function.
[[BoundArguments]]	List of Any	A list of values whose elements are used as the first arguments to any call to the wrapped function.

9.4.1.1 [[Call]] ( `thisArgument`, `argumentsList` )

When the [[Call]] internal method of a bound function exotic object, F, which was created using the bind function is called with parameters thisArgument and argumentsList, a List of ECMAScript language values, the following steps are taken:

Let target be F.[[BoundTargetFunction]].
Let boundThis be F.[[BoundThis]].
Let boundArgs be F.[[BoundArguments]].
Let args be a new list containing the same values as the list boundArgs in the same order followed by the same values as the list argumentsList in the same order.
Return ? Call(target, boundThis, args).

9.4.1.2 [[Construct]] ( `argumentsList`, `newTarget` )

When the [[Construct]] internal method of a bound function exotic object, F that was created using the bind function is called with a list of arguments argumentsList and newTarget, the following steps are taken:

Let target be F.[[BoundTargetFunction]].
Assert: IsConstructor(target) is true.
Let boundArgs be F.[[BoundArguments]].
Let args be a new list containing the same values as the list boundArgs in the same order followed by the same values as the list argumentsList in the same order.
If SameValue(F, newTarget) is true, set newTarget to target.
Return ? Construct(target, args, newTarget).

9.4.1.3 BoundFunctionCreate ( `targetFunction`, `boundThis`, `boundArgs` )

The abstract operation BoundFunctionCreate takes arguments targetFunction, boundThis, and boundArgs. It is used to specify the creation of new bound function exotic objects. It performs the following steps when called:

Assert: Type(targetFunction) is Object.
Let proto be ? targetFunction.[[GetPrototypeOf]]().
Let internalSlotsList be the internal slots listed in Table 28, plus [[Prototype]] and [[Extensible]].
Let obj be ! MakeBasicObject(internalSlotsList).
Set obj.[[Prototype]] to proto.
Set obj.[[Call]] as described in 9.4.1.1.
If IsConstructor(targetFunction) is true, then
1. Set obj.[[Construct]] as described in 9.4.1.2.
Set obj.[[BoundTargetFunction]] to targetFunction.
Set obj.[[BoundThis]] to boundThis.
Set obj.[[BoundArguments]] to boundArgs.
Return obj.

9.4.2 Array Exotic Objects

An Array object is an exotic object that gives special treatment to array index property keys (see 6.1.7). A property whose property name is an array index is also called an element. Every Array object has a non-configurable "length" property whose value is always a nonnegative integer less than 2³². The value of the "length" property is numerically greater than the name of every own property whose name is an array index; whenever an own property of an Array object is created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever an own property is added whose name is an array index, the value of the "length" property is changed, if necessary, to be one more than the numeric value of that array index; and whenever the value of the "length" property is changed, every own property whose name is an array index whose value is not smaller than the new length is deleted. This constraint applies only to own properties of an Array object and is unaffected by "length" or array index properties that may be inherited from its prototypes.

Note

A String property name P is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 2³² - 1.

An object is an Array exotic object (or simply, an Array object) if its [[DefineOwnProperty]] internal method uses the following implementation, and its other essential internal methods use the definitions found in 9.1. These methods are installed in ArrayCreate.

9.4.2.1 [[DefineOwnProperty]] ( `P`, `Desc` )

When the [[DefineOwnProperty]] internal method of an Array exotic object A is called with property key P, and Property Descriptor Desc, the following steps are taken:

Assert: IsPropertyKey(P) is true.
If P is "length", then
1. Return ? ArraySetLength(A, Desc).
Else if P is an array index, then
1. Let oldLenDesc be OrdinaryGetOwnProperty(A, "length").
2. Assert: ! IsDataDescriptor(oldLenDesc) is true.
3. Assert: oldLenDesc.[[Configurable]] is false.
4. Let oldLen be oldLenDesc.[[Value]].
5. Assert: IsNonNegativeInteger(oldLen) is true.
6. Let index be ! ToUint32(P).
7. If index ≥ oldLen and oldLenDesc.[[Writable]] is false, return false.
8. Let succeeded be ! OrdinaryDefineOwnProperty(A, P, Desc).
9. If succeeded is false, return false.
10. If index ≥ oldLen, then
  1. Set oldLenDesc.[[Value]] to index + 1.
  2. Let succeeded be OrdinaryDefineOwnProperty(A, "length", oldLenDesc).
  3. Assert: succeeded is true.
11. Return true.
Return OrdinaryDefineOwnProperty(A, P, Desc).

9.4.2.2 ArrayCreate ( `length` [ , `proto` ] )

The abstract operation ArrayCreate takes argument length (a non-negative integer) and optional argument proto. It is used to specify the creation of new Array exotic objects. It performs the following steps when called:

Assert: ! IsNonNegativeInteger(length) is true.
If length is -0, set length to +0.
If length > 2³² - 1, throw a RangeError exception.
If proto is not present, set proto to %Array.prototype%.
Let A be ! MakeBasicObject(« [[Prototype]], [[Extensible]] »).
Set A.[[Prototype]] to proto.
Set A.[[DefineOwnProperty]] as specified in 9.4.2.1.
Perform ! OrdinaryDefineOwnProperty(A, "length", PropertyDescriptor { [[Value]]: length, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Return A.

9.4.2.3 ArraySpeciesCreate ( `originalArray`, `length` )

The abstract operation ArraySpeciesCreate takes arguments originalArray and length. It is used to specify the creation of a new Array object using a constructor function that is derived from originalArray. It performs the following steps when called:

Assert: ! IsNonNegativeInteger(length) is true.
If length is -0, set length to +0.
Let isArray be ? IsArray(originalArray).
If isArray is false, return ? ArrayCreate(length).
Let C be ? Get(originalArray, "constructor").
If IsConstructor(C) is true, then
1. Let thisRealm be the current Realm Record.
2. Let realmC be ? GetFunctionRealm(C).
3. If thisRealm and realmC are not the same Realm Record, then
  1. If SameValue(C, realmC.[[Intrinsics]].[[%Array%]]) is true, set C to undefined.
If Type(C) is Object, then
1. Set C to ? Get(C, @@species).
2. If C is null, set C to undefined.
If C is undefined, return ? ArrayCreate(length).
If IsConstructor(C) is false, throw a TypeError exception.
Return ? Construct(C, « length »).

Note

If originalArray was created using the standard built-in Array constructor for a realm that is not the realm of the running execution context, then a new Array is created using the realm of the running execution context. This maintains compatibility with Web browsers that have historically had that behaviour for the Array.prototype methods that now are defined using ArraySpeciesCreate.

9.4.2.4 ArraySetLength ( `A`, `Desc` )

The abstract operation ArraySetLength takes arguments A (an Array object) and Desc (a Property Descriptor). It performs the following steps when called:

If Desc.[[Value]] is absent, then
1. Return OrdinaryDefineOwnProperty(A, "length", Desc).
Let newLenDesc be a copy of Desc.
Let newLen be ? ToUint32(Desc.[[Value]]).
Let numberLen be ? ToNumber(Desc.[[Value]]).
If newLen ≠ numberLen, throw a RangeError exception.
Set newLenDesc.[[Value]] to newLen.
Let oldLenDesc be OrdinaryGetOwnProperty(A, "length").
Assert: ! IsDataDescriptor(oldLenDesc) is true.
Assert: oldLenDesc.[[Configurable]] is false.
Let oldLen be oldLenDesc.[[Value]].
If newLen ≥ oldLen, then
1. Return OrdinaryDefineOwnProperty(A, "length", newLenDesc).
If oldLenDesc.[[Writable]] is false, return false.
If newLenDesc.[[Writable]] is absent or has the value true, let newWritable be true.
Else,
1. NOTE: Setting the [[Writable]] attribute to false is deferred in case any elements cannot be deleted.
2. Let newWritable be false.
3. Set newLenDesc.[[Writable]] to true.
Let succeeded be ! OrdinaryDefineOwnProperty(A, "length", newLenDesc).
If succeeded is false, return false.
For each own property key P of A that is an array index, whose numeric value is greater than or equal to newLen, in descending numeric index order, do
1. Let deleteSucceeded be ! A.[[Delete]](P).
2. If deleteSucceeded is false, then
  1. Set newLenDesc.[[Value]] to ! ToUint32(P) + 1.
  2. If newWritable is false, set newLenDesc.[[Writable]] to false.
  3. Perform ! OrdinaryDefineOwnProperty(A, "length", newLenDesc).
  4. Return false.
If newWritable is false, then
1. Let succeeded be ! OrdinaryDefineOwnProperty(A, "length", PropertyDescriptor { [[Writable]]: false }).
2. Assert: succeeded is true.
Return true.

Note

In steps 3 and 4, if Desc.[[Value]] is an object then its valueOf method is called twice. This is legacy behaviour that was specified with this effect starting with the 2^nd Edition of this specification.

9.4.3 String Exotic Objects

A String object is an exotic object that encapsulates a String value and exposes virtual integer-indexed data properties corresponding to the individual code unit elements of the String value. String exotic objects always have a data property named "length" whose value is the number of code unit elements in the encapsulated String value. Both the code unit data properties and the "length" property are non-writable and non-configurable.

An object is a String exotic object (or simply, a String object) if its [[GetOwnProperty]], [[DefineOwnProperty]], and [[OwnPropertyKeys]] internal methods use the following implementations, and its other essential internal methods use the definitions found in 9.1. These methods are installed in StringCreate.

String exotic objects have the same internal slots as ordinary objects. They also have a [[StringData]] internal slot.

9.4.3.1 [[GetOwnProperty]] ( `P` )

When the [[GetOwnProperty]] internal method of a String exotic object S is called with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let desc be OrdinaryGetOwnProperty(S, P).
If desc is not undefined, return desc.
Return ! StringGetOwnProperty(S, P).

9.4.3.2 [[DefineOwnProperty]] ( `P`, `Desc` )

When the [[DefineOwnProperty]] internal method of a String exotic object S is called with property key P, and Property Descriptor Desc, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let stringDesc be ! StringGetOwnProperty(S, P).
If stringDesc is not undefined, then
1. Let extensible be S.[[Extensible]].
2. Return ! IsCompatiblePropertyDescriptor(extensible, Desc, stringDesc).
Return ! OrdinaryDefineOwnProperty(S, P, Desc).

9.4.3.3 [[OwnPropertyKeys]] ( )

When the [[OwnPropertyKeys]] internal method of a String exotic object O is called, the following steps are taken:

Let keys be a new empty List.
Let str be O.[[StringData]].
Assert: Type(str) is String.
Let len be the length of str.
For each integer i starting with 0 such that i < len, in ascending order, do
1. Add ! ToString(i) as the last element of keys.
For each own property key P of O such that P is an array index and ToInteger(P) ≥ len, in ascending numeric index order, do
1. Add P as the last element of keys.
For each own property key P of O such that Type(P) is String and P is not an array index, in ascending chronological order of property creation, do
1. Add P as the last element of keys.
For each own property key P of O such that Type(P) is Symbol, in ascending chronological order of property creation, do
1. Add P as the last element of keys.
Return keys.

9.4.3.4 StringCreate ( `value`, `prototype` )

The abstract operation StringCreate takes arguments value and prototype. It is used to specify the creation of new String exotic objects. It performs the following steps when called:

Assert: Type(value) is String.
Let S be ! MakeBasicObject(« [[Prototype]], [[Extensible]], [[StringData]] »).
Set S.[[Prototype]] to prototype.
Set S.[[StringData]] to value.
Set S.[[GetOwnProperty]] as specified in 9.4.3.1.
Set S.[[DefineOwnProperty]] as specified in 9.4.3.2.
Set S.[[OwnPropertyKeys]] as specified in 9.4.3.3.
Let length be the number of code unit elements in value.
Perform ! DefinePropertyOrThrow(S, "length", PropertyDescriptor { [[Value]]: length, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }).
Return S.

9.4.3.5 StringGetOwnProperty ( `S`, `P` )

The abstract operation StringGetOwnProperty takes arguments S and P. It performs the following steps when called:

Assert: S is an Object that has a [[StringData]] internal slot.
Assert: IsPropertyKey(P) is true.
If Type(P) is not String, return undefined.
Let index be ! CanonicalNumericIndexString(P).
If index is undefined, return undefined.
If IsInteger(index) is false, return undefined.
If index is -0, return undefined.
Let str be S.[[StringData]].
Assert: Type(str) is String.
Let len be the length of str.
If index < 0 or len ≤ index, return undefined.
Let resultStr be the String value of length 1, containing one code unit from str, specifically the code unit at index index.
Return the PropertyDescriptor { [[Value]]: resultStr, [[Writable]]: false, [[Enumerable]]: true, [[Configurable]]: false }.

9.4.4 Arguments Exotic Objects

Most ECMAScript functions make an arguments object available to their code. Depending upon the characteristics of the function definition, its arguments object is either an ordinary object or an arguments exotic object. An arguments exotic object is an exotic object whose array index properties map to the formal parameters bindings of an invocation of its associated ECMAScript function.

An object is an arguments exotic object if its internal methods use the following implementations, with the ones not specified here using those found in 9.1. These methods are installed in CreateMappedArgumentsObject.

Note 1

While CreateUnmappedArgumentsObject is grouped into this clause, it creates an ordinary object, not an arguments exotic object.

Arguments exotic objects have the same internal slots as ordinary objects. They also have a [[ParameterMap]] internal slot. Ordinary arguments objects also have a [[ParameterMap]] internal slot whose value is always undefined. For ordinary argument objects the [[ParameterMap]] internal slot is only used by Object.prototype.toString (19.1.3.6) to identify them as such.

Note 2

The integer-indexed data properties of an arguments exotic object whose numeric name values are less than the number of formal parameters of the corresponding function object initially share their values with the corresponding argument bindings in the function's execution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into an accessor property. If the arguments object is an ordinary object, the values of its properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.

Note 3

The ParameterMap object and its property values are used as a device for specifying the arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are not directly observable from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.

Note 4

Ordinary arguments objects define a non-configurable accessor property named "callee" which throws a TypeError exception on access. The "callee" property has a more specific meaning for arguments exotic objects, which are created only for some class of non-strict functions. The definition of this property in the ordinary variant exists to ensure that it is not defined in any other manner by conforming ECMAScript implementations.

Note 5

ECMAScript implementations of arguments exotic objects have historically contained an accessor property named "caller". Prior to ECMAScript 2017, this specification included the definition of a throwing "caller" property on ordinary arguments objects. Since implementations do not contain this extension any longer, ECMAScript 2017 dropped the requirement for a throwing "caller" accessor.

9.4.4.1 [[GetOwnProperty]] ( `P` )

The [[GetOwnProperty]] internal method of an arguments exotic object when called with a property key P performs the following steps:

Let args be the arguments object.
Let desc be OrdinaryGetOwnProperty(args, P).
If desc is undefined, return desc.
Let map be args.[[ParameterMap]].
Let isMapped be ! HasOwnProperty(map, P).
If isMapped is true, then
1. Set desc.[[Value]] to Get(map, P).
Return desc.

9.4.4.2 [[DefineOwnProperty]] ( `P`, `Desc` )

The [[DefineOwnProperty]] internal method of an arguments exotic object when called with a property key P and Property Descriptor Desc performs the following steps:

Let args be the arguments object.
Let map be args.[[ParameterMap]].
Let isMapped be HasOwnProperty(map, P).
Let newArgDesc be Desc.
If isMapped is true and IsDataDescriptor(Desc) is true, then
1. If Desc.[[Value]] is not present and Desc.[[Writable]] is present and its value is false, then
  1. Set newArgDesc to a copy of Desc.
  2. Set newArgDesc.[[Value]] to Get(map, P).
Let allowed be ? OrdinaryDefineOwnProperty(args, P, newArgDesc).
If allowed is false, return false.
If isMapped is true, then
1. If IsAccessorDescriptor(Desc) is true, then
  1. Call map.[[Delete]](P).
2. Else,
  1. If Desc.[[Value]] is present, then
    1. Let setStatus be Set(map, P, Desc.[[Value]], false).
    2. Assert: setStatus is true because formal parameters mapped by argument objects are always writable.
  2. If Desc.[[Writable]] is present and its value is false, then
    1. Call map.[[Delete]](P).
Return true.

9.4.4.3 [[Get]] ( `P`, `Receiver` )

The [[Get]] internal method of an arguments exotic object when called with a property key P and ECMAScript language value Receiver performs the following steps:

Let args be the arguments object.
Let map be args.[[ParameterMap]].
Let isMapped be ! HasOwnProperty(map, P).
If isMapped is false, then
1. Return ? OrdinaryGet(args, P, Receiver).
Else,
1. Assert: map contains a formal parameter mapping for P.
2. Return Get(map, P).

9.4.4.4 [[Set]] ( `P`, `V`, `Receiver` )

The [[Set]] internal method of an arguments exotic object when called with property key P, value V, and ECMAScript language value Receiver performs the following steps:

Let args be the arguments object.
If SameValue(args, Receiver) is false, then
1. Let isMapped be false.
Else,
1. Let map be args.[[ParameterMap]].
2. Let isMapped be ! HasOwnProperty(map, P).
If isMapped is true, then
1. Let setStatus be Set(map, P, V, false).
2. Assert: setStatus is true because formal parameters mapped by argument objects are always writable.
Return ? OrdinarySet(args, P, V, Receiver).

9.4.4.5 [[Delete]] ( `P` )

The [[Delete]] internal method of an arguments exotic object when called with a property key P performs the following steps:

Let args be the arguments object.
Let map be args.[[ParameterMap]].
Let isMapped be ! HasOwnProperty(map, P).
Let result be ? OrdinaryDelete(args, P).
If result is true and isMapped is true, then
1. Call map.[[Delete]](P).
Return result.

9.4.4.6 CreateUnmappedArgumentsObject ( `argumentsList` )

The abstract operation CreateUnmappedArgumentsObject takes argument argumentsList. It performs the following steps when called:

Let len be the number of elements in argumentsList.
Let obj be OrdinaryObjectCreate(%Object.prototype%, « [[ParameterMap]] »).
Set obj.[[ParameterMap]] to undefined.
Perform DefinePropertyOrThrow(obj, "length", PropertyDescriptor { [[Value]]: len, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
Let index be 0.
Repeat, while index < len,
1. Let val be argumentsList[index].
2. Perform ! CreateDataPropertyOrThrow(obj, ! ToString(index), val).
3. Set index to index + 1.
Perform ! DefinePropertyOrThrow(obj, @@iterator, PropertyDescriptor { [[Value]]: %Array.prototype.values%, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
Perform ! DefinePropertyOrThrow(obj, "callee", PropertyDescriptor { [[Get]]: %ThrowTypeError%, [[Set]]: %ThrowTypeError%, [[Enumerable]]: false, [[Configurable]]: false }).
Return obj.

9.4.4.7 CreateMappedArgumentsObject ( `func`, `formals`, `argumentsList`, `env` )

The abstract operation CreateMappedArgumentsObject takes arguments func (an Object), formals (a Parse Node), argumentsList (a List), and env (an Environment Record). It performs the following steps when called:

Assert: formals does not contain a rest parameter, any binding patterns, or any initializers. It may contain duplicate identifiers.
Let len be the number of elements in argumentsList.
Let obj be ! MakeBasicObject(« [[Prototype]], [[Extensible]], [[ParameterMap]] »).
Set obj.[[GetOwnProperty]] as specified in 9.4.4.1.
Set obj.[[DefineOwnProperty]] as specified in 9.4.4.2.
Set obj.[[Get]] as specified in 9.4.4.3.
Set obj.[[Set]] as specified in 9.4.4.4.
Set obj.[[Delete]] as specified in 9.4.4.5.
Set obj.[[Prototype]] to %Object.prototype%.
Let map be OrdinaryObjectCreate(null).
Set obj.[[ParameterMap]] to map.
Let parameterNames be the BoundNames of formals.
Let numberOfParameters be the number of elements in parameterNames.
Let index be 0.
Repeat, while index < len,
1. Let val be argumentsList[index].
2. Perform ! CreateDataPropertyOrThrow(obj, ! ToString(index), val).
3. Set index to index + 1.
Perform ! DefinePropertyOrThrow(obj, "length", PropertyDescriptor { [[Value]]: len, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
Let mappedNames be a new empty List.
Let index be numberOfParameters - 1.
Repeat, while index ≥ 0,
1. Let name be parameterNames[index].
2. If name is not an element of mappedNames, then
  1. Add name as an element of the list mappedNames.
  2. If index < len, then
    1. Let g be MakeArgGetter(name, env).
    2. Let p be MakeArgSetter(name, env).
    3. Perform map.[[DefineOwnProperty]](! ToString(index), PropertyDescriptor { [[Set]]: p, [[Get]]: g, [[Enumerable]]: false, [[Configurable]]: true }).
3. Set index to index - 1.
Perform ! DefinePropertyOrThrow(obj, @@iterator, PropertyDescriptor { [[Value]]: %Array.prototype.values%, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
Perform ! DefinePropertyOrThrow(obj, "callee", PropertyDescriptor { [[Value]]: func, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
Return obj.

9.4.4.7.1 MakeArgGetter ( `name`, `env` )

The abstract operation MakeArgGetter takes arguments name (a String) and env (an Environment Record). It creates a built-in function object that when executed returns the value bound for name in env. It performs the following steps when called:

Let steps be the steps of an ArgGetter function as specified below.
Let getter be ! CreateBuiltinFunction(steps, « [[Name]], [[Env]] »).
Set getter.[[Name]] to name.
Set getter.[[Env]] to env.
Return getter.

An ArgGetter function is an anonymous built-in function with [[Name]] and [[Env]] internal slots. When an ArgGetter function that expects no arguments is called it performs the following steps:

Let f be the active function object.
Let name be f.[[Name]].
Let env be f.[[Env]].
Return env.GetBindingValue(name, false).

Note

ArgGetter functions are never directly accessible to ECMAScript code.

9.4.4.7.2 MakeArgSetter ( `name`, `env` )

The abstract operation MakeArgSetter takes arguments name (a String) and env (an Environment Record). It creates a built-in function object that when executed sets the value bound for name in env. It performs the following steps when called:

Let steps be the steps of an ArgSetter function as specified below.
Let setter be ! CreateBuiltinFunction(steps, « [[Name]], [[Env]] »).
Set setter.[[Name]] to name.
Set setter.[[Env]] to env.
Return setter.

An ArgSetter function is an anonymous built-in function with [[Name]] and [[Env]] internal slots. When an ArgSetter function is called with argument value it performs the following steps:

Let f be the active function object.
Let name be f.[[Name]].
Let env be f.[[Env]].
Return env.SetMutableBinding(name, value, false).

Note

ArgSetter functions are never directly accessible to ECMAScript code.

9.4.5 Integer-Indexed Exotic Objects

An Integer-Indexed exotic object is an exotic object that performs special handling of integer index property keys.

Integer-Indexed exotic objects have the same internal slots as ordinary objects and additionally [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], [[ContentType]], and [[TypedArrayName]] internal slots.

An object is an Integer-Indexed exotic object if its [[GetOwnProperty]], [[HasProperty]], [[DefineOwnProperty]], [[Get]], [[Set]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its other essential internal methods use the definitions found in 9.1. These methods are installed by IntegerIndexedObjectCreate.

9.4.5.1 [[GetOwnProperty]] ( `P` )

When the [[GetOwnProperty]] internal method of an Integer-Indexed exotic object O is called with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Assert: O is an Integer-Indexed exotic object.
If Type(P) is String, then
1. Let numericIndex be ! CanonicalNumericIndexString(P).
2. If numericIndex is not undefined, then
  1. Let value be ? IntegerIndexedElementGet(O, numericIndex).
  2. If value is undefined, return undefined.
  3. Return the PropertyDescriptor { [[Value]]: value, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: false }.
Return OrdinaryGetOwnProperty(O, P).

9.4.5.2 [[HasProperty]] ( `P` )

When the [[HasProperty]] internal method of an Integer-Indexed exotic object O is called with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Assert: O is an Integer-Indexed exotic object.
If Type(P) is String, then
1. Let numericIndex be ! CanonicalNumericIndexString(P).
2. If numericIndex is not undefined, then
  1. Let buffer be O.[[ViewedArrayBuffer]].
  2. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
  3. If ! IsValidIntegerIndex(O, numericIndex) is false, return false.
  4. Return true.
Return ? OrdinaryHasProperty(O, P).

9.4.5.3 [[DefineOwnProperty]] ( `P`, `Desc` )

When the [[DefineOwnProperty]] internal method of an Integer-Indexed exotic object O is called with property key P, and Property Descriptor Desc, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Assert: O is an Integer-Indexed exotic object.
If Type(P) is String, then
1. Let numericIndex be ! CanonicalNumericIndexString(P).
2. If numericIndex is not undefined, then
  1. If ! IsValidIntegerIndex(O, numericIndex) is false, return false.
  2. If IsAccessorDescriptor(Desc) is true, return false.
  3. If Desc has a [[Configurable]] field and if Desc.[[Configurable]] is true, return false.
  4. If Desc has an [[Enumerable]] field and if Desc.[[Enumerable]] is false, return false.
  5. If Desc has a [[Writable]] field and if Desc.[[Writable]] is false, return false.
  6. If Desc has a [[Value]] field, then
    1. Let value be Desc.[[Value]].
    2. Return ? IntegerIndexedElementSet(O, numericIndex, value).
  7. Return true.
Return ! OrdinaryDefineOwnProperty(O, P, Desc).

9.4.5.4 [[Get]] ( `P`, `Receiver` )

When the [[Get]] internal method of an Integer-Indexed exotic object O is called with property key P and ECMAScript language value Receiver, the following steps are taken:

Assert: IsPropertyKey(P) is true.
If Type(P) is String, then
1. Let numericIndex be ! CanonicalNumericIndexString(P).
2. If numericIndex is not undefined, then
  1. Return ? IntegerIndexedElementGet(O, numericIndex).
Return ? OrdinaryGet(O, P, Receiver).

9.4.5.5 [[Set]] ( `P`, `V`, `Receiver` )

When the [[Set]] internal method of an Integer-Indexed exotic object O is called with property key P, value V, and ECMAScript language value Receiver, the following steps are taken:

Assert: IsPropertyKey(P) is true.
If Type(P) is String, then
1. Let numericIndex be ! CanonicalNumericIndexString(P).
2. If numericIndex is not undefined, then
  1. Return ? IntegerIndexedElementSet(O, numericIndex, V).
Return ? OrdinarySet(O, P, V, Receiver).

9.4.5.6 [[OwnPropertyKeys]] ( )

When the [[OwnPropertyKeys]] internal method of an Integer-Indexed exotic object O is called, the following steps are taken:

Let keys be a new empty List.
Assert: O is an Integer-Indexed exotic object.
Let len be O.[[ArrayLength]].
For each integer i starting with 0 such that i < len, in ascending order, do
1. Add ! ToString(i) as the last element of keys.
For each own property key P of O such that Type(P) is String and P is not an integer index, in ascending chronological order of property creation, do
1. Add P as the last element of keys.
For each own property key P of O such that Type(P) is Symbol, in ascending chronological order of property creation, do
1. Add P as the last element of keys.
Return keys.

9.4.5.7 IntegerIndexedObjectCreate ( `prototype` )

The abstract operation IntegerIndexedObjectCreate takes argument prototype. It is used to specify the creation of new Integer-Indexed exotic objects. It performs the following steps when called:

Let internalSlotsList be « [[Prototype]], [[Extensible]], [[ViewedArrayBuffer]], [[TypedArrayName]], [[ContentType]], [[ByteLength]], [[ByteOffset]], [[ArrayLength]] ».
Let A be ! MakeBasicObject(internalSlotsList).
Set A.[[GetOwnProperty]] as specified in 9.4.5.1.
Set A.[[HasProperty]] as specified in 9.4.5.2.
Set A.[[DefineOwnProperty]] as specified in 9.4.5.3.
Set A.[[Get]] as specified in 9.4.5.4.
Set A.[[Set]] as specified in 9.4.5.5.
Set A.[[OwnPropertyKeys]] as specified in 9.4.5.6.
Set A.[[Prototype]] to prototype.
Return A.

9.4.5.8 IsValidIntegerIndex ( `O`, `index` )

The abstract operation IsValidIntegerIndex takes arguments O and index. It performs the following steps when called:

Assert: O is an Integer-Indexed exotic object.
Assert: Type(index) is Number.
If ! IsInteger(index) is false, return false.
If index is -0, return false.
If index < 0 or index ≥ O.[[ArrayLength]], return false.
Return true.

9.4.5.9 IntegerIndexedElementGet ( `O`, `index` )

The abstract operation IntegerIndexedElementGet takes arguments O and index. It performs the following steps when called:

Assert: O is an Integer-Indexed exotic object.
Assert: Type(index) is Number.
Let buffer be O.[[ViewedArrayBuffer]].
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
If ! IsValidIntegerIndex(O, index) is false, return undefined.
Let offset be O.[[ByteOffset]].
Let arrayTypeName be the String value of O.[[TypedArrayName]].
Let elementSize be the Element Size value specified in Table 61 for arrayTypeName.
Let indexedPosition be (index × elementSize) + offset.
Let elementType be the Element Type value in Table 61 for arrayTypeName.
Return GetValueFromBuffer(buffer, indexedPosition, elementType, true, Unordered).

9.4.5.10 IntegerIndexedElementSet ( `O`, `index`, `value` )

The abstract operation IntegerIndexedElementSet takes arguments O, index, and value. It performs the following steps when called:

Assert: O is an Integer-Indexed exotic object.
Assert: Type(index) is Number.
If O.[[ContentType]] is BigInt, let numValue be ? ToBigInt(value).
Otherwise, let numValue be ? ToNumber(value).
Let buffer be O.[[ViewedArrayBuffer]].
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
If ! IsValidIntegerIndex(O, index) is false, return false.
Let offset be O.[[ByteOffset]].
Let arrayTypeName be the String value of O.[[TypedArrayName]].
Let elementSize be the Element Size value specified in Table 61 for arrayTypeName.
Let indexedPosition be (index × elementSize) + offset.
Let elementType be the Element Type value in Table 61 for arrayTypeName.
Perform SetValueInBuffer(buffer, indexedPosition, elementType, numValue, true, Unordered).
Return true.

9.4.6 Module Namespace Exotic Objects

A module namespace exotic object is an exotic object that exposes the bindings exported from an ECMAScript Module (See 15.2.3). There is a one-to-one correspondence between the String-keyed own properties of a module namespace exotic object and the binding names exported by the Module. The exported bindings include any bindings that are indirectly exported using export * export items. Each String-valued own property key is the StringValue of the corresponding exported binding name. These are the only String-keyed properties of a module namespace exotic object. Each such property has the attributes { [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: false }. Module namespace exotic objects are not extensible.

An object is a module namespace exotic object if its [[SetPrototypeOf]], [[IsExtensible]], [[PreventExtensions]], [[GetOwnProperty]], [[DefineOwnProperty]], [[HasProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its other essential internal methods use the definitions found in 9.1. These methods are installed by ModuleNamespaceCreate.

Module namespace exotic objects have the internal slots defined in Table 29.

Table 29: Internal Slots of Module Namespace Exotic Objects

Internal Slot	Type	Description
[[Module]]	Module Record	The Module Record whose exports this namespace exposes.
[[Exports]]	List of String	A List containing the String values of the exported names exposed as own properties of this object. The list is ordered as if an Array of those String values had been sorted using `Array.prototype.sort` using undefined as `comparefn`.
[[Prototype]]	Null	This slot always contains the value null (see 9.4.6.1).

Module namespace exotic objects provide alternative definitions for all of the internal methods except [[GetPrototypeOf]], which behaves as defined in 9.1.1.

9.4.6.1 [[SetPrototypeOf]] ( `V` )

When the [[SetPrototypeOf]] internal method of a module namespace exotic object O is called with argument V, the following steps are taken:

Return ? SetImmutablePrototype(O, V).

9.4.6.2 [[IsExtensible]] ( )

When the [[IsExtensible]] internal method of a module namespace exotic object O is called, the following steps are taken:

Return false.

9.4.6.3 [[PreventExtensions]] ( )

When the [[PreventExtensions]] internal method of a module namespace exotic object O is called, the following steps are taken:

Return true.

9.4.6.4 [[GetOwnProperty]] ( `P` )

When the [[GetOwnProperty]] internal method of a module namespace exotic object O is called with property key P, the following steps are taken:

If Type(P) is Symbol, return OrdinaryGetOwnProperty(O, P).
Let exports be O.[[Exports]].
If P is not an element of exports, return undefined.
Let value be ? O.[[Get]](P, O).
Return PropertyDescriptor { [[Value]]: value, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: false }.

9.4.6.5 [[DefineOwnProperty]] ( `P`, `Desc` )

When the [[DefineOwnProperty]] internal method of a module namespace exotic object O is called with property key P and Property Descriptor Desc, the following steps are taken:

If Type(P) is Symbol, return OrdinaryDefineOwnProperty(O, P, Desc).
Let current be ? O.[[GetOwnProperty]](P).
If current is undefined, return false.
If IsAccessorDescriptor(Desc) is true, return false.
If Desc.[[Writable]] is present and has value false, return false.
If Desc.[[Enumerable]] is present and has value false, return false.
If Desc.[[Configurable]] is present and has value true, return false.
If Desc.[[Value]] is present, return SameValue(Desc.[[Value]], current.[[Value]]).
Return true.

9.4.6.6 [[HasProperty]] ( `P` )

When the [[HasProperty]] internal method of a module namespace exotic object O is called with property key P, the following steps are taken:

If Type(P) is Symbol, return OrdinaryHasProperty(O, P).
Let exports be O.[[Exports]].
If P is an element of exports, return true.
Return false.

9.4.6.7 [[Get]] ( `P`, `Receiver` )

When the [[Get]] internal method of a module namespace exotic object O is called with property key P and ECMAScript language value Receiver, the following steps are taken:

Assert: IsPropertyKey(P) is true.
If Type(P) is Symbol, then
1. Return ? OrdinaryGet(O, P, Receiver).
Let exports be O.[[Exports]].
If P is not an element of exports, return undefined.
Let m be O.[[Module]].
Let binding be ! m.ResolveExport(P).
Assert: binding is a ResolvedBinding Record.
Let targetModule be binding.[[Module]].
Assert: targetModule is not undefined.
If binding.[[BindingName]] is "*namespace*", then
1. Return ? GetModuleNamespace(targetModule).
Let targetEnv be targetModule.[[Environment]].
If targetEnv is undefined, throw a ReferenceError exception.
Return ? targetEnv.GetBindingValue(binding.[[BindingName]], true).

Note

ResolveExport is side-effect free. Each time this operation is called with a specific exportName, resolveSet pair as arguments it must return the same result. An implementation might choose to pre-compute or cache the ResolveExport results for the [[Exports]] of each module namespace exotic object.

9.4.6.8 [[Set]] ( `P`, `V`, `Receiver` )

When the [[Set]] internal method of a module namespace exotic object O is called with property key P, value V, and ECMAScript language value Receiver, the following steps are taken:

Return false.

9.4.6.9 [[Delete]] ( `P` )

When the [[Delete]] internal method of a module namespace exotic object O is called with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
If Type(P) is Symbol, then
1. Return ? OrdinaryDelete(O, P).
Let exports be O.[[Exports]].
If P is an element of exports, return false.
Return true.

9.4.6.10 [[OwnPropertyKeys]] ( )

When the [[OwnPropertyKeys]] internal method of a module namespace exotic object O is called, the following steps are taken:

Let exports be a copy of O.[[Exports]].
Let symbolKeys be ! OrdinaryOwnPropertyKeys(O).
Append all the entries of symbolKeys to the end of exports.
Return exports.

9.4.6.11 ModuleNamespaceCreate ( `module`, `exports` )

The abstract operation ModuleNamespaceCreate takes arguments module and exports. It is used to specify the creation of new module namespace exotic objects. It performs the following steps when called:

Assert: module is a Module Record.
Assert: module.[[Namespace]] is undefined.
Assert: exports is a List of String values.
Let internalSlotsList be the internal slots listed in Table 29.
Let M be ! MakeBasicObject(internalSlotsList).
Set M's essential internal methods to the definitions specified in 9.4.6.
Set M.[[Prototype]] to null.
Set M.[[Module]] to module.
Let sortedExports be a new List containing the same values as the list exports where the values are ordered as if an Array of the same values had been sorted using Array.prototype.sort using undefined as comparefn.
Set M.[[Exports]] to sortedExports.
Create own properties of M corresponding to the definitions in 26.3.
Set module.[[Namespace]] to M.
Return M.

9.4.7 Immutable Prototype Exotic Objects

An immutable prototype exotic object is an exotic object that has a [[Prototype]] internal slot that will not change once it is initialized.

An object is an immutable prototype exotic object if its [[SetPrototypeOf]] internal method uses the following implementation. (Its other essential internal methods may use any implementation, depending on the specific immutable prototype exotic object in question.)

Note

Unlike other exotic objects, there is not a dedicated creation abstract operation provided for immutable prototype exotic objects. This is because they are only used by %ObjectPrototype% and by host environments, and in host environments, the relevant objects are potentially exotic in other ways and thus need their own dedicated creation operation.

9.4.7.1 [[SetPrototypeOf]] ( `V` )

When the [[SetPrototypeOf]] internal method of an immutable prototype exotic object O is called with argument V, the following steps are taken:

Return ? SetImmutablePrototype(O, V).

9.4.7.2 SetImmutablePrototype ( `O`, `V` )

The abstract operation SetImmutablePrototype takes arguments O and V. It performs the following steps when called:

Assert: Either Type(V) is Object or Type(V) is Null.
Let current be ? O.[[GetPrototypeOf]]().
If SameValue(V, current) is true, return true.
Return false.

9.5 Proxy Object Internal Methods and Internal Slots

A proxy object is an exotic object whose essential internal methods are partially implemented using ECMAScript code. Every proxy object has an internal slot called [[ProxyHandler]]. The value of [[ProxyHandler]] is an object, called the proxy's handler object, or null. Methods (see Table 30) of a handler object may be used to augment the implementation for one or more of the proxy object's internal methods. Every proxy object also has an internal slot called [[ProxyTarget]] whose value is either an object or the null value. This object is called the proxy's target object.

An object is a Proxy exotic object if its essential internal methods (including [[Call]] and [[Construct]], if applicable) use the definitions in this section. These internal methods are installed in ProxyCreate.

Table 30: Proxy Handler Methods

Internal Method	Handler Method
[[GetPrototypeOf]]	`getPrototypeOf`
[[SetPrototypeOf]]	`setPrototypeOf`
[[IsExtensible]]	`isExtensible`
[[PreventExtensions]]	`preventExtensions`
[[GetOwnProperty]]	`getOwnPropertyDescriptor`
[[DefineOwnProperty]]	`defineProperty`
[[HasProperty]]	`has`
[[Get]]	`get`
[[Set]]	`set`
[[Delete]]	`deleteProperty`
[[OwnPropertyKeys]]	`ownKeys`
[[Call]]	`apply`
[[Construct]]	`construct`

When a handler method is called to provide the implementation of a proxy object internal method, the handler method is passed the proxy's target object as a parameter. A proxy's handler object does not necessarily have a method corresponding to every essential internal method. Invoking an internal method on the proxy results in the invocation of the corresponding internal method on the proxy's target object if the handler object does not have a method corresponding to the internal trap.

The [[ProxyHandler]] and [[ProxyTarget]] internal slots of a proxy object are always initialized when the object is created and typically may not be modified. Some proxy objects are created in a manner that permits them to be subsequently revoked. When a proxy is revoked, its [[ProxyHandler]] and [[ProxyTarget]] internal slots are set to null causing subsequent invocations of internal methods on that proxy object to throw a TypeError exception.

Because proxy objects permit the implementation of internal methods to be provided by arbitrary ECMAScript code, it is possible to define a proxy object whose handler methods violates the invariants defined in 6.1.7.3. Some of the internal method invariants defined in 6.1.7.3 are essential integrity invariants. These invariants are explicitly enforced by the proxy object internal methods specified in this section. An ECMAScript implementation must be robust in the presence of all possible invariant violations.

In the following algorithm descriptions, assume O is an ECMAScript proxy object, P is a property key value, V is any ECMAScript language value and Desc is a Property Descriptor record.

9.5.1 [[GetPrototypeOf]] ( )

When the [[GetPrototypeOf]] internal method of a Proxy exotic object O is called, the following steps are taken:

Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "getPrototypeOf").
If trap is undefined, then
1. Return ? target.[[GetPrototypeOf]]().
Let handlerProto be ? Call(trap, handler, « target »).
If Type(handlerProto) is neither Object nor Null, throw a TypeError exception.
Let extensibleTarget be ? IsExtensible(target).
If extensibleTarget is true, return handlerProto.
Let targetProto be ? target.[[GetPrototypeOf]]().
If SameValue(handlerProto, targetProto) is false, throw a TypeError exception.
Return handlerProto.

Note

[[GetPrototypeOf]] for proxy objects enforces the following invariants:

The result of [[GetPrototypeOf]] must be either an Object or null.
If the target object is not extensible, [[GetPrototypeOf]] applied to the proxy object must return the same value as [[GetPrototypeOf]] applied to the proxy object's target object.

9.5.2 [[SetPrototypeOf]] ( `V` )

When the [[SetPrototypeOf]] internal method of a Proxy exotic object O is called with argument V, the following steps are taken:

Assert: Either Type(V) is Object or Type(V) is Null.
Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "setPrototypeOf").
If trap is undefined, then
1. Return ? target.[[SetPrototypeOf]](V).
Let booleanTrapResult be ! ToBoolean(? Call(trap, handler, « target, V »)).
If booleanTrapResult is false, return false.
Let extensibleTarget be ? IsExtensible(target).
If extensibleTarget is true, return true.
Let targetProto be ? target.[[GetPrototypeOf]]().
If SameValue(V, targetProto) is false, throw a TypeError exception.
Return true.

Note

[[SetPrototypeOf]] for proxy objects enforces the following invariants:

The result of [[SetPrototypeOf]] is a Boolean value.
If the target object is not extensible, the argument value must be the same as the result of [[GetPrototypeOf]] applied to target object.

9.5.3 [[IsExtensible]] ( )

When the [[IsExtensible]] internal method of a Proxy exotic object O is called, the following steps are taken:

Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "isExtensible").
If trap is undefined, then
1. Return ? IsExtensible(target).
Let booleanTrapResult be ! ToBoolean(? Call(trap, handler, « target »)).
Let targetResult be ? IsExtensible(target).
If SameValue(booleanTrapResult, targetResult) is false, throw a TypeError exception.
Return booleanTrapResult.

Note

[[IsExtensible]] for proxy objects enforces the following invariants:

The result of [[IsExtensible]] is a Boolean value.
[[IsExtensible]] applied to the proxy object must return the same value as [[IsExtensible]] applied to the proxy object's target object with the same argument.

9.5.4 [[PreventExtensions]] ( )

When the [[PreventExtensions]] internal method of a Proxy exotic object O is called, the following steps are taken:

Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "preventExtensions").
If trap is undefined, then
1. Return ? target.[[PreventExtensions]]().
Let booleanTrapResult be ! ToBoolean(? Call(trap, handler, « target »)).
If booleanTrapResult is true, then
1. Let extensibleTarget be ? IsExtensible(target).
2. If extensibleTarget is true, throw a TypeError exception.
Return booleanTrapResult.

Note

[[PreventExtensions]] for proxy objects enforces the following invariants:

The result of [[PreventExtensions]] is a Boolean value.
[[PreventExtensions]] applied to the proxy object only returns true if [[IsExtensible]] applied to the proxy object's target object is false.

9.5.5 [[GetOwnProperty]] ( `P` )

When the [[GetOwnProperty]] internal method of a Proxy exotic object O is called with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "getOwnPropertyDescriptor").
If trap is undefined, then
1. Return ? target.[[GetOwnProperty]](P).
Let trapResultObj be ? Call(trap, handler, « target, P »).
If Type(trapResultObj) is neither Object nor Undefined, throw a TypeError exception.
Let targetDesc be ? target.[[GetOwnProperty]](P).
If trapResultObj is undefined, then
1. If targetDesc is undefined, return undefined.
2. If targetDesc.[[Configurable]] is false, throw a TypeError exception.
3. Let extensibleTarget be ? IsExtensible(target).
4. If extensibleTarget is false, throw a TypeError exception.
5. Return undefined.
Let extensibleTarget be ? IsExtensible(target).
Let resultDesc be ? ToPropertyDescriptor(trapResultObj).
Call CompletePropertyDescriptor(resultDesc).
Let valid be IsCompatiblePropertyDescriptor(extensibleTarget, resultDesc, targetDesc).
If valid is false, throw a TypeError exception.
If resultDesc.[[Configurable]] is false, then
1. If targetDesc is undefined or targetDesc.[[Configurable]] is true, then
  1. Throw a TypeError exception.
2. If resultDesc has a [[Writable]] field and resultDesc.[[Writable]] is false, then
  1. If targetDesc.[[Writable]] is true, throw a TypeError exception.
Return resultDesc.

Note

[[GetOwnProperty]] for proxy objects enforces the following invariants:

The result of [[GetOwnProperty]] must be either an Object or undefined.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
A property cannot be reported as non-existent, if the target object is not extensible, unless it does not exist as an own property of the target object.
A property cannot be reported as existent, if the target object is not extensible, unless it exists as an own property of the target object.
A property cannot be reported as non-configurable, unless it exists as a non-configurable own property of the target object.
A property cannot be reported as both non-configurable and non-writable, unless it exists as a non-configurable, non-writable own property of the target object.

9.5.6 [[DefineOwnProperty]] ( `P`, `Desc` )

When the [[DefineOwnProperty]] internal method of a Proxy exotic object O is called with property key P and Property Descriptor Desc, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "defineProperty").
If trap is undefined, then
1. Return ? target.[[DefineOwnProperty]](P, Desc).
Let descObj be FromPropertyDescriptor(Desc).
Let booleanTrapResult be ! ToBoolean(? Call(trap, handler, « target, P, descObj »)).
If booleanTrapResult is false, return false.
Let targetDesc be ? target.[[GetOwnProperty]](P).
Let extensibleTarget be ? IsExtensible(target).
If Desc has a [[Configurable]] field and if Desc.[[Configurable]] is false, then
1. Let settingConfigFalse be true.
Else, let settingConfigFalse be false.
If targetDesc is undefined, then
1. If extensibleTarget is false, throw a TypeError exception.
2. If settingConfigFalse is true, throw a TypeError exception.
Else,
1. If IsCompatiblePropertyDescriptor(extensibleTarget, Desc, targetDesc) is false, throw a TypeError exception.
2. If settingConfigFalse is true and targetDesc.[[Configurable]] is true, throw a TypeError exception.
3. If IsDataDescriptor(targetDesc) is true, targetDesc.[[Configurable]] is false, and targetDesc.[[Writable]] is true, then
  1. If Desc has a [[Writable]] field and Desc.[[Writable]] is false, throw a TypeError exception.
Return true.

Note

[[DefineOwnProperty]] for proxy objects enforces the following invariants:

The result of [[DefineOwnProperty]] is a Boolean value.
A property cannot be added, if the target object is not extensible.
A property cannot be non-configurable, unless there exists a corresponding non-configurable own property of the target object.
A non-configurable property cannot be non-writable, unless there exists a corresponding non-configurable, non-writable own property of the target object.
If a property has a corresponding target object property then applying the Property Descriptor of the property to the target object using [[DefineOwnProperty]] will not throw an exception.

9.5.7 [[HasProperty]] ( `P` )

When the [[HasProperty]] internal method of a Proxy exotic object O is called with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "has").
If trap is undefined, then
1. Return ? target.[[HasProperty]](P).
Let booleanTrapResult be ! ToBoolean(? Call(trap, handler, « target, P »)).
If booleanTrapResult is false, then
1. Let targetDesc be ? target.[[GetOwnProperty]](P).
2. If targetDesc is not undefined, then
  1. If targetDesc.[[Configurable]] is false, throw a TypeError exception.
  2. Let extensibleTarget be ? IsExtensible(target).
  3. If extensibleTarget is false, throw a TypeError exception.
Return booleanTrapResult.

Note

[[HasProperty]] for proxy objects enforces the following invariants:

The result of [[HasProperty]] is a Boolean value.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.

9.5.8 [[Get]] ( `P`, `Receiver` )

When the [[Get]] internal method of a Proxy exotic object O is called with property key P and ECMAScript language value Receiver, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "get").
If trap is undefined, then
1. Return ? target.[[Get]](P, Receiver).
Let trapResult be ? Call(trap, handler, « target, P, Receiver »).
Let targetDesc be ? target.[[GetOwnProperty]](P).
If targetDesc is not undefined and targetDesc.[[Configurable]] is false, then
1. If IsDataDescriptor(targetDesc) is true and targetDesc.[[Writable]] is false, then
  1. If SameValue(trapResult, targetDesc.[[Value]]) is false, throw a TypeError exception.
2. If IsAccessorDescriptor(targetDesc) is true and targetDesc.[[Get]] is undefined, then
  1. If trapResult is not undefined, throw a TypeError exception.
Return trapResult.

Note

[[Get]] for proxy objects enforces the following invariants:

The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable own data property.
The value reported for a property must be undefined if the corresponding target object property is a non-configurable own accessor property that has undefined as its [[Get]] attribute.

9.5.9 [[Set]] ( `P`, `V`, `Receiver` )

When the [[Set]] internal method of a Proxy exotic object O is called with property key P, value V, and ECMAScript language value Receiver, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "set").
If trap is undefined, then
1. Return ? target.[[Set]](P, V, Receiver).
Let booleanTrapResult be ! ToBoolean(? Call(trap, handler, « target, P, V, Receiver »)).
If booleanTrapResult is false, return false.
Let targetDesc be ? target.[[GetOwnProperty]](P).
If targetDesc is not undefined and targetDesc.[[Configurable]] is false, then
1. If IsDataDescriptor(targetDesc) is true and targetDesc.[[Writable]] is false, then
  1. If SameValue(V, targetDesc.[[Value]]) is false, throw a TypeError exception.
2. If IsAccessorDescriptor(targetDesc) is true, then
  1. If targetDesc.[[Set]] is undefined, throw a TypeError exception.
Return true.

Note

[[Set]] for proxy objects enforces the following invariants:

The result of [[Set]] is a Boolean value.
Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable own data property.
Cannot set the value of a property if the corresponding target object property is a non-configurable own accessor property that has undefined as its [[Set]] attribute.

9.5.10 [[Delete]] ( `P` )

When the [[Delete]] internal method of a Proxy exotic object O is called with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "deleteProperty").
If trap is undefined, then
1. Return ? target.[[Delete]](P).
Let booleanTrapResult be ! ToBoolean(? Call(trap, handler, « target, P »)).
If booleanTrapResult is false, return false.
Let targetDesc be ? target.[[GetOwnProperty]](P).
If targetDesc is undefined, return true.
If targetDesc.[[Configurable]] is false, throw a TypeError exception.
Let extensibleTarget be ? IsExtensible(target).
If extensibleTarget is false, throw a TypeError exception.
Return true.

Note

[[Delete]] for proxy objects enforces the following invariants:

The result of [[Delete]] is a Boolean value.
A property cannot be reported as deleted, if it exists as a non-configurable own property of the target object.
A property cannot be reported as deleted, if it exists as an own property of the target object and the target object is non-extensible.

9.5.11 [[OwnPropertyKeys]] ( )

When the [[OwnPropertyKeys]] internal method of a Proxy exotic object O is called, the following steps are taken:

Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "ownKeys").
If trap is undefined, then
1. Return ? target.[[OwnPropertyKeys]]().
Let trapResultArray be ? Call(trap, handler, « target »).
Let trapResult be ? CreateListFromArrayLike(trapResultArray, « String, Symbol »).
If trapResult contains any duplicate entries, throw a TypeError exception.
Let extensibleTarget be ? IsExtensible(target).
Let targetKeys be ? target.[[OwnPropertyKeys]]().
Assert: targetKeys is a List containing only String and Symbol values.
Assert: targetKeys contains no duplicate entries.
Let targetConfigurableKeys be a new empty List.
Let targetNonconfigurableKeys be a new empty List.
For each element key of targetKeys, do
1. Let desc be ? target.[[GetOwnProperty]](key).
2. If desc is not undefined and desc.[[Configurable]] is false, then
  1. Append key as an element of targetNonconfigurableKeys.
3. Else,
  1. Append key as an element of targetConfigurableKeys.
If extensibleTarget is true and targetNonconfigurableKeys is empty, then
1. Return trapResult.
Let uncheckedResultKeys be a new List which is a copy of trapResult.
For each key that is an element of targetNonconfigurableKeys, do
1. If key is not an element of uncheckedResultKeys, throw a TypeError exception.
2. Remove key from uncheckedResultKeys.
If extensibleTarget is true, return trapResult.
For each key that is an element of targetConfigurableKeys, do
1. If key is not an element of uncheckedResultKeys, throw a TypeError exception.
2. Remove key from uncheckedResultKeys.
If uncheckedResultKeys is not empty, throw a TypeError exception.
Return trapResult.

Note

[[OwnPropertyKeys]] for proxy objects enforces the following invariants:

The result of [[OwnPropertyKeys]] is a List.
The returned List contains no duplicate entries.
The Type of each result List element is either String or Symbol.
The result List must contain the keys of all non-configurable own properties of the target object.
If the target object is not extensible, then the result List must contain all the keys of the own properties of the target object and no other values.

9.5.12 [[Call]] ( `thisArgument`, `argumentsList` )

The [[Call]] internal method of a Proxy exotic object O is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:

Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "apply").
If trap is undefined, then
1. Return ? Call(target, thisArgument, argumentsList).
Let argArray be ! CreateArrayFromList(argumentsList).
Return ? Call(trap, handler, « target, thisArgument, argArray »).

Note

A Proxy exotic object only has a [[Call]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Call]] internal method.

9.5.13 [[Construct]] ( `argumentsList`, `newTarget` )

The [[Construct]] internal method of a Proxy exotic object O is called with parameters argumentsList which is a possibly empty List of ECMAScript language values and newTarget. The following steps are taken:

Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Assert: IsConstructor(target) is true.
Let trap be ? GetMethod(handler, "construct").
If trap is undefined, then
1. Return ? Construct(target, argumentsList, newTarget).
Let argArray be ! CreateArrayFromList(argumentsList).
Let newObj be ? Call(trap, handler, « target, argArray, newTarget »).
If Type(newObj) is not Object, throw a TypeError exception.
Return newObj.

Note 1

A Proxy exotic object only has a [[Construct]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Construct]] internal method.

Note 2

[[Construct]] for proxy objects enforces the following invariants:

The result of [[Construct]] must be an Object.

9.5.14 ProxyCreate ( `target`, `handler` )

The abstract operation ProxyCreate takes arguments target and handler. It is used to specify the creation of new Proxy exotic objects. It performs the following steps when called:

If Type(target) is not Object, throw a TypeError exception.
If Type(handler) is not Object, throw a TypeError exception.
Let P be ! MakeBasicObject(« [[ProxyHandler]], [[ProxyTarget]] »).
Set P's essential internal methods, except for [[Call]] and [[Construct]], to the definitions specified in 9.5.
If IsCallable(target) is true, then
1. Set P.[[Call]] as specified in 9.5.12.
2. If IsConstructor(target) is true, then
  1. Set P.[[Construct]] as specified in 9.5.13.
Set P.[[ProxyTarget]] to target.
Set P.[[ProxyHandler]] to handler.
Return P.

10 ECMAScript Language: Source Code

10.1 Source Text

Syntax

any Unicode code point

ECMAScript code is expressed using Unicode. ECMAScript source text is a sequence of code points. All Unicode code point values from U+0000 to U+10FFFF, including surrogate code points, may occur in source text where permitted by the ECMAScript grammars. The actual encodings used to store and interchange ECMAScript source text is not relevant to this specification. Regardless of the external source text encoding, a conforming ECMAScript implementation processes the source text as if it was an equivalent sequence of SourceCharacter values, each SourceCharacter being a Unicode code point. Conforming ECMAScript implementations are not required to perform any normalization of source text, or behave as though they were performing normalization of source text.

The components of a combining character sequence are treated as individual Unicode code points even though a user might think of the whole sequence as a single character.

Note

In string literals, regular expression literals, template literals and identifiers, any Unicode code point may also be expressed using Unicode escape sequences that explicitly express a code point's numeric value. Within a comment, such an escape sequence is effectively ignored as part of the comment.

ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a Java program, if the Unicode escape sequence \u000A, for example, occurs within a single-line comment, it is interpreted as a line terminator (Unicode code point U+000A is LINE FEED (LF)) and therefore the next code point is not part of the comment. Similarly, if the Unicode escape sequence \u000A occurs within a string literal in a Java program, it is likewise interpreted as a line terminator, which is not allowed within a string literal—one must write \n instead of \u000A to cause a LINE FEED (LF) to be part of the String value of a string literal. In an ECMAScript program, a Unicode escape sequence occurring within a comment is never interpreted and therefore cannot contribute to termination of the comment. Similarly, a Unicode escape sequence occurring within a string literal in an ECMAScript program always contributes to the literal and is never interpreted as a line terminator or as a code point that might terminate the string literal.

10.1.1 Static Semantics: UTF16Encoding ( `cp` )

The abstract operation UTF16Encoding takes argument cp (a numeric code point value). It performs the following steps when called:

Assert: 0 ≤ cp ≤ 0x10FFFF.
If cp ≤ 0xFFFF, return cp.
Let cu1 be floor((cp - 0x10000) / 0x400) + 0xD800.
Let cu2 be ((cp - 0x10000) modulo 0x400) + 0xDC00.
Return the code unit sequence consisting of cu1 followed by cu2.

10.1.2 Static Semantics: UTF16Encode ( `text` )

The abstract operation UTF16Encode takes argument text (a sequence of Unicode code points). It converts text into a String value, as described in 6.1.4. It performs the following steps when called:

Return the string-concatenation of the code units that are the UTF16Encoding of each code point in text, in order.

10.1.3 Static Semantics: UTF16DecodeSurrogatePair ( `lead`, `trail` )

The abstract operation UTF16DecodeSurrogatePair takes arguments lead (a code unit) and trail (a code unit). Two code units that form a UTF-16 surrogate pair are converted to a code point. It performs the following steps when called:

Assert: lead is a leading surrogate and trail is a trailing surrogate.
Let cp be (lead - 0xD800) × 0x400 + (trail - 0xDC00) + 0x10000.
Return the code point cp.

10.1.4 Static Semantics: CodePointAt ( `string`, `position` )

The abstract operation CodePointAt takes arguments string (a String) and position (a non-negative integer). It interprets string as a sequence of UTF-16 encoded code points, as described in 6.1.4, and reads from it a single code point starting with the code unit at index position. It performs the following steps when called:

Let size be the length of string.
Assert: position ≥ 0 and position < size.
Let first be the code unit at index position within string.
Let cp be the code point whose numeric value is that of first.
If first is not a leading surrogate or trailing surrogate, then
1. Return the Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]: false }.
If first is a trailing surrogate or position + 1 = size, then
1. Return the Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]: true }.
Let second be the code unit at index position + 1 within string.
If second is not a trailing surrogate, then
1. Return the Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]: true }.
Set cp to ! UTF16DecodeSurrogatePair(first, second).
Return the Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 2, [[IsUnpairedSurrogate]]: false }.

10.1.5 Static Semantics: UTF16DecodeString ( `string` )

The abstract operation UTF16DecodeString takes argument string (a String). It returns the sequence of Unicode code points that results from interpreting string as UTF-16 encoded Unicode text as described in 6.1.4. It performs the following steps when called:

Let codePoints be a new empty List.
Let size be the length of string.
Let position be 0.
Repeat, while position < size,
1. Let cp be ! CodePointAt(string, position).
2. Append cp.[[CodePoint]] to codePoints.
3. Set position to position + cp.[[CodeUnitCount]].
Return codePoints.

10.2 Types of Source Code

There are four types of ECMAScript code:

Global code is source text that is treated as an ECMAScript Script. The global code of a particular Script does not include any source text that is parsed as part of a FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, AsyncArrowFunction, ClassDeclaration, or ClassExpression.
Eval code is the source text supplied to the built-in eval function. More precisely, if the parameter to the built-in eval function is a String, it is treated as an ECMAScript Script. The eval code for a particular invocation of eval is the global code portion of that Script.
Function code is source text that is parsed to supply the value of the [[ECMAScriptCode]] and [[FormalParameters]] internal slots (see 9.2) of an ECMAScript function object. The function code of a particular ECMAScript function does not include any source text that is parsed as the function code of a nested FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, AsyncArrowFunction, ClassDeclaration, or ClassExpression.

In addition, if the source text referred to above is parsed as:
- the FormalParameters and FunctionBody of a FunctionDeclaration or FunctionExpression,
- the FormalParameters and GeneratorBody of a GeneratorDeclaration or GeneratorExpression,
- the FormalParameters and AsyncFunctionBody of an AsyncFunctionDeclaration or AsyncFunctionExpression, or
- the FormalParameters and AsyncGeneratorBody of an AsyncGeneratorDeclaration or AsyncGeneratorExpression,
then the source text matching the BindingIdentifier (if any) of that declaration or expression is also included in the function code of the corresponding function.
Module code is source text that is code that is provided as a ModuleBody. It is the code that is directly evaluated when a module is initialized. The module code of a particular module does not include any source text that is parsed as part of a nested FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, AsyncArrowFunction, ClassDeclaration, or ClassExpression.

Note 1

Function code is generally provided as the bodies of Function Definitions (14.1), Arrow Function Definitions (14.2), Method Definitions (14.3), Generator Function Definitions (14.4), Async Function Definitions (14.7), Async Generator Function Definitions (14.5), and Async Arrow Functions (14.8). Function code is also derived from the arguments to the Function constructor (19.2.1.1), the GeneratorFunction constructor (25.2.1.1), and the AsyncFunction constructor (25.7.1.1).

Note 2

The practical effect of including the BindingIdentifier in function code is that the Early Errors for strict mode code are applied to a BindingIdentifier that is the name of a function whose body contains a "use strict" directive, even if the surrounding code is not strict mode code.

10.2.1 Strict Mode Code

An ECMAScript Script syntactic unit may be processed using either unrestricted or strict mode syntax and semantics. Code is interpreted as strict mode code in the following situations:

Global code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive.
Module code is always strict mode code.
All parts of a ClassDeclaration or a ClassExpression are strict mode code.
Eval code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct eval that is contained in strict mode code.
Function code is strict mode code if the associated FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, or AsyncArrowFunction is contained in strict mode code or if the code that produces the value of the function's [[ECMAScriptCode]] internal slot begins with a Directive Prologue that contains a Use Strict Directive.
Function code that is supplied as the arguments to the built-in Function, Generator, AsyncFunction, and AsyncGenerator constructors is strict mode code if the last argument is a String that when processed is a FunctionBody that begins with a Directive Prologue that contains a Use Strict Directive.

ECMAScript code that is not strict mode code is called non-strict code.

10.2.2 Non-ECMAScript Functions

An ECMAScript implementation may support the evaluation of function exotic objects whose evaluative behaviour is expressed in some implementation-defined form of executable code other than via ECMAScript code. Whether a function object is an ECMAScript code function or a non-ECMAScript function is not semantically observable from the perspective of an ECMAScript code function that calls or is called by such a non-ECMAScript function.

11 ECMAScript Language: Lexical Grammar

The source text of an ECMAScript Script or Module is first converted into a sequence of input elements, which are tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of code points as the next input element.

There are several situations where the identification of lexical input elements is sensitive to the syntactic grammar context that is consuming the input elements. This requires multiple goal symbols for the lexical grammar. The InputElementRegExpOrTemplateTail goal is used in syntactic grammar contexts where a RegularExpressionLiteral, a TemplateMiddle, or a TemplateTail is permitted. The InputElementRegExp goal symbol is used in all syntactic grammar contexts where a RegularExpressionLiteral is permitted but neither a TemplateMiddle, nor a TemplateTail is permitted. The InputElementTemplateTail goal is used in all syntactic grammar contexts where a TemplateMiddle or a TemplateTail is permitted but a RegularExpressionLiteral is not permitted. In all other contexts, InputElementDiv is used as the lexical goal symbol.

Note

The use of multiple lexical goals ensures that there are no lexical ambiguities that would affect automatic semicolon insertion. For example, there are no syntactic grammar contexts where both a leading division or division-assignment, and a leading RegularExpressionLiteral are permitted. This is not affected by semicolon insertion (see 11.9); in examples such as the following:

a = b
/hi/g.exec(c).map(d);

where the first non-whitespace, non-comment code point after a LineTerminator is U+002F (SOLIDUS) and the syntactic context allows division or division-assignment, no semicolon is inserted at the LineTerminator. That is, the above example is interpreted in the same way as:

a = b / hi / g.exec(c).map(d);

Syntax

InputElementRegExpOrTemplateTail

InputElementTemplateTail

11.1 Unicode Format-Control Characters

The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the formatting of a range of text in the absence of higher-level protocols for this (such as mark-up languages).

It is useful to allow format-control characters in source text to facilitate editing and display. All format control characters may be used within comments, and within string literals, template literals, and regular expression literals.

U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are format-control characters that are used to make necessary distinctions when forming words or phrases in certain languages. In ECMAScript source text these code points may also be used in an IdentifierName after the first character.

U+FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character used primarily at the start of a text to mark it as Unicode and to allow detection of the text's encoding and byte order. <ZWNBSP> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. In ECMAScript source text <ZWNBSP> code points are treated as white space characters (see 11.2).

The special treatment of certain format-control characters outside of comments, string literals, and regular expression literals is summarized in Table 31.

Table 31: Format-Control Code Point Usage

Code Point	Name	Abbreviation	Usage
`U+200C`	ZERO WIDTH NON-JOINER	<ZWNJ>	IdentifierPart
`U+200D`	ZERO WIDTH JOINER	<ZWJ>	IdentifierPart
`U+FEFF`	ZERO WIDTH NO-BREAK SPACE	<ZWNBSP>	WhiteSpace

11.2 White Space

White space code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space code points may occur between any two tokens and at the start or end of input. White space code points may occur within a StringLiteral, a RegularExpressionLiteral, a Template, or a TemplateSubstitutionTail where they are considered significant code points forming part of a literal value. They may also occur within a Comment, but cannot appear within any other kind of token.

The ECMAScript white space code points are listed in Table 32.

Table 32: White Space Code Points

Code Point	Name	Abbreviation
`U+0009`	CHARACTER TABULATION	<TAB>
`U+000B`	LINE TABULATION	<VT>
`U+000C`	FORM FEED (FF)	<FF>
`U+0020`	SPACE	<SP>
`U+00A0`	NO-BREAK SPACE	<NBSP>
`U+FEFF`	ZERO WIDTH NO-BREAK SPACE	<ZWNBSP>
Other category “Zs”	Any other Unicode “Space_Separator” code point	<USP>

ECMAScript implementations must recognize as WhiteSpace code points listed in the “Space_Separator” (“Zs”) category.

Note

Other than for the code points listed in Table 32, ECMAScript WhiteSpace intentionally excludes all code points that have the Unicode “White_Space” property but which are not classified in category “Space_Separator” (“Zs”).

Syntax

WhiteSpace

<TAB>

<VT>

<FF>

<SP>

<NBSP>

<USP>

11.3 Line Terminators

Like white space code points, line terminator code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space code points, line terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar. Line terminators also affect the process of automatic semicolon insertion (11.9). A line terminator cannot occur within any token except a StringLiteral, Template, or TemplateSubstitutionTail. <LF> and <CR> line terminators cannot occur within a StringLiteral token except as part of a LineContinuation.

A line terminator can occur within a MultiLineComment but cannot occur within a SingleLineComment.

Line terminators are included in the set of white space code points that are matched by the \s class in regular expressions.

The ECMAScript line terminator code points are listed in Table 33.

Table 33: Line Terminator Code Points

Code Point	Unicode Name	Abbreviation
`U+000A`	LINE FEED (LF)	<LF>
`U+000D`	CARRIAGE RETURN (CR)	<CR>
`U+2028`	LINE SEPARATOR	<LS>
`U+2029`	PARAGRAPH SEPARATOR	<PS>

Only the Unicode code points in Table 33 are treated as line terminators. Other new line or line breaking Unicode code points are not treated as line terminators but are treated as white space if they meet the requirements listed in Table 32. The sequence <CR><LF> is commonly used as a line terminator. It should be considered a single SourceCharacter for the purpose of reporting line numbers.

Syntax

LineTerminator

<LF>

<CR>

<LS>

<PS>

<LF>

<CR>

[lookahead ≠ <LF>]

<LS>

<PS>

<CR>

<LF>

11.4 Comments

Comments can be either single or multi-line. Multi-line comments cannot nest.

Because a single-line comment can contain any Unicode code point except a LineTerminator code point, and because of the general rule that a token is always as long as possible, a single-line comment always consists of all code points from the // marker to the end of the line. However, the LineTerminator at the end of the line is not considered to be part of the single-line comment; it is recognized separately by the lexical grammar and becomes part of the stream of input elements for the syntactic grammar. This point is very important, because it implies that the presence or absence of single-line comments does not affect the process of automatic semicolon insertion (see 11.9).

Comments behave like white space and are discarded except that, if a MultiLineComment contains a line terminator code point, then the entire comment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.

Syntax

opt

opt

opt

opt

opt

but not *

but not one of / or *

SingleLineComment

opt

opt

but not LineTerminator

11.5 Tokens

Syntax

Note

The DivPunctuator, RegularExpressionLiteral, RightBracePunctuator, and TemplateSubstitutionTail productions derive additional tokens that are not included in the CommonToken production.

11.6 Names and Keywords

IdentifierName and ReservedWord are tokens that are interpreted according to the Default Identifier Syntax given in Unicode Standard Annex #31, Identifier and Pattern Syntax, with some small modifications. ReservedWord is an enumerated subset of IdentifierName. The syntactic grammar defines Identifier as an IdentifierName that is not a ReservedWord. The Unicode identifier grammar is based on character properties specified by the Unicode Standard. The Unicode code points in the specified categories in the latest version of the Unicode standard must be treated as in those categories by all conforming ECMAScript implementations. ECMAScript implementations may recognize identifier code points defined in later editions of the Unicode Standard.

Note 1

This standard specifies specific code point additions: U+0024 (DOLLAR SIGN) and U+005F (LOW LINE) are permitted anywhere in an IdentifierName, and the code points U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are permitted anywhere after the first code point of an IdentifierName.

Unicode escape sequences are permitted in an IdentifierName, where they contribute a single Unicode code point to the IdentifierName. The code point is expressed by the CodePoint of the UnicodeEscapeSequence (see 11.8.4). The \ preceding the UnicodeEscapeSequence and the u and { } code units, if they appear, do not contribute code points to the IdentifierName. A UnicodeEscapeSequence cannot be used to put a code point into an IdentifierName that would otherwise be illegal. In other words, if a \ UnicodeEscapeSequence sequence were replaced by the SourceCharacter it contributes, the result must still be a valid IdentifierName that has the exact same sequence of SourceCharacter elements as the original IdentifierName. All interpretations of IdentifierName within this specification are based upon their actual code points regardless of whether or not an escape sequence was used to contribute any particular code point.

Two IdentifierNames that are canonically equivalent according to the Unicode standard are not equal unless, after replacement of each UnicodeEscapeSequence, they are represented by the exact same sequence of code points.

Syntax

<ZWNJ>

<ZWJ>

any Unicode code point with the Unicode property “ID_Start”

any Unicode code point with the Unicode property “ID_Continue”

The definitions of the nonterminal UnicodeEscapeSequence is given in 11.8.4.

Note 2

The nonterminal IdentifierPart derives _ via UnicodeIDContinue.

Note 3

The sets of code points with Unicode properties “ID_Start” and “ID_Continue” include, respectively, the code points with Unicode properties “Other_ID_Start” and “Other_ID_Continue”.

11.6.1 Identifier Names

11.6.1.1 Static Semantics: Early Errors

IdentifierStart

It is a Syntax Error if the SV of UnicodeEscapeSequence is none of "$", or "_", or the UTF16Encoding of a code point matched by the UnicodeIDStart lexical grammar production.

It is a Syntax Error if the SV of UnicodeEscapeSequence is none of "$", or "_", or the UTF16Encoding of either <ZWNJ> or <ZWJ>, or the UTF16Encoding of a Unicode code point that would be matched by the UnicodeIDContinue lexical grammar production.

11.6.1.2 Static Semantics: StringValue

Let idText be the source text matched by IdentifierName.
Let idTextUnescaped be the result of replacing any occurrences of \ UnicodeEscapeSequence in idText with the code point represented by the UnicodeEscapeSequence.
Return ! UTF16Encode(idTextUnescaped).

11.6.2 Keywords and Reserved Words

A keyword is a token that matches IdentifierName, but also has a syntactic use; that is, it appears literally, in a fixed width font, in some syntactic production. The keywords of ECMAScript include if, while, async, await, and many others.

A reserved word is an IdentifierName that cannot be used as an identifier. Many keywords are reserved words, but some are not, and some are reserved only in certain contexts. if and while are reserved words. await is reserved only inside async functions and modules. async is not reserved; it can be used as a variable name or statement label without restriction.

This specification uses a combination of grammatical productions and early error rules to specify which names are valid identifiers and which are reserved words. All tokens in the ReservedWord list below, except for await and yield, are unconditionally reserved. Exceptions for await and yield are specified in 12.1, using parameterized syntactic productions. Lastly, several early error rules restrict the set of valid identifiers. See 12.1.1, 13.3.1.1, 13.7.5.1, and 14.6.1. In summary, there are five categories of identifier names:

Those that are always allowed as identifiers, and are not keywords, such as Math, window, toString, and _;
Those that are never allowed as identifiers, namely the ReservedWords listed below except await and yield;
Those that are contextually allowed as identifiers, namely await and yield;
Those that are contextually disallowed as identifiers, in strict mode code: let, static, implements, interface, package, private, protected, and public;
Those that are always allowed as identifiers, but also appear as keywords within certain syntactic productions, at places where Identifier is not allowed: as, async, from, get, of, set, and target.

The term conditional keyword, or contextual keyword, is sometimes used to refer to the keywords that fall in the last three categories, and thus can be used as identifiers in some contexts and as keywords in others.

Syntax

ReservedWord

one of

await

break

case

catch

class

const

continue

debugger

default

delete

else

enum

export

extends

false

finally

for

function

import

instanceof

new

null

return

super

switch

this

throw

true

try

typeof

var

void

while

with

yield

Note 1

Per 5.1.5, keywords in the grammar match literal sequences of specific SourceCharacter elements. A code point in a keyword cannot be expressed by a \ UnicodeEscapeSequence.

An IdentifierName can contain \ UnicodeEscapeSequences, but it is not possible to declare a variable named "else" by spelling it els\u{65}. The early error rules in 12.1.1 rule out identifiers with the same StringValue as a reserved word.

Note 2

enum is not currently used as a keyword in this specification. It is a future reserved word, set aside for use as a keyword in future language extensions.

Similarly, implements, interface, package, private, protected, and public are future reserved words in strict mode code.

Note 3

The names arguments and eval are not keywords, but they are subject to some restrictions in strict mode code. See 12.1.1, 12.1.3, 14.1.2, 14.4.1, 14.5.1, and 14.7.1.

11.7 Punctuators

Syntax

Punctuator

[lookahead ∉ DecimalDigit]

one of

{

(

)

[

]

...

;

===

!==

>>>

**=

<<=

>>=

>>>=

DivPunctuator

RightBracePunctuator

}

11.8 Literals

11.8.1 Null Literals

Syntax

NullLiteral

null

11.8.2 Boolean Literals

Syntax

BooleanLiteral

true

false

11.8.3 Numeric Literals

Syntax

opt

opt

opt

opt

ExponentPart

opt

opt

one of

one of

one of

one of

one of

one of

The SourceCharacter immediately following a NumericLiteral must not be an IdentifierStart or DecimalDigit.

Note

For example: 3in is an error and not the two input elements 3 and in.

A conforming implementation, when processing strict mode code, must not extend, as described in B.1.1, the syntax of NumericLiteral to include LegacyOctalIntegerLiteral, nor extend the syntax of DecimalIntegerLiteral to include NonOctalDecimalIntegerLiteral.

11.8.3.1 Static Semantics: MV

A numeric literal stands for a value of the Number type or the BigInt type.

The MV of NumericLiteral::DecimalLiteral is the MV of DecimalLiteral.
The MV of NonDecimalIntegerLiteral::BinaryIntegerLiteral is the MV of BinaryIntegerLiteral.
The MV of NonDecimalIntegerLiteral::OctalIntegerLiteral is the MV of OctalIntegerLiteral.
The MV of NonDecimalIntegerLiteral::HexIntegerLiteral is the MV of HexIntegerLiteral.
The MV of DecimalLiteral::DecimalIntegerLiteral. is the MV of DecimalIntegerLiteral.
The MV of DecimalLiteral::DecimalIntegerLiteral.DecimalDigits is the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits × 10_ℝ^-_ℝn), where n is the mathematical value of the number of code points in DecimalDigits.
The MV of DecimalLiteral::DecimalIntegerLiteral.ExponentPart is the MV of DecimalIntegerLiteral × 10_ℝ^e, where e is the MV of ExponentPart.
The MV of DecimalLiteral::DecimalIntegerLiteral.DecimalDigitsExponentPart is (the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits × 10_ℝ^-_ℝn)) × 10_ℝ^e, where n is the mathematical integer number of code points in DecimalDigits and e is the MV of ExponentPart.
The MV of DecimalLiteral::.DecimalDigits is the MV of DecimalDigits × 10_ℝ^-_ℝn, where n is the mathematical integer number of code points in DecimalDigits.
The MV of DecimalLiteral::.DecimalDigitsExponentPart is the MV of DecimalDigits × 10_ℝ^{e -_ℝ n}, where n is the mathematical integer number of code points in DecimalDigits and e is the MV of ExponentPart.
The MV of DecimalLiteral::DecimalIntegerLiteral is the MV of DecimalIntegerLiteral.
The MV of DecimalLiteral::DecimalIntegerLiteralExponentPart is the MV of DecimalIntegerLiteral × 10_ℝ^e, where e is the MV of ExponentPart.
The MV of DecimalIntegerLiteral::0 is 0_ℝ.
The MV of DecimalIntegerLiteral::NonZeroDigit is the MV of NonZeroDigit.
The MV of DecimalIntegerLiteral::NonZeroDigitDecimalDigits is (the MV of NonZeroDigit × 10_ℝⁿ) plus the MV of DecimalDigits, where n is the mathematical integer number of code points in DecimalDigits.
The MV of DecimalDigits::DecimalDigit is the MV of DecimalDigit.
The MV of DecimalDigits::DecimalDigitsDecimalDigit is (the MV of DecimalDigits × 10_ℝ) plus the MV of DecimalDigit.
The MV of ExponentPart::ExponentIndicatorSignedInteger is the MV of SignedInteger.
The MV of SignedInteger::DecimalDigits is the MV of DecimalDigits.
The MV of SignedInteger::+DecimalDigits is the MV of DecimalDigits.
The MV of SignedInteger::-DecimalDigits is the negative of the MV of DecimalDigits.
The MV of DecimalDigit::0 or of HexDigit::0 or of OctalDigit::0 or of BinaryDigit::0 is 0_ℝ.
The MV of DecimalDigit::1 or of NonZeroDigit::1 or of HexDigit::1 or of OctalDigit::1 or of BinaryDigit::1 is 1_ℝ.
The MV of DecimalDigit::2 or of NonZeroDigit::2 or of HexDigit::2 or of OctalDigit::2 is 2_ℝ.
The MV of DecimalDigit::3 or of NonZeroDigit::3 or of HexDigit::3 or of OctalDigit::3 is 3_ℝ.
The MV of DecimalDigit::4 or of NonZeroDigit::4 or of HexDigit::4 or of OctalDigit::4 is 4_ℝ.
The MV of DecimalDigit::5 or of NonZeroDigit::5 or of HexDigit::5 or of OctalDigit::5 is 5_ℝ.
The MV of DecimalDigit::6 or of NonZeroDigit::6 or of HexDigit::6 or of OctalDigit::6 is 6_ℝ.
The MV of DecimalDigit::7 or of NonZeroDigit::7 or of HexDigit::7 or of OctalDigit::7 is 7_ℝ.
The MV of DecimalDigit::8 or of NonZeroDigit::8 or of HexDigit::8 is 8_ℝ.
The MV of DecimalDigit::9 or of NonZeroDigit::9 or of HexDigit::9 is 9_ℝ.
The MV of HexDigit::a or of HexDigit::A is 10_ℝ.
The MV of HexDigit::b or of HexDigit::B is 11_ℝ.
The MV of HexDigit::c or of HexDigit::C is 12_ℝ.
The MV of HexDigit::d or of HexDigit::D is 13_ℝ.
The MV of HexDigit::e or of HexDigit::E is 14_ℝ.
The MV of HexDigit::f or of HexDigit::F is 15_ℝ.
The MV of BinaryIntegerLiteral::0bBinaryDigits is the MV of BinaryDigits.
The MV of BinaryIntegerLiteral::0BBinaryDigits is the MV of BinaryDigits.
The MV of BinaryDigits::BinaryDigit is the MV of BinaryDigit.
The MV of BinaryDigits::BinaryDigitsBinaryDigit is (the MV of BinaryDigits × 2_ℝ) plus the MV of BinaryDigit.
The MV of OctalIntegerLiteral::0oOctalDigits is the MV of OctalDigits.
The MV of OctalIntegerLiteral::0OOctalDigits is the MV of OctalDigits.
The MV of OctalDigits::OctalDigit is the MV of OctalDigit.
The MV of OctalDigits::OctalDigitsOctalDigit is (the MV of OctalDigits × 8_ℝ) plus the MV of OctalDigit.
The MV of HexIntegerLiteral::0xHexDigits is the MV of HexDigits.
The MV of HexIntegerLiteral::0XHexDigits is the MV of HexDigits.
The MV of HexDigits::HexDigit is the MV of HexDigit.
The MV of HexDigits::HexDigitsHexDigit is (the MV of HexDigits × 16_ℝ) plus the MV of HexDigit.
The MV of Hex4Digits::HexDigitHexDigitHexDigitHexDigit is (0x1000_ℝ times the MV of the first HexDigit) plus (0x100_ℝ times the MV of the second HexDigit) plus (0x10_ℝ times the MV of the third HexDigit) plus the MV of the fourth HexDigit.

11.8.3.2 Static Semantics: NumericValue

Return the Number value that results from rounding the MV of DecimalLiteral as described below.

Return the Number value that results from rounding the MV of NonDecimalIntegerLiteral as described below.

Once the exact MV for a numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0_ℝ, then the rounded value is +0; otherwise, the rounded value must be the Number value for the MV (as specified in 6.1.6.1), unless the literal is a DecimalLiteral and the literal has more than 20 significant digits, in which case the Number value may be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit and then incrementing the literal at the 20th significant digit position. A digit is significant if it is not part of an ExponentPart and

it is not 0; or
there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.

Return the BigInt value that represents the MV of NonDecimalIntegerLiteral.

Return the BigInt value that represents 0_ℝ.

Return the BigInt value that represents the MV of NonZeroDigit.

Let n be the mathematical integer number of code points in DecimalDigits.
Let mv be (the MV of NonZeroDigit × 10_ℝⁿ) plus the MV of DecimalDigits.
Return the BigInt value that represents mv.

11.8.4 String Literals

Note 1

A string literal is zero or more Unicode code points enclosed in single or double quotes. Unicode code points may also be represented by an escape sequence. All code points may appear literally in a string literal except for the closing quote code points, U+005C (REVERSE SOLIDUS), U+000D (CARRIAGE RETURN), and U+000A (LINE FEED). Any code points may appear in the form of an escape sequence. String literals evaluate to ECMAScript String values. When generating these String values Unicode code points are UTF-16 encoded as defined in 10.1.1. Code points belonging to the Basic Multilingual Plane are encoded as a single code unit element of the string. All other code points are encoded as two code unit elements of the string.

Syntax

opt

opt

opt

opt

but not one of " or \ or LineTerminator

<LS>

<PS>

but not one of ' or \ or LineTerminator

<LS>

<PS>

[lookahead ∉ DecimalDigit]

A conforming implementation, when processing strict mode code, must not extend the syntax of EscapeSequence to include LegacyOctalEscapeSequence as described in B.1.2.

one of

but not one of EscapeCharacter or LineTerminator

EscapeCharacter

}

The definition of the nonterminal HexDigit is given in 11.8.3. SourceCharacter is defined in 10.1.

Note 2

<LF> and <CR> cannot appear in a string literal, except as part of a LineContinuation to produce the empty code points sequence. The proper way to include either in the String value of a string literal is to use an escape sequence such as \n or \u000A.

11.8.4.1 Static Semantics: StringValue

opt

opt

Return the String value whose code units are the SV of this StringLiteral.

11.8.4.2 Static Semantics: SV

A string literal stands for a value of the String type. The String value (SV) of the literal is described in terms of code unit values contributed by the various parts of the string literal. As part of this process, some Unicode code points within the string literal are interpreted as having a mathematical value (MV), as described below or in 11.8.3.

The SV of StringLiteral::"" is the empty code unit sequence.
The SV of StringLiteral::'' is the empty code unit sequence.
The SV of DoubleStringCharacters::DoubleStringCharacter is a sequence of up to two code units that is the SV of DoubleStringCharacter.
The SV of DoubleStringCharacters::DoubleStringCharacterDoubleStringCharacters is a sequence of up to two code units that is the SV of DoubleStringCharacter followed by the code units of the SV of DoubleStringCharacters in order.
The SV of SingleStringCharacters::SingleStringCharacter is a sequence of up to two code units that is the SV of SingleStringCharacter.
The SV of SingleStringCharacters::SingleStringCharacterSingleStringCharacters is a sequence of up to two code units that is the SV of SingleStringCharacter followed by the code units of the SV of SingleStringCharacters in order.
The SV of DoubleStringCharacter::SourceCharacterbut not one of " or \ or LineTerminator is the UTF16Encoding of the code point value of SourceCharacter.
The SV of DoubleStringCharacter::<LS> is the code unit 0x2028 (LINE SEPARATOR).
The SV of DoubleStringCharacter::<PS> is the code unit 0x2029 (PARAGRAPH SEPARATOR).
The SV of DoubleStringCharacter::LineContinuation is the empty code unit sequence.
The SV of SingleStringCharacter::SourceCharacterbut not one of ' or \ or LineTerminator is the UTF16Encoding of the code point value of SourceCharacter.
The SV of SingleStringCharacter::<LS> is the code unit 0x2028 (LINE SEPARATOR).
The SV of SingleStringCharacter::<PS> is the code unit 0x2029 (PARAGRAPH SEPARATOR).
The SV of SingleStringCharacter::LineContinuation is the empty code unit sequence.
The SV of EscapeSequence::0 is the code unit 0x0000 (NULL).
The SV of CharacterEscapeSequence::SingleEscapeCharacter is the code unit whose value is determined by the SingleEscapeCharacter according to Table 34.

Table 34: String Single Character Escape Sequences

Escape Sequence	Code Unit Value	Unicode Character Name	Symbol
`\b`	`0x0008`	BACKSPACE	<BS>
`\t`	`0x0009`	CHARACTER TABULATION	<HT>
`\n`	`0x000A`	LINE FEED (LF)	<LF>
`\v`	`0x000B`	LINE TABULATION	<VT>
`\f`	`0x000C`	FORM FEED (FF)	<FF>
`\r`	`0x000D`	CARRIAGE RETURN (CR)	<CR>
`\"`	`0x0022`	QUOTATION MARK	`"`
`\'`	`0x0027`	APOSTROPHE	`'`
`\\`	`0x005C`	REVERSE SOLIDUS	`\`

The SV of NonEscapeCharacter::SourceCharacterbut not one of EscapeCharacter or LineTerminator is the UTF16Encoding of the code point value of SourceCharacter.
The SV of HexEscapeSequence::xHexDigitHexDigit is the code unit whose value is (16_ℝ times the MV of the first HexDigit) plus the MV of the second HexDigit.
The SV of Hex4Digits::HexDigitHexDigitHexDigitHexDigit is the code unit whose value is the MV of Hex4Digits.
The SV of UnicodeEscapeSequence::u{CodePoint} is the UTF16Encoding of the MV of CodePoint.

11.8.5 Regular Expression Literals

Note 1

A regular expression literal is an input element that is converted to a RegExp object (see 21.2) each time the literal is evaluated. Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by new RegExp or calling the RegExp constructor as a function (see 21.2.3).

The productions below describe the syntax for a regular expression literal and are used by the input element scanner to find the end of the regular expression literal. The source text comprising the RegularExpressionBody and the RegularExpressionFlags are subsequently parsed again using the more stringent ECMAScript Regular Expression grammar (21.2.1).

An implementation may extend the ECMAScript Regular Expression grammar defined in 21.2.1, but it must not extend the RegularExpressionBody and RegularExpressionFlags productions defined below or the productions used by these productions.

Syntax

[empty]

but not one of * or \ or / or [

but not one of \ or / or [

but not LineTerminator

[

]

[empty]

RegularExpressionClassChar

but not one of ] or \

[empty]

Note 2

Regular expression literals may not be empty; instead of representing an empty regular expression literal, the code unit sequence // starts a single-line comment. To specify an empty regular expression, use: /(?:)/.

11.8.5.1 Static Semantics: Early Errors

It is a Syntax Error if IdentifierPart contains a Unicode escape sequence.

11.8.5.2 Static Semantics: BodyText

Return the source text that was recognized as RegularExpressionBody.

11.8.5.3 Static Semantics: FlagText

Return the source text that was recognized as RegularExpressionFlags.

11.8.6 Template Literal Lexical Components

Syntax

Template

opt

opt

}

opt

}

opt

opt

[lookahead ≠ {]

but not one of ` or \ or $ or LineTerminator

NotEscapeSequence

but not 0

[lookahead ∉ HexDigit]

[lookahead ∉ HexDigit]

[lookahead ≠ {]

[lookahead ∉ HexDigit]

[lookahead ∉ HexDigit]

[lookahead ∉ HexDigit]

{

[lookahead ∉ HexDigit]

{

[lookahead ∉ HexDigit]

{

[lookahead ∉ HexDigit]

[lookahead ≠ }]

but only if MV of HexDigits > 0x10FFFF

but only if MV of HexDigits ≤ 0x10FFFF

A conforming implementation must not use the extended definition of EscapeSequence described in B.1.2 when parsing a TemplateCharacter.

Note

TemplateSubstitutionTail is used by the InputElementTemplateTail alternative lexical goal.

11.8.6.1 Static Semantics: TV and TRV

A template literal component is interpreted as a sequence of Unicode code points. The Template Value (TV) of a literal component is described in terms of code unit values (SV, 11.8.4) contributed by the various parts of the template literal component. As part of this process, some Unicode code points within the template component are interpreted as having a mathematical value (MV, 11.8.3). In determining a TV, escape sequences are replaced by the UTF-16 code unit(s) of the Unicode code point represented by the escape sequence. The Template Raw Value (TRV) is similar to a Template Value with the difference that in TRVs escape sequences are interpreted literally.

The TV and TRV of NoSubstitutionTemplate::`` is the empty code unit sequence.
The TV and TRV of TemplateHead::`${ is the empty code unit sequence.
The TV and TRV of TemplateMiddle::}${ is the empty code unit sequence.
The TV and TRV of TemplateTail::}` is the empty code unit sequence.
The TV of TemplateCharacters::TemplateCharacterTemplateCharacters is undefined if either the TV of TemplateCharacter is undefined or the TV of TemplateCharacters is undefined. Otherwise, it is a sequence consisting of the code units of the TV of TemplateCharacter followed by the code units of the TV of TemplateCharacters.
The TV of TemplateCharacter::SourceCharacterbut not one of ` or \ or $ or LineTerminator is the UTF16Encoding of the code point value of SourceCharacter.
The TV of TemplateCharacter::$ is the code unit 0x0024 (DOLLAR SIGN).
The TV of TemplateCharacter::\EscapeSequence is the SV of EscapeSequence.
The TV of TemplateCharacter::\NotEscapeSequence is undefined.
The TV of TemplateCharacter::LineTerminatorSequence is the TRV of LineTerminatorSequence.
The TV of LineContinuation::\LineTerminatorSequence is the empty code unit sequence.
The TRV of TemplateCharacters::TemplateCharacterTemplateCharacters is a sequence consisting of the code units of the TRV of TemplateCharacter followed by the code units of the TRV of TemplateCharacters.
The TRV of TemplateCharacter::SourceCharacterbut not one of ` or \ or $ or LineTerminator is the UTF16Encoding of the code point value of SourceCharacter.
The TRV of TemplateCharacter::$ is the code unit 0x0024 (DOLLAR SIGN).
The TRV of TemplateCharacter::\EscapeSequence is the sequence consisting of the code unit 0x005C (REVERSE SOLIDUS) followed by the code units of TRV of EscapeSequence.
The TRV of TemplateCharacter::\NotEscapeSequence is the sequence consisting of the code unit 0x005C (REVERSE SOLIDUS) followed by the code units of TRV of NotEscapeSequence.
The TRV of EscapeSequence::0 is the code unit 0x0030 (DIGIT ZERO).
The TRV of NotEscapeSequence::0DecimalDigit is the sequence consisting of the code unit 0x0030 (DIGIT ZERO) followed by the code units of the TRV of DecimalDigit.
The TRV of NotEscapeSequence::x[lookahead ∉ HexDigit] is the code unit 0x0078 (LATIN SMALL LETTER X).
The TRV of NotEscapeSequence::xHexDigit[lookahead ∉ HexDigit] is the sequence consisting of the code unit 0x0078 (LATIN SMALL LETTER X) followed by the code units of the TRV of HexDigit.
The TRV of NotEscapeSequence::u[lookahead ∉ HexDigit][lookahead ≠ {] is the code unit 0x0075 (LATIN SMALL LETTER U).
The TRV of NotEscapeSequence::uHexDigit[lookahead ∉ HexDigit] is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code units of the TRV of HexDigit.
The TRV of NotEscapeSequence::uHexDigitHexDigit[lookahead ∉ HexDigit] is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code units of the TRV of the first HexDigit followed by the code units of the TRV of the second HexDigit.
The TRV of NotEscapeSequence::uHexDigitHexDigitHexDigit[lookahead ∉ HexDigit] is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code units of the TRV of the first HexDigit followed by the code units of the TRV of the second HexDigit followed by the code units of the TRV of the third HexDigit.
The TRV of NotEscapeSequence::u{[lookahead ∉ HexDigit] is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET).
The TRV of NotEscapeSequence::u{NotCodePoint[lookahead ∉ HexDigit] is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET) followed by the code units of the TRV of NotCodePoint.
The TRV of NotEscapeSequence::u{CodePoint[lookahead ∉ HexDigit][lookahead ≠ }] is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET) followed by the code units of the TRV of CodePoint.
The TRV of DecimalDigit::one of0123456789 is the UTF16Encoding of the single code point matched by this production.
The TRV of CharacterEscapeSequence::NonEscapeCharacter is the SV of NonEscapeCharacter.
The TRV of SingleEscapeCharacter::one of'"\bfnrtv is the UTF16Encoding of the single code point matched by this production.
The TRV of HexEscapeSequence::xHexDigitHexDigit is the sequence consisting of the code unit 0x0078 (LATIN SMALL LETTER X) followed by TRV of the first HexDigit followed by the TRV of the second HexDigit.
The TRV of UnicodeEscapeSequence::uHex4Digits is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by TRV of Hex4Digits.
The TRV of UnicodeEscapeSequence::u{CodePoint} is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET) followed by TRV of CodePoint followed by the code unit 0x007D (RIGHT CURLY BRACKET).
The TRV of Hex4Digits::HexDigitHexDigitHexDigitHexDigit is the sequence consisting of the TRV of the first HexDigit followed by the TRV of the second HexDigit followed by the TRV of the third HexDigit followed by the TRV of the fourth HexDigit.
The TRV of HexDigits::HexDigitsHexDigit is the sequence consisting of TRV of HexDigits followed by TRV of HexDigit.
The TRV of HexDigit::one of0123456789abcdefABCDEF is the UTF16Encoding of the single code point matched by this production.
The TRV of LineContinuation::\LineTerminatorSequence is the sequence consisting of the code unit 0x005C (REVERSE SOLIDUS) followed by the code units of TRV of LineTerminatorSequence.
The TRV of LineTerminatorSequence::<LF> is the code unit 0x000A (LINE FEED).
The TRV of LineTerminatorSequence::<CR> is the code unit 0x000A (LINE FEED).
The TRV of LineTerminatorSequence::<LS> is the code unit 0x2028 (LINE SEPARATOR).
The TRV of LineTerminatorSequence::<PS> is the code unit 0x2029 (PARAGRAPH SEPARATOR).
The TRV of LineTerminatorSequence::<CR><LF> is the sequence consisting of the code unit 0x000A (LINE FEED).

Note

TV excludes the code units of LineContinuation while TRV includes them. <CR><LF> and <CR> LineTerminatorSequences are normalized to <LF> for both TV and TRV. An explicit EscapeSequence is needed to include a <CR> or <CR><LF> sequence.

11.9 Automatic Semicolon Insertion

Most ECMAScript statements and declarations must be terminated with a semicolon. Such semicolons may always appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source text in certain situations. These situations are described by saying that semicolons are automatically inserted into the source code token stream in those situations.

11.9.1 Rules of Automatic Semicolon Insertion

In the following rules, “token” means the actual recognized lexical token determined using the current lexical goal symbol as described in clause 11.

There are three basic rules of semicolon insertion:

When, as the source text is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:
- The offending token is separated from the previous token by at least one LineTerminator.
- The offending token is }.
- The previous token is ) and the inserted semicolon would then be parsed as the terminating semicolon of a do-while statement (13.7.2).
When, as the source text is parsed from left to right, the end of the input stream of tokens is encountered and the parser is unable to parse the input token stream as a single instance of the goal nonterminal, then a semicolon is automatically inserted at the end of the input stream.
When, as the source text is parsed from left to right, a token is encountered that is allowed by some production of the grammar, but the production is a restricted production and the token would be the first token for a terminal or nonterminal immediately following the annotation “[no LineTerminator here]” within the restricted production (and therefore such a token is called a restricted token), and the restricted token is separated from the previous token by at least one LineTerminator, then a semicolon is automatically inserted before the restricted token.

However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in the header of a for statement (see 13.7.4).

Note

The following are the only restricted productions in the grammar:

[Yield, Await]

[?Yield, ?Await]

[no LineTerminator here]

[?Yield, ?Await]

[no LineTerminator here]

[Yield, Await]

continue

;

continue

[no LineTerminator here]

[?Yield, ?Await]

;

[Yield, Await]

break

;

break

[no LineTerminator here]

[?Yield, ?Await]

;

[Yield, Await]

return

;

return

[no LineTerminator here]

[+In, ?Yield, ?Await]

;

[Yield, Await]

throw

[no LineTerminator here]

[+In, ?Yield, ?Await]

;

[In, Yield, Await]

[?Yield, ?Await]

[no LineTerminator here]

[?In]

[In, Await]

yield

[no LineTerminator here]

[?In, +Yield, ?Await]

yield

[no LineTerminator here]

[?In, +Yield, ?Await]

The practical effect of these restricted productions is as follows:

When a ++ or -- token is encountered where the parser would treat it as a postfix operator, and at least one LineTerminator occurred between the preceding token and the ++ or -- token, then a semicolon is automatically inserted before the ++ or -- token.
When a continue, break, return, throw, or yield token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the continue, break, return, throw, or yield token.

The resulting practical advice to ECMAScript programmers is:

A postfix ++ or -- operator should appear on the same line as its operand.
An Expression in a return or throw statement or an AssignmentExpression in a yield expression should start on the same line as the return, throw, or yield token.
A LabelIdentifier in a break or continue statement should be on the same line as the break or continue token.

11.9.2 Examples of Automatic Semicolon Insertion

This section is non-normative.

The source

{ 1 2 } 3

is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source

{ 1
2 } 3

is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:

{ 1
;2 ;} 3;

which is a valid ECMAScript sentence.

The source

for (a; b
)

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the semicolon is needed for the header of a for statement. Automatic semicolon insertion never inserts one of the two semicolons in the header of a for statement.

The source

return
a + b

is transformed by automatic semicolon insertion into the following:

return;
a + b;

Note 1

The expression a + b is not treated as a value to be returned by the return statement, because a LineTerminator separates it from the token return.

The source

a = b
++c

is transformed by automatic semicolon insertion into the following:

a = b;
++c;

Note 2

The token ++ is not treated as a postfix operator applying to the variable b, because a LineTerminator occurs between b and ++.

The source

if (a > b)
else c = d

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the else token, even though no production of the grammar applies at that point, because an automatically inserted semicolon would then be parsed as an empty statement.

The source

a = b + c
(d + e).print()

is not transformed by automatic semicolon insertion, because the parenthesized expression that begins the second line can be interpreted as an argument list for a function call:

a = b + c(d + e).print()

In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on automatic semicolon insertion.

11.10 Interesting Cases of Automatic Semicolon Insertion

This section is non-normative.

ECMAScript programs can be written in a style with very few semicolons by relying on automatic semicolon insertion. As described above, semicolons are not inserted at every newline, and automatic semicolon insertion can depend on multiple tokens across line terminators.

As new syntactic features are added to ECMAScript, additional grammar productions could be added that cause lines relying on automatic semicolon insertion preceding them to change grammar productions when parsed.

For the purposes of this section, a case of automatic semicolon insertion is considered interesting if it is a place where a semicolon may or may not be inserted, depending on the source text which precedes it. The rest of this section describes a number of interesting cases of automatic semicolon insertion in this version of ECMAScript.

11.10.1 Interesting Cases of Automatic Semicolon Insertion in Statement Lists

In a StatementList, many StatementListItems end in semicolons, which may be omitted using automatic semicolon insertion. As a consequence of the rules above, at the end of a line ending an expression, a semicolon is required if the following line begins with any of the following:

An opening parenthesis ((). Without a semicolon, the two lines together are treated as a CallExpression.
An opening square bracket ([). Without a semicolon, the two lines together are treated as property access, rather than an ArrayLiteral or ArrayAssignmentPattern.
A template literal (`). Without a semicolon, the two lines together are interpreted as a tagged Template (12.3.11), with the previous expression as the MemberExpression.
Unary + or -. Without a semicolon, the two lines together are interpreted as a usage of the corresponding binary operator.
A RegExp literal. Without a semicolon, the two lines together may be parsed instead as the / MultiplicativeOperator, for example if the RegExp has flags.

11.11 Cases of Automatic Semicolon Insertion and “[no LineTerminator here]”

This section is non-normative.

ECMAScript contains grammar productions which include “[no LineTerminator here]”. These productions are sometimes a means to have optional operands in the grammar. Introducing a LineTerminator in these locations would change the grammar production of a source text by using the grammar production without the optional operand.

The rest of this section describes a number of productions using “[no LineTerminator here]” in this version of ECMAScript.

11.11.1 List of Grammar Productions with Optional Operands and “[no LineTerminator here]”

UpdateExpression.
ContinueStatement.
BreakStatement.
ReturnStatement.
YieldExpression.
Async Function Definitions (14.7) with relation to Function Definitions (14.1)

12 ECMAScript Language: Expressions

12.1 Identifiers

Syntax

[Yield, Await]

[~Yield]

yield

[~Await]

await

[Yield, Await]

yield

await

[Yield, Await]

[~Yield]

yield

[~Await]

await

but not ReservedWord

Note

yield and await are permitted as BindingIdentifier in the grammar, and prohibited with static semantics below, to prohibit automatic semicolon insertion in cases such as

let
await 0;

12.1.1 Static Semantics: Early Errors

It is a Syntax Error if the code matched by this production is contained in strict mode code and the StringValue of Identifier is "arguments" or "eval".

yield

yield

yield

It is a Syntax Error if the code matched by this production is contained in strict mode code.

await

await

await

It is a Syntax Error if the goal symbol of the syntactic grammar is Module.

[Yield, Await]

yield

It is a Syntax Error if this production has a _[Yield] parameter.

[Yield, Await]

await

It is a Syntax Error if this production has an _[Await] parameter.

[Yield, Await]

[Yield, Await]

[Yield, Await]

It is a Syntax Error if this production has a _[Yield] parameter and StringValue of Identifier is "yield".
It is a Syntax Error if this production has an _[Await] parameter and StringValue of Identifier is "await".

but not ReservedWord

It is a Syntax Error if this phrase is contained in strict mode code and the StringValue of IdentifierName is: "implements", "interface", "let", "package", "private", "protected", "public", "static", or "yield".
It is a Syntax Error if the goal symbol of the syntactic grammar is Module and the StringValue of IdentifierName is "await".
It is a Syntax Error if StringValue of IdentifierName is the same String value as the StringValue of any ReservedWord except for yield or await.

Note

StringValue of IdentifierName normalizes any Unicode escape sequences in IdentifierName hence such escapes cannot be used to write an Identifier whose code point sequence is the same as a ReservedWord.

12.1.2 Static Semantics: BoundNames

Return a new List containing the StringValue of Identifier.

yield

Return a new List containing "yield".

await

Return a new List containing "await".

12.1.3 Static Semantics: AssignmentTargetType

If this IdentifierReference is contained in strict mode code and StringValue of Identifier is "eval" or "arguments", return invalid.
Return simple.

yield

Return simple.

await

Return simple.

12.1.4 Static Semantics: StringValue

yield

yield

yield

Return "yield".

await

await

await

Return "await".

but not ReservedWord

Return the StringValue of IdentifierName.

12.1.5 Runtime Semantics: BindingInitialization

With parameters value and environment.

Note

undefined is passed for environment to indicate that a PutValue operation should be used to assign the initialization value. This is the case for var statements and formal parameter lists of some non-strict functions (See 9.2.10). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.

Let name be StringValue of Identifier.
Return ? InitializeBoundName(name, value, environment).

yield

Return ? InitializeBoundName("yield", value, environment).

await

Return ? InitializeBoundName("await", value, environment).

12.1.5.1 Runtime Semantics: InitializeBoundName ( `name`, `value`, `environment` )

The abstract operation InitializeBoundName takes arguments name, value, and environment. It performs the following steps when called:

Assert: Type(name) is String.
If environment is not undefined, then
1. Perform environment.InitializeBinding(name, value).
2. Return NormalCompletion(undefined).
Else,
1. Let lhs be ResolveBinding(name).
2. Return ? PutValue(lhs, value).

12.1.6 Runtime Semantics: Evaluation

Return ? ResolveBinding(StringValue of Identifier).

yield

Return ? ResolveBinding("yield").

await

Return ? ResolveBinding("await").

Note 1

The result of evaluating an IdentifierReference is always a value of type Reference.

Note 2

In non-strict code, the keyword yield may be used as an identifier. Evaluating the IdentifierReference resolves the binding of yield as if it was an Identifier. Early Error restriction ensures that such an evaluation only can occur for non-strict code.

12.2 Primary Expression

Syntax

[Yield, Await]

this

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, ~Tagged]

[?Yield, ?Await]

[Yield, Await]

(

[+In, ?Yield, ?Await]

)

(

[+In, ?Yield, ?Await]

)

(

)

(

...

[?Yield, ?Await]

)

(

...

[?Yield, ?Await]

)

(

[+In, ?Yield, ?Await]

...

[?Yield, ?Await]

)

(

[+In, ?Yield, ?Await]

...

[?Yield, ?Await]

)

Supplemental Syntax

When processing an instance of the production
PrimaryExpression[Yield, Await]:CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
the interpretation of CoverParenthesizedExpressionAndArrowParameterList is refined using the following grammar:

[Yield, Await]

(

[+In, ?Yield, ?Await]

)

12.2.1 Semantics

12.2.1.1 Static Semantics: CoveredParenthesizedExpression

(

)

Return the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.

12.2.1.2 Static Semantics: HasName

Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
If IsFunctionDefinition of expr is false, return false.
Return HasName of expr.

12.2.1.3 Static Semantics: IsFunctionDefinition

this

Return false.

Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
Return IsFunctionDefinition of expr.

12.2.1.4 Static Semantics: IsIdentifierRef

Return true.

this

Return false.

12.2.1.5 Static Semantics: AssignmentTargetType

this

Return invalid.

Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
Return AssignmentTargetType of expr.

12.2.2 The `this` Keyword

12.2.2.1 Runtime Semantics: Evaluation

this

Return ? ResolveThisBinding().

12.2.3 Identifier Reference

See 12.1 for IdentifierReference.

12.2.4 Literals

Syntax

12.2.4.1 Runtime Semantics: Evaluation

NullLiteral

Return null.

BooleanLiteral

If BooleanLiteral is the token false, return false.
If BooleanLiteral is the token true, return true.

Return the NumericValue of NumericLiteral as defined in 11.8.3.

Return the StringValue of StringLiteral as defined in 11.8.4.1.

12.2.5 Array Initializer

Note

An ArrayLiteral is an expression describing the initialization of an Array object, using a list, of zero or more expressions each of which represents an array element, enclosed in square brackets. The elements need not be literals; they are evaluated each time the array initializer is evaluated.

Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is not preceded by an AssignmentExpression (i.e., a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array elements are not defined. If an element is elided at the end of an array, that element does not contribute to the length of the Array.

Syntax

[Yield, Await]

[

opt

]

[

[?Yield, ?Await]

]

[

[?Yield, ?Await]

opt

]

[Yield, Await]

opt

[+In, ?Yield, ?Await]

opt

[?Yield, ?Await]

[?Yield, ?Await]

opt

[+In, ?Yield, ?Await]

[?Yield, ?Await]

opt

[?Yield, ?Await]

[Yield, Await]

...

[+In, ?Yield, ?Await]

12.2.5.1 Runtime Semantics: ArrayAccumulation

With parameters array and nextIndex.

Let len be nextIndex + 1.
Perform ? Set(array, "length", len, true).
NOTE: The above Set throws if len exceeds 2³²-1.
Return len.

Return the result of performing ArrayAccumulation for Elision with arguments array and nextIndex + 1.

opt

If Elision is present, then
1. Set nextIndex to the result of performing ArrayAccumulation for Elision with arguments array and nextIndex.
2. ReturnIfAbrupt(nextIndex).
Let initResult be the result of evaluating AssignmentExpression.
Let initValue be ? GetValue(initResult).
Let created be ! CreateDataPropertyOrThrow(array, ! ToString(nextIndex), initValue).
Return nextIndex + 1.

opt

SpreadElement

If Elision is present, then
1. Set nextIndex to the result of performing ArrayAccumulation for Elision with arguments array and nextIndex.
2. ReturnIfAbrupt(nextIndex).
Return the result of performing ArrayAccumulation for SpreadElement with arguments array and nextIndex.

opt

Set nextIndex to the result of performing ArrayAccumulation for ElementList with arguments array and nextIndex.
ReturnIfAbrupt(nextIndex).
If Elision is present, then
1. Set nextIndex to the result of performing ArrayAccumulation for Elision with arguments array and nextIndex.
2. ReturnIfAbrupt(nextIndex).
Let initResult be the result of evaluating AssignmentExpression.
Let initValue be ? GetValue(initResult).
Let created be ! CreateDataPropertyOrThrow(array, ! ToString(nextIndex), initValue).
Return nextIndex + 1.

opt

SpreadElement

Set nextIndex to the result of performing ArrayAccumulation for ElementList with arguments array and nextIndex.
ReturnIfAbrupt(nextIndex).
If Elision is present, then
1. Set nextIndex to the result of performing ArrayAccumulation for Elision with arguments array and nextIndex.
2. ReturnIfAbrupt(nextIndex).
Return the result of performing ArrayAccumulation for SpreadElement with arguments array and nextIndex.

SpreadElement

...

Let spreadRef be the result of evaluating AssignmentExpression.
Let spreadObj be ? GetValue(spreadRef).
Let iteratorRecord be ? GetIterator(spreadObj).
Repeat,
1. Let next be ? IteratorStep(iteratorRecord).
2. If next is false, return nextIndex.
3. Let nextValue be ? IteratorValue(next).
4. Perform ! CreateDataPropertyOrThrow(array, ! ToString(nextIndex), nextValue).
5. Set nextIndex to nextIndex + 1.

Note

CreateDataPropertyOrThrow is used to ensure that own properties are defined for the array even if the standard built-in Array prototype object has been modified in a manner that would preclude the creation of new own properties using [[Set]].

12.2.5.2 Runtime Semantics: Evaluation

[

opt

]

Let array be ! ArrayCreate(0).
If Elision is present, then
1. Let len be the result of performing ArrayAccumulation for Elision with arguments array and 0.
2. ReturnIfAbrupt(len).
Return array.

[

]

Let array be ! ArrayCreate(0).
Let len be the result of performing ArrayAccumulation for ElementList with arguments array and 0.
ReturnIfAbrupt(len).
Return array.

[

opt

]

Let array be ! ArrayCreate(0).
Let nextIndex be the result of performing ArrayAccumulation for ElementList with arguments array and 0.
ReturnIfAbrupt(nextIndex).
If Elision is present, then
1. Let len be the result of performing ArrayAccumulation for Elision with arguments array and nextIndex.
2. ReturnIfAbrupt(len).
Return array.

12.2.6 Object Initializer

Note 1

An object initializer is an expression describing the initialization of an Object, written in a form resembling a literal. It is a list of zero or more pairs of property keys and associated values, enclosed in curly brackets. The values need not be literals; they are evaluated each time the object initializer is evaluated.

Syntax

[Yield, Await]

{

}

{

[?Yield, ?Await]

}

{

[?Yield, ?Await]

}

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

[?Yield, ?Await]

...

[+In, ?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[

[+In, ?Yield, ?Await]

]

CoverInitializedName

[Yield, Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

Note 2

MethodDefinition is defined in 14.3.

Note 3

In certain contexts, ObjectLiteral is used as a cover grammar for a more restricted secondary grammar. The CoverInitializedName production is necessary to fully cover these secondary grammars. However, use of this production results in an early Syntax Error in normal contexts where an actual ObjectLiteral is expected.

12.2.6.1 Static Semantics: Early Errors

It is a Syntax Error if HasDirectSuper of MethodDefinition is true.

In addition to describing an actual object initializer the ObjectLiteral productions are also used as a cover grammar for ObjectAssignmentPattern and may be recognized as part of a CoverParenthesizedExpressionAndArrowParameterList. When ObjectLiteral appears in a context where ObjectAssignmentPattern is required the following Early Error rules are not applied. In addition, they are not applied when initially parsing a CoverParenthesizedExpressionAndArrowParameterList or CoverCallExpressionAndAsyncArrowHead.

CoverInitializedName

Always throw a Syntax Error if code matches this production.

Note

This production exists so that ObjectLiteral can serve as a cover grammar for ObjectAssignmentPattern. It cannot occur in an actual object initializer.

12.2.6.2 Static Semantics: ComputedPropertyContains

With parameter symbol.

Return false.

Return the result of ComputedPropertyName Contains symbol.

12.2.6.3 Static Semantics: Contains

With parameter symbol.

If symbol is MethodDefinition, return true.
Return the result of ComputedPropertyContains for MethodDefinition with argument symbol.

Note

Static semantic rules that depend upon substructure generally do not look into function definitions.

If symbol is a ReservedWord, return false.
If symbol is an Identifier and StringValue of symbol is the same value as the StringValue of IdentifierName, return true.
Return false.

12.2.6.4 Static Semantics: IsComputedPropertyKey

Return false.

Return true.

12.2.6.5 Static Semantics: PropName

Return StringValue of IdentifierReference.

...

Return empty.

Return PropName of PropertyName.

Return StringValue of IdentifierName.

Return the String value whose code units are the SV of StringLiteral.

Let nbr be the NumericValue of NumericLiteral.
Return ! ToString(nbr).

[

]

Return empty.

12.2.6.6 Static Semantics: PropertyNameList

If PropName of PropertyDefinition is empty, return a new empty List.
Return a new List containing PropName of PropertyDefinition.

Let list be PropertyNameList of PropertyDefinitionList.
If PropName of PropertyDefinition is empty, return list.
Append PropName of PropertyDefinition to the end of list.
Return list.

12.2.6.7 Runtime Semantics: Evaluation

{

}

Return OrdinaryObjectCreate(%Object.prototype%).

{

}

{

}

Let obj be OrdinaryObjectCreate(%Object.prototype%).
Perform ? PropertyDefinitionEvaluation of PropertyDefinitionList with arguments obj and true.
Return obj.

Return StringValue of IdentifierName.

Return the String value whose code units are the SV of StringLiteral.

Let nbr be the NumericValue of NumericLiteral.
Return ! ToString(nbr).

[

]

Let exprValue be the result of evaluating AssignmentExpression.
Let propName be ? GetValue(exprValue).
Return ? ToPropertyKey(propName).

12.2.6.8 Runtime Semantics: PropertyDefinitionEvaluation

With parameters object and enumerable.

Perform ? PropertyDefinitionEvaluation of PropertyDefinitionList with arguments object and enumerable.
Return the result of performing PropertyDefinitionEvaluation of PropertyDefinition with arguments object and enumerable.

...

Let exprValue be the result of evaluating AssignmentExpression.
Let fromValue be ? GetValue(exprValue).
Let excludedNames be a new empty List.
Return ? CopyDataProperties(object, fromValue, excludedNames).

Let propName be StringValue of IdentifierReference.
Let exprValue be the result of evaluating IdentifierReference.
Let propValue be ? GetValue(exprValue).
Assert: enumerable is true.
Assert: object is an ordinary, extensible object with no non-configurable properties.
Return ! CreateDataPropertyOrThrow(object, propName, propValue).

Let propKey be the result of evaluating PropertyName.
ReturnIfAbrupt(propKey).
If IsAnonymousFunctionDefinition(AssignmentExpression) is true, then
1. Let propValue be NamedEvaluation of AssignmentExpression with argument propKey.
Else,
1. Let exprValueRef be the result of evaluating AssignmentExpression.
2. Let propValue be ? GetValue(exprValueRef).
Assert: enumerable is true.
Assert: object is an ordinary, extensible object with no non-configurable properties.
Return ! CreateDataPropertyOrThrow(object, propKey, propValue).

Note

An alternative semantics for this production is given in B.3.1.

12.2.7 Function Defining Expressions

See 14.1 for PrimaryExpression:FunctionExpression .

See 14.4 for PrimaryExpression:GeneratorExpression .

See 14.6 for PrimaryExpression:ClassExpression .

See 14.7 for PrimaryExpression:AsyncFunctionExpression .

See 14.5 for PrimaryExpression:AsyncGeneratorExpression .

12.2.8 Regular Expression Literals

Syntax

See 11.8.5.

12.2.8.1 Static Semantics: Early Errors

It is a Syntax Error if IsValidRegularExpressionLiteral(RegularExpressionLiteral) is false.

12.2.8.2 Static Semantics: IsValidRegularExpressionLiteral ( `literal` )

The abstract operation IsValidRegularExpressionLiteral takes argument literal. It determines if its argument is a valid regular expression literal. It performs the following steps when called:

Assert: literal is a RegularExpressionLiteral.
If FlagText of literal contains any code points other than g, i, m, s, u, or y, or if it contains the same code point more than once, return false.
Let P be BodyText of literal.
If FlagText of literal contains u, then
1. Parse P using the grammars in 21.2.1. The goal symbol for the parse is Pattern[+U, +N]. If P did not conform to the grammar, if any elements of P were not matched by the parse, or if any Early Error conditions exist, return false. Otherwise, return true.
Let stringValue be UTF16Encode(P).
Let pText be the sequence of code points resulting from interpreting each of the 16-bit elements of stringValue as a Unicode BMP code point. UTF-16 decoding is not applied to the elements.
Parse pText using the grammars in 21.2.1. The goal symbol for the parse is Pattern[~U, ~N]. If the result of parsing contains a GroupName, reparse with the goal symbol Pattern[~U, +N]. If pText did not conform to the grammar, if any elements of pText were not matched by the parse, or if any Early Error conditions exist, return false. Otherwise, return true.

12.2.8.3 Runtime Semantics: Evaluation

Let pattern be ! UTF16Encode(BodyText of RegularExpressionLiteral).
Let flags be ! UTF16Encode(FlagText of RegularExpressionLiteral).
Return RegExpCreate(pattern, flags).

12.2.9 Template Literals

Syntax

[Yield, Await, Tagged]

[?Yield, ?Await, ?Tagged]

[Yield, Await, Tagged]

[+In, ?Yield, ?Await]

[?Yield, ?Await, ?Tagged]

[Yield, Await, Tagged]

[?Yield, ?Await, ?Tagged]

[Yield, Await, Tagged]

[+In, ?Yield, ?Await]

[?Yield, ?Await, ?Tagged]

[+In, ?Yield, ?Await]

12.2.9.1 Static Semantics: Early Errors

[Yield, Await, Tagged]

It is a Syntax Error if the _[Tagged] parameter was not set and NoSubstitutionTemplate Contains NotEscapeSequence.

[Yield, Await, Tagged]

[?Yield, ?Await, ?Tagged]

It is a Syntax Error if the number of elements in the result of TemplateStrings of TemplateLiteral with argument false is greater than 2³² - 1.

[Yield, Await, Tagged]

[+In, ?Yield, ?Await]

[?Yield, ?Await, ?Tagged]

It is a Syntax Error if the _[Tagged] parameter was not set and TemplateHead Contains NotEscapeSequence.

[Yield, Await, Tagged]

It is a Syntax Error if the _[Tagged] parameter was not set and TemplateTail Contains NotEscapeSequence.

[Yield, Await, Tagged]

[+In, ?Yield, ?Await]

[?Yield, ?Await, ?Tagged]

[+In, ?Yield, ?Await]

It is a Syntax Error if the _[Tagged] parameter was not set and TemplateMiddle Contains NotEscapeSequence.

12.2.9.2 Static Semantics: TemplateStrings

With parameter raw.

If raw is false, then
1. Let string be the TV of NoSubstitutionTemplate.
Else,
1. Let string be the TRV of NoSubstitutionTemplate.
Return a List containing the single element, string.

If raw is false, then
1. Let head be the TV of TemplateHead.
Else,
1. Let head be the TRV of TemplateHead.
Let tail be TemplateStrings of TemplateSpans with argument raw.
Return a List containing head followed by the elements, in order, of tail.

If raw is false, then
1. Let tail be the TV of TemplateTail.
Else,
1. Let tail be the TRV of TemplateTail.
Return a List containing the single element, tail.

Let middle be TemplateStrings of TemplateMiddleList with argument raw.
If raw is false, then
1. Let tail be the TV of TemplateTail.
Else,
1. Let tail be the TRV of TemplateTail.
Return a List containing the elements, in order, of middle followed by tail.

If raw is false, then
1. Let string be the TV of TemplateMiddle.
Else,
1. Let string be the TRV of TemplateMiddle.
Return a List containing the single element, string.

Let front be TemplateStrings of TemplateMiddleList with argument raw.
If raw is false, then
1. Let last be the TV of TemplateMiddle.
Else,
1. Let last be the TRV of TemplateMiddle.
Append last as the last element of the List front.
Return front.

12.2.9.3 Runtime Semantics: ArgumentListEvaluation

Let templateLiteral be this TemplateLiteral.
Let siteObj be GetTemplateObject(templateLiteral).
Return a List containing the one element which is siteObj.

Let templateLiteral be this TemplateLiteral.
Let siteObj be GetTemplateObject(templateLiteral).
Let firstSubRef be the result of evaluating Expression.
Let firstSub be ? GetValue(firstSubRef).
Let restSub be ? SubstitutionEvaluation of TemplateSpans.
Assert: restSub is a List.
Return a List whose first element is siteObj, whose second elements is firstSub, and whose subsequent elements are the elements of restSub, in order. restSub may contain no elements.

12.2.9.4 Runtime Semantics: GetTemplateObject ( `templateLiteral` )

The abstract operation GetTemplateObject takes argument templateLiteral (a Parse Node). It performs the following steps when called:

Let realm be the current Realm Record.
Let templateRegistry be realm.[[TemplateMap]].
For each element e of templateRegistry, do
1. If e.[[Site]] is the same Parse Node as templateLiteral, then
  1. Return e.[[Array]].
Let rawStrings be TemplateStrings of templateLiteral with argument true.
Let cookedStrings be TemplateStrings of templateLiteral with argument false.
Let count be the number of elements in the List cookedStrings.
Assert: count ≤ 2³² - 1.
Let template be ! ArrayCreate(count).
Let rawObj be ! ArrayCreate(count).
Let index be 0.
Repeat, while index < count
1. Let prop be ! ToString(index).
2. Let cookedValue be the String value cookedStrings[index].
3. Call template.[[DefineOwnProperty]](prop, PropertyDescriptor { [[Value]]: cookedValue, [[Writable]]: false, [[Enumerable]]: true, [[Configurable]]: false }).
4. Let rawValue be the String value rawStrings[index].
5. Call rawObj.[[DefineOwnProperty]](prop, PropertyDescriptor { [[Value]]: rawValue, [[Writable]]: false, [[Enumerable]]: true, [[Configurable]]: false }).
6. Set index to index + 1.
Perform SetIntegrityLevel(rawObj, frozen).
Call template.[[DefineOwnProperty]]("raw", PropertyDescriptor { [[Value]]: rawObj, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }).
Perform SetIntegrityLevel(template, frozen).
Append the Record { [[Site]]: templateLiteral, [[Array]]: template } to templateRegistry.
Return template.

Note 1

The creation of a template object cannot result in an abrupt completion.

Note 2

Each TemplateLiteral in the program code of a realm is associated with a unique template object that is used in the evaluation of tagged Templates (12.2.9.6). The template objects are frozen and the same template object is used each time a specific tagged Template is evaluated. Whether template objects are created lazily upon first evaluation of the TemplateLiteral or eagerly prior to first evaluation is an implementation choice that is not observable to ECMAScript code.

Note 3

Future editions of this specification may define additional non-enumerable properties of template objects.

12.2.9.5 Runtime Semantics: SubstitutionEvaluation

Return a new empty List.

Return the result of SubstitutionEvaluation of TemplateMiddleList.

Let subRef be the result of evaluating Expression.
Let sub be ? GetValue(subRef).
Return a List containing only sub.

Let preceding be ? SubstitutionEvaluation of TemplateMiddleList.
Let nextRef be the result of evaluating Expression.
Let next be ? GetValue(nextRef).
Append next as the last element of the List preceding.
Return preceding.

12.2.9.6 Runtime Semantics: Evaluation

Return the String value whose code units are the elements of the TV of NoSubstitutionTemplate as defined in 11.8.6.

Let head be the TV of TemplateHead as defined in 11.8.6.
Let subRef be the result of evaluating Expression.
Let sub be ? GetValue(subRef).
Let middle be ? ToString(sub).
Let tail be the result of evaluating TemplateSpans.
ReturnIfAbrupt(tail).
Return the string-concatenation of head, middle, and tail.

Note 1

The string conversion semantics applied to the Expression value are like String.prototype.concat rather than the + operator.

Let tail be the TV of TemplateTail as defined in 11.8.6.
Return the String value consisting of the code units of tail.

Let head be the result of evaluating TemplateMiddleList.
ReturnIfAbrupt(head).
Let tail be the TV of TemplateTail as defined in 11.8.6.
Return the string-concatenation of head and tail.

Let head be the TV of TemplateMiddle as defined in 11.8.6.
Let subRef be the result of evaluating Expression.
Let sub be ? GetValue(subRef).
Let middle be ? ToString(sub).
Return the sequence of code units consisting of the code units of head followed by the elements of middle.

Note 2

The string conversion semantics applied to the Expression value are like String.prototype.concat rather than the + operator.

Let rest be the result of evaluating TemplateMiddleList.
ReturnIfAbrupt(rest).
Let middle be the TV of TemplateMiddle as defined in 11.8.6.
Let subRef be the result of evaluating Expression.
Let sub be ? GetValue(subRef).
Let last be ? ToString(sub).
Return the sequence of code units consisting of the elements of rest followed by the code units of middle followed by the elements of last.

Note 3

The string conversion semantics applied to the Expression value are like String.prototype.concat rather than the + operator.

12.2.10 The Grouping Operator

12.2.10.1 Static Semantics: Early Errors

It is a Syntax Error if CoverParenthesizedExpressionAndArrowParameterList is not covering a ParenthesizedExpression.
All Early Error rules for ParenthesizedExpression and its derived productions also apply to CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.

12.2.10.2 Static Semantics: IsFunctionDefinition

(

)

Return IsFunctionDefinition of Expression.

12.2.10.3 Static Semantics: AssignmentTargetType

(

)

Return AssignmentTargetType of Expression.

12.2.10.4 Runtime Semantics: NamedEvaluation

With parameter name.

Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
Return the result of performing NamedEvaluation for expr with argument name.

(

)

Assert: IsAnonymousFunctionDefinition(Expression) is true.
Return the result of performing NamedEvaluation for Expression with argument name.

12.2.10.5 Runtime Semantics: Evaluation

Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
Return the result of evaluating expr.

(

)

Return the result of evaluating Expression. This may be of type Reference.

Note

This algorithm does not apply GetValue to the result of evaluating Expression. The principal motivation for this is so that operators such as delete and typeof may be applied to parenthesized expressions.

12.3 Left-Hand-Side Expressions

Syntax

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[

[+In, ?Yield, ?Await]

]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, +Tagged]

[?Yield, ?Await]

new

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

super

[

[+In, ?Yield, ?Await]

]

super

new

target

import

meta

[Yield, Await]

[?Yield, ?Await]

new

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[

[+In, ?Yield, ?Await]

]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, +Tagged]

SuperCall

[Yield, Await]

super

[?Yield, ?Await]

ImportCall

[Yield, Await]

import

(

[+In, ?Yield, ?Await]

)

[Yield, Await]

(

)

(

[?Yield, ?Await]

)

(

[?Yield, ?Await]

)

[Yield, Await]

[+In, ?Yield, ?Await]

...

[+In, ?Yield, ?Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

[?Yield, ?Await]

...

[+In, ?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[

[+In, ?Yield, ?Await]

]

[?Yield, ?Await, +Tagged]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[

[+In, ?Yield, ?Await]

]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, +Tagged]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

Supplemental Syntax

When processing an instance of the production CallExpression:CoverCallExpressionAndAsyncArrowHead the interpretation of CoverCallExpressionAndAsyncArrowHead is refined using the following grammar:

CallMemberExpression

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

12.3.1 Static Semantics

12.3.1.1 Static Semantics: Early Errors

It is a Syntax Error if any code matches this production.

Note

This production exists in order to prevent automatic semicolon insertion rules (11.9) from being applied to the following code:

a?.b
`c`

so that it would be interpreted as two valid statements. The purpose is to maintain consistency with similar code without optional chaining:

a.b
`c`

which is a valid statement and where automatic semicolon insertion does not apply.

import

meta

It is a Syntax Error if the syntactic goal symbol is not Module.

12.3.1.2 Static Semantics: CoveredCallExpression

Return the CallMemberExpression that is covered by CoverCallExpressionAndAsyncArrowHead.

12.3.1.3 Static Semantics: Contains

With parameter symbol.

If MemberExpression Contains symbol is true, return true.
If symbol is a ReservedWord, return false.
If symbol is an Identifier and StringValue of symbol is the same value as the StringValue of IdentifierName, return true.
Return false.

SuperProperty

super

If symbol is the ReservedWord super, return true.
If symbol is a ReservedWord, return false.
If symbol is an Identifier and StringValue of symbol is the same value as the StringValue of IdentifierName, return true.
Return false.

If CallExpression Contains symbol is true, return true.
If symbol is a ReservedWord, return false.
If symbol is an Identifier and StringValue of symbol is the same value as the StringValue of IdentifierName, return true.
Return false.

If symbol is a ReservedWord, return false.
If symbol is an Identifier and StringValue of symbol is the same value as the StringValue of IdentifierName, return true.
Return false.

If OptionalChain Contains symbol is true, return true.
If symbol is a ReservedWord, return false.
If symbol is an Identifier and StringValue of symbol is the same value as the StringValue of IdentifierName, return true.
Return false.

12.3.1.4 Static Semantics: IsFunctionDefinition

[

]

new

new

Return false.

12.3.1.5 Static Semantics: IsDestructuring

If PrimaryExpression is either an ObjectLiteral or an ArrayLiteral, return true.
Return false.

[

]

new

new

Return false.

12.3.1.6 Static Semantics: IsIdentifierRef

[

]

new

new

Return false.

12.3.1.7 Static Semantics: AssignmentTargetType

[

]

[

]

Return simple.

new

new

new

target

import

meta

Return invalid.

12.3.2 Property Accessors

Note

Properties are accessed by name, using either the dot notation:

.

.

or the bracket notation:

[

]

[

]

The dot notation is explained by the following syntactic conversion:

.

is identical in its behaviour to

[ <identifier-name-string> ]

and similarly

.

is identical in its behaviour to

[ <identifier-name-string> ]

where <identifier-name-string> is the result of evaluating StringValue of IdentifierName.

12.3.2.1 Runtime Semantics: Evaluation

[

]

Let baseReference be the result of evaluating MemberExpression.
Let baseValue be ? GetValue(baseReference).
If the code matched by this MemberExpression is strict mode code, let strict be true; else let strict be false.
Return ? EvaluatePropertyAccessWithExpressionKey(baseValue, Expression, strict).

Let baseReference be the result of evaluating MemberExpression.
Let baseValue be ? GetValue(baseReference).
If the code matched by this MemberExpression is strict mode code, let strict be true; else let strict be false.
Return ? EvaluatePropertyAccessWithIdentifierKey(baseValue, IdentifierName, strict).

[

]

Let baseReference be the result of evaluating CallExpression.
Let baseValue be ? GetValue(baseReference).
If the code matched by this CallExpression is strict mode code, let strict be true; else let strict be false.
Return ? EvaluatePropertyAccessWithExpressionKey(baseValue, Expression, strict).

Let baseReference be the result of evaluating CallExpression.
Let baseValue be ? GetValue(baseReference).
If the code matched by this CallExpression is strict mode code, let strict be true; else let strict be false.
Return ? EvaluatePropertyAccessWithIdentifierKey(baseValue, IdentifierName, strict).

12.3.3 Runtime Semantics: EvaluatePropertyAccessWithExpressionKey ( `baseValue`, `expression`, `strict` )

The abstract operation EvaluatePropertyAccessWithExpressionKey takes arguments baseValue (an ECMAScript language value), expression (a Parse Node), and strict (a Boolean). It performs the following steps when called:

Let propertyNameReference be the result of evaluating expression.
Let propertyNameValue be ? GetValue(propertyNameReference).
Let bv be ? RequireObjectCoercible(baseValue).
Let propertyKey be ? ToPropertyKey(propertyNameValue).
Return a value of type Reference whose base value component is bv, whose referenced name component is propertyKey, and whose strict reference flag is strict.

12.3.4 Runtime Semantics: EvaluatePropertyAccessWithIdentifierKey ( `baseValue`, `identifierName`, `strict` )

The abstract operation EvaluatePropertyAccessWithIdentifierKey takes arguments baseValue (an ECMAScript language value), identifierName (a Parse Node), and strict (a Boolean). It performs the following steps when called:

Assert: identifierName is an IdentifierName.
Let bv be ? RequireObjectCoercible(baseValue).
Let propertyNameString be StringValue of identifierName.
Return a value of type Reference whose base value component is bv, whose referenced name component is propertyNameString, and whose strict reference flag is strict.

12.3.5 The `new` Operator

12.3.5.1 Runtime Semantics: Evaluation

new

Return ? EvaluateNew(NewExpression, empty).

new

Return ? EvaluateNew(MemberExpression, Arguments).

12.3.5.1.1 Runtime Semantics: EvaluateNew ( `constructExpr`, `arguments` )

The abstract operation EvaluateNew takes arguments constructExpr and arguments. It performs the following steps when called:

Assert: constructExpr is either a NewExpression or a MemberExpression.
Assert: arguments is either empty or an Arguments.
Let ref be the result of evaluating constructExpr.
Let constructor be ? GetValue(ref).
If arguments is empty, let argList be a new empty List.
Else,
1. Let argList be ? ArgumentListEvaluation of arguments.
If IsConstructor(constructor) is false, throw a TypeError exception.
Return ? Construct(constructor, argList).

12.3.6 Function Calls

12.3.6.1 Runtime Semantics: Evaluation

Let expr be CoveredCallExpression of CoverCallExpressionAndAsyncArrowHead.
Let memberExpr be the MemberExpression of expr.
Let arguments be the Arguments of expr.
Let ref be the result of evaluating memberExpr.
Let func be ? GetValue(ref).
If Type(ref) is Reference, IsPropertyReference(ref) is false, and GetReferencedName(ref) is "eval", then
1. If SameValue(func, %eval%) is true, then
  1. Let argList be ? ArgumentListEvaluation of arguments.
  2. If argList has no elements, return undefined.
  3. Let evalArg be the first element of argList.
  4. If the source code matching this CallExpression is strict mode code, let strictCaller be true. Otherwise let strictCaller be false.
  5. Let evalRealm be the current Realm Record.
  6. Return ? PerformEval(evalArg, evalRealm, strictCaller, true).
Let thisCall be this CallExpression.
Let tailCall be IsInTailPosition(thisCall).
Return ? EvaluateCall(func, ref, arguments, tailCall).

A CallExpression evaluation that executes step 6.a.vi is a direct eval.

Let ref be the result of evaluating CallExpression.
Let func be ? GetValue(ref).
Let thisCall be this CallExpression.
Let tailCall be IsInTailPosition(thisCall).
Return ? EvaluateCall(func, ref, Arguments, tailCall).

12.3.6.2 Runtime Semantics: EvaluateCall ( `func`, `ref`, `arguments`, `tailPosition` )

The abstract operation EvaluateCall takes arguments func (an ECMAScript language value), ref (an ECMAScript language value), arguments (a Parse Node), and tailPosition (a Boolean). It performs the following steps when called:

If Type(ref) is Reference, then
1. If IsPropertyReference(ref) is true, then
  1. Let thisValue be GetThisValue(ref).
2. Else,
  1. Assert: the base of ref is an Environment Record.
  2. Let refEnv be GetBase(ref).
  3. Let thisValue be refEnv.WithBaseObject().
Else,
1. Let thisValue be undefined.
Let argList be ? ArgumentListEvaluation of arguments.
If Type(func) is not Object, throw a TypeError exception.
If IsCallable(func) is false, throw a TypeError exception.
If tailPosition is true, perform PrepareForTailCall().
Let result be Call(func, thisValue, argList).
Assert: If tailPosition is true, the above call will not return here, but instead evaluation will continue as if the following return has already occurred.
Assert: If result is not an abrupt completion, then Type(result) is an ECMAScript language type.
Return result.

12.3.7 The `super` Keyword

12.3.7.1 Runtime Semantics: Evaluation

SuperProperty

super

[

]

Let env be GetThisEnvironment().
Let actualThis be ? env.GetThisBinding().
Let propertyNameReference be the result of evaluating Expression.
Let propertyNameValue be ? GetValue(propertyNameReference).
Let propertyKey be ? ToPropertyKey(propertyNameValue).
If the code matched by this SuperProperty is strict mode code, let strict be true; else let strict be false.
Return ? MakeSuperPropertyReference(actualThis, propertyKey, strict).

SuperProperty

super

Let env be GetThisEnvironment().
Let actualThis be ? env.GetThisBinding().
Let propertyKey be StringValue of IdentifierName.
If the code matched by this SuperProperty is strict mode code, let strict be true; else let strict be false.
Return ? MakeSuperPropertyReference(actualThis, propertyKey, strict).

SuperCall

super

Let newTarget be GetNewTarget().
Assert: Type(newTarget) is Object.
Let func be ! GetSuperConstructor().
Let argList be ? ArgumentListEvaluation of Arguments.
If IsConstructor(func) is false, throw a TypeError exception.
Let result be ? Construct(func, argList, newTarget).
Let thisER be GetThisEnvironment().
Return ? thisER.BindThisValue(result).

12.3.7.2 Runtime Semantics: GetSuperConstructor ( )

The abstract operation GetSuperConstructor takes no arguments. It performs the following steps when called:

Let envRec be GetThisEnvironment().
Assert: envRec is a function Environment Record.
Let activeFunction be envRec.[[FunctionObject]].
Assert: activeFunction is an ECMAScript function object.
Let superConstructor be ! activeFunction.[[GetPrototypeOf]]().
Return superConstructor.

12.3.7.3 Runtime Semantics: MakeSuperPropertyReference ( `actualThis`, `propertyKey`, `strict` )

The abstract operation MakeSuperPropertyReference takes arguments actualThis, propertyKey, and strict. It performs the following steps when called:

Let env be GetThisEnvironment().
Assert: env.HasSuperBinding() is true.
Let baseValue be ? env.GetSuperBase().
Let bv be ? RequireObjectCoercible(baseValue).
Return a value of type Reference that is a Super Reference whose base value component is bv, whose referenced name component is propertyKey, whose thisValue component is actualThis, and whose strict reference flag is strict.

12.3.8 Argument Lists

Note

The evaluation of an argument list produces a List of values.

12.3.8.1 Runtime Semantics: ArgumentListEvaluation

(

)

Return a new empty List.

Let ref be the result of evaluating AssignmentExpression.
Let arg be ? GetValue(ref).
Return a List whose sole item is arg.

...

Let list be a new empty List.
Let spreadRef be the result of evaluating AssignmentExpression.
Let spreadObj be ? GetValue(spreadRef).
Let iteratorRecord be ? GetIterator(spreadObj).
Repeat,
1. Let next be ? IteratorStep(iteratorRecord).
2. If next is false, return list.
3. Let nextArg be ? IteratorValue(next).
4. Append nextArg as the last element of list.

Let precedingArgs be ? ArgumentListEvaluation of ArgumentList.
Let ref be the result of evaluating AssignmentExpression.
Let arg be ? GetValue(ref).
Append arg to the end of precedingArgs.
Return precedingArgs.

...

Let precedingArgs be ? ArgumentListEvaluation of ArgumentList.
Let spreadRef be the result of evaluating AssignmentExpression.
Let iteratorRecord be ? GetIterator(? GetValue(spreadRef)).
Repeat,
1. Let next be ? IteratorStep(iteratorRecord).
2. If next is false, return precedingArgs.
3. Let nextArg be ? IteratorValue(next).
4. Append nextArg as the last element of precedingArgs.

12.3.9 Optional Chains

Note

An optional chain is a chain of one or more property accesses and function calls, the first of which begins with the token ?..

12.3.9.1 Runtime Semantics: Evaluation

Let baseReference be the result of evaluating MemberExpression.
Let baseValue be ? GetValue(baseReference).
If baseValue is undefined or null, then
1. Return undefined.
Return the result of performing ChainEvaluation of OptionalChain with arguments baseValue and baseReference.

Let baseReference be the result of evaluating CallExpression.
Let baseValue be ? GetValue(baseReference).
If baseValue is undefined or null, then
1. Return undefined.
Return the result of performing ChainEvaluation of OptionalChain with arguments baseValue and baseReference.

Let baseReference be the result of evaluating OptionalExpression.
Let baseValue be ? GetValue(baseReference).
If baseValue is undefined or null, then
1. Return undefined.
Return the result of performing ChainEvaluation of OptionalChain with arguments baseValue and baseReference.

12.3.9.2 Runtime Semantics: ChainEvaluation

With parameters baseValue and baseReference.

Let thisChain be this OptionalChain.
Let tailCall be IsInTailPosition(thisChain).
Return ? EvaluateCall(baseValue, baseReference, Arguments, tailCall).

[

]

If the code matched by this OptionalChain is strict mode code, let strict be true; else let strict be false.
Return ? EvaluatePropertyAccessWithExpressionKey(baseValue, Expression, strict).

If the code matched by this OptionalChain is strict mode code, let strict be true; else let strict be false.
Return ? EvaluatePropertyAccessWithIdentifierKey(baseValue, IdentifierName, strict).

Let optionalChain be OptionalChain.
Let newReference be ? ChainEvaluation of optionalChain with arguments baseValue and baseReference.
Let newValue be ? GetValue(newReference).
Let thisChain be this OptionalChain.
Let tailCall be IsInTailPosition(thisChain).
Return ? EvaluateCall(newValue, newReference, Arguments, tailCall).

[

]

Let optionalChain be OptionalChain.
Let newReference be ? ChainEvaluation of optionalChain with arguments baseValue and baseReference.
Let newValue be ? GetValue(newReference).
If the code matched by this OptionalChain is strict mode code, let strict be true; else let strict be false.
Return ? EvaluatePropertyAccessWithExpressionKey(newValue, Expression, strict).

Let optionalChain be OptionalChain.
Let newReference be ? ChainEvaluation of optionalChain with arguments baseValue and baseReference.
Let newValue be ? GetValue(newReference).
If the code matched by this OptionalChain is strict mode code, let strict be true; else let strict be false.
Return ? EvaluatePropertyAccessWithIdentifierKey(newValue, IdentifierName, strict).

12.3.10 Import Calls

12.3.10.1 Runtime Semantics: Evaluation

ImportCall

import

(

)

Let referencingScriptOrModule be ! GetActiveScriptOrModule().
Let argRef be the result of evaluating AssignmentExpression.
Let specifier be ? GetValue(argRef).
Let promiseCapability be ! NewPromiseCapability(%Promise%).
Let specifierString be ToString(specifier).
IfAbruptRejectPromise(specifierString, promiseCapability).
Perform ! HostImportModuleDynamically(referencingScriptOrModule, specifierString, promiseCapability).
Return promiseCapability.[[Promise]].

12.3.11 Tagged Templates

Note

A tagged template is a function call where the arguments of the call are derived from a TemplateLiteral (12.2.9). The actual arguments include a template object (12.2.9.4) and the values produced by evaluating the expressions embedded within the TemplateLiteral.

12.3.11.1 Runtime Semantics: Evaluation

Let tagRef be the result of evaluating MemberExpression.
Let tagFunc be ? GetValue(tagRef).
Let thisCall be this MemberExpression.
Let tailCall be IsInTailPosition(thisCall).
Return ? EvaluateCall(tagFunc, tagRef, TemplateLiteral, tailCall).

Let tagRef be the result of evaluating CallExpression.
Let tagFunc be ? GetValue(tagRef).
Let thisCall be this CallExpression.
Let tailCall be IsInTailPosition(thisCall).
Return ? EvaluateCall(tagFunc, tagRef, TemplateLiteral, tailCall).

12.3.12 Meta Properties

12.3.12.1 Runtime Semantics: Evaluation

NewTarget

new

target

Return GetNewTarget().

import

meta

Let module be ! GetActiveScriptOrModule().
Assert: module is a Source Text Module Record.
Let importMeta be module.[[ImportMeta]].
If importMeta is empty, then
1. Set importMeta to ! OrdinaryObjectCreate(null).
2. Let importMetaValues be ! HostGetImportMetaProperties(module).
3. For each Record { [[Key]], [[Value]] } p that is an element of importMetaValues, do
  1. Perform ! CreateDataPropertyOrThrow(importMeta, p.[[Key]], p.[[Value]]).
4. Perform ! HostFinalizeImportMeta(importMeta, module).
5. Set module.[[ImportMeta]] to importMeta.
6. Return importMeta.
Else,
1. Assert: Type(importMeta) is Object.
2. Return importMeta.

12.3.12.1.1 Runtime Semantics: HostGetImportMetaProperties ( `moduleRecord` )

HostGetImportMetaProperties is an implementation-defined abstract operation that allows hosts to provide property keys and values for the object returned from import.meta.

The implementation of HostGetImportMetaProperties must conform to the following requirements:

It must return a List, whose values are all Records with two fields, [[Key]] and [[Value]].
Each such Record's [[Key]] field must be a property key, i.e., IsPropertyKey must return true when applied to it.
Each such Record's [[Value]] field must be an ECMAScript value.
It must always complete normally (i.e., not return an abrupt completion).

The default implementation of HostGetImportMetaProperties is to return a new empty List.

12.3.12.1.2 Runtime Semantics: HostFinalizeImportMeta ( `importMeta`, `moduleRecord` )

HostFinalizeImportMeta is an implementation-defined abstract operation that allows hosts to perform any extraordinary operations to prepare the object returned from import.meta.

Most hosts will be able to simply define HostGetImportMetaProperties, and leave HostFinalizeImportMeta with its default behavior. However, HostFinalizeImportMeta provides an "escape hatch" for hosts which need to directly manipulate the object before it is exposed to ECMAScript code.

The implementation of HostFinalizeImportMeta must conform to the following requirements:

It must always complete normally (i.e., not return an abrupt completion).

The default implementation of HostFinalizeImportMeta is to return NormalCompletion(empty).

12.4 Update Expressions

Syntax

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[no LineTerminator here]

[?Yield, ?Await]

[no LineTerminator here]

[?Yield, ?Await]

[?Yield, ?Await]

12.4.1 Static Semantics: Early Errors

It is an early Syntax Error if AssignmentTargetType of LeftHandSideExpression is not simple.

It is an early Syntax Error if AssignmentTargetType of UnaryExpression is not simple.

12.4.2 Static Semantics: IsFunctionDefinition

Return false.

12.4.3 Static Semantics: AssignmentTargetType

Return invalid.

12.4.4 Postfix Increment Operator

12.4.4.1 Runtime Semantics: Evaluation

Let lhs be the result of evaluating LeftHandSideExpression.
Let oldValue be ? ToNumeric(? GetValue(lhs)).
Let newValue be ! Type(oldValue)::add(oldValue, Type(oldValue)::unit).
Perform ? PutValue(lhs, newValue).
Return oldValue.

12.4.5 Postfix Decrement Operator

12.4.5.1 Runtime Semantics: Evaluation

Let lhs be the result of evaluating LeftHandSideExpression.
Let oldValue be ? ToNumeric(? GetValue(lhs)).
Let newValue be ! Type(oldValue)::subtract(oldValue, Type(oldValue)::unit).
Perform ? PutValue(lhs, newValue).
Return oldValue.

12.4.6 Prefix Increment Operator

12.4.6.1 Runtime Semantics: Evaluation

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ? ToNumeric(? GetValue(expr)).
Let newValue be ! Type(oldValue)::add(oldValue, Type(oldValue)::unit).
Perform ? PutValue(expr, newValue).
Return newValue.

12.4.7 Prefix Decrement Operator

12.4.7.1 Runtime Semantics: Evaluation

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ? ToNumeric(? GetValue(expr)).
Let newValue be ! Type(oldValue)::subtract(oldValue, Type(oldValue)::unit).
Perform ? PutValue(expr, newValue).
Return newValue.

12.5 Unary Operators

Syntax

[Yield, Await]

[?Yield, ?Await]

delete

[?Yield, ?Await]

void

[?Yield, ?Await]

typeof

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[+Await]

AwaitExpression

[?Yield]

12.5.1 Static Semantics: IsFunctionDefinition

delete

void

typeof

Return false.

12.5.2 Static Semantics: AssignmentTargetType

delete

void

typeof

Return invalid.

12.5.3 The `delete` Operator

12.5.3.1 Static Semantics: Early Errors

delete

It is a Syntax Error if the UnaryExpression is contained in strict mode code and the derived UnaryExpression is PrimaryExpression:IdentifierReference .
It is a Syntax Error if the derived UnaryExpression is
PrimaryExpression:CoverParenthesizedExpressionAndArrowParameterList
and CoverParenthesizedExpressionAndArrowParameterList ultimately derives a phrase that, if used in place of UnaryExpression, would produce a Syntax Error according to these rules. This rule is recursively applied.

Note

The last rule means that expressions such as delete (((foo))) produce early errors because of recursive application of the first rule.

12.5.3.2 Runtime Semantics: Evaluation

delete

Let ref be the result of evaluating UnaryExpression.
ReturnIfAbrupt(ref).
If Type(ref) is not Reference, return true.
If IsUnresolvableReference(ref) is true, then
1. Assert: IsStrictReference(ref) is false.
2. Return true.
If IsPropertyReference(ref) is true, then
1. If IsSuperReference(ref) is true, throw a ReferenceError exception.
2. Let baseObj be ! ToObject(GetBase(ref)).
3. Let deleteStatus be ? baseObj.[[Delete]](GetReferencedName(ref)).
4. If deleteStatus is false and IsStrictReference(ref) is true, throw a TypeError exception.
5. Return deleteStatus.
Else,
1. Assert: ref is a Reference to an Environment Record binding.
2. Let bindings be GetBase(ref).
3. Return ? bindings.DeleteBinding(GetReferencedName(ref)).

Note

When a delete operator occurs within strict mode code, a SyntaxError exception is thrown if its UnaryExpression is a direct reference to a variable, function argument, or function name. In addition, if a delete operator occurs within strict mode code and the property to be deleted has the attribute { [[Configurable]]: false }, a TypeError exception is thrown.

12.5.4 The `void` Operator

12.5.4.1 Runtime Semantics: Evaluation

void

Let expr be the result of evaluating UnaryExpression.
Perform ? GetValue(expr).
Return undefined.

Note

GetValue must be called even though its value is not used because it may have observable side-effects.

12.5.5 The `typeof` Operator

12.5.5.1 Runtime Semantics: Evaluation

typeof

Let val be the result of evaluating UnaryExpression.
If Type(val) is Reference, then
1. If IsUnresolvableReference(val) is true, return "undefined".
Set val to ? GetValue(val).
Return a String according to Table 35.

Table 35: typeof Operator Results

Type of `val`	Result
Undefined	"undefined"
Null	"object"
Boolean	"boolean"
Number	"number"
String	"string"
Symbol	"symbol"
BigInt	"bigint"
Object (does not implement [[Call]])	"object"
Object (implements [[Call]])	"function"

12.5.6 Unary `+` Operator

Note

The unary + operator converts its operand to Number type.

12.5.6.1 Runtime Semantics: Evaluation

Let expr be the result of evaluating UnaryExpression.
Return ? ToNumber(? GetValue(expr)).

12.5.7 Unary `-` Operator

Note

The unary - operator converts its operand to Number type and then negates it. Negating +0 produces -0, and negating -0 produces +0.

12.5.7.1 Runtime Semantics: Evaluation

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ? ToNumeric(? GetValue(expr)).
Let T be Type(oldValue).
Return ! T::unaryMinus(oldValue).

12.5.8 Bitwise NOT Operator ( `~` )

12.5.8.1 Runtime Semantics: Evaluation

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ? ToNumeric(? GetValue(expr)).
Let T be Type(oldValue).
Return ! T::bitwiseNOT(oldValue).

12.5.9 Logical NOT Operator ( `!` )

12.5.9.1 Runtime Semantics: Evaluation

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ! ToBoolean(? GetValue(expr)).
If oldValue is true, return false.
Return true.

12.6 Exponentiation Operator

Syntax

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

12.6.1 Static Semantics: IsFunctionDefinition

Return false.

12.6.2 Static Semantics: AssignmentTargetType

Return invalid.

12.6.3 Runtime Semantics: Evaluation

Return ? EvaluateStringOrNumericBinaryExpression(UpdateExpression, **, ExponentiationExpression).

12.7 Multiplicative Operators

Syntax

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

one of

12.7.1 Static Semantics: IsFunctionDefinition

Return false.

12.7.2 Static Semantics: AssignmentTargetType

Return invalid.

12.7.3 Runtime Semantics: Evaluation

Let opText be the source text matched by MultiplicativeOperator.
Return ? EvaluateStringOrNumericBinaryExpression(MultiplicativeExpression, opText, ExponentiationExpression).

12.8 Additive Operators

Syntax

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

12.8.1 Static Semantics: IsFunctionDefinition

Return false.

12.8.2 Static Semantics: AssignmentTargetType

Return invalid.

12.8.3 The Addition Operator ( `+` )

Note

The addition operator either performs string concatenation or numeric addition.

12.8.3.1 Runtime Semantics: Evaluation

Return ? EvaluateStringOrNumericBinaryExpression(AdditiveExpression, +, MultiplicativeExpression).

12.8.4 The Subtraction Operator ( `-` )

12.8.4.1 Runtime Semantics: Evaluation

Return ? EvaluateStringOrNumericBinaryExpression(AdditiveExpression, -, MultiplicativeExpression).

12.9 Bitwise Shift Operators

Syntax

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

>>>

[?Yield, ?Await]

12.9.1 Static Semantics: IsFunctionDefinition

>>>

Return false.

12.9.2 Static Semantics: AssignmentTargetType

>>>

Return invalid.

12.9.3 The Left Shift Operator ( `<<` )

Note

Performs a bitwise left shift operation on the left operand by the amount specified by the right operand.

12.9.3.1 Runtime Semantics: Evaluation

Return ? EvaluateStringOrNumericBinaryExpression(ShiftExpression, <<, AdditiveExpression).

12.9.4 The Signed Right Shift Operator ( `>>` )

Note

Performs a sign-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

12.9.4.1 Runtime Semantics: Evaluation

Return ? EvaluateStringOrNumericBinaryExpression(ShiftExpression, >>, AdditiveExpression).

12.9.5 The Unsigned Right Shift Operator ( `>>>` )

Note

Performs a zero-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

12.9.5.1 Runtime Semantics: Evaluation

>>>

Return ? EvaluateStringOrNumericBinaryExpression(ShiftExpression, >>>, AdditiveExpression).

12.10 Relational Operators

Note 1

The result of evaluating a relational operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.

Syntax

[In, Yield, Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

instanceof

[?Yield, ?Await]

[+In]

[+In, ?Yield, ?Await]

[?Yield, ?Await]

Note 2

The _[In] grammar parameter is needed to avoid confusing the in operator in a relational expression with the in operator in a for statement.

12.10.1 Static Semantics: IsFunctionDefinition

instanceof

Return false.

12.10.2 Static Semantics: AssignmentTargetType

instanceof

Return invalid.

12.10.3 Runtime Semantics: Evaluation

Let lref be the result of evaluating RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be ? GetValue(rref).
Let r be the result of performing Abstract Relational Comparison lval < rval.
ReturnIfAbrupt(r).
If r is undefined, return false. Otherwise, return r.

Let lref be the result of evaluating RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be ? GetValue(rref).
Let r be the result of performing Abstract Relational Comparison rval < lval with LeftFirst equal to false.
ReturnIfAbrupt(r).
If r is undefined, return false. Otherwise, return r.

Let lref be the result of evaluating RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be ? GetValue(rref).
Let r be the result of performing Abstract Relational Comparison rval < lval with LeftFirst equal to false.
ReturnIfAbrupt(r).
If r is true or undefined, return false. Otherwise, return true.

Let lref be the result of evaluating RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be ? GetValue(rref).
Let r be the result of performing Abstract Relational Comparison lval < rval.
ReturnIfAbrupt(r).
If r is true or undefined, return false. Otherwise, return true.

instanceof

Let lref be the result of evaluating RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be ? GetValue(rref).
Return ? InstanceofOperator(lval, rval).

Let lref be the result of evaluating RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be ? GetValue(rref).
If Type(rval) is not Object, throw a TypeError exception.
Return ? HasProperty(rval, ? ToPropertyKey(lval)).

12.10.4 Runtime Semantics: InstanceofOperator ( `V`, `target` )

The abstract operation InstanceofOperator takes arguments V (an ECMAScript language value) and target (an ECMAScript language value). It implements the generic algorithm for determining if V is an instance of target either by consulting target's @@hasInstance method or, if absent, determining whether the value of target's "prototype" property is present in V's prototype chain. It performs the following steps when called:

If Type(target) is not Object, throw a TypeError exception.
Let instOfHandler be ? GetMethod(target, @@hasInstance).
If instOfHandler is not undefined, then
1. Return ! ToBoolean(? Call(instOfHandler, target, « V »)).
If IsCallable(target) is false, throw a TypeError exception.
Return ? OrdinaryHasInstance(target, V).

Note

Steps 4 and 5 provide compatibility with previous editions of ECMAScript that did not use a @@hasInstance method to define the instanceof operator semantics. If an object does not define or inherit @@hasInstance it uses the default instanceof semantics.

12.11 Equality Operators

Note

The result of evaluating an equality operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.

Syntax

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

===

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

!==

[?In, ?Yield, ?Await]

12.11.1 Static Semantics: IsFunctionDefinition

===

!==

Return false.

12.11.2 Static Semantics: AssignmentTargetType

===

!==

Return invalid.

12.11.3 Runtime Semantics: Evaluation

Let lref be the result of evaluating EqualityExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating RelationalExpression.
Let rval be ? GetValue(rref).
Return the result of performing Abstract Equality Comparison rval == lval.

Let lref be the result of evaluating EqualityExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating RelationalExpression.
Let rval be ? GetValue(rref).
Let r be the result of performing Abstract Equality Comparison rval == lval.
ReturnIfAbrupt(r).
If r is true, return false. Otherwise, return true.

===

Let lref be the result of evaluating EqualityExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating RelationalExpression.
Let rval be ? GetValue(rref).
Return the result of performing Strict Equality Comparison rval === lval.

!==

Let lref be the result of evaluating EqualityExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating RelationalExpression.
Let rval be ? GetValue(rref).
Let r be the result of performing Strict Equality Comparison rval === lval.
Assert: r is a normal completion.
If r.[[Value]] is true, return false. Otherwise, return true.

Note 1

Given the above definition of equality:

String comparison can be forced by: `${a}` == `${b}`.
Numeric comparison can be forced by: +a == +b.
Boolean comparison can be forced by: !a == !b.

Note 2

The equality operators maintain the following invariants:

A != B is equivalent to !(A == B).
A == B is equivalent to B == A, except in the order of evaluation of A and B.

Note 3

The equality operator is not always transitive. For example, there might be two distinct String objects, each representing the same String value; each String object would be considered equal to the String value by the == operator, but the two String objects would not be equal to each other. For example:

new String("a") == "a" and "a" == new String("a") are both true.
new String("a") == new String("a") is false.

Note 4

Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore Strings values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form.

12.12 Binary Bitwise Operators

Syntax

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

12.12.1 Static Semantics: IsFunctionDefinition

Return false.

12.12.2 Static Semantics: AssignmentTargetType

Return invalid.

12.12.3 Runtime Semantics: Evaluation

Return ? EvaluateStringOrNumericBinaryExpression(BitwiseANDExpression, &, EqualityExpression).

Return ? EvaluateStringOrNumericBinaryExpression(BitwiseXORExpression, ^, BitwiseANDExpression).

Return ? EvaluateStringOrNumericBinaryExpression(BitwiseORExpression, |, BitwiseXORExpression).

12.13 Binary Logical Operators

Syntax

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

Note

The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.

12.13.1 Static Semantics: IsFunctionDefinition

Return false.

12.13.2 Static Semantics: AssignmentTargetType

Return invalid.

12.13.3 Runtime Semantics: Evaluation

Let lref be the result of evaluating LogicalANDExpression.
Let lval be ? GetValue(lref).
Let lbool be ! ToBoolean(lval).
If lbool is false, return lval.
Let rref be the result of evaluating BitwiseORExpression.
Return ? GetValue(rref).

Let lref be the result of evaluating LogicalORExpression.
Let lval be ? GetValue(lref).
Let lbool be ! ToBoolean(lval).
If lbool is true, return lval.
Let rref be the result of evaluating LogicalANDExpression.
Return ? GetValue(rref).

Let lref be the result of evaluating CoalesceExpressionHead.
Let lval be ? GetValue(lref).
If lval is undefined or null, then
1. Let rref be the result of evaluating BitwiseORExpression.
2. Return ? GetValue(rref).
Otherwise, return lval.

12.14 Conditional Operator ( `? :` )

Syntax

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[+In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

Note

The grammar for a ConditionalExpression in ECMAScript is slightly different from that in C and Java, which each allow the second subexpression to be an Expression but restrict the third expression to be a ConditionalExpression. The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the centre expression.

12.14.1 Static Semantics: IsFunctionDefinition

Return false.

12.14.2 Static Semantics: AssignmentTargetType

Return invalid.

12.14.3 Runtime Semantics: Evaluation

Let lref be the result of evaluating ShortCircuitExpression.
Let lval be ! ToBoolean(? GetValue(lref)).
If lval is true, then
1. Let trueRef be the result of evaluating the first AssignmentExpression.
2. Return ? GetValue(trueRef).
Else,
1. Let falseRef be the result of evaluating the second AssignmentExpression.
2. Return ? GetValue(falseRef).

12.15 Assignment Operators

Syntax

[In, Yield, Await]

[?In, ?Yield, ?Await]

[+Yield]

[?In, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

one of

<<=

>>=

>>>=

**=

12.15.1 Static Semantics: Early Errors

If LeftHandSideExpression is an ObjectLiteral or an ArrayLiteral, the following Early Error rules are applied:

It is a Syntax Error if LeftHandSideExpression is not covering an AssignmentPattern.
All Early Error rules for AssignmentPattern and its derived productions also apply to the AssignmentPattern that is covered by LeftHandSideExpression.

If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, the following Early Error rule is applied:

It is a Syntax Error if AssignmentTargetType of LeftHandSideExpression is not simple.

It is a Syntax Error if AssignmentTargetType of LeftHandSideExpression is not simple.

12.15.2 Static Semantics: IsFunctionDefinition

Return true.

Return false.

12.15.3 Static Semantics: AssignmentTargetType

Return invalid.

12.15.4 Runtime Semantics: Evaluation

If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, then
1. Let lref be the result of evaluating LeftHandSideExpression.
2. ReturnIfAbrupt(lref).
3. If IsAnonymousFunctionDefinition(AssignmentExpression) and IsIdentifierRef of LeftHandSideExpression are both true, then
  1. Let rval be NamedEvaluation of AssignmentExpression with argument GetReferencedName(lref).
4. Else,
  1. Let rref be the result of evaluating AssignmentExpression.
  2. Let rval be ? GetValue(rref).
5. Perform ? PutValue(lref, rval).
6. Return rval.
Let assignmentPattern be the AssignmentPattern that is covered by LeftHandSideExpression.
Let rref be the result of evaluating AssignmentExpression.
Let rval be ? GetValue(rref).
Perform ? DestructuringAssignmentEvaluation of assignmentPattern using rval as the argument.
Return rval.

Let lref be the result of evaluating LeftHandSideExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating AssignmentExpression.
Let rval be ? GetValue(rref).
Let assignmentOpText be the source text matched by AssignmentOperator.
Let opText be the sequence of Unicode code points associated with assignmentOpText in the following table:

assignmentOpText opText

**= **

*= *

/= /

%= %

+= +

-= -

<<= <<

>>= >>

>>>= >>>

&= &

^= ^

|= |
Let r be ApplyStringOrNumericBinaryOperator(lval, opText, rval).
Perform ? PutValue(lref, r).
Return r.

`assignmentOpText`	`opText`
`**=`	`**`
`*=`	`*`
`/=`	`/`
`%=`	`%`
`+=`	`+`
`-=`	`-`
`<<=`	`<<`
`>>=`	`>>`
`>>>=`	`>>>`
`&=`	`&`
`^=`	`^`
`\|=`	`\|`

Note

When an assignment occurs within strict mode code, it is a runtime error if lref in step 1.e of the first algorithm or step 8 of the second algorithm is an unresolvable reference. If it is, a ReferenceError exception is thrown. The LeftHandSideExpression also may not be a reference to a data property with the attribute value { [[Writable]]: false }, to an accessor property with the attribute value { [[Set]]: undefined }, nor to a non-existent property of an object for which the IsExtensible predicate returns the value false. In these cases a TypeError exception is thrown.

12.15.5 Runtime Semantics: ApplyStringOrNumericBinaryOperator ( `lval`, `opText`, `rval` )

The abstract operation ApplyStringOrNumericBinaryOperator takes arguments lval (an ECMAScript language value), opText (a sequence of Unicode code points), and rval (an ECMAScript language value). It performs the following steps when called:

If opText is +, then
1. Let lprim be ? ToPrimitive(lval).
2. Let rprim be ? ToPrimitive(rval).
3. If Type(lprim) is String or Type(rprim) is String, then
  1. Let lstr be ? ToString(lprim).
  2. Let rstr be ? ToString(rprim).
  3. Return the string-concatenation of lstr and rstr.
4. Set lval to lprim.
5. Set rval to rprim.
NOTE: At this point, it must be a numeric operation.
Let lnum be ? ToNumeric(lval).
Let rnum be ? ToNumeric(rval).
If Type(lnum) is different from Type(rnum), throw a TypeError exception.
Let T be Type(lnum).

Let operation be the abstract operation associated with opText in the following table:

`opText`	`operation`
`**`	`T`::exponentiate
`*`	`T`::multiply
`/`	`T`::divide
`%`	`T`::remainder
`+`	`T`::add
`-`	`T`::subtract
`<<`	`T`::leftShift
`>>`	`T`::signedRightShift
`>>>`	`T`::unsignedRightShift
`&`	`T`::bitwiseAND
`^`	`T`::bitwiseXOR
`\|`	`T`::bitwiseOR

Return ? operation(lnum, rnum).

Note 1

No hint is provided in the calls to ToPrimitive in steps 1.a and 1.b. All standard objects except Date objects handle the absence of a hint as if the hint Number were given; Date objects handle the absence of a hint as if the hint String were given. Exotic objects may handle the absence of a hint in some other manner.

Note 2

Step 1.c differs from step 3 of the Abstract Relational Comparison algorithm, by using the logical-or operation instead of the logical-and operation.

12.15.6 Runtime Semantics: EvaluateStringOrNumericBinaryExpression ( `leftOperand`, `opText`, `rightOperand` )

The abstract operation EvaluateStringOrNumericBinaryExpression takes arguments leftOperand (a Parse Node), opText (a sequence of Unicode code points), and rightOperand (a Parse Node). It performs the following steps when called:

Let lref be the result of evaluating leftOperand.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating rightOperand.
Let rval be ? GetValue(rref).
Return ? ApplyStringOrNumericBinaryOperator(lval, opText, rval).

12.15.7 Destructuring Assignment

Supplemental Syntax

In certain circumstances when processing an instance of the production AssignmentExpression:LeftHandSideExpression=AssignmentExpression the following grammar is used to refine the interpretation of LeftHandSideExpression.

AssignmentPattern

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

{

}

{

[?Yield, ?Await]

}

{

[?Yield, ?Await]

}

{

[?Yield, ?Await]

[?Yield, ?Await]

opt

}

[Yield, Await]

[

opt

[?Yield, ?Await]

opt

]

[

[?Yield, ?Await]

]

[

[?Yield, ?Await]

opt

[?Yield, ?Await]

opt

]

[Yield, Await]

...

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

opt

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

[Yield, Await]

...

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

12.15.7.1 Static Semantics: Early Errors

opt

It is a Syntax Error if AssignmentTargetType of IdentifierReference is not simple.

...

It is a Syntax Error if DestructuringAssignmentTarget is an ArrayLiteral or an ObjectLiteral.

If LeftHandSideExpression is an ObjectLiteral or an ArrayLiteral, the following Early Error rules are applied:

It is a Syntax Error if LeftHandSideExpression is not covering an AssignmentPattern.
All Early Error rules for AssignmentPattern and its derived productions also apply to the AssignmentPattern that is covered by LeftHandSideExpression.

If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, the following Early Error rule is applied:

It is a Syntax Error if AssignmentTargetType of LeftHandSideExpression is not simple.

12.15.7.2 Runtime Semantics: DestructuringAssignmentEvaluation

With parameter value.

{

}

Perform ? RequireObjectCoercible(value).
Return NormalCompletion(empty).

{

}

{

}

Perform ? RequireObjectCoercible(value).
Perform ? PropertyDestructuringAssignmentEvaluation for AssignmentPropertyList using value as the argument.
Return NormalCompletion(empty).

[

]

Let iteratorRecord be ? GetIterator(value).
Return ? IteratorClose(iteratorRecord, NormalCompletion(empty)).

[

]

Let iteratorRecord be ? GetIterator(value).
Let result be IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
Return result.

[

opt

]

Let iteratorRecord be ? GetIterator(value).
If Elision is present, then
1. Let status be IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
2. If status is an abrupt completion, then
  1. Assert: iteratorRecord.[[Done]] is true.
  2. Return Completion(status).
Let result be IteratorDestructuringAssignmentEvaluation of AssignmentRestElement with argument iteratorRecord.
If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
Return result.

[

]

Let iteratorRecord be ? GetIterator(value).
Let result be IteratorDestructuringAssignmentEvaluation of AssignmentElementList with argument iteratorRecord.
If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
Return result.

[

opt

opt

]

Let iteratorRecord be ? GetIterator(value).
Let status be IteratorDestructuringAssignmentEvaluation of AssignmentElementList with argument iteratorRecord.
If status is an abrupt completion, then
1. If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, status).
2. Return Completion(status).
If Elision is present, then
1. Set status to the result of performing IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
2. If status is an abrupt completion, then
  1. Assert: iteratorRecord.[[Done]] is true.
  2. Return Completion(status).
If AssignmentRestElement is present, then
1. Set status to the result of performing IteratorDestructuringAssignmentEvaluation of AssignmentRestElement with iteratorRecord as the argument.
If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, status).
Return Completion(status).

{

}

Perform ? RequireObjectCoercible(value).
Let excludedNames be a new empty List.
Return the result of performing RestDestructuringAssignmentEvaluation of AssignmentRestProperty with value and excludedNames as the arguments.

{

}

Perform ? RequireObjectCoercible(value).
Let excludedNames be ? PropertyDestructuringAssignmentEvaluation of AssignmentPropertyList with argument value.
Return the result of performing RestDestructuringAssignmentEvaluation of AssignmentRestProperty with arguments value and excludedNames.

12.15.7.3 Runtime Semantics: PropertyDestructuringAssignmentEvaluation

With parameter value.

Note

The following operations collect a list of all destructured property names.

Let propertyNames be ? PropertyDestructuringAssignmentEvaluation of AssignmentPropertyList with argument value.
Let nextNames be ? PropertyDestructuringAssignmentEvaluation of AssignmentProperty with argument value.
Append each item in nextNames to the end of propertyNames.
Return propertyNames.

opt

Let P be StringValue of IdentifierReference.
Let lref be ? ResolveBinding(P).
Let v be ? GetV(value, P).
If Initializeropt is present and v is undefined, then
1. If IsAnonymousFunctionDefinition(Initializer) is true, then
  1. Set v to the result of performing NamedEvaluation for Initializer with argument P.
2. Else,
  1. Let defaultValue be the result of evaluating Initializer.
  2. Set v to ? GetValue(defaultValue).
Perform ? PutValue(lref, v).
Return a new List containing P.

Let name be the result of evaluating PropertyName.
ReturnIfAbrupt(name).
Perform ? KeyedDestructuringAssignmentEvaluation of AssignmentElement with value and name as the arguments.
Return a new List containing name.

12.15.7.4 Runtime Semantics: RestDestructuringAssignmentEvaluation

With parameters value and excludedNames.

...

Let lref be the result of evaluating DestructuringAssignmentTarget.
ReturnIfAbrupt(lref).
Let restObj be OrdinaryObjectCreate(%Object.prototype%).
Perform ? CopyDataProperties(restObj, value, excludedNames).
Return PutValue(lref, restObj).

12.15.7.5 Runtime Semantics: IteratorDestructuringAssignmentEvaluation

With parameter iteratorRecord.

Return the result of performing IteratorDestructuringAssignmentEvaluation of AssignmentElisionElement using iteratorRecord as the argument.

Perform ? IteratorDestructuringAssignmentEvaluation of AssignmentElementList using iteratorRecord as the argument.
Return the result of performing IteratorDestructuringAssignmentEvaluation of AssignmentElisionElement using iteratorRecord as the argument.

Return the result of performing IteratorDestructuringAssignmentEvaluation of AssignmentElement with iteratorRecord as the argument.

Perform ? IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
Return the result of performing IteratorDestructuringAssignmentEvaluation of AssignmentElement with iteratorRecord as the argument.

If iteratorRecord.[[Done]] is false, then
1. Let next be IteratorStep(iteratorRecord).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, set iteratorRecord.[[Done]] to true.
Return NormalCompletion(empty).

Perform ? IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
If iteratorRecord.[[Done]] is false, then
1. Let next be IteratorStep(iteratorRecord).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, set iteratorRecord.[[Done]] to true.
Return NormalCompletion(empty).

opt

If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
1. Let lref be the result of evaluating DestructuringAssignmentTarget.
2. ReturnIfAbrupt(lref).
If iteratorRecord.[[Done]] is false, then
1. Let next be IteratorStep(iteratorRecord).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, set iteratorRecord.[[Done]] to true.
5. Else,
  1. Let value be IteratorValue(next).
  2. If value is an abrupt completion, set iteratorRecord.[[Done]] to true.
  3. ReturnIfAbrupt(value).
If iteratorRecord.[[Done]] is true, let value be undefined.
If Initializer is present and value is undefined, then
1. If IsAnonymousFunctionDefinition(Initializer) and IsIdentifierRef of DestructuringAssignmentTarget are both true, then
  1. Let v be NamedEvaluation of Initializer with argument GetReferencedName(lref).
2. Else,
  1. Let defaultValue be the result of evaluating Initializer.
  2. Let v be ? GetValue(defaultValue).
Else, let v be value.
If DestructuringAssignmentTarget is an ObjectLiteral or an ArrayLiteral, then
1. Let nestedAssignmentPattern be the AssignmentPattern that is covered by DestructuringAssignmentTarget.
2. Return the result of performing DestructuringAssignmentEvaluation of nestedAssignmentPattern with v as the argument.
Return ? PutValue(lref, v).

Note

Left to right evaluation order is maintained by evaluating a DestructuringAssignmentTarget that is not a destructuring pattern prior to accessing the iterator or evaluating the Initializer.

...

If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
1. Let lref be the result of evaluating DestructuringAssignmentTarget.
2. ReturnIfAbrupt(lref).
Let A be ! ArrayCreate(0).
Let n be 0.
Repeat, while iteratorRecord.[[Done]] is false,
1. Let next be IteratorStep(iteratorRecord).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, set iteratorRecord.[[Done]] to true.
5. Else,
  1. Let nextValue be IteratorValue(next).
  2. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
  3. ReturnIfAbrupt(nextValue).
  4. Perform ! CreateDataPropertyOrThrow(A, ! ToString(n), nextValue).
  5. Set n to n + 1.
If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
1. Return ? PutValue(lref, A).
Let nestedAssignmentPattern be the AssignmentPattern that is covered by DestructuringAssignmentTarget.
Return the result of performing DestructuringAssignmentEvaluation of nestedAssignmentPattern with A as the argument.

12.15.7.6 Runtime Semantics: KeyedDestructuringAssignmentEvaluation

With parameters value and propertyName.

opt

If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
1. Let lref be the result of evaluating DestructuringAssignmentTarget.
2. ReturnIfAbrupt(lref).
Let v be ? GetV(value, propertyName).
If Initializer is present and v is undefined, then
1. If IsAnonymousFunctionDefinition(Initializer) and IsIdentifierRef of DestructuringAssignmentTarget are both true, then
  1. Let rhsValue be NamedEvaluation of Initializer with argument GetReferencedName(lref).
2. Else,
  1. Let defaultValue be the result of evaluating Initializer.
  2. Let rhsValue be ? GetValue(defaultValue).
Else, let rhsValue be v.
If DestructuringAssignmentTarget is an ObjectLiteral or an ArrayLiteral, then
1. Let assignmentPattern be the AssignmentPattern that is covered by DestructuringAssignmentTarget.
2. Return the result of performing DestructuringAssignmentEvaluation of assignmentPattern with rhsValue as the argument.
Return ? PutValue(lref, rhsValue).

12.16 Comma Operator ( `,` )

Syntax

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

12.16.1 Static Semantics: IsFunctionDefinition

Return false.

12.16.2 Static Semantics: AssignmentTargetType

Return invalid.

12.16.3 Runtime Semantics: Evaluation

Let lref be the result of evaluating Expression.
Perform ? GetValue(lref).
Let rref be the result of evaluating AssignmentExpression.
Return ? GetValue(rref).

Note

GetValue must be called even though its value is not used because it may have observable side-effects.

13 ECMAScript Language: Statements and Declarations

Syntax

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await]

[?Yield, ?Await]

[+Return]

[?Yield, ?Await]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await]

[?Yield, ?Await, ?Return]

[Yield, Await]

[?Yield, ?Await, ~Default]

[?Yield, ?Await, ~Default]

[+In, ?Yield, ?Await]

[Yield, Await, Default]

[?Yield, ?Await, ?Default]

[?Yield, ?Await, ?Default]

[?Yield, ?Await, ?Default]

[?Yield, ?Await, ?Default]

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

13.1 Statement Semantics

13.1.1 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

Return false.

13.1.2 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

Return false.

13.1.3 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

Return false.

Let newIterationSet be a copy of iterationSet with all the elements of labelSet appended.
Return ContainsUndefinedContinueTarget of IterationStatement with arguments newIterationSet and « ».

13.1.4 Static Semantics: DeclarationPart

Return FunctionDeclaration.

Return GeneratorDeclaration.

Return AsyncFunctionDeclaration.

Return AsyncGeneratorDeclaration.

Return ClassDeclaration.

Return LexicalDeclaration.

13.1.5 Static Semantics: VarDeclaredNames

Return a new empty List.

13.1.6 Static Semantics: VarScopedDeclarations

Return a new empty List.

13.1.7 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

Let stmtResult be LabelledEvaluation of IterationStatement with argument labelSet.
If stmtResult.[[Type]] is break, then
1. If stmtResult.[[Target]] is empty, then
  1. If stmtResult.[[Value]] is empty, set stmtResult to NormalCompletion(undefined).
  2. Else, set stmtResult to NormalCompletion(stmtResult.[[Value]]).
Return Completion(stmtResult).

Let stmtResult be the result of evaluating SwitchStatement.
If stmtResult.[[Type]] is break, then
1. If stmtResult.[[Target]] is empty, then
  1. If stmtResult.[[Value]] is empty, set stmtResult to NormalCompletion(undefined).
  2. Else, set stmtResult to NormalCompletion(stmtResult.[[Value]]).
Return Completion(stmtResult).

Note

A BreakableStatement is one that can be exited via an unlabelled BreakStatement.

13.1.8 Runtime Semantics: Evaluation

Return NormalCompletion(empty).

Return the result of evaluating FunctionDeclaration.

Let newLabelSet be a new empty List.
Return the result of performing LabelledEvaluation of this BreakableStatement with argument newLabelSet.

13.2 Block

Syntax

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

{

[?Yield, ?Await, ?Return]

opt

}

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await]

13.2.1 Static Semantics: Early Errors

{

}

It is a Syntax Error if the LexicallyDeclaredNames of StatementList contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of StatementList also occurs in the VarDeclaredNames of StatementList.

13.2.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

{

}

Return false.

Let hasDuplicates be ContainsDuplicateLabels of StatementList with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of StatementListItem with argument labelSet.

Return false.

13.2.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

{

}

Return false.

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of StatementList with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of StatementListItem with argument labelSet.

Return false.

13.2.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

{

}

Return false.

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of StatementList with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of StatementListItem with arguments iterationSet and « ».

Return false.

13.2.5 Static Semantics: LexicallyDeclaredNames

{

}

Return a new empty List.

Let names be LexicallyDeclaredNames of StatementList.
Append to names the elements of the LexicallyDeclaredNames of StatementListItem.
Return names.

If Statement is Statement:LabelledStatement , return LexicallyDeclaredNames of LabelledStatement.
Return a new empty List.

Return the BoundNames of Declaration.

13.2.6 Static Semantics: LexicallyScopedDeclarations

Let declarations be LexicallyScopedDeclarations of StatementList.
Append to declarations the elements of the LexicallyScopedDeclarations of StatementListItem.
Return declarations.

If Statement is Statement:LabelledStatement , return LexicallyScopedDeclarations of LabelledStatement.
Return a new empty List.

Return a new List containing DeclarationPart of Declaration.

13.2.7 Static Semantics: TopLevelLexicallyDeclaredNames

Let names be TopLevelLexicallyDeclaredNames of StatementList.
Append to names the elements of the TopLevelLexicallyDeclaredNames of StatementListItem.
Return names.

Return a new empty List.

If Declaration is Declaration:HoistableDeclaration , then
1. Return « ».
Return the BoundNames of Declaration.

Note

At the top level of a function, or script, function declarations are treated like var declarations rather than like lexical declarations.

13.2.8 Static Semantics: TopLevelLexicallyScopedDeclarations

{

}

Return a new empty List.

Let declarations be TopLevelLexicallyScopedDeclarations of StatementList.
Append to declarations the elements of the TopLevelLexicallyScopedDeclarations of StatementListItem.
Return declarations.

Return a new empty List.

If Declaration is Declaration:HoistableDeclaration , then
1. Return « ».
Return a new List containing Declaration.

13.2.9 Static Semantics: TopLevelVarDeclaredNames

{

}

Return a new empty List.

Let names be TopLevelVarDeclaredNames of StatementList.
Append to names the elements of the TopLevelVarDeclaredNames of StatementListItem.
Return names.

If Declaration is Declaration:HoistableDeclaration , then
1. Return the BoundNames of HoistableDeclaration.
Return a new empty List.

If Statement is Statement:LabelledStatement , return TopLevelVarDeclaredNames of Statement.
Return VarDeclaredNames of Statement.

Note

At the top level of a function or script, inner function declarations are treated like var declarations.

13.2.10 Static Semantics: TopLevelVarScopedDeclarations

{

}

Return a new empty List.

Let declarations be TopLevelVarScopedDeclarations of StatementList.
Append to declarations the elements of the TopLevelVarScopedDeclarations of StatementListItem.
Return declarations.

If Statement is Statement:LabelledStatement , return TopLevelVarScopedDeclarations of Statement.
Return VarScopedDeclarations of Statement.

If Declaration is Declaration:HoistableDeclaration , then
1. Let declaration be DeclarationPart of HoistableDeclaration.
2. Return « declaration ».
Return a new empty List.

13.2.11 Static Semantics: VarDeclaredNames

{

}

Return a new empty List.

Let names be VarDeclaredNames of StatementList.
Append to names the elements of the VarDeclaredNames of StatementListItem.
Return names.

Return a new empty List.

13.2.12 Static Semantics: VarScopedDeclarations

{

}

Return a new empty List.

Let declarations be VarScopedDeclarations of StatementList.
Append to declarations the elements of the VarScopedDeclarations of StatementListItem.
Return declarations.

Return a new empty List.

13.2.13 Runtime Semantics: Evaluation

{

}

Return NormalCompletion(empty).

{

}

Let oldEnv be the running execution context's LexicalEnvironment.
Let blockEnv be NewDeclarativeEnvironment(oldEnv).
Perform BlockDeclarationInstantiation(StatementList, blockEnv).
Set the running execution context's LexicalEnvironment to blockEnv.
Let blockValue be the result of evaluating StatementList.
Set the running execution context's LexicalEnvironment to oldEnv.
Return blockValue.

Note 1

No matter how control leaves the Block the LexicalEnvironment is always restored to its former state.

Let sl be the result of evaluating StatementList.
ReturnIfAbrupt(sl).
Let s be the result of evaluating StatementListItem.
Return Completion(UpdateEmpty(s, sl)).

Note 2

The value of a StatementList is the value of the last value-producing item in the StatementList. For example, the following calls to the eval function all return the value 1:

eval("1;;;;;")
eval("1;{}")
eval("1;var a;")

13.2.14 Runtime Semantics: BlockDeclarationInstantiation ( `code`, `env` )

Note

When a Block or CaseBlock is evaluated a new declarative Environment Record is created and bindings for each block scoped variable, constant, function, or class declared in the block are instantiated in the Environment Record.

The abstract operation BlockDeclarationInstantiation takes arguments code (a Parse Node) and env (an Environment Record). code is the Parse Node corresponding to the body of the block. env is the Environment Record in which bindings are to be created. It performs the following steps when called:

Assert: env is a declarative Environment Record.
Let declarations be the LexicallyScopedDeclarations of code.
For each element d in declarations, do
1. For each element dn of the BoundNames of d, do
  1. If IsConstantDeclaration of d is true, then
    1. Perform ! env.CreateImmutableBinding(dn, true).
  2. Else,
    1. Perform ! env.CreateMutableBinding(dn, false).
2. If d is a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration, then
  1. Let fn be the sole element of the BoundNames of d.
  2. Let fo be InstantiateFunctionObject of d with argument env.
  3. Perform env.InitializeBinding(fn, fo).

13.3 Declarations and the Variable Statement

13.3.1 Let and Const Declarations

Note

let and const declarations define variables that are scoped to the running execution context's LexicalEnvironment. The variables are created when their containing Environment Record is instantiated but may not be accessed in any way until the variable's LexicalBinding is evaluated. A variable defined by a LexicalBinding with an Initializer is assigned the value of its Initializer's AssignmentExpression when the LexicalBinding is evaluated, not when the variable is created. If a LexicalBinding in a let declaration does not have an Initializer the variable is assigned the value undefined when the LexicalBinding is evaluated.

Syntax

[In, Yield, Await]

[?In, ?Yield, ?Await]

;

let

const

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

opt

[?Yield, ?Await]

[?In, ?Yield, ?Await]

13.3.1.1 Static Semantics: Early Errors

;

It is a Syntax Error if the BoundNames of BindingList contains "let".
It is a Syntax Error if the BoundNames of BindingList contains any duplicate entries.

opt

It is a Syntax Error if Initializer is not present and IsConstantDeclaration of the LexicalDeclaration containing this LexicalBinding is true.

13.3.1.2 Static Semantics: BoundNames

;

Return the BoundNames of BindingList.

Let names be the BoundNames of BindingList.
Append to names the elements of the BoundNames of LexicalBinding.
Return names.

opt

Return the BoundNames of BindingIdentifier.

Return the BoundNames of BindingPattern.

13.3.1.3 Static Semantics: IsConstantDeclaration

;

Return IsConstantDeclaration of LetOrConst.

let

Return false.

const

Return true.

13.3.1.4 Runtime Semantics: Evaluation

;

Let next be the result of evaluating BindingList.
ReturnIfAbrupt(next).
Return NormalCompletion(empty).

Let next be the result of evaluating BindingList.
ReturnIfAbrupt(next).
Return the result of evaluating LexicalBinding.

Let lhs be ResolveBinding(StringValue of BindingIdentifier).
Return InitializeReferencedBinding(lhs, undefined).

Note

A static semantics rule ensures that this form of LexicalBinding never occurs in a const declaration.

Let bindingId be StringValue of BindingIdentifier.
Let lhs be ResolveBinding(bindingId).
If IsAnonymousFunctionDefinition(Initializer) is true, then
1. Let value be NamedEvaluation of Initializer with argument bindingId.
Else,
1. Let rhs be the result of evaluating Initializer.
2. Let value be ? GetValue(rhs).
Return InitializeReferencedBinding(lhs, value).

Let rhs be the result of evaluating Initializer.
Let value be ? GetValue(rhs).
Let env be the running execution context's LexicalEnvironment.
Return the result of performing BindingInitialization for BindingPattern using value and env as the arguments.

13.3.2 Variable Statement

Note

A var statement declares variables that are scoped to the running execution context's VariableEnvironment. Var variables are created when their containing Environment Record is instantiated and are initialized to undefined when created. Within the scope of any VariableEnvironment a common BindingIdentifier may appear in more than one VariableDeclaration but those declarations collectively define only one variable. A variable defined by a VariableDeclaration with an Initializer is assigned the value of its Initializer's AssignmentExpression when the VariableDeclaration is executed, not when the variable is created.

Syntax

[Yield, Await]

var

[+In, ?Yield, ?Await]

;

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

opt

[?Yield, ?Await]

[?In, ?Yield, ?Await]

13.3.2.1 Static Semantics: BoundNames

Let names be BoundNames of VariableDeclarationList.
Append to names the elements of BoundNames of VariableDeclaration.
Return names.

opt

Return the BoundNames of BindingIdentifier.

Return the BoundNames of BindingPattern.

13.3.2.2 Static Semantics: VarDeclaredNames

var

;

Return BoundNames of VariableDeclarationList.

13.3.2.3 Static Semantics: VarScopedDeclarations

Return a new List containing VariableDeclaration.

Let declarations be VarScopedDeclarations of VariableDeclarationList.
Append VariableDeclaration to declarations.
Return declarations.

13.3.2.4 Runtime Semantics: Evaluation

var

;

Let next be the result of evaluating VariableDeclarationList.
ReturnIfAbrupt(next).
Return NormalCompletion(empty).

Let next be the result of evaluating VariableDeclarationList.
ReturnIfAbrupt(next).
Return the result of evaluating VariableDeclaration.

Return NormalCompletion(empty).

Let bindingId be StringValue of BindingIdentifier.
Let lhs be ? ResolveBinding(bindingId).
If IsAnonymousFunctionDefinition(Initializer) is true, then
1. Let value be NamedEvaluation of Initializer with argument bindingId.
Else,
1. Let rhs be the result of evaluating Initializer.
2. Let value be ? GetValue(rhs).
Return ? PutValue(lhs, value).

Note

If a VariableDeclaration is nested within a with statement and the BindingIdentifier in the VariableDeclaration is the same as a property name of the binding object of the with statement's object Environment Record, then step 5 will assign value to the property instead of assigning to the VariableEnvironment binding of the Identifier.

Let rhs be the result of evaluating Initializer.
Let rval be ? GetValue(rhs).
Return the result of performing BindingInitialization for BindingPattern passing rval and undefined as arguments.

13.3.3 Destructuring Binding Patterns

Syntax

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

{

}

{

[?Yield, ?Await]

}

{

[?Yield, ?Await]

}

{

[?Yield, ?Await]

[?Yield, ?Await]

opt

}

[Yield, Await]

[

opt

[?Yield, ?Await]

opt

]

[

[?Yield, ?Await]

]

[

[?Yield, ?Await]

opt

[?Yield, ?Await]

opt

]

[Yield, Await]

...

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

opt

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

[Yield, Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

[Yield, Await]

...

[?Yield, ?Await]

...

[?Yield, ?Await]

13.3.3.1 Static Semantics: BoundNames

{

}

Return a new empty List.

{

}

Let names be BoundNames of BindingPropertyList.
Append to names the elements of BoundNames of BindingRestProperty.
Return names.

[

opt

]

Return a new empty List.

[

opt

]

Return the BoundNames of BindingRestElement.

[

opt

]

Return the BoundNames of BindingElementList.

[

opt

]

Let names be BoundNames of BindingElementList.
Append to names the elements of BoundNames of BindingRestElement.
Return names.

Let names be BoundNames of BindingPropertyList.
Append to names the elements of BoundNames of BindingProperty.
Return names.

Let names be BoundNames of BindingElementList.
Append to names the elements of BoundNames of BindingElisionElement.
Return names.

opt

Return BoundNames of BindingElement.

Return the BoundNames of BindingElement.

opt

Return the BoundNames of BindingIdentifier.

opt

Return the BoundNames of BindingPattern.

13.3.3.2 Static Semantics: ContainsExpression

{

}

{

}

Return false.

{

}

Return ContainsExpression of BindingPropertyList.

[

opt

]

Return false.

[

opt

]

Return ContainsExpression of BindingRestElement.

[

opt

]

Return ContainsExpression of BindingElementList.

[

opt

]

Let has be ContainsExpression of BindingElementList.
If has is true, return true.
Return ContainsExpression of BindingRestElement.

Let has be ContainsExpression of BindingPropertyList.
If has is true, return true.
Return ContainsExpression of BindingProperty.

Let has be ContainsExpression of BindingElementList.
If has is true, return true.
Return ContainsExpression of BindingElisionElement.

opt

Return ContainsExpression of BindingElement.

Let has be IsComputedPropertyKey of PropertyName.
If has is true, return true.
Return ContainsExpression of BindingElement.

Return true.

Return false.

Return true.

...

Return false.

...

Return ContainsExpression of BindingPattern.

13.3.3.3 Static Semantics: HasInitializer

Return false.

Return true.

Return false.

Return true.

13.3.3.4 Static Semantics: IsSimpleParameterList

Return false.

Return false.

Return true.

Return false.

13.3.3.5 Runtime Semantics: BindingInitialization

With parameters value and environment.

Note

When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.

Perform ? RequireObjectCoercible(value).
Return the result of performing BindingInitialization for ObjectBindingPattern using value and environment as arguments.

Let iteratorRecord be ? GetIterator(value).
Let result be IteratorBindingInitialization of ArrayBindingPattern with arguments iteratorRecord and environment.
If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
Return result.

{

}

Return NormalCompletion(empty).

{

}

{

}

Perform ? PropertyBindingInitialization for BindingPropertyList using value and environment as the arguments.
Return NormalCompletion(empty).

{

}

Let excludedNames be a new empty List.
Return the result of performing RestBindingInitialization of BindingRestProperty with value, environment, and excludedNames as the arguments.

{

}

Let excludedNames be ? PropertyBindingInitialization of BindingPropertyList with arguments value and environment.
Return the result of performing RestBindingInitialization of BindingRestProperty with arguments value, environment, and excludedNames.

13.3.3.6 Runtime Semantics: PropertyBindingInitialization

With parameters value and environment.

Note

These collect a list of all bound property names rather than just empty completion.

Let boundNames be ? PropertyBindingInitialization of BindingPropertyList with arguments value and environment.
Let nextNames be ? PropertyBindingInitialization of BindingProperty with arguments value and environment.
Append each item in nextNames to the end of boundNames.
Return boundNames.

Let name be the string that is the only element of BoundNames of SingleNameBinding.
Perform ? KeyedBindingInitialization for SingleNameBinding using value, environment, and name as the arguments.
Return a new List containing name.

Let P be the result of evaluating PropertyName.
ReturnIfAbrupt(P).
Perform ? KeyedBindingInitialization of BindingElement with value, environment, and P as the arguments.
Return a new List containing P.

13.3.3.7 Runtime Semantics: RestBindingInitialization

With parameters value, environment, and excludedNames.

...

Let lhs be ? ResolveBinding(StringValue of BindingIdentifier, environment).
Let restObj be OrdinaryObjectCreate(%Object.prototype%).
Perform ? CopyDataProperties(restObj, value, excludedNames).
If environment is undefined, return PutValue(lhs, restObj).
Return InitializeReferencedBinding(lhs, restObj).

13.3.3.8 Runtime Semantics: IteratorBindingInitialization

With parameters iteratorRecord and environment.

Note

[

]

Return NormalCompletion(empty).

[

]

Return the result of performing IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.

[

opt

]

If Elision is present, then
1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
Return the result of performing IteratorBindingInitialization for BindingRestElement with iteratorRecord and environment as arguments.

[

]

Return the result of performing IteratorBindingInitialization for BindingElementList with iteratorRecord and environment as arguments.

[

]

Return the result of performing IteratorBindingInitialization for BindingElementList with iteratorRecord and environment as arguments.

[

]

Perform ? IteratorBindingInitialization for BindingElementList with iteratorRecord and environment as arguments.
Return the result of performing IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.

[

opt

]

Perform ? IteratorBindingInitialization for BindingElementList with iteratorRecord and environment as arguments.
If Elision is present, then
1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
Return the result of performing IteratorBindingInitialization for BindingRestElement with iteratorRecord and environment as arguments.

Return the result of performing IteratorBindingInitialization for BindingElisionElement with iteratorRecord and environment as arguments.

Perform ? IteratorBindingInitialization for BindingElementList with iteratorRecord and environment as arguments.
Return the result of performing IteratorBindingInitialization for BindingElisionElement using iteratorRecord and environment as arguments.

Return the result of performing IteratorBindingInitialization of BindingElement with iteratorRecord and environment as the arguments.

Perform ? IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
Return the result of performing IteratorBindingInitialization of BindingElement with iteratorRecord and environment as the arguments.

Return the result of performing IteratorBindingInitialization for SingleNameBinding with iteratorRecord and environment as the arguments.

opt

Let bindingId be StringValue of BindingIdentifier.
Let lhs be ? ResolveBinding(bindingId, environment).
If iteratorRecord.[[Done]] is false, then
1. Let next be IteratorStep(iteratorRecord).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, set iteratorRecord.[[Done]] to true.
5. Else,
  1. Let v be IteratorValue(next).
  2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
  3. ReturnIfAbrupt(v).
If iteratorRecord.[[Done]] is true, let v be undefined.
If Initializer is present and v is undefined, then
1. If IsAnonymousFunctionDefinition(Initializer) is true, then
  1. Set v to the result of performing NamedEvaluation for Initializer with argument bindingId.
2. Else,
  1. Let defaultValue be the result of evaluating Initializer.
  2. Set v to ? GetValue(defaultValue).
If environment is undefined, return ? PutValue(lhs, v).
Return InitializeReferencedBinding(lhs, v).

opt

If iteratorRecord.[[Done]] is false, then
1. Let next be IteratorStep(iteratorRecord).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, set iteratorRecord.[[Done]] to true.
5. Else,
  1. Let v be IteratorValue(next).
  2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
  3. ReturnIfAbrupt(v).
If iteratorRecord.[[Done]] is true, let v be undefined.
If Initializer is present and v is undefined, then
1. Let defaultValue be the result of evaluating Initializer.
2. Set v to ? GetValue(defaultValue).
Return the result of performing BindingInitialization of BindingPattern with v and environment as the arguments.

...

Let lhs be ? ResolveBinding(StringValue of BindingIdentifier, environment).
Let A be ! ArrayCreate(0).
Let n be 0.
Repeat,
1. If iteratorRecord.[[Done]] is false, then
  1. Let next be IteratorStep(iteratorRecord).
  2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
  3. ReturnIfAbrupt(next).
  4. If next is false, set iteratorRecord.[[Done]] to true.
2. If iteratorRecord.[[Done]] is true, then
  1. If environment is undefined, return ? PutValue(lhs, A).
  2. Return InitializeReferencedBinding(lhs, A).
3. Let nextValue be IteratorValue(next).
4. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
5. ReturnIfAbrupt(nextValue).
6. Perform ! CreateDataPropertyOrThrow(A, ! ToString(n), nextValue).
7. Set n to n + 1.

...

Let A be ! ArrayCreate(0).
Let n be 0.
Repeat,
1. If iteratorRecord.[[Done]] is false, then
  1. Let next be IteratorStep(iteratorRecord).
  2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
  3. ReturnIfAbrupt(next).
  4. If next is false, set iteratorRecord.[[Done]] to true.
2. If iteratorRecord.[[Done]] is true, then
  1. Return the result of performing BindingInitialization of BindingPattern with A and environment as the arguments.
3. Let nextValue be IteratorValue(next).
4. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
5. ReturnIfAbrupt(nextValue).
6. Perform ! CreateDataPropertyOrThrow(A, ! ToString(n), nextValue).
7. Set n to n + 1.

13.3.3.9 Runtime Semantics: KeyedBindingInitialization

With parameters value, environment, and propertyName.

Note

opt

Let v be ? GetV(value, propertyName).
If Initializer is present and v is undefined, then
1. Let defaultValue be the result of evaluating Initializer.
2. Set v to ? GetValue(defaultValue).
Return the result of performing BindingInitialization for BindingPattern passing v and environment as arguments.

opt

Let bindingId be StringValue of BindingIdentifier.
Let lhs be ? ResolveBinding(bindingId, environment).
Let v be ? GetV(value, propertyName).
If Initializer is present and v is undefined, then
1. If IsAnonymousFunctionDefinition(Initializer) is true, then
  1. Set v to the result of performing NamedEvaluation for Initializer with argument bindingId.
2. Else,
  1. Let defaultValue be the result of evaluating Initializer.
  2. Set v to ? GetValue(defaultValue).
If environment is undefined, return ? PutValue(lhs, v).
Return InitializeReferencedBinding(lhs, v).

13.4 Empty Statement

Syntax

EmptyStatement

;

13.4.1 Runtime Semantics: Evaluation

EmptyStatement

;

Return NormalCompletion(empty).

13.5 Expression Statement

Syntax

[Yield, Await]

[lookahead ∉ { {, function, async

[no LineTerminator here]

function, class, let [ }]

[+In, ?Yield, ?Await]

;

Note

An ExpressionStatement cannot start with a U+007B (LEFT CURLY BRACKET) because that might make it ambiguous with a Block. An ExpressionStatement cannot start with the function or class keywords because that would make it ambiguous with a FunctionDeclaration, a GeneratorDeclaration, or a ClassDeclaration. An ExpressionStatement cannot start with async function because that would make it ambiguous with an AsyncFunctionDeclaration or a AsyncGeneratorDeclaration. An ExpressionStatement cannot start with the two token sequence let [ because that would make it ambiguous with a let LexicalDeclaration whose first LexicalBinding was an ArrayBindingPattern.

13.5.1 Runtime Semantics: Evaluation

;

Let exprRef be the result of evaluating Expression.
Return ? GetValue(exprRef).

13.6 The `if` Statement

Syntax

[Yield, Await, Return]

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

else

[?Yield, ?Await, ?Return]

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

Each else for which the choice of associated if is ambiguous shall be associated with the nearest possible if that would otherwise have no corresponding else.

13.6.1 Static Semantics: Early Errors

(

)

else

(

)

It is a Syntax Error if IsLabelledFunction(Statement) is true.

Note

It is only necessary to apply this rule if the extension specified in B.3.2 is implemented.

13.6.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

(

)

else

Let hasDuplicate be ContainsDuplicateLabels of the first Statement with argument labelSet.
If hasDuplicate is true, return true.
Return ContainsDuplicateLabels of the second Statement with argument labelSet.

(

)

Return ContainsDuplicateLabels of Statement with argument labelSet.

13.6.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

(

)

else

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of the first Statement with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of the second Statement with argument labelSet.

(

)

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

13.6.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

(

)

else

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of the first Statement with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of the second Statement with arguments iterationSet and « ».

(

)

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

13.6.5 Static Semantics: VarDeclaredNames

(

)

else

Let names be VarDeclaredNames of the first Statement.
Append to names the elements of the VarDeclaredNames of the second Statement.
Return names.

(

)

Return the VarDeclaredNames of Statement.

13.6.6 Static Semantics: VarScopedDeclarations

(

)

else

Let declarations be VarScopedDeclarations of the first Statement.
Append to declarations the elements of the VarScopedDeclarations of the second Statement.
Return declarations.

(

)

Return the VarScopedDeclarations of Statement.

13.6.7 Runtime Semantics: Evaluation

(

)

else

Let exprRef be the result of evaluating Expression.
Let exprValue be ! ToBoolean(? GetValue(exprRef)).
If exprValue is true, then
1. Let stmtCompletion be the result of evaluating the first Statement.
Else,
1. Let stmtCompletion be the result of evaluating the second Statement.
Return Completion(UpdateEmpty(stmtCompletion, undefined)).

(

)

Let exprRef be the result of evaluating Expression.
Let exprValue be ! ToBoolean(? GetValue(exprRef)).
If exprValue is false, then
1. Return NormalCompletion(undefined).
Else,
1. Let stmtCompletion be the result of evaluating Statement.
2. Return Completion(UpdateEmpty(stmtCompletion, undefined)).

13.7 Iteration Statements

Syntax

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

while

(

[+In, ?Yield, ?Await]

)

;

while

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

[lookahead ≠ let []

[~In, ?Yield, ?Await]

opt

;

[+In, ?Yield, ?Await]

opt

;

[+In, ?Yield, ?Await]

opt

)

[?Yield, ?Await, ?Return]

for

(

var

[~In, ?Yield, ?Await]

;

[+In, ?Yield, ?Await]

opt

;

[+In, ?Yield, ?Await]

opt

)

[?Yield, ?Await, ?Return]

for

(

[~In, ?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

;

[+In, ?Yield, ?Await]

opt

)

[?Yield, ?Await, ?Return]

for

(

[lookahead ≠ let []

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

var

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

[lookahead ≠ let]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

var

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[+Await]

for

await

(

[lookahead ≠ let]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[+Await]

for

await

(

var

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[+Await]

for

await

(

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

Note

This section is extended by Annex B.3.6.

13.7.1 Semantics

13.7.1.1 Static Semantics: Early Errors

while

(

)

;

while

(

)

for

(

opt

;

opt

;

opt

)

for

(

var

;

opt

;

opt

)

for

(

opt

;

opt

)

for

(

)

for

(

var

)

for

(

)

for

(

)

for

(

var

)

for

(

)

for

await

(

)

for

await

(

var

)

for

await

(

)

It is a Syntax Error if IsLabelledFunction(Statement) is true.

Note

It is only necessary to apply this rule if the extension specified in B.3.2 is implemented.

13.7.1.2 Runtime Semantics: LoopContinues ( `completion`, `labelSet` )

The abstract operation LoopContinues takes arguments completion and labelSet. It performs the following steps when called:

If completion.[[Type]] is normal, return true.
If completion.[[Type]] is not continue, return false.
If completion.[[Target]] is empty, return true.
If completion.[[Target]] is an element of labelSet, return true.
Return false.

Note

Within the Statement part of an IterationStatement a ContinueStatement may be used to begin a new iteration.

13.7.2 The `do`-`while` Statement

13.7.2.1 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

while

(

)

;

Return ContainsDuplicateLabels of Statement with argument labelSet.

13.7.2.2 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

while

(

)

;

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

13.7.2.3 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

while

(

)

;

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

13.7.2.4 Static Semantics: VarDeclaredNames

while

(

)

;

Return the VarDeclaredNames of Statement.

13.7.2.5 Static Semantics: VarScopedDeclarations

while

(

)

;

Return the VarScopedDeclarations of Statement.

13.7.2.6 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

while

(

)

;

Let V be undefined.
Repeat,
1. Let stmtResult be the result of evaluating Statement.
2. If LoopContinues(stmtResult, labelSet) is false, return Completion(UpdateEmpty(stmtResult, V)).
3. If stmtResult.[[Value]] is not empty, set V to stmtResult.[[Value]].
4. Let exprRef be the result of evaluating Expression.
5. Let exprValue be ? GetValue(exprRef).
6. If ! ToBoolean(exprValue) is false, return NormalCompletion(V).

13.7.3 The `while` Statement

13.7.3.1 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

while

(

)

Return ContainsDuplicateLabels of Statement with argument labelSet.

13.7.3.2 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

while

(

)

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

13.7.3.3 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

while

(

)

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

13.7.3.4 Static Semantics: VarDeclaredNames

while

(

)

Return the VarDeclaredNames of Statement.

13.7.3.5 Static Semantics: VarScopedDeclarations

while

(

)

Return the VarScopedDeclarations of Statement.

13.7.3.6 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

while

(

)

Let V be undefined.
Repeat,
1. Let exprRef be the result of evaluating Expression.
2. Let exprValue be ? GetValue(exprRef).
3. If ! ToBoolean(exprValue) is false, return NormalCompletion(V).
4. Let stmtResult be the result of evaluating Statement.
5. If LoopContinues(stmtResult, labelSet) is false, return Completion(UpdateEmpty(stmtResult, V)).
6. If stmtResult.[[Value]] is not empty, set V to stmtResult.[[Value]].

13.7.4 The `for` Statement

13.7.4.1 Static Semantics: Early Errors

for

(

opt

;

opt

)

It is a Syntax Error if any element of the BoundNames of LexicalDeclaration also occurs in the VarDeclaredNames of Statement.

13.7.4.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

for

(

opt

;

opt

;

opt

)

for

(

var

;

opt

;

opt

)

for

(

opt

;

opt

)

Return ContainsDuplicateLabels of Statement with argument labelSet.

13.7.4.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

for

(

opt

;

opt

;

opt

)

for

(

var

;

opt

;

opt

)

for

(

opt

;

opt

)

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

13.7.4.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

for

(

opt

;

opt

;

opt

)

for

(

var

;

opt

;

opt

)

for

(

opt

;

opt

)

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

13.7.4.5 Static Semantics: VarDeclaredNames

for

(

opt

;

opt

;

opt

)

Return the VarDeclaredNames of Statement.

for

(

var

;

opt

;

opt

)

Let names be BoundNames of VariableDeclarationList.
Append to names the elements of the VarDeclaredNames of Statement.
Return names.

for

(

opt

;

opt

)

Return the VarDeclaredNames of Statement.

13.7.4.6 Static Semantics: VarScopedDeclarations

for

(

opt

;

opt

;

opt

)

Return the VarScopedDeclarations of Statement.

for

(

var

;

opt

;

opt

)

Let declarations be VarScopedDeclarations of VariableDeclarationList.
Append to declarations the elements of the VarScopedDeclarations of Statement.
Return declarations.

for

(

opt

;

opt

)

Return the VarScopedDeclarations of Statement.

13.7.4.7 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

for

(

opt

;

opt

;

opt

)

If the first Expression is present, then
1. Let exprRef be the result of evaluating the first Expression.
2. Perform ? GetValue(exprRef).
Return ? ForBodyEvaluation(the second Expression, the third Expression, Statement, « », labelSet).

for

(

var

;

opt

;

opt

)

Let varDcl be the result of evaluating VariableDeclarationList.
ReturnIfAbrupt(varDcl).
Return ? ForBodyEvaluation(the first Expression, the second Expression, Statement, « », labelSet).

for

(

opt

;

opt

)

Let oldEnv be the running execution context's LexicalEnvironment.
Let loopEnv be NewDeclarativeEnvironment(oldEnv).
Let isConst be IsConstantDeclaration of LexicalDeclaration.
Let boundNames be the BoundNames of LexicalDeclaration.
For each element dn of boundNames, do
1. If isConst is true, then
  1. Perform ! loopEnv.CreateImmutableBinding(dn, true).
2. Else,
  1. Perform ! loopEnv.CreateMutableBinding(dn, false).
Set the running execution context's LexicalEnvironment to loopEnv.
Let forDcl be the result of evaluating LexicalDeclaration.
If forDcl is an abrupt completion, then
1. Set the running execution context's LexicalEnvironment to oldEnv.
2. Return Completion(forDcl).
If isConst is false, let perIterationLets be boundNames; otherwise let perIterationLets be « ».
Let bodyResult be ForBodyEvaluation(the first Expression, the second Expression, Statement, perIterationLets, labelSet).
Set the running execution context's LexicalEnvironment to oldEnv.
Return Completion(bodyResult).

13.7.4.8 Runtime Semantics: ForBodyEvaluation ( `test`, `increment`, `stmt`, `perIterationBindings`, `labelSet` )

The abstract operation ForBodyEvaluation takes arguments test, increment, stmt, perIterationBindings, and labelSet. It performs the following steps when called:

Let V be undefined.
Perform ? CreatePerIterationEnvironment(perIterationBindings).
Repeat,
1. If test is not [empty], then
  1. Let testRef be the result of evaluating test.
  2. Let testValue be ? GetValue(testRef).
  3. If ! ToBoolean(testValue) is false, return NormalCompletion(V).
2. Let result be the result of evaluating stmt.
3. If LoopContinues(result, labelSet) is false, return Completion(UpdateEmpty(result, V)).
4. If result.[[Value]] is not empty, set V to result.[[Value]].
5. Perform ? CreatePerIterationEnvironment(perIterationBindings).
6. If increment is not [empty], then
  1. Let incRef be the result of evaluating increment.
  2. Perform ? GetValue(incRef).

13.7.4.9 Runtime Semantics: CreatePerIterationEnvironment ( `perIterationBindings` )

The abstract operation CreatePerIterationEnvironment takes argument perIterationBindings. It performs the following steps when called:

If perIterationBindings has any elements, then
1. Let lastIterationEnv be the running execution context's LexicalEnvironment.
2. Let outer be lastIterationEnv.[[OuterEnv]].
3. Assert: outer is not null.
4. Let thisIterationEnv be NewDeclarativeEnvironment(outer).
5. For each element bn of perIterationBindings, do
  1. Perform ! thisIterationEnv.CreateMutableBinding(bn, false).
  2. Let lastValue be ? lastIterationEnv.GetBindingValue(bn, true).
  3. Perform thisIterationEnv.InitializeBinding(bn, lastValue).
6. Set the running execution context's LexicalEnvironment to thisIterationEnv.
Return undefined.

13.7.5 The `for`-`in`, `for`-`of`, and `for`-`await`-`of` Statements

13.7.5.1 Static Semantics: Early Errors

for

(

)

for

(

)

for

await

(

)

If LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral, the following Early Error rules are applied:

It is a Syntax Error if LeftHandSideExpression is not covering an AssignmentPattern.
All Early Error rules for AssignmentPattern and its derived productions also apply to the AssignmentPattern that is covered by LeftHandSideExpression.

If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, the following Early Error rule is applied:

It is a Syntax Error if AssignmentTargetType of LeftHandSideExpression is not simple.

for

(

)

for

(

)

for

await

(

)

It is a Syntax Error if the BoundNames of ForDeclaration contains "let".
It is a Syntax Error if any element of the BoundNames of ForDeclaration also occurs in the VarDeclaredNames of Statement.
It is a Syntax Error if the BoundNames of ForDeclaration contains any duplicate entries.

13.7.5.2 Static Semantics: BoundNames

Return the BoundNames of ForBinding.

13.7.5.3 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

for

(

)

for

(

var

)

for

(

)

for

(

)

for

(

var

)

for

(

)

for

await

(

)

for

await

(

var

)

for

await

(

)

Return ContainsDuplicateLabels of Statement with argument labelSet.

Note

This section is extended by Annex B.3.6.

13.7.5.4 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

for

(

)

for

(

var

)

for

(

)

for

(

)

for

(

var

)

for

(

)

for

await

(

)

for

await

(

var

)

for

await

(

)

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

Note

This section is extended by Annex B.3.6.

13.7.5.5 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

for

(

)

for

(

var

)

for

(

)

for

(

)

for

(

var

)

for

(

)

for

await

(

)

for

await

(

var

)

for

await

(

)

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

Note

This section is extended by Annex B.3.6.

13.7.5.6 Static Semantics: IsDestructuring

Return IsDestructuring of ForBinding.

Return false.

Return true.

Note

This section is extended by Annex B.3.6.

13.7.5.7 Static Semantics: VarDeclaredNames

for

(

)

for

(

)

for

(

)

for

(

)

for

await

(

)

for

await

(

)

Return the VarDeclaredNames of Statement.

for

(

var

)

for

(

var

)

for

await

(

var

)

Let names be the BoundNames of ForBinding.
Append to names the elements of the VarDeclaredNames of Statement.
Return names.

Note

This section is extended by Annex B.3.6.

13.7.5.8 Static Semantics: VarScopedDeclarations

for

(

)

for

(

)

for

(

)

for

(

)

for

await

(

)

for

await

(

)

Return the VarScopedDeclarations of Statement.

for

(

var

)

for

(

var

)

for

await

(

var

)

Let declarations be a List containing ForBinding.
Append to declarations the elements of the VarScopedDeclarations of Statement.
Return declarations.

Note

This section is extended by Annex B.3.6.

13.7.5.9 Runtime Semantics: BindingInitialization

With parameters value and environment.

Note

undefined is passed for environment to indicate that a PutValue operation should be used to assign the initialization value. This is the case for var statements and the formal parameter lists of some non-strict functions (see 9.2.10). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.

Return the result of performing BindingInitialization for ForBinding passing value and environment as the arguments.

13.7.5.10 Runtime Semantics: BindingInstantiation

With parameter environment.

Assert: environment is a declarative Environment Record.
For each element name of the BoundNames of ForBinding, do
1. If IsConstantDeclaration of LetOrConst is true, then
  1. Perform ! environment.CreateImmutableBinding(name, true).
2. Else,
  1. Perform ! environment.CreateMutableBinding(name, false).

13.7.5.11 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

for

(

)

Let keyResult be ? ForIn/OfHeadEvaluation(« », Expression, enumerate).
Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement, keyResult, enumerate, assignment, labelSet).

for

(

var

)

Let keyResult be ? ForIn/OfHeadEvaluation(« », Expression, enumerate).
Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult, enumerate, varBinding, labelSet).

for

(

)

Let keyResult be ? ForIn/OfHeadEvaluation(BoundNames of ForDeclaration, Expression, enumerate).
Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult, enumerate, lexicalBinding, labelSet).

for

(

)

Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, iterate).
Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement, keyResult, iterate, assignment, labelSet).

for

(

var

)

Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, iterate).
Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult, iterate, varBinding, labelSet).

for

(

)

Let keyResult be ? ForIn/OfHeadEvaluation(BoundNames of ForDeclaration, AssignmentExpression, iterate).
Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult, iterate, lexicalBinding, labelSet).

for

await

(

)

Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, async-iterate).
Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement, keyResult, iterate, assignment, labelSet, async).

for

await

(

var

)

Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, async-iterate).
Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult, iterate, varBinding, labelSet, async).

for

await

(

)

Let keyResult be ? ForIn/OfHeadEvaluation(BoundNames of ForDeclaration, AssignmentExpression, async-iterate).
Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult, iterate, lexicalBinding, labelSet, async).

Note

This section is extended by Annex B.3.6.

13.7.5.12 Runtime Semantics: ForIn/OfHeadEvaluation ( `uninitializedBoundNames`, `expr`, `iterationKind` )

The abstract operation ForIn/OfHeadEvaluation takes arguments uninitializedBoundNames, expr, and iterationKind (either enumerate, iterate, or async-iterate). It performs the following steps when called:

Let oldEnv be the running execution context's LexicalEnvironment.
If uninitializedBoundNames is not an empty List, then
1. Assert: uninitializedBoundNames has no duplicate entries.
2. Let newEnv be NewDeclarativeEnvironment(oldEnv).
3. For each string name in uninitializedBoundNames, do
  1. Perform ! newEnv.CreateMutableBinding(name, false).
4. Set the running execution context's LexicalEnvironment to newEnv.
Let exprRef be the result of evaluating expr.
Set the running execution context's LexicalEnvironment to oldEnv.
Let exprValue be ? GetValue(exprRef).
If iterationKind is enumerate, then
1. If exprValue is undefined or null, then
  1. Return Completion { [[Type]]: break, [[Value]]: empty, [[Target]]: empty }.
2. Let obj be ! ToObject(exprValue).
3. Let iterator be ? EnumerateObjectProperties(obj).
4. Let nextMethod be ! GetV(iterator, "next").
5. Return the Record { [[Iterator]]: iterator, [[NextMethod]]: nextMethod, [[Done]]: false }.
Else,
1. Assert: iterationKind is iterate or async-iterate.
2. If iterationKind is async-iterate, let iteratorHint be async.
3. Else, let iteratorHint be sync.
4. Return ? GetIterator(exprValue, iteratorHint).

13.7.5.13 Runtime Semantics: ForIn/OfBodyEvaluation ( `lhs`, `stmt`, `iteratorRecord`, `iterationKind`, `lhsKind`, `labelSet` [ , `iteratorKind` ] )

The abstract operation ForIn/OfBodyEvaluation takes arguments lhs, stmt, iteratorRecord, iterationKind, lhsKind (either assignment, varBinding or lexicalBinding), and labelSet and optional argument iteratorKind (either sync or async). It performs the following steps when called:

If iteratorKind is not present, set iteratorKind to sync.
Let oldEnv be the running execution context's LexicalEnvironment.
Let V be undefined.
Let destructuring be IsDestructuring of lhs.
If destructuring is true and if lhsKind is assignment, then
1. Assert: lhs is a LeftHandSideExpression.
2. Let assignmentPattern be the AssignmentPattern that is covered by lhs.
Repeat,
1. Let nextResult be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]]).
2. If iteratorKind is async, then set nextResult to ? Await(nextResult).
3. If Type(nextResult) is not Object, throw a TypeError exception.
4. Let done be ? IteratorComplete(nextResult).
5. If done is true, return NormalCompletion(V).
6. Let nextValue be ? IteratorValue(nextResult).
7. If lhsKind is either assignment or varBinding, then
  1. If destructuring is false, then
    1. Let lhsRef be the result of evaluating lhs. (It may be evaluated repeatedly.)
8. Else,
  1. Assert: lhsKind is lexicalBinding.
  2. Assert: lhs is a ForDeclaration.
  3. Let iterationEnv be NewDeclarativeEnvironment(oldEnv).
  4. Perform BindingInstantiation for lhs passing iterationEnv as the argument.
  5. Set the running execution context's LexicalEnvironment to iterationEnv.
  6. If destructuring is false, then
    1. Assert: lhs binds a single name.
    2. Let lhsName be the sole element of BoundNames of lhs.
    3. Let lhsRef be ! ResolveBinding(lhsName).
9. If destructuring is false, then
  1. If lhsRef is an abrupt completion, then
    1. Let status be lhsRef.
  2. Else if lhsKind is lexicalBinding, then
    1. Let status be InitializeReferencedBinding(lhsRef, nextValue).
  3. Else,
    1. Let status be PutValue(lhsRef, nextValue).
10. Else,
  1. If lhsKind is assignment, then
    1. Let status be DestructuringAssignmentEvaluation of assignmentPattern with argument nextValue.
  2. Else if lhsKind is varBinding, then
    1. Assert: lhs is a ForBinding.
    2. Let status be BindingInitialization of lhs with arguments nextValue and undefined.
  3. Else,
    1. Assert: lhsKind is lexicalBinding.
    2. Assert: lhs is a ForDeclaration.
    3. Let status be BindingInitialization of lhs with arguments nextValue and iterationEnv.
11. If status is an abrupt completion, then
  1. Set the running execution context's LexicalEnvironment to oldEnv.
  2. If iteratorKind is async, return ? AsyncIteratorClose(iteratorRecord, status).
  3. If iterationKind is enumerate, then
    1. Return status.
  4. Else,
    1. Assert: iterationKind is iterate.
    2. Return ? IteratorClose(iteratorRecord, status).
12. Let result be the result of evaluating stmt.
13. Set the running execution context's LexicalEnvironment to oldEnv.
14. If LoopContinues(result, labelSet) is false, then
  1. If iterationKind is enumerate, then
    1. Return Completion(UpdateEmpty(result, V)).
  2. Else,
    1. Assert: iterationKind is iterate.
    2. Set status to UpdateEmpty(result, V).
    3. If iteratorKind is async, return ? AsyncIteratorClose(iteratorRecord, status).
    4. Return ? IteratorClose(iteratorRecord, status).
15. If result.[[Value]] is not empty, set V to result.[[Value]].

13.7.5.14 Runtime Semantics: Evaluation

Let bindingId be StringValue of BindingIdentifier.
Return ? ResolveBinding(bindingId).

13.7.5.15 EnumerateObjectProperties ( `O` )

The abstract operation EnumerateObjectProperties takes argument O. It performs the following steps when called:

Assert: Type(O) is Object.
Return an Iterator object (25.1.1.2) whose next method iterates over all the String-valued keys of enumerable properties of O. The iterator object is never directly accessible to ECMAScript code. The mechanics and order of enumerating the properties is not specified but must conform to the rules specified below.

The iterator's throw and return methods are null and are never invoked. The iterator's next method processes object properties to determine whether the property key should be returned as an iterator value. Returned property keys do not include keys that are Symbols. Properties of the target object may be deleted during enumeration. A property that is deleted before it is processed by the iterator's next method is ignored. If new properties are added to the target object during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration. A property name will be returned by the iterator's next method at most once in any enumeration.

Enumerating the properties of the target object includes enumerating properties of its prototype, and the prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if it has the same name as a property that has already been processed by the iterator's next method. The values of [[Enumerable]] attributes are not considered when determining if a property of a prototype object has already been processed. The enumerable property names of prototype objects must be obtained by invoking EnumerateObjectProperties passing the prototype object as the argument. EnumerateObjectProperties must obtain the own property keys of the target object by calling its [[OwnPropertyKeys]] internal method. Property attributes of the target object must be obtained by calling its [[GetOwnProperty]] internal method.

In addition, if neither O nor any object in its prototype chain is a Proxy exotic object, Integer-Indexed exotic object, module namespace exotic object, or implementation provided exotic object, then the iterator must behave as would the iterator given by CreateForInIterator(O) until one of the following occurs:

the value of the [[Prototype]] internal slot of O or an object in its prototype chain changes,
a property is removed from O or an object in its prototype chain,
a property is added to an object in O's prototype chain, or
the value of the [[Enumerable]] attribute of a property of O or an object in its prototype chain changes.

Note 1

Hosts are not required to implement the algorithm in 13.7.5.16.2.1 directly. They may choose any implementation whose behaviour will not deviate from that algorithm unless one of the constraints in the previous paragraph is violated.

The following is an informative definition of an ECMAScript generator function that conforms to these rules:

function* EnumerateObjectProperties(obj) {
  const visited = new Set();
  for (const key of Reflect.ownKeys(obj)) {
    if (typeof key === "symbol") continue;
    const desc = Reflect.getOwnPropertyDescriptor(obj, key);
    if (desc) {
      visited.add(key);
      if (desc.enumerable) yield key;
    }
  }
  const proto = Reflect.getPrototypeOf(obj);
  if (proto === null) return;
  for (const protoKey of EnumerateObjectProperties(proto)) {
    if (!visited.has(protoKey)) yield protoKey;
  }
}

Note 2

The list of exotic objects for which implementations are not required to match CreateForInIterator was chosen because implementations historically differed in behaviour for those cases, and agreed in all others.

13.7.5.16 For-In Iterator Objects

A For-In Iterator is an object that represents a specific iteration over some specific object. For-In Iterator objects are never directly accessible to ECMAScript code; they exist solely to illustrate the behaviour of EnumerateObjectProperties.

13.7.5.16.1 CreateForInIterator ( `object` )

The abstract operation CreateForInIterator takes argument object. It is used to create a For-In Iterator object which iterates over the own and inherited enumerable string properties of object in a specific order. It performs the following steps when called:

Assert: Type(object) is Object.
Let iterator be OrdinaryObjectCreate(%ForInIteratorPrototype%, « [[Object]], [[ObjectWasVisited]], [[VisitedKeys]], [[RemainingKeys]] »).
Set iterator.[[Object]] to object.
Set iterator.[[ObjectWasVisited]] to false.
Set iterator.[[VisitedKeys]] to a new empty List.
Set iterator.[[RemainingKeys]] to a new empty List.
Return iterator.

13.7.5.16.2 The %ForInIteratorPrototype% Object

The %ForInIteratorPrototype% object:

has properties that are inherited by all For-In Iterator Objects.
is an ordinary object.
has a [[Prototype]] internal slot whose value is the intrinsic object %IteratorPrototype%.
is never directly accessible to ECMAScript code.
has the following properties:

13.7.5.16.2.1 %ForInIteratorPrototype%.next ( )

Let O be the this value.
Assert: Type(O) is Object.
Assert: O has all of the internal slots of a For-In Iterator Instance (13.7.5.16.3).
Let object be O.[[Object]].
Let visited be O.[[VisitedKeys]].
Let remaining be O.[[RemainingKeys]].
Repeat,
1. If O.[[ObjectWasVisited]] is false, then
  1. Let keys be ? object.[[OwnPropertyKeys]]().
  2. For each key of keys in List order, do
    1. If Type(key) is String, then
      1. Append key to remaining.
  3. Set O.[[ObjectWasVisited]] to true.
2. Repeat, while remaining is not empty,
  1. Remove the first element from remaining and let r be the value of the element.
  2. If there does not exist an element v of visited such that SameValue(r, v) is true, then
    1. Let desc be ? object.[[GetOwnProperty]](r).
    2. If desc is not undefined, then
      1. Append r to visited.
      2. If desc.[[Enumerable]] is true, return CreateIterResultObject(r, false).
3. Set object to ? object.[[GetPrototypeOf]]().
4. Set O.[[Object]] to object.
5. Set O.[[ObjectWasVisited]] to false.
6. If object is null, return CreateIterResultObject(undefined, true).

13.7.5.16.3 Properties of For-In Iterator Instances

For-In Iterator instances are ordinary objects that inherit properties from the %ForInIteratorPrototype% intrinsic object. For-In Iterator instances are initially created with the internal slots listed in Table 36.

Table 36: Internal Slots of For-In Iterator Instances

Internal Slot	Description
[[Object]]	The Object value whose properties are being iterated.
[[ObjectWasVisited]]	true if the iterator has invoked [[OwnPropertyKeys]] on [[Object]], false otherwise.
[[VisitedKeys]]	A list of String values which have been emitted by this iterator thus far.
[[RemainingKeys]]	A list of String values remaining to be emitted for the current object, before iterating the properties of its prototype (if its prototype is not null).

13.8 The `continue` Statement

Syntax

[Yield, Await]

continue

;

continue

[no LineTerminator here]

[?Yield, ?Await]

;

13.8.1 Static Semantics: Early Errors

continue

;

continue

;

It is a Syntax Error if this ContinueStatement is not nested, directly or indirectly (but not crossing function boundaries), within an IterationStatement.

13.8.2 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

continue

;

Return false.

continue

;

If the StringValue of LabelIdentifier is not an element of iterationSet, return true.
Return false.

13.8.3 Runtime Semantics: Evaluation

continue

;

Return Completion { [[Type]]: continue, [[Value]]: empty, [[Target]]: empty }.

continue

;

Let label be the StringValue of LabelIdentifier.
Return Completion { [[Type]]: continue, [[Value]]: empty, [[Target]]: label }.

13.9 The `break` Statement

Syntax

[Yield, Await]

break

;

break

[no LineTerminator here]

[?Yield, ?Await]

;

13.9.1 Static Semantics: Early Errors

break

;

It is a Syntax Error if this BreakStatement is not nested, directly or indirectly (but not crossing function boundaries), within an IterationStatement or a SwitchStatement.

13.9.2 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

break

;

Return false.

break

;

If the StringValue of LabelIdentifier is not an element of labelSet, return true.
Return false.

13.9.3 Runtime Semantics: Evaluation

break

;

Return Completion { [[Type]]: break, [[Value]]: empty, [[Target]]: empty }.

break

;

Let label be the StringValue of LabelIdentifier.
Return Completion { [[Type]]: break, [[Value]]: empty, [[Target]]: label }.

13.10 The `return` Statement

Syntax

[Yield, Await]

return

;

return

[no LineTerminator here]

[+In, ?Yield, ?Await]

;

Note

A return statement causes a function to cease execution and, in most cases, returns a value to the caller. If Expression is omitted, the return value is undefined. Otherwise, the return value is the value of Expression. A return statement may not actually return a value to the caller depending on surrounding context. For example, in a try block, a return statement's completion record may be replaced with another completion record during evaluation of the finally block.

13.10.1 Runtime Semantics: Evaluation

return

;

Return Completion { [[Type]]: return, [[Value]]: undefined, [[Target]]: empty }.

return

;

Let exprRef be the result of evaluating Expression.
Let exprValue be ? GetValue(exprRef).
If ! GetGeneratorKind() is async, set exprValue to ? Await(exprValue).
Return Completion { [[Type]]: return, [[Value]]: exprValue, [[Target]]: empty }.

13.11 The `with` Statement

Syntax

[Yield, Await, Return]

with

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

Note

The with statement adds an object Environment Record for a computed object to the lexical environment of the running execution context. It then executes a statement using this augmented lexical environment. Finally, it restores the original lexical environment.

13.11.1 Static Semantics: Early Errors

with

(

)

It is a Syntax Error if the code that matches this production is contained in strict mode code.
It is a Syntax Error if IsLabelledFunction(Statement) is true.

Note

It is only necessary to apply the second rule if the extension specified in B.3.2 is implemented.

13.11.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

with

(

)

Return ContainsDuplicateLabels of Statement with argument labelSet.

13.11.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

with

(

)

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

13.11.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

with

(

)

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

13.11.5 Static Semantics: VarDeclaredNames

with

(

)

Return the VarDeclaredNames of Statement.

13.11.6 Static Semantics: VarScopedDeclarations

with

(

)

Return the VarScopedDeclarations of Statement.

13.11.7 Runtime Semantics: Evaluation

with

(

)

Let val be the result of evaluating Expression.
Let obj be ? ToObject(? GetValue(val)).
Let oldEnv be the running execution context's LexicalEnvironment.
Let newEnv be NewObjectEnvironment(obj, oldEnv).
Set the withEnvironment flag of newEnv to true.
Set the running execution context's LexicalEnvironment to newEnv.
Let C be the result of evaluating Statement.
Set the running execution context's LexicalEnvironment to oldEnv.
Return Completion(UpdateEmpty(C, undefined)).

Note

No matter how control leaves the embedded Statement, whether normally or by some form of abrupt completion or exception, the LexicalEnvironment is always restored to its former state.

13.12 The `switch` Statement

Syntax

[Yield, Await, Return]

switch

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

{

[?Yield, ?Await, ?Return]

opt

}

{

[?Yield, ?Await, ?Return]

opt

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

opt

}

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

case

[+In, ?Yield, ?Await]

[?Yield, ?Await, ?Return]

opt

[Yield, Await, Return]

default

[?Yield, ?Await, ?Return]

opt

13.12.1 Static Semantics: Early Errors

switch

(

)

It is a Syntax Error if the LexicallyDeclaredNames of CaseBlock contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of CaseBlock also occurs in the VarDeclaredNames of CaseBlock.

13.12.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

switch

(

)

Return ContainsDuplicateLabels of CaseBlock with argument labelSet.

{

}

Return false.

{

opt

opt

}

If the first CaseClauses is present, then
1. Let hasDuplicates be ContainsDuplicateLabels of the first CaseClauses with argument labelSet.
2. If hasDuplicates is true, return true.
Let hasDuplicates be ContainsDuplicateLabels of DefaultClause with argument labelSet.
If hasDuplicates is true, return true.
If the second CaseClauses is not present, return false.
Return ContainsDuplicateLabels of the second CaseClauses with argument labelSet.

Let hasDuplicates be ContainsDuplicateLabels of CaseClauses with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of CaseClause with argument labelSet.

case

opt

If the StatementList is present, return ContainsDuplicateLabels of StatementList with argument labelSet.
Return false.

default

opt

If the StatementList is present, return ContainsDuplicateLabels of StatementList with argument labelSet.
Return false.

13.12.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

switch

(

)

Return ContainsUndefinedBreakTarget of CaseBlock with argument labelSet.

{

}

Return false.

{

opt

opt

}

If the first CaseClauses is present, then
1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of the first CaseClauses with argument labelSet.
2. If hasUndefinedLabels is true, return true.
Let hasUndefinedLabels be ContainsUndefinedBreakTarget of DefaultClause with argument labelSet.
If hasUndefinedLabels is true, return true.
If the second CaseClauses is not present, return false.
Return ContainsUndefinedBreakTarget of the second CaseClauses with argument labelSet.

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of CaseClauses with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of CaseClause with argument labelSet.

case

opt

If the StatementList is present, return ContainsUndefinedBreakTarget of StatementList with argument labelSet.
Return false.

default

opt

If the StatementList is present, return ContainsUndefinedBreakTarget of StatementList with argument labelSet.
Return false.

13.12.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

switch

(

)

Return ContainsUndefinedContinueTarget of CaseBlock with arguments iterationSet and « ».

{

}

Return false.

{

opt

opt

}

If the first CaseClauses is present, then
1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of the first CaseClauses with arguments iterationSet and « ».
2. If hasUndefinedLabels is true, return true.
Let hasUndefinedLabels be ContainsUndefinedContinueTarget of DefaultClause with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
If the second CaseClauses is not present, return false.
Return ContainsUndefinedContinueTarget of the second CaseClauses with arguments iterationSet and « ».

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of CaseClauses with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of CaseClause with arguments iterationSet and « ».

case

opt

If the StatementList is present, return ContainsUndefinedContinueTarget of StatementList with arguments iterationSet and « ».
Return false.

default

opt

If the StatementList is present, return ContainsUndefinedContinueTarget of StatementList with arguments iterationSet and « ».
Return false.

13.12.5 Static Semantics: LexicallyDeclaredNames

{

}

Return a new empty List.

{

opt

opt

}

If the first CaseClauses is present, let names be the LexicallyDeclaredNames of the first CaseClauses.
Else, let names be a new empty List.
Append to names the elements of the LexicallyDeclaredNames of DefaultClause.
If the second CaseClauses is not present, return names.
Return the result of appending to names the elements of the LexicallyDeclaredNames of the second CaseClauses.

Let names be LexicallyDeclaredNames of CaseClauses.
Append to names the elements of the LexicallyDeclaredNames of CaseClause.
Return names.

case

opt

If the StatementList is present, return the LexicallyDeclaredNames of StatementList.
Return a new empty List.

default

opt

If the StatementList is present, return the LexicallyDeclaredNames of StatementList.
Return a new empty List.

13.12.6 Static Semantics: LexicallyScopedDeclarations

{

}

Return a new empty List.

{

opt

opt

}

If the first CaseClauses is present, let declarations be the LexicallyScopedDeclarations of the first CaseClauses.
Else, let declarations be a new empty List.
Append to declarations the elements of the LexicallyScopedDeclarations of DefaultClause.
If the second CaseClauses is not present, return declarations.
Return the result of appending to declarations the elements of the LexicallyScopedDeclarations of the second CaseClauses.

Let declarations be LexicallyScopedDeclarations of CaseClauses.
Append to declarations the elements of the LexicallyScopedDeclarations of CaseClause.
Return declarations.

case

opt

If the StatementList is present, return the LexicallyScopedDeclarations of StatementList.
Return a new empty List.

default

opt

If the StatementList is present, return the LexicallyScopedDeclarations of StatementList.
Return a new empty List.

13.12.7 Static Semantics: VarDeclaredNames

switch

(

)

Return the VarDeclaredNames of CaseBlock.

{

}

Return a new empty List.

{

opt

opt

}

If the first CaseClauses is present, let names be the VarDeclaredNames of the first CaseClauses.
Else, let names be a new empty List.
Append to names the elements of the VarDeclaredNames of DefaultClause.
If the second CaseClauses is not present, return names.
Return the result of appending to names the elements of the VarDeclaredNames of the second CaseClauses.

Let names be VarDeclaredNames of CaseClauses.
Append to names the elements of the VarDeclaredNames of CaseClause.
Return names.

case

opt

If the StatementList is present, return the VarDeclaredNames of StatementList.
Return a new empty List.

default

opt

If the StatementList is present, return the VarDeclaredNames of StatementList.
Return a new empty List.

13.12.8 Static Semantics: VarScopedDeclarations

switch

(

)

Return the VarScopedDeclarations of CaseBlock.

{

}

Return a new empty List.

{

opt

opt

}

If the first CaseClauses is present, let declarations be the VarScopedDeclarations of the first CaseClauses.
Else, let declarations be a new empty List.
Append to declarations the elements of the VarScopedDeclarations of DefaultClause.
If the second CaseClauses is not present, return declarations.
Return the result of appending to declarations the elements of the VarScopedDeclarations of the second CaseClauses.

Let declarations be VarScopedDeclarations of CaseClauses.
Append to declarations the elements of the VarScopedDeclarations of CaseClause.
Return declarations.

case

opt

If the StatementList is present, return the VarScopedDeclarations of StatementList.
Return a new empty List.

default

opt

If the StatementList is present, return the VarScopedDeclarations of StatementList.
Return a new empty List.

13.12.9 Runtime Semantics: CaseBlockEvaluation

With parameter input.

{

}

Return NormalCompletion(undefined).

{

}

Let V be undefined.
Let A be the List of CaseClause items in CaseClauses, in source text order.
Let found be false.
For each CaseClause C in A, do
1. If found is false, then
  1. Set found to ? CaseClauseIsSelected(C, input).
2. If found is true, then
  1. Let R be the result of evaluating C.
  2. If R.[[Value]] is not empty, set V to R.[[Value]].
  3. If R is an abrupt completion, return Completion(UpdateEmpty(R, V)).
Return NormalCompletion(V).

{

opt

opt

}

Let V be undefined.
If the first CaseClauses is present, then
1. Let A be the List of CaseClause items in the first CaseClauses, in source text order.
Else,
1. Let A be « ».
Let found be false.
For each CaseClause C in A, do
1. If found is false, then
  1. Set found to ? CaseClauseIsSelected(C, input).
2. If found is true, then
  1. Let R be the result of evaluating C.
  2. If R.[[Value]] is not empty, set V to R.[[Value]].
  3. If R is an abrupt completion, return Completion(UpdateEmpty(R, V)).
Let foundInB be false.
If the second CaseClauses is present, then
1. Let B be the List of CaseClause items in the second CaseClauses, in source text order.
Else,
1. Let B be « ».
If found is false, then
1. For each CaseClause C in B, do
  1. If foundInB is false, then
    1. Set foundInB to ? CaseClauseIsSelected(C, input).
  2. If foundInB is true, then
    1. Let R be the result of evaluating CaseClause C.
    2. If R.[[Value]] is not empty, set V to R.[[Value]].
    3. If R is an abrupt completion, return Completion(UpdateEmpty(R, V)).
If foundInB is true, return NormalCompletion(V).
Let R be the result of evaluating DefaultClause.
If R.[[Value]] is not empty, set V to R.[[Value]].
If R is an abrupt completion, return Completion(UpdateEmpty(R, V)).
NOTE: The following is another complete iteration of the second CaseClauses.
For each CaseClause C in B, do
1. Let R be the result of evaluating CaseClause C.
2. If R.[[Value]] is not empty, set V to R.[[Value]].
3. If R is an abrupt completion, return Completion(UpdateEmpty(R, V)).
Return NormalCompletion(V).

13.12.10 Runtime Semantics: CaseClauseIsSelected ( `C`, `input` )

The abstract operation CaseClauseIsSelected takes arguments C (a Parse Node for CaseClause) and input (an ECMAScript language value). It determines whether C matches input. It performs the following steps when called:

Assert: C is an instance of the production CaseClause:caseExpression:StatementListopt .
Let exprRef be the result of evaluating the Expression of C.
Let clauseSelector be ? GetValue(exprRef).
Return the result of performing Strict Equality Comparison input === clauseSelector.

Note

This operation does not execute C's StatementList (if any). The CaseBlock algorithm uses its return value to determine which StatementList to start executing.

13.12.11 Runtime Semantics: Evaluation

switch

(

)

Let exprRef be the result of evaluating Expression.
Let switchValue be ? GetValue(exprRef).
Let oldEnv be the running execution context's LexicalEnvironment.
Let blockEnv be NewDeclarativeEnvironment(oldEnv).
Perform BlockDeclarationInstantiation(CaseBlock, blockEnv).
Set the running execution context's LexicalEnvironment to blockEnv.
Let R be CaseBlockEvaluation of CaseBlock with argument switchValue.
Set the running execution context's LexicalEnvironment to oldEnv.
Return R.

Note

No matter how control leaves the SwitchStatement the LexicalEnvironment is always restored to its former state.

case

Return NormalCompletion(empty).

case

Return the result of evaluating StatementList.

default

Return NormalCompletion(empty).

default

Return the result of evaluating StatementList.

13.13 Labelled Statements

Syntax

[Yield, Await, Return]

[?Yield, ?Await]

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ~Default]

Note

A Statement may be prefixed by a label. Labelled statements are only used in conjunction with labelled break and continue statements. ECMAScript has no goto statement. A Statement can be part of a LabelledStatement, which itself can be part of a LabelledStatement, and so on. The labels introduced this way are collectively referred to as the “current label set” when describing the semantics of individual statements.

13.13.1 Static Semantics: Early Errors

It is a Syntax Error if any source text matches this rule.

Note

An alternative definition for this rule is provided in B.3.2.

13.13.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

Let label be the StringValue of LabelIdentifier.
If label is an element of labelSet, return true.
Let newLabelSet be a copy of labelSet with label appended.
Return ContainsDuplicateLabels of LabelledItem with argument newLabelSet.

Return false.

13.13.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

Let label be the StringValue of LabelIdentifier.
Let newLabelSet be a copy of labelSet with label appended.
Return ContainsUndefinedBreakTarget of LabelledItem with argument newLabelSet.

Return false.

13.13.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

Let label be the StringValue of LabelIdentifier.
Let newLabelSet be a copy of labelSet with label appended.
Return ContainsUndefinedContinueTarget of LabelledItem with arguments iterationSet and newLabelSet.

Return false.

13.13.5 Static Semantics: IsLabelledFunction ( `stmt` )

The abstract operation IsLabelledFunction takes argument stmt. It performs the following steps when called:

If stmt is not a LabelledStatement, return false.
Let item be the LabelledItem of stmt.
If item is LabelledItem:FunctionDeclaration , return true.
Let subStmt be the Statement of item.
Return IsLabelledFunction(subStmt).

13.13.6 Static Semantics: LexicallyDeclaredNames

Return the LexicallyDeclaredNames of LabelledItem.

Return a new empty List.

Return BoundNames of FunctionDeclaration.

13.13.7 Static Semantics: LexicallyScopedDeclarations

Return the LexicallyScopedDeclarations of LabelledItem.

Return a new empty List.

Return a new List containing FunctionDeclaration.

13.13.8 Static Semantics: TopLevelLexicallyDeclaredNames

Return a new empty List.

13.13.9 Static Semantics: TopLevelLexicallyScopedDeclarations

Return a new empty List.

13.13.10 Static Semantics: TopLevelVarDeclaredNames

Return the TopLevelVarDeclaredNames of LabelledItem.

If Statement is Statement:LabelledStatement , return TopLevelVarDeclaredNames of Statement.
Return VarDeclaredNames of Statement.

Return BoundNames of FunctionDeclaration.

13.13.11 Static Semantics: TopLevelVarScopedDeclarations

Return the TopLevelVarScopedDeclarations of LabelledItem.

If Statement is Statement:LabelledStatement , return TopLevelVarScopedDeclarations of Statement.
Return VarScopedDeclarations of Statement.

Return a new List containing FunctionDeclaration.

13.13.12 Static Semantics: VarDeclaredNames

Return the VarDeclaredNames of LabelledItem.

Return a new empty List.

13.13.13 Static Semantics: VarScopedDeclarations

Return the VarScopedDeclarations of LabelledItem.

Return a new empty List.

13.13.14 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

Let label be the StringValue of LabelIdentifier.
Append label as an element of labelSet.
Let stmtResult be LabelledEvaluation of LabelledItem with argument labelSet.
If stmtResult.[[Type]] is break and SameValue(stmtResult.[[Target]], label) is true, then
1. Set stmtResult to NormalCompletion(stmtResult.[[Value]]).
Return Completion(stmtResult).

If Statement is either a LabelledStatement or a BreakableStatement, then
1. Return LabelledEvaluation of Statement with argument labelSet.
Else,
1. Return the result of evaluating Statement.

Return the result of evaluating FunctionDeclaration.

13.13.15 Runtime Semantics: Evaluation

Let newLabelSet be a new empty List.
Return LabelledEvaluation of this LabelledStatement with argument newLabelSet.

13.14 The `throw` Statement

Syntax

[Yield, Await]

throw

[no LineTerminator here]

[+In, ?Yield, ?Await]

;

13.14.1 Runtime Semantics: Evaluation

throw

;

Let exprRef be the result of evaluating Expression.
Let exprValue be ? GetValue(exprRef).
Return ThrowCompletion(exprValue).

13.15 The `try` Statement

Syntax

[Yield, Await, Return]

try

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

try

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

try

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

catch

(

[?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

catch

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

finally

[?Yield, ?Await, ?Return]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

Note

The try statement encloses a block of code in which an exceptional condition can occur, such as a runtime error or a throw statement. The catch clause provides the exception-handling code. When a catch clause catches an exception, its CatchParameter is bound to that exception.

13.15.1 Static Semantics: Early Errors

catch

(

)

It is a Syntax Error if BoundNames of CatchParameter contains any duplicate elements.
It is a Syntax Error if any element of the BoundNames of CatchParameter also occurs in the LexicallyDeclaredNames of Block.
It is a Syntax Error if any element of the BoundNames of CatchParameter also occurs in the VarDeclaredNames of Block.

Note

An alternative static semantics for this production is given in B.3.5.

13.15.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

try

Let hasDuplicates be ContainsDuplicateLabels of Block with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of Catch with argument labelSet.

try

Let hasDuplicates be ContainsDuplicateLabels of Block with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of Finally with argument labelSet.

try

Let hasDuplicates be ContainsDuplicateLabels of Block with argument labelSet.
If hasDuplicates is true, return true.
Let hasDuplicates be ContainsDuplicateLabels of Catch with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of Finally with argument labelSet.

catch

(

)

Return ContainsDuplicateLabels of Block with argument labelSet.

13.15.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

try

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Block with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of Catch with argument labelSet.

try

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Block with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of Finally with argument labelSet.

try

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Block with argument labelSet.
If hasUndefinedLabels is true, return true.
Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Catch with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of Finally with argument labelSet.

catch

(

)

Return ContainsUndefinedBreakTarget of Block with argument labelSet.

13.15.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

try

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of Catch with arguments iterationSet and « ».

try

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of Finally with arguments iterationSet and « ».

try

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Catch with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of Finally with arguments iterationSet and « ».

catch

(

)

Return ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».

13.15.5 Static Semantics: VarDeclaredNames

try

Let names be VarDeclaredNames of Block.
Append to names the elements of the VarDeclaredNames of Catch.
Return names.

try

Let names be VarDeclaredNames of Block.
Append to names the elements of the VarDeclaredNames of Finally.
Return names.

try

Let names be VarDeclaredNames of Block.
Append to names the elements of the VarDeclaredNames of Catch.
Append to names the elements of the VarDeclaredNames of Finally.
Return names.

catch

(

)

Return the VarDeclaredNames of Block.

13.15.6 Static Semantics: VarScopedDeclarations

try

Let declarations be VarScopedDeclarations of Block.
Append to declarations the elements of the VarScopedDeclarations of Catch.
Return declarations.

try

Let declarations be VarScopedDeclarations of Block.
Append to declarations the elements of the VarScopedDeclarations of Finally.
Return declarations.

try

Let declarations be VarScopedDeclarations of Block.
Append to declarations the elements of the VarScopedDeclarations of Catch.
Append to declarations the elements of the VarScopedDeclarations of Finally.
Return declarations.

catch

(

)

Return the VarScopedDeclarations of Block.

13.15.7 Runtime Semantics: CatchClauseEvaluation

With parameter thrownValue.

catch

(

)

Let oldEnv be the running execution context's LexicalEnvironment.
Let catchEnv be NewDeclarativeEnvironment(oldEnv).
For each element argName of the BoundNames of CatchParameter, do
1. Perform ! catchEnv.CreateMutableBinding(argName, false).
Set the running execution context's LexicalEnvironment to catchEnv.
Let status be BindingInitialization of CatchParameter with arguments thrownValue and catchEnv.
If status is an abrupt completion, then
1. Set the running execution context's LexicalEnvironment to oldEnv.
2. Return Completion(status).
Let B be the result of evaluating Block.
Set the running execution context's LexicalEnvironment to oldEnv.
Return Completion(B).

catch

Return the result of evaluating Block.

Note

No matter how control leaves the Block the LexicalEnvironment is always restored to its former state.

13.15.8 Runtime Semantics: Evaluation

try

Let B be the result of evaluating Block.
If B.[[Type]] is throw, let C be CatchClauseEvaluation of Catch with argument B.[[Value]].
Else, let C be B.
Return Completion(UpdateEmpty(C, undefined)).

try

Let B be the result of evaluating Block.
Let F be the result of evaluating Finally.
If F.[[Type]] is normal, set F to B.
Return Completion(UpdateEmpty(F, undefined)).

try

Let B be the result of evaluating Block.
If B.[[Type]] is throw, let C be CatchClauseEvaluation of Catch with argument B.[[Value]].
Else, let C be B.
Let F be the result of evaluating Finally.
If F.[[Type]] is normal, set F to C.
Return Completion(UpdateEmpty(F, undefined)).

13.16 The `debugger` Statement

Syntax

debugger

;

13.16.1 Runtime Semantics: Evaluation

Note

Evaluating a DebuggerStatement may allow an implementation to cause a breakpoint when run under a debugger. If a debugger is not present or active this statement has no observable effect.

debugger

;

If an implementation-defined debugging facility is available and enabled, then
1. Perform an implementation-defined debugging action.
2. Let result be an implementation-defined Completion value.
Else,
1. Let result be NormalCompletion(empty).
Return result.

14 ECMAScript Language: Functions and Classes

Note

Various ECMAScript language elements cause the creation of ECMAScript function objects (9.2). Evaluation of such functions starts with the execution of their [[Call]] internal method (9.2.1).

14.1 Function Definitions

Syntax

[Yield, Await, Default]

function

[?Yield, ?Await]

(

[~Yield, ~Await]

)

{

[~Yield, ~Await]

}

[+Default]

function

(

[~Yield, ~Await]

)

{

[~Yield, ~Await]

}

function

[~Yield, ~Await]

opt

(

[~Yield, ~Await]

)

{

[~Yield, ~Await]

}

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[empty]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await, +Return]

opt

14.1.1 Directive Prologues and the Use Strict Directive

A Directive Prologue is the longest sequence of ExpressionStatements occurring as the initial StatementListItems or ModuleItems of a FunctionBody, a ScriptBody, or a ModuleBody and where each ExpressionStatement in the sequence consists entirely of a StringLiteral token followed by a semicolon. The semicolon may appear explicitly or may be inserted by automatic semicolon insertion. A Directive Prologue may be an empty sequence.

A Use Strict Directive is an ExpressionStatement in a Directive Prologue whose StringLiteral is either of the exact code point sequences "use strict" or 'use strict'. A Use Strict Directive may not contain an EscapeSequence or LineContinuation.

A Directive Prologue may contain more than one Use Strict Directive. However, an implementation may issue a warning if this occurs.

Note

The ExpressionStatements of a Directive Prologue are evaluated normally during evaluation of the containing production. Implementations may define implementation specific meanings for ExpressionStatements which are not a Use Strict Directive and which occur in a Directive Prologue. If an appropriate notification mechanism exists, an implementation should issue a warning if it encounters in a Directive Prologue an ExpressionStatement that is not a Use Strict Directive and which does not have a meaning defined by the implementation.

14.1.2 Static Semantics: Early Errors

function

(

)

{

}

function

(

)

{

}

function

opt

(

)

{

}

If the source code matching FormalParameters is strict mode code, the Early Error rules for UniqueFormalParameters:FormalParameters are applied.
If BindingIdentifier is present and the source code matching BindingIdentifier is strict mode code, it is a Syntax Error if the StringValue of BindingIdentifier is "eval" or "arguments".
It is a Syntax Error if ContainsUseStrict of FunctionBody is true and IsSimpleParameterList of FormalParameters is false.
It is a Syntax Error if any element of the BoundNames of FormalParameters also occurs in the LexicallyDeclaredNames of FunctionBody.
It is a Syntax Error if FormalParameters Contains SuperProperty is true.
It is a Syntax Error if FunctionBody Contains SuperProperty is true.
It is a Syntax Error if FormalParameters Contains SuperCall is true.
It is a Syntax Error if FunctionBody Contains SuperCall is true.

Note 1

The LexicallyDeclaredNames of a FunctionBody does not include identifiers bound using var or function declarations.

It is a Syntax Error if BoundNames of FormalParameters contains any duplicate elements.

It is a Syntax Error if IsSimpleParameterList of FormalParameterList is false and BoundNames of FormalParameterList contains any duplicate elements.

Note 2

Multiple occurrences of the same BindingIdentifier in a FormalParameterList is only allowed for functions which have simple parameter lists and which are not defined in strict mode code.

It is a Syntax Error if the LexicallyDeclaredNames of FunctionStatementList contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of FunctionStatementList also occurs in the VarDeclaredNames of FunctionStatementList.
It is a Syntax Error if ContainsDuplicateLabels of FunctionStatementList with argument « » is true.
It is a Syntax Error if ContainsUndefinedBreakTarget of FunctionStatementList with argument « » is true.
It is a Syntax Error if ContainsUndefinedContinueTarget of FunctionStatementList with arguments « » and « » is true.

14.1.3 Static Semantics: BoundNames

function

(

)

{

}

Return the BoundNames of BindingIdentifier.

function

(

)

{

}

Return « "*default*" ».

Note

"*default*" is used within this specification as a synthetic name for hoistable anonymous functions that are defined using export declarations.

[empty]

Return a new empty List.

Let names be BoundNames of FormalParameterList.
Append to names the BoundNames of FunctionRestParameter.
Return names.

Let names be BoundNames of FormalParameterList.
Append to names the BoundNames of FormalParameter.
Return names.

14.1.4 Static Semantics: Contains

With parameter symbol.

function

(

)

{

}

function

(

)

{

}

function

opt

(

)

{

}

Return false.

Note

Static semantic rules that depend upon substructure generally do not look into function definitions.

14.1.5 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

[empty]

Return false.

14.1.6 Static Semantics: ContainsExpression

[empty]

Return false.

If ContainsExpression of FormalParameterList is true, return true.
Return ContainsExpression of FunctionRestParameter.

If ContainsExpression of FormalParameterList is true, return true.
Return ContainsExpression of FormalParameter.

14.1.7 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

[empty]

Return false.

14.1.8 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

[empty]

Return false.

14.1.9 Static Semantics: ContainsUseStrict

If the Directive Prologue of FunctionBody contains a Use Strict Directive, return true; otherwise, return false.

14.1.10 Static Semantics: ExpectedArgumentCount

[empty]

Return 0.

Return ExpectedArgumentCount of FormalParameterList.

Note

The ExpectedArgumentCount of a FormalParameterList is the number of FormalParameters to the left of either the rest parameter or the first FormalParameter with an Initializer. A FormalParameter without an initializer is allowed after the first parameter with an initializer but such parameters are considered to be optional with undefined as their default value.

If HasInitializer of FormalParameter is true, return 0.
Return 1.

Let count be ExpectedArgumentCount of FormalParameterList.
If HasInitializer of FormalParameterList is true or HasInitializer of FormalParameter is true, return count.
Return count + 1.

14.1.11 Static Semantics: HasInitializer

If HasInitializer of FormalParameterList is true, return true.
Return HasInitializer of FormalParameter.

14.1.12 Static Semantics: HasName

function

(

)

{

}

Return false.

function

(

)

{

}

Return true.

14.1.13 Static Semantics: IsAnonymousFunctionDefinition ( `expr` )

The abstract operation IsAnonymousFunctionDefinition takes argument expr (a Parse Node for AssignmentExpression or a Parse Node for Initializer). It determines if its argument is a function definition that does not bind a name. It performs the following steps when called:

If IsFunctionDefinition of expr is false, return false.
Let hasName be HasName of expr.
If hasName is true, return false.
Return true.

14.1.14 Static Semantics: IsConstantDeclaration

function

(

)

{

}

function

(

)

{

}

Return false.

14.1.15 Static Semantics: IsFunctionDefinition

function

opt

(

)

{

}

Return true.

14.1.16 Static Semantics: IsSimpleParameterList

[empty]

Return true.

Return false.

Return false.

If IsSimpleParameterList of FormalParameterList is false, return false.
Return IsSimpleParameterList of FormalParameter.

Return IsSimpleParameterList of BindingElement.

14.1.17 Static Semantics: LexicallyDeclaredNames

[empty]

Return a new empty List.

Return TopLevelLexicallyDeclaredNames of StatementList.

14.1.18 Static Semantics: LexicallyScopedDeclarations

[empty]

Return a new empty List.

Return the TopLevelLexicallyScopedDeclarations of StatementList.

14.1.19 Static Semantics: VarDeclaredNames

[empty]

Return a new empty List.

Return TopLevelVarDeclaredNames of StatementList.

14.1.20 Static Semantics: VarScopedDeclarations

[empty]

Return a new empty List.

Return the TopLevelVarScopedDeclarations of StatementList.

14.1.21 Runtime Semantics: EvaluateBody

With parameters functionObject and List argumentsList.

Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
Return the result of evaluating FunctionStatementList.

14.1.22 Runtime Semantics: IteratorBindingInitialization

With parameters iteratorRecord and environment.

Note

[empty]

Return NormalCompletion(empty).

Perform ? IteratorBindingInitialization for FormalParameterList using iteratorRecord and environment as the arguments.
Return the result of performing IteratorBindingInitialization for FunctionRestParameter using iteratorRecord and environment as the arguments.

Perform ? IteratorBindingInitialization for FormalParameterList using iteratorRecord and environment as the arguments.
Return the result of performing IteratorBindingInitialization for FormalParameter using iteratorRecord and environment as the arguments.

Return the result of performing IteratorBindingInitialization for BindingElement with arguments iteratorRecord and environment.

Return the result of performing IteratorBindingInitialization for BindingRestElement with arguments iteratorRecord and environment.

14.1.23 Runtime Semantics: InstantiateFunctionObject

With parameter scope.

function

(

)

{

}

Let name be StringValue of BindingIdentifier.
Let sourceText be the source text matched by FunctionDeclaration.
Let F be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, non-lexical-this, scope).
Perform SetFunctionName(F, name).
Perform MakeConstructor(F).
Return F.

function

(

)

{

}

Let sourceText be the source text matched by FunctionDeclaration.
Let F be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, non-lexical-this, scope).
Perform SetFunctionName(F, "default").
Perform MakeConstructor(F).
Return F.

Note

An anonymous FunctionDeclaration can only occur as part of an export default declaration, and its function code is therefore always strict mode code.

14.1.24 Runtime Semantics: NamedEvaluation

With parameter name.

function

(

)

{

}

Let scope be the LexicalEnvironment of the running execution context.
Let sourceText be the source text matched by FunctionExpression.
Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, non-lexical-this, scope).
Perform SetFunctionName(closure, name).
Perform MakeConstructor(closure).
Return closure.

14.1.25 Runtime Semantics: Evaluation

function

(

)

{

}

Return NormalCompletion(empty).

Note 1

An alternative semantics is provided in B.3.3.

function

(

)

{

}

Return NormalCompletion(empty).

function

(

)

{

}

Return the result of performing NamedEvaluation for this FunctionExpression with argument "".

function

(

)

{

}

Let scope be the running execution context's LexicalEnvironment.
Let funcEnv be NewDeclarativeEnvironment(scope).
Let name be StringValue of BindingIdentifier.
Perform funcEnv.CreateImmutableBinding(name, false).
Let sourceText be the source text matched by FunctionExpression.
Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, non-lexical-this, funcEnv).
Perform SetFunctionName(closure, name).
Perform MakeConstructor(closure).
Perform funcEnv.InitializeBinding(name, closure).
Return closure.

Note 2

The BindingIdentifier in a FunctionExpression can be referenced from inside the FunctionExpression's FunctionBody to allow the function to call itself recursively. However, unlike in a FunctionDeclaration, the BindingIdentifier in a FunctionExpression cannot be referenced from and does not affect the scope enclosing the FunctionExpression.

Note 3

A "prototype" property is automatically created for every function defined using a FunctionDeclaration or FunctionExpression, to allow for the possibility that the function will be used as a constructor.

[empty]

Return NormalCompletion(undefined).

14.2 Arrow Function Definitions

Syntax

[In, Yield, Await]

[?Yield, ?Await]

[no LineTerminator here]

[?In]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[In]

[lookahead ≠ {]

[?In, ~Await]

{

[~Yield, ~Await]

}

[In, Await]

[?In, ~Yield, ?Await]

Supplemental Syntax

When the production
ArrowParameters[Yield, Await]:CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
is recognized the following grammar is used to refine the interpretation of CoverParenthesizedExpressionAndArrowParameterList:

[Yield, Await]

(

[?Yield, ?Await]

)

14.2.1 Static Semantics: Early Errors

It is a Syntax Error if ArrowParameters Contains YieldExpression is true.
It is a Syntax Error if ArrowParameters Contains AwaitExpression is true.
It is a Syntax Error if ContainsUseStrict of ConciseBody is true and IsSimpleParameterList of ArrowParameters is false.
It is a Syntax Error if any element of the BoundNames of ArrowParameters also occurs in the LexicallyDeclaredNames of ConciseBody.

It is a Syntax Error if CoverParenthesizedExpressionAndArrowParameterList is not covering an ArrowFormalParameters.
All early error rules for ArrowFormalParameters and its derived productions also apply to CoveredFormalsList of CoverParenthesizedExpressionAndArrowParameterList.

14.2.2 Static Semantics: BoundNames

Let formals be CoveredFormalsList of CoverParenthesizedExpressionAndArrowParameterList.
Return the BoundNames of formals.

14.2.3 Static Semantics: Contains

With parameter symbol.

If symbol is not one of NewTarget, SuperProperty, SuperCall, super or this, return false.
If ArrowParameters Contains symbol is true, return true.
Return ConciseBody Contains symbol.

Note

Normally, Contains does not look inside most function forms. However, Contains is used to detect new.target, this, and super usage within an ArrowFunction.

Let formals be CoveredFormalsList of CoverParenthesizedExpressionAndArrowParameterList.
Return formals Contains symbol.

14.2.4 Static Semantics: ContainsExpression

Return false.

Let formals be CoveredFormalsList of CoverParenthesizedExpressionAndArrowParameterList.
Return ContainsExpression of formals.

14.2.5 Static Semantics: ContainsUseStrict

Return false.

14.2.6 Static Semantics: ExpectedArgumentCount

Return 1.

Let formals be CoveredFormalsList of CoverParenthesizedExpressionAndArrowParameterList.
Return ExpectedArgumentCount of formals.

14.2.7 Static Semantics: HasName

Return false.

14.2.8 Static Semantics: IsSimpleParameterList

Return true.

Let formals be CoveredFormalsList of CoverParenthesizedExpressionAndArrowParameterList.
Return IsSimpleParameterList of formals.

14.2.9 Static Semantics: CoveredFormalsList

Return this ArrowParameters.

(

)

(

)

(

)

(

...

)

(

...

)

(

...

)

(

...

)

Return the ArrowFormalParameters that is covered by CoverParenthesizedExpressionAndArrowParameterList.

14.2.10 Static Semantics: LexicallyDeclaredNames

Return a new empty List.

14.2.11 Static Semantics: LexicallyScopedDeclarations

Return a new empty List.

14.2.12 Static Semantics: VarDeclaredNames

Return a new empty List.

14.2.13 Static Semantics: VarScopedDeclarations

Return a new empty List.

14.2.14 Runtime Semantics: IteratorBindingInitialization

With parameters iteratorRecord and environment.

Note

Assert: iteratorRecord.[[Done]] is false.
Let next be IteratorStep(iteratorRecord).
If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
ReturnIfAbrupt(next).
If next is false, set iteratorRecord.[[Done]] to true.
Else,
1. Let v be IteratorValue(next).
2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(v).
If iteratorRecord.[[Done]] is true, let v be undefined.
Return the result of performing BindingInitialization for BindingIdentifier using v and environment as the arguments.

Let formals be CoveredFormalsList of CoverParenthesizedExpressionAndArrowParameterList.
Return IteratorBindingInitialization of formals with arguments iteratorRecord and environment.

14.2.15 Runtime Semantics: EvaluateBody

With parameters functionObject and List argumentsList.

Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
Return the result of evaluating ExpressionBody.

14.2.16 Runtime Semantics: NamedEvaluation

With parameter name.

Let scope be the LexicalEnvironment of the running execution context.
Let sourceText be the source text matched by ArrowFunction.
Let parameters be CoveredFormalsList of ArrowParameters.
Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, parameters, ConciseBody, lexical-this, scope).
Perform SetFunctionName(closure, name).
Return closure.

Note

An ArrowFunction does not define local bindings for arguments, super, this, or new.target. Any reference to arguments, super, this, or new.target within an ArrowFunction must resolve to a binding in a lexically enclosing environment. Typically this will be the Function Environment of an immediately enclosing function. Even though an ArrowFunction may contain references to super, the function object created in step 4 is not made into a method by performing MakeMethod. An ArrowFunction that references super is always contained within a non-ArrowFunction and the necessary state to implement super is accessible via the scope that is captured by the function object of the ArrowFunction.

14.2.17 Runtime Semantics: Evaluation

Return the result of performing NamedEvaluation for this ArrowFunction with argument "".

Let exprRef be the result of evaluating AssignmentExpression.
Let exprValue be ? GetValue(exprRef).
Return Completion { [[Type]]: return, [[Value]]: exprValue, [[Target]]: empty }.

14.3 Method Definitions

Syntax

[Yield, Await]

[?Yield, ?Await]

(

[~Yield, ~Await]

)

{

[~Yield, ~Await]

}

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

get

[?Yield, ?Await]

(

)

{

[~Yield, ~Await]

}

set

[?Yield, ?Await]

(

)

{

[~Yield, ~Await]

}

[~Yield, ~Await]

14.3.1 Static Semantics: Early Errors

(

)

{

}

It is a Syntax Error if ContainsUseStrict of FunctionBody is true and IsSimpleParameterList of UniqueFormalParameters is false.
It is a Syntax Error if any element of the BoundNames of UniqueFormalParameters also occurs in the LexicallyDeclaredNames of FunctionBody.

set

(

)

{

}

It is a Syntax Error if BoundNames of PropertySetParameterList contains any duplicate elements.
It is a Syntax Error if ContainsUseStrict of FunctionBody is true and IsSimpleParameterList of PropertySetParameterList is false.
It is a Syntax Error if any element of the BoundNames of PropertySetParameterList also occurs in the LexicallyDeclaredNames of FunctionBody.

14.3.2 Static Semantics: ComputedPropertyContains

With parameter symbol.

(

)

{

}

get

(

)

{

}

set

(

)

{

}

Return the result of ComputedPropertyContains for PropertyName with argument symbol.

14.3.3 Static Semantics: ExpectedArgumentCount

If HasInitializer of FormalParameter is true, return 0.
Return 1.

14.3.4 Static Semantics: HasDirectSuper

(

)

{

}

If UniqueFormalParameters Contains SuperCall is true, return true.
Return FunctionBody Contains SuperCall.

get

(

)

{

}

Return FunctionBody Contains SuperCall.

set

(

)

{

}

If PropertySetParameterList Contains SuperCall is true, return true.
Return FunctionBody Contains SuperCall.

14.3.5 Static Semantics: PropName

(

)

{

}

get

(

)

{

}

set

(

)

{

}

Return PropName of PropertyName.

14.3.6 Static Semantics: SpecialMethod

(

)

{

}

Return false.

get

(

)

{

}

set

(

)

{

}

Return true.

14.3.7 Runtime Semantics: DefineMethod

With parameter object and optional parameter functionPrototype.

(

)

{

}

Let propKey be the result of evaluating PropertyName.
ReturnIfAbrupt(propKey).
Let scope be the running execution context's LexicalEnvironment.
If functionPrototype is present, then
1. Let prototype be functionPrototype.
Else,
1. Let prototype be %Function.prototype%.
Let sourceText be the source text matched by MethodDefinition.
Let closure be OrdinaryFunctionCreate(prototype, sourceText, UniqueFormalParameters, FunctionBody, non-lexical-this, scope).
Perform MakeMethod(closure, object).
Return the Record { [[Key]]: propKey, [[Closure]]: closure }.

14.3.8 Runtime Semantics: PropertyDefinitionEvaluation

With parameters object and enumerable.

(

)

{

}

Let methodDef be ? DefineMethod of MethodDefinition with argument object.
Perform SetFunctionName(methodDef.[[Closure]], methodDef.[[Key]]).
Let desc be the PropertyDescriptor { [[Value]]: methodDef.[[Closure]], [[Writable]]: true, [[Enumerable]]: enumerable, [[Configurable]]: true }.
Return ? DefinePropertyOrThrow(object, methodDef.[[Key]], desc).

get

(

)

{

}

Let propKey be the result of evaluating PropertyName.
ReturnIfAbrupt(propKey).
Let scope be the running execution context's LexicalEnvironment.
Let sourceText be the source text matched by MethodDefinition.
Let formalParameterList be an instance of the production FormalParameters:[empty] .
Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, formalParameterList, FunctionBody, non-lexical-this, scope).
Perform MakeMethod(closure, object).
Perform SetFunctionName(closure, propKey, "get").
Let desc be the PropertyDescriptor { [[Get]]: closure, [[Enumerable]]: enumerable, [[Configurable]]: true }.
Return ? DefinePropertyOrThrow(object, propKey, desc).

set

(

)

{

}

Let propKey be the result of evaluating PropertyName.
ReturnIfAbrupt(propKey).
Let scope be the running execution context's LexicalEnvironment.
Let sourceText be the source text matched by MethodDefinition.
Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, PropertySetParameterList, FunctionBody, non-lexical-this, scope).
Perform MakeMethod(closure, object).
Perform SetFunctionName(closure, propKey, "set").
Let desc be the PropertyDescriptor { [[Set]]: closure, [[Enumerable]]: enumerable, [[Configurable]]: true }.
Return ? DefinePropertyOrThrow(object, propKey, desc).

14.4 Generator Function Definitions

Syntax

[Yield, Await]

[?Yield, ?Await]

(

[+Yield, ~Await]

)

{

}

[Yield, Await, Default]

function

[?Yield, ?Await]

(

[+Yield, ~Await]

)

{

}

[+Default]

function

(

[+Yield, ~Await]

)

{

}

function

[+Yield, ~Await]

opt

(

[+Yield, ~Await]

)

{

}

[+Yield, ~Await]

[In, Await]

yield

[no LineTerminator here]

[?In, +Yield, ?Await]

yield

[no LineTerminator here]

[?In, +Yield, ?Await]

Note 1

The syntactic context immediately following yield requires use of the InputElementRegExpOrTemplateTail lexical goal.

Note 2

YieldExpression cannot be used within the FormalParameters of a generator function because any expressions that are part of FormalParameters are evaluated before the resulting generator object is in a resumable state.

Note 3

Abstract operations relating to generator objects are defined in 25.4.3.

14.4.1 Static Semantics: Early Errors

(

)

{

}

It is a Syntax Error if HasDirectSuper of GeneratorMethod is true.
It is a Syntax Error if UniqueFormalParameters Contains YieldExpression is true.
It is a Syntax Error if ContainsUseStrict of GeneratorBody is true and IsSimpleParameterList of UniqueFormalParameters is false.
It is a Syntax Error if any element of the BoundNames of UniqueFormalParameters also occurs in the LexicallyDeclaredNames of GeneratorBody.

function

(

)

{

}

function

(

)

{

}

function

opt

(

)

{

}

If the source code matching FormalParameters is strict mode code, the Early Error rules for UniqueFormalParameters:FormalParameters are applied.
If BindingIdentifier is present and the source code matching BindingIdentifier is strict mode code, it is a Syntax Error if the StringValue of BindingIdentifier is "eval" or "arguments".
It is a Syntax Error if ContainsUseStrict of GeneratorBody is true and IsSimpleParameterList of FormalParameters is false.
It is a Syntax Error if any element of the BoundNames of FormalParameters also occurs in the LexicallyDeclaredNames of GeneratorBody.
It is a Syntax Error if FormalParameters Contains YieldExpression is true.
It is a Syntax Error if FormalParameters Contains SuperProperty is true.
It is a Syntax Error if GeneratorBody Contains SuperProperty is true.
It is a Syntax Error if FormalParameters Contains SuperCall is true.
It is a Syntax Error if GeneratorBody Contains SuperCall is true.

14.4.2 Static Semantics: BoundNames

function

(

)

{

}

Return the BoundNames of BindingIdentifier.

function

(

)

{

}

Return « "*default*" ».

Note

"*default*" is used within this specification as a synthetic name for hoistable anonymous functions that are defined using export declarations.

14.4.3 Static Semantics: ComputedPropertyContains

With parameter symbol.

(

)

{

}

Return the result of ComputedPropertyContains for PropertyName with argument symbol.

14.4.4 Static Semantics: Contains

With parameter symbol.

function

(

)

{

}

function

(

)

{

}

function

opt

(

)

{

}

Return false.

Note

Static semantic rules that depend upon substructure generally do not look into function definitions.

14.4.5 Static Semantics: HasDirectSuper

(

)

{

}

If UniqueFormalParameters Contains SuperCall is true, return true.
Return GeneratorBody Contains SuperCall.

14.4.6 Static Semantics: HasName

function

(

)

{

}

Return false.

function

(

)

{

}

Return true.

14.4.7 Static Semantics: IsConstantDeclaration

function

(

)

{

}

function

(

)

{

}

Return false.

14.4.8 Static Semantics: IsFunctionDefinition

function

opt

(

)

{

}

Return true.

14.4.9 Static Semantics: PropName

(

)

{

}

Return PropName of PropertyName.

14.4.10 Runtime Semantics: EvaluateBody

With parameters functionObject and List argumentsList.

Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
Let G be ? OrdinaryCreateFromConstructor(functionObject, "%Generator.prototype%", « [[GeneratorState]], [[GeneratorContext]] »).
Perform GeneratorStart(G, FunctionBody).
Return Completion { [[Type]]: return, [[Value]]: G, [[Target]]: empty }.

14.4.11 Runtime Semantics: InstantiateFunctionObject

With parameter scope.

function

(

)

{

}

Let name be StringValue of BindingIdentifier.
Let sourceText be the source text matched by GeneratorDeclaration.
Let F be OrdinaryFunctionCreate(%Generator%, sourceText, FormalParameters, GeneratorBody, non-lexical-this, scope).
Perform SetFunctionName(F, name).
Let prototype be OrdinaryObjectCreate(%Generator.prototype%).
Perform DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Return F.

function

(

)

{

}

Let sourceText be the source text matched by GeneratorDeclaration.
Let F be OrdinaryFunctionCreate(%Generator%, sourceText, FormalParameters, GeneratorBody, non-lexical-this, scope).
Perform SetFunctionName(F, "default").
Let prototype be OrdinaryObjectCreate(%Generator.prototype%).
Perform DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Return F.

Note

An anonymous GeneratorDeclaration can only occur as part of an export default declaration, and its function code is therefore always strict mode code.

14.4.12 Runtime Semantics: PropertyDefinitionEvaluation

With parameters object and enumerable.

(

)

{

}

Let propKey be the result of evaluating PropertyName.
ReturnIfAbrupt(propKey).
Let scope be the running execution context's LexicalEnvironment.
Let sourceText be the source text matched by GeneratorMethod.
Let closure be OrdinaryFunctionCreate(%Generator%, sourceText, UniqueFormalParameters, GeneratorBody, non-lexical-this, scope).
Perform MakeMethod(closure, object).
Perform SetFunctionName(closure, propKey).
Let prototype be OrdinaryObjectCreate(%Generator.prototype%).
Perform DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Let desc be the PropertyDescriptor { [[Value]]: closure, [[Writable]]: true, [[Enumerable]]: enumerable, [[Configurable]]: true }.
Return ? DefinePropertyOrThrow(object, propKey, desc).

14.4.13 Runtime Semantics: NamedEvaluation

With parameter name.

function

(

)

{

}

Let scope be the LexicalEnvironment of the running execution context.
Let sourceText be the source text matched by GeneratorExpression.
Let closure be OrdinaryFunctionCreate(%Generator%, sourceText, FormalParameters, GeneratorBody, non-lexical-this, scope).
Perform SetFunctionName(closure, name).
Let prototype be OrdinaryObjectCreate(%Generator.prototype%).
Perform DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Return closure.

14.4.14 Runtime Semantics: Evaluation

function

(

)

{

}

Return the result of performing NamedEvaluation for this GeneratorExpression with argument "".

function

(

)

{

}

Let scope be the running execution context's LexicalEnvironment.
Let funcEnv be NewDeclarativeEnvironment(scope).
Let name be StringValue of BindingIdentifier.
Perform funcEnv.CreateImmutableBinding(name, false).
Let sourceText be the source text matched by GeneratorExpression.
Let closure be OrdinaryFunctionCreate(%Generator%, sourceText, FormalParameters, GeneratorBody, non-lexical-this, funcEnv).
Perform SetFunctionName(closure, name).
Let prototype be OrdinaryObjectCreate(%Generator.prototype%).
Perform DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Perform funcEnv.InitializeBinding(name, closure).
Return closure.

Note

The BindingIdentifier in a GeneratorExpression can be referenced from inside the GeneratorExpression's FunctionBody to allow the generator code to call itself recursively. However, unlike in a GeneratorDeclaration, the BindingIdentifier in a GeneratorExpression cannot be referenced from and does not affect the scope enclosing the GeneratorExpression.

yield

Let generatorKind be ! GetGeneratorKind().
If generatorKind is async, then return ? AsyncGeneratorYield(undefined).
Otherwise, return ? GeneratorYield(CreateIterResultObject(undefined, false)).

yield

Let generatorKind be ! GetGeneratorKind().
Let exprRef be the result of evaluating AssignmentExpression.
Let value be ? GetValue(exprRef).
If generatorKind is async, then return ? AsyncGeneratorYield(value).
Otherwise, return ? GeneratorYield(CreateIterResultObject(value, false)).

yield

Let generatorKind be ! GetGeneratorKind().
Let exprRef be the result of evaluating AssignmentExpression.
Let value be ? GetValue(exprRef).
Let iteratorRecord be ? GetIterator(value, generatorKind).
Let iterator be iteratorRecord.[[Iterator]].
Let received be NormalCompletion(undefined).
Repeat,
1. If received.[[Type]] is normal, then
  1. Let innerResult be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]], « received.[[Value]] »).
  2. If generatorKind is async, then set innerResult to ? Await(innerResult).
  3. If Type(innerResult) is not Object, throw a TypeError exception.
  4. Let done be ? IteratorComplete(innerResult).
  5. If done is true, then
    1. Return ? IteratorValue(innerResult).
  6. If generatorKind is async, then set received to AsyncGeneratorYield(? IteratorValue(innerResult)).
  7. Else, set received to GeneratorYield(innerResult).
2. Else if received.[[Type]] is throw, then
  1. Let throw be ? GetMethod(iterator, "throw").
  2. If throw is not undefined, then
    1. Let innerResult be ? Call(throw, iterator, « received.[[Value]] »).
    2. If generatorKind is async, then set innerResult to ? Await(innerResult).
    3. NOTE: Exceptions from the inner iterator throw method are propagated. Normal completions from an inner throw method are processed similarly to an inner next.
    4. If Type(innerResult) is not Object, throw a TypeError exception.
    5. Let done be ? IteratorComplete(innerResult).
    6. If done is true, then
      1. Return ? IteratorValue(innerResult).
    7. If generatorKind is async, then set received to AsyncGeneratorYield(? IteratorValue(innerResult)).
    8. Else, set received to GeneratorYield(innerResult).
  3. Else,
    1. NOTE: If iterator does not have a throw method, this throw is going to terminate the yield* loop. But first we need to give iterator a chance to clean up.
    2. Let closeCompletion be Completion { [[Type]]: normal, [[Value]]: empty, [[Target]]: empty }.
    3. If generatorKind is async, perform ? AsyncIteratorClose(iteratorRecord, closeCompletion).
    4. Else, perform ? IteratorClose(iteratorRecord, closeCompletion).
    5. NOTE: The next step throws a TypeError to indicate that there was a yield* protocol violation: iterator does not have a throw method.
    6. Throw a TypeError exception.
3. Else,
  1. Assert: received.[[Type]] is return.
  2. Let return be ? GetMethod(iterator, "return").
  3. If return is undefined, then
    1. If generatorKind is async, then set received.[[Value]] to ? Await(received.[[Value]]).
    2. Return Completion(received).
  4. Let innerReturnResult be ? Call(return, iterator, « received.[[Value]] »).
  5. If generatorKind is async, then set innerReturnResult to ? Await(innerReturnResult).
  6. If Type(innerReturnResult) is not Object, throw a TypeError exception.
  7. Let done be ? IteratorComplete(innerReturnResult).
  8. If done is true, then
    1. Let value be ? IteratorValue(innerReturnResult).
    2. Return Completion { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.
  9. If generatorKind is async, then set received to AsyncGeneratorYield(? IteratorValue(innerReturnResult)).
  10. Else, set received to GeneratorYield(innerReturnResult).

14.5 Async Generator Function Definitions

Syntax

[Yield, Await]

async

[no LineTerminator here]

[?Yield, ?Await]

(

[+Yield, +Await]

)

{

}

[Yield, Await, Default]

async

[no LineTerminator here]

function

[?Yield, ?Await]

(

[+Yield, +Await]

)

{

}

[+Default]

async

[no LineTerminator here]

function

(

[+Yield, +Await]

)

{

}

async

[no LineTerminator here]

function

[+Yield, +Await]

opt

(

[+Yield, +Await]

)

{

}

[+Yield, +Await]

Note 1

YieldExpression and AwaitExpression cannot be used within the FormalParameters of an async generator function because any expressions that are part of FormalParameters are evaluated before the resulting async generator object is in a resumable state.

Note 2

Abstract operations relating to async generator objects are defined in 25.5.3.

14.5.1 Static Semantics: Early Errors

async

(

)

{

}

It is a Syntax Error if HasDirectSuper of AsyncGeneratorMethod is true.
It is a Syntax Error if UniqueFormalParameters Contains YieldExpression is true.
It is a Syntax Error if UniqueFormalParameters Contains AwaitExpression is true.
It is a Syntax Error if ContainsUseStrict of AsyncGeneratorBody is true and IsSimpleParameterList of UniqueFormalParameters is false.
It is a Syntax Error if any element of the BoundNames of UniqueFormalParameters also occurs in the LexicallyDeclaredNames of AsyncGeneratorBody.

async

function

(

)

{

}

async

function

(

)

{

}

async

function

opt

(

)

{

}

If the source code matching FormalParameters is strict mode code, the Early Error rules for UniqueFormalParameters:FormalParameters are applied.
If BindingIdentifier is present and the source code matching BindingIdentifier is strict mode code, it is a Syntax Error if the StringValue of BindingIdentifier is "eval" or "arguments".
It is a Syntax Error if ContainsUseStrict of AsyncGeneratorBody is true and IsSimpleParameterList of FormalParameters is false.
It is a Syntax Error if any element of the BoundNames of FormalParameters also occurs in the LexicallyDeclaredNames of AsyncGeneratorBody.
It is a Syntax Error if FormalParameters Contains YieldExpression is true.
It is a Syntax Error if FormalParameters Contains AwaitExpression is true.
It is a Syntax Error if FormalParameters Contains SuperProperty is true.
It is a Syntax Error if AsyncGeneratorBody Contains SuperProperty is true.
It is a Syntax Error if FormalParameters Contains SuperCall is true.
It is a Syntax Error if AsyncGeneratorBody Contains SuperCall is true.

14.5.2 Static Semantics: BoundNames

async

function

(

)

{

}

Return the BoundNames of BindingIdentifier.

async

function

(

)

{

}

Return « "*default*" ».

Note

"*default*" is used within this specification as a synthetic name for hoistable anonymous functions that are defined using export declarations.

14.5.3 Static Semantics: ComputedPropertyContains

With parameter symbol.

async

(

)

{

}

Return the result of ComputedPropertyContains for PropertyName with argument symbol.

14.5.4 Static Semantics: Contains

With parameter symbol.

async

function

(

)

{

}

async

function

(

)

{

}

async

function

opt

(

)

{

}

Return false.

Note

Static semantic rules that depend upon substructure generally do not look into function definitions.

14.5.5 Static Semantics: HasDirectSuper

async

(

)

{

}

If UniqueFormalParameters Contains SuperCall is true, return true.
Return AsyncGeneratorBody Contains SuperCall.

14.5.6 Static Semantics: HasName

async

function

(

)

{

}

Return false.

async

function

(

)

{

}

Return true.

14.5.7 Static Semantics: IsConstantDeclaration

async

function

(

)

{

}

async

function

(

)

{

}

Return false.

14.5.8 Static Semantics: IsFunctionDefinition

async

function

opt

(

)

{

}

Return true.

14.5.9 Static Semantics: PropName

async

(

)

{

}

Return PropName of PropertyName.

14.5.10 Runtime Semantics: EvaluateBody

With parameters functionObject and List argumentsList.

Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
Let generator be ? OrdinaryCreateFromConstructor(functionObject, "%AsyncGenerator.prototype%", « [[AsyncGeneratorState]], [[AsyncGeneratorContext]], [[AsyncGeneratorQueue]] »).
Perform ! AsyncGeneratorStart(generator, FunctionBody).
Return Completion { [[Type]]: return, [[Value]]: generator, [[Target]]: empty }.

14.5.11 Runtime Semantics: InstantiateFunctionObject

With parameter scope.

async

function

(

)

{

}

Let name be StringValue of BindingIdentifier.
Let sourceText be the source text matched by AsyncGeneratorDeclaration.
Let F be ! OrdinaryFunctionCreate(%AsyncGenerator%, sourceText, FormalParameters, AsyncGeneratorBody, non-lexical-this, scope).
Perform ! SetFunctionName(F, name).
Let prototype be ! OrdinaryObjectCreate(%AsyncGenerator.prototype%).
Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Return F.

async

function

(

)

{

}

Let sourceText be the source text matched by AsyncGeneratorDeclaration.
Let F be OrdinaryFunctionCreate(%AsyncGenerator%, sourceText, FormalParameters, AsyncGeneratorBody, non-lexical-this, scope).
Perform SetFunctionName(F, "default").
Let prototype be OrdinaryObjectCreate(%AsyncGenerator.prototype%).
Perform DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Return F.

Note

An anonymous AsyncGeneratorDeclaration can only occur as part of an export default declaration.

14.5.12 Runtime Semantics: PropertyDefinitionEvaluation

With parameter object and enumerable.

async

(

)

{

}

Let propKey be the result of evaluating PropertyName.
ReturnIfAbrupt(propKey).
Let scope be the running execution context's LexicalEnvironment.
Let sourceText be the source text matched by AsyncGeneratorMethod.
Let closure be ! OrdinaryFunctionCreate(%AsyncGenerator%, sourceText, UniqueFormalParameters, AsyncGeneratorBody, non-lexical-this, scope).
Perform ! MakeMethod(closure, object).
Perform ! SetFunctionName(closure, propKey).
Let prototype be ! OrdinaryObjectCreate(%AsyncGenerator.prototype%).
Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Let desc be PropertyDescriptor { [[Value]]: closure, [[Writable]]: true, [[Enumerable]]: enumerable, [[Configurable]]: true }.
Return ? DefinePropertyOrThrow(object, propKey, desc).

14.5.13 Runtime Semantics: NamedEvaluation

With parameter name.

async

function

(

)

{

}

Let scope be the LexicalEnvironment of the running execution context.
Let sourceText be the source text matched by AsyncGeneratorExpression.
Let closure be ! OrdinaryFunctionCreate(%AsyncGenerator%, sourceText, FormalParameters, AsyncGeneratorBody, non-lexical-this, scope).
Perform SetFunctionName(closure, name).
Let prototype be ! OrdinaryObjectCreate(%AsyncGenerator.prototype%).
Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Return closure.

14.5.14 Runtime Semantics: Evaluation

async

function

(

)

{

}

Return the result of performing NamedEvaluation for this AsyncGeneratorExpression with argument "".

async

function

(

)

{

}

Let scope be the running execution context's LexicalEnvironment.
Let funcEnv be ! NewDeclarativeEnvironment(scope).
Let name be StringValue of BindingIdentifier.
Perform ! funcEnv.CreateImmutableBinding(name, false).
Let sourceText be the source text matched by AsyncGeneratorExpression.
Let closure be ! OrdinaryFunctionCreate(%AsyncGenerator%, sourceText, FormalParameters, AsyncGeneratorBody, non-lexical-this, funcEnv).
Perform ! SetFunctionName(closure, name).
Let prototype be ! OrdinaryObjectCreate(%AsyncGenerator.prototype%).
Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Perform ! funcEnv.InitializeBinding(name, closure).
Return closure.

Note

The BindingIdentifier in an AsyncGeneratorExpression can be referenced from inside the AsyncGeneratorExpression's AsyncGeneratorBody to allow the generator code to call itself recursively. However, unlike in an AsyncGeneratorDeclaration, the BindingIdentifier in an AsyncGeneratorExpression cannot be referenced from and does not affect the scope enclosing the AsyncGeneratorExpression.

14.6 Class Definitions

Syntax

[Yield, Await, Default]

class

[?Yield, ?Await]

[?Yield, ?Await]

[+Default]

class

[?Yield, ?Await]

[Yield, Await]

class

[?Yield, ?Await]

opt

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

opt

{

[?Yield, ?Await]

opt

}

[Yield, Await]

extends

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

static

[?Yield, ?Await]

;

Note

A class definition is always strict mode code.

14.6.1 Static Semantics: Early Errors

opt

{

}

It is a Syntax Error if ClassHeritage is not present and the following algorithm evaluates to true:
1. Let constructor be ConstructorMethod of ClassBody.
2. If constructor is empty, return false.
3. Return HasDirectSuper of constructor.

It is a Syntax Error if PrototypePropertyNameList of ClassElementList contains more than one occurrence of "constructor".

It is a Syntax Error if PropName of MethodDefinition is not "constructor" and HasDirectSuper of MethodDefinition is true.
It is a Syntax Error if PropName of MethodDefinition is "constructor" and SpecialMethod of MethodDefinition is true.

static

It is a Syntax Error if HasDirectSuper of MethodDefinition is true.
It is a Syntax Error if PropName of MethodDefinition is "prototype".

14.6.2 Static Semantics: BoundNames

class

Return the BoundNames of BindingIdentifier.

class

Return « "*default*" ».

14.6.3 Static Semantics: ClassElementKind

If PropName of MethodDefinition is "constructor", return ConstructorMethod.
Return NonConstructorMethod.

static

Return NonConstructorMethod.

;

Return empty.

14.6.4 Static Semantics: ConstructorMethod

If ClassElementKind of ClassElement is ConstructorMethod, return ClassElement.
Return empty.

Let head be ConstructorMethod of ClassElementList.
If head is not empty, return head.
If ClassElementKind of ClassElement is ConstructorMethod, return ClassElement.
Return empty.

Note

Early Error rules ensure that there is only one method definition named "constructor" and that it is not an accessor property or generator definition.

14.6.5 Static Semantics: Contains

With parameter symbol.

opt

{

}

If symbol is ClassBody, return true.
If symbol is ClassHeritage, then
1. If ClassHeritage is present, return true; otherwise return false.
Let inHeritage be ClassHeritage Contains symbol.
If inHeritage is true, return true.
Return the result of ComputedPropertyContains for ClassBody with argument symbol.

Note

Static semantic rules that depend upon substructure generally do not look into class bodies except for PropertyNames.

14.6.6 Static Semantics: ComputedPropertyContains

With parameter symbol.

Let inList be ComputedPropertyContains of ClassElementList with argument symbol.
If inList is true, return true.
Return the result of ComputedPropertyContains for ClassElement with argument symbol.

;

Return false.

14.6.7 Static Semantics: HasName

class

Return false.

class

Return true.

14.6.8 Static Semantics: IsConstantDeclaration

class

class

Return false.

14.6.9 Static Semantics: IsFunctionDefinition

class

opt

Return true.

14.6.10 Static Semantics: IsStatic

Return false.

static

Return true.

;

Return false.

14.6.11 Static Semantics: NonConstructorMethodDefinitions

If ClassElementKind of ClassElement is NonConstructorMethod, then
1. Return a List containing ClassElement.
Return a new empty List.

Let list be NonConstructorMethodDefinitions of ClassElementList.
If ClassElementKind of ClassElement is NonConstructorMethod, then
1. Append ClassElement to the end of list.
Return list.

14.6.12 Static Semantics: PrototypePropertyNameList

If PropName of ClassElement is empty, return a new empty List.
If IsStatic of ClassElement is true, return a new empty List.
Return a List containing PropName of ClassElement.

Let list be PrototypePropertyNameList of ClassElementList.
If PropName of ClassElement is empty, return list.
If IsStatic of ClassElement is true, return list.
Append PropName of ClassElement to the end of list.
Return list.

14.6.13 Static Semantics: PropName

;

Return empty.

14.6.14 Runtime Semantics: ClassDefinitionEvaluation

With parameters classBinding and className.

opt

{

opt

}

Let env be the LexicalEnvironment of the running execution context.
Let classScope be NewDeclarativeEnvironment(env).
If classBinding is not undefined, then
1. Perform classScope.CreateImmutableBinding(classBinding, true).
If ClassHeritageopt is not present, then
1. Let protoParent be %Object.prototype%.
2. Let constructorParent be %Function.prototype%.
Else,
1. Set the running execution context's LexicalEnvironment to classScope.
2. Let superclassRef be the result of evaluating ClassHeritage.
3. Set the running execution context's LexicalEnvironment to env.
4. Let superclass be ? GetValue(superclassRef).
5. If superclass is null, then
  1. Let protoParent be null.
  2. Let constructorParent be %Function.prototype%.
6. Else if IsConstructor(superclass) is false, throw a TypeError exception.
7. Else,
  1. Let protoParent be ? Get(superclass, "prototype").
  2. If Type(protoParent) is neither Object nor Null, throw a TypeError exception.
  3. Let constructorParent be superclass.
Let proto be OrdinaryObjectCreate(protoParent).
If ClassBodyopt is not present, let constructor be empty.
Else, let constructor be ConstructorMethod of ClassBody.
If constructor is empty, then
1. If ClassHeritageopt is present, then
  1. Set constructor to the result of parsing the source text
```
constructor(...args) { super(...args); }
```
    using the syntactic grammar with the goal symbol MethodDefinition[~Yield, ~Await].
2. Else,
  1. Set constructor to the result of parsing the source text
```
constructor() {}
```
    using the syntactic grammar with the goal symbol MethodDefinition[~Yield, ~Await].
Set the running execution context's LexicalEnvironment to classScope.
Let constructorInfo be ! DefineMethod of constructor with arguments proto and constructorParent.
Let F be constructorInfo.[[Closure]].
Perform SetFunctionName(F, className).
Perform MakeConstructor(F, false, proto).
If ClassHeritageopt is present, set F.[[ConstructorKind]] to derived.
Perform MakeClassConstructor(F).
Perform CreateMethodProperty(proto, "constructor", F).
If ClassBodyopt is not present, let methods be a new empty List.
Else, let methods be NonConstructorMethodDefinitions of ClassBody.
For each ClassElement m in order from methods, do
1. If IsStatic of m is false, then
  1. Let status be PropertyDefinitionEvaluation of m with arguments proto and false.
2. Else,
  1. Let status be PropertyDefinitionEvaluation of m with arguments F and false.
3. If status is an abrupt completion, then
  1. Set the running execution context's LexicalEnvironment to env.
  2. Return Completion(status).
Set the running execution context's LexicalEnvironment to env.
If classBinding is not undefined, then
1. Perform classScope.InitializeBinding(classBinding, F).
Return F.

14.6.15 Runtime Semantics: BindingClassDeclarationEvaluation

class

Let className be StringValue of BindingIdentifier.
Let value be ? ClassDefinitionEvaluation of ClassTail with arguments className and className.
Set value.[[SourceText]] to the source text matched by ClassDeclaration.
Let env be the running execution context's LexicalEnvironment.
Perform ? InitializeBoundName(className, value, env).
Return value.

class

Let value be ? ClassDefinitionEvaluation of ClassTail with arguments undefined and "default".
Set value.[[SourceText]] to the source text matched by ClassDeclaration.
Return value.

Note

ClassDeclaration:classClassTail only occurs as part of an ExportDeclaration and establishing its binding is handled as part of the evaluation action for that production. See 15.2.3.11.

14.6.16 Runtime Semantics: NamedEvaluation

With parameter name.

class

Let value be the result of ClassDefinitionEvaluation of ClassTail with arguments undefined and name.
ReturnIfAbrupt(value).
Set value.[[SourceText]] to the source text matched by ClassExpression.
Return value.

14.6.17 Runtime Semantics: Evaluation

class

Perform ? BindingClassDeclarationEvaluation of this ClassDeclaration.
Return NormalCompletion(empty).

Note

ClassDeclaration:classClassTail only occurs as part of an ExportDeclaration and is never directly evaluated.

class

Let value be ? ClassDefinitionEvaluation of ClassTail with arguments undefined and "".
Set value.[[SourceText]] to the source text matched by ClassExpression.
Return value.

class

Let className be StringValue of BindingIdentifier.
Let value be ? ClassDefinitionEvaluation of ClassTail with arguments className and className.
Set value.[[SourceText]] to the source text matched by ClassExpression.
Return value.

14.7 Async Function Definitions

Syntax

[Yield, Await, Default]

async

[no LineTerminator here]

function

[?Yield, ?Await]

(

[~Yield, +Await]

)

{

}

[+Default]

async

[no LineTerminator here]

function

(

[~Yield, +Await]

)

{

}

async

[no LineTerminator here]

function

(

[~Yield, +Await]

)

{

}

async

[no LineTerminator here]

function

[~Yield, +Await]

(

[~Yield, +Await]

)

{

}

[Yield, Await]

async

[no LineTerminator here]

[?Yield, ?Await]

(

[~Yield, +Await]

)

{

}

[~Yield, +Await]

[Yield]

await

[?Yield, +Await]

Note 1

await is parsed as an AwaitExpression when the _[Await] parameter is present. The _[Await] parameter is present in the following contexts:

In an AsyncFunctionBody.
In the FormalParameters of an AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, or AsyncGeneratorExpression. AwaitExpression in this position is a Syntax error via static semantics.

When Module is the syntactic goal symbol and the _[Await] parameter is absent, await is parsed as a keyword and will be a Syntax error. When Script is the syntactic goal symbol, await may be parsed as an identifier when the _[Await] parameter is absent. This includes the following contexts:

Anywhere outside of an AsyncFunctionBody or FormalParameters of an AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, or AsyncGeneratorExpression.
In the BindingIdentifier of a FunctionExpression, GeneratorExpression, or AsyncGeneratorExpression.

Note 2

Unlike YieldExpression, it is a Syntax Error to omit the operand of an AwaitExpression. You must await something.

14.7.1 Static Semantics: Early Errors

async

(

)

{

}

It is a Syntax Error if ContainsUseStrict of AsyncFunctionBody is true and IsSimpleParameterList of UniqueFormalParameters is false.
It is a Syntax Error if HasDirectSuper of AsyncMethod is true.
It is a Syntax Error if UniqueFormalParameters Contains AwaitExpression is true.
It is a Syntax Error if any element of the BoundNames of UniqueFormalParameters also occurs in the LexicallyDeclaredNames of AsyncFunctionBody.

async

function

(

)

{

}

async

function

(

)

{

}

async

function

(

)

{

}

async

function

(

)

{

}

It is a Syntax Error if ContainsUseStrict of AsyncFunctionBody is true and IsSimpleParameterList of FormalParameters is false.
It is a Syntax Error if FormalParameters Contains AwaitExpression is true.
If the source code matching FormalParameters is strict mode code, the Early Error rules for UniqueFormalParameters:FormalParameters are applied.
If BindingIdentifier is present and the source code matching BindingIdentifier is strict mode code, it is a Syntax Error if the StringValue of BindingIdentifier is "eval" or "arguments".
It is a Syntax Error if any element of the BoundNames of FormalParameters also occurs in the LexicallyDeclaredNames of AsyncFunctionBody.
It is a Syntax Error if FormalParameters Contains SuperProperty is true.
It is a Syntax Error if AsyncFunctionBody Contains SuperProperty is true.
It is a Syntax Error if FormalParameters Contains SuperCall is true.
It is a Syntax Error if AsyncFunctionBody Contains SuperCall is true.

14.7.2 Static Semantics: BoundNames

async

function

(

)

{

}

Return the BoundNames of BindingIdentifier.

async

function

(

)

{

}

Return « "*default*" ».

Note

"*default*" is used within this specification as a synthetic name for hoistable anonymous functions that are defined using export declarations.

14.7.3 Static Semantics: ComputedPropertyContains

With parameter symbol.

async

(

)

{

}

Return the result of ComputedPropertyContains for PropertyName with argument symbol.

14.7.4 Static Semantics: Contains

With parameter symbol.

async

function

(

)

{

}

async

function

(

)

{

}

async

function

(

)

{

}

async

function

(

)

{

}

Return false.

14.7.5 Static Semantics: HasDirectSuper

async

(

)

{

}

If UniqueFormalParameters Contains SuperCall is true, return true.
Return AsyncFunctionBody Contains SuperCall.

14.7.6 Static Semantics: HasName

async

function

(

)

{

}

Return false.

async

function

(

)

{

}

Return true.

14.7.7 Static Semantics: IsConstantDeclaration

async

function

(

)

{

}

async

function

(

)

{

}

Return false.

14.7.8 Static Semantics: IsFunctionDefinition

async

function

(

)

{

}

async

function

(

)

{

}

Return true.

14.7.9 Static Semantics: PropName

async

(

)

{

}

Return PropName of PropertyName.

14.7.10 Runtime Semantics: InstantiateFunctionObject

With parameter scope.

async

function

(

)

{

}

Let name be StringValue of BindingIdentifier.
Let sourceText be the source text matched by AsyncFunctionDeclaration.
Let F be ! OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, FormalParameters, AsyncFunctionBody, non-lexical-this, scope).
Perform ! SetFunctionName(F, name).
Return F.

async

function

(

)

{

}

Let sourceText be the source text matched by AsyncFunctionDeclaration.
Let F be ! OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, FormalParameters, AsyncFunctionBody, non-lexical-this, scope).
Perform ! SetFunctionName(F, "default").
Return F.

14.7.11 Runtime Semantics: EvaluateBody

With parameters functionObject and List argumentsList.

Let promiseCapability be ! NewPromiseCapability(%Promise%).
Let declResult be FunctionDeclarationInstantiation(functionObject, argumentsList).
If declResult is not an abrupt completion, then
1. Perform ! AsyncFunctionStart(promiseCapability, FunctionBody).
Else,
1. Perform ! Call(promiseCapability.[[Reject]], undefined, « declResult.[[Value]] »).
Return Completion { [[Type]]: return, [[Value]]: promiseCapability.[[Promise]], [[Target]]: empty }.

14.7.12 Runtime Semantics: PropertyDefinitionEvaluation

With parameters object and enumerable.

async

(

)

{

}

Let propKey be the result of evaluating PropertyName.
ReturnIfAbrupt(propKey).
Let scope be the LexicalEnvironment of the running execution context.
Let sourceText be the source text matched by AsyncMethod.
Let closure be ! OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, UniqueFormalParameters, AsyncFunctionBody, non-lexical-this, scope).
Perform ! MakeMethod(closure, object).
Perform ! SetFunctionName(closure, propKey).
Let desc be the PropertyDescriptor { [[Value]]: closure, [[Writable]]: true, [[Enumerable]]: enumerable, [[Configurable]]: true }.
Return ? DefinePropertyOrThrow(object, propKey, desc).

14.7.13 Runtime Semantics: NamedEvaluation

With parameter name.

async

function

(

)

{

}

Let scope be the LexicalEnvironment of the running execution context.
Let sourceText be the source text matched by AsyncFunctionExpression.
Let closure be ! OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, FormalParameters, AsyncFunctionBody, non-lexical-this, scope).
Perform SetFunctionName(closure, name).
Return closure.

14.7.14 Runtime Semantics: Evaluation

async

function

(

)

{

}

Return NormalCompletion(empty).

async

function

(

)

{

}

Return NormalCompletion(empty).

async

function

(

)

{

}

Return the result of performing NamedEvaluation for this AsyncFunctionExpression with argument "".

async

function

(

)

{

}

Let scope be the LexicalEnvironment of the running execution context.
Let funcEnv be ! NewDeclarativeEnvironment(scope).
Let name be StringValue of BindingIdentifier.
Perform ! funcEnv.CreateImmutableBinding(name, false).
Let sourceText be the source text matched by AsyncFunctionExpression.
Let closure be ! OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, FormalParameters, AsyncFunctionBody, non-lexical-this, funcEnv).
Perform ! SetFunctionName(closure, name).
Perform ! funcEnv.InitializeBinding(name, closure).
Return closure.

AwaitExpression

await

Let exprRef be the result of evaluating UnaryExpression.
Let value be ? GetValue(exprRef).
Return ? Await(value).

14.8 Async Arrow Function Definitions

Syntax

[In, Yield, Await]

async

[no LineTerminator here]

[?Yield]

[no LineTerminator here]

[?In]

[?Yield, ?Await]

[no LineTerminator here]

[?In]

[In]

[lookahead ≠ {]

[?In, +Await]

{

}

[Yield]

[?Yield, +Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

Supplemental Syntax

When processing an instance of the production AsyncArrowFunction:CoverCallExpressionAndAsyncArrowHead=>AsyncConciseBody the interpretation of CoverCallExpressionAndAsyncArrowHead is refined using the following grammar:

AsyncArrowHead

async

[no LineTerminator here]

[~Yield, +Await]

14.8.1 Static Semantics: Early Errors

async

It is a Syntax Error if any element of the BoundNames of AsyncArrowBindingIdentifier also occurs in the LexicallyDeclaredNames of AsyncConciseBody.

It is a Syntax Error if CoverCallExpressionAndAsyncArrowHead Contains YieldExpression is true.
It is a Syntax Error if CoverCallExpressionAndAsyncArrowHead Contains AwaitExpression is true.
It is a Syntax Error if CoverCallExpressionAndAsyncArrowHead is not covering an AsyncArrowHead.
It is a Syntax Error if any element of the BoundNames of CoverCallExpressionAndAsyncArrowHead also occurs in the LexicallyDeclaredNames of AsyncConciseBody.
It is a Syntax Error if ContainsUseStrict of AsyncConciseBody is true and IsSimpleParameterList of CoverCallExpressionAndAsyncArrowHead is false.
All Early Error rules for AsyncArrowHead and its derived productions apply to CoveredAsyncArrowHead of CoverCallExpressionAndAsyncArrowHead.

14.8.2 Static Semantics: CoveredAsyncArrowHead

Return the AsyncArrowHead that is covered by CoverCallExpressionAndAsyncArrowHead.

14.8.3 Static Semantics: BoundNames

Let head be CoveredAsyncArrowHead of CoverCallExpressionAndAsyncArrowHead.
Return the BoundNames of head.

14.8.4 Static Semantics: Contains

With parameter symbol.

async

If symbol is not one of NewTarget, SuperProperty, SuperCall, super, or this, return false.
Return AsyncConciseBody Contains symbol.

If symbol is not one of NewTarget, SuperProperty, SuperCall, super, or this, return false.
Let head be CoveredAsyncArrowHead of CoverCallExpressionAndAsyncArrowHead.
If head Contains symbol is true, return true.
Return AsyncConciseBody Contains symbol.

Note

Normally, Contains does not look inside most function forms. However, Contains is used to detect new.target, this, and super usage within an AsyncArrowFunction.

14.8.5 Static Semantics: ContainsExpression

Return false.

14.8.6 Static Semantics: ContainsUseStrict

Return false.

14.8.7 Static Semantics: ExpectedArgumentCount

Return 1.

14.8.8 Static Semantics: HasName

async

Return false.

14.8.9 Static Semantics: IsSimpleParameterList

[Yield]

[?Yield, +Await]

Return true.

Let head be CoveredAsyncArrowHead of CoverCallExpressionAndAsyncArrowHead.
Return IsSimpleParameterList of head.

14.8.10 Static Semantics: LexicallyDeclaredNames

Return a new empty List.

14.8.11 Static Semantics: LexicallyScopedDeclarations

Return a new empty List.

14.8.12 Static Semantics: VarDeclaredNames

Return a new empty List.

14.8.13 Static Semantics: VarScopedDeclarations

Return a new empty List.

14.8.14 Runtime Semantics: IteratorBindingInitialization

With parameters iteratorRecord and environment.

Assert: iteratorRecord.[[Done]] is false.
Let next be IteratorStep(iteratorRecord).
If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
ReturnIfAbrupt(next).
If next is false, set iteratorRecord.[[Done]] to true.
Else,
1. Let v be IteratorValue(next).
2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(v).
If iteratorRecord.[[Done]] is true, let v be undefined.
Return the result of performing BindingInitialization for BindingIdentifier using v and environment as the arguments.

14.8.15 Runtime Semantics: EvaluateBody

With parameters functionObject and List argumentsList.

Let promiseCapability be ! NewPromiseCapability(%Promise%).
Let declResult be FunctionDeclarationInstantiation(functionObject, argumentsList).
If declResult is not an abrupt completion, then
1. Perform ! AsyncFunctionStart(promiseCapability, ExpressionBody).
Else,
1. Perform ! Call(promiseCapability.[[Reject]], undefined, « declResult.[[Value]] »).
Return Completion { [[Type]]: return, [[Value]]: promiseCapability.[[Promise]], [[Target]]: empty }.

14.8.16 Runtime Semantics: NamedEvaluation

With parameter name.

async

Let scope be the LexicalEnvironment of the running execution context.
Let sourceText be the source text matched by AsyncArrowFunction.
Let parameters be AsyncArrowBindingIdentifier.
Let closure be ! OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, parameters, AsyncConciseBody, lexical-this, scope).
Perform SetFunctionName(closure, name).
Return closure.

Let scope be the LexicalEnvironment of the running execution context.
Let sourceText be the source text matched by AsyncArrowFunction.
Let head be CoveredAsyncArrowHead of CoverCallExpressionAndAsyncArrowHead.
Let parameters be the ArrowFormalParameters of head.
Let closure be ! OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, parameters, AsyncConciseBody, lexical-this, scope).
Perform SetFunctionName(closure, name).
Return closure.

14.8.17 Runtime Semantics: Evaluation

async

Return the result of performing NamedEvaluation for this AsyncArrowFunction with argument "".

Return the result of performing NamedEvaluation for this AsyncArrowFunction with argument "".

14.9 Tail Position Calls

14.9.1 Static Semantics: IsInTailPosition ( `call` )

The abstract operation IsInTailPosition takes argument call. It performs the following steps when called:

Assert: call is a Parse Node.
If the source code matching call is non-strict code, return false.
If call is not contained within a FunctionBody, ConciseBody, or AsyncConciseBody, return false.
Let body be the FunctionBody, ConciseBody, or AsyncConciseBody that most closely contains call.
If body is the FunctionBody of a GeneratorBody, return false.
If body is the FunctionBody of an AsyncFunctionBody, return false.
If body is the FunctionBody of an AsyncGeneratorBody, return false.
If body is an AsyncConciseBody, return false.
Return the result of HasCallInTailPosition of body with argument call.

Note

Tail Position calls are only defined in strict mode code because of a common non-standard language extension (see 9.2.4) that enables observation of the chain of caller contexts.

14.9.2 Static Semantics: HasCallInTailPosition

With parameter call.

Note

call is a Parse Node that represents a specific range of source text. When the following algorithms compare call to another Parse Node, it is a test of whether they represent the same source text.

14.9.2.1 Statement Rules

Let has be HasCallInTailPosition of StatementList with argument call.
If has is true, return true.
Return HasCallInTailPosition of StatementListItem with argument call.

[empty]

{

}

return

;

for

(

)

for

(

var

)

for

(

)

{

}

Return false.

(

)

else

Let has be HasCallInTailPosition of the first Statement with argument call.
If has is true, return true.
Return HasCallInTailPosition of the second Statement with argument call.

(

)

while

(

)

;

while

(

)

for

(

opt

;

opt

;

opt

)

for

(

var

;

opt

;

opt

)

for

(

opt

;

opt

)

for

(

)

for

(

var

)

for

(

)

for

await

(

)

for

await

(

var

)

for

await

(

)

with

(

)

Return HasCallInTailPosition of Statement with argument call.

Return HasCallInTailPosition of LabelledItem with argument call.

return

;

Return HasCallInTailPosition of Expression with argument call.

switch

(

)

Return HasCallInTailPosition of CaseBlock with argument call.

{

opt

opt

}

Let has be false.
If the first CaseClauses is present, let has be HasCallInTailPosition of the first CaseClauses with argument call.
If has is true, return true.
Let has be HasCallInTailPosition of DefaultClause with argument call.
If has is true, return true.
If the second CaseClauses is present, let has be HasCallInTailPosition of the second CaseClauses with argument call.
Return has.

Let has be HasCallInTailPosition of CaseClauses with argument call.
If has is true, return true.
Return HasCallInTailPosition of CaseClause with argument call.

case

opt

default

opt

If StatementList is present, return HasCallInTailPosition of StatementList with argument call.
Return false.

try

Return HasCallInTailPosition of Catch with argument call.

try

try

Return HasCallInTailPosition of Finally with argument call.

catch

(

)

Return HasCallInTailPosition of Block with argument call.

14.9.2.2 Expression Rules

Note

A potential tail position call that is immediately followed by return GetValue of the call result is also a possible tail position call. Function calls cannot return reference values, so such a GetValue operation will always return the same value as the actual function call result.

===

!==

instanceof

>>>

delete

void

typeof

[

]

new

[

]

new

this

Return false.

Return HasCallInTailPosition of AssignmentExpression with argument call.

Let has be HasCallInTailPosition of the first AssignmentExpression with argument call.
If has is true, return true.
Return HasCallInTailPosition of the second AssignmentExpression with argument call.

Return HasCallInTailPosition of BitwiseORExpression with argument call.

Return HasCallInTailPosition of LogicalANDExpression with argument call.

Return HasCallInTailPosition of BitwiseORExpression with argument call.

If this CallExpression is call, return true.
Return false.

Return HasCallInTailPosition of OptionalChain with argument call.

[

]

[

]

Return false.

If this OptionalChain is call, return true.
Return false.

If this MemberExpression is call, return true.
Return false.

Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
Return HasCallInTailPosition of expr with argument call.

(

)

Return HasCallInTailPosition of Expression with argument call.

14.9.3 Runtime Semantics: PrepareForTailCall ( )

The abstract operation PrepareForTailCall takes no arguments. It performs the following steps when called:

Let leafContext be the running execution context.
Suspend leafContext.
Pop leafContext from the execution context stack. The execution context now on the top of the stack becomes the running execution context.
Assert: leafContext has no further use. It will never be activated as the running execution context.

A tail position call must either release any transient internal resources associated with the currently executing function execution context before invoking the target function or reuse those resources in support of the target function.

Note

For example, a tail position call should only grow an implementation's activation record stack by the amount that the size of the target function's activation record exceeds the size of the calling function's activation record. If the target function's activation record is smaller, then the total size of the stack should decrease.

15 ECMAScript Language: Scripts and Modules

15.1 Scripts

Syntax

opt

[~Yield, ~Await, ~Return]

15.1.1 Static Semantics: Early Errors

Script

It is a Syntax Error if the LexicallyDeclaredNames of ScriptBody contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of ScriptBody also occurs in the VarDeclaredNames of ScriptBody.

It is a Syntax Error if StatementList Contains super unless the source code containing super is eval code that is being processed by a direct eval. Additional early error rules for super within direct eval are defined in 18.2.1.1.
It is a Syntax Error if StatementList Contains NewTarget unless the source code containing NewTarget is eval code that is being processed by a direct eval. Additional early error rules for NewTarget in direct eval are defined in 18.2.1.1.
It is a Syntax Error if ContainsDuplicateLabels of StatementList with argument « » is true.
It is a Syntax Error if ContainsUndefinedBreakTarget of StatementList with argument « » is true.
It is a Syntax Error if ContainsUndefinedContinueTarget of StatementList with arguments « » and « » is true.

15.1.2 Static Semantics: IsStrict

Script

opt

If ScriptBody is present and the Directive Prologue of ScriptBody contains a Use Strict Directive, return true; otherwise, return false.

15.1.3 Static Semantics: LexicallyDeclaredNames

Return TopLevelLexicallyDeclaredNames of StatementList.

Note

At the top level of a Script, function declarations are treated like var declarations rather than like lexical declarations.

15.1.4 Static Semantics: LexicallyScopedDeclarations

Return TopLevelLexicallyScopedDeclarations of StatementList.

15.1.5 Static Semantics: VarDeclaredNames

Return TopLevelVarDeclaredNames of StatementList.

15.1.6 Static Semantics: VarScopedDeclarations

Return TopLevelVarScopedDeclarations of StatementList.

15.1.7 Runtime Semantics: Evaluation

Script

[empty]

Return NormalCompletion(undefined).

15.1.8 Script Records

A Script Record encapsulates information about a script being evaluated. Each script record contains the fields listed in Table 37.

Table 37: Script Record Fields

Field Name	Value Type	Meaning
[[Realm]]	Realm Record \| undefined	The realm within which this script was created. undefined if not yet assigned.
[[Environment]]	Environment Record \| undefined	The Environment Record containing the top level bindings for this script. This field is set when the script is instantiated.
[[ECMAScriptCode]]	a Parse Node	The result of parsing the source text of this script using Script as the goal symbol.
[[HostDefined]]	Any, default value is undefined.	Field reserved for use by host environments that need to associate additional information with a script.

15.1.9 ParseScript ( `sourceText`, `realm`, `hostDefined` )

The abstract operation ParseScript takes arguments sourceText, realm, and hostDefined. It creates a Script Record based upon the result of parsing sourceText as a Script. It performs the following steps when called:

Assert: sourceText is an ECMAScript source text (see clause 10).
Parse sourceText using Script as the goal symbol and analyse the parse result for any Early Error conditions. If the parse was successful and no early errors were found, let body be the resulting parse tree. Otherwise, let body be a List of one or more SyntaxError objects representing the parsing errors and/or early errors. Parsing and early error detection may be interweaved in an implementation-dependent manner. If more than one parsing error or early error is present, the number and ordering of error objects in the list is implementation-dependent, but at least one must be present.
If body is a List of errors, return body.
Return Script Record { [[Realm]]: realm, [[Environment]]: undefined, [[ECMAScriptCode]]: body, [[HostDefined]]: hostDefined }.

Note

An implementation may parse script source text and analyse it for Early Error conditions prior to evaluation of ParseScript for that script source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseScript upon that source text.

15.1.10 ScriptEvaluation ( `scriptRecord` )

The abstract operation ScriptEvaluation takes argument scriptRecord. It performs the following steps when called:

Let globalEnv be scriptRecord.[[Realm]].[[GlobalEnv]].
Let scriptContext be a new ECMAScript code execution context.
Set the Function of scriptContext to null.
Set the Realm of scriptContext to scriptRecord.[[Realm]].
Set the ScriptOrModule of scriptContext to scriptRecord.
Set the VariableEnvironment of scriptContext to globalEnv.
Set the LexicalEnvironment of scriptContext to globalEnv.
Suspend the currently running execution context.
Push scriptContext onto the execution context stack; scriptContext is now the running execution context.
Let scriptBody be scriptRecord.[[ECMAScriptCode]].
Let result be GlobalDeclarationInstantiation(scriptBody, globalEnv).
If result.[[Type]] is normal, then
1. Set result to the result of evaluating scriptBody.
If result.[[Type]] is normal and result.[[Value]] is empty, then
1. Set result to NormalCompletion(undefined).
Suspend scriptContext and remove it from the execution context stack.
Assert: The execution context stack is not empty.
Resume the context that is now on the top of the execution context stack as the running execution context.
Return Completion(result).

15.1.11 Runtime Semantics: GlobalDeclarationInstantiation ( `script`, `env` )

Note 1

When an execution context is established for evaluating scripts, declarations are instantiated in the current global environment. Each global binding declared in the code is instantiated.

The abstract operation GlobalDeclarationInstantiation takes arguments script (a Parse Node for ScriptBody) and env (an Environment Record). script is the ScriptBody for which the execution context is being established. env is the global environment in which bindings are to be created. It performs the following steps when called:

Assert: env is a global Environment Record.
Let lexNames be the LexicallyDeclaredNames of script.
Let varNames be the VarDeclaredNames of script.
For each name in lexNames, do
1. If env.HasVarDeclaration(name) is true, throw a SyntaxError exception.
2. If env.HasLexicalDeclaration(name) is true, throw a SyntaxError exception.
3. Let hasRestrictedGlobal be ? env.HasRestrictedGlobalProperty(name).
4. If hasRestrictedGlobal is true, throw a SyntaxError exception.
For each name in varNames, do
1. If env.HasLexicalDeclaration(name) is true, throw a SyntaxError exception.
Let varDeclarations be the VarScopedDeclarations of script.
Let functionsToInitialize be a new empty List.
Let declaredFunctionNames be a new empty List.
For each d in varDeclarations, in reverse list order, do
1. If d is neither a VariableDeclaration nor a ForBinding nor a BindingIdentifier, then
  1. Assert: d is either a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration.
  2. NOTE: If there are multiple function declarations for the same name, the last declaration is used.
  3. Let fn be the sole element of the BoundNames of d.
  4. If fn is not an element of declaredFunctionNames, then
    1. Let fnDefinable be ? env.CanDeclareGlobalFunction(fn).
    2. If fnDefinable is false, throw a TypeError exception.
    3. Append fn to declaredFunctionNames.
    4. Insert d as the first element of functionsToInitialize.
Let declaredVarNames be a new empty List.
For each d in varDeclarations, do
1. If d is a VariableDeclaration, a ForBinding, or a BindingIdentifier, then
  1. For each String vn in the BoundNames of d, do
    1. If vn is not an element of declaredFunctionNames, then
      1. Let vnDefinable be ? env.CanDeclareGlobalVar(vn).
      2. If vnDefinable is false, throw a TypeError exception.
      3. If vn is not an element of declaredVarNames, then
        Append vn to declaredVarNames.
NOTE: No abnormal terminations occur after this algorithm step if the global object is an ordinary object. However, if the global object is a Proxy exotic object it may exhibit behaviours that cause abnormal terminations in some of the following steps.
NOTE: Annex B.3.3.2 adds additional steps at this point.
Let lexDeclarations be the LexicallyScopedDeclarations of script.
For each element d in lexDeclarations, do
1. NOTE: Lexically declared names are only instantiated here but not initialized.
2. For each element dn of the BoundNames of d, do
  1. If IsConstantDeclaration of d is true, then
    1. Perform ? env.CreateImmutableBinding(dn, true).
  2. Else,
    1. Perform ? env.CreateMutableBinding(dn, false).
For each Parse Node f in functionsToInitialize, do
1. Let fn be the sole element of the BoundNames of f.
2. Let fo be InstantiateFunctionObject of f with argument env.
3. Perform ? env.CreateGlobalFunctionBinding(fn, fo, false).
For each String vn in declaredVarNames, in list order, do
1. Perform ? env.CreateGlobalVarBinding(vn, false).
Return NormalCompletion(empty).

Note 2

Early errors specified in 15.1.1 prevent name conflicts between function/var declarations and let/const/class declarations as well as redeclaration of let/const/class bindings for declaration contained within a single Script. However, such conflicts and redeclarations that span more than one Script are detected as runtime errors during GlobalDeclarationInstantiation. If any such errors are detected, no bindings are instantiated for the script. However, if the global object is defined using Proxy exotic objects then the runtime tests for conflicting declarations may be unreliable resulting in an abrupt completion and some global declarations not being instantiated. If this occurs, the code for the Script is not evaluated.

Unlike explicit var or function declarations, properties that are directly created on the global object result in global bindings that may be shadowed by let/const/class declarations.

15.2 Modules

Syntax

opt

[~Yield, ~Await, ~Return]

15.2.1 Module Semantics

15.2.1.1 Static Semantics: Early Errors

ModuleBody

It is a Syntax Error if the LexicallyDeclaredNames of ModuleItemList contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of ModuleItemList also occurs in the VarDeclaredNames of ModuleItemList.
It is a Syntax Error if the ExportedNames of ModuleItemList contains any duplicate entries.
It is a Syntax Error if any element of the ExportedBindings of ModuleItemList does not also occur in either the VarDeclaredNames of ModuleItemList, or the LexicallyDeclaredNames of ModuleItemList.
It is a Syntax Error if ModuleItemList Contains super.
It is a Syntax Error if ModuleItemList Contains NewTarget.
It is a Syntax Error if ContainsDuplicateLabels of ModuleItemList with argument « » is true.
It is a Syntax Error if ContainsUndefinedBreakTarget of ModuleItemList with argument « » is true.
It is a Syntax Error if ContainsUndefinedContinueTarget of ModuleItemList with arguments « » and « » is true.

Note

The duplicate ExportedNames rule implies that multiple export default ExportDeclaration items within a ModuleBody is a Syntax Error. Additional error conditions relating to conflicting or duplicate declarations are checked during module linking prior to evaluation of a Module. If any such errors are detected the Module is not evaluated.

15.2.1.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

Let hasDuplicates be ContainsDuplicateLabels of ModuleItemList with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of ModuleItem with argument labelSet.

Return false.

15.2.1.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of ModuleItemList with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of ModuleItem with argument labelSet.

Return false.

15.2.1.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of ModuleItemList with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of ModuleItem with arguments iterationSet and « ».

Return false.

15.2.1.5 Static Semantics: ExportedBindings

Note

ExportedBindings are the locally bound names that are explicitly associated with a Module's ExportedNames.

Let names be ExportedBindings of ModuleItemList.
Append to names the elements of the ExportedBindings of ModuleItem.
Return names.

Return a new empty List.

15.2.1.6 Static Semantics: ExportedNames

Note

ExportedNames are the externally visible names that a Module explicitly maps to one of its local name bindings.

Let names be ExportedNames of ModuleItemList.
Append to names the elements of the ExportedNames of ModuleItem.
Return names.

Return the ExportedNames of ExportDeclaration.

Return a new empty List.

15.2.1.7 Static Semantics: ExportEntries

[empty]

Return a new empty List.

Let entries be ExportEntries of ModuleItemList.
Append to entries the elements of the ExportEntries of ModuleItem.
Return entries.

Return a new empty List.

15.2.1.8 Static Semantics: ImportEntries

[empty]

Return a new empty List.

Let entries be ImportEntries of ModuleItemList.
Append to entries the elements of the ImportEntries of ModuleItem.
Return entries.

Return a new empty List.

15.2.1.9 Static Semantics: ImportedLocalNames ( `importEntries` )

The abstract operation ImportedLocalNames takes argument importEntries (a List of ImportEntry Records (see Table 43)). It creates a List of all of the local name bindings defined by importEntries. It performs the following steps when called:

Let localNames be a new empty List.
For each ImportEntry Record i in importEntries, do
1. Append i.[[LocalName]] to localNames.
Return localNames.

15.2.1.10 Static Semantics: ModuleRequests

[empty]

Return a new empty List.

Return ModuleRequests of ModuleItem.

Let moduleNames be ModuleRequests of ModuleItemList.
Let additionalNames be ModuleRequests of ModuleItem.
Append to moduleNames each element of additionalNames that is not already an element of moduleNames.
Return moduleNames.

Return a new empty List.

15.2.1.11 Static Semantics: LexicallyDeclaredNames

Note 1

The LexicallyDeclaredNames of a Module includes the names of all of its imported bindings.

Let names be LexicallyDeclaredNames of ModuleItemList.
Append to names the elements of the LexicallyDeclaredNames of ModuleItem.
Return names.

Return the BoundNames of ImportDeclaration.

If ExportDeclaration is export VariableStatement, return a new empty List.
Return the BoundNames of ExportDeclaration.

Return LexicallyDeclaredNames of StatementListItem.

Note 2

At the top level of a Module, function declarations are treated like lexical declarations rather than like var declarations.

15.2.1.12 Static Semantics: LexicallyScopedDeclarations

[empty]

Return a new empty List.

Let declarations be LexicallyScopedDeclarations of ModuleItemList.
Append to declarations the elements of the LexicallyScopedDeclarations of ModuleItem.
Return declarations.

Return a new empty List.

15.2.1.13 Static Semantics: VarDeclaredNames

[empty]

Return a new empty List.

Let names be VarDeclaredNames of ModuleItemList.
Append to names the elements of the VarDeclaredNames of ModuleItem.
Return names.

Return a new empty List.

If ExportDeclaration is export VariableStatement, return BoundNames of ExportDeclaration.
Return a new empty List.

15.2.1.14 Static Semantics: VarScopedDeclarations

[empty]

Return a new empty List.

Let declarations be VarScopedDeclarations of ModuleItemList.
Append to declarations the elements of the VarScopedDeclarations of ModuleItem.
Return declarations.

Return a new empty List.

If ExportDeclaration is export VariableStatement, return VarScopedDeclarations of VariableStatement.
Return a new empty List.

15.2.1.15 Abstract Module Records

A Module Record encapsulates structural information about the imports and exports of a single module. This information is used to link the imports and exports of sets of connected modules. A Module Record includes four fields that are only used when evaluating a module.

For specification purposes Module Record values are values of the Record specification type and can be thought of as existing in a simple object-oriented hierarchy where Module Record is an abstract class with both abstract and concrete subclasses. This specification defines the abstract subclass named Cyclic Module Record and its concrete subclass named Source Text Module Record. Other specifications and implementations may define additional Module Record subclasses corresponding to alternative module definition facilities that they defined.

Module Record defines the fields listed in Table 38. All Module Definition subclasses include at least those fields. Module Record also defines the abstract method list in Table 39. All Module definition subclasses must provide concrete implementations of these abstract methods.

Table 38: Module Record Fields

Field Name	Value Type	Meaning
[[Realm]]	Realm Record \| undefined	The Realm within which this module was created. undefined if not yet assigned.
[[Environment]]	module Environment Record \| undefined	The Environment Record containing the top level bindings for this module. This field is set when the module is linked.
[[Namespace]]	Object \| undefined	The Module Namespace Object (26.3) if one has been created for this module. Otherwise undefined.
[[HostDefined]]	Any, default value is undefined.	Field reserved for use by host environments that need to associate additional information with a module.

Table 39: Abstract Methods of Module Records

Method	Purpose
GetExportedNames([`exportStarSet`])	Return a list of all names that are either directly or indirectly exported from this module.
ResolveExport(`exportName` [, `resolveSet`])	Return the binding of a name exported by this module. Bindings are represented by a ResolvedBinding Record, of the form { [[Module]]: Module Record, [[BindingName]]: String }. If the export is a Module Namespace Object without a direct binding in any module, [[BindingName]] will be set to "namespace". Return null if the name cannot be resolved, or "ambiguous" if multiple bindings were found. Each time this operation is called with a specific `exportName`, `resolveSet` pair as arguments it must return the same result if it completes normally.
Link()	Prepare the module for evaluation by transitively resolving all module dependencies and creating a module Environment Record.
Evaluate()	If this module has already been evaluated successfully, return undefined; if it has already been evaluated unsuccessfully, throw the exception that was produced. Otherwise, transitively evaluate all module dependencies of this module and then evaluate this module. Link must have completed successfully prior to invoking this method.

15.2.1.16 Cyclic Module Records

A Cyclic Module Record is used to represent information about a module that can participate in dependency cycles with other modules that are subclasses of the Cyclic Module Record type. Module Records that are not subclasses of the Cyclic Module Record type must not participate in dependency cycles with Source Text Module Records.

In addition to the fields defined in Table 38 Cyclic Module Records have the additional fields listed in Table 40

Table 40: Additional Fields of Cyclic Module Records

Field Name	Value Type	Meaning
[[Status]]	unlinked \| linking \| linked \| evaluating \| evaluated	Initially unlinked. Transitions to linking, linked, evaluating, evaluated (in that order) as the module progresses throughout its lifecycle.
[[EvaluationError]]	An abrupt completion \| undefined	A completion of type throw representing the exception that occurred during evaluation. undefined if no exception occurred or if [[Status]] is not evaluated.
[[DFSIndex]]	Integer \| undefined	Auxiliary field used during Link and Evaluate only. If [[Status]] is linking or evaluating, this nonnegative number records the point at which the module was first visited during the ongoing depth-first traversal of the dependency graph.
[[DFSAncestorIndex]]	Integer \| undefined	Auxiliary field used during Link and Evaluate only. If [[Status]] is linking or evaluating, this is either the module's own [[DFSIndex]] or that of an "earlier" module in the same strongly connected component.
[[RequestedModules]]	List of String	A List of all the ModuleSpecifier strings used by the module represented by this record to request the importation of a module. The List is source code occurrence ordered.

In addition to the methods defined in Table 39 Cyclic Module Records have the additional methods listed in Table 41

Table 41: Additional Abstract Methods of Cyclic Module Records

Method	Purpose
InitializeEnvironment()	Initialize the Environment Record of the module, including resolving all imported bindings, and create the module's execution context.
ExecuteModule()	Evaluate the module's code within its execution context.

15.2.1.16.1 Link ( ) Concrete Method

The Link concrete method of a Cyclic Module Record implements the corresponding Module Record abstract method.

On success, Link transitions this module's [[Status]] from unlinked to linked. On failure, an exception is thrown and this module's [[Status]] remains unlinked.

This abstract method performs the following steps (most of the work is done by the auxiliary function InnerModuleLinking):

Let module be this Cyclic Module Record.
Assert: module.[[Status]] is not linking or evaluating.
Let stack be a new empty List.
Let result be InnerModuleLinking(module, stack, 0).
If result is an abrupt completion, then
1. For each Cyclic Module Record m in stack, do
  1. Assert: m.[[Status]] is linking.
  2. Set m.[[Status]] to unlinked.
  3. Set m.[[Environment]] to undefined.
  4. Set m.[[DFSIndex]] to undefined.
  5. Set m.[[DFSAncestorIndex]] to undefined.
2. Assert: module.[[Status]] is unlinked.
3. Return result.
Assert: module.[[Status]] is linked or evaluated.
Assert: stack is empty.
Return undefined.

15.2.1.16.1.1 InnerModuleLinking ( `module`, `stack`, `index` )

The abstract operation InnerModuleLinking takes arguments module (a Cyclic Module Record), stack, and index. It is used by Link to perform the actual linking process for module, as well as recursively on all other modules in the dependency graph. The stack and index parameters, as well as a module's [[DFSIndex]] and [[DFSAncestorIndex]] fields, keep track of the depth-first search (DFS) traversal. In particular, [[DFSAncestorIndex]] is used to discover strongly connected components (SCCs), such that all modules in an SCC transition to linked together. It performs the following steps when called:

If module is not a Cyclic Module Record, then
1. Perform ? module.Link().
2. Return index.
If module.[[Status]] is linking, linked, or evaluated, then
1. Return index.
Assert: module.[[Status]] is unlinked.
Set module.[[Status]] to linking.
Set module.[[DFSIndex]] to index.
Set module.[[DFSAncestorIndex]] to index.
Set index to index + 1.
Append module to stack.
For each String required that is an element of module.[[RequestedModules]], do
1. Let requiredModule be ? HostResolveImportedModule(module, required).
2. Set index to ? InnerModuleLinking(requiredModule, stack, index).
3. If requiredModule is a Cyclic Module Record, then
  1. Assert: requiredModule.[[Status]] is either linking, linked, or evaluated.
  2. Assert: requiredModule.[[Status]] is linking if and only if requiredModule is in stack.
  3. If requiredModule.[[Status]] is linking, then
    1. Set module.[[DFSAncestorIndex]] to min(module.[[DFSAncestorIndex]], requiredModule.[[DFSAncestorIndex]]).
Perform ? module.InitializeEnvironment().
Assert: module occurs exactly once in stack.
Assert: module.[[DFSAncestorIndex]] is less than or equal to module.[[DFSIndex]].
If module.[[DFSAncestorIndex]] equals module.[[DFSIndex]], then
1. Let done be false.
2. Repeat, while done is false,
  1. Let requiredModule be the last element in stack.
  2. Remove the last element of stack.
  3. Assert: requiredModule is a Cyclic Module Record.
  4. Set requiredModule.[[Status]] to linked.
  5. If requiredModule and module are the same Module Record, set done to true.
Return index.

15.2.1.16.2 Evaluate ( ) Concrete Method

The Evaluate concrete method of a Cyclic Module Record implements the corresponding Module Record abstract method.

Evaluate transitions this module's [[Status]] from linked to evaluated.

If execution results in an exception, that exception is recorded in the [[EvaluationError]] field and rethrown by future invocations of Evaluate.

This abstract method performs the following steps (most of the work is done by the auxiliary function InnerModuleEvaluation):

Assert: This call to Evaluate is not happening at the same time as another call to Evaluate within the surrounding agent.
Let module be this Cyclic Module Record.
Assert: module.[[Status]] is linked or evaluated.
Let stack be a new empty List.
Let result be InnerModuleEvaluation(module, stack, 0).
If result is an abrupt completion, then
1. For each Cyclic Module Record m in stack, do
  1. Assert: m.[[Status]] is evaluating.
  2. Set m.[[Status]] to evaluated.
  3. Set m.[[EvaluationError]] to result.
2. Assert: module.[[Status]] is evaluated and module.[[EvaluationError]] is result.
3. Return result.
Assert: module.[[Status]] is evaluated and module.[[EvaluationError]] is undefined.
Assert: stack is empty.
Return undefined.

15.2.1.16.2.1 InnerModuleEvaluation ( `module`, `stack`, `index` )

The abstract operation InnerModuleEvaluation takes arguments module (a Source Text Module Record), stack, and index. It is used by Evaluate to perform the actual evaluation process for module, as well as recursively on all other modules in the dependency graph. The stack and index parameters, as well as module's [[DFSIndex]] and [[DFSAncestorIndex]] fields, are used the same way as in InnerModuleLinking. It performs the following steps when called:

If module is not a Cyclic Module Record, then
1. Perform ? module.Evaluate().
2. Return index.
If module.[[Status]] is evaluated, then
1. If module.[[EvaluationError]] is undefined, return index.
2. Otherwise, return module.[[EvaluationError]].
If module.[[Status]] is evaluating, return index.
Assert: module.[[Status]] is linked.
Set module.[[Status]] to evaluating.
Set module.[[DFSIndex]] to index.
Set module.[[DFSAncestorIndex]] to index.
Set index to index + 1.
Append module to stack.
For each String required that is an element of module.[[RequestedModules]], do
1. Let requiredModule be ! HostResolveImportedModule(module, required).
2. NOTE: Link must be completed successfully prior to invoking this method, so every requested module is guaranteed to resolve successfully.
3. Set index to ? InnerModuleEvaluation(requiredModule, stack, index).
4. If requiredModule is a Cyclic Module Record, then
  1. Assert: requiredModule.[[Status]] is either evaluating or evaluated.
  2. Assert: requiredModule.[[Status]] is evaluating if and only if requiredModule is in stack.
  3. If requiredModule.[[Status]] is evaluating, then
    1. Set module.[[DFSAncestorIndex]] to min(module.[[DFSAncestorIndex]], requiredModule.[[DFSAncestorIndex]]).
Perform ? module.ExecuteModule().
Assert: module occurs exactly once in stack.
Assert: module.[[DFSAncestorIndex]] is less than or equal to module.[[DFSIndex]].
If module.[[DFSAncestorIndex]] equals module.[[DFSIndex]], then
1. Let done be false.
2. Repeat, while done is false,
  1. Let requiredModule be the last element in stack.
  2. Remove the last element of stack.
  3. Assert: requiredModule is a Cyclic Module Record.
  4. Set requiredModule.[[Status]] to evaluated.
  5. If requiredModule and module are the same Module Record, set done to true.
Return index.

15.2.1.16.3 Example Cyclic Module Record Graphs

This non-normative section gives a series of examples of the linking and evaluation of a few common module graphs, with a specific focus on how errors can occur.

First consider the following simple module graph:

A module graph in which module A depends on module B, and module B depends on module C — Figure 2: A simple module graph

Let's first assume that there are no error conditions. When a host first calls A.Link(), this will complete successfully by assumption, and recursively link modules B and C as well, such that A.[[Status]] = B.[[Status]] = C.[[Status]] = linked. This preparatory step can be performed at any time. Later, when the host is ready to incur any possible side effects of the modules, it can call A.Evaluate(), which will complete successfully (again by assumption), recursively having evaluated first C and then B. Each module's [[Status]] at this point will be evaluated.

Consider then cases involving linking errors. If InnerModuleLinking of C succeeds but, thereafter, fails for B, for example because it imports something that C does not provide, then the original A.Link() will fail, and both A and B's [[Status]] remain unlinked. C's [[Status]] has become linked, though.

Finally, consider a case involving evaluation errors. If InnerModuleEvaluation of C succeeds but, thereafter, fails for B, for example because B contains code that throws an exception, then the original A.Evaluate() will fail. The resulting exception will be recorded in both A and B's [[EvaluationError]] fields, and their [[Status]] will become evaluated. C will also become evaluated but, in contrast to A and B, will remain without an [[EvaluationError]], as it successfully completed evaluation. Storing the exception ensures that any time a host tries to reuse A or B by calling their Evaluate() method, it will encounter the same exception. (Hosts are not required to reuse Cyclic Module Records; similarly, hosts are not required to expose the exception objects thrown by these methods. However, the specification enables such uses.)

The difference here between linking and evaluation errors is due to how evaluation must be only performed once, as it can cause side effects; it is thus important to remember whether evaluation has already been performed, even if unsuccessfully. (In the error case, it makes sense to also remember the exception because otherwise subsequent Evaluate() calls would have to synthesize a new one.) Linking, on the other hand, is side-effect-free, and thus even if it fails, it can be retried at a later time with no issues.

Now consider a different type of error condition:

A module graph in which module A depends on a missing (unresolvable) module, represented by ??? — Figure 3: A module graph with an unresolvable module

In this scenario, module A declares a dependency on some other module, but no Module Record exists for that module, i.e. HostResolveImportedModule throws an exception when asked for it. This could occur for a variety of reasons, such as the corresponding resource not existing, or the resource existing but ParseModule throwing an exception when trying to parse the resulting source text. Hosts can choose to expose the cause of failure via the exception they throw from HostResolveImportedModule. In any case, this exception causes a linking failure, which as before results in A's [[Status]] remaining unlinked.

Lastly, consider a module graph with a cycle:

A module graph in which module A depends on module B and C, but module B also depends on module A — Figure 4: A cyclic module graph

Here we assume that the entry point is module A, so that the host proceeds by calling A.Link(), which performs InnerModuleLinking on A. This in turn calls InnerModuleLinking on B. Because of the cycle, this again triggers InnerModuleLinking on A, but at this point it is a no-op since A.[[Status]] is already linking. B.[[Status]] itself remains linking when control gets back to A and InnerModuleLinking is triggered on C. After this returns with C.[[Status]] being linked , both A and B transition from linking to linked together; this is by design, since they form a strongly connected component.

An analogous story occurs for the evaluation phase of a cyclic module graph, in the success case.

Now consider a case where A has an linking error; for example, it tries to import a binding from C that does not exist. In that case, the above steps still occur, including the early return from the second call to InnerModuleLinking on A. However, once we unwind back to the original InnerModuleLinking on A, it fails during InitializeEnvironment, namely right after C.ResolveExport(). The thrown SyntaxError exception propagates up to A.Link, which resets all modules that are currently on its stack (these are always exactly the modules that are still linking). Hence both A and B become unlinked. Note that C is left as linked.

Finally, consider a case where A has an evaluation error; for example, its source code throws an exception. In that case, the evaluation-time analog of the above steps still occurs, including the early return from the second call to InnerModuleEvaluation on A. However, once we unwind back to the original InnerModuleEvaluation on A, it fails by assumption. The exception thrown propagates up to A.Evaluate(), which records the error in all modules that are currently on its stack (i.e., the modules that are still evaluating). Hence both A and B become evaluated and the exception is recorded in both A and B's [[EvaluationError]] fields, while C is left as evaluated with no [[EvaluationError]].

15.2.1.17 Source Text Module Records

A Source Text Module Record is used to represent information about a module that was defined from ECMAScript source text (10) that was parsed using the goal symbol Module. Its fields contain digested information about the names that are imported by the module and its concrete methods use this digest to link, link, and evaluate the module.

A Source Text Module Record can exist in a module graph with other subclasses of the abstract Module Record type, and can participate in cycles with other subclasses of the Cyclic Module Record type.

In addition to the fields defined in Table 40, Source Text Module Records have the additional fields listed in Table 42. Each of these fields is initially set in ParseModule.

Table 42: Additional Fields of Source Text Module Records

Field Name	Value Type	Meaning
[[ECMAScriptCode]]	a Parse Node	The result of parsing the source text of this module using Module as the goal symbol.
[[Context]]	An ECMAScript execution context.	The execution context associated with this module.
[[ImportMeta]]	Object	An object exposed through the `import.meta` meta property. It is empty until it is accessed by ECMAScript code.
[[ImportEntries]]	List of ImportEntry Records	A List of ImportEntry records derived from the code of this module.
[[LocalExportEntries]]	List of ExportEntry Records	A List of ExportEntry records derived from the code of this module that correspond to declarations that occur within the module.
[[IndirectExportEntries]]	List of ExportEntry Records	A List of ExportEntry records derived from the code of this module that correspond to reexported imports that occur within the module or exports from `export * as namespace` declarations.
[[StarExportEntries]]	List of ExportEntry Records	A List of ExportEntry records derived from the code of this module that correspond to `export ` declarations that occur within the module, not including `export as namespace` declarations.

An ImportEntry Record is a Record that digests information about a single declarative import. Each ImportEntry Record has the fields defined in Table 43:

Table 43: ImportEntry Record Fields

Field Name	Value Type	Meaning
[[ModuleRequest]]	String	String value of the ModuleSpecifier of the ImportDeclaration.
[[ImportName]]	String	The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. The value "*" indicates that the import request is for the target module's namespace object.
[[LocalName]]	String	The name that is used to locally access the imported value from within the importing module.

Note 1

Table 44 gives examples of ImportEntry records fields used to represent the syntactic import forms:

Table 44 (Informative): Import Forms Mappings to ImportEntry Records

Import Statement Form	[[ModuleRequest]]	[[ImportName]]	[[LocalName]]
`import v from "mod";`	"mod"	"default"	"v"
`import * as ns from "mod";`	"mod"	"*"	"ns"
`import {x} from "mod";`	"mod"	"x"	"x"
`import {x as v} from "mod";`	"mod"	"x"	"v"
`import "mod";`	An ImportEntry Record is not created.

An ExportEntry Record is a Record that digests information about a single declarative export. Each ExportEntry Record has the fields defined in Table 45:

Table 45: ExportEntry Record Fields

Field Name	Value Type	Meaning
[[ExportName]]	String \| null	The name used to export this binding by this module.
[[ModuleRequest]]	String \| null	The String value of the ModuleSpecifier of the ExportDeclaration. null if the ExportDeclaration does not have a ModuleSpecifier.
[[ImportName]]	String \| null	The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. null if the ExportDeclaration does not have a ModuleSpecifier. "*" indicates that the export request is for all exported bindings.
[[LocalName]]	String \| null	The name that is used to locally access the exported value from within the importing module. null if the exported value is not locally accessible from within the module.

Note 2

Table 46 gives examples of the ExportEntry record fields used to represent the syntactic export forms:

Table 46 (Informative): Export Forms Mappings to ExportEntry Records

Export Statement Form	[[ExportName]]	[[ModuleRequest]]	[[ImportName]]	[[LocalName]]
`export var v;`	"v"	null	null	"v"
`export default function f() {}`	"default"	null	null	"f"
`export default function () {}`	"default"	null	null	"default"
`export default 42;`	"default"	null	null	"default"
`export {x};`	"x"	null	null	"x"
`export {v as x};`	"x"	null	null	"v"
`export {x} from "mod";`	"x"	"mod"	"x"	null
`export {v as x} from "mod";`	"x"	"mod"	"v"	null
`export * from "mod";`	null	"mod"	"*"	null
`export * as ns from "mod";`	"ns"	"mod"	"*"	null

The following definitions specify the required concrete methods and other abstract operations for Source Text Module Records

15.2.1.17.1 ParseModule ( `sourceText`, `realm`, `hostDefined` )

The abstract operation ParseModule takes arguments sourceText (ECMAScript source text), realm, and hostDefined. It creates a Source Text Module Record based upon the result of parsing sourceText as a Module. It performs the following steps when called:

Assert: sourceText is an ECMAScript source text (see clause 10).
Parse sourceText using Module as the goal symbol and analyse the parse result for any Early Error conditions. If the parse was successful and no early errors were found, let body be the resulting parse tree. Otherwise, let body be a List of one or more SyntaxError objects representing the parsing errors and/or early errors. Parsing and early error detection may be interweaved in an implementation-dependent manner. If more than one parsing error or early error is present, the number and ordering of error objects in the list is implementation-dependent, but at least one must be present.
If body is a List of errors, return body.
Let requestedModules be the ModuleRequests of body.
Let importEntries be ImportEntries of body.
Let importedBoundNames be ImportedLocalNames(importEntries).
Let indirectExportEntries be a new empty List.
Let localExportEntries be a new empty List.
Let starExportEntries be a new empty List.
Let exportEntries be ExportEntries of body.
For each ExportEntry Record ee in exportEntries, do
1. If ee.[[ModuleRequest]] is null, then
  1. If ee.[[LocalName]] is not an element of importedBoundNames, then
    1. Append ee to localExportEntries.
  2. Else,
    1. Let ie be the element of importEntries whose [[LocalName]] is the same as ee.[[LocalName]].
    2. If ie.[[ImportName]] is "*", then
      1. NOTE: This is a re-export of an imported module namespace object.
      2. Append ee to localExportEntries.
    3. Else,
      1. NOTE: This is a re-export of a single name.
      2. Append the ExportEntry Record { [[ModuleRequest]]: ie.[[ModuleRequest]], [[ImportName]]: ie.[[ImportName]], [[LocalName]]: null, [[ExportName]]: ee.[[ExportName]] } to indirectExportEntries.
2. Else if ee.[[ImportName]] is "*" and ee.[[ExportName]] is null, then
  1. Append ee to starExportEntries.
3. Else,
  1. Append ee to indirectExportEntries.
Return Source Text Module Record { [[Realm]]: realm, [[Environment]]: undefined, [[Namespace]]: undefined, [[Status]]: unlinked, [[EvaluationError]]: undefined, [[HostDefined]]: hostDefined, [[ECMAScriptCode]]: body, [[Context]]: empty, [[ImportMeta]]: empty, [[RequestedModules]]: requestedModules, [[ImportEntries]]: importEntries, [[LocalExportEntries]]: localExportEntries, [[IndirectExportEntries]]: indirectExportEntries, [[StarExportEntries]]: starExportEntries, [[DFSIndex]]: undefined, [[DFSAncestorIndex]]: undefined }.

Note

An implementation may parse module source text and analyse it for Early Error conditions prior to the evaluation of ParseModule for that module source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseModule upon that source text.

15.2.1.17.2 GetExportedNames ( [ `exportStarSet` ] ) Concrete Method

The GetExportedNames concrete method of a Source Text Module Record implements the corresponding Module Record abstract method.

It performs the following steps when called:

If exportStarSet is not present, set exportStarSet to a new empty List.
Assert: exportStarSet is a List of Source Text Module Records.
Let module be this Source Text Module Record.
If exportStarSet contains module, then
1. Assert: We've reached the starting point of an export * circularity.
2. Return a new empty List.
Append module to exportStarSet.
Let exportedNames be a new empty List.
For each ExportEntry Record e in module.[[LocalExportEntries]], do
1. Assert: module provides the direct binding for this export.
2. Append e.[[ExportName]] to exportedNames.
For each ExportEntry Record e in module.[[IndirectExportEntries]], do
1. Assert: module imports a specific binding for this export.
2. Append e.[[ExportName]] to exportedNames.
For each ExportEntry Record e in module.[[StarExportEntries]], do
1. Let requestedModule be ? HostResolveImportedModule(module, e.[[ModuleRequest]]).
2. Let starNames be ? requestedModule.GetExportedNames(exportStarSet).
3. For each element n of starNames, do
  1. If SameValue(n, "default") is false, then
    1. If n is not an element of exportedNames, then
      1. Append n to exportedNames.
Return exportedNames.

Note

GetExportedNames does not filter out or throw an exception for names that have ambiguous star export bindings.

15.2.1.17.3 ResolveExport ( `exportName` [ , `resolveSet` ] ) Concrete Method

The ResolveExport concrete method of a Source Text Module Record implements the corresponding Module Record abstract method.

ResolveExport attempts to resolve an imported binding to the actual defining module and local binding name. The defining module may be the module represented by the Module Record this method was invoked on or some other module that is imported by that module. The parameter resolveSet is used to detect unresolved circular import/export paths. If a pair consisting of specific Module Record and exportName is reached that is already in resolveSet, an import circularity has been encountered. Before recursively calling ResolveExport, a pair consisting of module and exportName is added to resolveSet.

If a defining module is found, a ResolvedBinding Record { [[Module]], [[BindingName]] } is returned. This record identifies the resolved binding of the originally requested export, unless this is the export of a namespace with no local binding. In this case, [[BindingName]] will be set to "*namespace*". If no definition was found or the request is found to be circular, null is returned. If the request is found to be ambiguous, the string "ambiguous" is returned.

This abstract method performs the following steps:

If resolveSet is not present, set resolveSet to a new empty List.
Assert: resolveSet is a List of Record { [[Module]], [[ExportName]] }.
Let module be this Source Text Module Record.
For each Record { [[Module]], [[ExportName]] } r in resolveSet, do
1. If module and r.[[Module]] are the same Module Record and SameValue(exportName, r.[[ExportName]]) is true, then
  1. Assert: This is a circular import request.
  2. Return null.
Append the Record { [[Module]]: module, [[ExportName]]: exportName } to resolveSet.
For each ExportEntry Record e in module.[[LocalExportEntries]], do
1. If SameValue(exportName, e.[[ExportName]]) is true, then
  1. Assert: module provides the direct binding for this export.
  2. Return ResolvedBinding Record { [[Module]]: module, [[BindingName]]: e.[[LocalName]] }.
For each ExportEntry Record e in module.[[IndirectExportEntries]], do
1. If SameValue(exportName, e.[[ExportName]]) is true, then
  1. Let importedModule be ? HostResolveImportedModule(module, e.[[ModuleRequest]]).
  2. If e.[[ImportName]] is "*", then
    1. Assert: module does not provide the direct binding for this export.
    2. Return ResolvedBinding Record { [[Module]]: importedModule, [[BindingName]]: "*namespace*" }.
  3. Else,
    1. Assert: module imports a specific binding for this export.
    2. Return importedModule.ResolveExport(e.[[ImportName]], resolveSet).
If SameValue(exportName, "default") is true, then
1. Assert: A default export was not explicitly defined by this module.
2. Return null.
3. NOTE: A default export cannot be provided by an export * or export * from "mod" declaration.
Let starResolution be null.
For each ExportEntry Record e in module.[[StarExportEntries]], do
1. Let importedModule be ? HostResolveImportedModule(module, e.[[ModuleRequest]]).
2. Let resolution be ? importedModule.ResolveExport(exportName, resolveSet).
3. If resolution is "ambiguous", return "ambiguous".
4. If resolution is not null, then
  1. Assert: resolution is a ResolvedBinding Record.
  2. If starResolution is null, set starResolution to resolution.
  3. Else,
    1. Assert: There is more than one * import that includes the requested name.
    2. If resolution.[[Module]] and starResolution.[[Module]] are not the same Module Record or SameValue(resolution.[[BindingName]], starResolution.[[BindingName]]) is false, return "ambiguous".
Return starResolution.

15.2.1.17.4 InitializeEnvironment ( ) Concrete Method

The InitializeEnvironment concrete method of a Source Text Module Record implements the corresponding Cyclic Module Record abstract method.

This abstract method performs the following steps:

Let module be this Source Text Module Record.
For each ExportEntry Record e in module.[[IndirectExportEntries]], do
1. Let resolution be ? module.ResolveExport(e.[[ExportName]]).
2. If resolution is null or "ambiguous", throw a SyntaxError exception.
3. Assert: resolution is a ResolvedBinding Record.
Assert: All named exports from module are resolvable.
Let realm be module.[[Realm]].
Assert: realm is not undefined.
Let env be NewModuleEnvironment(realm.[[GlobalEnv]]).
Set module.[[Environment]] to env.
For each ImportEntry Record in in module.[[ImportEntries]], do
1. Let importedModule be ! HostResolveImportedModule(module, in.[[ModuleRequest]]).
2. NOTE: The above call cannot fail because imported module requests are a subset of module.[[RequestedModules]], and these have been resolved earlier in this algorithm.
3. If in.[[ImportName]] is "*", then
  1. Let namespace be ? GetModuleNamespace(importedModule).
  2. Perform ! env.CreateImmutableBinding(in.[[LocalName]], true).
  3. Call env.InitializeBinding(in.[[LocalName]], namespace).
4. Else,
  1. Let resolution be ? importedModule.ResolveExport(in.[[ImportName]]).
  2. If resolution is null or "ambiguous", throw a SyntaxError exception.
  3. If resolution.[[BindingName]] is "*namespace*", then
    1. Let namespace be ? GetModuleNamespace(resolution.[[Module]]).
    2. Perform ! env.CreateImmutableBinding(in.[[LocalName]], true).
    3. Call env.InitializeBinding(in.[[LocalName]], namespace).
  4. Else,
    1. Call env.CreateImportBinding(in.[[LocalName]], resolution.[[Module]], resolution.[[BindingName]]).
Let moduleContext be a new ECMAScript code execution context.
Set the Function of moduleContext to null.
Assert: module.[[Realm]] is not undefined.
Set the Realm of moduleContext to module.[[Realm]].
Set the ScriptOrModule of moduleContext to module.
Set the VariableEnvironment of moduleContext to module.[[Environment]].
Set the LexicalEnvironment of moduleContext to module.[[Environment]].
Set module.[[Context]] to moduleContext.
Push moduleContext onto the execution context stack; moduleContext is now the running execution context.
Let code be module.[[ECMAScriptCode]].
Let varDeclarations be the VarScopedDeclarations of code.
Let declaredVarNames be a new empty List.
For each element d in varDeclarations, do
1. For each element dn of the BoundNames of d, do
  1. If dn is not an element of declaredVarNames, then
    1. Perform ! env.CreateMutableBinding(dn, false).
    2. Call env.InitializeBinding(dn, undefined).
    3. Append dn to declaredVarNames.
Let lexDeclarations be the LexicallyScopedDeclarations of code.
For each element d in lexDeclarations, do
1. For each element dn of the BoundNames of d, do
  1. If IsConstantDeclaration of d is true, then
    1. Perform ! env.CreateImmutableBinding(dn, true).
  2. Else,
    1. Perform ! env.CreateMutableBinding(dn, false).
  3. If d is a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration, then
    1. Let fo be InstantiateFunctionObject of d with argument env.
    2. Call env.InitializeBinding(dn, fo).
Remove moduleContext from the execution context stack.
Return NormalCompletion(empty).

15.2.1.17.5 ExecuteModule ( ) Concrete Method

The ExecuteModule concrete method of a Source Text Module Record implements the corresponding Cyclic Module Record abstract method.

This abstract method performs the following steps:

Let module be this Source Text Module Record.
Suspend the currently running execution context.
Let moduleContext be module.[[Context]].
Push moduleContext onto the execution context stack; moduleContext is now the running execution context.
Let result be the result of evaluating module.[[ECMAScriptCode]].
Suspend moduleContext and remove it from the execution context stack.
Resume the context that is now on the top of the execution context stack as the running execution context.
Return Completion(result).

15.2.1.18 Runtime Semantics: HostResolveImportedModule ( `referencingScriptOrModule`, `specifier` )

HostResolveImportedModule is an implementation-defined abstract operation that provides the concrete Module Record subclass instance that corresponds to the ModuleSpecifier String, specifier, occurring within the context of the script or module represented by the Script Record or Module Record referencingScriptOrModule. referencingScriptOrModule may also be null, if the resolution is being performed in the context of an import() expression, and there is no active script or module at that time.

Note

An example of when referencingScriptOrModule can be null is in a web browser host. There, if a user clicks on a control given by

<button type="button" onclick="import('./foo.mjs')">Click me</button>

there will be no active script or module at the time the import() expression runs. More generally, this can happen in any situation where the host pushes execution contexts with null ScriptOrModule components onto the execution context stack.

The implementation of HostResolveImportedModule must conform to the following requirements:

The normal return value must be an instance of a concrete subclass of Module Record.
If a Module Record corresponding to the pair referencingScriptOrModule, specifier does not exist or cannot be created, an exception must be thrown.
Each time this operation is called with a specific referencingScriptOrModule, specifier pair as arguments it must return the same Module Record instance if it completes normally.

Multiple different referencingScriptOrModule, specifier pairs may map to the same Module Record instance. The actual mapping semantic is implementation-defined but typically a normalization process is applied to specifier as part of the mapping process. A typical normalization process would include actions such as alphabetic case folding and expansion of relative and abbreviated path specifiers.

15.2.1.19 Runtime Semantics: HostImportModuleDynamically ( `referencingScriptOrModule`, `specifier`, `promiseCapability` )

HostImportModuleDynamically is an implementation-defined abstract operation that performs any necessary setup work in order to make available the module corresponding to the ModuleSpecifier String, specifier, occurring within the context of the script or module represented by the Script Record or Module Record referencingScriptOrModule. (referencingScriptOrModule may also be null, if there is no active script or module when the import() expression occurs.) It then performs FinishDynamicImport to finish the dynamic import process.

The implementation of HostImportModuleDynamically must conform to the following requirements:

The abstract operation must always complete normally with undefined. Success or failure must instead be signaled as discussed below.
The host environment must conform to one of the two following sets of requirements:
Success path
- At some future time, the host environment must perform FinishDynamicImport(referencingScriptOrModule, specifier, promiseCapability, NormalCompletion(undefined)).
- Any subsequent call to HostResolveImportedModule after FinishDynamicImport has completed, given the arguments referencingScriptOrModule and specifier, must complete normally.
- The completion value of any subsequent call to HostResolveImportedModule after FinishDynamicImport has completed, given the arguments referencingScriptOrModule and specifier, must be a module which has already been evaluated, i.e. whose Evaluate concrete method has already been called and returned a normal completion.
Failure path
- At some future time, the host environment must perform FinishDynamicImport(referencingScriptOrModule, specifier, promiseCapability, an abrupt completion), with the abrupt completion representing the cause of failure.
If the host environment takes the success path once for a given referencingScriptOrModule, specifier pair, it must always do so for subsequent calls.
The operation must not call promiseCapability.[[Resolve]] or promiseCapability.[[Reject]], but instead must treat promiseCapability as an opaque identifying value to be passed through to FinishDynamicImport.

The actual process performed is implementation-defined, but typically consists of performing whatever I/O operations are necessary to allow HostResolveImportedModule to synchronously retrieve the appropriate Module Record, and then calling its Evaluate concrete method. This might require performing similar normalization as HostResolveImportedModule does.

15.2.1.20 Runtime Semantics: FinishDynamicImport ( `referencingScriptOrModule`, `specifier`, `promiseCapability`, `completion` )

The abstract operation FinishDynamicImport takes arguments referencingScriptOrModule, specifier, promiseCapability, and completion. FinishDynamicImport completes the process of a dynamic import originally started by an import() call, resolving or rejecting the promise returned by that call as appropriate according to completion. It is performed by host environments as part of HostImportModuleDynamically. It performs the following steps when called:

If completion is an abrupt completion, then perform ! Call(promiseCapability.[[Reject]], undefined, « completion.[[Value]] »).
Else,
1. Assert: completion is a normal completion and completion.[[Value]] is undefined.
2. Let moduleRecord be ! HostResolveImportedModule(referencingScriptOrModule, specifier).
3. Assert: Evaluate has already been invoked on moduleRecord and successfully completed.
4. Let namespace be GetModuleNamespace(moduleRecord).
5. If namespace is an abrupt completion, perform ! Call(promiseCapability.[[Reject]], undefined, « namespace.[[Value]] »).
6. Else, perform ! Call(promiseCapability.[[Resolve]], undefined, « namespace.[[Value]] »).

15.2.1.21 Runtime Semantics: GetModuleNamespace ( `module` )

The abstract operation GetModuleNamespace takes argument module. It retrieves the Module Namespace Object representing module's exports, lazily creating it the first time it was requested, and storing it in module.[[Namespace]] for future retrieval. It performs the following steps when called:

Assert: module is an instance of a concrete subclass of Module Record.
Assert: If module is a Cyclic Module Record, then module.[[Status]] is not unlinked.
Let namespace be module.[[Namespace]].
If namespace is undefined, then
1. Let exportedNames be ? module.GetExportedNames().
2. Let unambiguousNames be a new empty List.
3. For each name that is an element of exportedNames, do
  1. Let resolution be ? module.ResolveExport(name).
  2. If resolution is a ResolvedBinding Record, append name to unambiguousNames.
4. Set namespace to ModuleNamespaceCreate(module, unambiguousNames).
Return namespace.

Note

The only way GetModuleNamespace can throw is via one of the triggered HostResolveImportedModule calls. Unresolvable names are simply excluded from the namespace at this point. They will lead to a real linking error later unless they are all ambiguous star exports that are not explicitly requested anywhere.

15.2.1.22 Runtime Semantics: Evaluation

[empty]

Return NormalCompletion(undefined).

ModuleBody

Let result be the result of evaluating ModuleItemList.
If result.[[Type]] is normal and result.[[Value]] is empty, then
1. Return NormalCompletion(undefined).
Return Completion(result).

Let sl be the result of evaluating ModuleItemList.
ReturnIfAbrupt(sl).
Let s be the result of evaluating ModuleItem.
Return Completion(UpdateEmpty(s, sl)).

Note

The value of a ModuleItemList is the value of the last value-producing item in the ModuleItemList.

Return NormalCompletion(empty).

15.2.2 Imports

Syntax

import

;

import

;

{

}

{

}

{

}

from

[~Yield, ~Await]

15.2.2.1 Static Semantics: Early Errors

It is a Syntax Error if the BoundNames of ImportDeclaration contains any duplicate entries.

15.2.2.2 Static Semantics: BoundNames

import

;

Return the BoundNames of ImportClause.

import

;

Return a new empty List.

Let names be the BoundNames of ImportedDefaultBinding.
Append to names the elements of the BoundNames of NameSpaceImport.
Return names.

Let names be the BoundNames of ImportedDefaultBinding.
Append to names the elements of the BoundNames of NamedImports.
Return names.

{

}

Return a new empty List.

Let names be the BoundNames of ImportsList.
Append to names the elements of the BoundNames of ImportSpecifier.
Return names.

Return the BoundNames of ImportedBinding.

15.2.2.3 Static Semantics: ImportEntries

import

;

Let module be the sole element of ModuleRequests of FromClause.
Return ImportEntriesForModule of ImportClause with argument module.

import

;

Return a new empty List.

15.2.2.4 Static Semantics: ImportEntriesForModule

With parameter module.

Let entries be ImportEntriesForModule of ImportedDefaultBinding with argument module.
Append to entries the elements of the ImportEntriesForModule of NameSpaceImport with argument module.
Return entries.

Let entries be ImportEntriesForModule of ImportedDefaultBinding with argument module.
Append to entries the elements of the ImportEntriesForModule of NamedImports with argument module.
Return entries.

Let localName be the sole element of BoundNames of ImportedBinding.
Let defaultEntry be the ImportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: "default", [[LocalName]]: localName }.
Return a new List containing defaultEntry.

Let localName be the StringValue of ImportedBinding.
Let entry be the ImportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: "*", [[LocalName]]: localName }.
Return a new List containing entry.

{

}

Return a new empty List.

Let specs be the ImportEntriesForModule of ImportsList with argument module.
Append to specs the elements of the ImportEntriesForModule of ImportSpecifier with argument module.
Return specs.

Let localName be the sole element of BoundNames of ImportedBinding.
Let entry be the ImportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: localName, [[LocalName]]: localName }.
Return a new List containing entry.

Let importName be the StringValue of IdentifierName.
Let localName be the StringValue of ImportedBinding.
Let entry be the ImportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName }.
Return a new List containing entry.

15.2.2.5 Static Semantics: ModuleRequests

import

;

Return ModuleRequests of FromClause.

Return a List containing the StringValue of StringLiteral.

15.2.3 Exports

Syntax

export

;

export

;

export

[~Yield, ~Await]

export

[~Yield, ~Await]

export

default

[~Yield, ~Await, +Default]

export

default

[~Yield, ~Await, +Default]

export

default

[lookahead ∉ { function, async

[no LineTerminator here]

function, class }]

[+In, ~Yield, ~Await]

;

{

}

{

}

{

}

15.2.3.1 Static Semantics: Early Errors

export

;

For each IdentifierName n in ReferencedBindings of NamedExports: It is a Syntax Error if StringValue of n is a ReservedWord or if the StringValue of n is one of: "implements", "interface", "let", "package", "private", "protected", "public", or "static".

Note

The above rule means that each ReferencedBindings of NamedExports is treated as an IdentifierReference.

15.2.3.2 Static Semantics: BoundNames

export

;

export

;

Return a new empty List.

export

Return the BoundNames of VariableStatement.

export

Return the BoundNames of Declaration.

export

default

Let declarationNames be the BoundNames of HoistableDeclaration.
If declarationNames does not include the element "*default*", append "*default*" to declarationNames.
Return declarationNames.

export

default

Let declarationNames be the BoundNames of ClassDeclaration.
If declarationNames does not include the element "*default*", append "*default*" to declarationNames.
Return declarationNames.

export

default

;

Return « "*default*" ».

15.2.3.3 Static Semantics: ExportedBindings

export

;

Return a new empty List.

export

;

Return the ExportedBindings of NamedExports.

export

Return the BoundNames of VariableStatement.

export

Return the BoundNames of Declaration.

export

default

export

default

export

default

;

Return the BoundNames of this ExportDeclaration.

{

}

Return a new empty List.

Let names be the ExportedBindings of ExportsList.
Append to names the elements of the ExportedBindings of ExportSpecifier.
Return names.

Return a List containing the StringValue of IdentifierName.

Return a List containing the StringValue of the first IdentifierName.

15.2.3.4 Static Semantics: ExportedNames

export

;

Return the ExportedNames of ExportFromClause.

Return a new empty List.

Return a List containing the StringValue of IdentifierName.

Return the ExportedNames of NamedExports.

export

Return the BoundNames of VariableStatement.

export

Return the BoundNames of Declaration.

export

default

export

default

export

default

;

Return « "default" ».

{

}

Return a new empty List.

Let names be the ExportedNames of ExportsList.
Append to names the elements of the ExportedNames of ExportSpecifier.
Return names.

Return a List containing the StringValue of IdentifierName.

Return a List containing the StringValue of the second IdentifierName.

15.2.3.5 Static Semantics: ExportEntries

export

;

Let module be the sole element of ModuleRequests of FromClause.
Return ExportEntriesForModule of ExportFromClause with argument module.

export

;

Return ExportEntriesForModule of NamedExports with argument null.

export

Let entries be a new empty List.
Let names be the BoundNames of VariableStatement.
For each name in names, do
1. Append the ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: name, [[ExportName]]: name } to entries.
Return entries.

export

Let entries be a new empty List.
Let names be the BoundNames of Declaration.
For each name in names, do
1. Append the ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: name, [[ExportName]]: name } to entries.
Return entries.

export

default

Let names be BoundNames of HoistableDeclaration.
Let localName be the sole element of names.
Return a new List containing the ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: localName, [[ExportName]]: "default" }.

export

default

Let names be BoundNames of ClassDeclaration.
Let localName be the sole element of names.
Return a new List containing the ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: localName, [[ExportName]]: "default" }.

export

default

;

Let entry be the ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: "*default*", [[ExportName]]: "default" }.
Return a new List containing entry.

Note

"*default*" is used within this specification as a synthetic name for anonymous default export values.

15.2.3.6 Static Semantics: ExportEntriesForModule

With parameter module.

Let entry be the ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: "*", [[LocalName]]: null, [[ExportName]]: null }.
Return a new List containing entry.

Let exportName be the StringValue of IdentifierName.
Let entry be the ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: "*", [[LocalName]]: null, [[ExportName]]: exportName }.
Return a new List containing entry.

{

}

Return a new empty List.

Let specs be the ExportEntriesForModule of ExportsList with argument module.
Append to specs the elements of the ExportEntriesForModule of ExportSpecifier with argument module.
Return specs.

Let sourceName be the StringValue of IdentifierName.
If module is null, then
1. Let localName be sourceName.
2. Let importName be null.
Else,
1. Let localName be null.
2. Let importName be sourceName.
Return a new List containing the ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: sourceName }.

Let sourceName be the StringValue of the first IdentifierName.
Let exportName be the StringValue of the second IdentifierName.
If module is null, then
1. Let localName be sourceName.
2. Let importName be null.
Else,
1. Let localName be null.
2. Let importName be sourceName.
Return a new List containing the ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: exportName }.

15.2.3.7 Static Semantics: IsConstantDeclaration

export

;

export

;

export

default

;

Return false.

Note

It is not necessary to treat export default AssignmentExpression as a constant declaration because there is no syntax that permits assignment to the internal bound name used to reference a module's default object.

15.2.3.8 Static Semantics: LexicallyScopedDeclarations

export

;

export

;

export

Return a new empty List.

export

Return a new List containing DeclarationPart of Declaration.

export

default

Return a new List containing DeclarationPart of HoistableDeclaration.

export

default

Return a new List containing ClassDeclaration.

export

default

;

Return a new List containing this ExportDeclaration.

15.2.3.9 Static Semantics: ModuleRequests

export

;

Return the ModuleRequests of FromClause.

export

;

export

export

export

default

export

default

export

default

;

Return a new empty List.

15.2.3.10 Static Semantics: ReferencedBindings

{

}

Return a new empty List.

Let names be the ReferencedBindings of ExportsList.
Append to names the elements of the ReferencedBindings of ExportSpecifier.
Return names.

Return a List containing the IdentifierName.

Return a List containing the first IdentifierName.

15.2.3.11 Runtime Semantics: Evaluation

export

;

export

;

Return NormalCompletion(empty).

export

Return the result of evaluating VariableStatement.

export

Return the result of evaluating Declaration.

export

default

Return the result of evaluating HoistableDeclaration.

export

default

Let value be ? BindingClassDeclarationEvaluation of ClassDeclaration.
Let className be the sole element of BoundNames of ClassDeclaration.
If className is "*default*", then
1. Let env be the running execution context's LexicalEnvironment.
2. Perform ? InitializeBoundName("*default*", value, env).
Return NormalCompletion(empty).

export

default

;

If IsAnonymousFunctionDefinition(AssignmentExpression) is true, then
1. Let value be NamedEvaluation of AssignmentExpression with argument "default".
Else,
1. Let rhs be the result of evaluating AssignmentExpression.
2. Let value be ? GetValue(rhs).
Let env be the running execution context's LexicalEnvironment.
Perform ? InitializeBoundName("*default*", value, env).
Return NormalCompletion(empty).

16 Error Handling and Language Extensions

An implementation must report most errors at the time the relevant ECMAScript language construct is evaluated. An early error is an error that can be detected and reported prior to the evaluation of any construct in the Script containing the error. The presence of an early error prevents the evaluation of the construct. An implementation must report early errors in a Script as part of parsing that Script in ParseScript. Early errors in a Module are reported at the point when the Module would be evaluated and the Module is never initialized. Early errors in eval code are reported at the time eval is called and prevent evaluation of the eval code. All errors that are not early errors are runtime errors.

An implementation must report as an early error any occurrence of a condition that is listed in a “Static Semantics: Early Errors” subclause of this specification.

An implementation shall not treat other kinds of errors as early errors even if the compiler can prove that a construct cannot execute without error under any circumstances. An implementation may issue an early warning in such a case, but it should not report the error until the relevant construct is actually executed.

An implementation shall report all errors as specified, except for the following:

Except as restricted in 16.1, an implementation may extend Script syntax, Module syntax, and regular expression pattern or flag syntax. To permit this, all operations (such as calling eval, using a regular expression literal, or using the Function or RegExp constructor) that are allowed to throw SyntaxError are permitted to exhibit implementation-defined behaviour instead of throwing SyntaxError when they encounter an implementation-defined extension to the script syntax or regular expression pattern or flag syntax.
Except as restricted in 16.1, an implementation may provide additional types, values, objects, properties, and functions beyond those described in this specification. This may cause constructs (such as looking up a variable in the global scope) to have implementation-defined behaviour instead of throwing an error (such as ReferenceError).

16.1 Forbidden Extensions

An implementation must not extend this specification in the following ways:

ECMAScript function objects defined using syntactic constructors in strict mode code must not be created with own properties named "caller" or "arguments". Such own properties also must not be created for function objects defined using an ArrowFunction, MethodDefinition, GeneratorDeclaration, GeneratorExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, ClassDeclaration, ClassExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, or AsyncArrowFunction regardless of whether the definition is contained in strict mode code. Built-in functions, strict functions created using the Function constructor, generator functions created using the Generator constructor, async functions created using the AsyncFunction constructor, and functions created using the bind method also must not be created with such own properties.
If an implementation extends any function object with an own property named "caller" the value of that property, as observed using [[Get]] or [[GetOwnProperty]], must not be a strict function object. If it is an accessor property, the function that is the value of the property's [[Get]] attribute must never return a strict function when called.
Neither mapped nor unmapped arguments objects may be created with an own property named "caller".
The behaviour of built-in methods which are specified in ECMA-402, such as those named toLocaleString, must not be extended except as specified in ECMA-402.
The RegExp pattern grammars in 21.2.1 and B.1.4 must not be extended to recognize any of the source characters A-Z or a-z as IdentityEscape[+U] when the _[U] grammar parameter is present.
The Syntactic Grammar must not be extended in any manner that allows the token : to immediately follow source text that matches the BindingIdentifier nonterminal symbol.
When processing strict mode code, the syntax of NumericLiteral must not be extended to include LegacyOctalIntegerLiteral and the syntax of DecimalIntegerLiteral must not be extended to include NonOctalDecimalIntegerLiteral as described in B.1.1.
TemplateCharacter must not be extended to include LegacyOctalEscapeSequence as defined in B.1.2.
When processing strict mode code, the extensions defined in B.3.2, B.3.3, B.3.4, and B.3.6 must not be supported.
When parsing for the Module goal symbol, the lexical grammar extensions defined in B.1.3 must not be supported.
ImportCall must not be extended.

17 ECMAScript Standard Built-in Objects

There are certain built-in objects available whenever an ECMAScript Script or Module begins execution. One, the global object, is part of the global environment of the executing program. Others are accessible as initial properties of the global object or indirectly as properties of accessible built-in objects.

Unless specified otherwise, a built-in object that is callable as a function is a built-in function object with the characteristics described in 9.3. Unless specified otherwise, the [[Extensible]] internal slot of a built-in object initially has the value true. Every built-in function object has a [[Realm]] internal slot whose value is the Realm Record of the realm for which the object was initially created.

Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they are functions intended for use with the new operator. For each built-in function, this specification describes the arguments required by that function and the properties of that function object. For each built-in constructor, this specification furthermore describes properties of the prototype object of that constructor and properties of specific object instances returned by a new expression that invokes that constructor.

Unless otherwise specified in the description of a particular function, if a built-in function or constructor is given fewer arguments than the function is specified to require, the function or constructor shall behave exactly as if it had been given sufficient additional arguments, each such argument being the undefined value. Such missing arguments are considered to be “not present” and may be identified in that manner by specification algorithms. In the description of a particular function, the terms “this value” and “NewTarget” have the meanings given in 9.3.

Unless otherwise specified in the description of a particular function, if a built-in function or constructor described is given more arguments than the function is specified to allow, the extra arguments are evaluated by the call and then ignored by the function. However, an implementation may define implementation specific behaviour relating to such arguments as long as the behaviour is not the throwing of a TypeError exception that is predicated simply on the presence of an extra argument.

Note 1

Implementations that add additional capabilities to the set of built-in functions are encouraged to do so by adding new functions rather than adding new parameters to existing functions.

Unless otherwise specified every built-in function and every built-in constructor has the Function prototype object, which is the initial value of the expression Function.prototype (19.2.3), as the value of its [[Prototype]] internal slot.

Unless otherwise specified every built-in prototype object has the Object prototype object, which is the initial value of the expression Object.prototype (19.1.3), as the value of its [[Prototype]] internal slot, except the Object prototype object itself.

Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function.

Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation (9.3.3).

Every built-in function object, including constructors, has a "length" property whose value is an integer. Unless otherwise specified, this value is equal to the largest number of named arguments shown in the subclause headings for the function description. Optional parameters (which are indicated with brackets: [ ]) or rest parameters (which are shown using the form «...name») are not included in the default argument count.

Note 2

For example, the function object that is the initial value of the "map" property of the Array prototype object is described under the subclause heading «Array.prototype.map (callbackFn [ , thisArg])» which shows the two named arguments callbackFn and thisArg, the latter being optional; therefore the value of the "length" property of that function object is 1.

Unless otherwise specified, the "length" property of a built-in function object has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

Every built-in function object, including constructors, has a "name" property whose value is a String. Unless otherwise specified, this value is the name that is given to the function in this specification. Functions that are identified as anonymous functions use the empty string as the value of the "name" property. For functions that are specified as properties of objects, the name value is the property name string used to access the function. Functions that are specified as get or set accessor functions of built-in properties have "get " or "set " prepended to the property name string. The value of the "name" property is explicitly specified for each built-in functions whose property key is a Symbol value.

Unless otherwise specified, the "name" property of a built-in function object has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

Every other data property described in clauses 18 through 26 and in Annex B.2 has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified.

Every accessor property described in clauses 18 through 26 and in Annex B.2 has the attributes { [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified. If only a get accessor function is described, the set accessor function is the default value, undefined. If only a set accessor is described the get accessor is the default value, undefined.

18 The Global Object

The global object:

is created before control enters any execution context.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
has a [[Prototype]] internal slot whose value is implementation-dependent.
may have host defined properties in addition to the properties defined in this specification. This may include a property whose value is the global object itself.

18.1 Value Properties of the Global Object

18.1.1 globalThis

The initial value of the "globalThis" property of the global object in a Realm Record realm is realm.[[GlobalEnv]].[[GlobalThisValue]].

This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }.

18.1.2 Infinity

The value of Infinity is +∞ (see 6.1.6.1). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

18.1.3 NaN

The value of NaN is NaN (see 6.1.6.1). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

18.1.4 undefined

The value of undefined is undefined (see 6.1.1). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

18.2 Function Properties of the Global Object

18.2.1 eval ( `x` )

The eval function is the %eval% intrinsic object. When the eval function is called with one argument x, the following steps are taken:

Assert: The execution context stack has at least two elements.
Let callerContext be the second to top element of the execution context stack.
Let callerRealm be callerContext's Realm.
Return ? PerformEval(x, callerRealm, false, false).

18.2.1.1 Runtime Semantics: PerformEval ( `x`, `callerRealm`, `strictCaller`, `direct` )

The abstract operation PerformEval takes arguments x, callerRealm, strictCaller, and direct. It performs the following steps when called:

Assert: If direct is false, then strictCaller is also false.
If Type(x) is not String, return x.
Let evalRealm be the current Realm Record.
Perform ? HostEnsureCanCompileStrings(callerRealm, evalRealm).
Let inFunction be false.
Let inMethod be false.
Let inDerivedConstructor be false.
If direct is true, then
1. Let thisEnvRec be ! GetThisEnvironment().
2. If thisEnvRec is a function Environment Record, then
  1. Let F be thisEnvRec.[[FunctionObject]].
  2. Set inFunction to true.
  3. Set inMethod to thisEnvRec.HasSuperBinding().
  4. If F.[[ConstructorKind]] is derived, set inDerivedConstructor to true.
Perform the following substeps in an implementation-dependent order, possibly interleaving parsing and error detection:
1. Let script be the ECMAScript code that is the result of parsing ! UTF16DecodeString(x), for the goal symbol Script. If the parse fails, throw a SyntaxError exception. If any early errors are detected, throw a SyntaxError exception (but see also clause 16).
2. If script Contains ScriptBody is false, return undefined.
3. Let body be the ScriptBody of script.
4. If inFunction is false, and body Contains NewTarget, throw a SyntaxError exception.
5. If inMethod is false, and body Contains SuperProperty, throw a SyntaxError exception.
6. If inDerivedConstructor is false, and body Contains SuperCall, throw a SyntaxError exception.
If strictCaller is true, let strictEval be true.
Else, let strictEval be IsStrict of script.
Let runningContext be the running execution context.
NOTE: If direct is true, runningContext will be the execution context that performed the direct eval. If direct is false, runningContext will be the execution context for the invocation of the eval function.
If direct is true, then
1. Let lexEnv be NewDeclarativeEnvironment(runningContext's LexicalEnvironment).
2. Let varEnv be runningContext's VariableEnvironment.
Else,
1. Let lexEnv be NewDeclarativeEnvironment(evalRealm.[[GlobalEnv]]).
2. Let varEnv be evalRealm.[[GlobalEnv]].
If strictEval is true, set varEnv to lexEnv.
If runningContext is not already suspended, suspend runningContext.
Let evalContext be a new ECMAScript code execution context.
Set evalContext's Function to null.
Set evalContext's Realm to evalRealm.
Set evalContext's ScriptOrModule to runningContext's ScriptOrModule.
Set evalContext's VariableEnvironment to varEnv.
Set evalContext's LexicalEnvironment to lexEnv.
Push evalContext onto the execution context stack; evalContext is now the running execution context.
Let result be EvalDeclarationInstantiation(body, varEnv, lexEnv, strictEval).
If result.[[Type]] is normal, then
1. Set result to the result of evaluating body.
If result.[[Type]] is normal and result.[[Value]] is empty, then
1. Set result to NormalCompletion(undefined).
Suspend evalContext and remove it from the execution context stack.
Resume the context that is now on the top of the execution context stack as the running execution context.
Return Completion(result).

Note

The eval code cannot instantiate variable or function bindings in the variable environment of the calling context that invoked the eval if the calling context is evaluating formal parameter initializers or if either the code of the calling context or the eval code is strict mode code. Instead such bindings are instantiated in a new VariableEnvironment that is only accessible to the eval code. Bindings introduced by let, const, or class declarations are always instantiated in a new LexicalEnvironment.

18.2.1.2 HostEnsureCanCompileStrings ( `callerRealm`, `calleeRealm` )

HostEnsureCanCompileStrings is an implementation-defined abstract operation that allows host environments to block certain ECMAScript functions which allow developers to compile strings into ECMAScript code.

An implementation of HostEnsureCanCompileStrings may complete normally or abruptly. Any abrupt completions will be propagated to its callers. The default implementation of HostEnsureCanCompileStrings is to unconditionally return an empty normal completion.

18.2.1.3 Runtime Semantics: EvalDeclarationInstantiation ( `body`, `varEnv`, `lexEnv`, `strict` )

The abstract operation EvalDeclarationInstantiation takes arguments body, varEnv, lexEnv, and strict. It performs the following steps when called:

Let varNames be the VarDeclaredNames of body.
Let varDeclarations be the VarScopedDeclarations of body.
If strict is false, then
1. If varEnv is a global Environment Record, then
  1. For each name in varNames, do
    1. If varEnv.HasLexicalDeclaration(name) is true, throw a SyntaxError exception.
    2. NOTE: eval will not create a global var declaration that would be shadowed by a global lexical declaration.
2. Let thisEnv be lexEnv.
3. Assert: The following loop will terminate.
4. Repeat, while thisEnv is not the same as varEnv,
  1. If thisEnv is not an object Environment Record, then
    1. NOTE: The environment of with statements cannot contain any lexical declaration so it doesn't need to be checked for var/let hoisting conflicts.
    2. For each name in varNames, do
      1. If thisEnv.HasBinding(name) is true, then
        Throw a SyntaxError exception.
        NOTE: Annex B.3.5 defines alternate semantics for the above step.
      2. NOTE: A direct eval will not hoist var declaration over a like-named lexical declaration.
  2. Set thisEnv to thisEnv.[[OuterEnv]].
Let functionsToInitialize be a new empty List.
Let declaredFunctionNames be a new empty List.
For each d in varDeclarations, in reverse list order, do
1. If d is neither a VariableDeclaration nor a ForBinding nor a BindingIdentifier, then
  1. Assert: d is either a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration.
  2. NOTE: If there are multiple function declarations for the same name, the last declaration is used.
  3. Let fn be the sole element of the BoundNames of d.
  4. If fn is not an element of declaredFunctionNames, then
    1. If varEnv is a global Environment Record, then
      1. Let fnDefinable be ? varEnv.CanDeclareGlobalFunction(fn).
      2. If fnDefinable is false, throw a TypeError exception.
    2. Append fn to declaredFunctionNames.
    3. Insert d as the first element of functionsToInitialize.
NOTE: Annex B.3.3.3 adds additional steps at this point.
Let declaredVarNames be a new empty List.
For each d in varDeclarations, do
1. If d is a VariableDeclaration, a ForBinding, or a BindingIdentifier, then
  1. For each String vn in the BoundNames of d, do
    1. If vn is not an element of declaredFunctionNames, then
      1. If varEnv is a global Environment Record, then
        Let vnDefinable be ? varEnv.CanDeclareGlobalVar(vn).
        If vnDefinable is false, throw a TypeError exception.
      2. If vn is not an element of declaredVarNames, then
        Append vn to declaredVarNames.
NOTE: No abnormal terminations occur after this algorithm step unless varEnv is a global Environment Record and the global object is a Proxy exotic object.
Let lexDeclarations be the LexicallyScopedDeclarations of body.
For each element d in lexDeclarations, do
1. NOTE: Lexically declared names are only instantiated here but not initialized.
2. For each element dn of the BoundNames of d, do
  1. If IsConstantDeclaration of d is true, then
    1. Perform ? lexEnv.CreateImmutableBinding(dn, true).
  2. Else,
    1. Perform ? lexEnv.CreateMutableBinding(dn, false).
For each Parse Node f in functionsToInitialize, do
1. Let fn be the sole element of the BoundNames of f.
2. Let fo be InstantiateFunctionObject of f with argument lexEnv.
3. If varEnv is a global Environment Record, then
  1. Perform ? varEnv.CreateGlobalFunctionBinding(fn, fo, true).
4. Else,
  1. Let bindingExists be varEnv.HasBinding(fn).
  2. If bindingExists is false, then
    1. Let status be ! varEnv.CreateMutableBinding(fn, true).
    2. Assert: status is not an abrupt completion because of validation preceding step 10.
    3. Perform ! varEnv.InitializeBinding(fn, fo).
  3. Else,
    1. Perform ! varEnv.SetMutableBinding(fn, fo, false).
For each String vn in declaredVarNames, in list order, do
1. If varEnv is a global Environment Record, then
  1. Perform ? varEnv.CreateGlobalVarBinding(vn, true).
2. Else,
  1. Let bindingExists be varEnv.HasBinding(vn).
  2. If bindingExists is false, then
    1. Let status be ! varEnv.CreateMutableBinding(vn, true).
    2. Assert: status is not an abrupt completion because of validation preceding step 10.
    3. Perform ! varEnv.InitializeBinding(vn, undefined).
Return NormalCompletion(empty).

Note

An alternative version of this algorithm is described in B.3.5.

18.2.2 isFinite ( `number` )

The isFinite function is the %isFinite% intrinsic object. When the isFinite function is called with one argument number, the following steps are taken:

Let num be ? ToNumber(number).
If num is NaN, +∞, or -∞, return false.
Otherwise, return true.

18.2.3 isNaN ( `number` )

The isNaN function is the %isNaN% intrinsic object. When the isNaN function is called with one argument number, the following steps are taken:

Let num be ? ToNumber(number).
If num is NaN, return true.
Otherwise, return false.

Note

A reliable way for ECMAScript code to test if a value X is a NaN is an expression of the form X !== X. The result will be true if and only if X is a NaN.

18.2.4 parseFloat ( `string` )

The parseFloat function produces a Number value dictated by interpretation of the contents of the string argument as a decimal literal.

The parseFloat function is the %parseFloat% intrinsic object. When the parseFloat function is called with one argument string, the following steps are taken:

Let inputString be ? ToString(string).
Let trimmedString be ! TrimString(inputString, start).
If neither trimmedString nor any prefix of trimmedString satisfies the syntax of a StrDecimalLiteral (see 7.1.4.1), return NaN.
Let numberString be the longest prefix of trimmedString, which might be trimmedString itself, that satisfies the syntax of a StrDecimalLiteral.
Let mathFloat be MV of numberString.
If mathFloat = 0_ℝ, then
1. If the first code unit of trimmedString is the code unit 0x002D (HYPHEN-MINUS), return -0.
2. Return +0.
Return the Number value for mathFloat.

Note

parseFloat may interpret only a leading portion of string as a Number value; it ignores any code units that cannot be interpreted as part of the notation of a decimal literal, and no indication is given that any such code units were ignored.

18.2.5 parseInt ( `string`, `radix` )

The parseInt function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix. Leading white space in string is ignored. If radix is undefined or 0, it is assumed to be 10 except when the number begins with the code unit pairs 0x or 0X, in which case a radix of 16 is assumed. If radix is 16, the number may also optionally begin with the code unit pairs 0x or 0X.

The parseInt function is the %parseInt% intrinsic object. When the parseInt function is called, the following steps are taken:

Let inputString be ? ToString(string).
Let S be ! TrimString(inputString, start).
Let sign be 1.
If S is not empty and the first code unit of S is the code unit 0x002D (HYPHEN-MINUS), set sign to -1.
If S is not empty and the first code unit of S is the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS), remove the first code unit from S.
Let R be ? ToInt32(radix).
Let stripPrefix be true.
If R ≠ 0, then
1. If R < 2 or R > 36, return NaN.
2. If R ≠ 16, set stripPrefix to false.
Else,
1. Set R to 10.
If stripPrefix is true, then
1. If the length of S is at least 2 and the first two code units of S are either "0x" or "0X", then
  1. Remove the first two code units from S.
  2. Set R to 16.
If S contains a code unit that is not a radix-R digit, let Z be the substring of S consisting of all code units before the first such code unit; otherwise, let Z be S.
If Z is empty, return NaN.
Let mathInt be the mathematical integer value that is represented by Z in radix-R notation, using the letters A-Z and a-z for digits with values 10 through 35. (However, if R is 10 and Z contains more than 20 significant digits, every significant digit after the 20th may be replaced by a 0 digit, at the option of the implementation; and if R is not 2, 4, 8, 10, 16, or 32, then mathInt may be an implementation-dependent approximation to the mathematical integer value that is represented by Z in radix-R notation.)
If mathInt = 0_ℝ, then
1. If sign = -1, return -0.
2. Return +0.
Let number be the Number value for mathInt.
Return sign × number.

Note

parseInt may interpret only a leading portion of string as an integer value; it ignores any code units that cannot be interpreted as part of the notation of an integer, and no indication is given that any such code units were ignored.

18.2.6 URI Handling Functions

Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) and transport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language itself does not provide any support for using URIs except for functions that encode and decode URIs as described in 18.2.6.2, 18.2.6.3, 18.2.6.4 and 18.2.6.5

Note

Many implementations of ECMAScript provide additional functions and methods that manipulate web pages; these functions are beyond the scope of this standard.

18.2.6.1 URI Syntax and Semantics

A URI is composed of a sequence of components separated by component separators. The general form is:

Scheme

:

First

/

Second

;

Third

?

Fourth

where the italicized names represent components and “:”, “/”, “;” and “?” are reserved for use as separators. The encodeURI and decodeURI functions are intended to work with complete URIs; they assume that any reserved code units in the URI are intended to have special meaning and so are not encoded. The encodeURIComponent and decodeURIComponent functions are intended to work with the individual component parts of a URI; they assume that any reserved code units represent text and so must be encoded so that they are not interpreted as reserved code units when the component is part of a complete URI.

The following lexical grammar specifies the form of encoded URIs.

Syntax

:::

opt

:::

opt

:::

:::

one of

;

:::

:::

:::

one of

uriMark

:::

one of

(

)

Note

The above syntax is based upon RFC 2396 and does not reflect changes introduced by the more recent RFC 3986.

Runtime Semantics

When a code unit to be included in a URI is not listed above or is not intended to have the special meaning sometimes given to the reserved code units, that code unit must be encoded. The code unit is transformed into its UTF-8 encoding, with surrogate pairs first converted from UTF-16 to the corresponding code point value. (Note that for code units in the range [0, 127] this results in a single octet with the same value.) The resulting sequence of octets is then transformed into a String with each octet represented by an escape sequence of the form "%xx".

18.2.6.1.1 Runtime Semantics: Encode ( `string`, `unescapedSet` )

The abstract operation Encode takes arguments string (a String) and unescapedSet (a String). It performs URI encoding and escaping. It performs the following steps when called:

Let strLen be the number of code units in string.
Let R be the empty String.
Let k be 0.
Repeat,
1. If k equals strLen, return R.
2. Let C be the code unit at index k within string.
3. If C is in unescapedSet, then
  1. Set k to k + 1.
  2. Set R to the string-concatenation of the previous value of R and C.
4. Else,
  1. Let cp be ! CodePointAt(string, k).
  2. If cp.[[IsUnpairedSurrogate]] is true, throw a URIError exception.
  3. Set k to k + cp.[[CodeUnitCount]].
  4. Let Octets be the List of octets resulting by applying the UTF-8 transformation to cp.[[CodePoint]].
  5. For each element octet of Octets in List order, do
    1. Set R to the string-concatenation of:
      - the previous value of R
      - "%"
      - the String representation of octet, formatted as a two-digit uppercase hexadecimal number, padded to the left with a zero if necessary

18.2.6.1.2 Runtime Semantics: Decode ( `string`, `reservedSet` )

The abstract operation Decode takes arguments string (a String) and reservedSet (a String). It performs URI unescaping and decoding. It performs the following steps when called:

Let strLen be the number of code units in string.
Let R be the empty String.
Let k be 0.
Repeat,
1. If k equals strLen, return R.
2. Let C be the code unit at index k within string.
3. If C is not the code unit 0x0025 (PERCENT SIGN), then
  1. Let S be the String value containing only the code unit C.
4. Else,
  1. Let start be k.
  2. If k + 2 is greater than or equal to strLen, throw a URIError exception.
  3. If the code units at index (k + 1) and (k + 2) within string do not represent hexadecimal digits, throw a URIError exception.
  4. Let B be the 8-bit value represented by the two hexadecimal digits at index (k + 1) and (k + 2).
  5. Set k to k + 2.
  6. If the most significant bit in B is 0, then
    1. Let C be the code unit whose value is B.
    2. If C is not in reservedSet, then
      1. Let S be the String value containing only the code unit C.
    3. Else,
      1. Let S be the substring of string from index start to index k inclusive.
  7. Else,
    1. Assert: the most significant bit in B is 1.
    2. Let n be the smallest nonnegative integer such that (B << n) & 0x80 is equal to 0.
    3. If n equals 1 or n is greater than 4, throw a URIError exception.
    4. Let Octets be a List of 8-bit integers of size n.
    5. Set Octets[0] to B.
    6. If k + (3 × (n - 1)) is greater than or equal to strLen, throw a URIError exception.
    7. Let j be 1.
    8. Repeat, while j < n
      1. Set k to k + 1.
      2. If the code unit at index k within string is not the code unit 0x0025 (PERCENT SIGN), throw a URIError exception.
      3. If the code units at index (k + 1) and (k + 2) within string do not represent hexadecimal digits, throw a URIError exception.
      4. Let B be the 8-bit value represented by the two hexadecimal digits at index (k + 1) and (k + 2).
      5. If the two most significant bits in B are not 10, throw a URIError exception.
      6. Set k to k + 2.
      7. Set Octets[j] to B.
      8. Set j to j + 1.
    9. If Octets does not contain a valid UTF-8 encoding of a Unicode code point, throw a URIError exception.
    10. Let V be the value obtained by applying the UTF-8 transformation to Octets, that is, from a List of octets into a 21-bit value.
    11. Let S be the String value whose code units are, in order, the elements in UTF16Encoding(V).
5. Set R to the string-concatenation of the previous value of R and S.
6. Set k to k + 1.

Note

This syntax of Uniform Resource Identifiers is based upon RFC 2396 and does not reflect the more recent RFC 3986 which replaces RFC 2396. A formal description and implementation of UTF-8 is given in RFC 3629.

In UTF-8, characters are encoded using sequences of 1 to 6 octets. The only octet of a sequence of one has the higher-order bit set to 0, the remaining 7 bits being used to encode the character value. In a sequence of n octets, n > 1, the initial octet has the n higher-order bits set to 1, followed by a bit set to 0. The remaining bits of that octet contain bits from the value of the character to be encoded. The following octets all have the higher-order bit set to 1 and the following bit set to 0, leaving 6 bits in each to contain bits from the character to be encoded. The possible UTF-8 encodings of ECMAScript characters are specified in Table 47.

Table 47 (Informative): UTF-8 Encodings

Code Unit Value	Representation	1^st Octet	2^nd Octet	3^rd Octet	4^th Octet
`0x0000 - 0x007F`	`00000000 0zzzzzzz`	`0zzzzzzz`
`0x0080 - 0x07FF`	`00000yyy yyzzzzzz`	`110yyyyy`	`10zzzzzz`
`0x0800 - 0xD7FF`	`xxxxyyyy yyzzzzzz`	`1110xxxx`	`10yyyyyy`	`10zzzzzz`
`0xD800 - 0xDBFF` followed by `0xDC00 - 0xDFFF`	`110110vv vvwwwwxx` followed by `110111yy yyzzzzzz`	`11110uuu`	`10uuwwww`	`10xxyyyy`	`10zzzzzz`
`0xD800 - 0xDBFF` not followed by `0xDC00 - 0xDFFF`	causes `URIError`
`0xDC00 - 0xDFFF`	causes `URIError`
`0xE000 - 0xFFFF`	`xxxxyyyy yyzzzzzz`	`1110xxxx`	`10yyyyyy`	`10zzzzzz`

Where
uuuuu = vvvv + 1
to account for the addition of 0x10000 as in section 3.8 of the Unicode Standard (Surrogates).

The above transformation combines each surrogate pair (for which code unit values in the inclusive range 0xD800 to 0xDFFF are reserved) into a UTF-32 representation and encodes the resulting 21-bit value into UTF-8. Decoding reconstructs the surrogate pair.

RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence C0 80 must not decode into the code unit 0x0000. Implementations of the Decode algorithm are required to throw a URIError when encountering such invalid sequences.

18.2.6.2 decodeURI ( `encodedURI` )

The decodeURI function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURI function is replaced with the UTF-16 encoding of the code points that it represents. Escape sequences that could not have been introduced by encodeURI are not replaced.

The decodeURI function is the %decodeURI% intrinsic object. When the decodeURI function is called with one argument encodedURI, the following steps are taken:

Let uriString be ? ToString(encodedURI).
Let reservedURISet be a String containing one instance of each code unit valid in uriReserved plus "#".
Return ? Decode(uriString, reservedURISet).

Note

The code point # is not decoded from escape sequences even though it is not a reserved URI code point.

18.2.6.3 decodeURIComponent ( `encodedURIComponent` )

The decodeURIComponent function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURIComponent function is replaced with the UTF-16 encoding of the code points that it represents.

The decodeURIComponent function is the %decodeURIComponent% intrinsic object. When the decodeURIComponent function is called with one argument encodedURIComponent, the following steps are taken:

Let componentString be ? ToString(encodedURIComponent).
Let reservedURIComponentSet be the empty String.
Return ? Decode(componentString, reservedURIComponentSet).

18.2.6.4 encodeURI ( `uri` )

The encodeURI function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the code points.

The encodeURI function is the %encodeURI% intrinsic object. When the encodeURI function is called with one argument uri, the following steps are taken:

Let uriString be ? ToString(uri).
Let unescapedURISet be a String containing one instance of each code unit valid in uriReserved and uriUnescaped plus "#".
Return ? Encode(uriString, unescapedURISet).

Note

The code point # is not encoded to an escape sequence even though it is not a reserved or unescaped URI code point.

18.2.6.5 encodeURIComponent ( `uriComponent` )

The encodeURIComponent function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the code point.

The encodeURIComponent function is the %encodeURIComponent% intrinsic object. When the encodeURIComponent function is called with one argument uriComponent, the following steps are taken:

Let componentString be ? ToString(uriComponent).
Let unescapedURIComponentSet be a String containing one instance of each code unit valid in uriUnescaped.
Return ? Encode(componentString, unescapedURIComponentSet).

18.3 Constructor Properties of the Global Object

18.3.1 Array ( . . . )

See 22.1.1.

18.3.2 ArrayBuffer ( . . . )

See 24.1.2.

18.3.3 BigInt ( . . . )

See 20.2.1.

18.3.4 BigInt64Array ( . . . )

See 22.2.4.

18.3.5 BigUint64Array ( . . . )

See 22.2.4.

18.3.6 Boolean ( . . . )

See 19.3.1.

18.3.7 DataView ( . . . )

See 24.3.2.

18.3.8 Date ( . . . )

See 20.4.2.

18.3.9 Error ( . . . )

See 19.5.1.

18.3.10 EvalError ( . . . )

See 19.5.5.1.

18.3.11 Float32Array ( . . . )

See 22.2.4.

18.3.12 Float64Array ( . . . )

See 22.2.4.

18.3.13 Function ( . . . )

See 19.2.1.

18.3.14 Int8Array ( . . . )

See 22.2.4.

18.3.15 Int16Array ( . . . )

See 22.2.4.

18.3.16 Int32Array ( . . . )

See 22.2.4.

18.3.17 Map ( . . . )

See 23.1.1.

18.3.18 Number ( . . . )

See 20.1.1.

18.3.19 Object ( . . . )

See 19.1.1.

18.3.20 Promise ( . . . )

See 25.6.3.

18.3.21 Proxy ( . . . )

See 26.2.1.

18.3.22 RangeError ( . . . )

See 19.5.5.2.

18.3.23 ReferenceError ( . . . )

See 19.5.5.3.

18.3.24 RegExp ( . . . )

See 21.2.3.

18.3.25 Set ( . . . )

See 23.2.1.

18.3.26 SharedArrayBuffer ( . . . )

See 24.2.2.

18.3.27 String ( . . . )

See 21.1.1.

18.3.28 Symbol ( . . . )

See 19.4.1.

18.3.29 SyntaxError ( . . . )

See 19.5.5.4.

18.3.30 TypeError ( . . . )

See 19.5.5.5.

18.3.31 Uint8Array ( . . . )

See 22.2.4.

18.3.32 Uint8ClampedArray ( . . . )

See 22.2.4.

18.3.33 Uint16Array ( . . . )

See 22.2.4.

18.3.34 Uint32Array ( . . . )

See 22.2.4.

18.3.35 URIError ( . . . )

See 19.5.5.6.

18.3.36 WeakMap ( . . . )

See 23.3.1.

18.3.37 WeakSet ( . . . )

See 23.4.

18.4 Other Properties of the Global Object

18.4.1 Atomics

See 24.4.

18.4.2 JSON

See 24.5.

18.4.3 Math

See 20.3.

18.4.4 Reflect

See 26.1.

19 Fundamental Objects

19.1 Object Objects

19.1.1 The Object Constructor

The Object constructor:

is the intrinsic object %Object%.
is the initial value of the "Object" property of the global object.
creates a new ordinary object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition.

19.1.1.1 Object ( [ `value` ] )

When the Object function is called with optional argument value, the following steps are taken:

If NewTarget is neither undefined nor the active function, then
1. Return ? OrdinaryCreateFromConstructor(NewTarget, "%Object.prototype%").
If value is undefined or null, return OrdinaryObjectCreate(%Object.prototype%).
Return ! ToObject(value).

The "length" property of the Object function is 1.

19.1.2 Properties of the Object Constructor

The Object constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has a "length" property.
has the following additional properties:

19.1.2.1 Object.assign ( `target`, ...`sources` )

The assign function is used to copy the values of all of the enumerable own properties from one or more source objects to a target object. When the assign function is called, the following steps are taken:

Let to be ? ToObject(target).
If only one argument was passed, return to.
Let sources be the List of argument values starting with the second argument.
For each element nextSource of sources, in ascending index order, do
1. If nextSource is neither undefined nor null, then
  1. Let from be ! ToObject(nextSource).
  2. Let keys be ? from.[[OwnPropertyKeys]]().
  3. For each element nextKey of keys in List order, do
    1. Let desc be ? from.[[GetOwnProperty]](nextKey).
    2. If desc is not undefined and desc.[[Enumerable]] is true, then
      1. Let propValue be ? Get(from, nextKey).
      2. Perform ? Set(to, nextKey, propValue, true).
Return to.

The "length" property of the assign function is 2.

19.1.2.2 Object.create ( `O`, `Properties` )

The create function creates a new object with a specified prototype. When the create function is called, the following steps are taken:

If Type(O) is neither Object nor Null, throw a TypeError exception.
Let obj be OrdinaryObjectCreate(O).
If Properties is not undefined, then
1. Return ? ObjectDefineProperties(obj, Properties).
Return obj.

19.1.2.3 Object.defineProperties ( `O`, `Properties` )

The defineProperties function is used to add own properties and/or update the attributes of existing own properties of an object. When the defineProperties function is called, the following steps are taken:

Return ? ObjectDefineProperties(O, Properties).

19.1.2.3.1 Runtime Semantics: ObjectDefineProperties ( `O`, `Properties` )

The abstract operation ObjectDefineProperties takes arguments O and Properties. It performs the following steps when called:

If Type(O) is not Object, throw a TypeError exception.
Let props be ? ToObject(Properties).
Let keys be ? props.[[OwnPropertyKeys]]().
Let descriptors be a new empty List.
For each element nextKey of keys in List order, do
1. Let propDesc be ? props.[[GetOwnProperty]](nextKey).
2. If propDesc is not undefined and propDesc.[[Enumerable]] is true, then
  1. Let descObj be ? Get(props, nextKey).
  2. Let desc be ? ToPropertyDescriptor(descObj).
  3. Append the pair (a two element List) consisting of nextKey and desc to the end of descriptors.
For each pair from descriptors in list order, do
1. Let P be the first element of pair.
2. Let desc be the second element of pair.
3. Perform ? DefinePropertyOrThrow(O, P, desc).
Return O.

19.1.2.4 Object.defineProperty ( `O`, `P`, `Attributes` )

The defineProperty function is used to add an own property and/or update the attributes of an existing own property of an object. When the defineProperty function is called, the following steps are taken:

If Type(O) is not Object, throw a TypeError exception.
Let key be ? ToPropertyKey(P).
Let desc be ? ToPropertyDescriptor(Attributes).
Perform ? DefinePropertyOrThrow(O, key, desc).
Return O.

19.1.2.5 Object.entries ( `O` )

When the entries function is called with argument O, the following steps are taken:

Let obj be ? ToObject(O).
Let nameList be ? EnumerableOwnPropertyNames(obj, key+value).
Return CreateArrayFromList(nameList).

19.1.2.6 Object.freeze ( `O` )

When the freeze function is called, the following steps are taken:

If Type(O) is not Object, return O.
Let status be ? SetIntegrityLevel(O, frozen).
If status is false, throw a TypeError exception.
Return O.

19.1.2.7 Object.fromEntries ( `iterable` )

When the fromEntries method is called with argument iterable, the following steps are taken:

Perform ? RequireObjectCoercible(iterable).
Let obj be OrdinaryObjectCreate(%Object.prototype%).
Assert: obj is an extensible ordinary object with no own properties.
Let stepsDefine be the algorithm steps defined in CreateDataPropertyOnObject Functions.
Let adder be ! CreateBuiltinFunction(stepsDefine, « »).
Return ? AddEntriesFromIterable(obj, iterable, adder).

Note

The function created for adder is never directly accessible to ECMAScript code.

19.1.2.7.1 CreateDataPropertyOnObject Functions

A CreateDataPropertyOnObject function is an anonymous built-in function. When a CreateDataPropertyOnObject function is called with arguments key and value, the following steps are taken:

Let O be the this value.
Assert: Type(O) is Object.
Assert: O is an extensible ordinary object.
Let propertyKey be ? ToPropertyKey(key).
Perform ! CreateDataPropertyOrThrow(O, propertyKey, value).
Return undefined.

19.1.2.8 Object.getOwnPropertyDescriptor ( `O`, `P` )

When the getOwnPropertyDescriptor function is called, the following steps are taken:

Let obj be ? ToObject(O).
Let key be ? ToPropertyKey(P).
Let desc be ? obj.[[GetOwnProperty]](key).
Return FromPropertyDescriptor(desc).

19.1.2.9 Object.getOwnPropertyDescriptors ( `O` )

When the getOwnPropertyDescriptors function is called, the following steps are taken:

Let obj be ? ToObject(O).
Let ownKeys be ? obj.[[OwnPropertyKeys]]().
Let descriptors be ! OrdinaryObjectCreate(%Object.prototype%).
For each element key of ownKeys in List order, do
1. Let desc be ? obj.[[GetOwnProperty]](key).
2. Let descriptor be ! FromPropertyDescriptor(desc).
3. If descriptor is not undefined, perform ! CreateDataPropertyOrThrow(descriptors, key, descriptor).
Return descriptors.

19.1.2.10 Object.getOwnPropertyNames ( `O` )

When the getOwnPropertyNames function is called, the following steps are taken:

Return ? GetOwnPropertyKeys(O, string).

19.1.2.11 Object.getOwnPropertySymbols ( `O` )

When the getOwnPropertySymbols function is called with argument O, the following steps are taken:

Return ? GetOwnPropertyKeys(O, symbol).

19.1.2.11.1 Runtime Semantics: GetOwnPropertyKeys ( `O`, `type` )

The abstract operation GetOwnPropertyKeys takes arguments O and type (either string or symbol). It performs the following steps when called:

Let obj be ? ToObject(O).
Let keys be ? obj.[[OwnPropertyKeys]]().
Let nameList be a new empty List.
For each element nextKey of keys in List order, do
1. If Type(nextKey) is Symbol and type is symbol or Type(nextKey) is String and type is string, then
  1. Append nextKey as the last element of nameList.
Return CreateArrayFromList(nameList).

19.1.2.12 Object.getPrototypeOf ( `O` )

When the getPrototypeOf function is called with argument O, the following steps are taken:

Let obj be ? ToObject(O).
Return ? obj.[[GetPrototypeOf]]().

19.1.2.13 Object.is ( `value1`, `value2` )

When the is function is called with arguments value1 and value2, the following steps are taken:

Return SameValue(value1, value2).

19.1.2.14 Object.isExtensible ( `O` )

When the isExtensible function is called with argument O, the following steps are taken:

If Type(O) is not Object, return false.
Return ? IsExtensible(O).

19.1.2.15 Object.isFrozen ( `O` )

When the isFrozen function is called with argument O, the following steps are taken:

If Type(O) is not Object, return true.
Return ? TestIntegrityLevel(O, frozen).

19.1.2.16 Object.isSealed ( `O` )

When the isSealed function is called with argument O, the following steps are taken:

If Type(O) is not Object, return true.
Return ? TestIntegrityLevel(O, sealed).

19.1.2.17 Object.keys ( `O` )

When the keys function is called with argument O, the following steps are taken:

Let obj be ? ToObject(O).
Let nameList be ? EnumerableOwnPropertyNames(obj, key).
Return CreateArrayFromList(nameList).

19.1.2.18 Object.preventExtensions ( `O` )

When the preventExtensions function is called, the following steps are taken:

If Type(O) is not Object, return O.
Let status be ? O.[[PreventExtensions]]().
If status is false, throw a TypeError exception.
Return O.

19.1.2.19 Object.prototype

The initial value of Object.prototype is %Object.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.1.2.20 Object.seal ( `O` )

When the seal function is called, the following steps are taken:

If Type(O) is not Object, return O.
Let status be ? SetIntegrityLevel(O, sealed).
If status is false, throw a TypeError exception.
Return O.

19.1.2.21 Object.setPrototypeOf ( `O`, `proto` )

When the setPrototypeOf function is called with arguments O and proto, the following steps are taken:

Set O to ? RequireObjectCoercible(O).
If Type(proto) is neither Object nor Null, throw a TypeError exception.
If Type(O) is not Object, return O.
Let status be ? O.[[SetPrototypeOf]](proto).
If status is false, throw a TypeError exception.
Return O.

19.1.2.22 Object.values ( `O` )

When the values function is called with argument O, the following steps are taken:

Let obj be ? ToObject(O).
Let nameList be ? EnumerableOwnPropertyNames(obj, value).
Return CreateArrayFromList(nameList).

19.1.3 Properties of the Object Prototype Object

The Object prototype object:

is the intrinsic object %ObjectPrototype%.
has an [[Extensible]] internal slot whose value is true.
has the internal methods defined for ordinary objects, except for the [[SetPrototypeOf]] method, which is as defined in 9.4.7.1. (Thus, it is an immutable prototype exotic object.)
has a [[Prototype]] internal slot whose value is null.

19.1.3.1 Object.prototype.constructor

The initial value of Object.prototype.constructor is %Object%.

19.1.3.2 Object.prototype.hasOwnProperty ( `V` )

When the hasOwnProperty method is called with argument V, the following steps are taken:

Let P be ? ToPropertyKey(V).
Let O be ? ToObject(this value).
Return ? HasOwnProperty(O, P).

Note

The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this value is undefined or null.

19.1.3.3 Object.prototype.isPrototypeOf ( `V` )

When the isPrototypeOf method is called with argument V, the following steps are taken:

If Type(V) is not Object, return false.
Let O be ? ToObject(this value).
Repeat,
1. Set V to ? V.[[GetPrototypeOf]]().
2. If V is null, return false.
3. If SameValue(O, V) is true, return true.

Note

The ordering of steps 1 and 2 preserves the behaviour specified by previous editions of this specification for the case where V is not an object and the this value is undefined or null.

19.1.3.4 Object.prototype.propertyIsEnumerable ( `V` )

When the propertyIsEnumerable method is called with argument V, the following steps are taken:

Let P be ? ToPropertyKey(V).
Let O be ? ToObject(this value).
Let desc be ? O.[[GetOwnProperty]](P).
If desc is undefined, return false.
Return desc.[[Enumerable]].

Note 1

This method does not consider objects in the prototype chain.

Note 2

19.1.3.5 Object.prototype.toLocaleString ( [ `reserved1` [ , `reserved2` ] ] )

When the toLocaleString method is called, the following steps are taken:

Let O be the this value.
Return ? Invoke(O, "toString").

The optional parameters to this function are not used but are intended to correspond to the parameter pattern used by ECMA-402 toLocaleString functions. Implementations that do not include ECMA-402 support must not use those parameter positions for other purposes.

Note 1

This function provides a generic toLocaleString implementation for objects that have no locale-specific toString behaviour. Array, Number, Date, and %TypedArray% provide their own locale-sensitive toLocaleString methods.

Note 2

ECMA-402 intentionally does not provide an alternative to this default implementation.

19.1.3.6 Object.prototype.toString ( )

When the toString method is called, the following steps are taken:

If the this value is undefined, return "[object Undefined]".
If the this value is null, return "[object Null]".
Let O be ! ToObject(this value).
Let isArray be ? IsArray(O).
If isArray is true, let builtinTag be "Array".
Else if O has a [[ParameterMap]] internal slot, let builtinTag be "Arguments".
Else if O has a [[Call]] internal method, let builtinTag be "Function".
Else if O has an [[ErrorData]] internal slot, let builtinTag be "Error".
Else if O has a [[BooleanData]] internal slot, let builtinTag be "Boolean".
Else if O has a [[NumberData]] internal slot, let builtinTag be "Number".
Else if O has a [[StringData]] internal slot, let builtinTag be "String".
Else if O has a [[DateValue]] internal slot, let builtinTag be "Date".
Else if O has a [[RegExpMatcher]] internal slot, let builtinTag be "RegExp".
Else, let builtinTag be "Object".
Let tag be ? Get(O, @@toStringTag).
If Type(tag) is not String, set tag to builtinTag.
Return the string-concatenation of "[object ", tag, and "]".

This function is the %ObjProto_toString% intrinsic object.

Note

Historically, this function was occasionally used to access the String value of the [[Class]] internal slot that was used in previous editions of this specification as a nominal type tag for various built-in objects. The above definition of toString preserves compatibility for legacy code that uses toString as a test for those specific kinds of built-in objects. It does not provide a reliable type testing mechanism for other kinds of built-in or program defined objects. In addition, programs can use @@toStringTag in ways that will invalidate the reliability of such legacy type tests.

19.1.3.7 Object.prototype.valueOf ( )

When the valueOf method is called, the following steps are taken:

Return ? ToObject(this value).

This function is the %ObjProto_valueOf% intrinsic object.

19.1.4 Properties of Object Instances

Object instances have no special properties beyond those inherited from the Object prototype object.

19.2 Function Objects

19.2.1 The Function Constructor

The Function constructor:

is the intrinsic object %Function%.
is the initial value of the "Function" property of the global object.
creates and initializes a new function object when called as a function rather than as a constructor. Thus the function call Function(…) is equivalent to the object creation expression new Function(…) with the same arguments.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Function behaviour must include a super call to the Function constructor to create and initialize a subclass instance with the internal slots necessary for built-in function behaviour. All ECMAScript syntactic forms for defining function objects create instances of Function. There is no syntactic means to create instances of Function subclasses except for the built-in GeneratorFunction, AsyncFunction, and AsyncGeneratorFunction subclasses.

19.2.1.1 Function ( `p1`, `p2`, … , `pn`, `body` )

The last argument specifies the body (executable code) of a function; any preceding arguments specify formal parameters.

When the Function function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “ p ” arguments, and where body might also not be provided), the following steps are taken:

Let C be the active function object.
Let args be the argumentsList that was passed to this function by [[Call]] or [[Construct]].
Return ? CreateDynamicFunction(C, NewTarget, normal, args).

Note

It is permissible but not necessary to have one argument for each formal parameter to be specified. For example, all three of the following expressions produce the same result:

new Function("a", "b", "c", "return a+b+c")
new Function("a, b, c", "return a+b+c")
new Function("a,b", "c", "return a+b+c")

19.2.1.1.1 Runtime Semantics: CreateDynamicFunction ( `constructor`, `newTarget`, `kind`, `args` )

The abstract operation CreateDynamicFunction takes arguments constructor (a constructor), newTarget (a constructor), kind (either normal, generator, async, or asyncGenerator), and args (a List of ECMAScript language values). constructor is the constructor function that is performing this action. newTarget is the constructor that new was initially applied to. args is the argument values that were passed to constructor. It performs the following steps when called:

Assert: The execution context stack has at least two elements.
Let callerContext be the second to top element of the execution context stack.
Let callerRealm be callerContext's Realm.
Let calleeRealm be the current Realm Record.
Perform ? HostEnsureCanCompileStrings(callerRealm, calleeRealm).
If newTarget is undefined, set newTarget to constructor.
If kind is normal, then
1. Let goal be the grammar symbol FunctionBody[~Yield, ~Await].
2. Let parameterGoal be the grammar symbol FormalParameters[~Yield, ~Await].
3. Let fallbackProto be "%Function.prototype%".
Else if kind is generator, then
1. Let goal be the grammar symbol GeneratorBody.
2. Let parameterGoal be the grammar symbol FormalParameters[+Yield, ~Await].
3. Let fallbackProto be "%Generator%".
Else if kind is async, then
1. Let goal be the grammar symbol AsyncFunctionBody.
2. Let parameterGoal be the grammar symbol FormalParameters[~Yield, +Await].
3. Let fallbackProto be "%AsyncFunction.prototype%".
Else,
1. Assert: kind is asyncGenerator.
2. Let goal be the grammar symbol AsyncGeneratorBody.
3. Let parameterGoal be the grammar symbol FormalParameters[+Yield, +Await].
4. Let fallbackProto be "%AsyncGenerator%".
Let argCount be the number of elements in args.
Let P be the empty String.
If argCount = 0, let bodyArg be the empty String.
Else if argCount = 1, let bodyArg be args[0].
Else,
1. Assert: argCount > 1.
2. Let firstArg be args[0].
3. Set P to ? ToString(firstArg).
4. Let k be 1.
5. Repeat, while k < argCount - 1
  1. Let nextArg be args[k].
  2. Let nextArgString be ? ToString(nextArg).
  3. Set P to the string-concatenation of the previous value of P, "," (a comma), and nextArgString.
  4. Set k to k + 1.
6. Let bodyArg be args[k].
Let bodyString be the string-concatenation of 0x000A (LINE FEED), ? ToString(bodyArg), and 0x000A (LINE FEED).
Let prefix be the prefix associated with kind in Table 48.
Let sourceString be the string-concatenation of prefix, " anonymous(", P, 0x000A (LINE FEED), ") {", bodyString, and "}".
Let sourceText be ! UTF16DecodeString(sourceString).
Perform the following substeps in an implementation-dependent order, possibly interleaving parsing and error detection:
1. Let parameters be the result of parsing ! UTF16DecodeString(P), using parameterGoal as the goal symbol. Throw a SyntaxError exception if the parse fails.
2. Let body be the result of parsing ! UTF16DecodeString(bodyString), using goal as the goal symbol. Throw a SyntaxError exception if the parse fails.
3. Let strict be ContainsUseStrict of body.
4. If any static semantics errors are detected for parameters or body, throw a SyntaxError exception. If strict is true, the Early Error rules for UniqueFormalParameters:FormalParameters are applied.
5. If strict is true and IsSimpleParameterList of parameters is false, throw a SyntaxError exception.
6. If any element of the BoundNames of parameters also occurs in the LexicallyDeclaredNames of body, throw a SyntaxError exception.
7. If body Contains SuperCall is true, throw a SyntaxError exception.
8. If parameters Contains SuperCall is true, throw a SyntaxError exception.
9. If body Contains SuperProperty is true, throw a SyntaxError exception.
10. If parameters Contains SuperProperty is true, throw a SyntaxError exception.
11. If kind is generator or asyncGenerator, then
  1. If parameters Contains YieldExpression is true, throw a SyntaxError exception.
12. If kind is async or asyncGenerator, then
  1. If parameters Contains AwaitExpression is true, throw a SyntaxError exception.
13. If strict is true, then
  1. If BoundNames of parameters contains any duplicate elements, throw a SyntaxError exception.
Let proto be ? GetPrototypeFromConstructor(newTarget, fallbackProto).
Let realmF be the current Realm Record.
Let scope be realmF.[[GlobalEnv]].
Let F be ! OrdinaryFunctionCreate(proto, sourceText, parameters, body, non-lexical-this, scope).
Perform SetFunctionName(F, "anonymous").
If kind is generator, then
1. Let prototype be OrdinaryObjectCreate(%Generator.prototype%).
2. Perform DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Else if kind is asyncGenerator, then
1. Let prototype be OrdinaryObjectCreate(%AsyncGenerator.prototype%).
2. Perform DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Else if kind is normal, perform MakeConstructor(F).
NOTE: Functions whose kind is async are not constructible and do not have a [[Construct]] internal method or a "prototype" property.
Return F.

Note

CreateDynamicFunction defines a "prototype" property on any function it creates whose kind is not async to provide for the possibility that the function will be used as a constructor.

Table 48: Dynamic Function SourceText Prefixes

Kind	Prefix
normal	"function"
generator	"function*"
async	"async function"
asyncGenerator	"async function*"

19.2.2 Properties of the Function Constructor

The Function constructor:

is itself a built-in function object.
has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

19.2.2.1 Function.length

This is a data property with a value of 1. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

19.2.2.2 Function.prototype

The value of Function.prototype is %Function.prototype%, the intrinsic Function prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.2.3 Properties of the Function Prototype Object

The Function prototype object:

is %Function.prototype%.
is itself a built-in function object.
accepts any arguments and returns undefined when invoked.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
has a [[Prototype]] internal slot whose value is %Object.prototype%.
does not have a "prototype" property.
has a "length" property whose value is 0.
has a "name" property whose value is the empty String.

Note

The Function prototype object is specified to be a function object to ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.

19.2.3.1 Function.prototype.apply ( `thisArg`, `argArray` )

When the apply method is called with arguments thisArg and argArray, the following steps are taken:

Let func be the this value.
If IsCallable(func) is false, throw a TypeError exception.
If argArray is undefined or null, then
1. Perform PrepareForTailCall().
2. Return ? Call(func, thisArg).
Let argList be ? CreateListFromArrayLike(argArray).
Perform PrepareForTailCall().
Return ? Call(func, thisArg, argList).

Note 1

The thisArg value is passed without modification as the this value. This is a change from Edition 3, where an undefined or null thisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value. Even though the thisArg is passed without modification, non-strict functions still perform these transformations upon entry to the function.

Note 2

If func is an arrow function or a bound function exotic object then the thisArg will be ignored by the function [[Call]] in step 6.

19.2.3.2 Function.prototype.bind ( `thisArg`, ...`args` )

When the bind method is called with argument thisArg and zero or more args, it performs the following steps:

Let Target be the this value.
If IsCallable(Target) is false, throw a TypeError exception.
Let args be a new (possibly empty) List consisting of all of the argument values provided after thisArg in order.
Let F be ? BoundFunctionCreate(Target, thisArg, args).
Let targetHasLength be ? HasOwnProperty(Target, "length").
If targetHasLength is true, then
1. Let targetLen be ? Get(Target, "length").
2. If Type(targetLen) is not Number, let L be 0.
3. Else,
  1. Set targetLen to ! ToInteger(targetLen).
  2. Let L be the larger of 0 and the result of targetLen minus the number of elements of args.
Else, let L be 0.
Perform ! SetFunctionLength(F, L).
Let targetName be ? Get(Target, "name").
If Type(targetName) is not String, set targetName to the empty String.
Perform SetFunctionName(F, targetName, "bound").
Return F.

Note 1

Function objects created using Function.prototype.bind are exotic objects. They also do not have a "prototype" property.

Note 2

If Target is an arrow function or a bound function exotic object then the thisArg passed to this method will not be used by subsequent calls to F.

19.2.3.3 Function.prototype.call ( `thisArg`, ...`args` )

When the call method is called with argument thisArg and zero or more args, the following steps are taken:

Let func be the this value.
If IsCallable(func) is false, throw a TypeError exception.
Let argList be a new empty List.
If this method was called with more than one argument, then in left to right order, starting with the second argument, append each argument as the last element of argList.
Perform PrepareForTailCall().
Return ? Call(func, thisArg, argList).

Note 1

Note 2

If func is an arrow function or a bound function exotic object then the thisArg will be ignored by the function [[Call]] in step 6.

19.2.3.4 Function.prototype.constructor

The initial value of Function.prototype.constructor is %Function%.

19.2.3.5 Function.prototype.toString ( )

When the toString method is called, the following steps are taken:

Let func be the this value.
If func is a bound function exotic object or a built-in function object, then return an implementation-dependent String source code representation of func. The representation must have the syntax of a NativeFunction. Additionally, if func is a Well-known Intrinsic Object and is not identified as an anonymous function, the portion of the returned String that would be matched by PropertyName must be the initial value of the "name" property of func.
If Type(func) is Object and func has a [[SourceText]] internal slot and func.[[SourceText]] is a sequence of Unicode code points and ! HostHasSourceTextAvailable(func) is true, then
1. Return ! UTF16Encode(func.[[SourceText]]).
If Type(func) is Object and IsCallable(func) is true, then return an implementation-dependent String source code representation of func. The representation must have the syntax of a NativeFunction.
Throw a TypeError exception.

NativeFunction

function

[~Yield, ~Await]

opt

(

[~Yield, ~Await]

)

{

[

native

code

]

}

19.2.3.6 Function.prototype [ @@hasInstance ] ( `V` )

When the @@hasInstance method of an object F is called with value V, the following steps are taken:

Let F be the this value.
Return ? OrdinaryHasInstance(F, V).

The value of the "name" property of this function is "[Symbol.hasInstance]".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

Note

This is the default implementation of @@hasInstance that most functions inherit. @@hasInstance is called by the instanceof operator to determine whether a value is an instance of a specific constructor. An expression such as

v instanceof F

evaluates as

F[@@hasInstance](v)

A constructor function can control which objects are recognized as its instances by instanceof by exposing a different @@hasInstance method on the function.

This property is non-writable and non-configurable to prevent tampering that could be used to globally expose the target function of a bound function.

19.2.4 Function Instances

Every Function instance is an ECMAScript function object and has the internal slots listed in Table 27. Function objects created using the Function.prototype.bind method (19.2.3.2) have the internal slots listed in Table 28.

Function instances have the following properties:

19.2.4.1 length

The value of the "length" property is an integer that indicates the typical number of arguments expected by the function. However, the language permits the function to be invoked with some other number of arguments. The behaviour of a function when invoked on a number of arguments other than the number specified by its "length" property depends on the function. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

19.2.4.2 name

The value of the "name" property is a String that is descriptive of the function. The name has no semantic significance but is typically a variable or property name that is used to refer to the function at its point of definition in ECMAScript code. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

Anonymous functions objects that do not have a contextual name associated with them by this specification use the empty string as the value of the "name" property.

19.2.4.3 prototype

Function instances that can be used as a constructor have a "prototype" property. Whenever such a Function instance is created another ordinary object is also created and is the initial value of the function's "prototype" property. Unless otherwise specified, the value of the "prototype" property is used to initialize the [[Prototype]] internal slot of the object created when that function is invoked as a constructor.

This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

Note

Function objects created using Function.prototype.bind, or by evaluating a MethodDefinition (that is not a GeneratorMethod or AsyncGeneratorMethod) or an ArrowFunction do not have a "prototype" property.

19.2.5 HostHasSourceTextAvailable ( `func` )

HostHasSourceTextAvailable is an implementation-defined abstract operation that allows host environments to prevent the source text from being provided for a given function.

An implementation of HostHasSourceTextAvailable must complete normally in all cases. This operation must be deterministic with respect to its parameters. Each time it is called with a specific func as its argument, it must return the same completion record. The default implementation of HostHasSourceTextAvailable is to unconditionally return a normal completion with a value of true.

19.3 Boolean Objects

19.3.1 The Boolean Constructor

The Boolean constructor:

is the intrinsic object %Boolean%.
is the initial value of the "Boolean" property of the global object.
creates and initializes a new Boolean object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Boolean behaviour must include a super call to the Boolean constructor to create and initialize the subclass instance with a [[BooleanData]] internal slot.

19.3.1.1 Boolean ( `value` )

When Boolean is called with argument value, the following steps are taken:

Let b be ! ToBoolean(value).
If NewTarget is undefined, return b.
Let O be ? OrdinaryCreateFromConstructor(NewTarget, "%Boolean.prototype%", « [[BooleanData]] »).
Set O.[[BooleanData]] to b.
Return O.

19.3.2 Properties of the Boolean Constructor

The Boolean constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

19.3.2.1 Boolean.prototype

The initial value of Boolean.prototype is %Boolean.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.3.3 Properties of the Boolean Prototype Object

The Boolean prototype object:

is the intrinsic object %BooleanPrototype%.
is an ordinary object.
is itself a Boolean object; it has a [[BooleanData]] internal slot with the value false.
has a [[Prototype]] internal slot whose value is %Object.prototype%.

The abstract operation thisBooleanValue takes argument value. It performs the following steps when called:

If Type(value) is Boolean, return value.
If Type(value) is Object and value has a [[BooleanData]] internal slot, then
1. Let b be value.[[BooleanData]].
2. Assert: Type(b) is Boolean.
3. Return b.
Throw a TypeError exception.

19.3.3.1 Boolean.prototype.constructor

The initial value of Boolean.prototype.constructor is %Boolean%.

19.3.3.2 Boolean.prototype.toString ( )

The following steps are taken:

Let b be ? thisBooleanValue(this value).
If b is true, return "true"; else return "false".

19.3.3.3 Boolean.prototype.valueOf ( )

The following steps are taken:

Return ? thisBooleanValue(this value).

19.3.4 Properties of Boolean Instances

Boolean instances are ordinary objects that inherit properties from the Boolean prototype object. Boolean instances have a [[BooleanData]] internal slot. The [[BooleanData]] internal slot is the Boolean value represented by this Boolean object.

19.4 Symbol Objects

19.4.1 The Symbol Constructor

The Symbol constructor:

is the intrinsic object %Symbol%.
is the initial value of the "Symbol" property of the global object.
returns a new Symbol value when called as a function.
is not intended to be used with the new operator.
is not intended to be subclassed.
may be used as the value of an extends clause of a class definition but a super call to it will cause an exception.

19.4.1.1 Symbol ( [ `description` ] )

When Symbol is called with optional argument description, the following steps are taken:

If NewTarget is not undefined, throw a TypeError exception.
If description is undefined, let descString be undefined.
Else, let descString be ? ToString(description).
Return a new unique Symbol value whose [[Description]] value is descString.

19.4.2 Properties of the Symbol Constructor

The Symbol constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

19.4.2.1 Symbol.asyncIterator

The initial value of Symbol.asyncIterator is the well known symbol @@asyncIterator (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.2 Symbol.for ( `key` )

When Symbol.for is called with argument key it performs the following steps:

Let stringKey be ? ToString(key).
For each element e of the GlobalSymbolRegistry List, do
1. If SameValue(e.[[Key]], stringKey) is true, return e.[[Symbol]].
Assert: GlobalSymbolRegistry does not currently contain an entry for stringKey.
Let newSymbol be a new unique Symbol value whose [[Description]] value is stringKey.
Append the Record { [[Key]]: stringKey, [[Symbol]]: newSymbol } to the GlobalSymbolRegistry List.
Return newSymbol.

The GlobalSymbolRegistry is a List that is globally available. It is shared by all realms. Prior to the evaluation of any ECMAScript code it is initialized as a new empty List. Elements of the GlobalSymbolRegistry are Records with the structure defined in Table 49.

Table 49: GlobalSymbolRegistry Record Fields

Field Name	Value	Usage
[[Key]]	A String	A string key used to globally identify a Symbol.
[[Symbol]]	A Symbol	A symbol that can be retrieved from any realm.

19.4.2.3 Symbol.hasInstance

The initial value of Symbol.hasInstance is the well-known symbol @@hasInstance (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.4 Symbol.isConcatSpreadable

The initial value of Symbol.isConcatSpreadable is the well-known symbol @@isConcatSpreadable (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.5 Symbol.iterator

The initial value of Symbol.iterator is the well-known symbol @@iterator (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.6 Symbol.keyFor ( `sym` )

When Symbol.keyFor is called with argument sym it performs the following steps:

If Type(sym) is not Symbol, throw a TypeError exception.
For each element e of the GlobalSymbolRegistry List (see 19.4.2.2), do
1. If SameValue(e.[[Symbol]], sym) is true, return e.[[Key]].
Assert: GlobalSymbolRegistry does not currently contain an entry for sym.
Return undefined.

19.4.2.7 Symbol.match

The initial value of Symbol.match is the well-known symbol @@match (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.8 Symbol.matchAll

The initial value of Symbol.matchAll is the well-known symbol @@matchAll (Table 3).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.9 Symbol.prototype

The initial value of Symbol.prototype is %Symbol.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.10 Symbol.replace

The initial value of Symbol.replace is the well-known symbol @@replace (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.11 Symbol.search

The initial value of Symbol.search is the well-known symbol @@search (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.12 Symbol.species

The initial value of Symbol.species is the well-known symbol @@species (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.13 Symbol.split

The initial value of Symbol.split is the well-known symbol @@split (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.14 Symbol.toPrimitive

The initial value of Symbol.toPrimitive is the well-known symbol @@toPrimitive (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.15 Symbol.toStringTag

The initial value of Symbol.toStringTag is the well-known symbol @@toStringTag (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.16 Symbol.unscopables

The initial value of Symbol.unscopables is the well-known symbol @@unscopables (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.3 Properties of the Symbol Prototype Object

The Symbol prototype object:

is the intrinsic object %SymbolPrototype%.
is an ordinary object.
is not a Symbol instance and does not have a [[SymbolData]] internal slot.
has a [[Prototype]] internal slot whose value is %Object.prototype%.

The abstract operation thisSymbolValue takes argument value. It performs the following steps when called:

If Type(value) is Symbol, return value.
If Type(value) is Object and value has a [[SymbolData]] internal slot, then
1. Let s be value.[[SymbolData]].
2. Assert: Type(s) is Symbol.
3. Return s.
Throw a TypeError exception.

19.4.3.1 Symbol.prototype.constructor

The initial value of Symbol.prototype.constructor is %Symbol%.

19.4.3.2 get Symbol.prototype.description

Symbol.prototype.description is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let s be the this value.
Let sym be ? thisSymbolValue(s).
Return sym.[[Description]].

19.4.3.3 Symbol.prototype.toString ( )

The following steps are taken:

Let sym be ? thisSymbolValue(this value).
Return SymbolDescriptiveString(sym).

19.4.3.3.1 Runtime Semantics: SymbolDescriptiveString ( `sym` )

The abstract operation SymbolDescriptiveString takes argument sym. It performs the following steps when called:

Assert: Type(sym) is Symbol.
Let desc be sym's [[Description]] value.
If desc is undefined, set desc to the empty String.
Assert: Type(desc) is String.
Return the string-concatenation of "Symbol(", desc, and ")".

19.4.3.4 Symbol.prototype.valueOf ( )

The following steps are taken:

Return ? thisSymbolValue(this value).

19.4.3.5 Symbol.prototype [ @@toPrimitive ] ( `hint` )

This function is called by ECMAScript language operators to convert a Symbol object to a primitive value. The allowed values for hint are "default", "number", and "string".

When the @@toPrimitive method is called with argument hint, the following steps are taken:

Return ? thisSymbolValue(this value).

The value of the "name" property of this function is "[Symbol.toPrimitive]".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

19.4.3.6 Symbol.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Symbol".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

19.4.4 Properties of Symbol Instances

Symbol instances are ordinary objects that inherit properties from the Symbol prototype object. Symbol instances have a [[SymbolData]] internal slot. The [[SymbolData]] internal slot is the Symbol value represented by this Symbol object.

19.5 Error Objects

Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also serve as base objects for user-defined exception classes.

19.5.1 The Error Constructor

The Error constructor:

is the intrinsic object %Error%.
is the initial value of the "Error" property of the global object.
creates and initializes a new Error object when called as a function rather than as a constructor. Thus the function call Error(…) is equivalent to the object creation expression new Error(…) with the same arguments.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Error behaviour must include a super call to the Error constructor to create and initialize subclass instances with an [[ErrorData]] internal slot.

19.5.1.1 Error ( `message` )

When the Error function is called with argument message, the following steps are taken:

If NewTarget is undefined, let newTarget be the active function object; else let newTarget be NewTarget.
Let O be ? OrdinaryCreateFromConstructor(newTarget, "%Error.prototype%", « [[ErrorData]] »).
If message is not undefined, then
1. Let msg be ? ToString(message).
2. Let msgDesc be the PropertyDescriptor { [[Value]]: msg, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }.
3. Perform ! DefinePropertyOrThrow(O, "message", msgDesc).
Return O.

19.5.2 Properties of the Error Constructor

The Error constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

19.5.2.1 Error.prototype

The initial value of Error.prototype is %Error.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.5.3 Properties of the Error Prototype Object

The Error prototype object:

is the intrinsic object %ErrorPrototype%.
is an ordinary object.
is not an Error instance and does not have an [[ErrorData]] internal slot.
has a [[Prototype]] internal slot whose value is %Object.prototype%.

19.5.3.1 Error.prototype.constructor

The initial value of Error.prototype.constructor is %Error%.

19.5.3.2 Error.prototype.message

The initial value of Error.prototype.message is the empty String.

19.5.3.3 Error.prototype.name

The initial value of Error.prototype.name is "Error".

19.5.3.4 Error.prototype.toString ( )

The following steps are taken:

Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
Let name be ? Get(O, "name").
If name is undefined, set name to "Error"; otherwise set name to ? ToString(name).
Let msg be ? Get(O, "message").
If msg is undefined, set msg to the empty String; otherwise set msg to ? ToString(msg).
If name is the empty String, return msg.
If msg is the empty String, return name.
Return the string-concatenation of name, the code unit 0x003A (COLON), the code unit 0x0020 (SPACE), and msg.

19.5.4 Properties of Error Instances

Error instances are ordinary objects that inherit properties from the Error prototype object and have an [[ErrorData]] internal slot whose value is undefined. The only specified uses of [[ErrorData]] is to identify Error and NativeError instances as Error objects within Object.prototype.toString.

19.5.5 Native Error Types Used in This Standard

A new instance of one of the NativeError objects below is thrown when a runtime error is detected. All of these objects share the same structure, as described in 19.5.6.

19.5.5.1 EvalError

This exception is not currently used within this specification. This object remains for compatibility with previous editions of this specification.

19.5.5.2 RangeError

Indicates a value that is not in the set or range of allowable values.

19.5.5.3 ReferenceError

Indicate that an invalid reference value has been detected.

19.5.5.4 SyntaxError

Indicates that a parsing error has occurred.

19.5.5.5 TypeError

TypeError is used to indicate an unsuccessful operation when none of the other NativeError objects are an appropriate indication of the failure cause.

19.5.5.6 URIError

Indicates that one of the global URI handling functions was used in a way that is incompatible with its definition.

19.5.6 `NativeError` Object Structure

When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the NativeError objects defined in 19.5.5. Each of these objects has the structure described below, differing only in the name used as the constructor name instead of NativeError, in the "name" property of the prototype object, and in the implementation-defined "message" property of the prototype object.

For each error object, references to NativeError in the definition should be replaced with the appropriate error object name from 19.5.5.

19.5.6.1 The `NativeError` Constructors

Each NativeError constructor:

creates and initializes a new NativeError object when called as a function rather than as a constructor. A call of the object as a function is equivalent to calling it as a constructor with the same arguments. Thus the function call NativeError(…) is equivalent to the object creation expression new NativeError(…) with the same arguments.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified NativeError behaviour must include a super call to the NativeError constructor to create and initialize subclass instances with an [[ErrorData]] internal slot.

19.5.6.1.1 `NativeError` ( `message` )

When a NativeError function is called with argument message, the following steps are taken:

If NewTarget is undefined, let newTarget be the active function object; else let newTarget be NewTarget.
Let O be ? OrdinaryCreateFromConstructor(newTarget, "%NativeError.prototype%", « [[ErrorData]] »).
If message is not undefined, then
1. Let msg be ? ToString(message).
2. Let msgDesc be the PropertyDescriptor { [[Value]]: msg, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }.
3. Perform ! DefinePropertyOrThrow(O, "message", msgDesc).
Return O.

The actual value of the string passed in step 2 is either "%EvalError.prototype%", "%RangeError.prototype%", "%ReferenceError.prototype%", "%SyntaxError.prototype%", "%TypeError.prototype%", or "%URIError.prototype%" corresponding to which NativeError constructor is being defined.

19.5.6.2 Properties of the `NativeError` Constructors

Each NativeError constructor:

has a [[Prototype]] internal slot whose value is %Error%.
has a "name" property whose value is the String value "NativeError".
has the following properties:

19.5.6.2.1 `NativeError`.prototype

The initial value of NativeError.prototype is a NativeError prototype object (19.5.6.3). Each NativeError constructor has a distinct prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.5.6.3 Properties of the `NativeError` Prototype Objects

Each NativeError prototype object:

is an ordinary object.
is not an Error instance and does not have an [[ErrorData]] internal slot.
has a [[Prototype]] internal slot whose value is %Error.prototype%.

19.5.6.3.1 `NativeError`.prototype.constructor

The initial value of the "constructor" property of the prototype for a given NativeError constructor is the corresponding intrinsic object %NativeError% (19.5.6.1).

19.5.6.3.2 `NativeError`.prototype.message

The initial value of the "message" property of the prototype for a given NativeError constructor is the empty String.

19.5.6.3.3 `NativeError`.prototype.name

The initial value of the "name" property of the prototype for a given NativeError constructor is the String value consisting of the name of the constructor (the name used instead of NativeError).

19.5.6.4 Properties of `NativeError` Instances

NativeError instances are ordinary objects that inherit properties from their NativeError prototype object and have an [[ErrorData]] internal slot whose value is undefined. The only specified use of [[ErrorData]] is by Object.prototype.toString (19.1.3.6) to identify Error or NativeError instances.

20 Numbers and Dates

20.1 Number Objects

20.1.1 The Number Constructor

The Number constructor:

is the intrinsic object %Number%.
is the initial value of the "Number" property of the global object.
creates and initializes a new Number object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Number behaviour must include a super call to the Number constructor to create and initialize the subclass instance with a [[NumberData]] internal slot.

20.1.1.1 Number ( `value` )

When Number is called with argument value, the following steps are taken:

If value is present, then
1. Let prim be ? ToNumeric(value).
2. If Type(prim) is BigInt, let n be the Number value for the mathematical value of prim.
3. Otherwise, let n be prim.
Else,
1. Let n be +0.
If NewTarget is undefined, return n.
Let O be ? OrdinaryCreateFromConstructor(NewTarget, "%Number.prototype%", « [[NumberData]] »).
Set O.[[NumberData]] to n.
Return O.

20.1.2 Properties of the Number Constructor

The Number constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

20.1.2.1 Number.EPSILON

The value of Number.EPSILON is the difference between 1 and the smallest value greater than 1 that is representable as a Number value, which is approximately 2.2204460492503130808472633361816 x 10^{- 16}.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.2 Number.isFinite ( `number` )

When Number.isFinite is called with one argument number, the following steps are taken:

If Type(number) is not Number, return false.
If number is NaN, +∞, or -∞, return false.
Otherwise, return true.

20.1.2.3 Number.isInteger ( `number` )

When Number.isInteger is called with one argument number, the following steps are taken:

Return ! IsInteger(number).

20.1.2.4 Number.isNaN ( `number` )

When Number.isNaN is called with one argument number, the following steps are taken:

If Type(number) is not Number, return false.
If number is NaN, return true.
Otherwise, return false.

Note

This function differs from the global isNaN function (18.2.3) in that it does not convert its argument to a Number before determining whether it is NaN.

20.1.2.5 Number.isSafeInteger ( `number` )

When Number.isSafeInteger is called with one argument number, the following steps are taken:

If ! IsInteger(number) is true, then
1. If abs(number) ≤ 2⁵³ - 1, return true.
Return false.

20.1.2.6 Number.MAX_SAFE_INTEGER

Note

The value of Number.MAX_SAFE_INTEGER is the largest integer n such that n and n + 1 are both exactly representable as a Number value.

The value of Number.MAX_SAFE_INTEGER is 9007199254740991 (2⁵³ - 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.7 Number.MAX_VALUE

The value of Number.MAX_VALUE is the largest positive finite value of the Number type, which is approximately 1.7976931348623157 × 10³⁰⁸.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.8 Number.MIN_SAFE_INTEGER

Note

The value of Number.MIN_SAFE_INTEGER is the smallest integer n such that n and n - 1 are both exactly representable as a Number value.

The value of Number.MIN_SAFE_INTEGER is -9007199254740991 (-(2⁵³ - 1)).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.9 Number.MIN_VALUE

The value of Number.MIN_VALUE is the smallest positive value of the Number type, which is approximately 5 × 10^-324.

In the IEEE 754-2019 double precision binary representation, the smallest possible value is a denormalized number. If an implementation does not support denormalized values, the value of Number.MIN_VALUE must be the smallest non-zero positive value that can actually be represented by the implementation.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.10 Number.NaN

The value of Number.NaN is NaN.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.11 Number.NEGATIVE_INFINITY

The value of Number.NEGATIVE_INFINITY is -∞.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.12 Number.parseFloat ( `string` )

The value of the Number.parseFloat data property is the same built-in function object that is the value of the "parseFloat" property of the global object defined in 18.2.4.

20.1.2.13 Number.parseInt ( `string`, `radix` )

The value of the Number.parseInt data property is the same built-in function object that is the value of the "parseInt" property of the global object defined in 18.2.5.

20.1.2.14 Number.POSITIVE_INFINITY

The value of Number.POSITIVE_INFINITY is +∞.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.15 Number.prototype

The initial value of Number.prototype is %Number.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.3 Properties of the Number Prototype Object

The Number prototype object:

is the intrinsic object %NumberPrototype%.
is an ordinary object.
is itself a Number object; it has a [[NumberData]] internal slot with the value +0.
has a [[Prototype]] internal slot whose value is %Object.prototype%.

Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not generic and the this value passed to them must be either a Number value or an object that has a [[NumberData]] internal slot that has been initialized to a Number value.

The abstract operation thisNumberValue takes argument value. It performs the following steps when called:

If Type(value) is Number, return value.
If Type(value) is Object and value has a [[NumberData]] internal slot, then
1. Let n be value.[[NumberData]].
2. Assert: Type(n) is Number.
3. Return n.
Throw a TypeError exception.

The phrase “this Number value” within the specification of a method refers to the result returned by calling the abstract operation thisNumberValue with the this value of the method invocation passed as the argument.

20.1.3.1 Number.prototype.constructor

The initial value of Number.prototype.constructor is %Number%.

20.1.3.2 Number.prototype.toExponential ( `fractionDigits` )

Return a String containing this Number value represented in decimal exponential notation with one digit before the significand's decimal point and fractionDigits digits after the significand's decimal point. If fractionDigits is undefined, include as many significand digits as necessary to uniquely specify the Number (just like in ToString except that in this case the Number is always output in exponential notation). Specifically, perform the following steps:

Let x be ? thisNumberValue(this value).
Let f be ? ToInteger(fractionDigits).
Assert: If fractionDigits is undefined, then f is 0.
If x is NaN, return the String "NaN".
Let s be the empty String.
If x < 0, then
1. Set s to "-".
2. Set x to -x.
If x = +∞, then
1. Return the string-concatenation of s and "Infinity".
If f < 0 or f > 100, throw a RangeError exception.
If x = 0, then
1. Let m be the String value consisting of f + 1 occurrences of the code unit 0x0030 (DIGIT ZERO).
2. Let e be 0.
Else,
1. If fractionDigits is not undefined, then
  1. Let e and n be integers such that 10^f ≤ n < 10^{f + 1} and for which ℝ(n) × 10_ℝ^{ℝ(e) - ℝ(n)} - ℝ(x) is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which ℝ(n) × 10_ℝ^{ℝ(e) - ℝ(f)} is larger.
2. Else,
  1. Let e, n, and f be integers such that f ≥ 0, 10^f ≤ n < 10^{f + 1}, the Number value for ℝ(n) × 10_ℝ^{ℝ(e) - ℝ(f)} is x, and f is as small as possible. Note that the decimal representation of n has f + 1_ℝ digits, n is not divisible by 10, and the least significant digit of n is not necessarily uniquely determined by these criteria.
3. Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
If f ≠ 0, then
1. Let a be the first code unit of m, and let b be the remaining f code units of m.
2. Set m to the string-concatenation of a, ".", and b.
If e = 0, then
1. Let c be "+".
2. Let d be "0".
Else,
1. If e > 0, let c be "+".
2. Else,
  1. Assert: e < 0.
  2. Let c be "-".
  3. Set e to -e.
3. Let d be the String value consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
Set m to the string-concatenation of m, "e", c, and d.
Return the string-concatenation of s and m.

Note

For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 10.b.i be used as a guideline:

Let e, n, and f be integers such that f ≥ 0, 10^f ≤ n < 10^{f + 1}, the Number value for ℝ(n) × 10_ℝ^{ℝ(e) - ℝ(f)} is x, and f is as small as possible. If there are multiple possibilities for n, choose the value of n for which ℝ(n) × 10_ℝ^{ℝ(e) - ℝ(f)} is closest in value to x. If there are two such possible values of n, choose the one that is even.

20.1.3.3 Number.prototype.toFixed ( `fractionDigits` )

Note 1

toFixed returns a String containing this Number value represented in decimal fixed-point notation with fractionDigits digits after the decimal point. If fractionDigits is undefined, 0 is assumed.

The following steps are performed:

Let x be ? thisNumberValue(this value).
Let f be ? ToInteger(fractionDigits).
Assert: If fractionDigits is undefined, then f is 0.
If f < 0 or f > 100, throw a RangeError exception.
If x is NaN, return the String "NaN".
Let s be the empty String.
If x < 0, then
1. Set s to "-".
2. Set x to -x.
If x ≥ 10²¹, then
1. Let m be ! ToString(x).
Else,
1. Let n be an integer for which ℝ(n) ÷ 10_ℝ^ℝ(f) - ℝ(x) is as close to zero as possible. If there are two such n, pick the larger n.
2. If n = 0, let m be the String "0". Otherwise, let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
3. If f ≠ 0, then
  1. Let k be the length of m.
  2. If k ≤ f, then
    1. Let z be the String value consisting of f + 1 - k occurrences of the code unit 0x0030 (DIGIT ZERO).
    2. Set m to the string-concatenation of z and m.
    3. Set k to f + 1.
  3. Let a be the first k - f code units of m, and let b be the remaining f code units of m.
  4. Set m to the string-concatenation of a, ".", and b.
Return the string-concatenation of s and m.

Note 2

The output of toFixed may be more precise than toString for some values because toString only prints enough significant digits to distinguish the number from adjacent number values. For example,

(1000000000000000128).toString() returns "1000000000000000100", while
(1000000000000000128).toFixed(0) returns "1000000000000000128".

20.1.3.4 Number.prototype.toLocaleString ( [ `reserved1` [ , `reserved2` ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Number.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.

Produces a String value that represents this Number value formatted according to the conventions of the host environment's current locale. This function is implementation-dependent, and it is permissible, but not encouraged, for it to return the same thing as toString.

The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

20.1.3.5 Number.prototype.toPrecision ( `precision` )

Return a String containing this Number value represented either in decimal exponential notation with one digit before the significand's decimal point and precision - 1 digits after the significand's decimal point or in decimal fixed notation with precision significant digits. If precision is undefined, call ToString instead. Specifically, perform the following steps:

Let x be ? thisNumberValue(this value).
If precision is undefined, return ! ToString(x).
Let p be ? ToInteger(precision).
If x is NaN, return the String "NaN".
Let s be the empty String.
If x < 0, then
1. Set s to the code unit 0x002D (HYPHEN-MINUS).
2. Set x to -x.
If x = +∞, then
1. Return the string-concatenation of s and "Infinity".
If p < 1 or p > 100, throw a RangeError exception.
If x = 0, then
1. Let m be the String value consisting of p occurrences of the code unit 0x0030 (DIGIT ZERO).
2. Let e be 0.
Else,
1. Let e and n be integers such that 10^{p - 1} ≤ n < 10^p and for which ℝ(n) × 10_ℝ^{ℝ(e) - ℝ(p) + 1_ℝ} - ℝ(x) is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which ℝ(n) × 10_ℝ^{ℝ(e) - ℝ(p) + 1_ℝ} is larger.
2. Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
3. If e < -6 or e ≥ p, then
  1. Assert: e ≠ 0.
  2. If p ≠ 1, then
    1. Let a be the first code unit of m, and let b be the remaining p - 1 code units of m.
    2. Set m to the string-concatenation of a, ".", and b.
  3. If e > 0, then
    1. Let c be the code unit 0x002B (PLUS SIGN).
  4. Else,
    1. Assert: e < 0.
    2. Let c be the code unit 0x002D (HYPHEN-MINUS).
    3. Set e to -e.
  5. Let d be the String value consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
  6. Return the string-concatenation of s, m, the code unit 0x0065 (LATIN SMALL LETTER E), c, and d.
If e = p - 1, return the string-concatenation of s and m.
If e ≥ 0, then
1. Set m to the string-concatenation of the first e + 1 code units of m, the code unit 0x002E (FULL STOP), and the remaining p - (e + 1) code units of m.
Else,
1. Set m to the string-concatenation of the code unit 0x0030 (DIGIT ZERO), the code unit 0x002E (FULL STOP), -(e + 1) occurrences of the code unit 0x0030 (DIGIT ZERO), and the String m.
Return the string-concatenation of s and m.

20.1.3.6 Number.prototype.toString ( [ `radix` ] )

Note

The optional radix should be an integer value in the inclusive range 2 to 36. If radix is undefined the Number 10 is used as the value of radix.

The following steps are performed:

Let x be ? thisNumberValue(this value).
If radix is undefined, let radixNumber be 10.
Else, let radixNumber be ? ToInteger(radix).
If radixNumber < 2 or radixNumber > 36, throw a RangeError exception.
If radixNumber = 10, return ! ToString(x).
Return the String representation of this Number value using the radix specified by radixNumber. Letters a-z are used for digits with values 10 through 35. The precise algorithm is implementation-dependent, however the algorithm should be a generalization of that specified in 6.1.6.1.20.

The toString function is not generic; it throws a TypeError exception if its this value is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

The "length" property of the toString method is 1.

20.1.3.7 Number.prototype.valueOf ( )

Return ? thisNumberValue(this value).

20.1.4 Properties of Number Instances

Number instances are ordinary objects that inherit properties from the Number prototype object. Number instances also have a [[NumberData]] internal slot. The [[NumberData]] internal slot is the Number value represented by this Number object.

20.2 BigInt Objects

20.2.1 The BigInt Constructor

The BigInt constructor:

is the intrinsic object %BigInt%.
is the initial value of the "BigInt" property of the global object.
performs a type conversion when called as a function rather than as a constructor.
is not intended to be used with the new operator or to be subclassed. It may be used as the value of an extends clause of a class definition but a super call to the BigInt constructor will cause an exception.

20.2.1.1 BigInt ( `value` )

When BigInt is called with argument value, the following steps are taken:

If NewTarget is not undefined, throw a TypeError exception.
Let prim be ? ToPrimitive(value, hint Number).
If Type(prim) is Number, return ? NumberToBigInt(prim).
Otherwise, return ? ToBigInt(value).

20.2.1.1.1 Runtime Semantics: NumberToBigInt ( `number` )

The abstract operation NumberToBigInt takes argument number. It performs the following steps when called:

Assert: Type(number) is Number.
If IsInteger(number) is false, throw a RangeError exception.
Return the BigInt value that represents the mathematical value of number.

20.2.2 Properties of the BigInt Constructor

The value of the [[Prototype]] internal slot of the BigInt constructor is the intrinsic object %Function.prototype%.

The BigInt constructor has the following properties:

20.2.2.1 BigInt.asIntN ( `bits`, `bigint` )

When the BigInt.asIntN function is called with two arguments bits and bigint, the following steps are taken:

Set bits to ? ToIndex(bits).
Set bigint to ? ToBigInt(bigint).
Let mod be the BigInt value that represents bigint modulo 2^bits.
If mod ≥ 2^{bits - 1}, return mod - 2^bits; otherwise, return mod.

20.2.2.2 BigInt.asUintN ( `bits`, `bigint` )

When the BigInt.asUintN function is called with two arguments bits and bigint, the following steps are taken:

Set bits to ? ToIndex(bits).
Set bigint to ? ToBigInt(bigint).
Return the BigInt value that represents bigint modulo 2^bits.

20.2.2.3 BigInt.prototype

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.2.3 Properties of the BigInt Prototype Object

The BigInt prototype object:

is an ordinary object.
is not a BigInt object; it does not have a [[BigIntData]] internal slot.
has a [[Prototype]] internal slot whose value is the intrinsic object %Object.prototype%.

The abstract operation thisBigIntValue takes argument value. It performs the following steps when called:

If Type(value) is BigInt, return value.
If Type(value) is Object and value has a [[BigIntData]] internal slot, then
1. Assert: Type(value.[[BigIntData]]) is BigInt.
2. Return value.[[BigIntData]].
Throw a TypeError exception.

The phrase “this BigInt value” within the specification of a method refers to the result returned by calling the abstract operation thisBigIntValue with the this value of the method invocation passed as the argument.

20.2.3.1 BigInt.prototype.constructor

The initial value of BigInt.prototype.constructor is the intrinsic object %BigInt%.

20.2.3.2 BigInt.prototype.toLocaleString ( [ `reserved1` [ , `reserved2` ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the BigInt.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.

Produces a String value that represents this BigInt value formatted according to the conventions of the host environment's current locale. This function is implementation-dependent, and it is permissible, but not encouraged, for it to return the same thing as toString.

20.2.3.3 BigInt.prototype.toString ( [ `radix` ] )

Note

The optional radix should be an integer value in the inclusive range 2 to 36. If radix not present or is undefined the Number 10 is used as the value of radix.

The following steps are performed:

Let x be ? thisBigIntValue(this value).
If radix is not present, let radixNumber be 10.
Else if radix is undefined, let radixNumber be 10.
Else, let radixNumber be ? ToInteger(radix).
If radixNumber < 2 or radixNumber > 36, throw a RangeError exception.
If radixNumber = 10, return ! ToString(x).
Return the String representation of this Number value using the radix specified by radixNumber. Letters a-z are used for digits with values 10 through 35. The precise algorithm is implementation-dependent, however the algorithm should be a generalization of that specified in 6.1.6.2.23.

The toString function is not generic; it throws a TypeError exception if its this value is not a BigInt or a BigInt object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

20.2.3.4 BigInt.prototype.valueOf ( )

Return ? thisBigIntValue(this value).

20.2.3.5 BigInt.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "BigInt".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

20.3 The Math Object

The Math object:

is the intrinsic object %Math%.
is the initial value of the "Math" property of the global object.
is an ordinary object.
has a [[Prototype]] internal slot whose value is %Object.prototype%.
is not a function object.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.

Note

In this specification, the phrase “the Number value for x” has a technical meaning defined in 6.1.6.1.

20.3.1 Value Properties of the Math Object

20.3.1.1 Math.E

The Number value for e_ℝ, the base of the natural logarithms, which is approximately 2.7182818284590452354.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.3.1.2 Math.LN10

The Number value for the natural logarithm of 10_ℝ, which is approximately 2.302585092994046.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.3.1.3 Math.LN2

The Number value for the natural logarithm of 2_ℝ, which is approximately 0.6931471805599453.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.3.1.4 Math.LOG10E

The Number value for the base-10 logarithm of e_ℝ, the base of the natural logarithms; this value is approximately 0.4342944819032518.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

Note

The value of Math.LOG10E is approximately the reciprocal of the value of Math.LN10.

20.3.1.5 Math.LOG2E

The Number value for the base-2 logarithm of e_ℝ, the base of the natural logarithms; this value is approximately 1.4426950408889634.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

Note

The value of Math.LOG2E is approximately the reciprocal of the value of Math.LN2.

20.3.1.6 Math.PI

The Number value for π_ℝ, the ratio of the circumference of a circle to its diameter, which is approximately 3.1415926535897932.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.3.1.7 Math.SQRT1_2

The Number value for the square root of ½_ℝ, which is approximately 0.7071067811865476.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

Note

The value of Math.SQRT1_2 is approximately the reciprocal of the value of Math.SQRT2.

20.3.1.8 Math.SQRT2

The Number value for the square root of 2_ℝ, which is approximately 1.4142135623730951.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.3.1.9 Math [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Math".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

20.3.2 Function Properties of the Math Object

Each of the following Math object functions applies the ToNumber abstract operation to each of its arguments (in left-to-right order if there is more than one). If ToNumber returns an abrupt completion, that Completion Record is immediately returned. Otherwise, the function performs a computation on the resulting Number value(s). The value returned by each function is a Number.

In the function descriptions below, the symbols NaN, -0, +0, -∞ and +∞ refer to the Number values described in 6.1.6.1.

Note

The behaviour of the functions acos, acosh, asin, asinh, atan, atanh, atan2, cbrt, cos, cosh, exp, expm1, hypot, log,log1p, log2, log10, pow, random, sin, sinh, sqrt, tan, and tanh is not precisely specified here except to require specific results for certain argument values that represent boundary cases of interest. For other argument values, these functions are intended to compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of approximation algorithms. The general intent is that an implementer should be able to use the same mathematical library for ECMAScript on a given hardware platform that is available to C programmers on that platform.

Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard) that implementations use the approximation algorithms for IEEE 754-2019 arithmetic contained in fdlibm, the freely distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).

20.3.2.1 Math.abs ( `x` )

Returns the absolute value of x; the result has the same magnitude as x but has positive sign.

If x is NaN, the result is NaN.
If x is -0, the result is +0.
If x is -∞, the result is +∞.

20.3.2.2 Math.acos ( `x` )

Returns an implementation-dependent approximation to the arc cosine of x. The result is expressed in radians and ranges from +0 to +π.

If x is NaN, the result is NaN.
If x is greater than 1, the result is NaN.
If x is less than -1, the result is NaN.
If x is exactly 1, the result is +0.

20.3.2.3 Math.acosh ( `x` )

Returns an implementation-dependent approximation to the inverse hyperbolic cosine of x.

If x is NaN, the result is NaN.
If x is less than 1, the result is NaN.
If x is 1, the result is +0.
If x is +∞, the result is +∞.

20.3.2.4 Math.asin ( `x` )

Returns an implementation-dependent approximation to the arc sine of x. The result is expressed in radians and ranges from -π / 2 to +π / 2.

If x is NaN, the result is NaN.
If x is greater than 1, the result is NaN.
If x is less than -1, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.

20.3.2.5 Math.asinh ( `x` )

Returns an implementation-dependent approximation to the inverse hyperbolic sine of x.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.

20.3.2.6 Math.atan ( `x` )

Returns an implementation-dependent approximation to the arc tangent of x. The result is expressed in radians and ranges from -π / 2 to +π / 2.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is an implementation-dependent approximation to +π / 2.
If x is -∞, the result is an implementation-dependent approximation to -π / 2.

20.3.2.7 Math.atanh ( `x` )

Returns an implementation-dependent approximation to the inverse hyperbolic tangent of x.

If x is NaN, the result is NaN.
If x is less than -1, the result is NaN.
If x is greater than 1, the result is NaN.
If x is -1, the result is -∞.
If x is +1, the result is +∞.
If x is +0, the result is +0.
If x is -0, the result is -0.

20.3.2.8 Math.atan2 ( `y`, `x` )

Returns an implementation-dependent approximation to the arc tangent of the quotient y / x of the arguments y and x, where the signs of y and x are used to determine the quadrant of the result. Note that it is intentional and traditional for the two-argument arc tangent function that the argument named y be first and the argument named x be second. The result is expressed in radians and ranges from -π to +π.

If either x or y is NaN, the result is NaN.
If y > 0 and x is +0, the result is an implementation-dependent approximation to +π / 2.
If y > 0 and x is -0, the result is an implementation-dependent approximation to +π / 2.
If y is +0 and x > 0, the result is +0.
If y is +0 and x is +0, the result is +0.
If y is +0 and x is -0, the result is an implementation-dependent approximation to +π.
If y is +0 and x < 0, the result is an implementation-dependent approximation to +π.
If y is -0 and x > 0, the result is -0.
If y is -0 and x is +0, the result is -0.
If y is -0 and x is -0, the result is an implementation-dependent approximation to -π.
If y is -0 and x < 0, the result is an implementation-dependent approximation to -π.
If y < 0 and x is +0, the result is an implementation-dependent approximation to -π / 2.
If y < 0 and x is -0, the result is an implementation-dependent approximation to -π / 2.
If y > 0 and y is finite and x is +∞, the result is +0.
If y > 0 and y is finite and x is -∞, the result is an implementation-dependent approximation to +π.
If y < 0 and y is finite and x is +∞, the result is -0.
If y < 0 and y is finite and x is -∞, the result is an implementation-dependent approximation to -π.
If y is +∞ and x is finite, the result is an implementation-dependent approximation to +π / 2.
If y is -∞ and x is finite, the result is an implementation-dependent approximation to -π / 2.
If y is +∞ and x is +∞, the result is an implementation-dependent approximation to +π / 4.
If y is +∞ and x is -∞, the result is an implementation-dependent approximation to +3π / 4.
If y is -∞ and x is +∞, the result is an implementation-dependent approximation to -π / 4.
If y is -∞ and x is -∞, the result is an implementation-dependent approximation to -3π / 4.

20.3.2.9 Math.cbrt ( `x` )

Returns an implementation-dependent approximation to the cube root of x.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.

20.3.2.10 Math.ceil ( `x` )

Returns the smallest (closest to -∞) Number value that is not less than x and is an integer. If x is already an integer, the result is x.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.
If x is less than 0 but greater than -1, the result is -0.

The value of Math.ceil(x) is the same as the value of -Math.floor(-x).

20.3.2.11 Math.clz32 ( `x` )

When Math.clz32 is called with one argument x, the following steps are taken:

Let n be ? ToUint32(x).
Let p be the number of leading zero bits in the 32-bit binary representation of n.
Return p.

Note

If n is 0, p will be 32. If the most significant bit of the 32-bit binary encoding of n is 1, p will be 0.

20.3.2.12 Math.cos ( `x` )

Returns an implementation-dependent approximation to the cosine of x. The argument is expressed in radians.

If x is NaN, the result is NaN.
If x is +0, the result is 1.
If x is -0, the result is 1.
If x is +∞, the result is NaN.
If x is -∞, the result is NaN.

20.3.2.13 Math.cosh ( `x` )

Returns an implementation-dependent approximation to the hyperbolic cosine of x.

If x is NaN, the result is NaN.
If x is +0, the result is 1.
If x is -0, the result is 1.
If x is +∞, the result is +∞.
If x is -∞, the result is +∞.

Note

The value of Math.cosh(x) is the same as the value of (Math.exp(x) + Math.exp(-x)) / 2.

20.3.2.14 Math.exp ( `x` )

Returns an implementation-dependent approximation to the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms).

If x is NaN, the result is NaN.
If x is +0, the result is 1.
If x is -0, the result is 1.
If x is +∞, the result is +∞.
If x is -∞, the result is +0.

20.3.2.15 Math.expm1 ( `x` )

Returns an implementation-dependent approximation to subtracting 1 from the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms). The result is computed in a way that is accurate even when the value of x is close 0.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -1.

20.3.2.16 Math.floor ( `x` )

Returns the greatest (closest to +∞) Number value that is not greater than x and is an integer. If x is already an integer, the result is x.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.
If x is greater than 0 but less than 1, the result is +0.

Note

The value of Math.floor(x) is the same as the value of -Math.ceil(-x).

20.3.2.17 Math.fround ( `x` )

When Math.fround is called with argument x, the following steps are taken:

If x is NaN, return NaN.
If x is one of +0, -0, +∞, -∞, return x.
Let x32 be the result of converting x to a value in IEEE 754-2019 binary32 format using roundTiesToEven mode.
Let x64 be the result of converting x32 to a value in IEEE 754-2019 binary64 format.
Return the ECMAScript Number value corresponding to x64.

20.3.2.18 Math.hypot ( `value1`, `value2`, ...`values` )

Math.hypot returns an implementation-dependent approximation of the square root of the sum of squares of its arguments.

If no arguments are passed, the result is +0.
If any argument is +∞, the result is +∞.
If any argument is -∞, the result is +∞.
If no argument is +∞ or -∞, and any argument is NaN, the result is NaN.
If all arguments are either +0 or -0, the result is +0.

Note

Implementations should take care to avoid the loss of precision from overflows and underflows that are prone to occur in naive implementations when this function is called with two or more arguments.

20.3.2.19 Math.imul ( `x`, `y` )

When Math.imul is called with arguments x and y, the following steps are taken:

Let a be ? ToUint32(x).
Let b be ? ToUint32(y).
Let product be (a × b) modulo 2³².
If product ≥ 2³¹, return product - 2³²; otherwise return product.

20.3.2.20 Math.log ( `x` )

Returns an implementation-dependent approximation to the natural logarithm of x.

If x is NaN, the result is NaN.
If x is less than 0, the result is NaN.
If x is +0 or -0, the result is -∞.
If x is 1, the result is +0.
If x is +∞, the result is +∞.

20.3.2.21 Math.log1p ( `x` )

Returns an implementation-dependent approximation to the natural logarithm of 1 + x. The result is computed in a way that is accurate even when the value of x is close to zero.

If x is NaN, the result is NaN.
If x is less than -1, the result is NaN.
If x is -1, the result is -∞.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.

20.3.2.22 Math.log10 ( `x` )

Returns an implementation-dependent approximation to the base 10 logarithm of x.

If x is NaN, the result is NaN.
If x is less than 0, the result is NaN.
If x is +0, the result is -∞.
If x is -0, the result is -∞.
If x is 1, the result is +0.
If x is +∞, the result is +∞.

20.3.2.23 Math.log2 ( `x` )

Returns an implementation-dependent approximation to the base 2 logarithm of x.

If x is NaN, the result is NaN.
If x is less than 0, the result is NaN.
If x is +0, the result is -∞.
If x is -0, the result is -∞.
If x is 1, the result is +0.
If x is +∞, the result is +∞.

20.3.2.24 Math.max ( `value1`, `value2`, ...`values` )

Given zero or more arguments, calls ToNumber on each of the arguments and returns the largest of the resulting values.

If no arguments are given, the result is -∞.
If any value is NaN, the result is NaN.
The comparison of values to determine the largest value is done using the Abstract Relational Comparison algorithm except that +0 is considered to be larger than -0.

20.3.2.25 Math.min ( `value1`, `value2`, ...`values` )

Given zero or more arguments, calls ToNumber on each of the arguments and returns the smallest of the resulting values.

If no arguments are given, the result is +∞.
If any value is NaN, the result is NaN.
The comparison of values to determine the smallest value is done using the Abstract Relational Comparison algorithm except that +0 is considered to be larger than -0.

20.3.2.26 Math.pow ( `base`, `exponent` )

Set base to ? ToNumber(base).
Set exponent to ? ToNumber(exponent).
Return ! Number::exponentiate(base, exponent).

20.3.2.27 Math.random ( )

Returns a Number value with positive sign, greater than or equal to 0 but less than 1, chosen randomly or pseudo randomly with approximately uniform distribution over that range, using an implementation-dependent algorithm or strategy. This function takes no arguments.

Each Math.random function created for distinct realms must produce a distinct sequence of values from successive calls.

20.3.2.28 Math.round ( `x` )

Returns the Number value that is closest to x and is an integer. If two integers are equally close to x, then the result is the Number value that is closer to +∞. If x is already an integer, the result is x.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.
If x is greater than 0 but less than 0.5, the result is +0.
If x is less than 0 but greater than or equal to -0.5, the result is -0.

Note 1

Math.round(3.5) returns 4, but Math.round(-3.5) returns -3.

Note 2

The value of Math.round(x) is not always the same as the value of Math.floor(x + 0.5). When x is -0 or is less than 0 but greater than or equal to -0.5, Math.round(x) returns -0, but Math.floor(x + 0.5) returns +0. Math.round(x) may also differ from the value of Math.floor(x + 0.5)because of internal rounding when computing x + 0.5.

20.3.2.29 Math.sign ( `x` )

Returns the sign of x, indicating whether x is positive, negative, or zero.

If x is NaN, the result is NaN.
If x is -0, the result is -0.
If x is +0, the result is +0.
If x is negative and not -0, the result is -1.
If x is positive and not +0, the result is +1.

20.3.2.30 Math.sin ( `x` )

Returns an implementation-dependent approximation to the sine of x. The argument is expressed in radians.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞ or -∞, the result is NaN.

20.3.2.31 Math.sinh ( `x` )

Returns an implementation-dependent approximation to the hyperbolic sine of x.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.

Note

The value of Math.sinh(x) is the same as the value of (Math.exp(x) - Math.exp(-x)) / 2.

20.3.2.32 Math.sqrt ( `x` )

Returns an implementation-dependent approximation to the square root of x.

If x is NaN, the result is NaN.
If x is less than 0, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.

20.3.2.33 Math.tan ( `x` )

Returns an implementation-dependent approximation to the tangent of x. The argument is expressed in radians.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞ or -∞, the result is NaN.

20.3.2.34 Math.tanh ( `x` )

Returns an implementation-dependent approximation to the hyperbolic tangent of x.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +1.
If x is -∞, the result is -1.

Note

The value of Math.tanh(x) is the same as the value of (Math.exp(x) - Math.exp(-x)) / (Math.exp(x) + Math.exp(-x)).

20.3.2.35 Math.trunc ( `x` )

Returns the integral part of the number x, removing any fractional digits. If x is already an integer, the result is x.

If x is NaN, the result is NaN.
If x is -0, the result is -0.
If x is +0, the result is +0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.
If x is greater than 0 but less than 1, the result is +0.
If x is less than 0 but greater than -1, the result is -0.

20.4 Date Objects

20.4.1 Overview of Date Objects and Definitions of Abstract Operations

The following functions are abstract operations that operate on time values (defined in 20.4.1.1). Note that, in every case, if any argument to one of these functions is NaN, the result will be NaN.

20.4.1.1 Time Values and Time Range

Time measurement in ECMAScript is analogous to time measurement in POSIX, in particular sharing definition in terms of the proleptic Gregorian calendar, an epoch of midnight at the beginning of 01 January, 1970 UTC, and an accounting of every day as comprising exactly 86,400 seconds (each of which is 1000 milliseconds long).

An ECMAScript time value is a Number, either a finite integer representing an instant in time to millisecond precision or NaN representing no specific instant. A time value that is a multiple of 24 × 60 × 60 × 1000 = 86,400,000 (i.e., is equal to 86,400,000 × d for some integer d) represents the instant at the start of the UTC day that follows the epoch by d whole UTC days (preceding the epoch for negative d). Every other finite time value t is defined relative to the greatest preceding time value s that is such a multiple, and represents the instant that occurs within the same UTC day as s but follows it by t − s milliseconds.

Time values do not account for UTC leap seconds—there are no time values representing instants within positive leap seconds, and there are time values representing instants removed from the UTC timeline by negative leap seconds. However, the definition of time values nonetheless yields piecewise alignment with UTC, with discontinuities only at leap second boundaries and zero difference outside of leap seconds.

A Number can exactly represent all integers from -9,007,199,254,740,992 to 9,007,199,254,740,992 (20.1.2.8 and 20.1.2.6). A time value supports a slightly smaller range of -8,640,000,000,000,000 to 8,640,000,000,000,000 milliseconds. This yields a supported time value range of exactly -100,000,000 days to 100,000,000 days relative to midnight at the beginning of 01 January, 1970 UTC.

The exact moment of midnight at the beginning of 01 January, 1970 UTC is represented by the time value +0.

Note

The 400 year cycle of the proleptic Gregorian calendar contains 97 leap years. This yields an average of 365.2425 days per year, which is 31,556,952,000 milliseconds. Therefore, the maximum range a Number could represent exactly with millisecond precision is approximately -285,426 to 285,426 years relative to 1970. The smaller range supported by a time value as specified in this section is approximately -273,790 to 273,790 years relative to 1970.

20.4.1.2 Day Number and Time within Day

A given time value t belongs to day number

Day(t) = floor(t / msPerDay)

where the number of milliseconds per day is

msPerDay = 86400000

The remainder is called the time within the day:

TimeWithinDay(t) = t

modulo

msPerDay

20.4.1.3 Year Number

ECMAScript uses a proleptic Gregorian calendar to map a day number to a year number and to determine the month and date within that year. In this calendar, leap years are precisely those which are (divisible by 4) and ((not divisible by 100) or (divisible by 400)). The number of days in year number y is therefore defined by

DaysInYear(y)

= 365 if (y modulo 4) ≠ 0

= 366 if (y modulo 4) = 0 and (y modulo 100) ≠ 0

= 365 if (y modulo 100) = 0 and (y modulo 400) ≠ 0

= 366 if (y modulo 400) = 0

All non-leap years have 365 days with the usual number of days per month and leap years have an extra day in February. The day number of the first day of year y is given by:

DayFromYear(y) = 365 × (y - 1970) + floor((y - 1969) / 4) - floor((y - 1901) / 100) + floor((y - 1601) / 400)

The time value of the start of a year is:

TimeFromYear(y) = msPerDay × DayFromYear(y)

A time value determines a year by:

YearFromTime(t) = the largest integer y (closest to positive infinity) such that TimeFromYear(y) ≤ t

The leap-year function is 1 for a time within a leap year and otherwise is zero:

InLeapYear(t)

= 0 if DaysInYear(YearFromTime(t)) = 365

= 1 if DaysInYear(YearFromTime(t)) = 366

20.4.1.4 Month Number

Months are identified by an integer in the range 0 to 11, inclusive. The mapping MonthFromTime(t) from a time value t to a month number is defined by:

MonthFromTime(t)

= 0 if 0 ≤ DayWithinYear(t) < 31

= 1 if 31 ≤ DayWithinYear(t) < 59 + InLeapYear(t)

= 2 if 59 + InLeapYear(t) ≤ DayWithinYear(t) < 90 + InLeapYear(t)

= 3 if 90 + InLeapYear(t) ≤ DayWithinYear(t) < 120 + InLeapYear(t)

= 4 if 120 + InLeapYear(t) ≤ DayWithinYear(t) < 151 + InLeapYear(t)

= 5 if 151 + InLeapYear(t) ≤ DayWithinYear(t) < 181 + InLeapYear(t)

= 6 if 181 + InLeapYear(t) ≤ DayWithinYear(t) < 212 + InLeapYear(t)

= 7 if 212 + InLeapYear(t) ≤ DayWithinYear(t) < 243 + InLeapYear(t)

= 8 if 243 + InLeapYear(t) ≤ DayWithinYear(t) < 273 + InLeapYear(t)

= 9 if 273 + InLeapYear(t) ≤ DayWithinYear(t) < 304 + InLeapYear(t)

= 10 if 304 + InLeapYear(t) ≤ DayWithinYear(t) < 334 + InLeapYear(t)

= 11 if 334 + InLeapYear(t) ≤ DayWithinYear(t) < 365 + InLeapYear(t)

where

DayWithinYear(t) = Day(t) - DayFromYear(YearFromTime(t))

A month value of 0 specifies January; 1 specifies February; 2 specifies March; 3 specifies April; 4 specifies May; 5 specifies June; 6 specifies July; 7 specifies August; 8 specifies September; 9 specifies October; 10 specifies November; and 11 specifies December. Note that MonthFromTime(0) = 0, corresponding to Thursday, 01 January, 1970.

20.4.1.5 Date Number

A date number is identified by an integer in the range 1 through 31, inclusive. The mapping DateFromTime(t) from a time value t to a date number is defined by:

DateFromTime(t)

= DayWithinYear(t) + 1 if MonthFromTime(t) = 0

= DayWithinYear(t) - 30 if MonthFromTime(t) = 1

= DayWithinYear(t) - 58 - InLeapYear(t) if MonthFromTime(t) = 2

= DayWithinYear(t) - 89 - InLeapYear(t) if MonthFromTime(t) = 3

= DayWithinYear(t) - 119 - InLeapYear(t) if MonthFromTime(t) = 4

= DayWithinYear(t) - 150 - InLeapYear(t) if MonthFromTime(t) = 5

= DayWithinYear(t) - 180 - InLeapYear(t) if MonthFromTime(t) = 6

= DayWithinYear(t) - 211 - InLeapYear(t) if MonthFromTime(t) = 7

= DayWithinYear(t) - 242 - InLeapYear(t) if MonthFromTime(t) = 8

= DayWithinYear(t) - 272 - InLeapYear(t) if MonthFromTime(t) = 9

= DayWithinYear(t) - 303 - InLeapYear(t) if MonthFromTime(t) = 10

= DayWithinYear(t) - 333 - InLeapYear(t) if MonthFromTime(t) = 11

20.4.1.6 Week Day

The weekday for a particular time value t is defined as

WeekDay(t) = (Day(t) + 4) modulo 7

A weekday value of 0 specifies Sunday; 1 specifies Monday; 2 specifies Tuesday; 3 specifies Wednesday; 4 specifies Thursday; 5 specifies Friday; and 6 specifies Saturday. Note that WeekDay(0) = 4, corresponding to Thursday, 01 January, 1970.

20.4.1.7 LocalTZA ( `t`, `isUTC` )

LocalTZA( t, isUTC ) is an implementation-defined algorithm that returns the local time zone adjustment, or offset, in milliseconds. The local political rules for standard time and daylight saving time in effect at t should be used to determine the result in the way specified in this section.

When isUTC is true, LocalTZA( t_UTC, true ) should return the offset of the local time zone from UTC measured in milliseconds at time represented by time value t_UTC. When the result is added to t_UTC, it should yield the corresponding Number t_local.

When isUTC is false, LocalTZA( t_local, false ) should return the offset of the local time zone from UTC measured in milliseconds at local time represented by Number t_local. When the result is subtracted from t_local, it should yield the corresponding time value t_UTC.

Input t is nominally a time value but may be any Number value. This can occur when isUTC is false and t_local represents a time value that is already offset outside of the time value range at the range boundaries. The algorithm must not limit t_local to the time value range, so that such inputs are supported.

When t_local represents local time repeating multiple times at a negative time zone transition (e.g. when the daylight saving time ends or the time zone offset is decreased due to a time zone rule change) or skipped local time at a positive time zone transitions (e.g. when the daylight saving time starts or the time zone offset is increased due to a time zone rule change), t_local must be interpreted using the time zone offset before the transition.

If an implementation does not support a conversion described above or if political rules for time t are not available within the implementation, the result must be 0.

Note

It is recommended that implementations use the time zone information of the IANA Time Zone Database https://www.iana.org/time-zones/.

1:30 AM on November 5, 2017 in America/New_York is repeated twice (fall backward), but it must be interpreted as 1:30 AM UTC-04 instead of 1:30 AM UTC-05. LocalTZA(TimeClip(MakeDate(MakeDay(2017, 10, 5), MakeTime(1, 30, 0, 0))), false) is -4 × msPerHour.

2:30 AM on March 12, 2017 in America/New_York does not exist, but it must be interpreted as 2:30 AM UTC-05 (equivalent to 3:30 AM UTC-04). LocalTZA(TimeClip(MakeDate(MakeDay(2017, 2, 12), MakeTime(2, 30, 0, 0))), false) is -5 × msPerHour.

Local time zone offset values may be positive or negative.

20.4.1.8 LocalTime ( `t` )

The abstract operation LocalTime takes argument t. It converts t from UTC to local time. It performs the following steps when called:

Return t + LocalTZA(t, true).

Note

Two different input time values t_UTC are converted to the same local time t_local at a negative time zone transition when there are repeated times (e.g. the daylight saving time ends or the time zone adjustment is decreased.).

LocalTime(UTC(t_local)) is not necessarily always equal to t_local. Correspondingly, UTC(LocalTime(t_UTC)) is not necessarily always equal to t_UTC.

20.4.1.9 UTC ( `t` )

The abstract operation UTC takes argument t. It converts t from local time to UTC. It performs the following steps when called:

Return t - LocalTZA(t, false).

Note

UTC(LocalTime(t_UTC)) is not necessarily always equal to t_UTC. Correspondingly, LocalTime(UTC(t_local)) is not necessarily always equal to t_local.

20.4.1.10 Hours, Minutes, Second, and Milliseconds

The following abstract operations are useful in decomposing time values:

HourFromTime(t) = floor(t / msPerHour) modulo

HoursPerDay

MinFromTime(t) = floor(t / msPerMinute) modulo

MinutesPerHour

SecFromTime(t) = floor(t / msPerSecond) modulo

SecondsPerMinute

msFromTime(t) = t

modulo

msPerSecond

where

HoursPerDay = 24

MinutesPerHour = 60

SecondsPerMinute = 60

msPerSecond = 1000

msPerMinute = 60000 = msPerSecond × SecondsPerMinute

msPerHour = 3600000 = msPerMinute × MinutesPerHour

20.4.1.11 MakeTime ( `hour`, `min`, `sec`, `ms` )

The abstract operation MakeTime takes arguments hour (a Number), min (a Number), sec (a Number), and ms (a Number). It calculates a number of milliseconds. It performs the following steps when called:

If hour is not finite or min is not finite or sec is not finite or ms is not finite, return NaN.
Let h be ! ToInteger(hour).
Let m be ! ToInteger(min).
Let s be ! ToInteger(sec).
Let milli be ! ToInteger(ms).
Let t be h * msPerHour + m * msPerMinute + s * msPerSecond + milli, performing the arithmetic according to IEEE 754-2019 rules (that is, as if using the ECMAScript operators * and +).
Return t.

20.4.1.12 MakeDay ( `year`, `month`, `date` )

The abstract operation MakeDay takes arguments year (a Number), month (a Number), and date (a Number). It calculates a number of days. It performs the following steps when called:

If year is not finite or month is not finite or date is not finite, return NaN.
Let y be ! ToInteger(year).
Let m be ! ToInteger(month).
Let dt be ! ToInteger(date).
Let ym be y + floor(m / 12).
Let mn be m modulo 12.
Find a value t such that YearFromTime(t) is ym and MonthFromTime(t) is mn and DateFromTime(t) is 1; but if this is not possible (because some argument is out of range), return NaN.
Return Day(t) + dt - 1.

20.4.1.13 MakeDate ( `day`, `time` )

The abstract operation MakeDate takes arguments day (a Number) and time (a Number). It calculates a number of milliseconds. It performs the following steps when called:

If day is not finite or time is not finite, return NaN.
Return day × msPerDay + time.

20.4.1.14 TimeClip ( `time` )

The abstract operation TimeClip takes argument time (a Number). It calculates a number of milliseconds. It performs the following steps when called:

If time is not finite, return NaN.
If abs(time) > 8.64 × 10¹⁵, return NaN.
Return ! ToInteger(time).

20.4.1.15 Date Time String Format

ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 calendar date extended format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ

Where the elements are as follows:

`YYYY`	is the year in the proleptic Gregorian calendar as four decimal digits from 0000 to 9999, or as an expanded year of "+" or "-" followed by six decimal digits.
`-`	"-" (hyphen) appears literally twice in the string.
`MM`	is the month of the year as two decimal digits from 01 (January) to 12 (December).
`DD`	is the day of the month as two decimal digits from 01 to 31.
`T`	"T" appears literally in the string, to indicate the beginning of the time element.
`HH`	is the number of complete hours that have passed since midnight as two decimal digits from 00 to 24.
`:`	":" (colon) appears literally twice in the string.
`mm`	is the number of complete minutes since the start of the hour as two decimal digits from 00 to 59.
`ss`	is the number of complete seconds since the start of the minute as two decimal digits from 00 to 59.
`.`	"." (dot) appears literally in the string.
`sss`	is the number of complete milliseconds since the start of the second as three decimal digits.
`Z`	is the UTC offset representation specified as "Z" (for UTC with no offset) or an offset of either "+" or "-" followed by a time expression `HH:mm` (indicating local time ahead of or behind UTC, respectively)

This format includes date-only forms:

YYYY
YYYY-MM
YYYY-MM-DD

It also includes “date-time” forms that consist of one of the above date-only forms immediately followed by one of the following time forms with an optional UTC offset representation appended:

THH:mm
THH:mm:ss
THH:mm:ss.sss

A string containing out-of-bounds or nonconforming elements is not a valid instance of this format.

Note 1

As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are available to distinguish the two midnights that can be associated with one date. This means that the following two notations refer to exactly the same point in time: 1995-02-04T24:00 and 1995-02-05T00:00. This interpretation of the latter form as "end of a calendar day" is consistent with ISO 8601, even though that specification reserves it for describing time intervals and does not permit it within representations of single points in time.

Note 2

There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones. For this reason, both ISO 8601 and this format specify numeric representations of time zone offsets.

20.4.1.15.1 Expanded Years

Covering the full time value range of approximately 273,790 years forward or backward from 01 January, 1970 (20.4.1.1) requires representing years before 0 or after 9999. ISO 8601 permits expansion of the year representation, but only by mutual agreement of the partners in information interchange. In the simplified ECMAScript format, such an expanded year representation shall have 6 digits and is always prefixed with a + or - sign. The year 0 is considered positive and hence prefixed with a + sign. Strings matching the Date Time String Format with expanded years representing instants in time outside the range of a time value are treated as unrecognizable by Date.parse and cause that function to return NaN without falling back to implementation-specific behaviour or heuristics.

Note

Examples of date-time values with expanded years:

-271821-04-20T00:00:00Z	271822 B.C.
-000001-01-01T00:00:00Z	2 B.C.
+000000-01-01T00:00:00Z	1 B.C.
+000001-01-01T00:00:00Z	1 A.D.
+001970-01-01T00:00:00Z	1970 A.D.
+002009-12-15T00:00:00Z	2009 A.D.
+275760-09-13T00:00:00Z	275760 A.D.

20.4.2 The Date Constructor

The Date constructor:

is the intrinsic object %Date%.
is the initial value of the "Date" property of the global object.
creates and initializes a new Date object when called as a constructor.
returns a String representing the current time (UTC) when called as a function rather than as a constructor.
is a single function whose behaviour is overloaded based upon the number and types of its arguments.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Date behaviour must include a super call to the Date constructor to create and initialize the subclass instance with a [[DateValue]] internal slot.
has a "length" property whose value is 7.

20.4.2.1 Date ( `year`, `month` [ , `date` [ , `hours` [ , `minutes` [ , `seconds` [ , `ms` ] ] ] ] ] )

This description applies only if the Date constructor is called with at least two arguments.

When the Date function is called, the following steps are taken:

Let numberOfArgs be the number of arguments passed to this function call.
Assert: numberOfArgs ≥ 2.
If NewTarget is undefined, then
1. Let now be the Number that is the time value (UTC) identifying the current time.
2. Return ToDateString(now).
Else,
1. Let y be ? ToNumber(year).
2. Let m be ? ToNumber(month).
3. If date is present, let dt be ? ToNumber(date); else let dt be 1.
4. If hours is present, let h be ? ToNumber(hours); else let h be 0.
5. If minutes is present, let min be ? ToNumber(minutes); else let min be 0.
6. If seconds is present, let s be ? ToNumber(seconds); else let s be 0.
7. If ms is present, let milli be ? ToNumber(ms); else let milli be 0.
8. If y is NaN, let yr be NaN.
9. Else,
  1. Let yi be ! ToInteger(y).
  2. If 0 ≤ yi ≤ 99, let yr be 1900 + yi; otherwise, let yr be y.
10. Let finalDate be MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli)).
11. Let O be ? OrdinaryCreateFromConstructor(NewTarget, "%Date.prototype%", « [[DateValue]] »).
12. Set O.[[DateValue]] to TimeClip(UTC(finalDate)).
13. Return O.

20.4.2.2 Date ( `value` )

This description applies only if the Date constructor is called with exactly one argument.

When the Date function is called, the following steps are taken:

Let numberOfArgs be the number of arguments passed to this function call.
Assert: numberOfArgs = 1.
If NewTarget is undefined, then
1. Let now be the Number that is the time value (UTC) identifying the current time.
2. Return ToDateString(now).
Else,
1. If Type(value) is Object and value has a [[DateValue]] internal slot, then
  1. Let tv be thisTimeValue(value).
2. Else,
  1. Let v be ? ToPrimitive(value).
  2. If Type(v) is String, then
    1. Assert: The next step never returns an abrupt completion because Type(v) is String.
    2. Let tv be the result of parsing v as a date, in exactly the same manner as for the parse method (20.4.3.2).
  3. Else,
    1. Let tv be ? ToNumber(v).
3. Let O be ? OrdinaryCreateFromConstructor(NewTarget, "%Date.prototype%", « [[DateValue]] »).
4. Set O.[[DateValue]] to TimeClip(tv).
5. Return O.

20.4.2.3 Date ( )

This description applies only if the Date constructor is called with no arguments.

When the Date function is called, the following steps are taken:

Let numberOfArgs be the number of arguments passed to this function call.
Assert: numberOfArgs = 0.
If NewTarget is undefined, then
1. Let now be the Number that is the time value (UTC) identifying the current time.
2. Return ToDateString(now).
Else,
1. Let O be ? OrdinaryCreateFromConstructor(NewTarget, "%Date.prototype%", « [[DateValue]] »).
2. Set O.[[DateValue]] to the time value (UTC) identifying the current time.
3. Return O.

20.4.3 Properties of the Date Constructor

The Date constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

20.4.3.1 Date.now ( )

The now function returns a Number value that is the time value designating the UTC date and time of the occurrence of the call to now.

20.4.3.2 Date.parse ( `string` )

The parse function applies the ToString operator to its argument. If ToString results in an abrupt completion the Completion Record is immediately returned. Otherwise, parse interprets the resulting String as a date and time; it returns a Number, the UTC time value corresponding to the date and time. The String may be interpreted as a local time, a UTC time, or a time in some other time zone, depending on the contents of the String. The function first attempts to parse the String according to the format described in Date Time String Format (20.4.1.15), including expanded years. If the String does not conform to that format the function may fall back to any implementation-specific heuristics or implementation-specific date formats. Strings that are unrecognizable or contain out-of-bounds format element values shall cause Date.parse to return NaN.

If the String conforms to the Date Time String Format, substitute values take the place of absent format elements. When the MM or DD elements are absent, "01" is used. When the HH, mm, or ss elements are absent, "00" is used. When the sss element is absent, "000" is used. When the UTC offset representation is absent, date-only forms are interpreted as a UTC time and date-time forms are interpreted as a local time.

If x is any Date object whose milliseconds amount is zero within a particular implementation of ECMAScript, then all of the following expressions should produce the same numeric value in that implementation, if all the properties referenced have their initial values:

x.valueOf()
Date.parse(x.toString())
Date.parse(x.toUTCString())
Date.parse(x.toISOString())

However, the expression

Date.parse(x.toLocaleString())

is not required to produce the same Number value as the preceding three expressions and, in general, the value produced by Date.parse is implementation-dependent when given any String value that does not conform to the Date Time String Format (20.4.1.15) and that could not be produced in that implementation by the toString or toUTCString method.

20.4.3.3 Date.prototype

The initial value of Date.prototype is %Date.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.4.3.4 Date.UTC ( `year` [ , `month` [ , `date` [ , `hours` [ , `minutes` [ , `seconds` [ , `ms` ] ] ] ] ] ] )

When the UTC function is called, the following steps are taken:

Let y be ? ToNumber(year).
If month is present, let m be ? ToNumber(month); else let m be 0.
If date is present, let dt be ? ToNumber(date); else let dt be 1.
If hours is present, let h be ? ToNumber(hours); else let h be 0.
If minutes is present, let min be ? ToNumber(minutes); else let min be 0.
If seconds is present, let s be ? ToNumber(seconds); else let s be 0.
If ms is present, let milli be ? ToNumber(ms); else let milli be 0.
If y is NaN, let yr be NaN.
Else,
1. Let yi be ! ToInteger(y).
2. If 0 ≤ yi ≤ 99, let yr be 1900 + yi; otherwise, let yr be y.
Return TimeClip(MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli))).

The "length" property of the UTC function is 7.

Note

The UTC function differs from the Date constructor in two ways: it returns a time value as a Number, rather than creating a Date object, and it interprets the arguments in UTC rather than as local time.

20.4.4 Properties of the Date Prototype Object

The Date prototype object:

is the intrinsic object %DatePrototype%.
is itself an ordinary object.
is not a Date instance and does not have a [[DateValue]] internal slot.
has a [[Prototype]] internal slot whose value is %Object.prototype%.

Unless explicitly defined otherwise, the methods of the Date prototype object defined below are not generic and the this value passed to them must be an object that has a [[DateValue]] internal slot that has been initialized to a time value.

The abstract operation thisTimeValue takes argument value. It performs the following steps when called:

If Type(value) is Object and value has a [[DateValue]] internal slot, then
1. Return value.[[DateValue]].
Throw a TypeError exception.

In following descriptions of functions that are properties of the Date prototype object, the phrase “this Date object” refers to the object that is the this value for the invocation of the function. If the Type of the this value is not Object, a TypeError exception is thrown. The phrase “this time value” within the specification of a method refers to the result returned by calling the abstract operation thisTimeValue with the this value of the method invocation passed as the argument.

20.4.4.1 Date.prototype.constructor

The initial value of Date.prototype.constructor is %Date%.

20.4.4.2 Date.prototype.getDate ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return DateFromTime(LocalTime(t)).

20.4.4.3 Date.prototype.getDay ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return WeekDay(LocalTime(t)).

20.4.4.4 Date.prototype.getFullYear ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return YearFromTime(LocalTime(t)).

20.4.4.5 Date.prototype.getHours ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return HourFromTime(LocalTime(t)).

20.4.4.6 Date.prototype.getMilliseconds ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return msFromTime(LocalTime(t)).

20.4.4.7 Date.prototype.getMinutes ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return MinFromTime(LocalTime(t)).

20.4.4.8 Date.prototype.getMonth ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return MonthFromTime(LocalTime(t)).

20.4.4.9 Date.prototype.getSeconds ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return SecFromTime(LocalTime(t)).

20.4.4.10 Date.prototype.getTime ( )

The following steps are performed:

Return ? thisTimeValue(this value).

20.4.4.11 Date.prototype.getTimezoneOffset ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return (t - LocalTime(t)) / msPerMinute.

20.4.4.12 Date.prototype.getUTCDate ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return DateFromTime(t).

20.4.4.13 Date.prototype.getUTCDay ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return WeekDay(t).

20.4.4.14 Date.prototype.getUTCFullYear ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return YearFromTime(t).

20.4.4.15 Date.prototype.getUTCHours ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return HourFromTime(t).

20.4.4.16 Date.prototype.getUTCMilliseconds ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return msFromTime(t).

20.4.4.17 Date.prototype.getUTCMinutes ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return MinFromTime(t).

20.4.4.18 Date.prototype.getUTCMonth ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return MonthFromTime(t).

20.4.4.19 Date.prototype.getUTCSeconds ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return SecFromTime(t).

20.4.4.20 Date.prototype.setDate ( `date` )

The following steps are performed:

Let t be LocalTime(? thisTimeValue(this value)).
Let dt be ? ToNumber(date).
Let newDate be MakeDate(MakeDay(YearFromTime(t), MonthFromTime(t), dt), TimeWithinDay(t)).
Let u be TimeClip(UTC(newDate)).
Set the [[DateValue]] internal slot of this Date object to u.
Return u.

20.4.4.21 Date.prototype.setFullYear ( `year` [ , `month` [ , `date` ] ] )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, set t to +0; otherwise, set t to LocalTime(t).
Let y be ? ToNumber(year).
If month is not present, let m be MonthFromTime(t); otherwise, let m be ? ToNumber(month).
If date is not present, let dt be DateFromTime(t); otherwise, let dt be ? ToNumber(date).
Let newDate be MakeDate(MakeDay(y, m, dt), TimeWithinDay(t)).
Let u be TimeClip(UTC(newDate)).
Set the [[DateValue]] internal slot of this Date object to u.
Return u.

The "length" property of the setFullYear method is 3.

Note

If month is not present, this method behaves as if month was present with the value getMonth(). If date is not present, it behaves as if date was present with the value getDate().

20.4.4.22 Date.prototype.setHours ( `hour` [ , `min` [ , `sec` [ , `ms` ] ] ] )

The following steps are performed:

Let t be LocalTime(? thisTimeValue(this value)).
Let h be ? ToNumber(hour).
If min is not present, let m be MinFromTime(t); otherwise, let m be ? ToNumber(min).
If sec is not present, let s be SecFromTime(t); otherwise, let s be ? ToNumber(sec).
If ms is not present, let milli be msFromTime(t); otherwise, let milli be ? ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(h, m, s, milli)).
Let u be TimeClip(UTC(date)).
Set the [[DateValue]] internal slot of this Date object to u.
Return u.

The "length" property of the setHours method is 4.

Note

If min is not present, this method behaves as if min was present with the value getMinutes(). If sec is not present, it behaves as if sec was present with the value getSeconds(). If ms is not present, it behaves as if ms was present with the value getMilliseconds().

20.4.4.23 Date.prototype.setMilliseconds ( `ms` )

The following steps are performed:

Let t be LocalTime(? thisTimeValue(this value)).
Set ms to ? ToNumber(ms).
Let time be MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t), ms).
Let u be TimeClip(UTC(MakeDate(Day(t), time))).
Set the [[DateValue]] internal slot of this Date object to u.
Return u.

20.4.4.24 Date.prototype.setMinutes ( `min` [ , `sec` [ , `ms` ] ] )

The following steps are performed:

Let t be LocalTime(? thisTimeValue(this value)).
Let m be ? ToNumber(min).
If sec is not present, let s be SecFromTime(t); otherwise, let s be ? ToNumber(sec).
If ms is not present, let milli be msFromTime(t); otherwise, let milli be ? ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), m, s, milli)).
Let u be TimeClip(UTC(date)).
Set the [[DateValue]] internal slot of this Date object to u.
Return u.

The "length" property of the setMinutes method is 3.

Note

If sec is not present, this method behaves as if sec was present with the value getSeconds(). If ms is not present, this behaves as if ms was present with the value getMilliseconds().

20.4.4.25 Date.prototype.setMonth ( `month` [ , `date` ] )

The following steps are performed:

Let t be LocalTime(? thisTimeValue(this value)).
Let m be ? ToNumber(month).
If date is not present, let dt be DateFromTime(t); otherwise, let dt be ? ToNumber(date).
Let newDate be MakeDate(MakeDay(YearFromTime(t), m, dt), TimeWithinDay(t)).
Let u be TimeClip(UTC(newDate)).
Set the [[DateValue]] internal slot of this Date object to u.
Return u.

The "length" property of the setMonth method is 2.

Note

If date is not present, this method behaves as if date was present with the value getDate().

20.4.4.26 Date.prototype.setSeconds ( `sec` [ , `ms` ] )

The following steps are performed:

Let t be LocalTime(? thisTimeValue(this value)).
Let s be ? ToNumber(sec).
If ms is not present, let milli be msFromTime(t); otherwise, let milli be ? ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), MinFromTime(t), s, milli)).
Let u be TimeClip(UTC(date)).
Set the [[DateValue]] internal slot of this Date object to u.
Return u.

The "length" property of the setSeconds method is 2.

Note

If ms is not present, this method behaves as if ms was present with the value getMilliseconds().

20.4.4.27 Date.prototype.setTime ( `time` )

The following steps are performed:

Perform ? thisTimeValue(this value).
Let t be ? ToNumber(time).
Let v be TimeClip(t).
Set the [[DateValue]] internal slot of this Date object to v.
Return v.

20.4.4.28 Date.prototype.setUTCDate ( `date` )

The following steps are performed:

Let t be ? thisTimeValue(this value).
Let dt be ? ToNumber(date).
Let newDate be MakeDate(MakeDay(YearFromTime(t), MonthFromTime(t), dt), TimeWithinDay(t)).
Let v be TimeClip(newDate).
Set the [[DateValue]] internal slot of this Date object to v.
Return v.

20.4.4.29 Date.prototype.setUTCFullYear ( `year` [ , `month` [ , `date` ] ] )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, set t to +0.
Let y be ? ToNumber(year).
If month is not present, let m be MonthFromTime(t); otherwise, let m be ? ToNumber(month).
If date is not present, let dt be DateFromTime(t); otherwise, let dt be ? ToNumber(date).
Let newDate be MakeDate(MakeDay(y, m, dt), TimeWithinDay(t)).
Let v be TimeClip(newDate).
Set the [[DateValue]] internal slot of this Date object to v.
Return v.

The "length" property of the setUTCFullYear method is 3.

Note

If month is not present, this method behaves as if month was present with the value getUTCMonth(). If date is not present, it behaves as if date was present with the value getUTCDate().

20.4.4.30 Date.prototype.setUTCHours ( `hour` [ , `min` [ , `sec` [ , `ms` ] ] ] )

The following steps are performed:

Let t be ? thisTimeValue(this value).
Let h be ? ToNumber(hour).
If min is not present, let m be MinFromTime(t); otherwise, let m be ? ToNumber(min).
If sec is not present, let s be SecFromTime(t); otherwise, let s be ? ToNumber(sec).
If ms is not present, let milli be msFromTime(t); otherwise, let milli be ? ToNumber(ms).
Let newDate be MakeDate(Day(t), MakeTime(h, m, s, milli)).
Let v be TimeClip(newDate).
Set the [[DateValue]] internal slot of this Date object to v.
Return v.

The "length" property of the setUTCHours method is 4.

Note

If min is not present, this method behaves as if min was present with the value getUTCMinutes(). If sec is not present, it behaves as if sec was present with the value getUTCSeconds(). If ms is not present, it behaves as if ms was present with the value getUTCMilliseconds().

20.4.4.31 Date.prototype.setUTCMilliseconds ( `ms` )

The following steps are performed:

Let t be ? thisTimeValue(this value).
Let milli be ? ToNumber(ms).
Let time be MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t), milli).
Let v be TimeClip(MakeDate(Day(t), time)).
Set the [[DateValue]] internal slot of this Date object to v.
Return v.

20.4.4.32 Date.prototype.setUTCMinutes ( `min` [ , `sec` [ , `ms` ] ] )

The following steps are performed:

Let t be ? thisTimeValue(this value).
Let m be ? ToNumber(min).
If sec is not present, let s be SecFromTime(t).
Else,
1. Let s be ? ToNumber(sec).
If ms is not present, let milli be msFromTime(t).
Else,
1. Let milli be ? ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), m, s, milli)).
Let v be TimeClip(date).
Set the [[DateValue]] internal slot of this Date object to v.
Return v.

The "length" property of the setUTCMinutes method is 3.

Note

If sec is not present, this method behaves as if sec was present with the value getUTCSeconds(). If ms is not present, it function behaves as if ms was present with the value return by getUTCMilliseconds().

20.4.4.33 Date.prototype.setUTCMonth ( `month` [ , `date` ] )

The following steps are performed:

Let t be ? thisTimeValue(this value).
Let m be ? ToNumber(month).
If date is not present, let dt be DateFromTime(t).
Else,
1. Let dt be ? ToNumber(date).
Let newDate be MakeDate(MakeDay(YearFromTime(t), m, dt), TimeWithinDay(t)).
Let v be TimeClip(newDate).
Set the [[DateValue]] internal slot of this Date object to v.
Return v.

The "length" property of the setUTCMonth method is 2.

Note

If date is not present, this method behaves as if date was present with the value getUTCDate().

20.4.4.34 Date.prototype.setUTCSeconds ( `sec` [ , `ms` ] )

The following steps are performed:

Let t be ? thisTimeValue(this value).
Let s be ? ToNumber(sec).
If ms is not present, let milli be msFromTime(t).
Else,
1. Let milli be ? ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), MinFromTime(t), s, milli)).
Let v be TimeClip(date).
Set the [[DateValue]] internal slot of this Date object to v.
Return v.

The "length" property of the setUTCSeconds method is 2.

Note

If ms is not present, this method behaves as if ms was present with the value getUTCMilliseconds().

20.4.4.35 Date.prototype.toDateString ( )

The following steps are performed:

Let O be this Date object.
Let tv be ? thisTimeValue(O).
If tv is NaN, return "Invalid Date".
Let t be LocalTime(tv).
Return DateString(t).

20.4.4.36 Date.prototype.toISOString ( )

If this time value is not a finite Number or if it corresponds with a year that cannot be represented in the Date Time String Format, this function throws a RangeError exception. Otherwise, it returns a String representation of this time value in that format on the UTC time scale, including all format elements and the UTC offset representation "Z".

20.4.4.37 Date.prototype.toJSON ( `key` )

This function provides a String representation of a Date object for use by JSON.stringify (24.5.2).

When the toJSON method is called with argument key, the following steps are taken:

Let O be ? ToObject(this value).
Let tv be ? ToPrimitive(O, hint Number).
If Type(tv) is Number and tv is not finite, return null.
Return ? Invoke(O, "toISOString").

Note 1

The argument is ignored.

Note 2

The toJSON function is intentionally generic; it does not require that its this value be a Date object. Therefore, it can be transferred to other kinds of objects for use as a method. However, it does require that any such object have a toISOString method.

20.4.4.38 Date.prototype.toLocaleDateString ( [ `reserved1` [ , `reserved2` ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleDateString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleDateString method is used.

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “date” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.

The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

20.4.4.39 Date.prototype.toLocaleString ( [ `reserved1` [ , `reserved2` ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.

20.4.4.40 Date.prototype.toLocaleTimeString ( [ `reserved1` [ , `reserved2` ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleTimeString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleTimeString method is used.

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “time” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.

20.4.4.41 Date.prototype.toString ( )

The following steps are performed:

Let tv be ? thisTimeValue(this value).
Return ToDateString(tv).

Note 1

For any Date object d whose milliseconds amount is zero, the result of Date.parse(d.toString()) is equal to d.valueOf(). See 20.4.3.2.

Note 2

The toString function is not generic; it throws a TypeError exception if its this value is not a Date object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

20.4.4.41.1 Runtime Semantics: TimeString ( `tv` )

The abstract operation TimeString takes argument tv. It performs the following steps when called:

Assert: Type(tv) is Number.
Assert: tv is not NaN.
Let hour be the String representation of HourFromTime(tv), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let minute be the String representation of MinFromTime(tv), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let second be the String representation of SecFromTime(tv), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Return the string-concatenation of hour, ":", minute, ":", second, the code unit 0x0020 (SPACE), and "GMT".

20.4.4.41.2 Runtime Semantics: DateString ( `tv` )

The abstract operation DateString takes argument tv. It performs the following steps when called:

Assert: Type(tv) is Number.
Assert: tv is not NaN.
Let weekday be the Name of the entry in Table 50 with the Number WeekDay(tv).
Let month be the Name of the entry in Table 51 with the Number MonthFromTime(tv).
Let day be the String representation of DateFromTime(tv), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let yv be YearFromTime(tv).
If yv ≥ 0, let yearSign be the empty String; otherwise, let yearSign be "-".
Let year be the String representation of abs(yv), formatted as a decimal number.
Let paddedYear be ! StringPad(year, 4, "0", start).
Return the string-concatenation of weekday, the code unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE), day, the code unit 0x0020 (SPACE), yearSign, and paddedYear.

Table 50: Names of days of the week

Number	Name
0	"Sun"
1	"Mon"
2	"Tue"
3	"Wed"
4	"Thu"
5	"Fri"
6	"Sat"

Table 51: Names of months of the year

Number	Name
0	"Jan"
1	"Feb"
2	"Mar"
3	"Apr"
4	"May"
5	"Jun"
6	"Jul"
7	"Aug"
8	"Sep"
9	"Oct"
10	"Nov"
11	"Dec"

20.4.4.41.3 Runtime Semantics: TimeZoneString ( `tv` )

The abstract operation TimeZoneString takes argument tv. It performs the following steps when called:

Assert: Type(tv) is Number.
Assert: tv is not NaN.
Let offset be LocalTZA(tv, true).
If offset ≥ 0, let offsetSign be "+"; otherwise, let offsetSign be "-".
Let offsetMin be the String representation of MinFromTime(abs(offset)), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let offsetHour be the String representation of HourFromTime(abs(offset)), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let tzName be an implementation-defined string that is either the empty String or the string-concatenation of the code unit 0x0020 (SPACE), the code unit 0x0028 (LEFT PARENTHESIS), an implementation-dependent timezone name, and the code unit 0x0029 (RIGHT PARENTHESIS).
Return the string-concatenation of offsetSign, offsetHour, offsetMin, and tzName.

20.4.4.41.4 Runtime Semantics: ToDateString ( `tv` )

The abstract operation ToDateString takes argument tv. It performs the following steps when called:

Assert: Type(tv) is Number.
If tv is NaN, return "Invalid Date".
Let t be LocalTime(tv).
Return the string-concatenation of DateString(t), the code unit 0x0020 (SPACE), TimeString(t), and TimeZoneString(tv).

20.4.4.42 Date.prototype.toTimeString ( )

The following steps are performed:

Let O be this Date object.
Let tv be ? thisTimeValue(O).
If tv is NaN, return "Invalid Date".
Let t be LocalTime(tv).
Return the string-concatenation of TimeString(t) and TimeZoneString(tv).

20.4.4.43 Date.prototype.toUTCString ( )

The toUTCString method returns a String value representing the instance in time corresponding to this time value. The format of the String is based upon "HTTP-date" from RFC 7231, generalized to support the full range of times supported by ECMAScript Date objects. It performs the following steps when called:

Let O be this Date object.
Let tv be ? thisTimeValue(O).
If tv is NaN, return "Invalid Date".
Let weekday be the Name of the entry in Table 50 with the Number WeekDay(tv).
Let month be the Name of the entry in Table 51 with the Number MonthFromTime(tv).
Let day be the String representation of DateFromTime(tv), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let yv be YearFromTime(tv).
If yv ≥ 0, let yearSign be the empty String; otherwise, let yearSign be "-".
Let year be the String representation of abs(yv), formatted as a decimal number.
Let paddedYear be ! StringPad(year, 4, "0", start).
Return the string-concatenation of weekday, ",", the code unit 0x0020 (SPACE), day, the code unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE), yearSign, paddedYear, the code unit 0x0020 (SPACE), and TimeString(tv).

20.4.4.44 Date.prototype.valueOf ( )

The following steps are performed:

Return ? thisTimeValue(this value).

20.4.4.45 Date.prototype [ @@toPrimitive ] ( `hint` )

This function is called by ECMAScript language operators to convert a Date object to a primitive value. The allowed values for hint are "default", "number", and "string". Date objects, are unique among built-in ECMAScript object in that they treat "default" as being equivalent to "string", All other built-in ECMAScript objects treat "default" as being equivalent to "number".

When the @@toPrimitive method is called with argument hint, the following steps are taken:

Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If hint is the String value "string" or the String value "default", then
1. Let tryFirst be "string".
Else if hint is the String value "number", then
1. Let tryFirst be "number".
Else, throw a TypeError exception.
Return ? OrdinaryToPrimitive(O, tryFirst).

The value of the "name" property of this function is "[Symbol.toPrimitive]".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

20.4.5 Properties of Date Instances

Date instances are ordinary objects that inherit properties from the Date prototype object. Date instances also have a [[DateValue]] internal slot. The [[DateValue]] internal slot is the time value represented by this Date object.

21 Text Processing

21.1 String Objects

21.1.1 The String Constructor

The String constructor:

is the intrinsic object %String%.
is the initial value of the "String" property of the global object.
creates and initializes a new String object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified String behaviour must include a super call to the String constructor to create and initialize the subclass instance with a [[StringData]] internal slot.

21.1.1.1 String ( `value` )

When String is called with argument value, the following steps are taken:

If value is not present, let s be the empty String.
Else,
1. If NewTarget is undefined and Type(value) is Symbol, return SymbolDescriptiveString(value).
2. Let s be ? ToString(value).
If NewTarget is undefined, return s.
Return ! StringCreate(s, ? GetPrototypeFromConstructor(NewTarget, "%String.prototype%")).

21.1.2 Properties of the String Constructor

The String constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

21.1.2.1 String.fromCharCode ( ...`codeUnits` )

The String.fromCharCode function may be called with any number of arguments which form the rest parameter codeUnits. The following steps are taken:

Let codeUnits be a List containing the arguments passed to this function.
Let length be the number of elements in codeUnits.
Let elements be a new empty List.
Let nextIndex be 0.
Repeat, while nextIndex < length
1. Let next be codeUnits[nextIndex].
2. Let nextCU be ? ToUint16(next).
3. Append nextCU to the end of elements.
4. Set nextIndex to nextIndex + 1.
Return the String value whose code units are, in order, the elements in the List elements. If length is 0, the empty String is returned.

The "length" property of the fromCharCode function is 1.

21.1.2.2 String.fromCodePoint ( ...`codePoints` )

The String.fromCodePoint function may be called with any number of arguments which form the rest parameter codePoints. The following steps are taken:

Let codePoints be a List containing the arguments passed to this function.
Let length be the number of elements in codePoints.
Let elements be a new empty List.
Let nextIndex be 0.
Repeat, while nextIndex < length
1. Let next be codePoints[nextIndex].
2. Let nextCP be ? ToNumber(next).
3. If ! IsInteger(nextCP) is false, throw a RangeError exception.
4. If nextCP < 0 or nextCP > 0x10FFFF, throw a RangeError exception.
5. Append the elements of the UTF16Encoding of nextCP to the end of elements.
6. Set nextIndex to nextIndex + 1.
Return the String value whose code units are, in order, the elements in the List elements. If length is 0, the empty String is returned.

The "length" property of the fromCodePoint function is 1.

21.1.2.3 String.prototype

The initial value of String.prototype is %String.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.1.2.4 String.raw ( `template`, ...`substitutions` )

The String.raw function may be called with a variable number of arguments. The first argument is template and the remainder of the arguments form the List substitutions. The following steps are taken:

Let substitutions be a List consisting of all of the arguments passed to this function, starting with the second argument. If fewer than two arguments were passed, the List is empty.
Let numberOfSubstitutions be the number of elements in substitutions.
Let cooked be ? ToObject(template).
Let raw be ? ToObject(? Get(cooked, "raw")).
Let literalSegments be ? LengthOfArrayLike(raw).
If literalSegments ≤ 0, return the empty String.
Let stringElements be a new empty List.
Let nextIndex be 0.
Repeat,
1. Let nextKey be ! ToString(nextIndex).
2. Let nextSeg be ? ToString(? Get(raw, nextKey)).
3. Append in order the code unit elements of nextSeg to the end of stringElements.
4. If nextIndex + 1 = literalSegments, then
  1. Return the String value whose code units are, in order, the elements in the List stringElements. If stringElements has no elements, the empty String is returned.
5. If nextIndex < numberOfSubstitutions, let next be substitutions[nextIndex].
6. Else, let next be the empty String.
7. Let nextSub be ? ToString(next).
8. Append in order the code unit elements of nextSub to the end of stringElements.
9. Set nextIndex to nextIndex + 1.

Note

The raw function is intended for use as a tag function of a Tagged Template (12.3.11). When called as such, the first argument will be a well formed template object and the rest parameter will contain the substitution values.

21.1.3 Properties of the String Prototype Object

The String prototype object:

is the intrinsic object %StringPrototype%.
is a String exotic object and has the internal methods specified for such objects.
has a [[StringData]] internal slot whose value is the empty String.
has a "length" property whose initial value is 0 and whose attributes are { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
has a [[Prototype]] internal slot whose value is %Object.prototype%.

Unless explicitly stated otherwise, the methods of the String prototype object defined below are not generic and the this value passed to them must be either a String value or an object that has a [[StringData]] internal slot that has been initialized to a String value.

The abstract operation thisStringValue takes argument value. It performs the following steps when called:

If Type(value) is String, return value.
If Type(value) is Object and value has a [[StringData]] internal slot, then
1. Let s be value.[[StringData]].
2. Assert: Type(s) is String.
3. Return s.
Throw a TypeError exception.

21.1.3.1 String.prototype.charAt ( `pos` )

Note 1

Returns a single element String containing the code unit at index pos within the String value resulting from converting this object to a String. If there is no element at that index, the result is the empty String. The result is a String value, not a String object.

If pos is a value of Number type that is an integer, then the result of x.charAt(pos) is equal to the result of x.substring(pos, pos + 1).

When the charAt method is called with one argument pos, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let position be ? ToInteger(pos).
Let size be the length of S.
If position < 0 or position ≥ size, return the empty String.
Return the String value of length 1, containing one code unit from S, namely the code unit at index position.

Note 2

The charAt function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.2 String.prototype.charCodeAt ( `pos` )

Note 1

Returns a Number (a nonnegative integer less than 2¹⁶) that is the numeric value of the code unit at index pos within the String resulting from converting this object to a String. If there is no element at that index, the result is NaN.

When the charCodeAt method is called with one argument pos, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let position be ? ToInteger(pos).
Let size be the length of S.
If position < 0 or position ≥ size, return NaN.
Return a value of Number type, whose value is the numeric value of the code unit at index position within the String S.

Note 2

The charCodeAt function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

21.1.3.3 String.prototype.codePointAt ( `pos` )

Note 1

Returns a nonnegative integer Number less than or equal to 0x10FFFF that is the code point value of the UTF-16 encoded code point (6.1.4) starting at the string element at index pos within the String resulting from converting this object to a String. If there is no element at that index, the result is undefined. If a valid UTF-16 surrogate pair does not begin at pos, the result is the code unit at pos.

When the codePointAt method is called with one argument pos, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let position be ? ToInteger(pos).
Let size be the length of S.
If position < 0 or position ≥ size, return undefined.
Let cp be ! CodePointAt(S, position).
Return cp.[[CodePoint]].

Note 2

The codePointAt function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

21.1.3.4 String.prototype.concat ( ...`args` )

Note 1

When the concat method is called it returns the String value consisting of the code units of the this object (converted to a String) followed by the code units of each of the arguments converted to a String. The result is a String value, not a String object.

When the concat method is called with zero or more arguments, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let args be a List whose elements are the arguments passed to this function.
Let R be S.
Repeat, while args is not empty
1. Remove the first element from args and let next be the value of that element.
2. Let nextString be ? ToString(next).
3. Set R to the string-concatenation of the previous value of R and nextString.
Return R.

The "length" property of the concat method is 1.

Note 2

The concat function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

21.1.3.5 String.prototype.constructor

The initial value of String.prototype.constructor is %String%.

21.1.3.6 String.prototype.endsWith ( `searchString` [ , `endPosition` ] )

The following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let isRegExp be ? IsRegExp(searchString).
If isRegExp is true, throw a TypeError exception.
Let searchStr be ? ToString(searchString).
Let len be the length of S.
If endPosition is undefined, let pos be len; else let pos be ? ToInteger(endPosition).
Let end be min(max(pos, 0), len).
Let searchLength be the length of searchStr.
Let start be end - searchLength.
If start is less than 0, return false.
If the sequence of code units of S starting at start of length searchLength is the same as the full code unit sequence of searchStr, return true.
Otherwise, return false.

Note 1

Returns true if the sequence of code units of searchString converted to a String is the same as the corresponding code units of this object (converted to a String) starting at endPosition - length(this). Otherwise returns false.

Note 2

Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.

Note 3

The endsWith function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.7 String.prototype.includes ( `searchString` [ , `position` ] )

The includes method takes two arguments, searchString and position, and performs the following steps:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let isRegExp be ? IsRegExp(searchString).
If isRegExp is true, throw a TypeError exception.
Let searchStr be ? ToString(searchString).
Let pos be ? ToInteger(position).
Assert: If position is undefined, then pos is 0.
Let len be the length of S.
Let start be min(max(pos, 0), len).
Let searchLen be the length of searchStr.
If there exists any integer k not smaller than start such that k + searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the code unit at index k + j within S is the same as the code unit at index j within searchStr, return true; but if there is no such integer k, return false.

Note 1

If searchString appears as a substring of the result of converting this object to a String, at one or more indices that are greater than or equal to position, return true; otherwise, returns false. If position is undefined, 0 is assumed, so as to search all of the String.

Note 2

Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.

Note 3

The includes function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.8 String.prototype.indexOf ( `searchString` [ , `position` ] )

Note 1

If searchString appears as a substring of the result of converting this object to a String, at one or more indices that are greater than or equal to position, then the smallest such index is returned; otherwise, -1 is returned. If position is undefined, 0 is assumed, so as to search all of the String.

The indexOf method takes two arguments, searchString and position, and performs the following steps:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let searchStr be ? ToString(searchString).
Let pos be ? ToInteger(position).
Assert: If position is undefined, then pos is 0.
Let len be the length of S.
Let start be min(max(pos, 0), len).
Let searchLen be the length of searchStr.
Return the smallest possible integer k not smaller than start such that k + searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the code unit at index k + j within S is the same as the code unit at index j within searchStr; but if there is no such integer k, return the value -1.

Note 2

The indexOf function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.9 String.prototype.lastIndexOf ( `searchString` [ , `position` ] )

Note 1

If searchString appears as a substring of the result of converting this object to a String at one or more indices that are smaller than or equal to position, then the greatest such index is returned; otherwise, -1 is returned. If position is undefined, the length of the String value is assumed, so as to search all of the String.

The lastIndexOf method takes two arguments, searchString and position, and performs the following steps:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let searchStr be ? ToString(searchString).
Let numPos be ? ToNumber(position).
Assert: If position is undefined, then numPos is NaN.
If numPos is NaN, let pos be +∞; otherwise, let pos be ! ToInteger(numPos).
Let len be the length of S.
Let start be min(max(pos, 0), len).
Let searchLen be the length of searchStr.
Return the largest possible nonnegative integer k not larger than start such that k + searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the code unit at index k + j within S is the same as the code unit at index j within searchStr; but if there is no such integer k, return the value -1.

Note 2

The lastIndexOf function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.10 String.prototype.localeCompare ( `that` [ , `reserved1` [ , `reserved2` ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the localeCompare method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the localeCompare method is used.

When the localeCompare method is called with argument that, it returns a Number other than NaN that represents the result of a locale-sensitive String comparison of the this value (converted to a String) with that (converted to a String). The two Strings are S and That. The two Strings are compared in an implementation-defined fashion. The result is intended to order String values in the sort order specified by a host default locale, and will be negative, zero, or positive, depending on whether S comes before That in the sort order, the Strings are equal, or S comes after That in the sort order, respectively.

Before performing the comparisons, the following steps are performed to prepare the Strings:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let That be ? ToString(that).

The meaning of the optional second and third parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not assign any other interpretation to those parameter positions.

The localeCompare method, if considered as a function of two arguments this and that, is a consistent comparison function (as defined in 22.1.3.27) on the set of all Strings.

The actual return values are implementation-defined to permit implementers to encode additional information in the value, but the function is required to define a total ordering on all Strings. This function must treat Strings that are canonically equivalent according to the Unicode standard as identical and must return 0 when comparing Strings that are considered canonically equivalent.

Note 1

The localeCompare method itself is not directly suitable as an argument to Array.prototype.sort because the latter requires a function of two arguments.

Note 2

This function is intended to rely on whatever language-sensitive comparison functionality is available to the ECMAScript environment from the host environment, and to compare according to the rules of the host environment's current locale. However, regardless of the host provided comparison capabilities, this function must treat Strings that are canonically equivalent according to the Unicode standard as identical. It is recommended that this function should not honour Unicode compatibility equivalences or decompositions. For a definition and discussion of canonical equivalence see the Unicode Standard, chapters 2 and 3, as well as Unicode Standard Annex #15, Unicode Normalization Forms (https://unicode.org/reports/tr15/) and Unicode Technical Note #5, Canonical Equivalence in Applications (https://www.unicode.org/notes/tn5/). Also see Unicode Technical Standard #10, Unicode Collation Algorithm (https://unicode.org/reports/tr10/).

Note 3

The localeCompare function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.11 String.prototype.match ( `regexp` )

When the match method is called with argument regexp, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
If regexp is neither undefined nor null, then
1. Let matcher be ? GetMethod(regexp, @@match).
2. If matcher is not undefined, then
  1. Return ? Call(matcher, regexp, « O »).
Let S be ? ToString(O).
Let rx be ? RegExpCreate(regexp, undefined).
Return ? Invoke(rx, @@match, « S »).

Note

The match function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.12 String.prototype.matchAll ( `regexp` )

Performs a regular expression match of the String representing the this value against regexp and returns an iterator. Each iteration result's value is an Array object containing the results of the match, or null if the String did not match.

When the matchAll method is called, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
If regexp is neither undefined nor null, then
1. Let isRegExp be ? IsRegExp(regexp).
2. If isRegExp is true, then
  1. Let flags be ? Get(regexp, "flags").
  2. Perform ? RequireObjectCoercible(flags).
  3. If ? ToString(flags) does not contain "g", throw a TypeError exception.
3. Let matcher be ? GetMethod(regexp, @@matchAll).
4. If matcher is not undefined, then
  1. Return ? Call(matcher, regexp, « O »).
Let S be ? ToString(O).
Let rx be ? RegExpCreate(regexp, "g").
Return ? Invoke(rx, @@matchAll, « S »).

Note 1

The matchAll function is intentionally generic, it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

Note 2

Similarly to String.prototype.split, String.prototype.matchAll is designed to typically act without mutating its inputs.

21.1.3.13 String.prototype.normalize ( [ `form` ] )

When the normalize method is called with one argument form, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
If form is undefined, let f be "NFC".
Else, let f be ? ToString(form).
If f is not one of "NFC", "NFD", "NFKC", or "NFKD", throw a RangeError exception.
Let ns be the String value that is the result of normalizing S into the normalization form named by f as specified in https://unicode.org/reports/tr15/.
Return ns.

Note

The normalize function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

21.1.3.14 String.prototype.padEnd ( `maxLength` [ , `fillString` ] )

When the padEnd method is called, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Return ? StringPad(O, maxLength, fillString, end).

21.1.3.15 String.prototype.padStart ( `maxLength` [ , `fillString` ] )

When the padStart method is called, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Return ? StringPad(O, maxLength, fillString, start).

21.1.3.15.1 Runtime Semantics: StringPad ( `O`, `maxLength`, `fillString`, `placement` )

The abstract operation StringPad takes arguments O, maxLength, fillString, and placement. It performs the following steps when called:

Assert: placement is start or end.
Let S be ? ToString(O).
Let intMaxLength be ? ToLength(maxLength).
Let stringLength be the length of S.
If intMaxLength is not greater than stringLength, return S.
If fillString is undefined, let filler be the String value consisting solely of the code unit 0x0020 (SPACE).
Else, let filler be ? ToString(fillString).
If filler is the empty String, return S.
Let fillLen be intMaxLength - stringLength.
Let truncatedStringFiller be the String value consisting of repeated concatenations of filler truncated to length fillLen.
If placement is start, return the string-concatenation of truncatedStringFiller and S.
Else, return the string-concatenation of S and truncatedStringFiller.

Note 1

The argument maxLength will be clamped such that it can be no smaller than the length of S.

Note 2

The argument fillString defaults to " " (the String value consisting of the code unit 0x0020 SPACE).

21.1.3.16 String.prototype.repeat ( `count` )

The following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let n be ? ToInteger(count).
If n < 0, throw a RangeError exception.
If n is +∞, throw a RangeError exception.
If n is 0, return the empty String.
Return the String value that is made from n copies of S appended together.

Note 1

This method creates the String value consisting of the code units of the this object (converted to String) repeated count times.

Note 2

The repeat function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.17 String.prototype.replace ( `searchValue`, `replaceValue` )

When the replace method is called with arguments searchValue and replaceValue, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
If searchValue is neither undefined nor null, then
1. Let replacer be ? GetMethod(searchValue, @@replace).
2. If replacer is not undefined, then
  1. Return ? Call(replacer, searchValue, « O, replaceValue »).
Let string be ? ToString(O).
Let searchString be ? ToString(searchValue).
Let functionalReplace be IsCallable(replaceValue).
If functionalReplace is false, then
1. Set replaceValue to ? ToString(replaceValue).
Search string for the first occurrence of searchString and let pos be the index within string of the first code unit of the matched substring and let matched be searchString. If no occurrences of searchString were found, return string.
If functionalReplace is true, then
1. Let replValue be ? Call(replaceValue, undefined, « matched, pos, string »).
2. Let replStr be ? ToString(replValue).
Else,
1. Let captures be a new empty List.
2. Let replStr be ! GetSubstitution(matched, string, pos, captures, undefined, replaceValue).
Let tailPos be pos + the number of code units in matched.
Let newString be the string-concatenation of the first pos code units of string, replStr, and the trailing substring of string starting at index tailPos. If pos is 0, the first element of the concatenation will be the empty String.
Return newString.

Note

The replace function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.17.1 Runtime Semantics: GetSubstitution ( `matched`, `str`, `position`, `captures`, `namedCaptures`, `replacement` )

The abstract operation GetSubstitution takes arguments matched, str, position, captures, namedCaptures, and replacement. It performs the following steps when called:

Assert: Type(matched) is String.
Let matchLength be the number of code units in matched.
Assert: Type(str) is String.
Let stringLength be the number of code units in str.
Assert: ! IsNonNegativeInteger(position) is true.
Assert: position ≤ stringLength.
Assert: captures is a possibly empty List of Strings.
Assert: Type(replacement) is String.
Let tailPos be position + matchLength.
Let m be the number of elements in captures.
Let result be the String value derived from replacement by copying code unit elements from replacement to result while performing replacements as specified in Table 52. These $ replacements are done left-to-right, and, once such a replacement is performed, the new replacement text is not subject to further replacements.
Return result.

Table 52: Replacement Text Symbol Substitutions

Code units	Unicode Characters	Replacement text
0x0024, 0x0024	`$$`	`$`
0x0024, 0x0026	`$&`	`matched`
0x0024, 0x0060	$`	If `position` is 0, the replacement is the empty String. Otherwise the replacement is the substring of `str` that starts at index 0 and whose last code unit is at index `position` - 1.
0x0024, 0x0027	`$'`	If `tailPos` ≥ `stringLength`, the replacement is the empty String. Otherwise the replacement is the substring of `str` that starts at index `tailPos` and continues to the end of `str`.
0x0024, N Where 0x0031 ≤ N ≤ 0x0039	`$n` where `n` is one of `1 2 3 4 5 6 7 8 9` and `$n` is not followed by a decimal digit	The `n`^th element of `captures`, where `n` is a single digit in the range 1 to 9. If `n` ≤ `m` and the `n`^th element of `captures` is undefined, use the empty String instead. If `n` > `m`, no replacement is done.
0x0024, N, N Where 0x0030 ≤ N ≤ 0x0039	`$nn` where `n` is one of `0 1 2 3 4 5 6 7 8 9`	The `nn`^th element of `captures`, where `nn` is a two-digit decimal number in the range 01 to 99. If `nn` ≤ `m` and the `nn`^th element of `captures` is undefined, use the empty String instead. If `nn` is 00 or `nn` > `m`, no replacement is done.
0x0024, 0x003C	`$<`	If `namedCaptures` is undefined, the replacement text is the String "$<". Else, Assert: Type(`namedCaptures`) is Object. Scan until the next `>` U+003E (GREATER-THAN SIGN). If none is found, the replacement text is the String "$<". Else, Let `groupName` be the enclosed substring. Let `capture` be ? Get(`namedCaptures`, `groupName`). If `capture` is undefined, replace the text through `>` with the empty String. Otherwise, replace the text through `>` with ? ToString(`capture`).
0x0024	`$` in any context that does not match any of the above.	`$`

21.1.3.18 String.prototype.search ( `regexp` )

When the search method is called with argument regexp, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
If regexp is neither undefined nor null, then
1. Let searcher be ? GetMethod(regexp, @@search).
2. If searcher is not undefined, then
  1. Return ? Call(searcher, regexp, « O »).
Let string be ? ToString(O).
Let rx be ? RegExpCreate(regexp, undefined).
Return ? Invoke(rx, @@search, « string »).

Note

The search function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.19 String.prototype.slice ( `start`, `end` )

The slice method takes two arguments, start and end, and returns a substring of the result of converting this object to a String, starting from index start and running to, but not including, index end (or through the end of the String if end is undefined). If start is negative, it is treated as sourceLength + start where sourceLength is the length of the String. If end is negative, it is treated as sourceLength + end where sourceLength is the length of the String. The result is a String value, not a String object. The following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let len be the length of S.
Let intStart be ? ToInteger(start).
If end is undefined, let intEnd be len; else let intEnd be ? ToInteger(end).
If intStart < 0, let from be max(len + intStart, 0); otherwise let from be min(intStart, len).
If intEnd < 0, let to be max(len + intEnd, 0); otherwise let to be min(intEnd, len).
Let span be max(to - from, 0).
Return the String value containing span consecutive code units from S beginning with the code unit at index from.

Note

The slice function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

21.1.3.20 String.prototype.split ( `separator`, `limit` )

Returns an Array object into which substrings of the result of converting this object to a String have been stored. The substrings are determined by searching from left to right for occurrences of separator; these occurrences are not part of any substring in the returned array, but serve to divide up the String value. The value of separator may be a String of any length or it may be an object, such as a RegExp, that has a @@split method.

When the split method is called, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
If separator is neither undefined nor null, then
1. Let splitter be ? GetMethod(separator, @@split).
2. If splitter is not undefined, then
  1. Return ? Call(splitter, separator, « O, limit »).
Let S be ? ToString(O).
Let A be ! ArrayCreate(0).
Let lengthA be 0.
If limit is undefined, let lim be 2³² - 1; else let lim be ? ToUint32(limit).
Let R be ? ToString(separator).
If lim = 0, return A.
If separator is undefined, then
1. Perform ! CreateDataPropertyOrThrow(A, "0", S).
2. Return A.
Let s be the length of S.
If s = 0, then
1. Let z be SplitMatch(S, 0, R).
2. If z is not false, return A.
3. Perform ! CreateDataPropertyOrThrow(A, "0", S).
4. Return A.
Let p be 0.
Let q be p.
Repeat, while q ≠ s
1. Let e be SplitMatch(S, q, R).
2. If e is false, set q to q + 1.
3. Else,
  1. Assert: e is an integer index ≤ s.
  2. If e = p, set q to q + 1.
  3. Else,
    1. Let T be the String value equal to the substring of S consisting of the code units at indices p (inclusive) through q (exclusive).
    2. Perform ! CreateDataPropertyOrThrow(A, ! ToString(lengthA), T).
    3. Set lengthA to lengthA + 1.
    4. If lengthA = lim, return A.
    5. Set p to e.
    6. Set q to p.
Let T be the String value equal to the substring of S consisting of the code units at indices p (inclusive) through s (exclusive).
Perform ! CreateDataPropertyOrThrow(A, ! ToString(lengthA), T).
Return A.

Note 1

The value of separator may be an empty String. In this case, separator does not match the empty substring at the beginning or end of the input String, nor does it match the empty substring at the end of the previous separator match. If separator is the empty String, the String is split up into individual code unit elements; the length of the result array equals the length of the String, and each substring contains one code unit.

If the this object is (or converts to) the empty String, the result depends on whether separator can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.

If separator is undefined, then the result array contains just one String, which is the this value (converted to a String). If limit is not undefined, then the output array is truncated so that it contains no more than limit elements.

Note 2

The split function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.20.1 Runtime Semantics: SplitMatch ( `S`, `q`, `R` )

The abstract operation SplitMatch takes arguments S (a String), q (an integer), and R (a String). It returns either false or the end index of a match. It performs the following steps when called:

Assert: Type(R) is String.
Let r be the number of code units in R.
Let s be the number of code units in S.
If q + r > s, return false.
If there exists an integer i between 0 (inclusive) and r (exclusive) such that the code unit at index q + i within S is different from the code unit at index i within R, return false.
Return q + r.

21.1.3.21 String.prototype.startsWith ( `searchString` [ , `position` ] )

The following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let isRegExp be ? IsRegExp(searchString).
If isRegExp is true, throw a TypeError exception.
Let searchStr be ? ToString(searchString).
Let pos be ? ToInteger(position).
Assert: If position is undefined, then pos is 0.
Let len be the length of S.
Let start be min(max(pos, 0), len).
Let searchLength be the length of searchStr.
If searchLength + start is greater than len, return false.
If the sequence of code units of S starting at start of length searchLength is the same as the full code unit sequence of searchStr, return true.
Otherwise, return false.

Note 1

This method returns true if the sequence of code units of searchString converted to a String is the same as the corresponding code units of this object (converted to a String) starting at index position. Otherwise returns false.

Note 2

Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.

Note 3

The startsWith function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.22 String.prototype.substring ( `start`, `end` )

The substring method takes two arguments, start and end, and returns a substring of the result of converting this object to a String, starting from index start and running to, but not including, index end of the String (or through the end of the String if end is undefined). The result is a String value, not a String object.

If either argument is NaN or negative, it is replaced with zero; if either argument is larger than the length of the String, it is replaced with the length of the String.

If start is larger than end, they are swapped.

The following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let len be the length of S.
Let intStart be ? ToInteger(start).
If end is undefined, let intEnd be len; else let intEnd be ? ToInteger(end).
Let finalStart be min(max(intStart, 0), len).
Let finalEnd be min(max(intEnd, 0), len).
Let from be min(finalStart, finalEnd).
Let to be max(finalStart, finalEnd).
Return the String value whose length is to - from, containing code units from S, namely the code units with indices from through to - 1, in ascending order.

Note

The substring function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.23 String.prototype.toLocaleLowerCase ( [ `reserved1` [ , `reserved2` ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the toLocaleLowerCase method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleLowerCase method is used.

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

This function works exactly the same as toLowerCase except that its result is intended to yield the correct result for the host environment's current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.

Note

The toLocaleLowerCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.24 String.prototype.toLocaleUpperCase ( [ `reserved1` [ , `reserved2` ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the toLocaleUpperCase method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleUpperCase method is used.

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

This function works exactly the same as toUpperCase except that its result is intended to yield the correct result for the host environment's current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.

Note

The toLocaleUpperCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.25 String.prototype.toLowerCase ( )

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4. The following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let sText be ! UTF16DecodeString(S).
Let lowerText be the result of toLowercase(sText), according to the Unicode Default Case Conversion algorithm.
Let L be ! UTF16Encode(lowerText).
Return L.

The result must be derived according to the locale-insensitive case mappings in the Unicode Character Database (this explicitly includes not only the UnicodeData.txt file, but also all locale-insensitive mappings in the SpecialCasings.txt file that accompanies it).

Note 1

The case mapping of some code points may produce multiple code points. In this case the result String may not be the same length as the source String. Because both toUpperCase and toLowerCase have context-sensitive behaviour, the functions are not symmetrical. In other words, s.toUpperCase().toLowerCase() is not necessarily equal to s.toLowerCase().

Note 2

The toLowerCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.26 String.prototype.toString ( )

When the toString method is called, the following steps are taken:

Return ? thisStringValue(this value).

Note

For a String object, the toString method happens to return the same thing as the valueOf method.

21.1.3.27 String.prototype.toUpperCase ( )

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

This function behaves in exactly the same way as String.prototype.toLowerCase, except that the String is mapped using the toUppercase algorithm of the Unicode Default Case Conversion.

Note

The toUpperCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.28 String.prototype.trim ( )

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

The following steps are taken:

Let S be the this value.
Return ? TrimString(S, start+end).

Note

The trim function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.28.1 Runtime Semantics: TrimString ( `string`, `where` )

The abstract operation TrimString takes arguments string and where. It interprets string as a sequence of UTF-16 encoded code points, as described in 6.1.4. It performs the following steps when called:

Let str be ? RequireObjectCoercible(string).
Let S be ? ToString(str).
If where is start, let T be the String value that is a copy of S with leading white space removed.
Else if where is end, let T be the String value that is a copy of S with trailing white space removed.
Else,
1. Assert: where is start+end.
2. Let T be the String value that is a copy of S with both leading and trailing white space removed.
Return T.

The definition of white space is the union of WhiteSpace and LineTerminator. When determining whether a Unicode code point is in Unicode general category “Space_Separator” (“Zs”), code unit sequences are interpreted as UTF-16 encoded code point sequences as specified in 6.1.4.

21.1.3.29 String.prototype.trimEnd ( )

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

The following steps are taken:

Let S be the this value.
Return ? TrimString(S, end).

Note

The trimEnd function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.30 String.prototype.trimStart ( )

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

The following steps are taken:

Let S be the this value.
Return ? TrimString(S, start).

Note

The trimStart function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.31 String.prototype.valueOf ( )

When the valueOf method is called, the following steps are taken:

Return ? thisStringValue(this value).

21.1.3.32 String.prototype [ @@iterator ] ( )

When the @@iterator method is called it returns an Iterator object (25.1.1.2) that iterates over the code points of a String value, returning each code point as a String value. The following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Return CreateStringIterator(S).

The value of the "name" property of this function is "[Symbol.iterator]".

21.1.4 Properties of String Instances

String instances are String exotic objects and have the internal methods specified for such objects. String instances inherit properties from the String prototype object. String instances also have a [[StringData]] internal slot.

String instances have a "length" property, and a set of enumerable properties with integer-indexed names.

21.1.4.1 length

The number of elements in the String value represented by this String object.

Once a String object is initialized, this property is unchanging. It has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.1.5 String Iterator Objects

A String Iterator is an object, that represents a specific iteration over some specific String instance object. There is not a named constructor for String Iterator objects. Instead, String iterator objects are created by calling certain methods of String instance objects.

21.1.5.1 CreateStringIterator ( `string` )

The abstract operation CreateStringIterator takes argument string. This operation is used to create iterator objects for String methods that return such iterators. It performs the following steps when called:

Assert: Type(string) is String.
Let iterator be OrdinaryObjectCreate(%StringIteratorPrototype%, « [[IteratedString]], [[StringNextIndex]] »).
Set iterator.[[IteratedString]] to string.
Set iterator.[[StringNextIndex]] to 0.
Return iterator.

21.1.5.2 The %StringIteratorPrototype% Object

The %StringIteratorPrototype% object:

has properties that are inherited by all String Iterator Objects.
is an ordinary object.
has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
has the following properties:

21.1.5.2.1 %StringIteratorPrototype%.next ( )

Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have all of the internal slots of a String Iterator Instance (21.1.5.3), throw a TypeError exception.
Let s be O.[[IteratedString]].
If s is undefined, return CreateIterResultObject(undefined, true).
Let position be O.[[StringNextIndex]].
Let len be the length of s.
If position ≥ len, then
1. Set O.[[IteratedString]] to undefined.
2. Return CreateIterResultObject(undefined, true).
Let cp be ! CodePointAt(s, position).
Let resultString be the String value containing cp.[[CodeUnitCount]] consecutive code units from s beginning with the code unit at index position.
Set O.[[StringNextIndex]] to position + cp.[[CodeUnitCount]].
Return CreateIterResultObject(resultString, false).

21.1.5.2.2 %StringIteratorPrototype% [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "String Iterator".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

21.1.5.3 Properties of String Iterator Instances

String Iterator instances are ordinary objects that inherit properties from the %StringIteratorPrototype% intrinsic object. String Iterator instances are initially created with the internal slots listed in Table 53.

Table 53: Internal Slots of String Iterator Instances

Internal Slot	Description
[[IteratedString]]	The String value whose code units are being iterated.
[[StringNextIndex]]	The integer index of the next string element (code unit) to be examined by this iterator.

21.2 RegExp (Regular Expression) Objects

A RegExp object contains a regular expression and the associated flags.

Note

The form and functionality of regular expressions is modelled after the regular expression facility in the Perl 5 programming language.

21.2.1 Patterns

The RegExp constructor applies the following grammar to the input pattern String. An error occurs if the grammar cannot interpret the String as an expansion of Pattern.

Syntax

[U, N]

[?U, ?N]

[U, N]

[?U, ?N]

[?U, ?N]

[?U, ?N]

[U, N]

[empty]

[?U, ?N]

[?U, ?N]

[U, N]

[?U, ?N]

[?U, ?N]

[?U, ?N]

[U, N]

(

[?U, ?N]

)

(

[?U, ?N]

)

(

[?U, ?N]

)

(

[?U, ?N]

)

QuantifierPrefix

{

}

{

}

{

}

[U, N]

[?U, ?N]

[?U]

(

GroupSpecifier

[?U]

[?U, ?N]

)

(

[?U, ?N]

)

one of

(

)

[

]

{

}

PatternCharacter

but not SyntaxCharacter

[U, N]

[?U]

[?U]

[+N]

[?U]

[U]

[lookahead ∉ DecimalDigit]

[?U]

[?U]

one of

one of

[U]

[empty]

[?U]

[U]

[?U]

[U]

[?U]

[?U]

[?U]

[U]

[+U]

[~U]

[U]

[+U]

[~U]

<ZWNJ>

<ZWJ>

[U]

[+U]

[+U]

[+U]

[+U]

[~U]

[+U]

}

any Unicode code point in the inclusive range 0xD800 to 0xDBFF

any Unicode code point in the inclusive range 0xDC00 to 0xDFFF

Each \u TrailSurrogate for which the choice of associated u LeadSurrogate is ambiguous shall be associated with the nearest possible u LeadSurrogate that would otherwise have no corresponding \u TrailSurrogate.

LeadSurrogate

but only if the SV of Hex4Digits is in the inclusive range 0xD800 to 0xDBFF

TrailSurrogate

but only if the SV of Hex4Digits is in the inclusive range 0xDC00 to 0xDFFF

NonSurrogate

but only if the SV of Hex4Digits is not in the inclusive range 0xD800 to 0xDFFF

[U]

[+U]

[+U]

[~U]

but not UnicodeIDContinue

opt

[lookahead ∉ DecimalDigit]

[U]

[+U]

}

[+U]

}

opt

opt

CharacterClass

[U]

[

[lookahead ≠ ^]

[?U]

]

[

[?U]

]

[U]

[empty]

[?U]

[U]

[?U]

[?U]

[?U]

[?U]

[?U]

[?U]

[U]

[?U]

[?U]

[?U]

[?U]

[?U]

[?U]

[U]

[?U]

[U]

but not one of \ or ] or -

[?U]

[U]

[+U]

[?U]

[?U]

21.2.1.1 Static Semantics: Early Errors

Pattern

It is a Syntax Error if NcapturingParens ≥ 2³² - 1.
It is a Syntax Error if Pattern contains multiple GroupSpecifiers whose enclosed RegExpIdentifierNames have the same StringValue.

QuantifierPrefix

{

}

It is a Syntax Error if the MV of the first DecimalDigits is larger than the MV of the second DecimalDigits.

GroupName

It is a Syntax Error if the enclosing Pattern does not contain a GroupSpecifier with an enclosed RegExpIdentifierName whose StringValue equals the StringValue of the RegExpIdentifierName of this production's GroupName.

It is a Syntax Error if the CapturingGroupNumber of DecimalEscape is larger than NcapturingParens (21.2.2.1).

NonemptyClassRanges

It is a Syntax Error if IsCharacterClass of the first ClassAtom is true or IsCharacterClass of the second ClassAtom is true.
It is a Syntax Error if IsCharacterClass of the first ClassAtom is false and IsCharacterClass of the second ClassAtom is false and the CharacterValue of the first ClassAtom is larger than the CharacterValue of the second ClassAtom.

It is a Syntax Error if IsCharacterClass of ClassAtomNoDash is true or IsCharacterClass of ClassAtom is true.
It is a Syntax Error if IsCharacterClass of ClassAtomNoDash is false and IsCharacterClass of ClassAtom is false and the CharacterValue of ClassAtomNoDash is larger than the CharacterValue of ClassAtom.

[U]

[+U]

It is a Syntax Error if the CharacterValue of RegExpUnicodeEscapeSequence is not the code point value of "$", "_", or some code point matched by the UnicodeIDStart lexical grammar production.

[U]

It is a Syntax Error if the result of performing UTF16DecodeSurrogatePair on the two code points matched by UnicodeLeadSurrogate and UnicodeTrailSurrogate respectively is not matched by the UnicodeIDStart lexical grammar production.

[U]

[+U]

It is a Syntax Error if the CharacterValue of RegExpUnicodeEscapeSequence is not the code point value of "$", "_", <ZWNJ>, <ZWJ>, or some code point matched by the UnicodeIDContinue lexical grammar production.

[U]

It is a Syntax Error if the result of performing UTF16DecodeSurrogatePair on the two code points matched by UnicodeLeadSurrogate and UnicodeTrailSurrogate respectively is not matched by the UnicodeIDContinue lexical grammar production.

It is a Syntax Error if the List of Unicode code points that is SourceText of UnicodePropertyName is not identical to a List of Unicode code points that is a Unicode property name or property alias listed in the “Property name and aliases” column of Table 55.
It is a Syntax Error if the List of Unicode code points that is SourceText of UnicodePropertyValue is not identical to a List of Unicode code points that is a value or value alias for the Unicode property or property alias given by SourceText of UnicodePropertyName listed in the “Property value and aliases” column of the corresponding tables Table 57 or Table 58.

It is a Syntax Error if the List of Unicode code points that is SourceText of LoneUnicodePropertyNameOrValue is not identical to a List of Unicode code points that is a Unicode general category or general category alias listed in the “Property value and aliases” column of Table 57, nor a binary property or binary property alias listed in the “Property name and aliases” column of Table 56.

21.2.1.2 Static Semantics: CapturingGroupNumber

Return the Number value for the MV of NonZeroDigit.

Let n be the mathematical integer number of code points in DecimalDigits.
Return the Number value for (the MV of NonZeroDigit ×_ℝ 10_ℝⁿ plus the MV of DecimalDigits).

The definitions of “the MV of NonZeroDigit” and “the MV of DecimalDigits” are in 11.8.3.

21.2.1.3 Static Semantics: IsCharacterClass

but not one of \ or ] or -

Return false.

Return true.

21.2.1.4 Static Semantics: CharacterValue

Return the code point value of U+002D (HYPHEN-MINUS).

but not one of \ or ] or -

Let ch be the code point matched by SourceCharacter.
Return the code point value of ch.

Return the code point value of U+0008 (BACKSPACE).

Return the code point value of U+002D (HYPHEN-MINUS).

ControlEscape

Return the code point value according to Table 54.

Table 54: ControlEscape Code Point Values

ControlEscape	Code Point Value	Code Point	Unicode Name	Symbol
`t`	9	`U+0009`	CHARACTER TABULATION	<HT>
`n`	10	`U+000A`	LINE FEED (LF)	<LF>
`v`	11	`U+000B`	LINE TABULATION	<VT>
`f`	12	`U+000C`	FORM FEED (FF)	<FF>
`r`	13	`U+000D`	CARRIAGE RETURN (CR)	<CR>

Let ch be the code point matched by ControlLetter.
Let i be ch's code point value.
Return the remainder of dividing i by 32.

[lookahead ∉ DecimalDigit]

Return the code point value of U+0000 (NULL).

Note

\0 represents the <NUL> character and cannot be followed by a decimal digit.

Return the numeric value of the code unit that is the SV of HexEscapeSequence.

LeadSurrogate

TrailSurrogate

Let lead be the CharacterValue of LeadSurrogate.
Let trail be the CharacterValue of TrailSurrogate.
Let cp be UTF16DecodeSurrogatePair(lead, trail).
Return the code point value of cp.

Return the Number value for the MV of Hex4Digits.

}

Return the Number value for the MV of CodePoint.

Return the Number value for the MV of HexDigits.

Let ch be the code point matched by IdentityEscape.
Return the code point value of ch.

21.2.1.5 Static Semantics: SourceText

opt

opt

Return the List, in source text order, of Unicode code points in the source text matched by this production.

21.2.1.6 Static Semantics: StringValue

[U]

[?U]

[?U]

[?U]

Let idText be the source text matched by RegExpIdentifierName.
Let idTextUnescaped be the result of replacing any occurrences of \ RegExpUnicodeEscapeSequence in idText with the code point represented by the RegExpUnicodeEscapeSequence.
Return ! UTF16Encode(idTextUnescaped).

21.2.2 Pattern Semantics

A regular expression pattern is converted into an Abstract Closure using the process described below. An implementation is encouraged to use more efficient algorithms than the ones listed below, as long as the results are the same. The Abstract Closure is used as the value of a RegExp object's [[RegExpMatcher]] internal slot.

A Pattern is either a BMP pattern or a Unicode pattern depending upon whether or not its associated flags contain a u. A BMP pattern matches against a String interpreted as consisting of a sequence of 16-bit values that are Unicode code points in the range of the Basic Multilingual Plane. A Unicode pattern matches against a String interpreted as consisting of Unicode code points encoded using UTF-16. In the context of describing the behaviour of a BMP pattern “character” means a single 16-bit Unicode BMP code point. In the context of describing the behaviour of a Unicode pattern “character” means a UTF-16 encoded code point (6.1.4). In either context, “character value” means the numeric value of the corresponding non-encoded code point.

The syntax and semantics of Pattern is defined as if the source code for the Pattern was a List of SourceCharacter values where each SourceCharacter corresponds to a Unicode code point. If a BMP pattern contains a non-BMP SourceCharacter the entire pattern is encoded using UTF-16 and the individual code units of that encoding are used as the elements of the List.

Note

For example, consider a pattern expressed in source text as the single non-BMP character U+1D11E (MUSICAL SYMBOL G CLEF). Interpreted as a Unicode pattern, it would be a single element (character) List consisting of the single code point 0x1D11E. However, interpreted as a BMP pattern, it is first UTF-16 encoded to produce a two element List consisting of the code units 0xD834 and 0xDD1E.

Patterns are passed to the RegExp constructor as ECMAScript String values in which non-BMP characters are UTF-16 encoded. For example, the single character MUSICAL SYMBOL G CLEF pattern, expressed as a String value, is a String of length 2 whose elements were the code units 0xD834 and 0xDD1E. So no further translation of the string would be necessary to process it as a BMP pattern consisting of two pattern characters. However, to process it as a Unicode pattern UTF16DecodeSurrogatePair must be used in producing a List consisting of a single pattern character, the code point U+1D11E.

An implementation may not actually perform such translations to or from UTF-16, but the semantics of this specification requires that the result of pattern matching be as if such translations were performed.

21.2.2.1 Notation

The descriptions below use the following aliases:

Input is a List consisting of all of the characters, in order, of the String being matched by the regular expression pattern. Each character is either a code unit or a code point, depending upon the kind of pattern involved. The notation Input[n] means the n^th character of Input, where n can range between 0 (inclusive) and InputLength (exclusive).
InputLength is the number of characters in Input.
NcapturingParens is the total number of left-capturing parentheses (i.e. the total number of Atom::(GroupSpecifierDisjunction) Parse Nodes) in the pattern. A left-capturing parenthesis is any ( pattern character that is matched by the ( terminal of the Atom::(GroupSpecifierDisjunction) production.
DotAll is true if the RegExp object's [[OriginalFlags]] internal slot contains "s" and otherwise is false.
IgnoreCase is true if the RegExp object's [[OriginalFlags]] internal slot contains "i" and otherwise is false.
Multiline is true if the RegExp object's [[OriginalFlags]] internal slot contains "m" and otherwise is false.
Unicode is true if the RegExp object's [[OriginalFlags]] internal slot contains "u" and otherwise is false.

Furthermore, the descriptions below use the following internal data structures:

A CharSet is a mathematical set of characters, either code units or code points depending up the state of the Unicode flag. “All characters” means either all code unit values or all code point values also depending upon the state of Unicode.
A State is an ordered pair (endIndex, captures) where endIndex is an integer and captures is a List of NcapturingParens values. States are used to represent partial match states in the regular expression matching algorithms. The endIndex is one plus the index of the last input character matched so far by the pattern, while captures holds the results of capturing parentheses. The n^th element of captures is either a List that represents the value obtained by the n^th set of capturing parentheses or undefined if the n^th set of capturing parentheses hasn't been reached yet. Due to backtracking, many States may be in use at any time during the matching process.
A MatchResult is either a State or the special token failure that indicates that the match failed.
A Continuation is an Abstract Closure that takes one State argument and returns a MatchResult result. The Continuation attempts to match the remaining portion (specified by the closure's captured values) of the pattern against Input, starting at the intermediate state given by its State argument. If the match succeeds, the Continuation returns the final State that it reached; if the match fails, the Continuation returns failure.
A Matcher is an Abstract Closure that takes two arguments—a State and a Continuation—and returns a MatchResult result. A Matcher attempts to match a middle subpattern (specified by the closure's captured values) of the pattern against Input, starting at the intermediate state given by its State argument. The Continuation argument should be a closure that matches the rest of the pattern. After matching the subpattern of a pattern to obtain a new State, the Matcher then calls Continuation on that new State to test if the rest of the pattern can match as well. If it can, the Matcher returns the State returned by Continuation; if not, the Matcher may try different choices at its choice points, repeatedly calling Continuation until it either succeeds or all possibilities have been exhausted.

21.2.2.2 Pattern

The production Pattern::Disjunction evaluates as follows:

Evaluate Disjunction with +1 as its direction argument to obtain a Matcher m.
Return a new Abstract Closure with parameters (str, index) that captures m and performs the following steps when called:
1. Assert: Type(str) is String.
2. Assert: ! IsNonNegativeInteger(index) is true and index ≤ the length of str.
3. If Unicode is true, let Input be a List consisting of the sequence of code points of ! UTF16DecodeString(str). Otherwise, let Input be a List consisting of the sequence of code units that are the elements of str. Input will be used throughout the algorithms in 21.2.2. Each element of Input is considered to be a character.
4. Let InputLength be the number of characters contained in Input. This alias will be used throughout the algorithms in 21.2.2.
5. Let listIndex be the index into Input of the character that was obtained from element index of str.
6. Let c be a new Continuation with parameters (y) that captures nothing and performs the following steps when called:
  1. Assert: y is a State.
  2. Return y.
7. Let cap be a List of NcapturingParens undefined values, indexed 1 through NcapturingParens.
8. Let x be the State (listIndex, cap).
9. Call m(x, c) and return its result.

Note

A Pattern evaluates (“compiles”) to an Abstract Closure value. RegExpBuiltinExec can then apply this procedure to a String and an offset within the String to determine whether the pattern would match starting at exactly that offset within the String, and, if it does match, what the values of the capturing parentheses would be. The algorithms in 21.2.2 are designed so that compiling a pattern may throw a SyntaxError exception; on the other hand, once the pattern is successfully compiled, applying the resulting Abstract Closure to find a match in a String cannot throw an exception (except for any host-defined exceptions that can occur anywhere such as out-of-memory).

21.2.2.3 Disjunction

With parameter direction.

The production Disjunction::Alternative evaluates as follows:

Evaluate Alternative with argument direction to obtain a Matcher m.
Return m.

The production Disjunction::Alternative|Disjunction evaluates as follows:

Evaluate Alternative with argument direction to obtain a Matcher m1.
Evaluate Disjunction with argument direction to obtain a Matcher m2.
Return a new Matcher with parameters (x, c) that captures m1 and m2 and performs the following steps when called:
1. Assert: x is a State.
2. Assert: c is a Continuation.
3. Call m1(x, c) and let r be its result.
4. If r is not failure, return r.
5. Call m2(x, c) and return its result.

Note

The | regular expression operator separates two alternatives. The pattern first tries to match the left Alternative (followed by the sequel of the regular expression); if it fails, it tries to match the right Disjunction (followed by the sequel of the regular expression). If the left Alternative, the right Disjunction, and the sequel all have choice points, all choices in the sequel are tried before moving on to the next choice in the left Alternative. If choices in the left Alternative are exhausted, the right Disjunction is tried instead of the left Alternative. Any capturing parentheses inside a portion of the pattern skipped by | produce undefined values instead of Strings. Thus, for example,

/a|ab/.exec("abc")

returns the result "a" and not "ab". Moreover,

/((a)|(ab))((c)|(bc))/.exec("abc")

returns the array

["abc", "a", "a", undefined, "bc", undefined, "bc"]

and not

["abc", "ab", undefined, "ab", "c", "c", undefined]

The order in which the two alternatives are tried is independent of the value of direction.

21.2.2.4 Alternative

With parameter direction.

The production Alternative::[empty] evaluates as follows:

Return a new Matcher with parameters (x, c) that captures nothing and performs the following steps when called:
1. Assert: x is a State.
2. Assert: c is a Continuation.
3. Call c(x) and return its result.

The production Alternative::AlternativeTerm evaluates as follows:

Evaluate Alternative with argument direction to obtain a Matcher m1.
Evaluate Term with argument direction to obtain a Matcher m2.
If direction is equal to +1, then
1. Return a new Matcher with parameters (x, c) that captures m1 and m2 and performs the following steps when called:
  1. Assert: x is a State.
  2. Assert: c is a Continuation.
  3. Let d be a new Continuation with parameters (y) that captures c and m2 and performs the following steps when called:
    1. Assert: y is a State.
    2. Call m2(y, c) and return its result.
  4. Call m1(x, d) and return its result.
Else,
1. Assert: direction is equal to -1.
2. Return a new Matcher with parameters (x, c) that captures m1 and m2 and performs the following steps when called:
  1. Assert: x is a State.
  2. Assert: c is a Continuation.
  3. Let d be a new Continuation with parameters (y) that captures c and m1 and performs the following steps when called:
    1. Assert: y is a State.
    2. Call m1(y, c) and return its result.
  4. Call m2(x, d) and return its result.

Note

Consecutive Terms try to simultaneously match consecutive portions of Input. When direction is equal to +1, if the left Alternative, the right Term, and the sequel of the regular expression all have choice points, all choices in the sequel are tried before moving on to the next choice in the right Term, and all choices in the right Term are tried before moving on to the next choice in the left Alternative. When direction is equal to -1, the evaluation order of Alternative and Term are reversed.

21.2.2.5 Term

With parameter direction.

The production Term::Assertion evaluates as follows:

Return the Matcher that is the result of evaluating Assertion.

Note

The resulting Matcher is independent of direction.

The production Term::Atom evaluates as follows:

Return the Matcher that is the result of evaluating Atom with argument direction.

The production Term::AtomQuantifier evaluates as follows:

Evaluate Atom with argument direction to obtain a Matcher m.
Evaluate Quantifier to obtain the three results: an integer min, an integer (or ∞) max, and Boolean greedy.
Assert: If max is finite, then max is not less than min.
Let parenIndex be the number of left-capturing parentheses in the entire regular expression that occur to the left of this Term. This is the total number of Atom::(GroupSpecifierDisjunction) Parse Nodes prior to or enclosing this Term.
Let parenCount be the number of left-capturing parentheses in Atom. This is the total number of Atom::(GroupSpecifierDisjunction) Parse Nodes enclosed by Atom.
Return a new Matcher with parameters (x, c) that captures m, min, max, greedy, parenIndex, and parenCount and performs the following steps when called:
1. Assert: x is a State.
2. Assert: c is a Continuation.
3. Call RepeatMatcher(m, min, max, greedy, x, c, parenIndex, parenCount) and return its result.

21.2.2.5.1 Runtime Semantics: RepeatMatcher ( `m`, `min`, `max`, `greedy`, `x`, `c`, `parenIndex`, `parenCount` )

The abstract operation RepeatMatcher takes arguments m (a Matcher), min (an integer), max (an integer or ∞), greedy (a Boolean), x (a State), c (a Continuation), parenIndex (an integer), and parenCount (an integer). It performs the following steps when called:

If max is zero, return c(x).
Let d be a new Continuation with parameters (y) that captures m, min, max, greedy, x, c, parenIndex, and parenCount and performs the following steps when called:
1. Assert: y is a State.
2. If min is zero and y's endIndex is equal to x's endIndex, return failure.
3. If min is zero, let min2 be zero; otherwise let min2 be min - 1.
4. If max is ∞, let max2 be ∞; otherwise let max2 be max - 1.
5. Call RepeatMatcher(m, min2, max2, greedy, y, c, parenIndex, parenCount) and return its result.
Let cap be a copy of x's captures List.
For each integer k that satisfies parenIndex < k and k ≤ parenIndex + parenCount, set cap[k] to undefined.
Let e be x's endIndex.
Let xr be the State (e, cap).
If min is not zero, return m(xr, d).
If greedy is false, then
1. Call c(x) and let z be its result.
2. If z is not failure, return z.
3. Call m(xr, d) and return its result.
Call m(xr, d) and let z be its result.
If z is not failure, return z.
Call c(x) and return its result.

Note 1

An Atom followed by a Quantifier is repeated the number of times specified by the Quantifier. A Quantifier can be non-greedy, in which case the Atom pattern is repeated as few times as possible while still matching the sequel, or it can be greedy, in which case the Atom pattern is repeated as many times as possible while still matching the sequel. The Atom pattern is repeated rather than the input character sequence that it matches, so different repetitions of the Atom can match different input substrings.

Note 2

If the Atom and the sequel of the regular expression all have choice points, the Atom is first matched as many (or as few, if non-greedy) times as possible. All choices in the sequel are tried before moving on to the next choice in the last repetition of Atom. All choices in the last (n^th) repetition of Atom are tried before moving on to the next choice in the next-to-last (n - 1)^st repetition of Atom; at which point it may turn out that more or fewer repetitions of Atom are now possible; these are exhausted (again, starting with either as few or as many as possible) before moving on to the next choice in the (n - 1)^st repetition of Atom and so on.

Compare

/a[a-z]{2,4}/.exec("abcdefghi")

which returns "abcde" with

/a[a-z]{2,4}?/.exec("abcdefghi")

which returns "abc".

Consider also

/(aa|aabaac|ba|b|c)*/.exec("aabaac")

which, by the choice point ordering above, returns the array

["aaba", "ba"]

and not any of:

["aabaac", "aabaac"]
["aabaac", "c"]

The above ordering of choice points can be used to write a regular expression that calculates the greatest common divisor of two numbers (represented in unary notation). The following example calculates the gcd of 10 and 15:

"aaaaaaaaaa,aaaaaaaaaaaaaaa".replace(/^(a+)\1*,\1+$/, "$1")

which returns the gcd in unary notation "aaaaa".

Note 3

Step 4 of the RepeatMatcher clears Atom's captures each time Atom is repeated. We can see its behaviour in the regular expression

/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")

which returns the array

["zaacbbbcac", "z", "ac", "a", undefined, "c"]

and not

["zaacbbbcac", "z", "ac", "a", "bbb", "c"]

because each iteration of the outermost * clears all captured Strings contained in the quantified Atom, which in this case includes capture Strings numbered 2, 3, 4, and 5.

Note 4

Step 2.b of the RepeatMatcher states that once the minimum number of repetitions has been satisfied, any more expansions of Atom that match the empty character sequence are not considered for further repetitions. This prevents the regular expression engine from falling into an infinite loop on patterns such as:

/(a*)*/.exec("b")

or the slightly more complicated:

/(a*)b\1+/.exec("baaaac")

which returns the array

["b", ""]

21.2.2.6 Assertion

The production Assertion::^ evaluates as follows:

Return a new Matcher with parameters (x, c) that captures nothing and performs the following steps when called:
1. Assert: x is a State.
2. Assert: c is a Continuation.
3. Let e be x's endIndex.
4. If e is zero, or if Multiline is true and the character Input[e - 1] is one of LineTerminator, then
  1. Call c(x) and return its result.
5. Return failure.

Note

Even when the y flag is used with a pattern, ^ always matches only at the beginning of Input, or (if Multiline is true) at the beginning of a line.

The production Assertion::$ evaluates as follows:

Return a new Matcher with parameters (x, c) that captures nothing and performs the following steps when called:
1. Assert: x is a State.
2. Assert: c is a Continuation.
3. Let e be x's endIndex.
4. If e is equal to InputLength, or if Multiline is true and the character Input[e] is one of LineTerminator, then
  1. Call c(x) and return its result.
5. Return failure.

The production Assertion::\b evaluates as follows:

Return a new Matcher with parameters (x, c) that captures nothing and performs the following steps when called:
1. Assert: x is a State.
2. Assert: c is a Continuation.
3. Let e be x's endIndex.
4. Call IsWordChar(e - 1) and let a be the Boolean result.
5. Call IsWordChar(e) and let b be the Boolean result.
6. If a is true and b is false, or if a is false and b is true, then
  1. Call c(x) and return its result.
7. Return failure.

The production Assertion::\B evaluates as follows:

Return a new Matcher with parameters (x, c) that captures nothing and performs the following steps when called:
1. Assert: x is a State.
2. Assert: c is a Continuation.
3. Let e be x's endIndex.
4. Call IsWordChar(e - 1) and let a be the Boolean result.
5. Call IsWordChar(e) and let b be the Boolean result.
6. If a is true and b is true, or if a is false and b is false, then
  1. Call c(x) and return its result.
7. Return failure.

The production Assertion::(?=Disjunction) evaluates as follows:

Evaluate Disjunction with +1 as its direction argument to obtain a Matcher m.
Return a new Matcher with parameters (x, c) that captures m and performs the following steps when called:
1. Assert: x is a State.
2. Assert: c is a Continuation.
3. Let d be a new Continuation with parameters (y) that captures nothing and performs the following steps when called:
  1. Assert: y is a State.
  2. Return y.
4. Call m(x, d) and let r be its result.
5. If r is failure, return failure.
6. Let y be r's State.
7. Let cap be y's captures List.
8. Let xe be x's endIndex.
9. Let z be the State (xe, cap).
10. Call c(z) and return its result.

The production Assertion::(?!Disjunction) evaluates as follows:

Evaluate Disjunction with +1 as its direction argument to obtain a Matcher m.
Return a new Matcher with parameters (x, c) that captures m and performs the following steps when called:
1. Assert: x is a State.
2. Assert: c is a Continuation.
3. Let d be a new Continuation with parameters (y) that captures nothing and performs the following steps when called:
  1. Assert: y is a State.
  2. Return y.
4. Call m(x, d) and let r be its result.
5. If r is not failure, return failure.
6. Call c(x) and return its result.

The production Assertion::(?<=Disjunction) evaluates as follows:

Evaluate Disjunction with -1 as its direction argument to obtain a Matcher m.
Return a new Matcher with parameters (x, c) that captures m and performs the following steps when called:
1. Assert: x is a State.
2. Assert: c is a Continuation.
3. Let d be a new Continuation with parameters (y) that captures nothing and performs the following steps when called:
  1. Assert: y is a State.
  2. Return y.
4. Call m(x, d) and let r be its result.
5. If r is failure, return failure.
6. Let y be r's State.
7. Let cap be y's captures List.
8. Let xe be x's endIndex.
9. Let z be the State (xe, cap).
10. Call c(z) and return its result.

The production Assertion::(?<!Disjunction) evaluates as follows:

Evaluate Disjunction with -1 as its direction argument to obtain a Matcher m.
Return a new Matcher with parameters (x, c) that captures m and performs the following steps when called:
1. Assert: x is a State.
2. Assert: c is a Continuation.
3. Let d be a new Continuation with parameters (y) that captures nothing and performs the following steps when called:
  1. Assert: y is a State.
  2. Return y.
4. Call m(x, d) and let r be its result.
5. If r is not failure, return failure.
6. Call c(x) and return its result.

21.2.2.6.1 Runtime Semantics: WordCharacters ( )

The abstract operation WordCharacters takes no arguments. It performs the following steps when called:

Let A be a set of characters containing the sixty-three characters:

a b c d e f g h i j k l m n o p q r s t u v w x y z

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

0 1 2 3 4 5 6 7 8 9 _

Let U be an empty set.
For each character c not in set A where Canonicalize(c) is in A, add c to U.
Assert: Unless Unicode and IgnoreCase are both true, U is empty.
Add the characters in set U to set A.
Return A.

21.2.2.6.2 Runtime Semantics: IsWordChar ( `e` )

The abstract operation IsWordChar takes argument e (an integer). It performs the following steps when called:

If e is -1 or e is InputLength, return false.
Let c be the character Input[e].
Let wordChars be the result of ! WordCharacters().
If c is in wordChars, return true.
Return false.

21.2.2.7 Quantifier

The production Quantifier::QuantifierPrefix evaluates as follows:

Evaluate QuantifierPrefix to obtain the two results: an integer min and an integer (or ∞) max.
Return the three results min, max, and true.

The production Quantifier::QuantifierPrefix? evaluates as follows:

Evaluate QuantifierPrefix to obtain the two results: an integer min and an integer (or ∞) max.
Return the three results min, max, and false.

The production QuantifierPrefix::* evaluates as follows:

Return the two results 0 and ∞.

The production QuantifierPrefix::+ evaluates as follows:

Return the two results 1 and ∞.

The production QuantifierPrefix::? evaluates as follows:

Return the two results 0 and 1.

The production QuantifierPrefix::{DecimalDigits} evaluates as follows:

Let i be the MV of DecimalDigits (see 11.8.3).
Return the two results i and i.

The production QuantifierPrefix::{DecimalDigits,} evaluates as follows:

Let i be the MV of DecimalDigits.
Return the two results i and ∞.

The production QuantifierPrefix::{DecimalDigits,DecimalDigits} evaluates as follows:

Let i be the MV of the first DecimalDigits.
Let j be the MV of the second DecimalDigits.
Return the two results i and j.

21.2.2.8 Atom

With parameter direction.

The production Atom::PatternCharacter evaluates as follows:

Let ch be the character matched by PatternCharacter.
Let A be a one-element CharSet containing the character ch.
Call CharacterSetMatcher(A, false, direction) and return its Matcher result.

The production Atom::. evaluates as follows:

If DotAll is true, then
1. Let A be the set of all characters.
Otherwise, let A be the set of all characters except LineTerminator.
Call CharacterSetMatcher(A, false, direction) and return its Matcher result.

The production Atom::\AtomEscape evaluates as follows:

Return the Matcher that is the result of evaluating AtomEscape with argument direction.

The production Atom::CharacterClass evaluates as follows:

Evaluate CharacterClass to obtain a CharSet A and a Boolean invert.
Call CharacterSetMatcher(A, invert, direction) and return its Matcher result.

The production Atom::(GroupSpecifierDisjunction) evaluates as follows:

Evaluate Disjunction with argument direction to obtain a Matcher m.
Let parenIndex be the number of left-capturing parentheses in the entire regular expression that occur to the left of this Atom. This is the total number of Atom::(GroupSpecifierDisjunction) Parse Nodes prior to or enclosing this Atom.
Return a new Matcher with parameters (x, c) that captures direction, m, and parenIndex and performs the following steps when called:
1. Assert: x is a State.
2. Assert: c is a Continuation.
3. Let d be a new Continuation with parameters (y) that captures x, c, direction, and parenIndex and performs the following steps when called:
  1. Assert: y is a State.
  2. Let cap be a copy of y's captures List.
  3. Let xe be x's endIndex.
  4. Let ye be y's endIndex.
  5. If direction is equal to +1, then
    1. Assert: xe ≤ ye.
    2. Let s be a new List whose elements are the characters of Input at indices xe (inclusive) through ye (exclusive).
  6. Else,
    1. Assert: direction is equal to -1.
    2. Assert: ye ≤ xe.
    3. Let s be a new List whose elements are the characters of Input at indices ye (inclusive) through xe (exclusive).
  7. Set cap[parenIndex + 1] to s.
  8. Let z be the State (ye, cap).
  9. Call c(z) and return its result.
4. Call m(x, d) and return its result.

The production Atom::(?:Disjunction) evaluates as follows:

Return the Matcher that is the result of evaluating Disjunction with argument direction.

21.2.2.8.1 Runtime Semantics: CharacterSetMatcher ( `A`, `invert`, `direction` )

The abstract operation CharacterSetMatcher takes arguments A (a CharSet), invert (a Boolean), and direction (an integer). It performs the following steps when called:

Return a new Matcher with parameters (x, c) that captures A, invert, and direction and performs the following steps when called:
1. Assert: x is a State.
2. Assert: c is a Continuation.
3. Let e be x's endIndex.
4. Let f be e + direction.
5. If f < 0 or f > InputLength, return failure.
6. Let index be min(e, f).
7. Let ch be the character Input[index].
8. Let cc be Canonicalize(ch).
9. If invert is false, then
  1. If there does not exist a member a of set A such that Canonicalize(a) is cc, return failure.
10. Else,
  1. Assert: invert is true.
  2. If there exists a member a of set A such that Canonicalize(a) is cc, return failure.
11. Let cap be x's captures List.
12. Let y be the State (f, cap).
13. Call c(y) and return its result.

21.2.2.8.2 Runtime Semantics: Canonicalize ( `ch` )

The abstract operation Canonicalize takes argument ch (a character). It performs the following steps when called:

If IgnoreCase is false, return ch.
If Unicode is true, then
1. If the file CaseFolding.txt of the Unicode Character Database provides a simple or common case folding mapping for ch, return the result of applying that mapping to ch.
2. Return ch.
Else,
1. Assert: ch is a UTF-16 code unit.
2. Let s be the String value consisting of the single code unit ch.
3. Let u be the same result produced as if by performing the algorithm for String.prototype.toUpperCase using s as the this value.
4. Assert: Type(u) is String.
5. If u does not consist of a single code unit, return ch.
6. Let cu be u's single code unit element.
7. If the numeric value of ch ≥ 128 and the numeric value of cu < 128, return ch.
8. Return cu.

Note 1

Parentheses of the form ( Disjunction ) serve both to group the components of the Disjunction pattern together and to save the result of the match. The result can be used either in a backreference (\ followed by a nonzero decimal number), referenced in a replace String, or returned as part of an array from the regular expression matching Abstract Closure. To inhibit the capturing behaviour of parentheses, use the form (?: Disjunction ) instead.

Note 2

The form (?= Disjunction ) specifies a zero-width positive lookahead. In order for it to succeed, the pattern inside Disjunction must match at the current position, but the current position is not advanced before matching the sequel. If Disjunction can match at the current position in several ways, only the first one is tried. Unlike other regular expression operators, there is no backtracking into a (?= form (this unusual behaviour is inherited from Perl). This only matters when the Disjunction contains capturing parentheses and the sequel of the pattern contains backreferences to those captures.

For example,

/(?=(a+))/.exec("baaabac")

matches the empty String immediately after the first b and therefore returns the array:

["", "aaa"]

To illustrate the lack of backtracking into the lookahead, consider:

/(?=(a+))a*b\1/.exec("baaabac")

This expression returns

["aba", "a"]

and not:

["aaaba", "a"]

Note 3

The form (?! Disjunction ) specifies a zero-width negative lookahead. In order for it to succeed, the pattern inside Disjunction must fail to match at the current position. The current position is not advanced before matching the sequel. Disjunction can contain capturing parentheses, but backreferences to them only make sense from within Disjunction itself. Backreferences to these capturing parentheses from elsewhere in the pattern always return undefined because the negative lookahead must fail for the pattern to succeed. For example,

/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")

looks for an a not immediately followed by some positive number n of a's, a b, another n a's (specified by the first \2) and a c. The second \2 is outside the negative lookahead, so it matches against undefined and therefore always succeeds. The whole expression returns the array:

["baaabaac", "ba", undefined, "abaac"]

Note 4

In case-insignificant matches when Unicode is true, all characters are implicitly case-folded using the simple mapping provided by the Unicode standard immediately before they are compared. The simple mapping always maps to a single code point, so it does not map, for example, ß (U+00DF) to SS. It may however map a code point outside the Basic Latin range to a character within, for example, ſ (U+017F) to s. Such characters are not mapped if Unicode is false. This prevents Unicode code points such as U+017F and U+212A from matching regular expressions such as /[a-z]/i, but they will match /[a-z]/ui.

21.2.2.8.3 Runtime Semantics: UnicodeMatchProperty ( `p` )

The abstract operation UnicodeMatchProperty takes argument p (a List of Unicode code points). It performs the following steps when called:

Assert: p is a List of Unicode code points that is identical to a List of Unicode code points that is a Unicode property name or property alias listed in the “Property name and aliases” column of Table 55 or Table 56.
Let c be the canonical property name of p as given in the “Canonical property name” column of the corresponding row.
Return the List of Unicode code points of c.

Implementations must support the Unicode property names and aliases listed in Table 55 and Table 56. To ensure interoperability, implementations must not support any other property names or aliases.

Note 1

For example, Script_Extensions (property name) and scx (property alias) are valid, but script_extensions or Scx aren't.

Note 2

The listed properties form a superset of what UTS18 RL1.2 requires.

Table 55: Non-binary Unicode property aliases and their canonical property names

Property name and aliases	Canonical property name
`General_Category` `gc`	`General_Category`
`Script` `sc`	`Script`
`Script_Extensions` `scx`	`Script_Extensions`

Table 56: Binary Unicode property aliases and their canonical property names

Property name and aliases	Canonical property name
`ASCII`	`ASCII`
`ASCII_Hex_Digit` `AHex`	`ASCII_Hex_Digit`
`Alphabetic` `Alpha`	`Alphabetic`
`Any`	`Any`
`Assigned`	`Assigned`
`Bidi_Control` `Bidi_C`	`Bidi_Control`
`Bidi_Mirrored` `Bidi_M`	`Bidi_Mirrored`
`Case_Ignorable` `CI`	`Case_Ignorable`
`Cased`	`Cased`
`Changes_When_Casefolded` `CWCF`	`Changes_When_Casefolded`
`Changes_When_Casemapped` `CWCM`	`Changes_When_Casemapped`
`Changes_When_Lowercased` `CWL`	`Changes_When_Lowercased`
`Changes_When_NFKC_Casefolded` `CWKCF`	`Changes_When_NFKC_Casefolded`
`Changes_When_Titlecased` `CWT`	`Changes_When_Titlecased`
`Changes_When_Uppercased` `CWU`	`Changes_When_Uppercased`
`Dash`	`Dash`
`Default_Ignorable_Code_Point` `DI`	`Default_Ignorable_Code_Point`
`Deprecated` `Dep`	`Deprecated`
`Diacritic` `Dia`	`Diacritic`
`Emoji`	`Emoji`
`Emoji_Component`	`Emoji_Component`
`Emoji_Modifier`	`Emoji_Modifier`
`Emoji_Modifier_Base`	`Emoji_Modifier_Base`
`Emoji_Presentation`	`Emoji_Presentation`
`Extended_Pictographic`	`Extended_Pictographic`
`Extender` `Ext`	`Extender`
`Grapheme_Base` `Gr_Base`	`Grapheme_Base`
`Grapheme_Extend` `Gr_Ext`	`Grapheme_Extend`
`Hex_Digit` `Hex`	`Hex_Digit`
`IDS_Binary_Operator` `IDSB`	`IDS_Binary_Operator`
`IDS_Trinary_Operator` `IDST`	`IDS_Trinary_Operator`
`ID_Continue` `IDC`	`ID_Continue`
`ID_Start` `IDS`	`ID_Start`
`Ideographic` `Ideo`	`Ideographic`
`Join_Control` `Join_C`	`Join_Control`
`Logical_Order_Exception` `LOE`	`Logical_Order_Exception`
`Lowercase` `Lower`	`Lowercase`
`Math`	`Math`
`Noncharacter_Code_Point` `NChar`	`Noncharacter_Code_Point`
`Pattern_Syntax` `Pat_Syn`	`Pattern_Syntax`
`Pattern_White_Space` `Pat_WS`	`Pattern_White_Space`
`Quotation_Mark` `QMark`	`Quotation_Mark`
`Radical`	`Radical`
`Regional_Indicator` `RI`	`Regional_Indicator`
`Sentence_Terminal` `STerm`	`Sentence_Terminal`
`Soft_Dotted` `SD`	`Soft_Dotted`
`Terminal_Punctuation` `Term`	`Terminal_Punctuation`
`Unified_Ideograph` `UIdeo`	`Unified_Ideograph`
`Uppercase` `Upper`	`Uppercase`
`Variation_Selector` `VS`	`Variation_Selector`
`White_Space` `space`	`White_Space`
`XID_Continue` `XIDC`	`XID_Continue`
`XID_Start` `XIDS`	`XID_Start`

21.2.2.8.4 Runtime Semantics: UnicodeMatchPropertyValue ( `p`, `v` )

The abstract operation UnicodeMatchPropertyValue takes arguments p (a List of Unicode code points) and v (a List of Unicode code points). It performs the following steps when called:

Assert: p is a List of Unicode code points that is identical to a List of Unicode code points that is a canonical, unaliased Unicode property name listed in the “Canonical property name” column of Table 55.
Assert: v is a List of Unicode code points that is identical to a List of Unicode code points that is a property value or property value alias for Unicode property p listed in the “Property value and aliases” column of Table 57 or Table 58.
Let value be the canonical property value of v as given in the “Canonical property value” column of the corresponding row.
Return the List of Unicode code points of value.

Implementations must support the Unicode property value names and aliases listed in Table 57 and Table 58. To ensure interoperability, implementations must not support any other property value names or aliases.

Note 1

For example, Xpeo and Old_Persian are valid Script_Extensions values, but xpeo and Old Persian aren't.

Note 2

This algorithm differs from the matching rules for symbolic values listed in UAX44: case, white space, U+002D (HYPHEN-MINUS), and U+005F (LOW LINE) are not ignored, and the Is prefix is not supported.

Table 57: Value aliases and canonical values for the Unicode property General_Category

Property value and aliases	Canonical property value
`Cased_Letter` `LC`	`Cased_Letter`
`Close_Punctuation` `Pe`	`Close_Punctuation`
`Connector_Punctuation` `Pc`	`Connector_Punctuation`
`Control` `Cc` `cntrl`	`Control`
`Currency_Symbol` `Sc`	`Currency_Symbol`
`Dash_Punctuation` `Pd`	`Dash_Punctuation`
`Decimal_Number` `Nd` `digit`	`Decimal_Number`
`Enclosing_Mark` `Me`	`Enclosing_Mark`
`Final_Punctuation` `Pf`	`Final_Punctuation`
`Format` `Cf`	`Format`
`Initial_Punctuation` `Pi`	`Initial_Punctuation`
`Letter` `L`	`Letter`
`Letter_Number` `Nl`	`Letter_Number`
`Line_Separator` `Zl`	`Line_Separator`
`Lowercase_Letter` `Ll`	`Lowercase_Letter`
`Mark` `M` `Combining_Mark`	`Mark`
`Math_Symbol` `Sm`	`Math_Symbol`
`Modifier_Letter` `Lm`	`Modifier_Letter`
`Modifier_Symbol` `Sk`	`Modifier_Symbol`
`Nonspacing_Mark` `Mn`	`Nonspacing_Mark`
`Number` `N`	`Number`
`Open_Punctuation` `Ps`	`Open_Punctuation`
`Other` `C`	`Other`
`Other_Letter` `Lo`	`Other_Letter`
`Other_Number` `No`	`Other_Number`
`Other_Punctuation` `Po`	`Other_Punctuation`
`Other_Symbol` `So`	`Other_Symbol`
`Paragraph_Separator` `Zp`	`Paragraph_Separator`
`Private_Use` `Co`	`Private_Use`
`Punctuation` `P` `punct`	`Punctuation`
`Separator` `Z`	`Separator`
`Space_Separator` `Zs`	`Space_Separator`
`Spacing_Mark` `Mc`	`Spacing_Mark`
`Surrogate` `Cs`	`Surrogate`
`Symbol` `S`	`Symbol`
`Titlecase_Letter` `Lt`	`Titlecase_Letter`
`Unassigned` `Cn`	`Unassigned`
`Uppercase_Letter` `Lu`	`Uppercase_Letter`

Table 58: Value aliases and canonical values for the Unicode properties Script and Script_Extensions

Property value and aliases	Canonical property value
`Adlam` `Adlm`	`Adlam`
`Ahom` `Ahom`	`Ahom`
`Anatolian_Hieroglyphs` `Hluw`	`Anatolian_Hieroglyphs`
`Arabic` `Arab`	`Arabic`
`Armenian` `Armn`	`Armenian`
`Avestan` `Avst`	`Avestan`
`Balinese` `Bali`	`Balinese`
`Bamum` `Bamu`	`Bamum`
`Bassa_Vah` `Bass`	`Bassa_Vah`
`Batak` `Batk`	`Batak`
`Bengali` `Beng`	`Bengali`
`Bhaiksuki` `Bhks`	`Bhaiksuki`
`Bopomofo` `Bopo`	`Bopomofo`
`Brahmi` `Brah`	`Brahmi`
`Braille` `Brai`	`Braille`
`Buginese` `Bugi`	`Buginese`
`Buhid` `Buhd`	`Buhid`
`Canadian_Aboriginal` `Cans`	`Canadian_Aboriginal`
`Carian` `Cari`	`Carian`
`Caucasian_Albanian` `Aghb`	`Caucasian_Albanian`
`Chakma` `Cakm`	`Chakma`
`Cham` `Cham`	`Cham`
`Cherokee` `Cher`	`Cherokee`
`Common` `Zyyy`	`Common`
`Coptic` `Copt` `Qaac`	`Coptic`
`Cuneiform` `Xsux`	`Cuneiform`
`Cypriot` `Cprt`	`Cypriot`
`Cyrillic` `Cyrl`	`Cyrillic`
`Deseret` `Dsrt`	`Deseret`
`Devanagari` `Deva`	`Devanagari`
`Dogra` `Dogr`	`Dogra`
`Duployan` `Dupl`	`Duployan`
`Egyptian_Hieroglyphs` `Egyp`	`Egyptian_Hieroglyphs`
`Elbasan` `Elba`	`Elbasan`
`Elymaic` `Elym`	`Elymaic`
`Ethiopic` `Ethi`	`Ethiopic`
`Georgian` `Geor`	`Georgian`
`Glagolitic` `Glag`	`Glagolitic`
`Gothic` `Goth`	`Gothic`
`Grantha` `Gran`	`Grantha`
`Greek` `Grek`	`Greek`
`Gujarati` `Gujr`	`Gujarati`
`Gunjala_Gondi` `Gong`	`Gunjala_Gondi`
`Gurmukhi` `Guru`	`Gurmukhi`
`Han` `Hani`	`Han`
`Hangul` `Hang`	`Hangul`
`Hanifi_Rohingya` `Rohg`	`Hanifi_Rohingya`
`Hanunoo` `Hano`	`Hanunoo`
`Hatran` `Hatr`	`Hatran`
`Hebrew` `Hebr`	`Hebrew`
`Hiragana` `Hira`	`Hiragana`
`Imperial_Aramaic` `Armi`	`Imperial_Aramaic`
`Inherited` `Zinh` `Qaai`	`Inherited`
`Inscriptional_Pahlavi` `Phli`	`Inscriptional_Pahlavi`
`Inscriptional_Parthian` `Prti`	`Inscriptional_Parthian`
`Javanese` `Java`	`Javanese`
`Kaithi` `Kthi`	`Kaithi`
`Kannada` `Knda`	`Kannada`
`Katakana` `Kana`	`Katakana`
`Kayah_Li` `Kali`	`Kayah_Li`
`Kharoshthi` `Khar`	`Kharoshthi`
`Khmer` `Khmr`	`Khmer`
`Khojki` `Khoj`	`Khojki`
`Khudawadi` `Sind`	`Khudawadi`
`Lao` `Laoo`	`Lao`
`Latin` `Latn`	`Latin`
`Lepcha` `Lepc`	`Lepcha`
`Limbu` `Limb`	`Limbu`
`Linear_A` `Lina`	`Linear_A`
`Linear_B` `Linb`	`Linear_B`
`Lisu` `Lisu`	`Lisu`
`Lycian` `Lyci`	`Lycian`
`Lydian` `Lydi`	`Lydian`
`Mahajani` `Mahj`	`Mahajani`
`Makasar` `Maka`	`Makasar`
`Malayalam` `Mlym`	`Malayalam`
`Mandaic` `Mand`	`Mandaic`
`Manichaean` `Mani`	`Manichaean`
`Marchen` `Marc`	`Marchen`
`Medefaidrin` `Medf`	`Medefaidrin`
`Masaram_Gondi` `Gonm`	`Masaram_Gondi`
`Meetei_Mayek` `Mtei`	`Meetei_Mayek`
`Mende_Kikakui` `Mend`	`Mende_Kikakui`
`Meroitic_Cursive` `Merc`	`Meroitic_Cursive`
`Meroitic_Hieroglyphs` `Mero`	`Meroitic_Hieroglyphs`
`Miao` `Plrd`	`Miao`
`Modi` `Modi`	`Modi`
`Mongolian` `Mong`	`Mongolian`
`Mro` `Mroo`	`Mro`
`Multani` `Mult`	`Multani`
`Myanmar` `Mymr`	`Myanmar`
`Nabataean` `Nbat`	`Nabataean`
`Nandinagari` `Nand`	`Nandinagari`
`New_Tai_Lue` `Talu`	`New_Tai_Lue`
`Newa` `Newa`	`Newa`
`Nko` `Nkoo`	`Nko`
`Nushu` `Nshu`	`Nushu`
`Nyiakeng_Puachue_Hmong` `Hmnp`	`Nyiakeng_Puachue_Hmong`
`Ogham` `Ogam`	`Ogham`
`Ol_Chiki` `Olck`	`Ol_Chiki`
`Old_Hungarian` `Hung`	`Old_Hungarian`
`Old_Italic` `Ital`	`Old_Italic`
`Old_North_Arabian` `Narb`	`Old_North_Arabian`
`Old_Permic` `Perm`	`Old_Permic`
`Old_Persian` `Xpeo`	`Old_Persian`
`Old_Sogdian` `Sogo`	`Old_Sogdian`
`Old_South_Arabian` `Sarb`	`Old_South_Arabian`
`Old_Turkic` `Orkh`	`Old_Turkic`
`Oriya` `Orya`	`Oriya`
`Osage` `Osge`	`Osage`
`Osmanya` `Osma`	`Osmanya`
`Pahawh_Hmong` `Hmng`	`Pahawh_Hmong`
`Palmyrene` `Palm`	`Palmyrene`
`Pau_Cin_Hau` `Pauc`	`Pau_Cin_Hau`
`Phags_Pa` `Phag`	`Phags_Pa`
`Phoenician` `Phnx`	`Phoenician`
`Psalter_Pahlavi` `Phlp`	`Psalter_Pahlavi`
`Rejang` `Rjng`	`Rejang`
`Runic` `Runr`	`Runic`
`Samaritan` `Samr`	`Samaritan`
`Saurashtra` `Saur`	`Saurashtra`
`Sharada` `Shrd`	`Sharada`
`Shavian` `Shaw`	`Shavian`
`Siddham` `Sidd`	`Siddham`
`SignWriting` `Sgnw`	`SignWriting`
`Sinhala` `Sinh`	`Sinhala`
`Sogdian` `Sogd`	`Sogdian`
`Sora_Sompeng` `Sora`	`Sora_Sompeng`
`Soyombo` `Soyo`	`Soyombo`
`Sundanese` `Sund`	`Sundanese`
`Syloti_Nagri` `Sylo`	`Syloti_Nagri`
`Syriac` `Syrc`	`Syriac`
`Tagalog` `Tglg`	`Tagalog`
`Tagbanwa` `Tagb`	`Tagbanwa`
`Tai_Le` `Tale`	`Tai_Le`
`Tai_Tham` `Lana`	`Tai_Tham`
`Tai_Viet` `Tavt`	`Tai_Viet`
`Takri` `Takr`	`Takri`
`Tamil` `Taml`	`Tamil`
`Tangut` `Tang`	`Tangut`
`Telugu` `Telu`	`Telugu`
`Thaana` `Thaa`	`Thaana`
`Thai` `Thai`	`Thai`
`Tibetan` `Tibt`	`Tibetan`
`Tifinagh` `Tfng`	`Tifinagh`
`Tirhuta` `Tirh`	`Tirhuta`
`Ugaritic` `Ugar`	`Ugaritic`
`Vai` `Vaii`	`Vai`
`Wancho` `Wcho`	`Wancho`
`Warang_Citi` `Wara`	`Warang_Citi`
`Yi` `Yiii`	`Yi`
`Zanabazar_Square` `Zanb`	`Zanabazar_Square`

21.2.2.9 AtomEscape

With parameter direction.

The production AtomEscape::DecimalEscape evaluates as follows:

Evaluate DecimalEscape to obtain an integer n.
Assert: n ≤ NcapturingParens.
Call BackreferenceMatcher(n, direction) and return its Matcher result.

The production AtomEscape::CharacterEscape evaluates as follows:

Evaluate CharacterEscape to obtain a character ch.
Let A be a one-element CharSet containing the character ch.
Call CharacterSetMatcher(A, false, direction) and return its Matcher result.

The production AtomEscape::CharacterClassEscape evaluates as follows:

Evaluate CharacterClassEscape to obtain a CharSet A.
Call CharacterSetMatcher(A, false, direction) and return its Matcher result.

Note

An escape sequence of the form \ followed by a nonzero decimal number n matches the result of the n^th set of capturing parentheses (21.2.2.1). It is an error if the regular expression has fewer than n capturing parentheses. If the regular expression has n or more capturing parentheses but the n^th one is undefined because it has not captured anything, then the backreference always succeeds.

The production AtomEscape::kGroupName evaluates as follows:

Search the enclosing Pattern for an instance of a GroupSpecifier for a RegExpIdentifierName which has a StringValue equal to the StringValue of the RegExpIdentifierName contained in GroupName.
Assert: A unique such GroupSpecifier is found.
Let parenIndex be the number of left-capturing parentheses in the entire regular expression that occur to the left of the located GroupSpecifier. This is the total number of Atom::(GroupSpecifierDisjunction) Parse Nodes prior to or enclosing the located GroupSpecifier.
Call BackreferenceMatcher(parenIndex, direction) and return its Matcher result.

21.2.2.9.1 Runtime Semantics: BackreferenceMatcher ( `n`, `direction` )

The abstract operation BackreferenceMatcher takes arguments n (an integer) and direction (an integer). It performs the following steps when called:

Return a new Matcher with parameters (x, c) that captures n and direction and performs the following steps when called:
1. Assert: x is a State.
2. Assert: c is a Continuation.
3. Let cap be x's captures List.
4. Let s be cap[n].
5. If s is undefined, return c(x).
6. Let e be x's endIndex.
7. Let len be the number of elements in s.
8. Let f be e + direction × len.
9. If f < 0 or f > InputLength, return failure.
10. Let g be min(e, f).
11. If there exists an integer i between 0 (inclusive) and len (exclusive) such that Canonicalize(s[i]) is not the same character value as Canonicalize(Input[g + i]), return failure.
12. Let y be the State (f, cap).
13. Call c(y) and return its result.

21.2.2.10 CharacterEscape

The CharacterEscape productions evaluate as follows:

ControlEscape

[lookahead ∉ DecimalDigit]

Let cv be the CharacterValue of this CharacterEscape.
Return the character whose character value is cv.

21.2.2.11 DecimalEscape

The DecimalEscape productions evaluate as follows:

opt

Return the CapturingGroupNumber of this DecimalEscape.

Note

If \ is followed by a decimal number n whose first digit is not 0, then the escape sequence is considered to be a backreference. It is an error if n is greater than the total number of left-capturing parentheses in the entire regular expression.

21.2.2.12 CharacterClassEscape

The production CharacterClassEscape::d evaluates as follows:

Return the ten-element set of characters containing the characters 0 through 9 inclusive.

The production CharacterClassEscape::D evaluates as follows:

Return the set of all characters not included in the set returned by CharacterClassEscape::d .

The production CharacterClassEscape::s evaluates as follows:

Return the set of characters containing the characters that are on the right-hand side of the WhiteSpace or LineTerminator productions.

The production CharacterClassEscape::S evaluates as follows:

Return the set of all characters not included in the set returned by CharacterClassEscape::s .

The production CharacterClassEscape::w evaluates as follows:

Return the set of all characters returned by WordCharacters().

The production CharacterClassEscape::W evaluates as follows:

Return the set of all characters not included in the set returned by CharacterClassEscape::w .

The production CharacterClassEscape::p{UnicodePropertyValueExpression} evaluates as follows:

Return the CharSet containing all Unicode code points included in the CharSet returned by UnicodePropertyValueExpression.

The production CharacterClassEscape::P{UnicodePropertyValueExpression} evaluates as follows:

Return the CharSet containing all Unicode code points not included in the CharSet returned by UnicodePropertyValueExpression.

The production UnicodePropertyValueExpression::UnicodePropertyName=UnicodePropertyValue evaluates as follows:

Let ps be SourceText of UnicodePropertyName.
Let p be ! UnicodeMatchProperty(ps).
Assert: p is a Unicode property name or property alias listed in the “Property name and aliases” column of Table 55.
Let vs be SourceText of UnicodePropertyValue.
Let v be ! UnicodeMatchPropertyValue(p, vs).
Return the CharSet containing all Unicode code points whose character database definition includes the property p with value v.

The production UnicodePropertyValueExpression::LoneUnicodePropertyNameOrValue evaluates as follows:

Let s be SourceText of LoneUnicodePropertyNameOrValue.
If ! UnicodeMatchPropertyValue(General_Category, s) is identical to a List of Unicode code points that is the name of a Unicode general category or general category alias listed in the “Property value and aliases” column of Table 57, then
1. Return the CharSet containing all Unicode code points whose character database definition includes the property “General_Category” with value s.
Let p be ! UnicodeMatchProperty(s).
Assert: p is a binary Unicode property or binary property alias listed in the “Property name and aliases” column of Table 56.
Return the CharSet containing all Unicode code points whose character database definition includes the property p with value “True”.

21.2.2.13 CharacterClass

The production CharacterClass::[ClassRanges] evaluates as follows:

Evaluate ClassRanges to obtain a CharSet A.
Return the two results A and false.

The production CharacterClass::[^ClassRanges] evaluates as follows:

Evaluate ClassRanges to obtain a CharSet A.
Return the two results A and true.

21.2.2.14 ClassRanges

The production ClassRanges::[empty] evaluates as follows:

Return the empty CharSet.

The production ClassRanges::NonemptyClassRanges evaluates as follows:

Return the CharSet that is the result of evaluating NonemptyClassRanges.

21.2.2.15 NonemptyClassRanges

The production NonemptyClassRanges::ClassAtom evaluates as follows:

Return the CharSet that is the result of evaluating ClassAtom.

The production NonemptyClassRanges::ClassAtomNonemptyClassRangesNoDash evaluates as follows:

Evaluate ClassAtom to obtain a CharSet A.
Evaluate NonemptyClassRangesNoDash to obtain a CharSet B.
Return the union of CharSets A and B.

The production NonemptyClassRanges::ClassAtom-ClassAtomClassRanges evaluates as follows:

Evaluate the first ClassAtom to obtain a CharSet A.
Evaluate the second ClassAtom to obtain a CharSet B.
Evaluate ClassRanges to obtain a CharSet C.
Call CharacterRange(A, B) and let D be the resulting CharSet.
Return the union of CharSets D and C.

21.2.2.15.1 Runtime Semantics: CharacterRange ( `A`, `B` )

The abstract operation CharacterRange takes arguments A (a CharSet) and B (a CharSet). It performs the following steps when called:

Assert: A and B each contain exactly one character.
Let a be the one character in CharSet A.
Let b be the one character in CharSet B.
Let i be the character value of character a.
Let j be the character value of character b.
Assert: i ≤ j.
Return the set containing all characters numbered i through j, inclusive.

21.2.2.16 NonemptyClassRangesNoDash

The production NonemptyClassRangesNoDash::ClassAtom evaluates as follows:

Return the CharSet that is the result of evaluating ClassAtom.

The production NonemptyClassRangesNoDash::ClassAtomNoDashNonemptyClassRangesNoDash evaluates as follows:

Evaluate ClassAtomNoDash to obtain a CharSet A.
Evaluate NonemptyClassRangesNoDash to obtain a CharSet B.
Return the union of CharSets A and B.

The production NonemptyClassRangesNoDash::ClassAtomNoDash-ClassAtomClassRanges evaluates as follows:

Evaluate ClassAtomNoDash to obtain a CharSet A.
Evaluate ClassAtom to obtain a CharSet B.
Evaluate ClassRanges to obtain a CharSet C.
Call CharacterRange(A, B) and let D be the resulting CharSet.
Return the union of CharSets D and C.

Note 1

ClassRanges can expand into a single ClassAtom and/or ranges of two ClassAtom separated by dashes. In the latter case the ClassRanges includes all characters between the first ClassAtom and the second ClassAtom, inclusive; an error occurs if either ClassAtom does not represent a single character (for example, if one is \w) or if the first ClassAtom's character value is greater than the second ClassAtom's character value.

Note 2

Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches only the letters E, F, e, and f, while the pattern /[E-f]/i matches all upper and lower-case letters in the Unicode Basic Latin block as well as the symbols [, \, ], ^, _, and `.

Note 3

A - character can be treated literally or it can denote a range. It is treated literally if it is the first or last character of ClassRanges, the beginning or end limit of a range specification, or immediately follows a range specification.

21.2.2.17 ClassAtom

The production ClassAtom::- evaluates as follows:

Return the CharSet containing the single character - U+002D (HYPHEN-MINUS).

The production ClassAtom::ClassAtomNoDash evaluates as follows:

Return the CharSet that is the result of evaluating ClassAtomNoDash.

21.2.2.18 ClassAtomNoDash

The production ClassAtomNoDash::SourceCharacterbut not one of \ or ] or - evaluates as follows:

Return the CharSet containing the character matched by SourceCharacter.

The production ClassAtomNoDash::\ClassEscape evaluates as follows:

Return the CharSet that is the result of evaluating ClassEscape.

21.2.2.19 ClassEscape

The ClassEscape productions evaluate as follows:

Let cv be the CharacterValue of this ClassEscape.
Let c be the character whose character value is cv.
Return the CharSet containing the single character c.

Return the CharSet that is the result of evaluating CharacterClassEscape.

Note

A ClassAtom can use any of the escape sequences that are allowed in the rest of the regular expression except for \b, \B, and backreferences. Inside a CharacterClass, \b means the backspace character, while \B and backreferences raise errors. Using a backreference inside a ClassAtom causes an error.

21.2.3 The RegExp Constructor

The RegExp constructor:

is the intrinsic object %RegExp%.
is the initial value of the "RegExp" property of the global object.
creates and initializes a new RegExp object when called as a function rather than as a constructor. Thus the function call RegExp(…) is equivalent to the object creation expression new RegExp(…) with the same arguments.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified RegExp behaviour must include a super call to the RegExp constructor to create and initialize subclass instances with the necessary internal slots.

21.2.3.1 RegExp ( `pattern`, `flags` )

The following steps are taken:

Let patternIsRegExp be ? IsRegExp(pattern).
If NewTarget is undefined, then
1. Let newTarget be the active function object.
2. If patternIsRegExp is true and flags is undefined, then
  1. Let patternConstructor be ? Get(pattern, "constructor").
  2. If SameValue(newTarget, patternConstructor) is true, return pattern.
Else, let newTarget be NewTarget.
If Type(pattern) is Object and pattern has a [[RegExpMatcher]] internal slot, then
1. Let P be pattern.[[OriginalSource]].
2. If flags is undefined, let F be pattern.[[OriginalFlags]].
3. Else, let F be flags.
Else if patternIsRegExp is true, then
1. Let P be ? Get(pattern, "source").
2. If flags is undefined, then
  1. Let F be ? Get(pattern, "flags").
3. Else, let F be flags.
Else,
1. Let P be pattern.
2. Let F be flags.
Let O be ? RegExpAlloc(newTarget).
Return ? RegExpInitialize(O, P, F).

Note

If pattern is supplied using a StringLiteral, the usual escape sequence substitutions are performed before the String is processed by RegExp. If pattern must contain an escape sequence to be recognized by RegExp, any U+005C (REVERSE SOLIDUS) code points must be escaped within the StringLiteral to prevent them being removed when the contents of the StringLiteral are formed.

21.2.3.2 Abstract Operations for the RegExp Constructor

21.2.3.2.1 Runtime Semantics: RegExpAlloc ( `newTarget` )

The abstract operation RegExpAlloc takes argument newTarget. It performs the following steps when called:

Let obj be ? OrdinaryCreateFromConstructor(newTarget, "%RegExp.prototype%", « [[RegExpMatcher]], [[OriginalSource]], [[OriginalFlags]] »).
Perform ! DefinePropertyOrThrow(obj, "lastIndex", PropertyDescriptor { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Return obj.

21.2.3.2.2 Runtime Semantics: RegExpInitialize ( `obj`, `pattern`, `flags` )

The abstract operation RegExpInitialize takes arguments obj, pattern, and flags. It performs the following steps when called:

If pattern is undefined, let P be the empty String.
Else, let P be ? ToString(pattern).
If flags is undefined, let F be the empty String.
Else, let F be ? ToString(flags).
If F contains any code unit other than "g", "i", "m", "s", "u", or "y" or if it contains the same code unit more than once, throw a SyntaxError exception.
If F contains "u", let BMP be false; else let BMP be true.
If BMP is true, then
1. Let pText be the sequence of code points resulting from interpreting each of the 16-bit elements of P as a Unicode BMP code point. UTF-16 decoding is not applied to the elements.
2. Parse pText using the grammars in 21.2.1. The goal symbol for the parse is Pattern[~U, ~N]. If the result of parsing contains a GroupName, reparse with the goal symbol Pattern[~U, +N] and use this result instead. Throw a SyntaxError exception if pText did not conform to the grammar, if any elements of pText were not matched by the parse, or if any Early Error conditions exist.
3. Let patternCharacters be a List whose elements are the code unit elements of P.
Else,
1. Let pText be ! UTF16DecodeString(P).
2. Parse pText using the grammars in 21.2.1. The goal symbol for the parse is Pattern[+U, +N]. Throw a SyntaxError exception if pText did not conform to the grammar, if any elements of pText were not matched by the parse, or if any Early Error conditions exist.
3. Let patternCharacters be a List whose elements are the code points of pText.
Set obj.[[OriginalSource]] to P.
Set obj.[[OriginalFlags]] to F.
Set obj.[[RegExpMatcher]] to the Abstract Closure that evaluates the above parse by applying the semantics provided in 21.2.2 using patternCharacters as the pattern's List of SourceCharacter values and F as the flag parameters.
Perform ? Set(obj, "lastIndex", 0, true).
Return obj.

21.2.3.2.3 Runtime Semantics: RegExpCreate ( `P`, `F` )

The abstract operation RegExpCreate takes arguments P and F. It performs the following steps when called:

Let obj be ? RegExpAlloc(%RegExp%).
Return ? RegExpInitialize(obj, P, F).

21.2.3.2.4 Runtime Semantics: EscapeRegExpPattern ( `P`, `F` )

The abstract operation EscapeRegExpPattern takes arguments P and F. It performs the following steps when called:

Let S be a String in the form of a Pattern[~U] (Pattern[+U] if F contains "u") equivalent to P interpreted as UTF-16 encoded Unicode code points (6.1.4), in which certain code points are escaped as described below. S may or may not be identical to P; however, the Abstract Closure that would result from evaluating S as a Pattern[~U] (Pattern[+U] if F contains "u") must behave identically to the Abstract Closure given by the constructed object's [[RegExpMatcher]] internal slot. Multiple calls to this abstract operation using the same values for P and F must produce identical results.
The code points / or any LineTerminator occurring in the pattern shall be escaped in S as necessary to ensure that the string-concatenation of "/", S, "/", and F can be parsed (in an appropriate lexical context) as a RegularExpressionLiteral that behaves identically to the constructed regular expression. For example, if P is "/", then S could be "\/" or "\u002F", among other possibilities, but not "/", because /// followed by F would be parsed as a SingleLineComment rather than a RegularExpressionLiteral. If P is the empty String, this specification can be met by letting S be "(?:)".
Return S.

21.2.4 Properties of the RegExp Constructor

The RegExp constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

21.2.4.1 RegExp.prototype

The initial value of RegExp.prototype is %RegExp.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.2.4.2 get RegExp [ @@species ]

RegExp[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

Note

RegExp prototype methods normally use their this object's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.

21.2.5 Properties of the RegExp Prototype Object

The RegExp prototype object:

is the intrinsic object %RegExpPrototype%.
is an ordinary object.
is not a RegExp instance and does not have a [[RegExpMatcher]] internal slot or any of the other internal slots of RegExp instance objects.
has a [[Prototype]] internal slot whose value is %Object.prototype%.

Note

The RegExp prototype object does not have a "valueOf" property of its own; however, it inherits the "valueOf" property from the Object prototype object.

21.2.5.1 RegExp.prototype.constructor

The initial value of RegExp.prototype.constructor is %RegExp%.

21.2.5.2 RegExp.prototype.exec ( `string` )

Performs a regular expression match of string against the regular expression and returns an Array object containing the results of the match, or null if string did not match.

The String ToString(string) is searched for an occurrence of the regular expression pattern as follows:

Let R be the this value.
Perform ? RequireInternalSlot(R, [[RegExpMatcher]]).
Let S be ? ToString(string).
Return ? RegExpBuiltinExec(R, S).

21.2.5.2.1 Runtime Semantics: RegExpExec ( `R`, `S` )

The abstract operation RegExpExec takes arguments R and S. It performs the following steps when called:

Assert: Type(R) is Object.
Assert: Type(S) is String.
Let exec be ? Get(R, "exec").
If IsCallable(exec) is true, then
1. Let result be ? Call(exec, R, « S »).
2. If Type(result) is neither Object nor Null, throw a TypeError exception.
3. Return result.
Perform ? RequireInternalSlot(R, [[RegExpMatcher]]).
Return ? RegExpBuiltinExec(R, S).

Note

If a callable "exec" property is not found this algorithm falls back to attempting to use the built-in RegExp matching algorithm. This provides compatible behaviour for code written for prior editions where most built-in algorithms that use regular expressions did not perform a dynamic property lookup of "exec".

21.2.5.2.2 Runtime Semantics: RegExpBuiltinExec ( `R`, `S` )

The abstract operation RegExpBuiltinExec takes arguments R and S. It performs the following steps when called:

Assert: R is an initialized RegExp instance.
Assert: Type(S) is String.
Let length be the number of code units in S.
Let lastIndex be ? ToLength(? Get(R, "lastIndex")).
Let flags be R.[[OriginalFlags]].
If flags contains "g", let global be true; else let global be false.
If flags contains "y", let sticky be true; else let sticky be false.
If global is false and sticky is false, set lastIndex to 0.
Let matcher be R.[[RegExpMatcher]].
If flags contains "u", let fullUnicode be true; else let fullUnicode be false.
Let matchSucceeded be false.
Repeat, while matchSucceeded is false
1. If lastIndex > length, then
  1. If global is true or sticky is true, then
    1. Perform ? Set(R, "lastIndex", 0, true).
  2. Return null.
2. Let r be matcher(S, lastIndex).
3. If r is failure, then
  1. If sticky is true, then
    1. Perform ? Set(R, "lastIndex", 0, true).
    2. Return null.
  2. Set lastIndex to AdvanceStringIndex(S, lastIndex, fullUnicode).
4. Else,
  1. Assert: r is a State.
  2. Set matchSucceeded to true.
Let e be r's endIndex value.
If fullUnicode is true, then
1. e is an index into the Input character list, derived from S, matched by matcher. Let eUTF be the smallest index into S that corresponds to the character at element e of Input. If e is greater than or equal to the number of elements in Input, then eUTF is the number of code units in S.
2. Set e to eUTF.
If global is true or sticky is true, then
1. Perform ? Set(R, "lastIndex", e, true).
Let n be the number of elements in r's captures List. (This is the same value as 21.2.2.1's NcapturingParens.)
Assert: n < 2³² - 1.
Let A be ! ArrayCreate(n + 1).
Assert: The value of A's "length" property is n + 1.
Perform ! CreateDataPropertyOrThrow(A, "index", lastIndex).
Perform ! CreateDataPropertyOrThrow(A, "input", S).
Let matchedSubstr be the matched substring (i.e. the portion of S between offset lastIndex inclusive and offset e exclusive).
Perform ! CreateDataPropertyOrThrow(A, "0", matchedSubstr).
If R contains any GroupName, then
1. Let groups be OrdinaryObjectCreate(null).
Else,
1. Let groups be undefined.
Perform ! CreateDataPropertyOrThrow(A, "groups", groups).
For each integer i such that i > 0 and i ≤ n, do
1. Let captureI be i^th element of r's captures List.
2. If captureI is undefined, let capturedValue be undefined.
3. Else if fullUnicode is true, then
  1. Assert: captureI is a List of code points.
  2. Let capturedValue be ! UTF16Encode(captureI).
4. Else,
  1. Assert: fullUnicode is false.
  2. Assert: captureI is a List of code units.
  3. Let capturedValue be the String value consisting of the code units of captureI.
5. Perform ! CreateDataPropertyOrThrow(A, ! ToString(i), capturedValue).
6. If the i^th capture of R was defined with a GroupName, then
  1. Let s be the StringValue of the corresponding RegExpIdentifierName.
  2. Perform ! CreateDataPropertyOrThrow(groups, s, capturedValue).
Return A.

21.2.5.2.3 AdvanceStringIndex ( `S`, `index`, `unicode` )

The abstract operation AdvanceStringIndex takes arguments S, index, and unicode. It performs the following steps when called:

Assert: Type(S) is String.
Assert: 0 ≤ index ≤ 2⁵³ - 1 and ! IsInteger(index) is true.
Assert: Type(unicode) is Boolean.
If unicode is false, return index + 1.
Let length be the number of code units in S.
If index + 1 ≥ length, return index + 1.
Let cp be ! CodePointAt(S, index).
Return index + cp.[[CodeUnitCount]].

21.2.5.3 get RegExp.prototype.dotAll

RegExp.prototype.dotAll is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let R be the this value.
If Type(R) is not Object, throw a TypeError exception.
If R does not have an [[OriginalFlags]] internal slot, then
1. If SameValue(R, %RegExp.prototype%) is true, return undefined.
2. Otherwise, throw a TypeError exception.
Let flags be R.[[OriginalFlags]].
If flags contains the code unit 0x0073 (LATIN SMALL LETTER S), return true.
Return false.

21.2.5.4 get RegExp.prototype.flags

RegExp.prototype.flags is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let R be the this value.
If Type(R) is not Object, throw a TypeError exception.
Let result be the empty String.
Let global be ! ToBoolean(? Get(R, "global")).
If global is true, append the code unit 0x0067 (LATIN SMALL LETTER G) as the last code unit of result.
Let ignoreCase be ! ToBoolean(? Get(R, "ignoreCase")).
If ignoreCase is true, append the code unit 0x0069 (LATIN SMALL LETTER I) as the last code unit of result.
Let multiline be ! ToBoolean(? Get(R, "multiline")).
If multiline is true, append the code unit 0x006D (LATIN SMALL LETTER M) as the last code unit of result.
Let dotAll be ! ToBoolean(? Get(R, "dotAll")).
If dotAll is true, append the code unit 0x0073 (LATIN SMALL LETTER S) as the last code unit of result.
Let unicode be ! ToBoolean(? Get(R, "unicode")).
If unicode is true, append the code unit 0x0075 (LATIN SMALL LETTER U) as the last code unit of result.
Let sticky be ! ToBoolean(? Get(R, "sticky")).
If sticky is true, append the code unit 0x0079 (LATIN SMALL LETTER Y) as the last code unit of result.
Return result.

21.2.5.5 get RegExp.prototype.global

RegExp.prototype.global is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let R be the this value.
If Type(R) is not Object, throw a TypeError exception.
If R does not have an [[OriginalFlags]] internal slot, then
1. If SameValue(R, %RegExp.prototype%) is true, return undefined.
2. Otherwise, throw a TypeError exception.
Let flags be R.[[OriginalFlags]].
If flags contains the code unit 0x0067 (LATIN SMALL LETTER G), return true.
Return false.

21.2.5.6 get RegExp.prototype.ignoreCase

RegExp.prototype.ignoreCase is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let R be the this value.
If Type(R) is not Object, throw a TypeError exception.
If R does not have an [[OriginalFlags]] internal slot, then
1. If SameValue(R, %RegExp.prototype%) is true, return undefined.
2. Otherwise, throw a TypeError exception.
Let flags be R.[[OriginalFlags]].
If flags contains the code unit 0x0069 (LATIN SMALL LETTER I), return true.
Return false.

21.2.5.7 RegExp.prototype [ @@match ] ( `string` )

When the @@match method is called with argument string, the following steps are taken:

Let rx be the this value.
If Type(rx) is not Object, throw a TypeError exception.
Let S be ? ToString(string).
Let global be ! ToBoolean(? Get(rx, "global")).
If global is false, then
1. Return ? RegExpExec(rx, S).
Else,
1. Assert: global is true.
2. Let fullUnicode be ! ToBoolean(? Get(rx, "unicode")).
3. Perform ? Set(rx, "lastIndex", 0, true).
4. Let A be ! ArrayCreate(0).
5. Let n be 0.
6. Repeat,
  1. Let result be ? RegExpExec(rx, S).
  2. If result is null, then
    1. If n = 0, return null.
    2. Return A.
  3. Else,
    1. Let matchStr be ? ToString(? Get(result, "0")).
    2. Perform ! CreateDataPropertyOrThrow(A, ! ToString(n), matchStr).
    3. If matchStr is the empty String, then
      1. Let thisIndex be ? ToLength(? Get(rx, "lastIndex")).
      2. Let nextIndex be AdvanceStringIndex(S, thisIndex, fullUnicode).
      3. Perform ? Set(rx, "lastIndex", nextIndex, true).
    4. Set n to n + 1.

The value of the "name" property of this function is "[Symbol.match]".

Note

The @@match property is used by the IsRegExp abstract operation to identify objects that have the basic behaviour of regular expressions. The absence of a @@match property or the existence of such a property whose value does not Boolean coerce to true indicates that the object is not intended to be used as a regular expression object.

21.2.5.8 RegExp.prototype [ @@matchAll ] ( `string` )

When the @@matchAll method is called with argument string, the following steps are taken:

Let R be the this value.
If Type(R) is not Object, throw a TypeError exception.
Let S be ? ToString(string).
Let C be ? SpeciesConstructor(R, %RegExp%).
Let flags be ? ToString(? Get(R, "flags")).
Let matcher be ? Construct(C, « R, flags »).
Let lastIndex be ? ToLength(? Get(R, "lastIndex")).
Perform ? Set(matcher, "lastIndex", lastIndex, true).
If flags contains "g", let global be true.
Else, let global be false.
If flags contains "u", let fullUnicode be true.
Else, let fullUnicode be false.
Return ! CreateRegExpStringIterator(matcher, S, global, fullUnicode).

The value of the "name" property of this function is "[Symbol.matchAll]".

21.2.5.8.1 CreateRegExpStringIterator ( `R`, `S`, `global`, `fullUnicode` )

Assert: Type(S) is String.
Assert: Type(global) is Boolean.
Assert: Type(fullUnicode) is Boolean.
Let iterator be OrdinaryObjectCreate(%RegExpStringIteratorPrototype%, « [[IteratingRegExp]], [[IteratedString]], [[Global]], [[Unicode]], [[Done]] »).
Set iterator.[[IteratingRegExp]] to R.
Set iterator.[[IteratedString]] to S.
Set iterator.[[Global]] to global.
Set iterator.[[Unicode]] to fullUnicode.
Set iterator.[[Done]] to false.
Return iterator.

21.2.5.9 get RegExp.prototype.multiline

RegExp.prototype.multiline is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let R be the this value.
If Type(R) is not Object, throw a TypeError exception.
If R does not have an [[OriginalFlags]] internal slot, then
1. If SameValue(R, %RegExp.prototype%) is true, return undefined.
2. Otherwise, throw a TypeError exception.
Let flags be R.[[OriginalFlags]].
If flags contains the code unit 0x006D (LATIN SMALL LETTER M), return true.
Return false.

21.2.5.10 RegExp.prototype [ @@replace ] ( `string`, `replaceValue` )

When the @@replace method is called with arguments string and replaceValue, the following steps are taken:

Let rx be the this value.
If Type(rx) is not Object, throw a TypeError exception.
Let S be ? ToString(string).
Let lengthS be the number of code unit elements in S.
Let functionalReplace be IsCallable(replaceValue).
If functionalReplace is false, then
1. Set replaceValue to ? ToString(replaceValue).
Let global be ! ToBoolean(? Get(rx, "global")).
If global is true, then
1. Let fullUnicode be ! ToBoolean(? Get(rx, "unicode")).
2. Perform ? Set(rx, "lastIndex", 0, true).
Let results be a new empty List.
Let done be false.
Repeat, while done is false
1. Let result be ? RegExpExec(rx, S).
2. If result is null, set done to true.
3. Else,
  1. Append result to the end of results.
  2. If global is false, set done to true.
  3. Else,
    1. Let matchStr be ? ToString(? Get(result, "0")).
    2. If matchStr is the empty String, then
      1. Let thisIndex be ? ToLength(? Get(rx, "lastIndex")).
      2. Let nextIndex be AdvanceStringIndex(S, thisIndex, fullUnicode).
      3. Perform ? Set(rx, "lastIndex", nextIndex, true).
Let accumulatedResult be the empty String value.
Let nextSourcePosition be 0.
For each result in results, do
1. Let nCaptures be ? LengthOfArrayLike(result).
2. Set nCaptures to max(nCaptures - 1, 0).
3. Let matched be ? ToString(? Get(result, "0")).
4. Let matchLength be the number of code units in matched.
5. Let position be ? ToInteger(? Get(result, "index")).
6. Set position to max(min(position, lengthS), 0).
7. Let n be 1.
8. Let captures be a new empty List.
9. Repeat, while n ≤ nCaptures
  1. Let capN be ? Get(result, ! ToString(n)).
  2. If capN is not undefined, then
    1. Set capN to ? ToString(capN).
  3. Append capN as the last element of captures.
  4. Set n to n + 1.
10. Let namedCaptures be ? Get(result, "groups").
11. If functionalReplace is true, then
  1. Let replacerArgs be « matched ».
  2. Append in list order the elements of captures to the end of the List replacerArgs.
  3. Append position and S to replacerArgs.
  4. If namedCaptures is not undefined, then
    1. Append namedCaptures as the last element of replacerArgs.
  5. Let replValue be ? Call(replaceValue, undefined, replacerArgs).
  6. Let replacement be ? ToString(replValue).
12. Else,
  1. If namedCaptures is not undefined, then
    1. Set namedCaptures to ? ToObject(namedCaptures).
  2. Let replacement be ? GetSubstitution(matched, S, position, captures, namedCaptures, replaceValue).
13. If position ≥ nextSourcePosition, then
  1. NOTE: position should not normally move backwards. If it does, it is an indication of an ill-behaving RegExp subclass or use of an access triggered side-effect to change the global flag or other characteristics of rx. In such cases, the corresponding substitution is ignored.
  2. Set accumulatedResult to the string-concatenation of the current value of accumulatedResult, the substring of S consisting of the code units from nextSourcePosition (inclusive) up to position (exclusive), and replacement.
  3. Set nextSourcePosition to position + matchLength.
If nextSourcePosition ≥ lengthS, return accumulatedResult.
Return the string-concatenation of accumulatedResult and the substring of S consisting of the code units from nextSourcePosition (inclusive) up through the final code unit of S (inclusive).

The value of the "name" property of this function is "[Symbol.replace]".

21.2.5.11 RegExp.prototype [ @@search ] ( `string` )

When the @@search method is called with argument string, the following steps are taken:

Let rx be the this value.
If Type(rx) is not Object, throw a TypeError exception.
Let S be ? ToString(string).
Let previousLastIndex be ? Get(rx, "lastIndex").
If SameValue(previousLastIndex, 0) is false, then
1. Perform ? Set(rx, "lastIndex", 0, true).
Let result be ? RegExpExec(rx, S).
Let currentLastIndex be ? Get(rx, "lastIndex").
If SameValue(currentLastIndex, previousLastIndex) is false, then
1. Perform ? Set(rx, "lastIndex", previousLastIndex, true).
If result is null, return -1.
Return ? Get(result, "index").

The value of the "name" property of this function is "[Symbol.search]".

Note

The "lastIndex" and "global" properties of this RegExp object are ignored when performing the search. The "lastIndex" property is left unchanged.

21.2.5.12 get RegExp.prototype.source

RegExp.prototype.source is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let R be the this value.
If Type(R) is not Object, throw a TypeError exception.
If R does not have an [[OriginalSource]] internal slot, then
1. If SameValue(R, %RegExp.prototype%) is true, return "(?:)".
2. Otherwise, throw a TypeError exception.
Assert: R has an [[OriginalFlags]] internal slot.
Let src be R.[[OriginalSource]].
Let flags be R.[[OriginalFlags]].
Return EscapeRegExpPattern(src, flags).

21.2.5.13 RegExp.prototype [ @@split ] ( `string`, `limit` )

Note 1

Returns an Array object into which substrings of the result of converting string to a String have been stored. The substrings are determined by searching from left to right for matches of the this value regular expression; these occurrences are not part of any substring in the returned array, but serve to divide up the String value.

The this value may be an empty regular expression or a regular expression that can match an empty String. In this case, the regular expression does not match the empty substring at the beginning or end of the input String, nor does it match the empty substring at the end of the previous separator match. (For example, if the regular expression matches the empty String, the String is split up into individual code unit elements; the length of the result array equals the length of the String, and each substring contains one code unit.) Only the first match at a given index of the String is considered, even if backtracking could yield a non-empty-substring match at that index. (For example, /a*?/[Symbol.split]("ab") evaluates to the array ["a", "b"], while /a*/[Symbol.split]("ab") evaluates to the array ["","b"].)

If the string is (or converts to) the empty String, the result depends on whether the regular expression can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.

If the regular expression contains capturing parentheses, then each time separator is matched the results (including any undefined results) of the capturing parentheses are spliced into the output array. For example,

/<(\/)?([^<>]+)>/[Symbol.split]("A<B>bold</B>and<CODE>coded</CODE>")

evaluates to the array

["A", undefined, "B", "bold", "/", "B", "and", undefined, "CODE", "coded", "/", "CODE", ""]

If limit is not undefined, then the output array is truncated so that it contains no more than limit elements.

When the @@split method is called, the following steps are taken:

Let rx be the this value.
If Type(rx) is not Object, throw a TypeError exception.
Let S be ? ToString(string).
Let C be ? SpeciesConstructor(rx, %RegExp%).
Let flags be ? ToString(? Get(rx, "flags")).
If flags contains "u", let unicodeMatching be true.
Else, let unicodeMatching be false.
If flags contains "y", let newFlags be flags.
Else, let newFlags be the string-concatenation of flags and "y".
Let splitter be ? Construct(C, « rx, newFlags »).
Let A be ! ArrayCreate(0).
Let lengthA be 0.
If limit is undefined, let lim be 2³² - 1; else let lim be ? ToUint32(limit).
If lim = 0, return A.
Let size be the length of S.
If size = 0, then
1. Let z be ? RegExpExec(splitter, S).
2. If z is not null, return A.
3. Perform ! CreateDataPropertyOrThrow(A, "0", S).
4. Return A.
Let p be 0.
Let q be p.
Repeat, while q < size
1. Perform ? Set(splitter, "lastIndex", q, true).
2. Let z be ? RegExpExec(splitter, S).
3. If z is null, set q to AdvanceStringIndex(S, q, unicodeMatching).
4. Else,
  1. Let e be ? ToLength(? Get(splitter, "lastIndex")).
  2. Set e to min(e, size).
  3. If e = p, set q to AdvanceStringIndex(S, q, unicodeMatching).
  4. Else,
    1. Let T be the String value equal to the substring of S consisting of the code units at indices p (inclusive) through q (exclusive).
    2. Perform ! CreateDataPropertyOrThrow(A, ! ToString(lengthA), T).
    3. Set lengthA to lengthA + 1.
    4. If lengthA = lim, return A.
    5. Set p to e.
    6. Let numberOfCaptures be ? LengthOfArrayLike(z).
    7. Set numberOfCaptures to max(numberOfCaptures - 1, 0).
    8. Let i be 1.
    9. Repeat, while i ≤ numberOfCaptures,
      1. Let nextCapture be ? Get(z, ! ToString(i)).
      2. Perform ! CreateDataPropertyOrThrow(A, ! ToString(lengthA), nextCapture).
      3. Set i to i + 1.
      4. Set lengthA to lengthA + 1.
      5. If lengthA = lim, return A.
    10. Set q to p.
Let T be the String value equal to the substring of S consisting of the code units at indices p (inclusive) through size (exclusive).
Perform ! CreateDataPropertyOrThrow(A, ! ToString(lengthA), T).
Return A.

The value of the "name" property of this function is "[Symbol.split]".

Note 2

The @@split method ignores the value of the "global" and "sticky" properties of this RegExp object.

21.2.5.14 get RegExp.prototype.sticky

RegExp.prototype.sticky is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let R be the this value.
If Type(R) is not Object, throw a TypeError exception.
If R does not have an [[OriginalFlags]] internal slot, then
1. If SameValue(R, %RegExp.prototype%) is true, return undefined.
2. Otherwise, throw a TypeError exception.
Let flags be R.[[OriginalFlags]].
If flags contains the code unit 0x0079 (LATIN SMALL LETTER Y), return true.
Return false.

21.2.5.15 RegExp.prototype.test ( `S` )

The following steps are taken:

Let R be the this value.
If Type(R) is not Object, throw a TypeError exception.
Let string be ? ToString(S).
Let match be ? RegExpExec(R, string).
If match is not null, return true; else return false.

21.2.5.16 RegExp.prototype.toString ( )

Let R be the this value.
If Type(R) is not Object, throw a TypeError exception.
Let pattern be ? ToString(? Get(R, "source")).
Let flags be ? ToString(? Get(R, "flags")).
Let result be the string-concatenation of "/", pattern, "/", and flags.
Return result.

Note

The returned String has the form of a RegularExpressionLiteral that evaluates to another RegExp object with the same behaviour as this object.

21.2.5.17 get RegExp.prototype.unicode

RegExp.prototype.unicode is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let R be the this value.
If Type(R) is not Object, throw a TypeError exception.
If R does not have an [[OriginalFlags]] internal slot, then
1. If SameValue(R, %RegExp.prototype%) is true, return undefined.
2. Otherwise, throw a TypeError exception.
Let flags be R.[[OriginalFlags]].
If flags contains the code unit 0x0075 (LATIN SMALL LETTER U), return true.
Return false.

21.2.6 Properties of RegExp Instances

RegExp instances are ordinary objects that inherit properties from the RegExp prototype object. RegExp instances have internal slots [[RegExpMatcher]], [[OriginalSource]], and [[OriginalFlags]]. The value of the [[RegExpMatcher]] internal slot is an Abstract Closure representation of the Pattern of the RegExp object.

Note

Prior to ECMAScript 2015, RegExp instances were specified as having the own data properties "source", "global", "ignoreCase", and "multiline". Those properties are now specified as accessor properties of RegExp.prototype.

RegExp instances also have the following property:

21.2.6.1 lastIndex

The value of the "lastIndex" property specifies the String index at which to start the next match. It is coerced to an integer when used (see 21.2.5.2.2). This property shall have the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

21.2.7 RegExp String Iterator Objects

A RegExp String Iterator is an object, that represents a specific iteration over some specific String instance object, matching against some specific RegExp instance object. There is not a named constructor for RegExp String Iterator objects. Instead, RegExp String Iterator objects are created by calling certain methods of RegExp instance objects.

21.2.7.1 The %RegExpStringIteratorPrototype% Object

The %RegExpStringIteratorPrototype% object:

has properties that are inherited by all RegExp String Iterator Objects.
is an ordinary object.
has a [[Prototype]] internal slot whose value is the intrinsic object %IteratorPrototype%.
has the following properties:

21.2.7.1.1 %RegExpStringIteratorPrototype%.next ( )

Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have all of the internal slots of a RegExp String Iterator Object Instance (see 21.2.7.2), throw a TypeError exception.
If O.[[Done]] is true, then
1. Return ! CreateIterResultObject(undefined, true).
Let R be O.[[IteratingRegExp]].
Let S be O.[[IteratedString]].
Let global be O.[[Global]].
Let fullUnicode be O.[[Unicode]].
Let match be ? RegExpExec(R, S).
If match is null, then
1. Set O.[[Done]] to true.
2. Return ! CreateIterResultObject(undefined, true).
Else,
1. If global is true, then
  1. Let matchStr be ? ToString(? Get(match, "0")).
  2. If matchStr is the empty String, then
    1. Let thisIndex be ? ToLength(? Get(R, "lastIndex")).
    2. Let nextIndex be ! AdvanceStringIndex(S, thisIndex, fullUnicode).
    3. Perform ? Set(R, "lastIndex", nextIndex, true).
  3. Return ! CreateIterResultObject(match, false).
2. Else,
  1. Set O.[[Done]] to true.
  2. Return ! CreateIterResultObject(match, false).

21.2.7.1.2 %RegExpStringIteratorPrototype% [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "RegExp String Iterator".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

21.2.7.2 Properties of RegExp String Iterator Instances

RegExp String Iterator instances are ordinary objects that inherit properties from the %RegExpStringIteratorPrototype% intrinsic object. RegExp String Iterator instances are initially created with the internal slots listed in Table 59.

Table 59: Internal Slots of RegExp String Iterator Instances

Internal Slot	Description
[[IteratingRegExp]]	The regular expression used for iteration. IsRegExp([[IteratingRegExp]]) is initially true.
[[IteratedString]]	The String value being iterated upon.
[[Global]]	A Boolean value to indicate whether the [[IteratingRegExp]] is global or not.
[[Unicode]]	A Boolean value to indicate whether the [[IteratingRegExp]] is in Unicode mode or not.
[[Done]]	A Boolean value to indicate whether the iteration is complete or not.

22 Indexed Collections

22.1 Array Objects

Array objects are exotic objects that give special treatment to a certain class of property names. See 9.4.2 for a definition of this special treatment.

22.1.1 The Array Constructor

The Array constructor:

is the intrinsic object %Array%.
is the initial value of the "Array" property of the global object.
creates and initializes a new Array exotic object when called as a constructor.
also creates and initializes a new Array object when called as a function rather than as a constructor. Thus the function call Array(…) is equivalent to the object creation expression new Array(…) with the same arguments.
is a single function whose behaviour is overloaded based upon the number and types of its arguments.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the exotic Array behaviour must include a super call to the Array constructor to initialize subclass instances that are Array exotic objects. However, most of the Array.prototype methods are generic methods that are not dependent upon their this value being an Array exotic object.
has a "length" property whose value is 1.

22.1.1.1 Array ( )

This description applies if and only if the Array constructor is called with no arguments.

Let numberOfArgs be the number of arguments passed to this function call.
Assert: numberOfArgs = 0.
If NewTarget is undefined, let newTarget be the active function object; else let newTarget be NewTarget.
Let proto be ? GetPrototypeFromConstructor(newTarget, "%Array.prototype%").
Return ! ArrayCreate(0, proto).

22.1.1.2 Array ( `len` )

This description applies if and only if the Array constructor is called with exactly one argument.

Let numberOfArgs be the number of arguments passed to this function call.
Assert: numberOfArgs = 1.
If NewTarget is undefined, let newTarget be the active function object; else let newTarget be NewTarget.
Let proto be ? GetPrototypeFromConstructor(newTarget, "%Array.prototype%").
Let array be ! ArrayCreate(0, proto).
If Type(len) is not Number, then
1. Perform ! CreateDataPropertyOrThrow(array, "0", len).
2. Let intLen be 1.
Else,
1. Let intLen be ToUint32(len).
2. If intLen ≠ len, throw a RangeError exception.
Perform ! Set(array, "length", intLen, true).
Return array.

22.1.1.3 Array ( ...`items` )

This description applies if and only if the Array constructor is called with at least two arguments.

When the Array function is called, the following steps are taken:

Let numberOfArgs be the number of arguments passed to this function call.
Assert: numberOfArgs ≥ 2.
If NewTarget is undefined, let newTarget be the active function object; else let newTarget be NewTarget.
Let proto be ? GetPrototypeFromConstructor(newTarget, "%Array.prototype%").
Let array be ? ArrayCreate(numberOfArgs, proto).
Let k be 0.
Let items be a zero-origined List containing the argument items in order.
Repeat, while k < numberOfArgs
1. Let Pk be ! ToString(k).
2. Let itemK be items[k].
3. Perform ! CreateDataPropertyOrThrow(array, Pk, itemK).
4. Set k to k + 1.
Assert: The value of array's "length" property is numberOfArgs.
Return array.

22.1.2 Properties of the Array Constructor

The Array constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

22.1.2.1 Array.from ( `items` [ , `mapfn` [ , `thisArg` ] ] )

When the from method is called with argument items and optional arguments mapfn and thisArg, the following steps are taken:

Let C be the this value.
If mapfn is undefined, let mapping be false.
Else,
1. If IsCallable(mapfn) is false, throw a TypeError exception.
2. Let mapping be true.
Let usingIterator be ? GetMethod(items, @@iterator).
If usingIterator is not undefined, then
1. If IsConstructor(C) is true, then
  1. Let A be ? Construct(C).
2. Else,
  1. Let A be ! ArrayCreate(0).
3. Let iteratorRecord be ? GetIterator(items, sync, usingIterator).
4. Let k be 0.
5. Repeat,
  1. If k ≥ 2⁵³ - 1, then
    1. Let error be ThrowCompletion(a newly created TypeError object).
    2. Return ? IteratorClose(iteratorRecord, error).
  2. Let Pk be ! ToString(k).
  3. Let next be ? IteratorStep(iteratorRecord).
  4. If next is false, then
    1. Perform ? Set(A, "length", k, true).
    2. Return A.
  5. Let nextValue be ? IteratorValue(next).
  6. If mapping is true, then
    1. Let mappedValue be Call(mapfn, thisArg, « nextValue, k »).
    2. If mappedValue is an abrupt completion, return ? IteratorClose(iteratorRecord, mappedValue).
    3. Set mappedValue to mappedValue.[[Value]].
  7. Else, let mappedValue be nextValue.
  8. Let defineStatus be CreateDataPropertyOrThrow(A, Pk, mappedValue).
  9. If defineStatus is an abrupt completion, return ? IteratorClose(iteratorRecord, defineStatus).
  10. Set k to k + 1.
NOTE: items is not an Iterable so assume it is an array-like object.
Let arrayLike be ! ToObject(items).
Let len be ? LengthOfArrayLike(arrayLike).
If IsConstructor(C) is true, then
1. Let A be ? Construct(C, « len »).
Else,
1. Let A be ? ArrayCreate(len).
Let k be 0.
Repeat, while k < len
1. Let Pk be ! ToString(k).
2. Let kValue be ? Get(arrayLike, Pk).
3. If mapping is true, then
  1. Let mappedValue be ? Call(mapfn, thisArg, « kValue, k »).
4. Else, let mappedValue be kValue.
5. Perform ? CreateDataPropertyOrThrow(A, Pk, mappedValue).
6. Set k to k + 1.
Perform ? Set(A, "length", len, true).
Return A.

Note

The from function is an intentionally generic factory method; it does not require that its this value be the Array constructor. Therefore it can be transferred to or inherited by any other constructors that may be called with a single numeric argument.

22.1.2.2 Array.isArray ( `arg` )

The isArray function takes one argument arg, and performs the following steps:

Return ? IsArray(arg).

22.1.2.3 Array.of ( ...`items` )

When the of method is called with any number of arguments, the following steps are taken:

Let len be the actual number of arguments passed to this function.
Let items be the List of arguments passed to this function.
Let C be the this value.
If IsConstructor(C) is true, then
1. Let A be ? Construct(C, « len »).
Else,
1. Let A be ? ArrayCreate(len).
Let k be 0.
Repeat, while k < len
1. Let kValue be items[k].
2. Let Pk be ! ToString(k).
3. Perform ? CreateDataPropertyOrThrow(A, Pk, kValue).
4. Set k to k + 1.
Perform ? Set(A, "length", len, true).
Return A.

Note 1

The items argument is assumed to be a well-formed rest argument value.

Note 2

The of function is an intentionally generic factory method; it does not require that its this value be the Array constructor. Therefore it can be transferred to or inherited by other constructors that may be called with a single numeric argument.

22.1.2.4 Array.prototype

The value of Array.prototype is %Array.prototype%, the intrinsic Array prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

22.1.2.5 get Array [ @@species ]

Array[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

Note

Array prototype methods normally use their this object's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.

22.1.3 Properties of the Array Prototype Object

The Array prototype object:

is the intrinsic object %ArrayPrototype%.
is an Array exotic object and has the internal methods specified for such objects.
has a "length" property whose initial value is 0 and whose attributes are { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
has a [[Prototype]] internal slot whose value is %Object.prototype%.

Note

The Array prototype object is specified to be an Array exotic object to ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.

22.1.3.1 Array.prototype.concat ( ...`arguments` )

When the concat method is called with zero or more arguments, it returns an array containing the array elements of the object followed by the array elements of each argument in order.

The following steps are taken:

Let O be ? ToObject(this value).
Let A be ? ArraySpeciesCreate(O, 0).
Let n be 0.
Let items be a List whose first element is O and whose subsequent elements are, in left to right order, the arguments that were passed to this function invocation.
Repeat, while items is not empty
1. Remove the first element from items and let E be the value of the element.
2. Let spreadable be ? IsConcatSpreadable(E).
3. If spreadable is true, then
  1. Let k be 0.
  2. Let len be ? LengthOfArrayLike(E).
  3. If n + len > 2⁵³ - 1, throw a TypeError exception.
  4. Repeat, while k < len
    1. Let P be ! ToString(k).
    2. Let exists be ? HasProperty(E, P).
    3. If exists is true, then
      1. Let subElement be ? Get(E, P).
      2. Perform ? CreateDataPropertyOrThrow(A, ! ToString(n), subElement).
    4. Set n to n + 1.
    5. Set k to k + 1.
4. Else,
  1. NOTE: E is added as a single item rather than spread.
  2. If n ≥ 2⁵³ - 1, throw a TypeError exception.
  3. Perform ? CreateDataPropertyOrThrow(A, ! ToString(n), E).
  4. Set n to n + 1.
Perform ? Set(A, "length", n, true).
Return A.

The "length" property of the concat method is 1.

Note 1

The explicit setting of the "length" property in step 6 is necessary to ensure that its value is correct in situations where the trailing elements of the result Array are not present.

Note 2

The concat function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.1.1 Runtime Semantics: IsConcatSpreadable ( `O` )

The abstract operation IsConcatSpreadable takes argument O. It performs the following steps when called:

If Type(O) is not Object, return false.
Let spreadable be ? Get(O, @@isConcatSpreadable).
If spreadable is not undefined, return ! ToBoolean(spreadable).
Return ? IsArray(O).

22.1.3.2 Array.prototype.constructor

The initial value of Array.prototype.constructor is %Array%.

22.1.3.3 Array.prototype.copyWithin ( `target`, `start` [ , `end` ] )

The copyWithin method takes up to three arguments target, start and end.

Note 1

The end argument is optional with the length of the this object as its default value. If target is negative, it is treated as length + target where length is the length of the array. If start is negative, it is treated as length + start. If end is negative, it is treated as length + end.

The following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
Let relativeTarget be ? ToInteger(target).
If relativeTarget < 0, let to be max((len + relativeTarget), 0); else let to be min(relativeTarget, len).
Let relativeStart be ? ToInteger(start).
If relativeStart < 0, let from be max((len + relativeStart), 0); else let from be min(relativeStart, len).
If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToInteger(end).
If relativeEnd < 0, let final be max((len + relativeEnd), 0); else let final be min(relativeEnd, len).
Let count be min(final - from, len - to).
If from < to and to < from + count, then
1. Let direction be -1.
2. Set from to from + count - 1.
3. Set to to to + count - 1.
Else,
1. Let direction be 1.
Repeat, while count > 0
1. Let fromKey be ! ToString(from).
2. Let toKey be ! ToString(to).
3. Let fromPresent be ? HasProperty(O, fromKey).
4. If fromPresent is true, then
  1. Let fromVal be ? Get(O, fromKey).
  2. Perform ? Set(O, toKey, fromVal, true).
5. Else,
  1. Assert: fromPresent is false.
  2. Perform ? DeletePropertyOrThrow(O, toKey).
6. Set from to from + direction.
7. Set to to to + direction.
8. Set count to count - 1.
Return O.

Note 2

The copyWithin function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.4 Array.prototype.entries ( )

The following steps are taken:

Let O be ? ToObject(this value).
Return CreateArrayIterator(O, key+value).

This function is the %ArrayProto_entries% intrinsic object.

22.1.3.5 Array.prototype.every ( `callbackfn` [ , `thisArg` ] )

Note 1

callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. every calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returns false. If such an element is found, every immediately returns false. Otherwise, if callbackfn returned true for all elements, every will return true. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

every does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by every is set before the first call to callbackfn. Elements which are appended to the array after the call to every begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time every visits them; elements that are deleted after the call to every begins and before being visited are not visited. every acts like the "for all" quantifier in mathematics. In particular, for an empty array, it returns true.

When the every method is called with one or two arguments, the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
If IsCallable(callbackfn) is false, throw a TypeError exception.
Let k be 0.
Repeat, while k < len
1. Let Pk be ! ToString(k).
2. Let kPresent be ? HasProperty(O, Pk).
3. If kPresent is true, then
  1. Let kValue be ? Get(O, Pk).
  2. Let testResult be ! ToBoolean(? Call(callbackfn, thisArg, « kValue, k, O »)).
  3. If testResult is false, return false.
4. Set k to k + 1.
Return true.

Note 2

The every function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.6 Array.prototype.fill ( `value` [ , `start` [ , `end` ] ] )

The fill method takes up to three arguments value, start and end.

Note 1

The start and end arguments are optional with default values of 0 and the length of the this object. If start is negative, it is treated as length + start where length is the length of the array. If end is negative, it is treated as length + end.

The following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
Let relativeStart be ? ToInteger(start).
If relativeStart < 0, let k be max((len + relativeStart), 0); else let k be min(relativeStart, len).
If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToInteger(end).
If relativeEnd < 0, let final be max((len + relativeEnd), 0); else let final be min(relativeEnd, len).
Repeat, while k < final
1. Let Pk be ! ToString(k).
2. Perform ? Set(O, Pk, value, true).
3. Set k to k + 1.
Return O.

Note 2

The fill function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.7 Array.prototype.filter ( `callbackfn` [ , `thisArg` ] )

Note 1

callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. filter calls callbackfn once for each element in the array, in ascending order, and constructs a new array of all the values for which callbackfn returns true. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

filter does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by filter is set before the first call to callbackfn. Elements which are appended to the array after the call to filter begins will not be visited by callbackfn. If existing elements of the array are changed their value as passed to callbackfn will be the value at the time filter visits them; elements that are deleted after the call to filter begins and before being visited are not visited.

When the filter method is called with one or two arguments, the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
If IsCallable(callbackfn) is false, throw a TypeError exception.
Let A be ? ArraySpeciesCreate(O, 0).
Let k be 0.
Let to be 0.
Repeat, while k < len
1. Let Pk be ! ToString(k).
2. Let kPresent be ? HasProperty(O, Pk).
3. If kPresent is true, then
  1. Let kValue be ? Get(O, Pk).
  2. Let selected be ! ToBoolean(? Call(callbackfn, thisArg, « kValue, k, O »)).
  3. If selected is true, then
    1. Perform ? CreateDataPropertyOrThrow(A, ! ToString(to), kValue).
    2. Set to to to + 1.
4. Set k to k + 1.
Return A.

Note 2

The filter function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.8 Array.prototype.find ( `predicate` [ , `thisArg` ] )

The find method is called with one or two arguments, predicate and thisArg.

Note 1

predicate should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. find calls predicate once for each element of the array, in ascending order, until it finds one where predicate returns true. If such an element is found, find immediately returns that element value. Otherwise, find returns undefined.

If a thisArg parameter is provided, it will be used as the this value for each invocation of predicate. If it is not provided, undefined is used instead.

predicate is called with three arguments: the value of the element, the index of the element, and the object being traversed.

find does not directly mutate the object on which it is called but the object may be mutated by the calls to predicate.

The range of elements processed by find is set before the first call to predicate. Elements that are appended to the array after the call to find begins will not be visited by predicate. If existing elements of the array are changed, their value as passed to predicate will be the value at the time that find visits them.

When the find method is called, the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
If IsCallable(predicate) is false, throw a TypeError exception.
Let k be 0.
Repeat, while k < len
1. Let Pk be ! ToString(k).
2. Let kValue be ? Get(O, Pk).
3. Let testResult be ! ToBoolean(? Call(predicate, thisArg, « kValue, k, O »)).
4. If testResult is true, return kValue.
5. Set k to k + 1.
Return undefined.

Note 2

The find function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.9 Array.prototype.findIndex ( `predicate` [ , `thisArg` ] )

Note 1

predicate should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. findIndex calls predicate once for each element of the array, in ascending order, until it finds one where predicate returns true. If such an element is found, findIndex immediately returns the index of that element value. Otherwise, findIndex returns -1.

If a thisArg parameter is provided, it will be used as the this value for each invocation of predicate. If it is not provided, undefined is used instead.

predicate is called with three arguments: the value of the element, the index of the element, and the object being traversed.

findIndex does not directly mutate the object on which it is called but the object may be mutated by the calls to predicate.

The range of elements processed by findIndex is set before the first call to predicate. Elements that are appended to the array after the call to findIndex begins will not be visited by predicate. If existing elements of the array are changed, their value as passed to predicate will be the value at the time that findIndex visits them.

When the findIndex method is called with one or two arguments, the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
If IsCallable(predicate) is false, throw a TypeError exception.
Let k be 0.
Repeat, while k < len
1. Let Pk be ! ToString(k).
2. Let kValue be ? Get(O, Pk).
3. Let testResult be ! ToBoolean(? Call(predicate, thisArg, « kValue, k, O »)).
4. If testResult is true, return k.
5. Set k to k + 1.
Return -1.

Note 2

The findIndex function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.10 Array.prototype.flat ( [ `depth` ] )

When the flat method is called with zero or one arguments, the following steps are taken:

Let O be ? ToObject(this value).
Let sourceLen be ? LengthOfArrayLike(O).
Let depthNum be 1.
If depth is not undefined, then
1. Set depthNum to ? ToInteger(depth).
Let A be ? ArraySpeciesCreate(O, 0).
Perform ? FlattenIntoArray(A, O, sourceLen, 0, depthNum).
Return A.

22.1.3.10.1 FlattenIntoArray ( `target`, `source`, `sourceLen`, `start`, `depth` [ , `mapperFunction`, `thisArg` ] )

The abstract operation FlattenIntoArray takes arguments target, source, sourceLen, start, and depth and optional arguments mapperFunction and thisArg. It performs the following steps when called:

Assert: Type(target) is Object.
Assert: Type(source) is Object.
Assert: ! IsNonNegativeInteger(sourceLen) is true.
Assert: ! IsNonNegativeInteger(start) is true.
Assert: ! IsInteger(depth) is true, or depth is either +∞ or -∞.
Assert: If mapperFunction is present, then ! IsCallable(mapperFunction) is true, thisArg is present, and depth is 1.
Let targetIndex be start.
Let sourceIndex be 0.
Repeat, while sourceIndex < sourceLen
1. Let P be ! ToString(sourceIndex).
2. Let exists be ? HasProperty(source, P).
3. If exists is true, then
  1. Let element be ? Get(source, P).
  2. If mapperFunction is present, then
    1. Set element to ? Call(mapperFunction, thisArg, « element, sourceIndex, source »).
  3. Let shouldFlatten be false.
  4. If depth > 0, then
    1. Set shouldFlatten to ? IsArray(element).
  5. If shouldFlatten is true, then
    1. Let elementLen be ? LengthOfArrayLike(element).
    2. Set targetIndex to ? FlattenIntoArray(target, element, elementLen, targetIndex, depth - 1).
  6. Else,
    1. If targetIndex ≥ 2⁵³ - 1, throw a TypeError exception.
    2. Perform ? CreateDataPropertyOrThrow(target, ! ToString(targetIndex), element).
    3. Set targetIndex to targetIndex + 1.
4. Set sourceIndex to sourceIndex + 1.
Return targetIndex.

22.1.3.11 Array.prototype.flatMap ( `mapperFunction` [ , `thisArg` ] )

When the flatMap method is called with one or two arguments, the following steps are taken:

Let O be ? ToObject(this value).
Let sourceLen be ? LengthOfArrayLike(O).
If ! IsCallable(mapperFunction) is false, throw a TypeError exception.
Let A be ? ArraySpeciesCreate(O, 0).
Perform ? FlattenIntoArray(A, O, sourceLen, 0, 1, mapperFunction, thisArg).
Return A.

22.1.3.12 Array.prototype.forEach ( `callbackfn` [ , `thisArg` ] )

Note 1

callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each element present in the array, in ascending order. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

When the forEach method is called with one or two arguments, the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
If IsCallable(callbackfn) is false, throw a TypeError exception.
Let k be 0.
Repeat, while k < len
1. Let Pk be ! ToString(k).
2. Let kPresent be ? HasProperty(O, Pk).
3. If kPresent is true, then
  1. Let kValue be ? Get(O, Pk).
  2. Perform ? Call(callbackfn, thisArg, « kValue, k, O »).
4. Set k to k + 1.
Return undefined.

This function is the %ArrayProto_forEach% intrinsic object.

Note 2

The forEach function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.13 Array.prototype.includes ( `searchElement` [ , `fromIndex` ] )

Note 1

includes compares searchElement to the elements of the array, in ascending order, using the SameValueZero algorithm, and if found at any position, returns true; otherwise, false is returned.

The optional second argument fromIndex defaults to 0 (i.e. the whole array is searched). If it is greater than or equal to the length of the array, false is returned, i.e. the array will not be searched. If it is negative, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than 0, the whole array will be searched.

When the includes method is called, the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
If len is 0, return false.
Let n be ? ToInteger(fromIndex).
Assert: If fromIndex is undefined, then n is 0.
If n ≥ 0, then
1. Let k be n.
Else,
1. Let k be len + n.
2. If k < 0, set k to 0.
Repeat, while k < len
1. Let elementK be the result of ? Get(O, ! ToString(k)).
2. If SameValueZero(searchElement, elementK) is true, return true.
3. Set k to k + 1.
Return false.

Note 2

The includes function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

Note 3

The includes method intentionally differs from the similar indexOf method in two ways. First, it uses the SameValueZero algorithm, instead of Strict Equality Comparison, allowing it to detect NaN array elements. Second, it does not skip missing array elements, instead treating them as undefined.

22.1.3.14 Array.prototype.indexOf ( `searchElement` [ , `fromIndex` ] )

Note 1

indexOf compares searchElement to the elements of the array, in ascending order, using the Strict Equality Comparison algorithm, and if found at one or more indices, returns the smallest such index; otherwise, -1 is returned.

The optional second argument fromIndex defaults to 0 (i.e. the whole array is searched). If it is greater than or equal to the length of the array, -1 is returned, i.e. the array will not be searched. If it is negative, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than 0, the whole array will be searched.

When the indexOf method is called with one or two arguments, the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
If len is 0, return -1.
Let n be ? ToInteger(fromIndex).
Assert: If fromIndex is undefined, then n is 0.
If n ≥ len, return -1.
If n ≥ 0, then
1. Let k be n.
Else,
1. Let k be len + n.
2. If k < 0, set k to 0.
Repeat, while k < len
1. Let kPresent be ? HasProperty(O, ! ToString(k)).
2. If kPresent is true, then
  1. Let elementK be ? Get(O, ! ToString(k)).
  2. Let same be the result of performing Strict Equality Comparison searchElement === elementK.
  3. If same is true, return k.
3. Set k to k + 1.
Return -1.

Note 2

The indexOf function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.15 Array.prototype.join ( `separator` )

Note 1

The elements of the array are converted to Strings, and these Strings are then concatenated, separated by occurrences of the separator. If no separator is provided, a single comma is used as the separator.

The join method takes one argument, separator, and performs the following steps:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
If separator is undefined, let sep be the single-element String ",".
Else, let sep be ? ToString(separator).
Let R be the empty String.
Let k be 0.
Repeat, while k < len
1. If k > 0, set R to the string-concatenation of R and sep.
2. Let element be ? Get(O, ! ToString(k)).
3. If element is undefined or null, let next be the empty String; otherwise, let next be ? ToString(element).
4. Set R to the string-concatenation of R and next.
5. Set k to k + 1.
Return R.

Note 2

The join function is intentionally generic; it does not require that its this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.16 Array.prototype.keys ( )

The following steps are taken:

Let O be ? ToObject(this value).
Return CreateArrayIterator(O, key).

This function is the %ArrayProto_keys% intrinsic object.

22.1.3.17 Array.prototype.lastIndexOf ( `searchElement` [ , `fromIndex` ] )

Note 1

lastIndexOf compares searchElement to the elements of the array in descending order using the Strict Equality Comparison algorithm, and if found at one or more indices, returns the largest such index; otherwise, -1 is returned.

The optional second argument fromIndex defaults to the array's length minus one (i.e. the whole array is searched). If it is greater than or equal to the length of the array, the whole array will be searched. If it is negative, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than 0, -1 is returned.

When the lastIndexOf method is called with one or two arguments, the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
If len is 0, return -1.
If fromIndex is present, let n be ? ToInteger(fromIndex); else let n be len - 1.
If n ≥ 0, then
1. Let k be min(n, len - 1).
Else,
1. Let k be len + n.
Repeat, while k ≥ 0
1. Let kPresent be ? HasProperty(O, ! ToString(k)).
2. If kPresent is true, then
  1. Let elementK be ? Get(O, ! ToString(k)).
  2. Let same be the result of performing Strict Equality Comparison searchElement === elementK.
  3. If same is true, return k.
3. Set k to k - 1.
Return -1.

Note 2

The lastIndexOf function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.18 Array.prototype.map ( `callbackfn` [ , `thisArg` ] )

Note 1

callbackfn should be a function that accepts three arguments. map calls callbackfn once for each element in the array, in ascending order, and constructs a new Array from the results. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

map does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by map is set before the first call to callbackfn. Elements which are appended to the array after the call to map begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time map visits them; elements that are deleted after the call to map begins and before being visited are not visited.

When the map method is called with one or two arguments, the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
If IsCallable(callbackfn) is false, throw a TypeError exception.
Let A be ? ArraySpeciesCreate(O, len).
Let k be 0.
Repeat, while k < len
1. Let Pk be ! ToString(k).
2. Let kPresent be ? HasProperty(O, Pk).
3. If kPresent is true, then
  1. Let kValue be ? Get(O, Pk).
  2. Let mappedValue be ? Call(callbackfn, thisArg, « kValue, k, O »).
  3. Perform ? CreateDataPropertyOrThrow(A, Pk, mappedValue).
4. Set k to k + 1.
Return A.

Note 2

The map function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.19 Array.prototype.pop ( )

Note 1

The last element of the array is removed from the array and returned.

When the pop method is called, the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
If len is zero, then
1. Perform ? Set(O, "length", 0, true).
2. Return undefined.
Else,
1. Assert: len > 0.
2. Let newLen be len - 1.
3. Let index be ! ToString(newLen).
4. Let element be ? Get(O, index).
5. Perform ? DeletePropertyOrThrow(O, index).
6. Perform ? Set(O, "length", newLen, true).
7. Return element.

Note 2

The pop function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.20 Array.prototype.push ( ...`items` )

Note 1

The arguments are appended to the end of the array, in the order in which they appear. The new length of the array is returned as the result of the call.

When the push method is called with zero or more arguments, the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
Let items be a List whose elements are, in left to right order, the arguments that were passed to this function invocation.
Let argCount be the number of elements in items.
If len + argCount > 2⁵³ - 1, throw a TypeError exception.
Repeat, while items is not empty
1. Remove the first element from items and let E be the value of the element.
2. Perform ? Set(O, ! ToString(len), E, true).
3. Set len to len + 1.
Perform ? Set(O, "length", len, true).
Return len.

The "length" property of the push method is 1.

Note 2

The push function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.21 Array.prototype.reduce ( `callbackfn` [ , `initialValue` ] )

Note 1

callbackfn should be a function that takes four arguments. reduce calls the callback, as a function, once for each element after the first element present in the array, in ascending order.

callbackfn is called with four arguments: the previousValue (value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time that callback is called, the previousValue and currentValue can be one of two values. If an initialValue was supplied in the call to reduce, then previousValue will be equal to initialValue and currentValue will be equal to the first value in the array. If no initialValue was supplied, then previousValue will be equal to the first value in the array and currentValue will be equal to the second. It is a TypeError if the array contains no elements and initialValue is not provided.

reduce does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by reduce is set before the first call to callbackfn. Elements that are appended to the array after the call to reduce begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time reduce visits them; elements that are deleted after the call to reduce begins and before being visited are not visited.

When the reduce method is called with one or two arguments, the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
If IsCallable(callbackfn) is false, throw a TypeError exception.
If len is 0 and initialValue is not present, throw a TypeError exception.
Let k be 0.
Let accumulator be undefined.
If initialValue is present, then
1. Set accumulator to initialValue.
Else,
1. Let kPresent be false.
2. Repeat, while kPresent is false and k < len
  1. Let Pk be ! ToString(k).
  2. Set kPresent to ? HasProperty(O, Pk).
  3. If kPresent is true, then
    1. Set accumulator to ? Get(O, Pk).
  4. Set k to k + 1.
3. If kPresent is false, throw a TypeError exception.
Repeat, while k < len
1. Let Pk be ! ToString(k).
2. Let kPresent be ? HasProperty(O, Pk).
3. If kPresent is true, then
  1. Let kValue be ? Get(O, Pk).
  2. Set accumulator to ? Call(callbackfn, undefined, « accumulator, kValue, k, O »).
4. Set k to k + 1.
Return accumulator.

Note 2

The reduce function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.22 Array.prototype.reduceRight ( `callbackfn` [ , `initialValue` ] )

Note 1

callbackfn should be a function that takes four arguments. reduceRight calls the callback, as a function, once for each element after the first element present in the array, in descending order.

callbackfn is called with four arguments: the previousValue (value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time the function is called, the previousValue and currentValue can be one of two values. If an initialValue was supplied in the call to reduceRight, then previousValue will be equal to initialValue and currentValue will be equal to the last value in the array. If no initialValue was supplied, then previousValue will be equal to the last value in the array and currentValue will be equal to the second-to-last value. It is a TypeError if the array contains no elements and initialValue is not provided.

reduceRight does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by reduceRight is set before the first call to callbackfn. Elements that are appended to the array after the call to reduceRight begins will not be visited by callbackfn. If existing elements of the array are changed by callbackfn, their value as passed to callbackfn will be the value at the time reduceRight visits them; elements that are deleted after the call to reduceRight begins and before being visited are not visited.

When the reduceRight method is called with one or two arguments, the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
If IsCallable(callbackfn) is false, throw a TypeError exception.
If len is 0 and initialValue is not present, throw a TypeError exception.
Let k be len - 1.
Let accumulator be undefined.
If initialValue is present, then
1. Set accumulator to initialValue.
Else,
1. Let kPresent be false.
2. Repeat, while kPresent is false and k ≥ 0
  1. Let Pk be ! ToString(k).
  2. Set kPresent to ? HasProperty(O, Pk).
  3. If kPresent is true, then
    1. Set accumulator to ? Get(O, Pk).
  4. Set k to k - 1.
3. If kPresent is false, throw a TypeError exception.
Repeat, while k ≥ 0
1. Let Pk be ! ToString(k).
2. Let kPresent be ? HasProperty(O, Pk).
3. If kPresent is true, then
  1. Let kValue be ? Get(O, Pk).
  2. Set accumulator to ? Call(callbackfn, undefined, « accumulator, kValue, k, O »).
4. Set k to k - 1.
Return accumulator.

Note 2

The reduceRight function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.23 Array.prototype.reverse ( )

Note 1

The elements of the array are rearranged so as to reverse their order. The object is returned as the result of the call.

When the reverse method is called, the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
Let middle be floor(len / 2).
Let lower be 0.
Repeat, while lower ≠ middle
1. Let upper be len - lower - 1.
2. Let upperP be ! ToString(upper).
3. Let lowerP be ! ToString(lower).
4. Let lowerExists be ? HasProperty(O, lowerP).
5. If lowerExists is true, then
  1. Let lowerValue be ? Get(O, lowerP).
6. Let upperExists be ? HasProperty(O, upperP).
7. If upperExists is true, then
  1. Let upperValue be ? Get(O, upperP).
8. If lowerExists is true and upperExists is true, then
  1. Perform ? Set(O, lowerP, upperValue, true).
  2. Perform ? Set(O, upperP, lowerValue, true).
9. Else if lowerExists is false and upperExists is true, then
  1. Perform ? Set(O, lowerP, upperValue, true).
  2. Perform ? DeletePropertyOrThrow(O, upperP).
10. Else if lowerExists is true and upperExists is false, then
  1. Perform ? DeletePropertyOrThrow(O, lowerP).
  2. Perform ? Set(O, upperP, lowerValue, true).
11. Else,
  1. Assert: lowerExists and upperExists are both false.
  2. No action is required.
12. Set lower to lower + 1.
Return O.

Note 2

The reverse function is intentionally generic; it does not require that its this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.24 Array.prototype.shift ( )

Note 1

The first element of the array is removed from the array and returned.

When the shift method is called, the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
If len is zero, then
1. Perform ? Set(O, "length", 0, true).
2. Return undefined.
Let first be ? Get(O, "0").
Let k be 1.
Repeat, while k < len
1. Let from be ! ToString(k).
2. Let to be ! ToString(k - 1).
3. Let fromPresent be ? HasProperty(O, from).
4. If fromPresent is true, then
  1. Let fromVal be ? Get(O, from).
  2. Perform ? Set(O, to, fromVal, true).
5. Else,
  1. Assert: fromPresent is false.
  2. Perform ? DeletePropertyOrThrow(O, to).
6. Set k to k + 1.
Perform ? DeletePropertyOrThrow(O, ! ToString(len - 1)).
Perform ? Set(O, "length", len - 1, true).
Return first.

Note 2

The shift function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.25 Array.prototype.slice ( `start`, `end` )

Note 1

The slice method takes two arguments, start and end, and returns an array containing the elements of the array from element start up to, but not including, element end (or through the end of the array if end is undefined). If start is negative, it is treated as length + start where length is the length of the array. If end is negative, it is treated as length + end where length is the length of the array.

The following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
Let relativeStart be ? ToInteger(start).
If relativeStart < 0, let k be max((len + relativeStart), 0); else let k be min(relativeStart, len).
If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToInteger(end).
If relativeEnd < 0, let final be max((len + relativeEnd), 0); else let final be min(relativeEnd, len).
Let count be max(final - k, 0).
Let A be ? ArraySpeciesCreate(O, count).
Let n be 0.
Repeat, while k < final
1. Let Pk be ! ToString(k).
2. Let kPresent be ? HasProperty(O, Pk).
3. If kPresent is true, then
  1. Let kValue be ? Get(O, Pk).
  2. Perform ? CreateDataPropertyOrThrow(A, ! ToString(n), kValue).
4. Set k to k + 1.
5. Set n to n + 1.
Perform ? Set(A, "length", n, true).
Return A.

Note 2

The explicit setting of the "length" property of the result Array in step 11 was necessary in previous editions of ECMAScript to ensure that its length was correct in situations where the trailing elements of the result Array were not present. Setting "length" became unnecessary starting in ES2015 when the result Array was initialized to its proper length rather than an empty Array but is carried forward to preserve backward compatibility.

Note 3

The slice function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.26 Array.prototype.some ( `callbackfn` [ , `thisArg` ] )

Note 1

callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. some calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returns true. If such an element is found, some immediately returns true. Otherwise, some returns false. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

some does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by some is set before the first call to callbackfn. Elements that are appended to the array after the call to some begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time that some visits them; elements that are deleted after the call to some begins and before being visited are not visited. some acts like the "exists" quantifier in mathematics. In particular, for an empty array, it returns false.

When the some method is called with one or two arguments, the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
If IsCallable(callbackfn) is false, throw a TypeError exception.
Let k be 0.
Repeat, while k < len
1. Let Pk be ! ToString(k).
2. Let kPresent be ? HasProperty(O, Pk).
3. If kPresent is true, then
  1. Let kValue be ? Get(O, Pk).
  2. Let testResult be ! ToBoolean(? Call(callbackfn, thisArg, « kValue, k, O »)).
  3. If testResult is true, return true.
4. Set k to k + 1.
Return false.

Note 2

The some function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.27 Array.prototype.sort ( `comparefn` )

The elements of this array are sorted. The sort must be stable (that is, elements that compare equal must remain in their original order). If comparefn is not undefined, it should be a function that accepts two arguments x and y and returns a negative value if x < y, zero if x = y, or a positive value if x > y.

Upon entry, the following steps are performed to initialize evaluation of the sort function:

If comparefn is not undefined and IsCallable(comparefn) is false, throw a TypeError exception.
Let obj be ? ToObject(this value).
Let len be ? LengthOfArrayLike(obj).

Within this specification of the sort method, an object, obj, is said to be sparse if the following algorithm returns true:

For each integer i in the range 0 ≤ i < len, do
1. Let elem be obj.[[GetOwnProperty]](! ToString(i)).
2. If elem is undefined, return true.
Return false.

The sort order is the ordering, after completion of this function, of the integer-indexed property values of obj whose integer indexes are less than len. The result of the sort function is then determined as follows:

If comparefn is not undefined and is not a consistent comparison function for the elements of this array (see below), the sort order is implementation-defined. The sort order is also implementation-defined if comparefn is undefined and SortCompare does not act as a consistent comparison function.

Let proto be obj.[[GetPrototypeOf]](). If proto is not null and there exists an integer j such that all of the conditions below are satisfied then the sort order is implementation-defined:

obj is sparse
0 ≤ j < len
HasProperty(proto, ToString(j)) is true.

The sort order is also implementation-defined if obj is sparse and any of the following conditions are true:

IsExtensible(obj) is false.
Any integer index property of obj whose name is a nonnegative integer less than len is a data property whose [[Configurable]] attribute is false.

The sort order is also implementation-defined if any of the following conditions are true:

If obj is an exotic object (including Proxy exotic objects) whose behaviour for [[Get]], [[Set]], [[Delete]], and [[GetOwnProperty]] is not the ordinary object implementation of these internal methods.
If any index property of obj whose name is a nonnegative integer less than len is an accessor property or is a data property whose [[Writable]] attribute is false.
If comparefn is undefined and the application of ToString to any value passed as an argument to SortCompare modifies obj or any object on obj's prototype chain.
If comparefn is undefined and all applications of ToString, to any specific value passed as an argument to SortCompare, do not produce the same result.

The following steps are taken:

Perform an implementation-dependent sequence of calls to the Get, Set, DeletePropertyOrThrow, and HasOwnProperty abstract operation with obj as the first argument, and to SortCompare (described below), such that:
- The property key argument for each call to Get, Set, HasOwnProperty, or DeletePropertyOrThrow is the string representation of a nonnegative integer less than len.
- The Throw argument for every call to Set is true.
- The arguments for calls to SortCompare are values returned by a previous call to the Get abstract operation, unless the properties accessed by those previous calls did not exist according to HasOwnProperty. If both prospective arguments to SortCompare correspond to non-existent properties, use +0 instead of calling SortCompare. If only the first prospective argument is non-existent use +1. If only the second prospective argument is non-existent use -1.
- If obj is not sparse then DeletePropertyOrThrow must not be called.
- If an abrupt completion is returned from any of these operations, it is immediately returned as the value of this function.
Return obj.

Unless the sort order is specified above to be implementation-defined, the returned object must have the following two characteristics:

There must be some mathematical permutation π of the nonnegative integers less than len, such that for every nonnegative integer j less than len, if property old[j] existed, then new[π(j)] is exactly the same value as old[j]. But if property old[j] did not exist, then new[π(j)] does not exist.
Then for all nonnegative integers j and k, each less than len, if SortCompare(old[j], old[k]) < 0 (see SortCompare below), then new[π(j)] < new[π(k)].

Here the notation old[j] is used to refer to the hypothetical result of calling Get(obj, j) before this function is executed, and the notation new[j] to refer to the hypothetical result of calling Get(obj, j) after this function has been executed.

A function comparefn is a consistent comparison function for a set of values S if all of the requirements below are met for all values a, b, and c (possibly the same value) in the set S: The notation a <_CF b means comparefn(a, b) < 0; a =_CF b means comparefn(a, b) = 0 (of either sign); and a >_CF b means comparefn(a, b) > 0.

Calling comparefn(a, b) always returns the same value v when given a specific pair of values a and b as its two arguments. Furthermore, Type(v) is Number, and v is not NaN. Note that this implies that exactly one of a <_CF b, a =_CF b, and a >_CF b will be true for a given pair of a and b.
Calling comparefn(a, b) does not modify obj or any object on obj's prototype chain.
a =_CF a (reflexivity)
If a =_CF b, then b =_CF a (symmetry)
If a =_CF b and b =_CF c, then a =_CF c (transitivity of =_CF)
If a <_CF b and b <_CF c, then a <_CF c (transitivity of <_CF)
If a >_CF b and b >_CF c, then a >_CF c (transitivity of >_CF)

Note 1

The above conditions are necessary and sufficient to ensure that comparefn divides the set S into equivalence classes and that these equivalence classes are totally ordered.

Note 2

The sort function is intentionally generic; it does not require that its this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.27.1 Runtime Semantics: SortCompare ( `x`, `y` )

The abstract operation SortCompare takes arguments x and y. It also has access to the comparefn argument passed to the current invocation of the sort method. It performs the following steps when called:

If x and y are both undefined, return +0.
If x is undefined, return 1.
If y is undefined, return -1.
If comparefn is not undefined, then
1. Let v be ? ToNumber(? Call(comparefn, undefined, « x, y »)).
2. If v is NaN, return +0.
3. Return v.
Let xString be ? ToString(x).
Let yString be ? ToString(y).
Let xSmaller be the result of performing Abstract Relational Comparison xString < yString.
If xSmaller is true, return -1.
Let ySmaller be the result of performing Abstract Relational Comparison yString < xString.
If ySmaller is true, return 1.
Return +0.

Note 1

Because non-existent property values always compare greater than undefined property values, and undefined always compares greater than any other value, undefined property values always sort to the end of the result, followed by non-existent property values.

Note 2

Method calls performed by the ToString abstract operations in steps 5 and 6 have the potential to cause SortCompare to not behave as a consistent comparison function.

22.1.3.28 Array.prototype.splice ( `start`, `deleteCount`, ...`items` )

Note 1

When the splice method is called with two or more arguments start, deleteCount and zero or more items, the deleteCount elements of the array starting at integer index start are replaced by the arguments items. An Array object containing the deleted elements (if any) is returned.

The following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
Let relativeStart be ? ToInteger(start).
If relativeStart < 0, let actualStart be max((len + relativeStart), 0); else let actualStart be min(relativeStart, len).
If the number of actual arguments is 0, then
1. Let insertCount be 0.
2. Let actualDeleteCount be 0.
Else if the number of actual arguments is 1, then
1. Let insertCount be 0.
2. Let actualDeleteCount be len - actualStart.
Else,
1. Let insertCount be the number of actual arguments minus 2.
2. Let dc be ? ToInteger(deleteCount).
3. Let actualDeleteCount be min(max(dc, 0), len - actualStart).
If len + insertCount - actualDeleteCount > 2⁵³ - 1, throw a TypeError exception.
Let A be ? ArraySpeciesCreate(O, actualDeleteCount).
Let k be 0.
Repeat, while k < actualDeleteCount
1. Let from be ! ToString(actualStart + k).
2. Let fromPresent be ? HasProperty(O, from).
3. If fromPresent is true, then
  1. Let fromValue be ? Get(O, from).
  2. Perform ? CreateDataPropertyOrThrow(A, ! ToString(k), fromValue).
4. Set k to k + 1.
Perform ? Set(A, "length", actualDeleteCount, true).
Let items be a List whose elements are, in left to right order, the portion of the actual argument list starting with the third argument. The list is empty if fewer than three arguments were passed.
Let itemCount be the number of elements in items.
If itemCount < actualDeleteCount, then
1. Set k to actualStart.
2. Repeat, while k < (len - actualDeleteCount)
  1. Let from be ! ToString(k + actualDeleteCount).
  2. Let to be ! ToString(k + itemCount).
  3. Let fromPresent be ? HasProperty(O, from).
  4. If fromPresent is true, then
    1. Let fromValue be ? Get(O, from).
    2. Perform ? Set(O, to, fromValue, true).
  5. Else,
    1. Assert: fromPresent is false.
    2. Perform ? DeletePropertyOrThrow(O, to).
  6. Set k to k + 1.
3. Set k to len.
4. Repeat, while k > (len - actualDeleteCount + itemCount)
  1. Perform ? DeletePropertyOrThrow(O, ! ToString(k - 1)).
  2. Set k to k - 1.
Else if itemCount > actualDeleteCount, then
1. Set k to (len - actualDeleteCount).
2. Repeat, while k > actualStart
  1. Let from be ! ToString(k + actualDeleteCount - 1).
  2. Let to be ! ToString(k + itemCount - 1).
  3. Let fromPresent be ? HasProperty(O, from).
  4. If fromPresent is true, then
    1. Let fromValue be ? Get(O, from).
    2. Perform ? Set(O, to, fromValue, true).
  5. Else,
    1. Assert: fromPresent is false.
    2. Perform ? DeletePropertyOrThrow(O, to).
  6. Set k to k - 1.
Set k to actualStart.
Repeat, while items is not empty
1. Remove the first element from items and let E be the value of that element.
2. Perform ? Set(O, ! ToString(k), E, true).
3. Set k to k + 1.
Perform ? Set(O, "length", len - actualDeleteCount + itemCount, true).
Return A.

Note 2

The explicit setting of the "length" property of the result Array in step 19 was necessary in previous editions of ECMAScript to ensure that its length was correct in situations where the trailing elements of the result Array were not present. Setting "length" became unnecessary starting in ES2015 when the result Array was initialized to its proper length rather than an empty Array but is carried forward to preserve backward compatibility.

Note 3

The splice function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.29 Array.prototype.toLocaleString ( [ `reserved1` [ , `reserved2` ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Array.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.

Note 1

The first edition of ECMA-402 did not include a replacement specification for the Array.prototype.toLocaleString method.

The following steps are taken:

Let array be ? ToObject(this value).
Let len be ? LengthOfArrayLike(array).
Let separator be the String value for the list-separator String appropriate for the host environment's current locale (this is derived in an implementation-defined way).
Let R be the empty String.
Let k be 0.
Repeat, while k < len
1. If k > 0, then
  1. Set R to the string-concatenation of R and separator.
2. Let nextElement be ? Get(array, ! ToString(k)).
3. If nextElement is not undefined or null, then
  1. Let S be ? ToString(? Invoke(nextElement, "toLocaleString")).
  2. Set R to the string-concatenation of R and S.
4. Set k to k + 1.
Return R.

Note 2

The elements of the array are converted to Strings using their toLocaleString methods, and these Strings are then concatenated, separated by occurrences of a separator String that has been derived in an implementation-defined locale-specific way. The result of calling this function is intended to be analogous to the result of toString, except that the result of this function is intended to be locale-specific.

Note 3

The toLocaleString function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.30 Array.prototype.toString ( )

When the toString method is called, the following steps are taken:

Let array be ? ToObject(this value).
Let func be ? Get(array, "join").
If IsCallable(func) is false, set func to the intrinsic function %Object.prototype.toString%.
Return ? Call(func, array).

Note

The toString function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.31 Array.prototype.unshift ( ...`items` )

Note 1

The arguments are prepended to the start of the array, such that their order within the array is the same as the order in which they appear in the argument list.

When the unshift method is called with zero or more arguments item1, item2, etc., the following steps are taken:

Let O be ? ToObject(this value).
Let len be ? LengthOfArrayLike(O).
Let argCount be the number of actual arguments.
If argCount > 0, then
1. If len + argCount > 2⁵³ - 1, throw a TypeError exception.
2. Let k be len.
3. Repeat, while k > 0,
  1. Let from be ! ToString(k - 1).
  2. Let to be ! ToString(k + argCount - 1).
  3. Let fromPresent be ? HasProperty(O, from).
  4. If fromPresent is true, then
    1. Let fromValue be ? Get(O, from).
    2. Perform ? Set(O, to, fromValue, true).
  5. Else,
    1. Assert: fromPresent is false.
    2. Perform ? DeletePropertyOrThrow(O, to).
  6. Set k to k - 1.
4. Let j be 0.
5. Let items be a List whose elements are, in left to right order, the arguments that were passed to this function invocation.
6. Repeat, while items is not empty
  1. Remove the first element from items and let E be the value of that element.
  2. Perform ? Set(O, ! ToString(j), E, true).
  3. Set j to j + 1.
Perform ? Set(O, "length", len + argCount, true).
Return len + argCount.

The "length" property of the unshift method is 1.

Note 2

The unshift function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.32 Array.prototype.values ( )

The following steps are taken:

Let O be ? ToObject(this value).
Return CreateArrayIterator(O, value).

This function is the %ArrayProto_values% intrinsic object.

22.1.3.33 Array.prototype [ @@iterator ] ( )

The initial value of the @@iterator property is the same function object as the initial value of the Array.prototype.values property.

22.1.3.34 Array.prototype [ @@unscopables ]

The initial value of the @@unscopables data property is an object created by the following steps:

Let unscopableList be OrdinaryObjectCreate(null).
Perform ! CreateDataPropertyOrThrow(unscopableList, "copyWithin", true).
Perform ! CreateDataPropertyOrThrow(unscopableList, "entries", true).
Perform ! CreateDataPropertyOrThrow(unscopableList, "fill", true).
Perform ! CreateDataPropertyOrThrow(unscopableList, "find", true).
Perform ! CreateDataPropertyOrThrow(unscopableList, "findIndex", true).
Perform ! CreateDataPropertyOrThrow(unscopableList, "flat", true).
Perform ! CreateDataPropertyOrThrow(unscopableList, "flatMap", true).
Perform ! CreateDataPropertyOrThrow(unscopableList, "includes", true).
Perform ! CreateDataPropertyOrThrow(unscopableList, "keys", true).
Perform ! CreateDataPropertyOrThrow(unscopableList, "values", true).
Return unscopableList.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

Note

The own property names of this object are property names that were not included as standard properties of Array.prototype prior to the ECMAScript 2015 specification. These names are ignored for with statement binding purposes in order to preserve the behaviour of existing code that might use one of these names as a binding in an outer scope that is shadowed by a with statement whose binding object is an Array object.

22.1.4 Properties of Array Instances

Array instances are Array exotic objects and have the internal methods specified for such objects. Array instances inherit properties from the Array prototype object.

Array instances have a "length" property, and a set of enumerable properties with array index names.

22.1.4.1 length

The "length" property of an Array instance is a data property whose value is always numerically greater than the name of every configurable own property whose name is an array index.

The "length" property initially has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

Note

Reducing the value of the "length" property has the side-effect of deleting own array elements whose array index is between the old and new length values. However, non-configurable properties can not be deleted. Attempting to set the "length" property of an Array object to a value that is numerically less than or equal to the largest numeric own property name of an existing non-configurable array-indexed property of the array will result in the length being set to a numeric value that is one greater than that non-configurable numeric own property name. See 9.4.2.1.

22.1.5 Array Iterator Objects

An Array Iterator is an object, that represents a specific iteration over some specific Array instance object. There is not a named constructor for Array Iterator objects. Instead, Array iterator objects are created by calling certain methods of Array instance objects.

22.1.5.1 CreateArrayIterator ( `array`, `kind` )

The abstract operation CreateArrayIterator takes arguments array and kind. This operation is used to create iterator objects for Array methods that return such iterators. It performs the following steps when called:

Assert: Type(array) is Object.
Assert: kind is key+value, key, or value.
Let iterator be OrdinaryObjectCreate(%ArrayIteratorPrototype%, « [[IteratedArrayLike]], [[ArrayLikeNextIndex]], [[ArrayLikeIterationKind]] »).
Set iterator.[[IteratedArrayLike]] to array.
Set iterator.[[ArrayLikeNextIndex]] to 0.
Set iterator.[[ArrayLikeIterationKind]] to kind.
Return iterator.

22.1.5.2 The %ArrayIteratorPrototype% Object

The %ArrayIteratorPrototype% object:

has properties that are inherited by all Array Iterator Objects.
is an ordinary object.
has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
has the following properties:

22.1.5.2.1 %ArrayIteratorPrototype%.next ( )

Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have all of the internal slots of an Array Iterator Instance (22.1.5.3), throw a TypeError exception.
Let a be O.[[IteratedArrayLike]].
If a is undefined, return CreateIterResultObject(undefined, true).
Let index be O.[[ArrayLikeNextIndex]].
Let itemKind be O.[[ArrayLikeIterationKind]].
If a has a [[TypedArrayName]] internal slot, then
1. If IsDetachedBuffer(a.[[ViewedArrayBuffer]]) is true, throw a TypeError exception.
2. Let len be a.[[ArrayLength]].
Else,
1. Let len be ? LengthOfArrayLike(a).
If index ≥ len, then
1. Set O.[[IteratedArrayLike]] to undefined.
2. Return CreateIterResultObject(undefined, true).
Set O.[[ArrayLikeNextIndex]] to index + 1.
If itemKind is key, return CreateIterResultObject(index, false).
Let elementKey be ! ToString(index).
Let elementValue be ? Get(a, elementKey).
If itemKind is value, let result be elementValue.
Else,
1. Assert: itemKind is key+value.
2. Let result be ! CreateArrayFromList(« index, elementValue »).
Return CreateIterResultObject(result, false).

22.1.5.2.2 %ArrayIteratorPrototype% [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Array Iterator".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

22.1.5.3 Properties of Array Iterator Instances

Array Iterator instances are ordinary objects that inherit properties from the %ArrayIteratorPrototype% intrinsic object. Array Iterator instances are initially created with the internal slots listed in Table 60.

Table 60: Internal Slots of Array Iterator Instances

Internal Slot	Description
[[IteratedArrayLike]]	The array-like object that is being iterated.
[[ArrayLikeNextIndex]]	The integer index of the next element to be examined by this iterator.
[[ArrayLikeIterationKind]]	A String value that identifies what is returned for each element of the iteration. The possible values are: key, value, key+value.

22.2 TypedArray Objects

TypedArray objects present an array-like view of an underlying binary data buffer (24.1). Each element of a TypedArray instance has the same underlying binary scalar data type. There is a distinct TypedArray constructor, listed in Table 61, for each of the supported element types. Each constructor in Table 61 has a corresponding distinct prototype object.

Table 61: The TypedArray Constructors

Constructor Name and Intrinsic	Element Type	Element Size	Conversion Operation	Description
Int8Array %Int8Array%	Int8	1	ToInt8	8-bit 2's complement signed integer
Uint8Array %Uint8Array%	Uint8	1	ToUint8	8-bit unsigned integer
Uint8ClampedArray %Uint8ClampedArray%	Uint8C	1	ToUint8Clamp	8-bit unsigned integer (clamped conversion)
Int16Array %Int16Array%	Int16	2	ToInt16	16-bit 2's complement signed integer
Uint16Array %Uint16Array%	Uint16	2	ToUint16	16-bit unsigned integer
Int32Array %Int32Array%	Int32	4	ToInt32	32-bit 2's complement signed integer
Uint32Array %Uint32Array%	Uint32	4	ToUint32	32-bit unsigned integer
BigInt64Array %BigInt64Array%	BigInt64	8	ToBigInt64	64-bit two's complement signed integer
BigUint64Array %BigUint64Array%	BigUint64	8	ToBigUint64	64-bit unsigned integer
Float32Array %Float32Array%	Float32	4		32-bit IEEE floating point
Float64Array %Float64Array%	Float64	8		64-bit IEEE floating point

In the definitions below, references to TypedArray should be replaced with the appropriate constructor name from the above table.

22.2.1 The %TypedArray% Intrinsic Object

The %TypedArray% intrinsic object:

is a constructor function object that all of the TypedArray constructor objects inherit from.
along with its corresponding prototype object, provides common properties that are inherited by all TypedArray constructors and their instances.
does not have a global name or appear as a property of the global object.
acts as the abstract superclass of the various TypedArray constructors.
will throw an error when invoked, because it is an abstract class constructor. The TypedArray constructors do not perform a super call to it.

22.2.1.1 %TypedArray% ( )

The %TypedArray% constructor performs the following steps:

Throw a TypeError exception.

The "length" property of the %TypedArray% constructor function is 0.

22.2.2 Properties of the %TypedArray% Intrinsic Object

The %TypedArray% intrinsic object:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has a "name" property whose value is "TypedArray".
has the following properties:

22.2.2.1 %TypedArray%.from ( `source` [ , `mapfn` [ , `thisArg` ] ] )

When the from method is called with argument source, and optional arguments mapfn and thisArg, the following steps are taken:

Let C be the this value.
If IsConstructor(C) is false, throw a TypeError exception.
If mapfn is undefined, let mapping be false.
Else,
1. If IsCallable(mapfn) is false, throw a TypeError exception.
2. Let mapping be true.
Let usingIterator be ? GetMethod(source, @@iterator).
If usingIterator is not undefined, then
1. Let values be ? IterableToList(source, usingIterator).
2. Let len be the number of elements in values.
3. Let targetObj be ? TypedArrayCreate(C, « len »).
4. Let k be 0.
5. Repeat, while k < len
  1. Let Pk be ! ToString(k).
  2. Let kValue be the first element of values and remove that element from values.
  3. If mapping is true, then
    1. Let mappedValue be ? Call(mapfn, thisArg, « kValue, k »).
  4. Else, let mappedValue be kValue.
  5. Perform ? Set(targetObj, Pk, mappedValue, true).
  6. Set k to k + 1.
6. Assert: values is now an empty List.
7. Return targetObj.
NOTE: source is not an Iterable so assume it is already an array-like object.
Let arrayLike be ! ToObject(source).
Let len be ? LengthOfArrayLike(arrayLike).
Let targetObj be ? TypedArrayCreate(C, « len »).
Let k be 0.
Repeat, while k < len
1. Let Pk be ! ToString(k).
2. Let kValue be ? Get(arrayLike, Pk).
3. If mapping is true, then
  1. Let mappedValue be ? Call(mapfn, thisArg, « kValue, k »).
4. Else, let mappedValue be kValue.
5. Perform ? Set(targetObj, Pk, mappedValue, true).
6. Set k to k + 1.
Return targetObj.

22.2.2.1.1 Runtime Semantics: IterableToList ( `items`, `method` )

The abstract operation IterableToList takes arguments items and method. It performs the following steps when called:

Let iteratorRecord be ? GetIterator(items, sync, method).
Let values be a new empty List.
Let next be true.
Repeat, while next is not false
1. Set next to ? IteratorStep(iteratorRecord).
2. If next is not false, then
  1. Let nextValue be ? IteratorValue(next).
  2. Append nextValue to the end of the List values.
Return values.

22.2.2.2 %TypedArray%.of ( ...`items` )

When the of method is called with any number of arguments, the following steps are taken:

Let len be the actual number of arguments passed to this function.
Let items be the List of arguments passed to this function.
Let C be the this value.
If IsConstructor(C) is false, throw a TypeError exception.
Let newObj be ? TypedArrayCreate(C, « len »).
Let k be 0.
Repeat, while k < len
1. Let kValue be items[k].
2. Let Pk be ! ToString(k).
3. Perform ? Set(newObj, Pk, kValue, true).
4. Set k to k + 1.
Return newObj.

Note

The items argument is assumed to be a well-formed rest argument value.

22.2.2.3 %TypedArray%.prototype

The initial value of %TypedArray%.prototype is the %TypedArray.prototype% intrinsic object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

22.2.2.4 get %TypedArray% [ @@species ]

%TypedArray%[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

Note

%TypedArray.prototype% methods normally use their this object's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.

22.2.3 Properties of the %TypedArray.prototype% Object

The %TypedArray.prototype% object:

has a [[Prototype]] internal slot whose value is %Object.prototype%.
is an ordinary object.
does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to TypedArray instance objects.

22.2.3.1 get %TypedArray%.prototype.buffer

%TypedArray%.prototype.buffer is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let O be the this value.
Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
Assert: O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
Return buffer.

22.2.3.2 get %TypedArray%.prototype.byteLength

%TypedArray%.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let O be the this value.
Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
Assert: O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
If IsDetachedBuffer(buffer) is true, return 0.
Let size be O.[[ByteLength]].
Return size.

22.2.3.3 get %TypedArray%.prototype.byteOffset

%TypedArray%.prototype.byteOffset is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let O be the this value.
Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
Assert: O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
If IsDetachedBuffer(buffer) is true, return 0.
Let offset be O.[[ByteOffset]].
Return offset.

22.2.3.4 %TypedArray%.prototype.constructor

The initial value of %TypedArray%.prototype.constructor is the %TypedArray% intrinsic object.

22.2.3.5 %TypedArray%.prototype.copyWithin ( `target`, `start` [ , `end` ] )

The interpretation and use of the arguments of %TypedArray%.prototype.copyWithin are the same as for Array.prototype.copyWithin as defined in 22.1.3.3.

The following steps are taken:

Let O be the this value.
Perform ? ValidateTypedArray(O).
Let len be O.[[ArrayLength]].
Let relativeTarget be ? ToInteger(target).
If relativeTarget < 0, let to be max((len + relativeTarget), 0); else let to be min(relativeTarget, len).
Let relativeStart be ? ToInteger(start).
If relativeStart < 0, let from be max((len + relativeStart), 0); else let from be min(relativeStart, len).
If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToInteger(end).
If relativeEnd < 0, let final be max((len + relativeEnd), 0); else let final be min(relativeEnd, len).
Let count be min(final - from, len - to).
If count > 0, then
1. NOTE: The copying must be performed in a manner that preserves the bit-level encoding of the source data.
2. Let buffer be O.[[ViewedArrayBuffer]].
3. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
4. Let typedArrayName be the String value of O.[[TypedArrayName]].
5. Let elementSize be the Element Size value specified in Table 61 for typedArrayName.
6. Let byteOffset be O.[[ByteOffset]].
7. Let toByteIndex be to × elementSize + byteOffset.
8. Let fromByteIndex be from × elementSize + byteOffset.
9. Let countBytes be count × elementSize.
10. If fromByteIndex < toByteIndex and toByteIndex < fromByteIndex + countBytes, then
  1. Let direction be -1.
  2. Set fromByteIndex to fromByteIndex + countBytes - 1.
  3. Set toByteIndex to toByteIndex + countBytes - 1.
11. Else,
  1. Let direction be 1.
12. Repeat, while countBytes > 0
  1. Let value be GetValueFromBuffer(buffer, fromByteIndex, Uint8, true, Unordered).
  2. Perform SetValueInBuffer(buffer, toByteIndex, Uint8, value, true, Unordered).
  3. Set fromByteIndex to fromByteIndex + direction.
  4. Set toByteIndex to toByteIndex + direction.
  5. Set countBytes to countBytes - 1.
Return O.

22.2.3.5.1 Runtime Semantics: ValidateTypedArray ( `O` )

The abstract operation ValidateTypedArray takes argument O. It performs the following steps when called:

Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
Assert: O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
Return buffer.

22.2.3.6 %TypedArray%.prototype.entries ( )

The following steps are taken:

Let O be the this value.
Perform ? ValidateTypedArray(O).
Return CreateArrayIterator(O, key+value).

22.2.3.7 %TypedArray%.prototype.every ( `callbackfn` [ , `thisArg` ] )

%TypedArray%.prototype.every is a distinct function that implements the same algorithm as Array.prototype.every as defined in 22.1.3.5 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to callbackfn may cause the this value to become detached.

This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.

22.2.3.8 %TypedArray%.prototype.fill ( `value` [ , `start` [ , `end` ] ] )

The interpretation and use of the arguments of %TypedArray%.prototype.fill are the same as for Array.prototype.fill as defined in 22.1.3.6.

The following steps are taken:

Let O be the this value.
Perform ? ValidateTypedArray(O).
Let len be O.[[ArrayLength]].
If O.[[ContentType]] is BigInt, set value to ? ToBigInt(value).
Otherwise, set value to ? ToNumber(value).
Let relativeStart be ? ToInteger(start).
If relativeStart < 0, let k be max((len + relativeStart), 0); else let k be min(relativeStart, len).
If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToInteger(end).
If relativeEnd < 0, let final be max((len + relativeEnd), 0); else let final be min(relativeEnd, len).
If IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true, throw a TypeError exception.
Repeat, while k < final
1. Let Pk be ! ToString(k).
2. Perform ! Set(O, Pk, value, true).
3. Set k to k + 1.
Return O.

22.2.3.9 %TypedArray%.prototype.filter ( `callbackfn` [ , `thisArg` ] )

The interpretation and use of the arguments of %TypedArray%.prototype.filter are the same as for Array.prototype.filter as defined in 22.1.3.7.

When the filter method is called with one or two arguments, the following steps are taken:

Let O be the this value.
Perform ? ValidateTypedArray(O).
Let len be O.[[ArrayLength]].
If IsCallable(callbackfn) is false, throw a TypeError exception.
Let kept be a new empty List.
Let k be 0.
Let captured be 0.
Repeat, while k < len
1. Let Pk be ! ToString(k).
2. Let kValue be ? Get(O, Pk).
3. Let selected be ! ToBoolean(? Call(callbackfn, thisArg, « kValue, k, O »)).
4. If selected is true, then
  1. Append kValue to the end of kept.
  2. Set captured to captured + 1.
5. Set k to k + 1.
Let A be ? TypedArraySpeciesCreate(O, « captured »).
Let n be 0.
For each element e of kept, do
1. Perform ! Set(A, ! ToString(n), e, true).
2. Set n to n + 1.
Return A.

This function is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

22.2.3.10 %TypedArray%.prototype.find ( `predicate` [ , `thisArg` ] )

%TypedArray%.prototype.find is a distinct function that implements the same algorithm as Array.prototype.find as defined in 22.1.3.8 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to predicate may cause the this value to become detached.

22.2.3.11 %TypedArray%.prototype.findIndex ( `predicate` [ , `thisArg` ] )

%TypedArray%.prototype.findIndex is a distinct function that implements the same algorithm as Array.prototype.findIndex as defined in 22.1.3.9 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to predicate may cause the this value to become detached.

22.2.3.12 %TypedArray%.prototype.forEach ( `callbackfn` [ , `thisArg` ] )

%TypedArray%.prototype.forEach is a distinct function that implements the same algorithm as Array.prototype.forEach as defined in 22.1.3.12 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to callbackfn may cause the this value to become detached.

22.2.3.13 %TypedArray%.prototype.includes ( `searchElement` [ , `fromIndex` ] )

%TypedArray%.prototype.includes is a distinct function that implements the same algorithm as Array.prototype.includes as defined in 22.1.3.13 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.

22.2.3.14 %TypedArray%.prototype.indexOf ( `searchElement` [ , `fromIndex` ] )

%TypedArray%.prototype.indexOf is a distinct function that implements the same algorithm as Array.prototype.indexOf as defined in 22.1.3.14 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.

22.2.3.15 %TypedArray%.prototype.join ( `separator` )

%TypedArray%.prototype.join is a distinct function that implements the same algorithm as Array.prototype.join as defined in 22.1.3.15 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.

22.2.3.16 %TypedArray%.prototype.keys ( )

The following steps are taken:

Let O be the this value.
Perform ? ValidateTypedArray(O).
Return CreateArrayIterator(O, key).

22.2.3.17 %TypedArray%.prototype.lastIndexOf ( `searchElement` [ , `fromIndex` ] )

%TypedArray%.prototype.lastIndexOf is a distinct function that implements the same algorithm as Array.prototype.lastIndexOf as defined in 22.1.3.17 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.

22.2.3.18 get %TypedArray%.prototype.length

%TypedArray%.prototype.length is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let O be the this value.
Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
Assert: O has [[ViewedArrayBuffer]] and [[ArrayLength]] internal slots.
Let buffer be O.[[ViewedArrayBuffer]].
If IsDetachedBuffer(buffer) is true, return 0.
Let length be O.[[ArrayLength]].
Return length.

This function is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

22.2.3.19 %TypedArray%.prototype.map ( `callbackfn` [ , `thisArg` ] )

The interpretation and use of the arguments of %TypedArray%.prototype.map are the same as for Array.prototype.map as defined in 22.1.3.18.

When the map method is called with one or two arguments, the following steps are taken:

Let O be the this value.
Perform ? ValidateTypedArray(O).
Let len be O.[[ArrayLength]].
If IsCallable(callbackfn) is false, throw a TypeError exception.
Let A be ? TypedArraySpeciesCreate(O, « len »).
Let k be 0.
Repeat, while k < len
1. Let Pk be ! ToString(k).
2. Let kValue be ? Get(O, Pk).
3. Let mappedValue be ? Call(callbackfn, thisArg, « kValue, k, O »).
4. Perform ? Set(A, Pk, mappedValue, true).
5. Set k to k + 1.
Return A.

This function is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

22.2.3.20 %TypedArray%.prototype.reduce ( `callbackfn` [ , `initialValue` ] )

%TypedArray%.prototype.reduce is a distinct function that implements the same algorithm as Array.prototype.reduce as defined in 22.1.3.21 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to callbackfn may cause the this value to become detached.

22.2.3.21 %TypedArray%.prototype.reduceRight ( `callbackfn` [ , `initialValue` ] )

%TypedArray%.prototype.reduceRight is a distinct function that implements the same algorithm as Array.prototype.reduceRight as defined in 22.1.3.22 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to callbackfn may cause the this value to become detached.

22.2.3.22 %TypedArray%.prototype.reverse ( )

%TypedArray%.prototype.reverse is a distinct function that implements the same algorithm as Array.prototype.reverse as defined in 22.1.3.23 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.

22.2.3.23 %TypedArray%.prototype.set ( `overloaded` [ , `offset` ] )

%TypedArray%.prototype.set is a single function whose behaviour is overloaded based upon the type of its first argument.

This function is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

22.2.3.23.1 %TypedArray%.prototype.set ( `array` [ , `offset` ] )

Sets multiple values in this TypedArray, reading the values from the object array. The optional offset value indicates the first element index in this TypedArray where values are written. If omitted, it is assumed to be 0.

Assert: array is any ECMAScript language value other than an Object with a [[TypedArrayName]] internal slot. If it is such an Object, the definition in 22.2.3.23.2 applies.
Let target be the this value.
Perform ? RequireInternalSlot(target, [[TypedArrayName]]).
Assert: target has a [[ViewedArrayBuffer]] internal slot.
Let targetOffset be ? ToInteger(offset).
If targetOffset < 0, throw a RangeError exception.
Let targetBuffer be target.[[ViewedArrayBuffer]].
If IsDetachedBuffer(targetBuffer) is true, throw a TypeError exception.
Let targetLength be target.[[ArrayLength]].
Let targetName be the String value of target.[[TypedArrayName]].
Let targetElementSize be the Element Size value specified in Table 61 for targetName.
Let targetType be the Element Type value in Table 61 for targetName.
Let targetByteOffset be target.[[ByteOffset]].
Let src be ? ToObject(array).
Let srcLength be ? LengthOfArrayLike(src).
If srcLength + targetOffset > targetLength, throw a RangeError exception.
Let targetByteIndex be targetOffset × targetElementSize + targetByteOffset.
Let k be 0.
Let limit be targetByteIndex + targetElementSize × srcLength.
Repeat, while targetByteIndex < limit
1. Let Pk be ! ToString(k).
2. Let value be ? Get(src, Pk).
3. If target.[[ContentType]] is BigInt, set value to ? ToBigInt(value).
4. Otherwise, set value to ? ToNumber(value).
5. If IsDetachedBuffer(targetBuffer) is true, throw a TypeError exception.
6. Perform SetValueInBuffer(targetBuffer, targetByteIndex, targetType, value, true, Unordered).
7. Set k to k + 1.
8. Set targetByteIndex to targetByteIndex + targetElementSize.
Return undefined.

22.2.3.23.2 %TypedArray%.prototype.set ( `typedArray` [ , `offset` ] )

Sets multiple values in this TypedArray, reading the values from the typedArray argument object. The optional offset value indicates the first element index in this TypedArray where values are written. If omitted, it is assumed to be 0.

Assert: typedArray has a [[TypedArrayName]] internal slot. If it does not, the definition in 22.2.3.23.1 applies.
Let target be the this value.
Perform ? RequireInternalSlot(target, [[TypedArrayName]]).
Assert: target has a [[ViewedArrayBuffer]] internal slot.
Let targetOffset be ? ToInteger(offset).
If targetOffset < 0, throw a RangeError exception.
Let targetBuffer be target.[[ViewedArrayBuffer]].
If IsDetachedBuffer(targetBuffer) is true, throw a TypeError exception.
Let targetLength be target.[[ArrayLength]].
Let srcBuffer be typedArray.[[ViewedArrayBuffer]].
If IsDetachedBuffer(srcBuffer) is true, throw a TypeError exception.
Let targetName be the String value of target.[[TypedArrayName]].
Let targetType be the Element Type value in Table 61 for targetName.
Let targetElementSize be the Element Size value specified in Table 61 for targetName.
Let targetByteOffset be target.[[ByteOffset]].
Let srcName be the String value of typedArray.[[TypedArrayName]].
Let srcType be the Element Type value in Table 61 for srcName.
Let srcElementSize be the Element Size value specified in Table 61 for srcName.
Let srcLength be typedArray.[[ArrayLength]].
Let srcByteOffset be typedArray.[[ByteOffset]].
If srcLength + targetOffset > targetLength, throw a RangeError exception.
If target.[[ContentType]] is not equal to typedArray.[[ContentType]], throw a TypeError exception.
If both IsSharedArrayBuffer(srcBuffer) and IsSharedArrayBuffer(targetBuffer) are true, then
1. If srcBuffer.[[ArrayBufferData]] and targetBuffer.[[ArrayBufferData]] are the same Shared Data Block values, let same be true; else let same be false.
Else, let same be SameValue(srcBuffer, targetBuffer).
If same is true, then
1. Let srcByteLength be typedArray.[[ByteLength]].
2. Set srcBuffer to ? CloneArrayBuffer(srcBuffer, srcByteOffset, srcByteLength, %ArrayBuffer%).
3. NOTE: %ArrayBuffer% is used to clone srcBuffer because is it known to not have any observable side-effects.
4. Let srcByteIndex be 0.
Else, let srcByteIndex be srcByteOffset.
Let targetByteIndex be targetOffset × targetElementSize + targetByteOffset.
Let limit be targetByteIndex + targetElementSize × srcLength.
If srcType is the same as targetType, then
1. NOTE: If srcType and targetType are the same, the transfer must be performed in a manner that preserves the bit-level encoding of the source data.
2. Repeat, while targetByteIndex < limit
  1. Let value be GetValueFromBuffer(srcBuffer, srcByteIndex, Uint8, true, Unordered).
  2. Perform SetValueInBuffer(targetBuffer, targetByteIndex, Uint8, value, true, Unordered).
  3. Set srcByteIndex to srcByteIndex + 1.
  4. Set targetByteIndex to targetByteIndex + 1.
Else,
1. Repeat, while targetByteIndex < limit
  1. Let value be GetValueFromBuffer(srcBuffer, srcByteIndex, srcType, true, Unordered).
  2. Perform SetValueInBuffer(targetBuffer, targetByteIndex, targetType, value, true, Unordered).
  3. Set srcByteIndex to srcByteIndex + srcElementSize.
  4. Set targetByteIndex to targetByteIndex + targetElementSize.
Return undefined.

22.2.3.24 %TypedArray%.prototype.slice ( `start`, `end` )

The interpretation and use of the arguments of %TypedArray%.prototype.slice are the same as for Array.prototype.slice as defined in 22.1.3.25. The following steps are taken:

Let O be the this value.
Perform ? ValidateTypedArray(O).
Let len be O.[[ArrayLength]].
Let relativeStart be ? ToInteger(start).
If relativeStart < 0, let k be max((len + relativeStart), 0); else let k be min(relativeStart, len).
If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToInteger(end).
If relativeEnd < 0, let final be max((len + relativeEnd), 0); else let final be min(relativeEnd, len).
Let count be max(final - k, 0).
Let A be ? TypedArraySpeciesCreate(O, « count »).
Let srcName be the String value of O.[[TypedArrayName]].
Let srcType be the Element Type value in Table 61 for srcName.
Let targetName be the String value of A.[[TypedArrayName]].
Let targetType be the Element Type value in Table 61 for targetName.
If srcType is different from targetType, then
1. Let n be 0.
2. Repeat, while k < final
  1. Let Pk be ! ToString(k).
  2. Let kValue be ? Get(O, Pk).
  3. Perform ! Set(A, ! ToString(n), kValue, true).
  4. Set k to k + 1.
  5. Set n to n + 1.
Else if count > 0, then
1. Let srcBuffer be O.[[ViewedArrayBuffer]].
2. If IsDetachedBuffer(srcBuffer) is true, throw a TypeError exception.
3. Let targetBuffer be A.[[ViewedArrayBuffer]].
4. Let elementSize be the Element Size value specified in Table 61 for Element Type srcType.
5. NOTE: If srcType and targetType are the same, the transfer must be performed in a manner that preserves the bit-level encoding of the source data.
6. Let srcByteOffet be O.[[ByteOffset]].
7. Let targetByteIndex be A.[[ByteOffset]].
8. Let srcByteIndex be (k × elementSize) + srcByteOffet.
9. Let limit be targetByteIndex + count × elementSize.
10. Repeat, while targetByteIndex < limit
  1. Let value be GetValueFromBuffer(srcBuffer, srcByteIndex, Uint8, true, Unordered).
  2. Perform SetValueInBuffer(targetBuffer, targetByteIndex, Uint8, value, true, Unordered).
  3. Set srcByteIndex to srcByteIndex + 1.
  4. Set targetByteIndex to targetByteIndex + 1.
Return A.

This function is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

22.2.3.25 %TypedArray%.prototype.some ( `callbackfn` [ , `thisArg` ] )

%TypedArray%.prototype.some is a distinct function that implements the same algorithm as Array.prototype.some as defined in 22.1.3.26 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to callbackfn may cause the this value to become detached.

22.2.3.26 %TypedArray%.prototype.sort ( `comparefn` )

%TypedArray%.prototype.sort is a distinct function that, except as described below, implements the same requirements as those of Array.prototype.sort as defined in 22.1.3.27. The implementation of the %TypedArray%.prototype.sort specification may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse. The only internal methods of the this object that the algorithm may call are [[Get]] and [[Set]].

This function is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

Upon entry, the following steps are performed to initialize evaluation of the sort function. These steps are used instead of the entry steps in 22.1.3.27:

If comparefn is not undefined and IsCallable(comparefn) is false, throw a TypeError exception.
Let obj be the this value.
Let buffer be ? ValidateTypedArray(obj).
Let len be obj.[[ArrayLength]].

The implementation-defined sort order condition for exotic objects is not applied by %TypedArray%.prototype.sort.

The following version of SortCompare is used by %TypedArray%.prototype.sort. It performs a numeric comparison rather than the string comparison used in 22.1.3.27.

The abstract operation TypedArraySortCompare takes arguments x and y. It also has access to the comparefn and buffer values of the current invocation of the sort method. It performs the following steps when called:

Assert: Both Type(x) and Type(y) are Number or both are BigInt.
If comparefn is not undefined, then
1. Let v be ? ToNumber(? Call(comparefn, undefined, « x, y »)).
2. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
3. If v is NaN, return +0.
4. Return v.
If x and y are both NaN, return +0.
If x is NaN, return 1.
If y is NaN, return -1.
If x < y, return -1.
If x > y, return 1.
If x is -0 and y is +0, return -1.
If x is +0 and y is -0, return 1.
Return +0.

Note

Because NaN always compares greater than any other value, NaN property values always sort to the end of the result when comparefn is not provided.

22.2.3.27 %TypedArray%.prototype.subarray ( `begin`, `end` )

Returns a new TypedArray object whose element type is the same as this TypedArray and whose ArrayBuffer is the same as the ArrayBuffer of this TypedArray, referencing the elements at begin, inclusive, up to end, exclusive. If either begin or end is negative, it refers to an index from the end of the array, as opposed to from the beginning.

Let O be the this value.
Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
Assert: O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
Let srcLength be O.[[ArrayLength]].
Let relativeBegin be ? ToInteger(begin).
If relativeBegin < 0, let beginIndex be max((srcLength + relativeBegin), 0); else let beginIndex be min(relativeBegin, srcLength).
If end is undefined, let relativeEnd be srcLength; else let relativeEnd be ? ToInteger(end).
If relativeEnd < 0, let endIndex be max((srcLength + relativeEnd), 0); else let endIndex be min(relativeEnd, srcLength).
Let newLength be max(endIndex - beginIndex, 0).
Let constructorName be the String value of O.[[TypedArrayName]].
Let elementSize be the Element Size value specified in Table 61 for constructorName.
Let srcByteOffset be O.[[ByteOffset]].
Let beginByteOffset be srcByteOffset + beginIndex × elementSize.
Let argumentsList be « buffer, beginByteOffset, newLength ».
Return ? TypedArraySpeciesCreate(O, argumentsList).

This function is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

22.2.3.28 %TypedArray%.prototype.toLocaleString ( [ `reserved1` [ , `reserved2` ] ] )

%TypedArray%.prototype.toLocaleString is a distinct function that implements the same algorithm as Array.prototype.toLocaleString as defined in 22.1.3.29 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.

Note

If the ECMAScript implementation includes the ECMA-402 Internationalization API this function is based upon the algorithm for Array.prototype.toLocaleString that is in the ECMA-402 specification.

22.2.3.29 %TypedArray%.prototype.toString ( )

The initial value of the %TypedArray%.prototype.toString data property is the same built-in function object as the Array.prototype.toString method defined in 22.1.3.30.

22.2.3.30 %TypedArray%.prototype.values ( )

The following steps are taken:

Let O be the this value.
Perform ? ValidateTypedArray(O).
Return CreateArrayIterator(O, value).

22.2.3.31 %TypedArray%.prototype [ @@iterator ] ( )

The initial value of the @@iterator property is the same function object as the initial value of the %TypedArray%.prototype.values property.

22.2.3.32 get %TypedArray%.prototype [ @@toStringTag ]

%TypedArray%.prototype[@@toStringTag] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let O be the this value.
If Type(O) is not Object, return undefined.
If O does not have a [[TypedArrayName]] internal slot, return undefined.
Let name be O.[[TypedArrayName]].
Assert: Type(name) is String.
Return name.

This property has the attributes { [[Enumerable]]: false, [[Configurable]]: true }.

The initial value of the "name" property of this function is "get [Symbol.toStringTag]".

22.2.4 The `TypedArray` Constructors

Each TypedArray constructor:

is an intrinsic object that has the structure described below, differing only in the name used as the constructor name instead of TypedArray, in Table 61.
is a single function whose behaviour is overloaded based upon the number and types of its arguments. The actual behaviour of a call of TypedArray depends upon the number and kind of arguments that are passed to it.
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified TypedArray behaviour must include a super call to the TypedArray constructor to create and initialize the subclass instance with the internal state necessary to support the %TypedArray%.prototype built-in methods.
has a "length" property whose value is 3.

22.2.4.1 `TypedArray` ( )

This description applies only if the TypedArray function is called with no arguments.

If NewTarget is undefined, throw a TypeError exception.
Let constructorName be the String value of the Constructor Name value specified in Table 61 for this TypedArray constructor.
Return ? AllocateTypedArray(constructorName, NewTarget, "%TypedArray.prototype%", 0).

22.2.4.2 `TypedArray` ( `length` )

This description applies only if the TypedArray function is called with at least one argument and the Type of the first argument is not Object.

TypedArray called with argument length performs the following steps:

Assert: Type(length) is not Object.
If NewTarget is undefined, throw a TypeError exception.
Let elementLength be ? ToIndex(length).
Let constructorName be the String value of the Constructor Name value specified in Table 61 for this TypedArray constructor.
Return ? AllocateTypedArray(constructorName, NewTarget, "%TypedArray.prototype%", elementLength).

22.2.4.2.1 Runtime Semantics: AllocateTypedArray ( `constructorName`, `newTarget`, `defaultProto` [ , `length` ] )

The abstract operation AllocateTypedArray takes arguments constructorName (a String which is the name of a TypedArray constructor in Table 61), newTarget, and defaultProto and optional argument length. It is used to validate and create an instance of a TypedArray constructor. If the length argument is passed, an ArrayBuffer of that length is also allocated and associated with the new TypedArray instance. AllocateTypedArray provides common semantics that is used by all of the TypedArray overloads. It performs the following steps when called:

Let proto be ? GetPrototypeFromConstructor(newTarget, defaultProto).
Let obj be ! IntegerIndexedObjectCreate(proto).
Assert: obj.[[ViewedArrayBuffer]] is undefined.
Set obj.[[TypedArrayName]] to constructorName.
If constructorName is "BigInt64Array" or "BigUint64Array", set obj.[[ContentType]] to BigInt.
Otherwise, set obj.[[ContentType]] to Number.
If length is not present, then
1. Set obj.[[ByteLength]] to 0.
2. Set obj.[[ByteOffset]] to 0.
3. Set obj.[[ArrayLength]] to 0.
Else,
1. Perform ? AllocateTypedArrayBuffer(obj, length).
Return obj.

22.2.4.2.2 Runtime Semantics: AllocateTypedArrayBuffer ( `O`, `length` )

The abstract operation AllocateTypedArrayBuffer takes arguments O (a TypedArray object) and length. It allocates and associates an ArrayBuffer with O. It performs the following steps when called:

Assert: O is an Object that has a [[ViewedArrayBuffer]] internal slot.
Assert: O.[[ViewedArrayBuffer]] is undefined.
Assert: ! IsNonNegativeInteger(length) is true.
Let constructorName be the String value of O.[[TypedArrayName]].
Let elementSize be the Element Size value specified in Table 61 for constructorName.
Let byteLength be elementSize × length.
Let data be ? AllocateArrayBuffer(%ArrayBuffer%, byteLength).
Set O.[[ViewedArrayBuffer]] to data.
Set O.[[ByteLength]] to byteLength.
Set O.[[ByteOffset]] to 0.
Set O.[[ArrayLength]] to length.
Return O.

22.2.4.3 `TypedArray` ( `typedArray` )

This description applies only if the TypedArray function is called with at least one argument and the Type of the first argument is Object and that object has a [[TypedArrayName]] internal slot.

TypedArray called with argument typedArray performs the following steps:

Assert: Type(typedArray) is Object and typedArray has a [[TypedArrayName]] internal slot.
If NewTarget is undefined, throw a TypeError exception.
Let constructorName be the String value of the Constructor Name value specified in Table 61 for this TypedArray constructor.
Let O be ? AllocateTypedArray(constructorName, NewTarget, "%TypedArray.prototype%").
Let srcArray be typedArray.
Let srcData be srcArray.[[ViewedArrayBuffer]].
If IsDetachedBuffer(srcData) is true, throw a TypeError exception.
Let elementType be the Element Type value in Table 61 for constructorName.
Let elementLength be srcArray.[[ArrayLength]].
Let srcName be the String value of srcArray.[[TypedArrayName]].
Let srcType be the Element Type value in Table 61 for srcName.
Let srcElementSize be the Element Size value specified in Table 61 for srcName.
Let srcByteOffset be srcArray.[[ByteOffset]].
Let elementSize be the Element Size value specified in Table 61 for constructorName.
Let byteLength be elementSize × elementLength.
If IsSharedArrayBuffer(srcData) is false, then
1. Let bufferConstructor be ? SpeciesConstructor(srcData, %ArrayBuffer%).
Else,
1. Let bufferConstructor be %ArrayBuffer%.
If elementType is the same as srcType, then
1. Let data be ? CloneArrayBuffer(srcData, srcByteOffset, byteLength, bufferConstructor).
Else,
1. Let data be ? AllocateArrayBuffer(bufferConstructor, byteLength).
2. If IsDetachedBuffer(srcData) is true, throw a TypeError exception.
3. If srcArray.[[ContentType]] is not equal to O.[[ContentType]], throw a TypeError exception.
4. Let srcByteIndex be srcByteOffset.
5. Let targetByteIndex be 0.
6. Let count be elementLength.
7. Repeat, while count > 0
  1. Let value be GetValueFromBuffer(srcData, srcByteIndex, srcType, true, Unordered).
  2. Perform SetValueInBuffer(data, targetByteIndex, elementType, value, true, Unordered).
  3. Set srcByteIndex to srcByteIndex + srcElementSize.
  4. Set targetByteIndex to targetByteIndex + elementSize.
  5. Set count to count - 1.
Set O.[[ViewedArrayBuffer]] to data.
Set O.[[ByteLength]] to byteLength.
Set O.[[ByteOffset]] to 0.
Set O.[[ArrayLength]] to elementLength.
Return O.

22.2.4.4 `TypedArray` ( `object` )

This description applies only if the TypedArray function is called with at least one argument and the Type of the first argument is Object and that object does not have either a [[TypedArrayName]] or an [[ArrayBufferData]] internal slot.

TypedArray called with argument object performs the following steps:

Assert: Type(object) is Object and object does not have either a [[TypedArrayName]] or an [[ArrayBufferData]] internal slot.
If NewTarget is undefined, throw a TypeError exception.
Let constructorName be the String value of the Constructor Name value specified in Table 61 for this TypedArray constructor.
Let O be ? AllocateTypedArray(constructorName, NewTarget, "%TypedArray.prototype%").
Let usingIterator be ? GetMethod(object, @@iterator).
If usingIterator is not undefined, then
1. Let values be ? IterableToList(object, usingIterator).
2. Let len be the number of elements in values.
3. Perform ? AllocateTypedArrayBuffer(O, len).
4. Let k be 0.
5. Repeat, while k < len
  1. Let Pk be ! ToString(k).
  2. Let kValue be the first element of values and remove that element from values.
  3. Perform ? Set(O, Pk, kValue, true).
  4. Set k to k + 1.
6. Assert: values is now an empty List.
7. Return O.
NOTE: object is not an Iterable so assume it is already an array-like object.
Let arrayLike be object.
Let len be ? LengthOfArrayLike(arrayLike).
Perform ? AllocateTypedArrayBuffer(O, len).
Let k be 0.
Repeat, while k < len
1. Let Pk be ! ToString(k).
2. Let kValue be ? Get(arrayLike, Pk).
3. Perform ? Set(O, Pk, kValue, true).
4. Set k to k + 1.
Return O.

22.2.4.5 `TypedArray` ( `buffer` [ , `byteOffset` [ , `length` ] ] )

This description applies only if the TypedArray function is called with at least one argument and the Type of the first argument is Object and that object has an [[ArrayBufferData]] internal slot.

TypedArray called with at least one argument buffer performs the following steps:

Assert: Type(buffer) is Object and buffer has an [[ArrayBufferData]] internal slot.
If NewTarget is undefined, throw a TypeError exception.
Let constructorName be the String value of the Constructor Name value specified in Table 61 for this TypedArray constructor.
Let O be ? AllocateTypedArray(constructorName, NewTarget, "%TypedArray.prototype%").
Let elementSize be the Element Size value specified in Table 61 for constructorName.
Let offset be ? ToIndex(byteOffset).
If offset modulo elementSize ≠ 0, throw a RangeError exception.
If length is not undefined, then
1. Let newLength be ? ToIndex(length).
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
Let bufferByteLength be buffer.[[ArrayBufferByteLength]].
If length is undefined, then
1. If bufferByteLength modulo elementSize ≠ 0, throw a RangeError exception.
2. Let newByteLength be bufferByteLength - offset.
3. If newByteLength < 0, throw a RangeError exception.
Else,
1. Let newByteLength be newLength × elementSize.
2. If offset + newByteLength > bufferByteLength, throw a RangeError exception.
Set O.[[ViewedArrayBuffer]] to buffer.
Set O.[[ByteLength]] to newByteLength.
Set O.[[ByteOffset]] to offset.
Set O.[[ArrayLength]] to newByteLength / elementSize.
Return O.

22.2.4.6 TypedArrayCreate ( `constructor`, `argumentList` )

The abstract operation TypedArrayCreate takes arguments constructor and argumentList. It is used to specify the creation of a new TypedArray object using a constructor function. It performs the following steps when called:

Let newTypedArray be ? Construct(constructor, argumentList).
Perform ? ValidateTypedArray(newTypedArray).
If argumentList is a List of a single Number, then
1. If newTypedArray.[[ArrayLength]] < argumentList[0], throw a TypeError exception.
Return newTypedArray.

22.2.4.7 TypedArraySpeciesCreate ( `exemplar`, `argumentList` )

The abstract operation TypedArraySpeciesCreate takes arguments exemplar and argumentList. It is used to specify the creation of a new TypedArray object using a constructor function that is derived from exemplar. It performs the following steps when called:

Assert: exemplar is an Object that has [[TypedArrayName]] and [[ContentType]] internal slots.
Let defaultConstructor be the intrinsic object listed in column one of Table 61 for exemplar.[[TypedArrayName]].
Let constructor be ? SpeciesConstructor(exemplar, defaultConstructor).
Let result be ? TypedArrayCreate(constructor, argumentList).
Assert: result has [[TypedArrayName]] and [[ContentType]] internal slots.
If result.[[ContentType]] is not equal to exemplar.[[ContentType]], throw a TypeError exception.
Return result.

22.2.5 Properties of the `TypedArray` Constructors

Each TypedArray constructor:

has a [[Prototype]] internal slot whose value is %TypedArray%.
has a "name" property whose value is the String value of the constructor name specified for it in Table 61.
has the following properties:

22.2.5.1 `TypedArray`.BYTES_PER_ELEMENT

The value of TypedArray.BYTES_PER_ELEMENT is the Element Size value specified in Table 61 for TypedArray.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

22.2.5.2 `TypedArray`.prototype

The initial value of TypedArray.prototype is the corresponding TypedArray prototype intrinsic object (22.2.6).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

22.2.6 Properties of the `TypedArray` Prototype Objects

Each TypedArray prototype object:

has a [[Prototype]] internal slot whose value is %TypedArray.prototype%.
is an ordinary object.
does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to TypedArray instance objects.

22.2.6.1 `TypedArray`.prototype.BYTES_PER_ELEMENT

The value of TypedArray.prototype.BYTES_PER_ELEMENT is the Element Size value specified in Table 61 for TypedArray.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

22.2.6.2 `TypedArray`.prototype.constructor

The initial value of a TypedArray.prototype.constructor is the corresponding %TypedArray% intrinsic object.

22.2.7 Properties of `TypedArray` Instances

TypedArray instances are Integer-Indexed exotic objects. Each TypedArray instance inherits properties from the corresponding TypedArray prototype object. Each TypedArray instance has the following internal slots: [[TypedArrayName]], [[ViewedArrayBuffer]], [[ByteLength]], [[ByteOffset]], and [[ArrayLength]].

23 Keyed Collections

23.1 Map Objects

Map objects are collections of key/value pairs where both the keys and values may be arbitrary ECMAScript language values. A distinct key value may only occur in one key/value pair within the Map's collection. Distinct key values are discriminated using the SameValueZero comparison algorithm.

Map object must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structures used in this Map objects specification is only intended to describe the required observable semantics of Map objects. It is not intended to be a viable implementation model.

23.1.1 The Map Constructor

The Map constructor:

is the intrinsic object %Map%.
is the initial value of the "Map" property of the global object.
creates and initializes a new Map object when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Map behaviour must include a super call to the Map constructor to create and initialize the subclass instance with the internal state necessary to support the Map.prototype built-in methods.

23.1.1.1 Map ( [ `iterable` ] )

When the Map function is called with optional argument iterable, the following steps are taken:

If NewTarget is undefined, throw a TypeError exception.
Let map be ? OrdinaryCreateFromConstructor(NewTarget, "%Map.prototype%", « [[MapData]] »).
Set map.[[MapData]] to a new empty List.
If iterable is either undefined or null, return map.
Let adder be ? Get(map, "set").
Return ? AddEntriesFromIterable(map, iterable, adder).

Note

If the parameter iterable is present, it is expected to be an object that implements an @@iterator method that returns an iterator object that produces a two element array-like object whose first element is a value that will be used as a Map key and whose second element is the value to associate with that key.

23.1.1.2 AddEntriesFromIterable ( `target`, `iterable`, `adder` )

The abstract operation AddEntriesFromIterable takes arguments target, iterable, and adder (a function object). adder will be invoked, with target as the receiver. It performs the following steps when called:

If IsCallable(adder) is false, throw a TypeError exception.
Assert: iterable is present, and is neither undefined nor null.
Let iteratorRecord be ? GetIterator(iterable).
Repeat,
1. Let next be ? IteratorStep(iteratorRecord).
2. If next is false, return target.
3. Let nextItem be ? IteratorValue(next).
4. If Type(nextItem) is not Object, then
  1. Let error be ThrowCompletion(a newly created TypeError object).
  2. Return ? IteratorClose(iteratorRecord, error).
5. Let k be Get(nextItem, "0").
6. If k is an abrupt completion, return ? IteratorClose(iteratorRecord, k).
7. Let v be Get(nextItem, "1").
8. If v is an abrupt completion, return ? IteratorClose(iteratorRecord, v).
9. Let status be Call(adder, target, « k.[[Value]], v.[[Value]] »).
10. If status is an abrupt completion, return ? IteratorClose(iteratorRecord, status).

Note

The parameter iterable is expected to be an object that implements an @@iterator method that returns an iterator object that produces a two element array-like object whose first element is a value that will be used as a Map key and whose second element is the value to associate with that key.

23.1.2 Properties of the Map Constructor

The Map constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

23.1.2.1 Map.prototype

The initial value of Map.prototype is %Map.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

23.1.2.2 get Map [ @@species ]

Map[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

Note

Methods that create derived collection objects should call @@species to determine the constructor to use to create the derived objects. Subclass constructor may over-ride @@species to change the default constructor assignment.

23.1.3 Properties of the Map Prototype Object

The Map prototype object:

is the intrinsic object %MapPrototype%.
has a [[Prototype]] internal slot whose value is %Object.prototype%.
is an ordinary object.
does not have a [[MapData]] internal slot.

23.1.3.1 Map.prototype.clear ( )

The following steps are taken:

Let M be the this value.
Perform ? RequireInternalSlot(M, [[MapData]]).
Let entries be the List that is M.[[MapData]].
For each Record { [[Key]], [[Value]] } p that is an element of entries, do
1. Set p.[[Key]] to empty.
2. Set p.[[Value]] to empty.
Return undefined.

Note

The existing [[MapData]] List is preserved because there may be existing Map Iterator objects that are suspended midway through iterating over that List.

23.1.3.2 Map.prototype.constructor

The initial value of Map.prototype.constructor is %Map%.

23.1.3.3 Map.prototype.delete ( `key` )

The following steps are taken:

Let M be the this value.
Perform ? RequireInternalSlot(M, [[MapData]]).
Let entries be the List that is M.[[MapData]].
For each Record { [[Key]], [[Value]] } p that is an element of entries, do
1. If p.[[Key]] is not empty and SameValueZero(p.[[Key]], key) is true, then
  1. Set p.[[Key]] to empty.
  2. Set p.[[Value]] to empty.
  3. Return true.
Return false.

Note

The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.

23.1.3.4 Map.prototype.entries ( )

The following steps are taken:

Let M be the this value.
Return ? CreateMapIterator(M, key+value).

23.1.3.5 Map.prototype.forEach ( `callbackfn` [ , `thisArg` ] )

When the forEach method is called with one or two arguments, the following steps are taken:

Let M be the this value.
Perform ? RequireInternalSlot(M, [[MapData]]).
If IsCallable(callbackfn) is false, throw a TypeError exception.
Let entries be the List that is M.[[MapData]].
For each Record { [[Key]], [[Value]] } e that is an element of entries, in original key insertion order, do
1. If e.[[Key]] is not empty, then
  1. Perform ? Call(callbackfn, thisArg, « e.[[Value]], e.[[Key]], M »).
Return undefined.

Note

callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each key/value pair present in the map object, in key insertion order. callbackfn is called only for keys of the map which actually exist; it is not called for keys that have been deleted from the map.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the item, the key of the item, and the Map object being traversed.

forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn. Each entry of a map's [[MapData]] is only visited once. New keys added after the call to forEach begins are visited. A key will be revisited if it is deleted after it has been visited and then re-added before the forEach call completes. Keys that are deleted after the call to forEach begins and before being visited are not visited unless the key is added again before the forEach call completes.

23.1.3.6 Map.prototype.get ( `key` )

The following steps are taken:

Let M be the this value.
Perform ? RequireInternalSlot(M, [[MapData]]).
Let entries be the List that is M.[[MapData]].
For each Record { [[Key]], [[Value]] } p that is an element of entries, do
1. If p.[[Key]] is not empty and SameValueZero(p.[[Key]], key) is true, return p.[[Value]].
Return undefined.

23.1.3.7 Map.prototype.has ( `key` )

The following steps are taken:

Let M be the this value.
Perform ? RequireInternalSlot(M, [[MapData]]).
Let entries be the List that is M.[[MapData]].
For each Record { [[Key]], [[Value]] } p that is an element of entries, do
1. If p.[[Key]] is not empty and SameValueZero(p.[[Key]], key) is true, return true.
Return false.

23.1.3.8 Map.prototype.keys ( )

The following steps are taken:

Let M be the this value.
Return ? CreateMapIterator(M, key).

23.1.3.9 Map.prototype.set ( `key`, `value` )

The following steps are taken:

Let M be the this value.
Perform ? RequireInternalSlot(M, [[MapData]]).
Let entries be the List that is M.[[MapData]].
For each Record { [[Key]], [[Value]] } p that is an element of entries, do
1. If p.[[Key]] is not empty and SameValueZero(p.[[Key]], key) is true, then
  1. Set p.[[Value]] to value.
  2. Return M.
If key is -0, set key to +0.
Let p be the Record { [[Key]]: key, [[Value]]: value }.
Append p as the last element of entries.
Return M.

23.1.3.10 get Map.prototype.size

Map.prototype.size is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let M be the this value.
Perform ? RequireInternalSlot(M, [[MapData]]).
Let entries be the List that is M.[[MapData]].
Let count be 0.
For each Record { [[Key]], [[Value]] } p that is an element of entries, do
1. If p.[[Key]] is not empty, set count to count + 1.
Return count.

23.1.3.11 Map.prototype.values ( )

The following steps are taken:

Let M be the this value.
Return ? CreateMapIterator(M, value).

23.1.3.12 Map.prototype [ @@iterator ] ( )

The initial value of the @@iterator property is the same function object as the initial value of the "entries" property.

23.1.3.13 Map.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Map".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

23.1.4 Properties of Map Instances

Map instances are ordinary objects that inherit properties from the Map prototype. Map instances also have a [[MapData]] internal slot.

23.1.5 Map Iterator Objects

A Map Iterator is an object, that represents a specific iteration over some specific Map instance object. There is not a named constructor for Map Iterator objects. Instead, map iterator objects are created by calling certain methods of Map instance objects.

23.1.5.1 CreateMapIterator ( `map`, `kind` )

The abstract operation CreateMapIterator takes arguments map and kind. This operation is used to create iterator objects for Map methods that return such iterators. It performs the following steps when called:

Perform ? RequireInternalSlot(map, [[MapData]]).
Let iterator be OrdinaryObjectCreate(%MapIteratorPrototype%, « [[IteratedMap]], [[MapNextIndex]], [[MapIterationKind]] »).
Set iterator.[[IteratedMap]] to map.
Set iterator.[[MapNextIndex]] to 0.
Set iterator.[[MapIterationKind]] to kind.
Return iterator.

23.1.5.2 The %MapIteratorPrototype% Object

The %MapIteratorPrototype% object:

has properties that are inherited by all Map Iterator Objects.
is an ordinary object.
has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
has the following properties:

23.1.5.2.1 %MapIteratorPrototype%.next ( )

Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have all of the internal slots of a Map Iterator Instance (23.1.5.3), throw a TypeError exception.
Let m be O.[[IteratedMap]].
Let index be O.[[MapNextIndex]].
Let itemKind be O.[[MapIterationKind]].
If m is undefined, return CreateIterResultObject(undefined, true).
Assert: m has a [[MapData]] internal slot.
Let entries be the List that is m.[[MapData]].
Let numEntries be the number of elements of entries.
NOTE: numEntries must be redetermined each time this method is evaluated.
Repeat, while index is less than numEntries,
1. Let e be the Record { [[Key]], [[Value]] } that is the value of entries[index].
2. Set index to index + 1.
3. Set O.[[MapNextIndex]] to index.
4. If e.[[Key]] is not empty, then
  1. If itemKind is key, let result be e.[[Key]].
  2. Else if itemKind is value, let result be e.[[Value]].
  3. Else,
    1. Assert: itemKind is key+value.
    2. Let result be ! CreateArrayFromList(« e.[[Key]], e.[[Value]] »).
  4. Return CreateIterResultObject(result, false).
Set O.[[IteratedMap]] to undefined.
Return CreateIterResultObject(undefined, true).

23.1.5.2.2 %MapIteratorPrototype% [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Map Iterator".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

23.1.5.3 Properties of Map Iterator Instances

Map Iterator instances are ordinary objects that inherit properties from the %MapIteratorPrototype% intrinsic object. Map Iterator instances are initially created with the internal slots described in Table 62.

Table 62: Internal Slots of Map Iterator Instances

Internal Slot	Description
[[IteratedMap]]	The Map object that is being iterated.
[[MapNextIndex]]	The integer index of the next [[MapData]] element to be examined by this iterator.
[[MapIterationKind]]	A String value that identifies what is returned for each element of the iteration. The possible values are: key, value, key+value.

23.2 Set Objects

Set objects are collections of ECMAScript language values. A distinct value may only occur once as an element of a Set's collection. Distinct values are discriminated using the SameValueZero comparison algorithm.

Set objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structures used in this Set objects specification is only intended to describe the required observable semantics of Set objects. It is not intended to be a viable implementation model.

23.2.1 The Set Constructor

The Set constructor:

is the intrinsic object %Set%.
is the initial value of the "Set" property of the global object.
creates and initializes a new Set object when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Set behaviour must include a super call to the Set constructor to create and initialize the subclass instance with the internal state necessary to support the Set.prototype built-in methods.

23.2.1.1 Set ( [ `iterable` ] )

When the Set function is called with optional argument iterable, the following steps are taken:

If NewTarget is undefined, throw a TypeError exception.
Let set be ? OrdinaryCreateFromConstructor(NewTarget, "%Set.prototype%", « [[SetData]] »).
Set set.[[SetData]] to a new empty List.
If iterable is either undefined or null, return set.
Let adder be ? Get(set, "add").
If IsCallable(adder) is false, throw a TypeError exception.
Let iteratorRecord be ? GetIterator(iterable).
Repeat,
1. Let next be ? IteratorStep(iteratorRecord).
2. If next is false, return set.
3. Let nextValue be ? IteratorValue(next).
4. Let status be Call(adder, set, « nextValue »).
5. If status is an abrupt completion, return ? IteratorClose(iteratorRecord, status).

23.2.2 Properties of the Set Constructor

The Set constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

23.2.2.1 Set.prototype

The initial value of Set.prototype is the intrinsic %SetPrototype% object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

23.2.2.2 get Set [ @@species ]

Set[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

Note

23.2.3 Properties of the Set Prototype Object

The Set prototype object:

is the intrinsic object %SetPrototype%.
has a [[Prototype]] internal slot whose value is %Object.prototype%.
is an ordinary object.
does not have a [[SetData]] internal slot.

23.2.3.1 Set.prototype.add ( `value` )

The following steps are taken:

Let S be the this value.
Perform ? RequireInternalSlot(S, [[SetData]]).
Let entries be the List that is S.[[SetData]].
For each e that is an element of entries, do
1. If e is not empty and SameValueZero(e, value) is true, then
  1. Return S.
If value is -0, set value to +0.
Append value as the last element of entries.
Return S.

23.2.3.2 Set.prototype.clear ( )

The following steps are taken:

Let S be the this value.
Perform ? RequireInternalSlot(S, [[SetData]]).
Let entries be the List that is S.[[SetData]].
For each e that is an element of entries, do
1. Replace the element of entries whose value is e with an element whose value is empty.
Return undefined.

Note

The existing [[SetData]] List is preserved because there may be existing Set Iterator objects that are suspended midway through iterating over that List.

23.2.3.3 Set.prototype.constructor

The initial value of Set.prototype.constructor is %Set%.

23.2.3.4 Set.prototype.delete ( `value` )

The following steps are taken:

Let S be the this value.
Perform ? RequireInternalSlot(S, [[SetData]]).
Let entries be the List that is S.[[SetData]].
For each e that is an element of entries, do
1. If e is not empty and SameValueZero(e, value) is true, then
  1. Replace the element of entries whose value is e with an element whose value is empty.
  2. Return true.
Return false.

Note

23.2.3.5 Set.prototype.entries ( )

The following steps are taken:

Let S be the this value.
Return ? CreateSetIterator(S, key+value).

Note

For iteration purposes, a Set appears similar to a Map where each entry has the same value for its key and value.

23.2.3.6 Set.prototype.forEach ( `callbackfn` [ , `thisArg` ] )

When the forEach method is called with one or two arguments, the following steps are taken:

Let S be the this value.
Perform ? RequireInternalSlot(S, [[SetData]]).
If IsCallable(callbackfn) is false, throw a TypeError exception.
Let entries be the List that is S.[[SetData]].
For each e that is an element of entries, in original insertion order, do
1. If e is not empty, then
  1. Perform ? Call(callbackfn, thisArg, « e, e, S »).
Return undefined.

Note

callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each value present in the set object, in value insertion order. callbackfn is called only for values of the Set which actually exist; it is not called for keys that have been deleted from the set.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the first two arguments are a value contained in the Set. The same value is passed for both arguments. The Set object being traversed is passed as the third argument.

The callbackfn is called with three arguments to be consistent with the call back functions used by forEach methods for Map and Array. For Sets, each item value is considered to be both the key and the value.

forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

Each value is normally visited only once. However, a value will be revisited if it is deleted after it has been visited and then re-added before the forEach call completes. Values that are deleted after the call to forEach begins and before being visited are not visited unless the value is added again before the forEach call completes. New values added after the call to forEach begins are visited.

23.2.3.7 Set.prototype.has ( `value` )

The following steps are taken:

Let S be the this value.
Perform ? RequireInternalSlot(S, [[SetData]]).
Let entries be the List that is S.[[SetData]].
For each e that is an element of entries, do
1. If e is not empty and SameValueZero(e, value) is true, return true.
Return false.

23.2.3.8 Set.prototype.keys ( )

The initial value of the "keys" property is the same function object as the initial value of the "values" property.

Note

For iteration purposes, a Set appears similar to a Map where each entry has the same value for its key and value.

23.2.3.9 get Set.prototype.size

Set.prototype.size is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let S be the this value.
Perform ? RequireInternalSlot(S, [[SetData]]).
Let entries be the List that is S.[[SetData]].
Let count be 0.
For each e that is an element of entries, do
1. If e is not empty, set count to count + 1.
Return count.

23.2.3.10 Set.prototype.values ( )

The following steps are taken:

Let S be the this value.
Return ? CreateSetIterator(S, value).

23.2.3.11 Set.prototype [ @@iterator ] ( )

The initial value of the @@iterator property is the same function object as the initial value of the "values" property.

23.2.3.12 Set.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Set".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

23.2.4 Properties of Set Instances

Set instances are ordinary objects that inherit properties from the Set prototype. Set instances also have a [[SetData]] internal slot.

23.2.5 Set Iterator Objects

A Set Iterator is an ordinary object, with the structure defined below, that represents a specific iteration over some specific Set instance object. There is not a named constructor for Set Iterator objects. Instead, set iterator objects are created by calling certain methods of Set instance objects.

23.2.5.1 CreateSetIterator ( `set`, `kind` )

The abstract operation CreateSetIterator takes arguments set and kind. This operation is used to create iterator objects for Set methods that return such iterators. It performs the following steps when called:

Perform ? RequireInternalSlot(set, [[SetData]]).
Let iterator be OrdinaryObjectCreate(%SetIteratorPrototype%, « [[IteratedSet]], [[SetNextIndex]], [[SetIterationKind]] »).
Set iterator.[[IteratedSet]] to set.
Set iterator.[[SetNextIndex]] to 0.
Set iterator.[[SetIterationKind]] to kind.
Return iterator.

23.2.5.2 The %SetIteratorPrototype% Object

The %SetIteratorPrototype% object:

has properties that are inherited by all Set Iterator Objects.
is an ordinary object.
has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
has the following properties:

23.2.5.2.1 %SetIteratorPrototype%.next ( )

Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have all of the internal slots of a Set Iterator Instance (23.2.5.3), throw a TypeError exception.
Let s be O.[[IteratedSet]].
Let index be O.[[SetNextIndex]].
Let itemKind be O.[[SetIterationKind]].
If s is undefined, return CreateIterResultObject(undefined, true).
Assert: s has a [[SetData]] internal slot.
Let entries be the List that is s.[[SetData]].
Let numEntries be the number of elements of entries.
NOTE: numEntries must be redetermined each time this method is evaluated.
Repeat, while index is less than numEntries,
1. Let e be entries[index].
2. Set index to index + 1.
3. Set O.[[SetNextIndex]] to index.
4. If e is not empty, then
  1. If itemKind is key+value, then
    1. Return CreateIterResultObject(CreateArrayFromList(« e, e »), false).
  2. Assert: itemKind is value.
  3. Return CreateIterResultObject(e, false).
Set O.[[IteratedSet]] to undefined.
Return CreateIterResultObject(undefined, true).

23.2.5.2.2 %SetIteratorPrototype% [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Set Iterator".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

23.2.5.3 Properties of Set Iterator Instances

Set Iterator instances are ordinary objects that inherit properties from the %SetIteratorPrototype% intrinsic object. Set Iterator instances are initially created with the internal slots specified in Table 63.

Table 63: Internal Slots of Set Iterator Instances

Internal Slot	Description
[[IteratedSet]]	The Set object that is being iterated.
[[SetNextIndex]]	The integer index of the next [[SetData]] element to be examined by this iterator.
[[SetIterationKind]]	A String value that identifies what is returned for each element of the iteration. The possible values are value and key+value.

23.3 WeakMap Objects

WeakMap objects are collections of key/value pairs where the keys are objects and values may be arbitrary ECMAScript language values. A WeakMap may be queried to see if it contains a key/value pair with a specific key, but no mechanism is provided for enumerating the objects it holds as keys. If an object that is being used as the key of a WeakMap key/value pair is only reachable by following a chain of references that start within that WeakMap, then that key/value pair is inaccessible and is automatically removed from the WeakMap. WeakMap implementations must detect and remove such key/value pairs and any associated resources.

An implementation may impose an arbitrarily determined latency between the time a key/value pair of a WeakMap becomes inaccessible and the time when the key/value pair is removed from the WeakMap. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to observe a key of a WeakMap that does not require the observer to present the observed key.

WeakMap objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of key/value pairs in the collection. The data structure used in this WeakMap objects specification are only intended to describe the required observable semantics of WeakMap objects. It is not intended to be a viable implementation model.

Note

WeakMap and WeakSets are intended to provide mechanisms for dynamically associating state with an object in a manner that does not “leak” memory resources if, in the absence of the WeakMap or WeakSet, the object otherwise became inaccessible and subject to resource reclamation by the implementation's garbage collection mechanisms. This characteristic can be achieved by using an inverted per-object mapping of weak map instances to keys. Alternatively each weak map may internally store its key to value mappings but this approach requires coordination between the WeakMap or WeakSet implementation and the garbage collector. The following references describe mechanism that may be useful to implementations of WeakMap and WeakSets:

Barry Hayes. 1997. Ephemerons: a new finalization mechanism. In Proceedings of the 12th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications (OOPSLA '97), A. Michael Berman (Ed.). ACM, New York, NY, USA, 176-183, http://doi.acm.org/10.1145/263698.263733.

Alexandra Barros, Roberto Ierusalimschy, Eliminating Cycles in Weak Tables. Journal of Universal Computer Science - J.UCS, vol. 14, no. 21, pp. 3481-3497, 2008, http://www.jucs.org/jucs_14_21/eliminating_cycles_in_weak

23.3.1 The WeakMap Constructor

The WeakMap constructor:

is the intrinsic object %WeakMap%.
is the initial value of the "WeakMap" property of the global object.
creates and initializes a new WeakMap object when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakMap behaviour must include a super call to the WeakMap constructor to create and initialize the subclass instance with the internal state necessary to support the WeakMap.prototype built-in methods.

23.3.1.1 WeakMap ( [ `iterable` ] )

When the WeakMap function is called with optional argument iterable, the following steps are taken:

If NewTarget is undefined, throw a TypeError exception.
Let map be ? OrdinaryCreateFromConstructor(NewTarget, "%WeakMap.prototype%", « [[WeakMapData]] »).
Set map.[[WeakMapData]] to a new empty List.
If iterable is either undefined or null, return map.
Let adder be ? Get(map, "set").
Return ? AddEntriesFromIterable(map, iterable, adder).

Note

If the parameter iterable is present, it is expected to be an object that implements an @@iterator method that returns an iterator object that produces a two element array-like object whose first element is a value that will be used as a WeakMap key and whose second element is the value to associate with that key.

23.3.2 Properties of the WeakMap Constructor

The WeakMap constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

23.3.2.1 WeakMap.prototype

The initial value of WeakMap.prototype is %WeakMap.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

23.3.3 Properties of the WeakMap Prototype Object

The WeakMap prototype object:

is the intrinsic object %WeakMapPrototype%.
has a [[Prototype]] internal slot whose value is %Object.prototype%.
is an ordinary object.
does not have a [[WeakMapData]] internal slot.

23.3.3.1 WeakMap.prototype.constructor

The initial value of WeakMap.prototype.constructor is %WeakMap%.

23.3.3.2 WeakMap.prototype.delete ( `key` )

The following steps are taken:

Let M be the this value.
Perform ? RequireInternalSlot(M, [[WeakMapData]]).
Let entries be the List that is M.[[WeakMapData]].
If Type(key) is not Object, return false.
For each Record { [[Key]], [[Value]] } p that is an element of entries, do
1. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, then
  1. Set p.[[Key]] to empty.
  2. Set p.[[Value]] to empty.
  3. Return true.
Return false.

Note

23.3.3.3 WeakMap.prototype.get ( `key` )

The following steps are taken:

Let M be the this value.
Perform ? RequireInternalSlot(M, [[WeakMapData]]).
Let entries be the List that is M.[[WeakMapData]].
If Type(key) is not Object, return undefined.
For each Record { [[Key]], [[Value]] } p that is an element of entries, do
1. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, return p.[[Value]].
Return undefined.

23.3.3.4 WeakMap.prototype.has ( `key` )

The following steps are taken:

Let M be the this value.
Perform ? RequireInternalSlot(M, [[WeakMapData]]).
Let entries be the List that is M.[[WeakMapData]].
If Type(key) is not Object, return false.
For each Record { [[Key]], [[Value]] } p that is an element of entries, do
1. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, return true.
Return false.

23.3.3.5 WeakMap.prototype.set ( `key`, `value` )

The following steps are taken:

Let M be the this value.
Perform ? RequireInternalSlot(M, [[WeakMapData]]).
Let entries be the List that is M.[[WeakMapData]].
If Type(key) is not Object, throw a TypeError exception.
For each Record { [[Key]], [[Value]] } p that is an element of entries, do
1. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, then
  1. Set p.[[Value]] to value.
  2. Return M.
Let p be the Record { [[Key]]: key, [[Value]]: value }.
Append p as the last element of entries.
Return M.

23.3.3.6 WeakMap.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "WeakMap".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

23.3.4 Properties of WeakMap Instances

WeakMap instances are ordinary objects that inherit properties from the WeakMap prototype. WeakMap instances also have a [[WeakMapData]] internal slot.

23.4 WeakSet Objects

WeakSet objects are collections of objects. A distinct object may only occur once as an element of a WeakSet's collection. A WeakSet may be queried to see if it contains a specific object, but no mechanism is provided for enumerating the objects it holds. If an object that is contained by a WeakSet is only reachable by following a chain of references that start within that WeakSet, then that object is inaccessible and is automatically removed from the WeakSet. WeakSet implementations must detect and remove such objects and any associated resources.

An implementation may impose an arbitrarily determined latency between the time an object contained in a WeakSet becomes inaccessible and the time when the object is removed from the WeakSet. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to determine if a WeakSet contains a particular object that does not require the observer to present the observed object.

WeakSet objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structure used in this WeakSet objects specification is only intended to describe the required observable semantics of WeakSet objects. It is not intended to be a viable implementation model.

Note

See the NOTE in 23.3.

23.4.1 The WeakSet Constructor

The WeakSet constructor:

is the intrinsic object %WeakSet%.
is the initial value of the "WeakSet" property of the global object.
creates and initializes a new WeakSet object when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakSet behaviour must include a super call to the WeakSet constructor to create and initialize the subclass instance with the internal state necessary to support the WeakSet.prototype built-in methods.

23.4.1.1 WeakSet ( [ `iterable` ] )

When the WeakSet function is called with optional argument iterable, the following steps are taken:

If NewTarget is undefined, throw a TypeError exception.
Let set be ? OrdinaryCreateFromConstructor(NewTarget, "%WeakSet.prototype%", « [[WeakSetData]] »).
Set set.[[WeakSetData]] to a new empty List.
If iterable is either undefined or null, return set.
Let adder be ? Get(set, "add").
If IsCallable(adder) is false, throw a TypeError exception.
Let iteratorRecord be ? GetIterator(iterable).
Repeat,
1. Let next be ? IteratorStep(iteratorRecord).
2. If next is false, return set.
3. Let nextValue be ? IteratorValue(next).
4. Let status be Call(adder, set, « nextValue »).
5. If status is an abrupt completion, return ? IteratorClose(iteratorRecord, status).

23.4.2 Properties of the WeakSet Constructor

The WeakSet constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

23.4.2.1 WeakSet.prototype

The initial value of WeakSet.prototype is the intrinsic %WeakSetPrototype% object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

23.4.3 Properties of the WeakSet Prototype Object

The WeakSet prototype object:

is the intrinsic object %WeakSetPrototype%.
has a [[Prototype]] internal slot whose value is %Object.prototype%.
is an ordinary object.
does not have a [[WeakSetData]] internal slot.

23.4.3.1 WeakSet.prototype.add ( `value` )

The following steps are taken:

Let S be the this value.
Perform ? RequireInternalSlot(S, [[WeakSetData]]).
If Type(value) is not Object, throw a TypeError exception.
Let entries be the List that is S.[[WeakSetData]].
For each e that is an element of entries, do
1. If e is not empty and SameValue(e, value) is true, then
  1. Return S.
Append value as the last element of entries.
Return S.

23.4.3.2 WeakSet.prototype.constructor

The initial value of WeakSet.prototype.constructor is the %WeakSet% intrinsic object.

23.4.3.3 WeakSet.prototype.delete ( `value` )

The following steps are taken:

Let S be the this value.
Perform ? RequireInternalSlot(S, [[WeakSetData]]).
If Type(value) is not Object, return false.
Let entries be the List that is S.[[WeakSetData]].
For each e that is an element of entries, do
1. If e is not empty and SameValue(e, value) is true, then
  1. Replace the element of entries whose value is e with an element whose value is empty.
  2. Return true.
Return false.

Note

23.4.3.4 WeakSet.prototype.has ( `value` )

The following steps are taken:

Let S be the this value.
Perform ? RequireInternalSlot(S, [[WeakSetData]]).
Let entries be the List that is S.[[WeakSetData]].
If Type(value) is not Object, return false.
For each e that is an element of entries, do
1. If e is not empty and SameValue(e, value) is true, return true.
Return false.

23.4.3.5 WeakSet.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "WeakSet".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

23.4.4 Properties of WeakSet Instances

WeakSet instances are ordinary objects that inherit properties from the WeakSet prototype. WeakSet instances also have a [[WeakSetData]] internal slot.

24 Structured Data

24.1 ArrayBuffer Objects

24.1.1 Abstract Operations For ArrayBuffer Objects

24.1.1.1 AllocateArrayBuffer ( `constructor`, `byteLength` )

The abstract operation AllocateArrayBuffer takes arguments constructor and byteLength. It is used to create an ArrayBuffer object. It performs the following steps when called:

Let obj be ? OrdinaryCreateFromConstructor(constructor, "%ArrayBuffer.prototype%", « [[ArrayBufferData]], [[ArrayBufferByteLength]], [[ArrayBufferDetachKey]] »).
Assert: ! IsNonNegativeInteger(byteLength) is true.
Let block be ? CreateByteDataBlock(byteLength).
Set obj.[[ArrayBufferData]] to block.
Set obj.[[ArrayBufferByteLength]] to byteLength.
Return obj.

24.1.1.2 IsDetachedBuffer ( `arrayBuffer` )

The abstract operation IsDetachedBuffer takes argument arrayBuffer. It performs the following steps when called:

Assert: Type(arrayBuffer) is Object and it has an [[ArrayBufferData]] internal slot.
If arrayBuffer.[[ArrayBufferData]] is null, return true.
Return false.

24.1.1.3 DetachArrayBuffer ( `arrayBuffer` [ , `key` ] )

The abstract operation DetachArrayBuffer takes argument arrayBuffer and optional argument key. It performs the following steps when called:

Assert: Type(arrayBuffer) is Object and it has [[ArrayBufferData]], [[ArrayBufferByteLength]], and [[ArrayBufferDetachKey]] internal slots.
Assert: IsSharedArrayBuffer(arrayBuffer) is false.
If key is not present, set key to undefined.
If SameValue(arrayBuffer.[[ArrayBufferDetachKey]], key) is false, throw a TypeError exception.
Set arrayBuffer.[[ArrayBufferData]] to null.
Set arrayBuffer.[[ArrayBufferByteLength]] to 0.
Return NormalCompletion(null).

Note

Detaching an ArrayBuffer instance disassociates the Data Block used as its backing store from the instance and sets the byte length of the buffer to 0. No operations defined by this specification use the DetachArrayBuffer abstract operation. However, an ECMAScript implementation or host environment may define such operations.

24.1.1.4 CloneArrayBuffer ( `srcBuffer`, `srcByteOffset`, `srcLength`, `cloneConstructor` )

The abstract operation CloneArrayBuffer takes arguments srcBuffer (an ArrayBuffer object), srcByteOffset (an integer), srcLength (an integer), and cloneConstructor (a constructor). It creates a new ArrayBuffer whose data is a copy of srcBuffer's data over the range starting at srcByteOffset and continuing for srcLength bytes. It performs the following steps when called:

Assert: Type(srcBuffer) is Object and it has an [[ArrayBufferData]] internal slot.
Assert: IsConstructor(cloneConstructor) is true.
Let targetBuffer be ? AllocateArrayBuffer(cloneConstructor, srcLength).
If IsDetachedBuffer(srcBuffer) is true, throw a TypeError exception.
Let srcBlock be srcBuffer.[[ArrayBufferData]].
Let targetBlock be targetBuffer.[[ArrayBufferData]].
Perform CopyDataBlockBytes(targetBlock, 0, srcBlock, srcByteOffset, srcLength).
Return targetBuffer.

24.1.1.5 IsUnsignedElementType ( `type` )

The abstract operation IsUnsignedElementType takes argument type. It verifies if the argument type is an unsigned TypedArray element type. It performs the following steps when called:

If type is Uint8, Uint8C, Uint16, Uint32, or BigUint64, return true.
Return false.

24.1.1.6 IsUnclampedIntegerElementType ( `type` )

The abstract operation IsUnclampedIntegerElementType takes argument type. It verifies if the argument type is an Integer TypedArray element type not including Uint8C. It performs the following steps when called:

If type is Int8, Uint8, Int16, Uint16, Int32, or Uint32, return true.
Return false.

24.1.1.7 IsBigIntElementType ( `type` )

The abstract operation IsBigIntElementType takes argument type. It verifies if the argument type is a BigInt TypedArray element type. It performs the following steps when called:

If type is BigUint64 or BigInt64, return true.
Return false.

24.1.1.8 IsNoTearConfiguration ( `type`, `order` )

The abstract operation IsNoTearConfiguration takes arguments type and order. It performs the following steps when called:

If ! IsUnclampedIntegerElementType(type) is true, return true.
If ! IsBigIntElementType(type) is true and order is not Init or Unordered, return true.
Return false.

24.1.1.9 RawBytesToNumeric ( `type`, `rawBytes`, `isLittleEndian` )

The abstract operation RawBytesToNumeric takes arguments type (a TypedArray element type), rawBytes (a List), and isLittleEndian (a Boolean). It performs the following steps when called:

Let elementSize be the Element Size value specified in Table 61 for Element Type type.
If isLittleEndian is false, reverse the order of the elements of rawBytes.
If type is Float32, then
1. Let value be the byte elements of rawBytes concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2019 binary32 value.
2. If value is an IEEE 754-2019 binary32 NaN value, return the NaN Number value.
3. Return the Number value that corresponds to value.
If type is Float64, then
1. Let value be the byte elements of rawBytes concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2019 binary64 value.
2. If value is an IEEE 754-2019 binary64 NaN value, return the NaN Number value.
3. Return the Number value that corresponds to value.
If ! IsUnsignedElementType(type) is true, then
1. Let intValue be the byte elements of rawBytes concatenated and interpreted as a bit string encoding of an unsigned little-endian binary number.
Else,
1. Let intValue be the byte elements of rawBytes concatenated and interpreted as a bit string encoding of a binary little-endian 2's complement number of bit length elementSize × 8.
If ! IsBigIntElementType(type) is true, return the BigInt value that corresponds to intValue.
Otherwise, return the Number value that corresponds to intValue.

24.1.1.10 GetValueFromBuffer ( `arrayBuffer`, `byteIndex`, `type`, `isTypedArray`, `order` [ , `isLittleEndian` ] )

The abstract operation GetValueFromBuffer takes arguments arrayBuffer (an ArrayBuffer or SharedArrayBuffer), byteIndex (an integer), type (a TypedArray element type), isTypedArray (a Boolean), and order (either SeqCst or Unordered) and optional argument isLittleEndian (a Boolean). It performs the following steps when called:

Assert: IsDetachedBuffer(arrayBuffer) is false.
Assert: There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.
Assert: ! IsNonNegativeInteger(byteIndex) is true.
Let block be arrayBuffer.[[ArrayBufferData]].
Let elementSize be the Element Size value specified in Table 61 for Element Type type.
If IsSharedArrayBuffer(arrayBuffer) is true, then
1. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
2. Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
3. If isTypedArray is true and IsNoTearConfiguration(type, order) is true, let noTear be true; otherwise let noTear be false.
4. Let rawValue be a List of length elementSize of nondeterministically chosen byte values.
5. NOTE: In implementations, rawValue is the result of a non-atomic or atomic read instruction on the underlying hardware. The nondeterminism is a semantic prescription of the memory model to describe observable behaviour of hardware with weak consistency.
6. Let readEvent be ReadSharedMemory { [[Order]]: order, [[NoTear]]: noTear, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize }.
7. Append readEvent to eventList.
8. Append Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]: rawValue } to execution.[[ChosenValues]].
Else, let rawValue be a List of elementSize containing, in order, the elementSize sequence of bytes starting with block[byteIndex].
If isLittleEndian is not present, set isLittleEndian to the value of the [[LittleEndian]] field of the surrounding agent's Agent Record.
Return RawBytesToNumeric(type, rawValue, isLittleEndian).

24.1.1.11 NumericToRawBytes ( `type`, `value`, `isLittleEndian` )

The abstract operation NumericToRawBytes takes arguments type (a TypedArray element type), value (a BigInt or a Number), and isLittleEndian (a Boolean). It performs the following steps when called:

If type is Float32, then
1. Let rawBytes be a List containing the 4 bytes that are the result of converting value to IEEE 754-2019 binary32 format using roundTiesToEven mode. If isLittleEndian is false, the bytes are arranged in big endian order. Otherwise, the bytes are arranged in little endian order. If value is NaN, rawBytes may be set to any implementation chosen IEEE 754-2019 binary32 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishable NaN value.
Else if type is Float64, then
1. Let rawBytes be a List containing the 8 bytes that are the IEEE 754-2019 binary64 format encoding of value. If isLittleEndian is false, the bytes are arranged in big endian order. Otherwise, the bytes are arranged in little endian order. If value is NaN, rawBytes may be set to any implementation chosen IEEE 754-2019 binary64 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishable NaN value.
Else,
1. Let n be the Element Size value specified in Table 61 for Element Type type.
2. Let convOp be the abstract operation named in the Conversion Operation column in Table 61 for Element Type type.
3. Let intValue be convOp(value) treated as a mathematical value, whether the result is a BigInt or Number.
4. If intValue ≥ 0_ℝ, then
  1. Let rawBytes be a List containing the n-byte binary encoding of intValue. If isLittleEndian is false, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian order.
5. Else,
  1. Let rawBytes be a List containing the n-byte binary 2's complement encoding of intValue. If isLittleEndian is false, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian order.
Return rawBytes.

24.1.1.12 SetValueInBuffer ( `arrayBuffer`, `byteIndex`, `type`, `value`, `isTypedArray`, `order` [ , `isLittleEndian` ] )

The abstract operation SetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer or SharedArrayBuffer), byteIndex (an integer), type (a TypedArray element type), value (a Number or a BigInt), isTypedArray (a Boolean), and order (one of SeqCst, Unordered, or Init) and optional argument isLittleEndian (a Boolean). It performs the following steps when called:

Assert: IsDetachedBuffer(arrayBuffer) is false.
Assert: There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.
Assert: ! IsNonNegativeInteger(byteIndex) is true.
Assert: Type(value) is BigInt if ! IsBigIntElementType(type) is true; otherwise, Type(value) is Number.
Let block be arrayBuffer.[[ArrayBufferData]].
Let elementSize be the Element Size value specified in Table 61 for Element Type type.
If isLittleEndian is not present, set isLittleEndian to the value of the [[LittleEndian]] field of the surrounding agent's Agent Record.
Let rawBytes be NumericToRawBytes(type, value, isLittleEndian).
If IsSharedArrayBuffer(arrayBuffer) is true, then
1. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
2. Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
3. If isTypedArray is true and IsNoTearConfiguration(type, order) is true, let noTear be true; otherwise let noTear be false.
4. Append WriteSharedMemory { [[Order]]: order, [[NoTear]]: noTear, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize, [[Payload]]: rawBytes } to eventList.
Else, store the individual bytes of rawBytes into block, in order, starting at block[byteIndex].
Return NormalCompletion(undefined).

24.1.1.13 GetModifySetValueInBuffer ( `arrayBuffer`, `byteIndex`, `type`, `value`, `op` [ , `isLittleEndian` ] )

The abstract operation GetModifySetValueInBuffer takes arguments arrayBuffer (a SharedArrayBuffer object), byteIndex (a non-negative integer), type (a TypedArray element type), value (a Number or a BigInt), and op (a semantic function) and optional argument isLittleEndian (a Boolean). It performs the following steps when called:

Assert: IsSharedArrayBuffer(arrayBuffer) is true.
Assert: There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.
Assert: ! IsNonNegativeInteger(byteIndex) is true.
Assert: Type(value) is BigInt if ! IsBigIntElementType(type) is true; otherwise, Type(value) is Number.
Let block be arrayBuffer.[[ArrayBufferData]].
Let elementSize be the Element Size value specified in Table 61 for Element Type type.
If isLittleEndian is not present, set isLittleEndian to the value of the [[LittleEndian]] field of the surrounding agent's Agent Record.
Let rawBytes be NumericToRawBytes(type, value, isLittleEndian).
Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
Let rawBytesRead be a List of length elementSize of nondeterministically chosen byte values.
NOTE: In implementations, rawBytesRead is the result of a load-link, of a load-exclusive, or of an operand of a read-modify-write instruction on the underlying hardware. The nondeterminism is a semantic prescription of the memory model to describe observable behaviour of hardware with weak consistency.
Let rmwEvent be ReadModifyWriteSharedMemory { [[Order]]: SeqCst, [[NoTear]]: true, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize, [[Payload]]: rawBytes, [[ModifyOp]]: op }.
Append rmwEvent to eventList.
Append Chosen Value Record { [[Event]]: rmwEvent, [[ChosenValue]]: rawBytesRead } to execution.[[ChosenValues]].
Return RawBytesToNumeric(type, rawBytesRead, isLittleEndian).

24.1.2 The ArrayBuffer Constructor

The ArrayBuffer constructor:

is the intrinsic object %ArrayBuffer%.
is the initial value of the "ArrayBuffer" property of the global object.
creates and initializes a new ArrayBuffer object when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified ArrayBuffer behaviour must include a super call to the ArrayBuffer constructor to create and initialize subclass instances with the internal state necessary to support the ArrayBuffer.prototype built-in methods.

24.1.2.1 ArrayBuffer ( `length` )

When the ArrayBuffer function is called with argument length, the following steps are taken:

If NewTarget is undefined, throw a TypeError exception.
Let byteLength be ? ToIndex(length).
Return ? AllocateArrayBuffer(NewTarget, byteLength).

24.1.3 Properties of the ArrayBuffer Constructor

The ArrayBuffer constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

24.1.3.1 ArrayBuffer.isView ( `arg` )

The isView function takes one argument arg, and performs the following steps:

If Type(arg) is not Object, return false.
If arg has a [[ViewedArrayBuffer]] internal slot, return true.
Return false.

24.1.3.2 ArrayBuffer.prototype

The initial value of ArrayBuffer.prototype is %ArrayBuffer.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

24.1.3.3 get ArrayBuffer [ @@species ]

ArrayBuffer[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

Note

ArrayBuffer prototype methods normally use their this object's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.

24.1.4 Properties of the ArrayBuffer Prototype Object

The ArrayBuffer prototype object:

is the intrinsic object %ArrayBufferPrototype%.
has a [[Prototype]] internal slot whose value is %Object.prototype%.
is an ordinary object.
does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.

24.1.4.1 get ArrayBuffer.prototype.byteLength

ArrayBuffer.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let O be the this value.
Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
If IsSharedArrayBuffer(O) is true, throw a TypeError exception.
If IsDetachedBuffer(O) is true, throw a TypeError exception.
Let length be O.[[ArrayBufferByteLength]].
Return length.

24.1.4.2 ArrayBuffer.prototype.constructor

The initial value of ArrayBuffer.prototype.constructor is %ArrayBuffer%.

24.1.4.3 ArrayBuffer.prototype.slice ( `start`, `end` )

The following steps are taken:

Let O be the this value.
Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
If IsSharedArrayBuffer(O) is true, throw a TypeError exception.
If IsDetachedBuffer(O) is true, throw a TypeError exception.
Let len be O.[[ArrayBufferByteLength]].
Let relativeStart be ? ToInteger(start).
If relativeStart < 0, let first be max((len + relativeStart), 0); else let first be min(relativeStart, len).
If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToInteger(end).
If relativeEnd < 0, let final be max((len + relativeEnd), 0); else let final be min(relativeEnd, len).
Let newLen be max(final - first, 0).
Let ctor be ? SpeciesConstructor(O, %ArrayBuffer%).
Let new be ? Construct(ctor, « newLen »).
Perform ? RequireInternalSlot(new, [[ArrayBufferData]]).
If IsSharedArrayBuffer(new) is true, throw a TypeError exception.
If IsDetachedBuffer(new) is true, throw a TypeError exception.
If SameValue(new, O) is true, throw a TypeError exception.
If new.[[ArrayBufferByteLength]] < newLen, throw a TypeError exception.
NOTE: Side-effects of the above steps may have detached O.
If IsDetachedBuffer(O) is true, throw a TypeError exception.
Let fromBuf be O.[[ArrayBufferData]].
Let toBuf be new.[[ArrayBufferData]].
Perform CopyDataBlockBytes(toBuf, 0, fromBuf, first, newLen).
Return new.

24.1.4.4 ArrayBuffer.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "ArrayBuffer".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

24.1.5 Properties of ArrayBuffer Instances

ArrayBuffer instances inherit properties from the ArrayBuffer prototype object. ArrayBuffer instances each have an [[ArrayBufferData]] internal slot, an [[ArrayBufferByteLength]] internal slot, and an [[ArrayBufferDetachKey]] internal slot.

ArrayBuffer instances whose [[ArrayBufferData]] is null are considered to be detached and all operators to access or modify data contained in the ArrayBuffer instance will fail.

ArrayBuffer instances whose [[ArrayBufferDetachKey]] is set to a value other than undefined need to have all DetachArrayBuffer calls passing that same "detach key" as an argument, otherwise a TypeError will result. This internal slot is only ever set by certain embedding environments, not by algorithms in this specification.

24.2 SharedArrayBuffer Objects

24.2.1 Abstract Operations for SharedArrayBuffer Objects

24.2.1.1 AllocateSharedArrayBuffer ( `constructor`, `byteLength` )

The abstract operation AllocateSharedArrayBuffer takes arguments constructor and byteLength. It is used to create a SharedArrayBuffer object. It performs the following steps when called:

Let obj be ? OrdinaryCreateFromConstructor(constructor, "%SharedArrayBuffer.prototype%", « [[ArrayBufferData]], [[ArrayBufferByteLength]] »).
Assert: ! IsNonNegativeInteger(byteLength) is true.
Let block be ? CreateSharedByteDataBlock(byteLength).
Set obj.[[ArrayBufferData]] to block.
Set obj.[[ArrayBufferByteLength]] to byteLength.
Return obj.

24.2.1.2 IsSharedArrayBuffer ( `obj` )

The abstract operation IsSharedArrayBuffer takes argument obj. It tests whether an object is an ArrayBuffer, a SharedArrayBuffer, or a subtype of either. It performs the following steps when called:

Assert: Type(obj) is Object and it has an [[ArrayBufferData]] internal slot.
Let bufferData be obj.[[ArrayBufferData]].
If bufferData is null, return false.
If bufferData is a Data Block, return false.
Assert: bufferData is a Shared Data Block.
Return true.

24.2.2 The SharedArrayBuffer Constructor

The SharedArrayBuffer constructor:

is the intrinsic object %SharedArrayBuffer%.
is the initial value of the "SharedArrayBuffer" property of the global object.
creates and initializes a new SharedArrayBuffer object when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified SharedArrayBuffer behaviour must include a super call to the SharedArrayBuffer constructor to create and initialize subclass instances with the internal state necessary to support the SharedArrayBuffer.prototype built-in methods.

Note

Unlike an ArrayBuffer, a SharedArrayBuffer cannot become detached, and its internal [[ArrayBufferData]] slot is never null.

24.2.2.1 SharedArrayBuffer ( [ `length` ] )

When the SharedArrayBuffer function is called with optional argument length, the following steps are taken:

If NewTarget is undefined, throw a TypeError exception.
Let byteLength be ? ToIndex(length).
Return ? AllocateSharedArrayBuffer(NewTarget, byteLength).

24.2.3 Properties of the SharedArrayBuffer Constructor

The SharedArrayBuffer constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

24.2.3.1 SharedArrayBuffer.prototype

The initial value of SharedArrayBuffer.prototype is %SharedArrayBuffer.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

24.2.3.2 get SharedArrayBuffer [ @@species ]

SharedArrayBuffer[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

24.2.4 Properties of the SharedArrayBuffer Prototype Object

The SharedArrayBuffer prototype object:

is the intrinsic object %SharedArrayBufferPrototype%.
has a [[Prototype]] internal slot whose value is %Object.prototype%.
is an ordinary object.
does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.

24.2.4.1 get SharedArrayBuffer.prototype.byteLength

SharedArrayBuffer.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let O be the this value.
Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
If IsSharedArrayBuffer(O) is false, throw a TypeError exception.
Let length be O.[[ArrayBufferByteLength]].
Return length.

24.2.4.2 SharedArrayBuffer.prototype.constructor

The initial value of SharedArrayBuffer.prototype.constructor is %SharedArrayBuffer%.

24.2.4.3 SharedArrayBuffer.prototype.slice ( `start`, `end` )

The following steps are taken:

Let O be the this value.
Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
If IsSharedArrayBuffer(O) is false, throw a TypeError exception.
Let len be O.[[ArrayBufferByteLength]].
Let relativeStart be ? ToInteger(start).
If relativeStart < 0, let first be max((len + relativeStart), 0); else let first be min(relativeStart, len).
If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToInteger(end).
If relativeEnd < 0, let final be max((len + relativeEnd), 0); else let final be min(relativeEnd, len).
Let newLen be max(final - first, 0).
Let ctor be ? SpeciesConstructor(O, %SharedArrayBuffer%).
Let new be ? Construct(ctor, « newLen »).
Perform ? RequireInternalSlot(new, [[ArrayBufferData]]).
If IsSharedArrayBuffer(new) is false, throw a TypeError exception.
If new.[[ArrayBufferData]] and O.[[ArrayBufferData]] are the same Shared Data Block values, throw a TypeError exception.
If new.[[ArrayBufferByteLength]] < newLen, throw a TypeError exception.
Let fromBuf be O.[[ArrayBufferData]].
Let toBuf be new.[[ArrayBufferData]].
Perform CopyDataBlockBytes(toBuf, 0, fromBuf, first, newLen).
Return new.

24.2.4.4 SharedArrayBuffer.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "SharedArrayBuffer".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

24.2.5 Properties of SharedArrayBuffer Instances

SharedArrayBuffer instances inherit properties from the SharedArrayBuffer prototype object. SharedArrayBuffer instances each have an [[ArrayBufferData]] internal slot and an [[ArrayBufferByteLength]] internal slot.

Note

SharedArrayBuffer instances, unlike ArrayBuffer instances, are never detached.

24.3 DataView Objects

24.3.1 Abstract Operations For DataView Objects

24.3.1.1 GetViewValue ( `view`, `requestIndex`, `isLittleEndian`, `type` )

The abstract operation GetViewValue takes arguments view, requestIndex, isLittleEndian, and type. It is used by functions on DataView instances to retrieve values from the view's buffer. It performs the following steps when called:

Perform ? RequireInternalSlot(view, [[DataView]]).
Assert: view has a [[ViewedArrayBuffer]] internal slot.
Let getIndex be ? ToIndex(requestIndex).
Set isLittleEndian to ! ToBoolean(isLittleEndian).
Let buffer be view.[[ViewedArrayBuffer]].
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
Let viewOffset be view.[[ByteOffset]].
Let viewSize be view.[[ByteLength]].
Let elementSize be the Element Size value specified in Table 61 for Element Type type.
If getIndex + elementSize > viewSize, throw a RangeError exception.
Let bufferIndex be getIndex + viewOffset.
Return GetValueFromBuffer(buffer, bufferIndex, type, false, Unordered, isLittleEndian).

24.3.1.2 SetViewValue ( `view`, `requestIndex`, `isLittleEndian`, `type`, `value` )

The abstract operation SetViewValue takes arguments view, requestIndex, isLittleEndian, type, and value. It is used by functions on DataView instances to store values into the view's buffer. It performs the following steps when called:

Perform ? RequireInternalSlot(view, [[DataView]]).
Assert: view has a [[ViewedArrayBuffer]] internal slot.
Let getIndex be ? ToIndex(requestIndex).
If ! IsBigIntElementType(type) is true, let numberValue be ? ToBigInt(value).
Otherwise, let numberValue be ? ToNumber(value).
Set isLittleEndian to ! ToBoolean(isLittleEndian).
Let buffer be view.[[ViewedArrayBuffer]].
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
Let viewOffset be view.[[ByteOffset]].
Let viewSize be view.[[ByteLength]].
Let elementSize be the Element Size value specified in Table 61 for Element Type type.
If getIndex + elementSize > viewSize, throw a RangeError exception.
Let bufferIndex be getIndex + viewOffset.
Return SetValueInBuffer(buffer, bufferIndex, type, numberValue, false, Unordered, isLittleEndian).

24.3.2 The DataView Constructor

The DataView constructor:

is the intrinsic object %DataView%.
is the initial value of the "DataView" property of the global object.
creates and initializes a new DataView object when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified DataView behaviour must include a super call to the DataView constructor to create and initialize subclass instances with the internal state necessary to support the DataView.prototype built-in methods.

24.3.2.1 DataView ( `buffer` [ , `byteOffset` [ , `byteLength` ] ] )

When the DataView function is called with at least one argument buffer, the following steps are taken:

If NewTarget is undefined, throw a TypeError exception.
Perform ? RequireInternalSlot(buffer, [[ArrayBufferData]]).
Let offset be ? ToIndex(byteOffset).
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
Let bufferByteLength be buffer.[[ArrayBufferByteLength]].
If offset > bufferByteLength, throw a RangeError exception.
If byteLength is undefined, then
1. Let viewByteLength be bufferByteLength - offset.
Else,
1. Let viewByteLength be ? ToIndex(byteLength).
2. If offset + viewByteLength > bufferByteLength, throw a RangeError exception.
Let O be ? OrdinaryCreateFromConstructor(NewTarget, "%DataView.prototype%", « [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], [[ByteOffset]] »).
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
Set O.[[ViewedArrayBuffer]] to buffer.
Set O.[[ByteLength]] to viewByteLength.
Set O.[[ByteOffset]] to offset.
Return O.

24.3.3 Properties of the DataView Constructor

The DataView constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

24.3.3.1 DataView.prototype

The initial value of DataView.prototype is %DataView.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

24.3.4 Properties of the DataView Prototype Object

The DataView prototype object:

is the intrinsic object %DataViewPrototype%.
has a [[Prototype]] internal slot whose value is %Object.prototype%.
is an ordinary object.
does not have a [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], or [[ByteOffset]] internal slot.

24.3.4.1 get DataView.prototype.buffer

DataView.prototype.buffer is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let O be the this value.
Perform ? RequireInternalSlot(O, [[DataView]]).
Assert: O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
Return buffer.

24.3.4.2 get DataView.prototype.byteLength

DataView.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let O be the this value.
Perform ? RequireInternalSlot(O, [[DataView]]).
Assert: O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
Let size be O.[[ByteLength]].
Return size.

24.3.4.3 get DataView.prototype.byteOffset

DataView.prototype.byteOffset is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Let O be the this value.
Perform ? RequireInternalSlot(O, [[DataView]]).
Assert: O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
Let offset be O.[[ByteOffset]].
Return offset.

24.3.4.4 DataView.prototype.constructor

The initial value of DataView.prototype.constructor is %DataView%.

24.3.4.5 DataView.prototype.getBigInt64 ( `byteOffset` [ , `littleEndian` ] )

When the getBigInt64 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:

Let v be the this value.
If littleEndian is not present, set littleEndian to undefined.
Return ? GetViewValue(v, byteOffset, littleEndian, BigInt64).

24.3.4.6 DataView.prototype.getBigUint64 ( `byteOffset` [ , `littleEndian` ] )

When the getBigUint64 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:

Let v be the this value.
If littleEndian is not present, set littleEndian to undefined.
Return ? GetViewValue(v, byteOffset, littleEndian, BigUint64).

24.3.4.7 DataView.prototype.getFloat32 ( `byteOffset` [ , `littleEndian` ] )

When the getFloat32 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:

Let v be the this value.
If littleEndian is not present, set littleEndian to false.
Return ? GetViewValue(v, byteOffset, littleEndian, Float32).

24.3.4.8 DataView.prototype.getFloat64 ( `byteOffset` [ , `littleEndian` ] )

When the getFloat64 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:

Let v be the this value.
If littleEndian is not present, set littleEndian to false.
Return ? GetViewValue(v, byteOffset, littleEndian, Float64).

24.3.4.9 DataView.prototype.getInt8 ( `byteOffset` )

When the getInt8 method is called with argument byteOffset, the following steps are taken:

Let v be the this value.
Return ? GetViewValue(v, byteOffset, true, Int8).

24.3.4.10 DataView.prototype.getInt16 ( `byteOffset` [ , `littleEndian` ] )

When the getInt16 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:

Let v be the this value.
If littleEndian is not present, set littleEndian to false.
Return ? GetViewValue(v, byteOffset, littleEndian, Int16).

24.3.4.11 DataView.prototype.getInt32 ( `byteOffset` [ , `littleEndian` ] )

When the getInt32 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:

Let v be the this value.
If littleEndian is not present, set littleEndian to false.
Return ? GetViewValue(v, byteOffset, littleEndian, Int32).

24.3.4.12 DataView.prototype.getUint8 ( `byteOffset` )

When the getUint8 method is called with argument byteOffset, the following steps are taken:

Let v be the this value.
Return ? GetViewValue(v, byteOffset, true, Uint8).

24.3.4.13 DataView.prototype.getUint16 ( `byteOffset` [ , `littleEndian` ] )

When the getUint16 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:

Let v be the this value.
If littleEndian is not present, set littleEndian to false.
Return ? GetViewValue(v, byteOffset, littleEndian, Uint16).

24.3.4.14 DataView.prototype.getUint32 ( `byteOffset` [ , `littleEndian` ] )

When the getUint32 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:

Let v be the this value.
If littleEndian is not present, set littleEndian to false.
Return ? GetViewValue(v, byteOffset, littleEndian, Uint32).

24.3.4.15 DataView.prototype.setBigInt64 ( `byteOffset`, `value` [ , `littleEndian` ] )

When the setBigInt64 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:

Let v be the this value.
If littleEndian is not present, set littleEndian to undefined.
Return ? SetViewValue(v, byteOffset, littleEndian, BigInt64, value).

24.3.4.16 DataView.prototype.setBigUint64 ( `byteOffset`, `value` [ , `littleEndian` ] )

When the setBigUint64 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:

Let v be the this value.
If littleEndian is not present, set littleEndian to undefined.
Return ? SetViewValue(v, byteOffset, littleEndian, BigUint64, value).

24.3.4.17 DataView.prototype.setFloat32 ( `byteOffset`, `value` [ , `littleEndian` ] )

When the setFloat32 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:

Let v be the this value.
If littleEndian is not present, set littleEndian to false.
Return ? SetViewValue(v, byteOffset, littleEndian, Float32, value).

24.3.4.18 DataView.prototype.setFloat64 ( `byteOffset`, `value` [ , `littleEndian` ] )

When the setFloat64 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:

Let v be the this value.
If littleEndian is not present, set littleEndian to false.
Return ? SetViewValue(v, byteOffset, littleEndian, Float64, value).

24.3.4.19 DataView.prototype.setInt8 ( `byteOffset`, `value` )

When the setInt8 method is called with arguments byteOffset and value, the following steps are taken:

Let v be the this value.
Return ? SetViewValue(v, byteOffset, true, Int8, value).

24.3.4.20 DataView.prototype.setInt16 ( `byteOffset`, `value` [ , `littleEndian` ] )

When the setInt16 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:

Let v be the this value.
If littleEndian is not present, set littleEndian to false.
Return ? SetViewValue(v, byteOffset, littleEndian, Int16, value).

24.3.4.21 DataView.prototype.setInt32 ( `byteOffset`, `value` [ , `littleEndian` ] )

When the setInt32 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:

Let v be the this value.
If littleEndian is not present, set littleEndian to false.
Return ? SetViewValue(v, byteOffset, littleEndian, Int32, value).

24.3.4.22 DataView.prototype.setUint8 ( `byteOffset`, `value` )

When the setUint8 method is called with arguments byteOffset and value, the following steps are taken:

Let v be the this value.
Return ? SetViewValue(v, byteOffset, true, Uint8, value).

24.3.4.23 DataView.prototype.setUint16 ( `byteOffset`, `value` [ , `littleEndian` ] )

When the setUint16 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:

Let v be the this value.
If littleEndian is not present, set littleEndian to false.
Return ? SetViewValue(v, byteOffset, littleEndian, Uint16, value).

24.3.4.24 DataView.prototype.setUint32 ( `byteOffset`, `value` [ , `littleEndian` ] )

When the setUint32 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:

Let v be the this value.
If littleEndian is not present, set littleEndian to false.
Return ? SetViewValue(v, byteOffset, littleEndian, Uint32, value).

24.3.4.25 DataView.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "DataView".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

24.3.5 Properties of DataView Instances

DataView instances are ordinary objects that inherit properties from the DataView prototype object. DataView instances each have [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], and [[ByteOffset]] internal slots.

Note

The value of the [[DataView]] internal slot is not used within this specification. The simple presence of that internal slot is used within the specification to identify objects created using the DataView constructor.

24.4 The Atomics Object

The Atomics object:

is the intrinsic object %Atomics%.
is the initial value of the "Atomics" property of the global object.
is an ordinary object.
has a [[Prototype]] internal slot whose value is %Object.prototype%.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.

The Atomics object provides functions that operate indivisibly (atomically) on shared memory array cells as well as functions that let agents wait for and dispatch primitive events. When used with discipline, the Atomics functions allow multi-agent programs that communicate through shared memory to execute in a well-understood order even on parallel CPUs. The rules that govern shared-memory communication are provided by the memory model, defined below.

Note

For informative guidelines for programming and implementing shared memory in ECMAScript, please see the notes at the end of the memory model section.

24.4.1 Abstract Operations for Atomics

24.4.1.1 ValidateSharedIntegerTypedArray ( `typedArray` [ , `waitable` ] )

The abstract operation ValidateSharedIntegerTypedArray takes argument typedArray and optional argument waitable (a Boolean). It performs the following steps when called:

If waitable is not present, set waitable to false.
Perform ? RequireInternalSlot(typedArray, [[TypedArrayName]]).
Let typeName be typedArray.[[TypedArrayName]].
Let type be the Element Type value in Table 61 for typeName.
If waitable is true, then
1. If typeName is not "Int32Array" or "BigInt64Array", throw a TypeError exception.
Else,
1. If ! IsUnclampedIntegerElementType(type) is false and ! IsBigIntElementType(type) is false, throw a TypeError exception.
Assert: typedArray has a [[ViewedArrayBuffer]] internal slot.
Let buffer be typedArray.[[ViewedArrayBuffer]].
If IsSharedArrayBuffer(buffer) is false, throw a TypeError exception.
Return buffer.

24.4.1.2 ValidateAtomicAccess ( `typedArray`, `requestIndex` )

The abstract operation ValidateAtomicAccess takes arguments typedArray and requestIndex. It performs the following steps when called:

Assert: typedArray is an Object that has a [[ViewedArrayBuffer]] internal slot.
Let accessIndex be ? ToIndex(requestIndex).
Let length be typedArray.[[ArrayLength]].
Assert: accessIndex ≥ 0.
If accessIndex ≥ length, throw a RangeError exception.
Return accessIndex.

24.4.1.3 GetWaiterList ( `block`, `i` )

A WaiterList is a semantic object that contains an ordered list of those agents that are waiting on a location (block, i) in shared memory; block is a Shared Data Block and i a byte offset into the memory of block. A WaiterList object also optionally contains a Synchronize event denoting the previous leaving of its critical section.

Initially a WaiterList object has an empty list and no Synchronize event.

The agent cluster has a store of WaiterList objects; the store is indexed by (block, i). WaiterLists are agent-independent: a lookup in the store of WaiterLists by (block, i) will result in the same WaiterList object in any agent in the agent cluster.

Each WaiterList has a critical section that controls exclusive access to that WaiterList during evaluation. Only a single agent may enter a WaiterList's critical section at one time. Entering and leaving a WaiterList's critical section is controlled by the abstract operations EnterCriticalSection and LeaveCriticalSection. Operations on a WaiterList—adding and removing waiting agents, traversing the list of agents, suspending and notifying agents on the list, setting and retrieving the Synchronize event—may only be performed by agents that have entered the WaiterList's critical section.

The abstract operation GetWaiterList takes arguments block (a Shared Data Block) and i (a non-negative integer). It performs the following steps when called:

Assert: block is a Shared Data Block.
Assert: i and i + 3 are valid byte offsets within the memory of block.
Assert: i is divisible by 4.
Return the WaiterList that is referenced by the pair (block, i).

24.4.1.4 EnterCriticalSection ( `WL` )

The abstract operation EnterCriticalSection takes argument WL (a WaiterList). It performs the following steps when called:

Assert: The calling agent is not in the critical section for any WaiterList.
Wait until no agent is in the critical section for WL, then enter the critical section for WL (without allowing any other agent to enter).
If WL has a Synchronize event, then
1. NOTE: A WL whose critical section has been entered at least once has a Synchronize event set by LeaveCriticalSection.
2. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
3. Let eventsRecord be the Agent Events Record in execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
4. Let entererEventList be eventsRecord.[[EventList]].
5. Let enterEvent be a new Synchronize event.
6. Append enterEvent to entererEventList.
7. Let leaveEvent be the Synchronize event in WL.
8. Append (leaveEvent, enterEvent) to eventsRecord.[[AgentSynchronizesWith]].

EnterCriticalSection has contention when an agent attempting to enter the critical section must wait for another agent to leave it. When there is no contention, FIFO order of EnterCriticalSection calls is observable. When there is contention, an implementation may choose an arbitrary order but may not cause an agent to wait indefinitely.

24.4.1.5 LeaveCriticalSection ( `WL` )

The abstract operation LeaveCriticalSection takes argument WL (a WaiterList). It performs the following steps when called:

Assert: The calling agent is in the critical section for WL.
Let execution be the [[CandidateExecution]] field of the calling surrounding's Agent Record.
Let eventsRecord be the Agent Events Record in execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
Let leaverEventList be eventsRecord.[[EventList]].
Let leaveEvent be a new Synchronize event.
Append leaveEvent to leaverEventList.
Set the Synchronize event in WL to leaveEvent.
Leave the critical section for WL.

24.4.1.6 AddWaiter ( `WL`, `W` )

The abstract operation AddWaiter takes arguments WL (a WaiterList) and W (an agent signifier). It performs the following steps when called:

Assert: The calling agent is in the critical section for WL.
Assert: W is not on the list of waiters in any WaiterList.
Add W to the end of the list of waiters in WL.

24.4.1.7 RemoveWaiter ( `WL`, `W` )

The abstract operation RemoveWaiter takes arguments WL (a WaiterList) and W (an agent signifier). It performs the following steps when called:

Assert: The calling agent is in the critical section for WL.
Assert: W is on the list of waiters in WL.
Remove W from the list of waiters in WL.

24.4.1.8 RemoveWaiters ( `WL`, `c` )

The abstract operation RemoveWaiters takes arguments WL (a WaiterList) and c (a non-negative integer). It performs the following steps when called:

Assert: The calling agent is in the critical section for WL.
Let L be a new empty List.
Let S be a reference to the list of waiters in WL.
Repeat, while c > 0 and S is not an empty List,
1. Let W be the first waiter in S.
2. Add W to the end of L.
3. Remove W from S.
4. Set c to c - 1.
Return L.

24.4.1.9 Suspend ( `WL`, `W`, `timeout` )

The abstract operation Suspend takes arguments WL (a WaiterList), W (an agent signifier), and timeout (a Number). It performs the following steps when called:

Assert: The calling agent is in the critical section for WL.
Assert: W is equal to AgentSignifier().
Assert: W is on the list of waiters in WL.
Assert: AgentCanSuspend() is true.
Perform LeaveCriticalSection(WL) and suspend W for up to timeout milliseconds, performing the combined operation in such a way that a notification that arrives after the critical section is exited but before the suspension takes effect is not lost. W can notify either because the timeout expired or because it was notified explicitly by another agent calling NotifyWaiter(WL, W), and not for any other reasons at all.
Perform EnterCriticalSection(WL).
If W was notified explicitly by another agent calling NotifyWaiter(WL, W), return true.
Return false.

24.4.1.10 NotifyWaiter ( `WL`, `W` )

The abstract operation NotifyWaiter takes arguments WL (a WaiterList) and W (an agent signifier). It performs the following steps when called:

Assert: The calling agent is in the critical section for WL.
Notify the agent W.

Note

The embedding may delay notifying W, e.g. for resource management reasons, but W must eventually be notified in order to guarantee forward progress.

24.4.1.11 AtomicReadModifyWrite ( `typedArray`, `index`, `value`, `op` )

The abstract operation AtomicReadModifyWrite takes arguments typedArray, index, value, and op (a pure combining operation). op takes two List of byte values arguments and returns a List of byte values. This operation atomically loads a value, combines it with another value, and stores the result of the combination. It returns the loaded value. It performs the following steps when called:

Let buffer be ? ValidateSharedIntegerTypedArray(typedArray).
Let i be ? ValidateAtomicAccess(typedArray, index).
Let arrayTypeName be typedArray.[[TypedArrayName]].
If typedArray.[[ContentType]] is BigInt, let v be ? ToBigInt(value).
Otherwise, let v be ? ToInteger(value).
Let elementSize be the Element Size value specified in Table 61 for arrayTypeName.
Let elementType be the Element Type value in Table 61 for arrayTypeName.
Let offset be typedArray.[[ByteOffset]].
Let indexedPosition be (i × elementSize) + offset.
Return GetModifySetValueInBuffer(buffer, indexedPosition, elementType, v, op).

24.4.1.12 AtomicLoad ( `typedArray`, `index` )

The abstract operation AtomicLoad takes arguments typedArray and index. It atomically loads a value and returns the loaded value. It performs the following steps when called:

Let buffer be ? ValidateSharedIntegerTypedArray(typedArray).
Let i be ? ValidateAtomicAccess(typedArray, index).
Let arrayTypeName be typedArray.[[TypedArrayName]].
Let elementSize be the Element Size value specified in Table 61 for arrayTypeName.
Let elementType be the Element Type value in Table 61 for arrayTypeName.
Let offset be typedArray.[[ByteOffset]].
Let indexedPosition be (i × elementSize) + offset.
Return GetValueFromBuffer(buffer, indexedPosition, elementType, true, SeqCst).

24.4.2 Atomics.add ( `typedArray`, `index`, `value` )

Let add denote a semantic function of two List of byte values arguments that applies the addition operation to the Number values corresponding to the List of byte values arguments and returns a List of byte values corresponding to the result of that operation.

The following steps are taken:

Return ? AtomicReadModifyWrite(typedArray, index, value, add).

24.4.3 Atomics.and ( `typedArray`, `index`, `value` )

Let and denote a semantic function of two List of byte values arguments that applies the bitwise-and operation element-wise to the two arguments and returns a List of byte values corresponding to the result of that operation.

The following steps are taken:

Return ? AtomicReadModifyWrite(typedArray, index, value, and).

24.4.4 Atomics.compareExchange ( `typedArray`, `index`, `expectedValue`, `replacementValue` )

The following steps are taken:

Let buffer be ? ValidateSharedIntegerTypedArray(typedArray).
Let i be ? ValidateAtomicAccess(typedArray, index).
Let arrayTypeName be typedArray.[[TypedArrayName]].
If typedArray.[[ContentType]] is BigInt, then
1. Let expected be ? ToBigInt(expectedValue).
2. Let replacement be ? ToBigInt(replacementValue).
Else,
1. Let expected be ? ToInteger(expectedValue).
2. Let replacement be ? ToInteger(replacementValue).
Let elementType be the Element Type value in Table 61 for arrayTypeName.
Let isLittleEndian be the value of the [[LittleEndian]] field of the surrounding agent's Agent Record.
Let expectedBytes be NumericToRawBytes(elementType, expected, isLittleEndian).
Let elementSize be the Element Size value specified in Table 61 for arrayTypeName.
Let offset be typedArray.[[ByteOffset]].
Let indexedPosition be (i × elementSize) + offset.
Let compareExchange denote a semantic function of two List of byte values arguments that returns the second argument if the first argument is element-wise equal to expectedBytes.
Return GetModifySetValueInBuffer(buffer, indexedPosition, elementType, replacement, compareExchange).

24.4.5 Atomics.exchange ( `typedArray`, `index`, `value` )

Let second denote a semantic function of two List of byte values arguments that returns its second argument.

The following steps are taken:

Return ? AtomicReadModifyWrite(typedArray, index, value, second).

24.4.6 Atomics.isLockFree ( `size` )

The following steps are taken:

Let n be ? ToInteger(size).
Let AR be the Agent Record of the surrounding agent.
If n equals 1, return AR.[[IsLockFree1]].
If n equals 2, return AR.[[IsLockFree2]].
If n equals 4, return true.
If n equals 8, return AR.[[IsLockFree8]].
Return false.

Note

Atomics.isLockFree() is an optimization primitive. The intuition is that if the atomic step of an atomic primitive (compareExchange, load, store, add, sub, and, or, xor, or exchange) on a datum of size n bytes will be performed without the calling agent acquiring a lock outside the n bytes comprising the datum, then Atomics.isLockFree(n) will return true. High-performance algorithms will use Atomics.isLockFree to determine whether to use locks or atomic operations in critical sections. If an atomic primitive is not lock-free then it is often more efficient for an algorithm to provide its own locking.

Atomics.isLockFree(4) always returns true as that can be supported on all known relevant hardware. Being able to assume this will generally simplify programs.

Regardless of the value of Atomics.isLockFree, all atomic operations are guaranteed to be atomic. For example, they will never have a visible operation take place in the middle of the operation (e.g., "tearing").

24.4.7 Atomics.load ( `typedArray`, `index` )

The following steps are taken:

Return ? AtomicLoad(typedArray, index).

24.4.8 Atomics.or ( `typedArray`, `index`, `value` )

Let or denote a semantic function of two List of byte values arguments that applies the bitwise-or operation element-wise to the two arguments and returns a List of byte values corresponding to the result of that operation.

The following steps are taken:

Return ? AtomicReadModifyWrite(typedArray, index, value, or).

24.4.9 Atomics.store ( `typedArray`, `index`, `value` )

The following steps are taken:

Let buffer be ? ValidateSharedIntegerTypedArray(typedArray).
Let i be ? ValidateAtomicAccess(typedArray, index).
Let arrayTypeName be typedArray.[[TypedArrayName]].
If arrayTypeName is "BigUint64Array" or "BigInt64Array", let v be ? ToBigInt(value).
Otherwise, let v be ? ToInteger(value).
Let elementSize be the Element Size value specified in Table 61 for arrayTypeName.
Let elementType be the Element Type value in Table 61 for arrayTypeName.
Let offset be typedArray.[[ByteOffset]].
Let indexedPosition be (i × elementSize) + offset.
Perform SetValueInBuffer(buffer, indexedPosition, elementType, v, true, SeqCst).
Return v.

24.4.10 Atomics.sub ( `typedArray`, `index`, `value` )

Let subtract denote a semantic function of two List of byte values arguments that applies the subtraction operation to the Number values corresponding to the List of byte values arguments and returns a List of byte values corresponding to the result of that operation.

The following steps are taken:

Return ? AtomicReadModifyWrite(typedArray, index, value, subtract).

24.4.11 Atomics.wait ( `typedArray`, `index`, `value`, `timeout` )

Atomics.wait puts the calling agent in a wait queue and puts it to sleep until it is notified or the sleep times out. The following steps are taken:

Let buffer be ? ValidateSharedIntegerTypedArray(typedArray, true).
Let i be ? ValidateAtomicAccess(typedArray, index).
Let arrayTypeName be typedArray.[[TypedArrayName]].
If arrayTypeName is "BigInt64Array", let v be ? ToBigInt64(value).
Otherwise, let v be ? ToInt32(value).
Let q be ? ToNumber(timeout).
If q is NaN, let t be +∞; else let t be max(q, 0).
Let B be AgentCanSuspend().
If B is false, throw a TypeError exception.
Let block be buffer.[[ArrayBufferData]].
Let offset be typedArray.[[ByteOffset]].
Let elementSize be the Element Size value specified in Table 61 for arrayTypeName.
Let indexedPosition be (i × elementSize) + offset.
Let WL be GetWaiterList(block, indexedPosition).
Perform EnterCriticalSection(WL).
Let w be ! AtomicLoad(typedArray, i).
If v is not equal to w, then
1. Perform LeaveCriticalSection(WL).
2. Return the String "not-equal".
Let W be AgentSignifier().
Perform AddWaiter(WL, W).
Let notified be Suspend(WL, W, t).
If notified is true, then
1. Assert: W is not on the list of waiters in WL.
Else,
1. Perform RemoveWaiter(WL, W).
Perform LeaveCriticalSection(WL).
If notified is true, return the String "ok".
Return the String "timed-out".

24.4.12 Atomics.notify ( `typedArray`, `index`, `count` )

Atomics.notify notifies some agents that are sleeping in the wait queue. The following steps are taken:

Let buffer be ? ValidateSharedIntegerTypedArray(typedArray, true).
Let i be ? ValidateAtomicAccess(typedArray, index).
If count is undefined, let c be +∞.
Else,
1. Let intCount be ? ToInteger(count).
2. Let c be max(intCount, 0).
Let block be buffer.[[ArrayBufferData]].
Let offset be typedArray.[[ByteOffset]].
Let arrayTypeName be typedArray.[[TypedArrayName]].
Let elementSize be the Element Size value specified in Table 61 for arrayTypeName.
Let indexedPosition be (i × elementSize) + offset.
Let WL be GetWaiterList(block, indexedPosition).
Let n be 0.
Perform EnterCriticalSection(WL).
Let S be RemoveWaiters(WL, c).
Repeat, while S is not an empty List,
1. Let W be the first agent in S.
2. Remove W from the front of S.
3. Perform NotifyWaiter(WL, W).
4. Set n to n + 1.
Perform LeaveCriticalSection(WL).
Return n.

24.4.13 Atomics.xor ( `typedArray`, `index`, `value` )

Let xor denote a semantic function of two List of byte values arguments that applies the bitwise-xor operation element-wise to the two arguments and returns a List of byte values corresponding to the result of that operation.

The following steps are taken:

Return ? AtomicReadModifyWrite(typedArray, index, value, xor).

24.4.14 Atomics [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Atomics".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

24.5 The JSON Object

The JSON object:

is the intrinsic object %JSON%.
is the initial value of the "JSON" property of the global object.
is an ordinary object.
contains two functions, parse and stringify, that are used to parse and construct JSON texts.
has a [[Prototype]] internal slot whose value is %Object.prototype%.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.

The JSON Data Interchange Format is defined in ECMA-404. The JSON interchange format used in this specification is exactly that described by ECMA-404. Conforming implementations of JSON.parse and JSON.stringify must support the exact interchange format described in the ECMA-404 specification without any deletions or extensions to the format.

24.5.1 JSON.parse ( `text` [ , `reviver` ] )

The parse function parses a JSON text (a JSON-formatted String) and produces an ECMAScript value. The JSON format represents literals, arrays, and objects with a syntax similar to the syntax for ECMAScript literals, Array Initializers, and Object Initializers. After parsing, JSON objects are realized as ECMAScript objects. JSON arrays are realized as ECMAScript Array instances. JSON strings, numbers, booleans, and null are realized as ECMAScript Strings, Numbers, Booleans, and null.

The optional reviver parameter is a function that takes two parameters, key and value. It can filter and transform the results. It is called with each of the key/value pairs produced by the parse, and its return value is used instead of the original value. If it returns what it received, the structure is not modified. If it returns undefined then the property is deleted from the result.

Let jsonString be ? ToString(text).
Parse ! UTF16DecodeString(jsonString) as a JSON text as specified in ECMA-404. Throw a SyntaxError exception if it is not a valid JSON text as defined in that specification.
Let scriptString be the string-concatenation of "(", jsonString, and ");".
Let completion be the result of parsing and evaluating ! UTF16DecodeString(scriptString) as if it was the source text of an ECMAScript Script. The extended PropertyDefinitionEvaluation semantics defined in B.3.1 must not be used during the evaluation.
Let unfiltered be completion.[[Value]].
Assert: unfiltered is either a String, Number, Boolean, Null, or an Object that is defined by either an ArrayLiteral or an ObjectLiteral.
If IsCallable(reviver) is true, then
1. Let root be OrdinaryObjectCreate(%Object.prototype%).
2. Let rootName be the empty String.
3. Perform ! CreateDataPropertyOrThrow(root, rootName, unfiltered).
4. Return ? InternalizeJSONProperty(root, rootName, reviver).
Else,
1. Return unfiltered.

This function is the %JSONParse% intrinsic object.

The "length" property of the parse function is 2.

Note

Valid JSON text is a subset of the ECMAScript PrimaryExpression syntax as modified by Step 4 above. Step 2 verifies that jsonString conforms to that subset, and step 6 verifies that that parsing and evaluation returns a value of an appropriate type.

24.5.1.1 Runtime Semantics: InternalizeJSONProperty ( `holder`, `name`, `reviver` )

The abstract operation InternalizeJSONProperty takes arguments holder (an Object), name (a String), and reviver (a function object). It performs the following steps when called:

Note 1

This algorithm intentionally does not throw an exception if either [[Delete]] or CreateDataProperty return false.

Let val be ? Get(holder, name).
If Type(val) is Object, then
1. Let isArray be ? IsArray(val).
2. If isArray is true, then
  1. Let I be 0.
  2. Let len be ? LengthOfArrayLike(val).
  3. Repeat, while I < len,
    1. Let newElement be ? InternalizeJSONProperty(val, ! ToString(I), reviver).
    2. If newElement is undefined, then
      1. Perform ? val.[[Delete]](! ToString(I)).
    3. Else,
      1. Perform ? CreateDataProperty(val, ! ToString(I), newElement).
    4. Set I to I + 1.
3. Else,
  1. Let keys be ? EnumerableOwnPropertyNames(val, key).
  2. For each String P in keys, do
    1. Let newElement be ? InternalizeJSONProperty(val, P, reviver).
    2. If newElement is undefined, then
      1. Perform ? val.[[Delete]](P).
    3. Else,
      1. Perform ? CreateDataProperty(val, P, newElement).
Return ? Call(reviver, holder, « name, val »).

It is not permitted for a conforming implementation of JSON.parse to extend the JSON grammars. If an implementation wishes to support a modified or extended JSON interchange format it must do so by defining a different parse function.

Note 2

In the case where there are duplicate name Strings within an object, lexically preceding values for the same key shall be overwritten.

24.5.2 JSON.stringify ( `value` [ , `replacer` [ , `space` ] ] )

The stringify function returns a String in UTF-16 encoded JSON format representing an ECMAScript value, or undefined. It can take three parameters. The value parameter is an ECMAScript value, which is usually an object or array, although it can also be a String, Boolean, Number or null. The optional replacer parameter is either a function that alters the way objects and arrays are stringified, or an array of Strings and Numbers that acts as an inclusion list for selecting the object properties that will be stringified. The optional space parameter is a String or Number that allows the result to have white space injected into it to improve human readability.

These are the steps in stringifying an object:

Let stack be a new empty List.
Let indent be the empty String.
Let PropertyList and ReplacerFunction be undefined.
If Type(replacer) is Object, then
1. If IsCallable(replacer) is true, then
  1. Set ReplacerFunction to replacer.
2. Else,
  1. Let isArray be ? IsArray(replacer).
  2. If isArray is true, then
    1. Set PropertyList to a new empty List.
    2. Let len be ? LengthOfArrayLike(replacer).
    3. Let k be 0.
    4. Repeat, while k < len,
      1. Let v be ? Get(replacer, ! ToString(k)).
      2. Let item be undefined.
      3. If Type(v) is String, set item to v.
      4. Else if Type(v) is Number, set item to ! ToString(v).
      5. Else if Type(v) is Object, then
        If v has a [[StringData]] or [[NumberData]] internal slot, set item to ? ToString(v).
      6. If item is not undefined and item is not currently an element of PropertyList, then
        Append item to the end of PropertyList.
      7. Set k to k + 1.
If Type(space) is Object, then
1. If space has a [[NumberData]] internal slot, then
  1. Set space to ? ToNumber(space).
2. Else if space has a [[StringData]] internal slot, then
  1. Set space to ? ToString(space).
If Type(space) is Number, then
1. Set space to min(10, ! ToInteger(space)).
2. If space < 1, let gap be the empty String; otherwise let gap be the String value containing space occurrences of the code unit 0x0020 (SPACE).
Else if Type(space) is String, then
1. If the length of space is 10 or less, let gap be space; otherwise let gap be the String value consisting of the first 10 code units of space.
Else,
1. Let gap be the empty String.
Let wrapper be OrdinaryObjectCreate(%Object.prototype%).
Perform ! CreateDataPropertyOrThrow(wrapper, the empty String, value).
Let state be the Record { [[ReplacerFunction]]: ReplacerFunction, [[Stack]]: stack, [[Indent]]: indent, [[Gap]]: gap, [[PropertyList]]: PropertyList }.
Return ? SerializeJSONProperty(state, the empty String, wrapper).

This function is the %JSONStringify% intrinsic object.

The "length" property of the stringify function is 3.

Note 1

JSON structures are allowed to be nested to any depth, but they must be acyclic. If value is or contains a cyclic structure, then the stringify function must throw a TypeError exception. This is an example of a value that cannot be stringified:

a = [];
a[0] = a;
my_text = JSON.stringify(a); // This must throw a TypeError.

Note 2

Symbolic primitive values are rendered as follows:

The null value is rendered in JSON text as the String "null".
The undefined value is not rendered.
The true value is rendered in JSON text as the String "true".
The false value is rendered in JSON text as the String "false".

Note 3

String values are wrapped in QUOTATION MARK (") code units. The code units " and \ are escaped with \ prefixes. Control characters code units are replaced with escape sequences \uHHHH, or with the shorter forms, \b (BACKSPACE), \f (FORM FEED), \n (LINE FEED), \r (CARRIAGE RETURN), \t (CHARACTER TABULATION).

Note 4

Finite numbers are stringified as if by calling ToString(number). NaN and Infinity regardless of sign are represented as the String "null".

Note 5

Values that do not have a JSON representation (such as undefined and functions) do not produce a String. Instead they produce the undefined value. In arrays these values are represented as the String "null". In objects an unrepresentable value causes the property to be excluded from stringification.

Note 6

An object is rendered as U+007B (LEFT CURLY BRACKET) followed by zero or more properties, separated with a U+002C (COMMA), closed with a U+007D (RIGHT CURLY BRACKET). A property is a quoted String representing the key or property name, a U+003A (COLON), and then the stringified property value. An array is rendered as an opening U+005B (LEFT SQUARE BRACKET followed by zero or more values, separated with a U+002C (COMMA), closed with a U+005D (RIGHT SQUARE BRACKET).

24.5.2.1 Runtime Semantics: SerializeJSONProperty ( `state`, `key`, `holder` )

The abstract operation SerializeJSONProperty takes arguments state, key, and holder. It performs the following steps when called:

Let value be ? Get(holder, key).
If Type(value) is Object or BigInt, then
1. Let toJSON be ? GetV(value, "toJSON").
2. If IsCallable(toJSON) is true, then
  1. Set value to ? Call(toJSON, value, « key »).
If state.[[ReplacerFunction]] is not undefined, then
1. Set value to ? Call(state.[[ReplacerFunction]], holder, « key, value »).
If Type(value) is Object, then
1. If value has a [[NumberData]] internal slot, then
  1. Set value to ? ToNumber(value).
2. Else if value has a [[StringData]] internal slot, then
  1. Set value to ? ToString(value).
3. Else if value has a [[BooleanData]] internal slot, then
  1. Set value to value.[[BooleanData]].
4. Else if value has a [[BigIntData]] internal slot, then
  1. Set value to value.[[BigIntData]].
If value is null, return "null".
If value is true, return "true".
If value is false, return "false".
If Type(value) is String, return QuoteJSONString(value).
If Type(value) is Number, then
1. If value is finite, return ! ToString(value).
2. Return "null".
If Type(value) is BigInt, throw a TypeError exception.
If Type(value) is Object and IsCallable(value) is false, then
1. Let isArray be ? IsArray(value).
2. If isArray is true, return ? SerializeJSONArray(state, value).
3. Return ? SerializeJSONObject(state, value).
Return undefined.

24.5.2.2 Runtime Semantics: QuoteJSONString ( `value` )

The abstract operation QuoteJSONString takes argument value. It wraps value in 0x0022 (QUOTATION MARK) code units and escapes certain other code units within it. This operation interprets value as a sequence of UTF-16 encoded code points, as described in 6.1.4. It performs the following steps when called:

Let product be the String value consisting solely of the code unit 0x0022 (QUOTATION MARK).
For each code point C in ! UTF16DecodeString(value), do
1. If C is listed in the “Code Point” column of Table 64, then
  1. Set product to the string-concatenation of product and the escape sequence for C as specified in the “Escape Sequence” column of the corresponding row.
2. Else if C has a numeric value less than 0x0020 (SPACE), or if C has the same numeric value as a leading surrogate or trailing surrogate, then
  1. Let unit be the code unit whose numeric value is that of C.
  2. Set product to the string-concatenation of product and UnicodeEscape(unit).
3. Else,
  1. Set product to the string-concatenation of product and the UTF16Encoding of C.
Set product to the string-concatenation of product and the code unit 0x0022 (QUOTATION MARK).
Return product.

Table 64: JSON Single Character Escape Sequences

Code Point	Unicode Character Name	Escape Sequence
U+0008	BACKSPACE	`\b`
U+0009	CHARACTER TABULATION	`\t`
U+000A	LINE FEED (LF)	`\n`
U+000C	FORM FEED (FF)	`\f`
U+000D	CARRIAGE RETURN (CR)	`\r`
U+0022	QUOTATION MARK	`\"`
U+005C	REVERSE SOLIDUS	`\\`

24.5.2.3 Runtime Semantics: UnicodeEscape ( `C` )

The abstract operation UnicodeEscape takes argument C (a code unit). It represents C as a Unicode escape sequence. It performs the following steps when called:

Let n be the numeric value of C.
Assert: n ≤ 0xFFFF.
Return the string-concatenation of:
- the code unit 0x005C (REVERSE SOLIDUS)
- "u"
- the String representation of n, formatted as a four-digit lowercase hexadecimal number, padded to the left with zeroes if necessary

24.5.2.4 Runtime Semantics: SerializeJSONObject ( `state`, `value` )

The abstract operation SerializeJSONObject takes arguments state and value. It serializes an object. It performs the following steps when called:

If state.[[Stack]] contains value, throw a TypeError exception because the structure is cyclical.
Append value to state.[[Stack]].
Let stepback be state.[[Indent]].
Set state.[[Indent]] to the string-concatenation of state.[[Indent]] and state.[[Gap]].
If state.[[PropertyList]] is not undefined, then
1. Let K be state.[[PropertyList]].
Else,
1. Let K be ? EnumerableOwnPropertyNames(value, key).
Let partial be a new empty List.
For each element P of K, do
1. Let strP be ? SerializeJSONProperty(state, P, value).
2. If strP is not undefined, then
  1. Let member be QuoteJSONString(P).
  2. Set member to the string-concatenation of member and ":".
  3. If state.[[Gap]] is not the empty String, then
    1. Set member to the string-concatenation of member and the code unit 0x0020 (SPACE).
  4. Set member to the string-concatenation of member and strP.
  5. Append member to partial.
If partial is empty, then
1. Let final be "{}".
Else,
1. If state.[[Gap]] is the empty String, then
  1. Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with the code unit 0x002C (COMMA). A comma is not inserted either before the first String or after the last String.
  2. Let final be the string-concatenation of "{", properties, and "}".
2. Else,
  1. Let separator be the string-concatenation of the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED), and state.[[Indent]].
  2. Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with separator. The separator String is not inserted either before the first String or after the last String.
  3. Let final be the string-concatenation of "{", the code unit 0x000A (LINE FEED), state.[[Indent]], properties, the code unit 0x000A (LINE FEED), stepback, and "}".
Remove the last element of state.[[Stack]].
Set state.[[Indent]] to stepback.
Return final.

24.5.2.5 Runtime Semantics: SerializeJSONArray ( `state`, `value` )

The abstract operation SerializeJSONArray takes arguments state and value. It serializes an array. It performs the following steps when called:

If state.[[Stack]] contains value, throw a TypeError exception because the structure is cyclical.
Append value to state.[[Stack]].
Let stepback be state.[[Indent]].
Set state.[[Indent]] to the string-concatenation of state.[[Indent]] and state.[[Gap]].
Let partial be a new empty List.
Let len be ? LengthOfArrayLike(value).
Let index be 0.
Repeat, while index < len
1. Let strP be ? SerializeJSONProperty(state, ! ToString(index), value).
2. If strP is undefined, then
  1. Append "null" to partial.
3. Else,
  1. Append strP to partial.
4. Set index to index + 1.
If partial is empty, then
1. Let final be "[]".
Else,
1. If state.[[Gap]] is the empty String, then
  1. Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with the code unit 0x002C (COMMA). A comma is not inserted either before the first String or after the last String.
  2. Let final be the string-concatenation of "[", properties, and "]".
2. Else,
  1. Let separator be the string-concatenation of the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED), and state.[[Indent]].
  2. Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with separator. The separator String is not inserted either before the first String or after the last String.
  3. Let final be the string-concatenation of "[", the code unit 0x000A (LINE FEED), state.[[Indent]], properties, the code unit 0x000A (LINE FEED), stepback, and "]".
Remove the last element of state.[[Stack]].
Set state.[[Indent]] to stepback.
Return final.

Note

The representation of arrays includes only the elements between zero and array.length - 1 inclusive. Properties whose keys are not array indexes are excluded from the stringification. An array is stringified as an opening LEFT SQUARE BRACKET, elements separated by COMMA, and a closing RIGHT SQUARE BRACKET.

24.5.3 JSON [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "JSON".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25 Control Abstraction Objects

25.1 Iteration

25.1.1 Common Iteration Interfaces

An interface is a set of property keys whose associated values match a specific specification. Any object that provides all the properties as described by an interface's specification conforms to that interface. An interface is not represented by a distinct object. There may be many separately implemented objects that conform to any interface. An individual object may conform to multiple interfaces.

25.1.1.1 The Iterable Interface

The Iterable interface includes the property described in Table 65:

Table 65: Iterable Interface Required Properties

Property	Value	Requirements
`@@iterator`	A function that returns an Iterator object.	The returned object must conform to the Iterator interface.

25.1.1.2 The Iterator Interface

An object that implements the Iterator interface must include the property in Table 66. Such objects may also implement the properties in Table 67.

Table 66: Iterator Interface Required Properties

Property	Value	Requirements
"next"	A function that returns an IteratorResult object.	The returned object must conform to the IteratorResult interface. If a previous call to the `next` method of an Iterator has returned an IteratorResult object whose "done" property is true, then all subsequent calls to the `next` method of that object should also return an IteratorResult object whose "done" property is true. However, this requirement is not enforced.

Note 1

Arguments may be passed to the next function but their interpretation and validity is dependent upon the target Iterator. The for-of statement and other common users of Iterators do not pass any arguments, so Iterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.

Table 67: Iterator Interface Optional Properties

Property	Value	Requirements
"return"	A function that returns an IteratorResult object.	The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller does not intend to make any more `next` method calls to the Iterator. The returned IteratorResult object will typically have a "done" property whose value is true, and a "value" property with the value passed as the argument of the `return` method. However, this requirement is not enforced.
"throw"	A function that returns an IteratorResult object.	The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller has detected an error condition. The argument may be used to identify the error condition and typically will be an exception object. A typical response is to `throw` the value passed as the argument. If the method does not `throw`, the returned IteratorResult object will typically have a "done" property whose value is true.

Note 2

Typically callers of these methods should check for their existence before invoking them. Certain ECMAScript language features including for-of, yield*, and array destructuring call these methods after performing an existence check. Most ECMAScript library functions that accept Iterable objects as arguments also conditionally call them.

25.1.1.3 The AsyncIterable Interface

The AsyncIterable interface includes the properties described in Table 68:

Table 68: AsyncIterable Interface Required Properties

Property	Value	Requirements
`@@asyncIterator`	A function that returns an AsyncIterator object.	The returned object must conform to the AsyncIterator interface.

25.1.1.4 The AsyncIterator Interface

An object that implements the AsyncIterator interface must include the properties in Table 69. Such objects may also implement the properties in Table 70.

Table 69: AsyncIterator Interface Required Properties

Property Value Requirements

"next"

A function that returns a promise for an IteratorResult object.

The returned promise, when fulfilled, must fulfill with an object which conforms to the IteratorResult interface. If a previous call to the next method of an AsyncIterator has returned a promise for an IteratorResult object whose "done" property is true, then all subsequent calls to the next method of that object should also return a promise for an IteratorResult object whose "done" property is true. However, this requirement is not enforced.

Additionally, the IteratorResult object that serves as a fulfillment value should have a "value" property whose value is not a promise (or "thenable"). However, this requirement is also not enforced.

Note 1

Arguments may be passed to the next function but their interpretation and validity is dependent upon the target AsyncIterator. The for-await-of statement and other common users of AsyncIterators do not pass any arguments, so AsyncIterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.

Table 70: AsyncIterator Interface Optional Properties

Property Value Requirements

"return"

A function that returns a promise for an IteratorResult object.

The returned promise, when fulfilled, must fulfill with an object which conforms to the IteratorResult interface. Invoking this method notifies the AsyncIterator object that the caller does not intend to make any more next method calls to the AsyncIterator. The returned promise will fulfill with an IteratorResult object which will typically have a "done" property whose value is true, and a "value" property with the value passed as the argument of the return method. However, this requirement is not enforced.

Additionally, the IteratorResult object that serves as a fulfillment value should have a "value" property whose value is not a promise (or "thenable"). If the argument value is used in the typical manner, then if it is a rejected promise, a promise rejected with the same reason should be returned; if it is a fulfilled promise, then its fulfillment value should be used as the "value" property of the returned promise's IteratorResult object fulfillment value. However, these requirements are also not enforced.

"throw"

A function that returns a promise for an IteratorResult object.

The returned promise, when fulfilled, must fulfill with an object which conforms to the IteratorResult interface. Invoking this method notifies the AsyncIterator object that the caller has detected an error condition. The argument may be used to identify the error condition and typically will be an exception object. A typical response is to return a rejected promise which rejects with the value passed as the argument.

If the returned promise is fulfilled, the IteratorResult fulfillment value will typically have a "done" property whose value is true. Additionally, it should have a "value" property whose value is not a promise (or "thenable"), but this requirement is not enforced.

Note 2

Typically callers of these methods should check for their existence before invoking them. Certain ECMAScript language features including for-await-of and yield* call these methods after performing an existence check.

25.1.1.5 The IteratorResult Interface

The IteratorResult interface includes the properties listed in Table 71:

Table 71: IteratorResult Interface Properties

Property	Value	Requirements
"done"	Either true or false.	This is the result status of an iterator `next` method call. If the end of the iterator was reached "done" is true. If the end was not reached "done" is false and a value is available. If a "done" property (either own or inherited) does not exist, it is consider to have the value false.
"value"	Any ECMAScript language value.	If done is false, this is the current iteration element value. If done is true, this is the return value of the iterator, if it supplied one. If the iterator does not have a return value, "value" is undefined. In that case, the "value" property may be absent from the conforming object if it does not inherit an explicit "value" property.

25.1.2 The %IteratorPrototype% Object

The %IteratorPrototype% object:

has a [[Prototype]] internal slot whose value is %Object.prototype%.
is an ordinary object.

Note

All objects defined in this specification that implement the Iterator interface also inherit from %IteratorPrototype%. ECMAScript code may also define objects that inherit from %IteratorPrototype%. The %IteratorPrototype% object provides a place where additional methods that are applicable to all iterator objects may be added.

The following expression is one way that ECMAScript code can access the %IteratorPrototype% object:

Object.getPrototypeOf(Object.getPrototypeOf([][Symbol.iterator]()))

25.1.2.1 %IteratorPrototype% [ @@iterator ] ( )

The following steps are taken:

Return the this value.

The value of the "name" property of this function is "[Symbol.iterator]".

25.1.3 The %AsyncIteratorPrototype% Object

The %AsyncIteratorPrototype% object:

has a [[Prototype]] internal slot whose value is %Object.prototype%.
is an ordinary object.

Note

All objects defined in this specification that implement the AsyncIterator interface also inherit from %AsyncIteratorPrototype%. ECMAScript code may also define objects that inherit from %AsyncIteratorPrototype%. The %AsyncIteratorPrototype% object provides a place where additional methods that are applicable to all async iterator objects may be added.

25.1.3.1 %AsyncIteratorPrototype% [ @@asyncIterator ] ( )

The following steps are taken:

Return the this value.

The value of the "name" property of this function is "[Symbol.asyncIterator]".

25.1.4 Async-from-Sync Iterator Objects

An Async-from-Sync Iterator object is an async iterator that adapts a specific synchronous iterator. There is not a named constructor for Async-from-Sync Iterator objects. Instead, Async-from-Sync iterator objects are created by the CreateAsyncFromSyncIterator abstract operation as needed.

25.1.4.1 CreateAsyncFromSyncIterator ( `syncIteratorRecord` )

The abstract operation CreateAsyncFromSyncIterator takes argument syncIteratorRecord. It is used to create an async iterator Record from a synchronous iterator Record. It performs the following steps when called:

Let asyncIterator be ! OrdinaryObjectCreate(%AsyncFromSyncIteratorPrototype%, « [[SyncIteratorRecord]] »).
Set asyncIterator.[[SyncIteratorRecord]] to syncIteratorRecord.
Let nextMethod be ! Get(asyncIterator, "next").
Let iteratorRecord be the Record { [[Iterator]]: asyncIterator, [[NextMethod]]: nextMethod, [[Done]]: false }.
Return iteratorRecord.

25.1.4.2 The %AsyncFromSyncIteratorPrototype% Object

The %AsyncFromSyncIteratorPrototype% object:

has properties that are inherited by all Async-from-Sync Iterator Objects.
is an ordinary object.
has a [[Prototype]] internal slot whose value is %AsyncIteratorPrototype%.
has the following properties:

25.1.4.2.1 %AsyncFromSyncIteratorPrototype%.next ( `value` )

Let O be the this value.
Assert: Type(O) is Object and O has a [[SyncIteratorRecord]] internal slot.
Let promiseCapability be ! NewPromiseCapability(%Promise%).
Let syncIteratorRecord be O.[[SyncIteratorRecord]].
Let result be IteratorNext(syncIteratorRecord, value).
IfAbruptRejectPromise(result, promiseCapability).
Return ! AsyncFromSyncIteratorContinuation(result, promiseCapability).

25.1.4.2.2 %AsyncFromSyncIteratorPrototype%.return ( `value` )

Let O be the this value.
Assert: Type(O) is Object and O has a [[SyncIteratorRecord]] internal slot.
Let promiseCapability be ! NewPromiseCapability(%Promise%).
Let syncIterator be O.[[SyncIteratorRecord]].[[Iterator]].
Let return be GetMethod(syncIterator, "return").
IfAbruptRejectPromise(return, promiseCapability).
If return is undefined, then
1. Let iterResult be ! CreateIterResultObject(value, true).
2. Perform ! Call(promiseCapability.[[Resolve]], undefined, « iterResult »).
3. Return promiseCapability.[[Promise]].
Let result be Call(return, syncIterator, « value »).
IfAbruptRejectPromise(result, promiseCapability).
If Type(result) is not Object, then
1. Perform ! Call(promiseCapability.[[Reject]], undefined, « a newly created TypeError object »).
2. Return promiseCapability.[[Promise]].
Return ! AsyncFromSyncIteratorContinuation(result, promiseCapability).

25.1.4.2.3 %AsyncFromSyncIteratorPrototype%.throw ( `value` )

Let O be the this value.
Assert: Type(O) is Object and O has a [[SyncIteratorRecord]] internal slot.
Let promiseCapability be ! NewPromiseCapability(%Promise%).
Let syncIterator be O.[[SyncIteratorRecord]].[[Iterator]].
Let throw be GetMethod(syncIterator, "throw").
IfAbruptRejectPromise(throw, promiseCapability).
If throw is undefined, then
1. Perform ! Call(promiseCapability.[[Reject]], undefined, « value »).
2. Return promiseCapability.[[Promise]].
Let result be Call(throw, syncIterator, « value »).
IfAbruptRejectPromise(result, promiseCapability).
If Type(result) is not Object, then
1. Perform ! Call(promiseCapability.[[Reject]], undefined, « a newly created TypeError object »).
2. Return promiseCapability.[[Promise]].
Return ! AsyncFromSyncIteratorContinuation(result, promiseCapability).

25.1.4.2.4 Async-from-Sync Iterator Value Unwrap Functions

An async-from-sync iterator value unwrap function is an anonymous built-in function that is used by AsyncFromSyncIteratorContinuation when processing the "value" property of an IteratorResult object, in order to wait for its value if it is a promise and re-package the result in a new "unwrapped" IteratorResult object. Each async-from-sync iterator value unwrap function has a [[Done]] internal slot.

When an async-from-sync iterator value unwrap function is called with argument value, the following steps are taken:

Let F be the active function object.
Return ! CreateIterResultObject(value, F.[[Done]]).

25.1.4.3 Properties of Async-from-Sync Iterator Instances

Async-from-Sync Iterator instances are ordinary objects that inherit properties from the %AsyncFromSyncIteratorPrototype% intrinsic object. Async-from-Sync Iterator instances are initially created with the internal slots listed in Table 72. Async-from-Sync Iterator instances are not directly observable from ECMAScript code.

Table 72: Internal Slots of Async-from-Sync Iterator Instances

Internal Slot	Description
[[SyncIteratorRecord]]	A Record, of the type returned by GetIterator, representing the original synchronous iterator which is being adapted.

25.1.4.4 AsyncFromSyncIteratorContinuation ( `result`, `promiseCapability` )

The abstract operation AsyncFromSyncIteratorContinuation takes arguments result and promiseCapability. It performs the following steps when called:

Let done be IteratorComplete(result).
IfAbruptRejectPromise(done, promiseCapability).
Let value be IteratorValue(result).
IfAbruptRejectPromise(value, promiseCapability).
Let valueWrapper be PromiseResolve(%Promise%, value).
IfAbruptRejectPromise(valueWrapper, promiseCapability).
Let steps be the algorithm steps defined in Async-from-Sync Iterator Value Unwrap Functions.
Let onFulfilled be ! CreateBuiltinFunction(steps, « [[Done]] »).
Set onFulfilled.[[Done]] to done.
Perform ! PerformPromiseThen(valueWrapper, onFulfilled, undefined, promiseCapability).
Return promiseCapability.[[Promise]].

25.2 GeneratorFunction Objects

GeneratorFunction objects are functions that are usually created by evaluating GeneratorDeclarations, GeneratorExpressions, and GeneratorMethods. They may also be created by calling the %GeneratorFunction% intrinsic.

A staggering variety of boxes and arrows. — Figure 5 (Informative): Generator Objects Relationships

25.2.1 The GeneratorFunction Constructor

The GeneratorFunction constructor:

is the intrinsic object %GeneratorFunction%.
is a subclass of Function.
creates and initializes a new GeneratorFunction object when called as a function rather than as a constructor. Thus the function call GeneratorFunction (…) is equivalent to the object creation expression new GeneratorFunction (…) with the same arguments.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified GeneratorFunction behaviour must include a super call to the GeneratorFunction constructor to create and initialize subclass instances with the internal slots necessary for built-in GeneratorFunction behaviour. All ECMAScript syntactic forms for defining generator function objects create direct instances of GeneratorFunction. There is no syntactic means to create instances of GeneratorFunction subclasses.

25.2.1.1 GeneratorFunction ( `p1`, `p2`, … , `pn`, `body` )

The last argument specifies the body (executable code) of a generator function; any preceding arguments specify formal parameters.

When the GeneratorFunction function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “p” arguments, and where body might also not be provided), the following steps are taken:

Let C be the active function object.
Let args be the argumentsList that was passed to this function by [[Call]] or [[Construct]].
Return ? CreateDynamicFunction(C, NewTarget, generator, args).

Note

See NOTE for 19.2.1.1.

25.2.2 Properties of the GeneratorFunction Constructor

The GeneratorFunction constructor:

is a standard built-in function object that inherits from the Function constructor.
has a [[Prototype]] internal slot whose value is %Function%.
has a "name" property whose value is "GeneratorFunction".
has the following properties:

25.2.2.1 GeneratorFunction.length

This is a data property with a value of 1. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.2.2.2 GeneratorFunction.prototype

The initial value of GeneratorFunction.prototype is %Generator%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

25.2.3 Properties of the GeneratorFunction Prototype Object

The GeneratorFunction prototype object:

is an ordinary object.
is not a function object and does not have an [[ECMAScriptCode]] internal slot or any other of the internal slots listed in Table 27 or Table 73.
is the value of the "prototype" property of %GeneratorFunction%.
is the intrinsic object %Generator% (see Figure 2).
has a [[Prototype]] internal slot whose value is %Function.prototype%.

25.2.3.1 GeneratorFunction.prototype.constructor

The initial value of GeneratorFunction.prototype.constructor is %GeneratorFunction%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.2.3.2 GeneratorFunction.prototype.prototype

The value of GeneratorFunction.prototype.prototype is the %Generator.prototype% intrinsic object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.2.3.3 GeneratorFunction.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "GeneratorFunction".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.2.4 GeneratorFunction Instances

Every GeneratorFunction instance is an ECMAScript function object and has the internal slots listed in Table 27. The value of the [[IsClassConstructor]] internal slot for all such instances is false.

Each GeneratorFunction instance has the following own properties:

25.2.4.1 length

The specification for the "length" property of Function instances given in 19.2.4.1 also applies to GeneratorFunction instances.

25.2.4.2 name

The specification for the "name" property of Function instances given in 19.2.4.2 also applies to GeneratorFunction instances.

25.2.4.3 prototype

Whenever a GeneratorFunction instance is created another ordinary object is also created and is the initial value of the generator function's "prototype" property. The value of the prototype property is used to initialize the [[Prototype]] internal slot of a newly created Generator object when the generator function object is invoked using [[Call]].

This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

Note

Unlike Function instances, the object that is the value of the a GeneratorFunction's "prototype" property does not have a "constructor" property whose value is the GeneratorFunction instance.

25.3 AsyncGeneratorFunction Objects

AsyncGeneratorFunction objects are functions that are usually created by evaluating AsyncGeneratorDeclaration, AsyncGeneratorExpression, and AsyncGeneratorMethod syntactic productions. They may also be created by calling the %AsyncGeneratorFunction% intrinsic.

25.3.1 The AsyncGeneratorFunction Constructor

The AsyncGeneratorFunction constructor:

is the intrinsic object %AsyncGeneratorFunction%.
is a subclass of Function.
creates and initializes a new AsyncGeneratorFunction object when called as a function rather than as a constructor. Thus the function call AsyncGeneratorFunction (...) is equivalent to the object creation expression new AsyncGeneratorFunction (...) with the same arguments.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified AsyncGeneratorFunction behaviour must include a super call to the AsyncGeneratorFunction constructor to create and initialize subclass instances with the internal slots necessary for built-in AsyncGeneratorFunction behaviour. All ECMAScript syntactic forms for defining async generator function objects create direct instances of AsyncGeneratorFunction. There is no syntactic means to create instances of AsyncGeneratorFunction subclasses.

25.3.1.1 AsyncGeneratorFunction ( `p1`, `p2`, … , `pn`, `body` )

The last argument specifies the body (executable code) of an async generator function; any preceding arguments specify formal parameters.

When the AsyncGeneratorFunction function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no "p" arguments, and where body might also not be provided), the following steps are taken:

Let C be the active function object.
Let args be the argumentsList that was passed to this function by [[Call]] or [[Construct]].
Return ? CreateDynamicFunction(C, NewTarget, asyncGenerator, args).

Note

See NOTE for 19.2.1.1.

25.3.2 Properties of the AsyncGeneratorFunction Constructor

The AsyncGeneratorFunction constructor:

is a standard built-in function object that inherits from the Function constructor.
has a [[Prototype]] internal slot whose value is %Function%.
has a "name" property whose value is "AsyncGeneratorFunction".
has the following properties:

25.3.2.1 AsyncGeneratorFunction.length

This is a data property with a value of 1. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.3.2.2 AsyncGeneratorFunction.prototype

The initial value of AsyncGeneratorFunction.prototype is the intrinsic object %AsyncGenerator%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

25.3.3 Properties of the AsyncGeneratorFunction Prototype Object

The AsyncGeneratorFunction prototype object:

is an ordinary object.
is not a function object and does not have an [[ECMAScriptCode]] internal slot or any other of the internal slots listed in Table 27 or Table 74.
is the value of the "prototype" property of %AsyncGeneratorFunction%.
is %AsyncGenerator%.
has a [[Prototype]] internal slot whose value is %Function.prototype%.

25.3.3.1 AsyncGeneratorFunction.prototype.constructor

The initial value of AsyncGeneratorFunction.prototype.constructor is %AsyncGeneratorFunction%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.3.3.2 AsyncGeneratorFunction.prototype.prototype

The value of AsyncGeneratorFunction.prototype.prototype is the %AsyncGenerator.prototype% intrinsic object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.3.3.3 AsyncGeneratorFunction.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "AsyncGeneratorFunction".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.3.4 AsyncGeneratorFunction Instances

Every AsyncGeneratorFunction instance is an ECMAScript function object and has the internal slots listed in Table 27. The value of the [[IsClassConstructor]] internal slot for all such instances is false.

Each AsyncGeneratorFunction instance has the following own properties:

25.3.4.1 length

The value of the "length" property is an integer that indicates the typical number of arguments expected by the AsyncGeneratorFunction. However, the language permits the function to be invoked with some other number of arguments. The behaviour of an AsyncGeneratorFunction when invoked on a number of arguments other than the number specified by its "length" property depends on the function.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.3.4.2 name

The specification for the "name" property of Function instances given in 19.2.4.2 also applies to AsyncGeneratorFunction instances.

25.3.4.3 prototype

Whenever an AsyncGeneratorFunction instance is created another ordinary object is also created and is the initial value of the async generator function's "prototype" property. The value of the prototype property is used to initialize the [[Prototype]] internal slot of a newly created AsyncGenerator object when the generator function object is invoked using [[Call]].

This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

Note

Unlike function instances, the object that is the value of the an AsyncGeneratorFunction's "prototype" property does not have a "constructor" property whose value is the AsyncGeneratorFunction instance.

25.4 Generator Objects

A Generator object is an instance of a generator function and conforms to both the Iterator and Iterable interfaces.

Generator instances directly inherit properties from the object that is the value of the "prototype" property of the Generator function that created the instance. Generator instances indirectly inherit properties from the Generator Prototype intrinsic, %Generator.prototype%.

25.4.1 Properties of the Generator Prototype Object

The Generator prototype object:

is the intrinsic object %GeneratorPrototype%.
is the initial value of the "prototype" property of %Generator% (the GeneratorFunction.prototype).
is an ordinary object.
is not a Generator instance and does not have a [[GeneratorState]] internal slot.
has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
has properties that are indirectly inherited by all Generator instances.

25.4.1.1 Generator.prototype.constructor

The initial value of Generator.prototype.constructor is %Generator%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.4.1.2 Generator.prototype.next ( `value` )

The next method performs the following steps:

Let g be the this value.
Return ? GeneratorResume(g, value).

25.4.1.3 Generator.prototype.return ( `value` )

The return method performs the following steps:

Let g be the this value.
Let C be Completion { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.
Return ? GeneratorResumeAbrupt(g, C).

25.4.1.4 Generator.prototype.throw ( `exception` )

The throw method performs the following steps:

Let g be the this value.
Let C be ThrowCompletion(exception).
Return ? GeneratorResumeAbrupt(g, C).

25.4.1.5 Generator.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Generator".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.4.2 Properties of Generator Instances

Generator instances are initially created with the internal slots described in Table 73.

Table 73: Internal Slots of Generator Instances

Internal Slot	Description
[[GeneratorState]]	The current execution state of the generator. The possible values are: undefined, suspendedStart, suspendedYield, executing, and completed.
[[GeneratorContext]]	The execution context that is used when executing the code of this generator.

25.4.3 Generator Abstract Operations

25.4.3.1 GeneratorStart ( `generator`, `generatorBody` )

The abstract operation GeneratorStart takes arguments generator and generatorBody. It performs the following steps when called:

Assert: The value of generator.[[GeneratorState]] is undefined.
Let genContext be the running execution context.
Set the Generator component of genContext to generator.
Set the code evaluation state of genContext such that when evaluation is resumed for that execution context the following steps will be performed:
1. Let result be the result of evaluating generatorBody.
2. Assert: If we return here, the generator either threw an exception or performed either an implicit or explicit return.
3. Remove genContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
4. Set generator.[[GeneratorState]] to completed.
5. Once a generator enters the completed state it never leaves it and its associated execution context is never resumed. Any execution state associated with generator can be discarded at this point.
6. If result.[[Type]] is normal, let resultValue be undefined.
7. Else if result.[[Type]] is return, let resultValue be result.[[Value]].
8. Else,
  1. Assert: result.[[Type]] is throw.
  2. Return Completion(result).
9. Return CreateIterResultObject(resultValue, true).
Set generator.[[GeneratorContext]] to genContext.
Set generator.[[GeneratorState]] to suspendedStart.
Return NormalCompletion(undefined).

25.4.3.2 GeneratorValidate ( `generator` )

The abstract operation GeneratorValidate takes argument generator. It performs the following steps when called:

Perform ? RequireInternalSlot(generator, [[GeneratorState]]).
Assert: generator also has a [[GeneratorContext]] internal slot.
Let state be generator.[[GeneratorState]].
If state is executing, throw a TypeError exception.
Return state.

25.4.3.3 GeneratorResume ( `generator`, `value` )

The abstract operation GeneratorResume takes arguments generator and value. It performs the following steps when called:

Let state be ? GeneratorValidate(generator).
If state is completed, return CreateIterResultObject(undefined, true).
Assert: state is either suspendedStart or suspendedYield.
Let genContext be generator.[[GeneratorContext]].
Let methodContext be the running execution context.
Suspend methodContext.
Set generator.[[GeneratorState]] to executing.
Push genContext onto the execution context stack; genContext is now the running execution context.
Resume the suspended evaluation of genContext using NormalCompletion(value) as the result of the operation that suspended it. Let result be the value returned by the resumed computation.
Assert: When we return here, genContext has already been removed from the execution context stack and methodContext is the currently running execution context.
Return Completion(result).

25.4.3.4 GeneratorResumeAbrupt ( `generator`, `abruptCompletion` )

The abstract operation GeneratorResumeAbrupt takes arguments generator and abruptCompletion. It performs the following steps when called:

Let state be ? GeneratorValidate(generator).
If state is suspendedStart, then
1. Set generator.[[GeneratorState]] to completed.
2. Once a generator enters the completed state it never leaves it and its associated execution context is never resumed. Any execution state associated with generator can be discarded at this point.
3. Set state to completed.
If state is completed, then
1. If abruptCompletion.[[Type]] is return, then
  1. Return CreateIterResultObject(abruptCompletion.[[Value]], true).
2. Return Completion(abruptCompletion).
Assert: state is suspendedYield.
Let genContext be generator.[[GeneratorContext]].
Let methodContext be the running execution context.
Suspend methodContext.
Set generator.[[GeneratorState]] to executing.
Push genContext onto the execution context stack; genContext is now the running execution context.
Resume the suspended evaluation of genContext using abruptCompletion as the result of the operation that suspended it. Let result be the completion record returned by the resumed computation.
Assert: When we return here, genContext has already been removed from the execution context stack and methodContext is the currently running execution context.
Return Completion(result).

25.4.3.5 GetGeneratorKind ( )

The abstract operation GetGeneratorKind takes no arguments. It performs the following steps when called:

Let genContext be the running execution context.
If genContext does not have a Generator component, return non-generator.
Let generator be the Generator component of genContext.
If generator has an [[AsyncGeneratorState]] internal slot, return async.
Else, return sync.

25.4.3.6 GeneratorYield ( `iterNextObj` )

The abstract operation GeneratorYield takes argument iterNextObj. It performs the following steps when called:

Assert: iterNextObj is an Object that implements the IteratorResult interface.
Let genContext be the running execution context.
Assert: genContext is the execution context of a generator.
Let generator be the value of the Generator component of genContext.
Assert: GetGeneratorKind() is sync.
Set generator.[[GeneratorState]] to suspendedYield.
Remove genContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
Set the code evaluation state of genContext such that when evaluation is resumed with a Completion resumptionValue the following steps will be performed:
1. Return resumptionValue.
2. NOTE: This returns to the evaluation of the YieldExpression that originally called this abstract operation.
Return NormalCompletion(iterNextObj).
NOTE: This returns to the evaluation of the operation that had most previously resumed evaluation of genContext.

25.5 AsyncGenerator Objects

An AsyncGenerator object is an instance of an async generator function and conforms to both the AsyncIterator and AsyncIterable interfaces.

AsyncGenerator instances directly inherit properties from the object that is the value of the "prototype" property of the AsyncGenerator function that created the instance. AsyncGenerator instances indirectly inherit properties from the AsyncGenerator Prototype intrinsic, %AsyncGenerator.prototype%.

25.5.1 Properties of the AsyncGenerator Prototype Object

The AsyncGenerator prototype object:

is the intrinsic object %AsyncGeneratorPrototype%.
is the initial value of the "prototype" property of %AsyncGenerator% (the AsyncGeneratorFunction.prototype).
is an ordinary object.
is not an AsyncGenerator instance and does not have an [[AsyncGeneratorState]] internal slot.
has a [[Prototype]] internal slot whose value is %AsyncIteratorPrototype%.
has properties that are indirectly inherited by all AsyncGenerator instances.

25.5.1.1 AsyncGenerator.prototype.constructor

The initial value of AsyncGenerator.prototype.constructor is %AsyncGenerator%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.5.1.2 AsyncGenerator.prototype.next ( `value` )

Let generator be the this value.
Let completion be NormalCompletion(value).
Return ! AsyncGeneratorEnqueue(generator, completion).

25.5.1.3 AsyncGenerator.prototype.return ( `value` )

Let generator be the this value.
Let completion be Completion { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.
Return ! AsyncGeneratorEnqueue(generator, completion).

25.5.1.4 AsyncGenerator.prototype.throw ( `exception` )

Let generator be the this value.
Let completion be ThrowCompletion(exception).
Return ! AsyncGeneratorEnqueue(generator, completion).

25.5.1.5 AsyncGenerator.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "AsyncGenerator".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.5.2 Properties of AsyncGenerator Instances

AsyncGenerator instances are initially created with the internal slots described below:

Table 74: Internal Slots of AsyncGenerator Instances

Internal Slot	Description
[[AsyncGeneratorState]]	The current execution state of the async generator. The possible values are: undefined, suspendedStart, suspendedYield, executing, awaiting-return, and completed.
[[AsyncGeneratorContext]]	The execution context that is used when executing the code of this async generator.
[[AsyncGeneratorQueue]]	A List of AsyncGeneratorRequest records which represent requests to resume the async generator.

25.5.3 AsyncGenerator Abstract Operations

25.5.3.1 AsyncGeneratorRequest Records

The AsyncGeneratorRequest is a Record value used to store information about how an async generator should be resumed and contains capabilities for fulfilling or rejecting the corresponding promise.

They have the following fields:

Table 75: AsyncGeneratorRequest Record Fields

Field Name	Value	Meaning
[[Completion]]	A Completion record	The completion which should be used to resume the async generator.
[[Capability]]	A PromiseCapability record	The promise capabilities associated with this request.

25.5.3.2 AsyncGeneratorStart ( `generator`, `generatorBody` )

The abstract operation AsyncGeneratorStart takes arguments generator and generatorBody. It performs the following steps when called:

Assert: generator is an AsyncGenerator instance.
Assert: generator.[[AsyncGeneratorState]] is undefined.
Let genContext be the running execution context.
Set the Generator component of genContext to generator.
Set the code evaluation state of genContext such that when evaluation is resumed for that execution context the following steps will be performed:
1. Let result be the result of evaluating generatorBody.
2. Assert: If we return here, the async generator either threw an exception or performed either an implicit or explicit return.
3. Remove genContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
4. Set generator.[[AsyncGeneratorState]] to completed.
5. If result is a normal completion, let resultValue be undefined.
6. Else,
  1. Let resultValue be result.[[Value]].
  2. If result.[[Type]] is not return, then
    1. Return ! AsyncGeneratorReject(generator, resultValue).
7. Return ! AsyncGeneratorResolve(generator, resultValue, true).
Set generator.[[AsyncGeneratorContext]] to genContext.
Set generator.[[AsyncGeneratorState]] to suspendedStart.
Set generator.[[AsyncGeneratorQueue]] to a new empty List.
Return undefined.

25.5.3.3 AsyncGeneratorResolve ( `generator`, `value`, `done` )

The abstract operation AsyncGeneratorResolve takes arguments generator, value, and done. It performs the following steps when called:

Assert: generator is an AsyncGenerator instance.
Let queue be generator.[[AsyncGeneratorQueue]].
Assert: queue is not an empty List.
Remove the first element from queue and let next be the value of that element.
Let promiseCapability be next.[[Capability]].
Let iteratorResult be ! CreateIterResultObject(value, done).
Perform ! Call(promiseCapability.[[Resolve]], undefined, « iteratorResult »).
Perform ! AsyncGeneratorResumeNext(generator).
Return undefined.

25.5.3.4 AsyncGeneratorReject ( `generator`, `exception` )

The abstract operation AsyncGeneratorReject takes arguments generator and exception. It performs the following steps when called:

Assert: generator is an AsyncGenerator instance.
Let queue be generator.[[AsyncGeneratorQueue]].
Assert: queue is not an empty List.
Remove the first element from queue and let next be the value of that element.
Let promiseCapability be next.[[Capability]].
Perform ! Call(promiseCapability.[[Reject]], undefined, « exception »).
Perform ! AsyncGeneratorResumeNext(generator).
Return undefined.

25.5.3.5 AsyncGeneratorResumeNext ( `generator` )

The abstract operation AsyncGeneratorResumeNext takes argument generator. It performs the following steps when called:

Assert: generator is an AsyncGenerator instance.
Let state be generator.[[AsyncGeneratorState]].
Assert: state is not executing.
If state is awaiting-return, return undefined.
Let queue be generator.[[AsyncGeneratorQueue]].
If queue is an empty List, return undefined.
Let next be the value of the first element of queue.
Assert: next is an AsyncGeneratorRequest record.
Let completion be next.[[Completion]].
If completion is an abrupt completion, then
1. If state is suspendedStart, then
  1. Set generator.[[AsyncGeneratorState]] to completed.
  2. Set state to completed.
2. If state is completed, then
  1. If completion.[[Type]] is return, then
    1. Set generator.[[AsyncGeneratorState]] to awaiting-return.
    2. Let promise be ? PromiseResolve(%Promise%, completion.[[Value]]).
    3. Let stepsFulfilled be the algorithm steps defined in AsyncGeneratorResumeNext Return Processor Fulfilled Functions.
    4. Let onFulfilled be ! CreateBuiltinFunction(stepsFulfilled, « [[Generator]] »).
    5. Set onFulfilled.[[Generator]] to generator.
    6. Let stepsRejected be the algorithm steps defined in AsyncGeneratorResumeNext Return Processor Rejected Functions.
    7. Let onRejected be ! CreateBuiltinFunction(stepsRejected, « [[Generator]] »).
    8. Set onRejected.[[Generator]] to generator.
    9. Perform ! PerformPromiseThen(promise, onFulfilled, onRejected).
    10. Return undefined.
  2. Else,
    1. Assert: completion.[[Type]] is throw.
    2. Perform ! AsyncGeneratorReject(generator, completion.[[Value]]).
    3. Return undefined.
Else if state is completed, return ! AsyncGeneratorResolve(generator, undefined, true).
Assert: state is either suspendedStart or suspendedYield.
Let genContext be generator.[[AsyncGeneratorContext]].
Let callerContext be the running execution context.
Suspend callerContext.
Set generator.[[AsyncGeneratorState]] to executing.
Push genContext onto the execution context stack; genContext is now the running execution context.
Resume the suspended evaluation of genContext using completion as the result of the operation that suspended it. Let result be the completion record returned by the resumed computation.
Assert: result is never an abrupt completion.
Assert: When we return here, genContext has already been removed from the execution context stack and callerContext is the currently running execution context.
Return undefined.

25.5.3.5.1 AsyncGeneratorResumeNext Return Processor Fulfilled Functions

An AsyncGeneratorResumeNext return processor fulfilled function is an anonymous built-in function that is used as part of the AsyncGeneratorResumeNext specification device to unwrap promises passed in to the AsyncGenerator.prototype.return ( value ) method. Each AsyncGeneratorResumeNext return processor fulfilled function has a [[Generator]] internal slot.

When an AsyncGeneratorResumeNext return processor fulfilled function is called with argument value, the following steps are taken:

Let F be the active function object.
Set F.[[Generator]].[[AsyncGeneratorState]] to completed.
Return ! AsyncGeneratorResolve(F.[[Generator]], value, true).

The "length" property of an AsyncGeneratorResumeNext return processor fulfilled function is 1.

25.5.3.5.2 AsyncGeneratorResumeNext Return Processor Rejected Functions

An AsyncGeneratorResumeNext return processor rejected function is an anonymous built-in function that is used as part of the AsyncGeneratorResumeNext specification device to unwrap promises passed in to the AsyncGenerator.prototype.return ( value ) method. Each AsyncGeneratorResumeNext return processor rejected function has a [[Generator]] internal slot.

When an AsyncGeneratorResumeNext return processor rejected function is called with argument reason, the following steps are taken:

Let F be the active function object.
Set F.[[Generator]].[[AsyncGeneratorState]] to completed.
Return ! AsyncGeneratorReject(F.[[Generator]], reason).

The "length" property of an AsyncGeneratorResumeNext return processor rejected function is 1.

25.5.3.6 AsyncGeneratorEnqueue ( `generator`, `completion` )

The abstract operation AsyncGeneratorEnqueue takes arguments generator and completion. It performs the following steps when called:

Assert: completion is a Completion Record.
Let promiseCapability be ! NewPromiseCapability(%Promise%).
If Type(generator) is not Object, or if generator does not have an [[AsyncGeneratorState]] internal slot, then
1. Let badGeneratorError be a newly created TypeError object.
2. Perform ! Call(promiseCapability.[[Reject]], undefined, « badGeneratorError »).
3. Return promiseCapability.[[Promise]].
Let queue be generator.[[AsyncGeneratorQueue]].
Let request be AsyncGeneratorRequest { [[Completion]]: completion, [[Capability]]: promiseCapability }.
Append request to the end of queue.
Let state be generator.[[AsyncGeneratorState]].
If state is not executing, then
1. Perform ! AsyncGeneratorResumeNext(generator).
Return promiseCapability.[[Promise]].

25.5.3.7 AsyncGeneratorYield ( `value` )

The abstract operation AsyncGeneratorYield takes argument value. It performs the following steps when called:

Let genContext be the running execution context.
Assert: genContext is the execution context of a generator.
Let generator be the value of the Generator component of genContext.
Assert: GetGeneratorKind() is async.
Set value to ? Await(value).
Set generator.[[AsyncGeneratorState]] to suspendedYield.
Remove genContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
Set the code evaluation state of genContext such that when evaluation is resumed with a Completion resumptionValue the following steps will be performed:
1. If resumptionValue.[[Type]] is not return, return Completion(resumptionValue).
2. Let awaited be Await(resumptionValue.[[Value]]).
3. If awaited.[[Type]] is throw, return Completion(awaited).
4. Assert: awaited.[[Type]] is normal.
5. Return Completion { [[Type]]: return, [[Value]]: awaited.[[Value]], [[Target]]: empty }.
6. NOTE: When one of the above steps returns, it returns to the evaluation of the YieldExpression production that originally called this abstract operation.
Return ! AsyncGeneratorResolve(generator, value, false).
NOTE: This returns to the evaluation of the operation that had most previously resumed evaluation of genContext.

25.6 Promise Objects

A Promise is an object that is used as a placeholder for the eventual results of a deferred (and possibly asynchronous) computation.

Any Promise object is in one of three mutually exclusive states: fulfilled, rejected, and pending:

A promise p is fulfilled if p.then(f, r) will immediately enqueue a Job to call the function f.
A promise p is rejected if p.then(f, r) will immediately enqueue a Job to call the function r.
A promise is pending if it is neither fulfilled nor rejected.

A promise is said to be settled if it is not pending, i.e. if it is either fulfilled or rejected.

A promise is resolved if it is settled or if it has been “locked in” to match the state of another promise. Attempting to resolve or reject a resolved promise has no effect. A promise is unresolved if it is not resolved. An unresolved promise is always in the pending state. A resolved promise may be pending, fulfilled or rejected.

25.6.1 Promise Abstract Operations

25.6.1.1 PromiseCapability Records

A PromiseCapability is a Record value used to encapsulate a promise object along with the functions that are capable of resolving or rejecting that promise object. PromiseCapability Records are produced by the NewPromiseCapability abstract operation.

PromiseCapability Records have the fields listed in Table 76.

Table 76: PromiseCapability Record Fields

Field Name	Value	Meaning
[[Promise]]	An object	An object that is usable as a promise.
[[Resolve]]	A function object	The function that is used to resolve the given promise object.
[[Reject]]	A function object	The function that is used to reject the given promise object.

25.6.1.1.1 IfAbruptRejectPromise ( `value`, `capability` )

IfAbruptRejectPromise is a shorthand for a sequence of algorithm steps that use a PromiseCapability Record. An algorithm step of the form:

IfAbruptRejectPromise(value, capability).

means the same thing as:

If value is an abrupt completion, then
1. Perform ? Call(capability.[[Reject]], undefined, « value.[[Value]] »).
2. Return capability.[[Promise]].
Else if value is a Completion Record, set value to value.[[Value]].

25.6.1.2 PromiseReaction Records

The PromiseReaction is a Record value used to store information about how a promise should react when it becomes resolved or rejected with a given value. PromiseReaction records are created by the PerformPromiseThen abstract operation, and are used by the Abstract Closure returned by NewPromiseReactionJob.

PromiseReaction records have the fields listed in Table 77.

Table 77: PromiseReaction Record Fields

Field Name	Value	Meaning
[[Capability]]	A PromiseCapability Record, or undefined	The capabilities of the promise for which this record provides a reaction handler.
[[Type]]	Fulfill \| Reject	The [[Type]] is used when [[Handler]] is undefined to allow for behaviour specific to the settlement type.
[[Handler]]	A function object or undefined.	The function that should be applied to the incoming value, and whose return value will govern what happens to the derived promise. If [[Handler]] is undefined, a function that depends on the value of [[Type]] will be used instead.

25.6.1.3 CreateResolvingFunctions ( `promise` )

The abstract operation CreateResolvingFunctions takes argument promise. It performs the following steps when called:

Let alreadyResolved be the Record { [[Value]]: false }.
Let stepsResolve be the algorithm steps defined in Promise Resolve Functions.
Let resolve be ! CreateBuiltinFunction(stepsResolve, « [[Promise]], [[AlreadyResolved]] »).
Set resolve.[[Promise]] to promise.
Set resolve.[[AlreadyResolved]] to alreadyResolved.
Let stepsReject be the algorithm steps defined in Promise Reject Functions.
Let reject be ! CreateBuiltinFunction(stepsReject, « [[Promise]], [[AlreadyResolved]] »).
Set reject.[[Promise]] to promise.
Set reject.[[AlreadyResolved]] to alreadyResolved.
Return the Record { [[Resolve]]: resolve, [[Reject]]: reject }.

25.6.1.3.1 Promise Reject Functions

A promise reject function is an anonymous built-in function that has [[Promise]] and [[AlreadyResolved]] internal slots.

When a promise reject function is called with argument reason, the following steps are taken:

Let F be the active function object.
Assert: F has a [[Promise]] internal slot whose value is an Object.
Let promise be F.[[Promise]].
Let alreadyResolved be F.[[AlreadyResolved]].
If alreadyResolved.[[Value]] is true, return undefined.
Set alreadyResolved.[[Value]] to true.
Return RejectPromise(promise, reason).

The "length" property of a promise reject function is 1.

25.6.1.3.2 Promise Resolve Functions

A promise resolve function is an anonymous built-in function that has [[Promise]] and [[AlreadyResolved]] internal slots.

When a promise resolve function is called with argument resolution, the following steps are taken:

Let F be the active function object.
Assert: F has a [[Promise]] internal slot whose value is an Object.
Let promise be F.[[Promise]].
Let alreadyResolved be F.[[AlreadyResolved]].
If alreadyResolved.[[Value]] is true, return undefined.
Set alreadyResolved.[[Value]] to true.
If SameValue(resolution, promise) is true, then
1. Let selfResolutionError be a newly created TypeError object.
2. Return RejectPromise(promise, selfResolutionError).
If Type(resolution) is not Object, then
1. Return FulfillPromise(promise, resolution).
Let then be Get(resolution, "then").
If then is an abrupt completion, then
1. Return RejectPromise(promise, then.[[Value]]).
Let thenAction be then.[[Value]].
If IsCallable(thenAction) is false, then
1. Return FulfillPromise(promise, resolution).
Let job be NewPromiseResolveThenableJob(promise, resolution, thenAction).
Perform HostEnqueuePromiseJob(job.[[Job]], job.[[Realm]]).
Return undefined.

The "length" property of a promise resolve function is 1.

25.6.1.4 FulfillPromise ( `promise`, `value` )

The abstract operation FulfillPromise takes arguments promise and value. It performs the following steps when called:

Assert: The value of promise.[[PromiseState]] is pending.
Let reactions be promise.[[PromiseFulfillReactions]].
Set promise.[[PromiseResult]] to value.
Set promise.[[PromiseFulfillReactions]] to undefined.
Set promise.[[PromiseRejectReactions]] to undefined.
Set promise.[[PromiseState]] to fulfilled.
Return TriggerPromiseReactions(reactions, value).

25.6.1.5 NewPromiseCapability ( `C` )

The abstract operation NewPromiseCapability takes argument C. It attempts to use C as a constructor in the fashion of the built-in Promise constructor to create a Promise object and extract its resolve and reject functions. The Promise object plus the resolve and reject functions are used to initialize a new PromiseCapability Record. It performs the following steps when called:

If IsConstructor(C) is false, throw a TypeError exception.
NOTE: C is assumed to be a constructor function that supports the parameter conventions of the Promise constructor (see 25.6.3.1).
Let promiseCapability be the PromiseCapability { [[Promise]]: undefined, [[Resolve]]: undefined, [[Reject]]: undefined }.
Let steps be the algorithm steps defined in GetCapabilitiesExecutor Functions.
Let executor be ! CreateBuiltinFunction(steps, « [[Capability]] »).
Set executor.[[Capability]] to promiseCapability.
Let promise be ? Construct(C, « executor »).
If IsCallable(promiseCapability.[[Resolve]]) is false, throw a TypeError exception.
If IsCallable(promiseCapability.[[Reject]]) is false, throw a TypeError exception.
Set promiseCapability.[[Promise]] to promise.
Return promiseCapability.

Note

This abstract operation supports Promise subclassing, as it is generic on any constructor that calls a passed executor function argument in the same way as the Promise constructor. It is used to generalize static methods of the Promise constructor to any subclass.

25.6.1.5.1 GetCapabilitiesExecutor Functions

A GetCapabilitiesExecutor function is an anonymous built-in function that has a [[Capability]] internal slot.

When a GetCapabilitiesExecutor function is called with arguments resolve and reject, the following steps are taken:

Let F be the active function object.
Assert: F has a [[Capability]] internal slot whose value is a PromiseCapability Record.
Let promiseCapability be F.[[Capability]].
If promiseCapability.[[Resolve]] is not undefined, throw a TypeError exception.
If promiseCapability.[[Reject]] is not undefined, throw a TypeError exception.
Set promiseCapability.[[Resolve]] to resolve.
Set promiseCapability.[[Reject]] to reject.
Return undefined.

The "length" property of a GetCapabilitiesExecutor function is 2.

25.6.1.6 IsPromise ( `x` )

The abstract operation IsPromise takes argument x. It checks for the promise brand on an object. It performs the following steps when called:

If Type(x) is not Object, return false.
If x does not have a [[PromiseState]] internal slot, return false.
Return true.

25.6.1.7 RejectPromise ( `promise`, `reason` )

The abstract operation RejectPromise takes arguments promise and reason. It performs the following steps when called:

Assert: The value of promise.[[PromiseState]] is pending.
Let reactions be promise.[[PromiseRejectReactions]].
Set promise.[[PromiseResult]] to reason.
Set promise.[[PromiseFulfillReactions]] to undefined.
Set promise.[[PromiseRejectReactions]] to undefined.
Set promise.[[PromiseState]] to rejected.
If promise.[[PromiseIsHandled]] is false, perform HostPromiseRejectionTracker(promise, "reject").
Return TriggerPromiseReactions(reactions, reason).

25.6.1.8 TriggerPromiseReactions ( `reactions`, `argument` )

The abstract operation TriggerPromiseReactions takes arguments reactions (a collection of PromiseReaction Records) and argument. It enqueues a new Job for each record in reactions. Each such Job processes the [[Type]] and [[Handler]] of the PromiseReaction Record, and if the [[Handler]] is a function, calls it passing the given argument. If the [[Handler]] is undefined, the behaviour is determined by the [[Type]]. It performs the following steps when called:

For each reaction in reactions, in original insertion order, do
1. Let job be NewPromiseReactionJob(reaction, argument).
2. Perform HostEnqueuePromiseJob(job.[[Job]], job.[[Realm]]).
Return undefined.

25.6.1.9 HostPromiseRejectionTracker ( `promise`, `operation` )

HostPromiseRejectionTracker is an implementation-defined abstract operation that allows host environments to track promise rejections.

An implementation of HostPromiseRejectionTracker must complete normally in all cases. The default implementation of HostPromiseRejectionTracker is to unconditionally return an empty normal completion.

Note 1

HostPromiseRejectionTracker is called in two scenarios:

When a promise is rejected without any handlers, it is called with its operation argument set to "reject".
When a handler is added to a rejected promise for the first time, it is called with its operation argument set to "handle".

A typical implementation of HostPromiseRejectionTracker might try to notify developers of unhandled rejections, while also being careful to notify them if such previous notifications are later invalidated by new handlers being attached.

Note 2

If operation is "handle", an implementation should not hold a reference to promise in a way that would interfere with garbage collection. An implementation may hold a reference to promise if operation is "reject", since it is expected that rejections will be rare and not on hot code paths.

25.6.2 Promise Jobs

25.6.2.1 NewPromiseReactionJob ( `reaction`, `argument` )

The abstract operation NewPromiseReactionJob takes arguments reaction and argument. It returns a new Job Abstract Closure that applies the appropriate handler to the incoming value, and uses the handler's return value to resolve or reject the derived promise associated with that handler. It performs the following steps when called:

Let job be a new Job Abstract Closure with no parameters that captures reaction and argument and performs the following steps when called:
1. Assert: reaction is a PromiseReaction Record.
2. Let promiseCapability be reaction.[[Capability]].
3. Let type be reaction.[[Type]].
4. Let handler be reaction.[[Handler]].
5. If handler is undefined, then
  1. If type is Fulfill, let handlerResult be NormalCompletion(argument).
  2. Else,
    1. Assert: type is Reject.
    2. Let handlerResult be ThrowCompletion(argument).
6. Else, let handlerResult be Call(handler, undefined, « argument »).
7. If promiseCapability is undefined, then
  1. Assert: handlerResult is not an abrupt completion.
  2. Return NormalCompletion(empty).
8. If handlerResult is an abrupt completion, then
  1. Let status be Call(promiseCapability.[[Reject]], undefined, « handlerResult.[[Value]] »).
9. Else,
  1. Let status be Call(promiseCapability.[[Resolve]], undefined, « handlerResult.[[Value]] »).
10. Return Completion(status).
Let handlerRealm be null.
If reaction.[[Handler]] is not undefined, then
1. Let getHandlerRealmResult be GetFunctionRealm(reaction.[[Handler]]).
2. If getHandlerRealmResult is a normal completion, then set handlerRealm to getHandlerRealmResult.[[Value]].
Return the Record { [[Job]]: job, [[Realm]]: handlerRealm }.

25.6.2.2 NewPromiseResolveThenableJob ( `promiseToResolve`, `thenable`, `then` )

The abstract operation NewPromiseResolveThenableJob takes arguments promiseToResolve, thenable, and then. It performs the following steps when called:

Let job be a new Job Abstract Closure with no parameters that captures promiseToResolve, thenable, and then and performs the following steps when called:
1. Let resolvingFunctions be CreateResolvingFunctions(promiseToResolve).
2. Let thenCallResult be Call(then, thenable, « resolvingFunctions.[[Resolve]], resolvingFunctions.[[Reject]] »).
3. If thenCallResult is an abrupt completion, then
  1. Let status be Call(resolvingFunctions.[[Reject]], undefined, « thenCallResult.[[Value]] »).
  2. Return Completion(status).
4. Return Completion(thenCallResult).
Let getThenRealmResult be GetFunctionRealm(then).
If getThenRealmResult is a normal completion, then let thenRealm be getThenRealmResult.[[Value]].
Otherwise, let thenRealm be null.
Return the Record { [[Job]]: job, [[Realm]]: thenRealm }.

Note

This Job uses the supplied thenable and its then method to resolve the given promise. This process must take place as a Job to ensure that the evaluation of the then method occurs after evaluation of any surrounding code has completed.

25.6.3 The Promise Constructor

The Promise constructor:

is the intrinsic object %Promise%.
is the initial value of the "Promise" property of the global object.
creates and initializes a new Promise object when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Promise behaviour must include a super call to the Promise constructor to create and initialize the subclass instance with the internal state necessary to support the Promise and Promise.prototype built-in methods.

25.6.3.1 Promise ( `executor` )

When the Promise function is called with argument executor, the following steps are taken:

If NewTarget is undefined, throw a TypeError exception.
If IsCallable(executor) is false, throw a TypeError exception.
Let promise be ? OrdinaryCreateFromConstructor(NewTarget, "%Promise.prototype%", « [[PromiseState]], [[PromiseResult]], [[PromiseFulfillReactions]], [[PromiseRejectReactions]], [[PromiseIsHandled]] »).
Set promise.[[PromiseState]] to pending.
Set promise.[[PromiseFulfillReactions]] to a new empty List.
Set promise.[[PromiseRejectReactions]] to a new empty List.
Set promise.[[PromiseIsHandled]] to false.
Let resolvingFunctions be CreateResolvingFunctions(promise).
Let completion be Call(executor, undefined, « resolvingFunctions.[[Resolve]], resolvingFunctions.[[Reject]] »).
If completion is an abrupt completion, then
1. Perform ? Call(resolvingFunctions.[[Reject]], undefined, « completion.[[Value]] »).
Return promise.

Note

The executor argument must be a function object. It is called for initiating and reporting completion of the possibly deferred action represented by this Promise object. The executor is called with two arguments: resolve and reject. These are functions that may be used by the executor function to report eventual completion or failure of the deferred computation. Returning from the executor function does not mean that the deferred action has been completed but only that the request to eventually perform the deferred action has been accepted.

The resolve function that is passed to an executor function accepts a single argument. The executor code may eventually call the resolve function to indicate that it wishes to resolve the associated Promise object. The argument passed to the resolve function represents the eventual value of the deferred action and can be either the actual fulfillment value or another Promise object which will provide the value if it is fulfilled.

The reject function that is passed to an executor function accepts a single argument. The executor code may eventually call the reject function to indicate that the associated Promise is rejected and will never be fulfilled. The argument passed to the reject function is used as the rejection value of the promise. Typically it will be an Error object.

The resolve and reject functions passed to an executor function by the Promise constructor have the capability to actually resolve and reject the associated promise. Subclasses may have different constructor behaviour that passes in customized values for resolve and reject.

25.6.4 Properties of the Promise Constructor

The Promise constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
has the following properties:

25.6.4.1 Promise.all ( `iterable` )

The all function returns a new promise which is fulfilled with an array of fulfillment values for the passed promises, or rejects with the reason of the first passed promise that rejects. It resolves all elements of the passed iterable to promises as it runs this algorithm.

Let C be the this value.
Let promiseCapability be ? NewPromiseCapability(C).
Let iteratorRecord be GetIterator(iterable).
IfAbruptRejectPromise(iteratorRecord, promiseCapability).
Let result be PerformPromiseAll(iteratorRecord, C, promiseCapability).
If result is an abrupt completion, then
1. If iteratorRecord.[[Done]] is false, set result to IteratorClose(iteratorRecord, result).
2. IfAbruptRejectPromise(result, promiseCapability).
Return Completion(result).

This function is the %Promise_all% intrinsic object.

Note

The all function requires its this value to be a constructor function that supports the parameter conventions of the Promise constructor.

25.6.4.1.1 Runtime Semantics: PerformPromiseAll ( `iteratorRecord`, `constructor`, `resultCapability` )

The abstract operation PerformPromiseAll takes arguments iteratorRecord, constructor, and resultCapability. It performs the following steps when called:

Assert: IsConstructor(constructor) is true.
Assert: resultCapability is a PromiseCapability Record.
Let values be a new empty List.
Let remainingElementsCount be the Record { [[Value]]: 1 }.
Let promiseResolve be ? Get(constructor, "resolve").
If ! IsCallable(promiseResolve) is false, throw a TypeError exception.
Let index be 0.
Repeat,
1. Let next be IteratorStep(iteratorRecord).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, then
  1. Set iteratorRecord.[[Done]] to true.
  2. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  3. If remainingElementsCount.[[Value]] is 0, then
    1. Let valuesArray be ! CreateArrayFromList(values).
    2. Perform ? Call(resultCapability.[[Resolve]], undefined, « valuesArray »).
  4. Return resultCapability.[[Promise]].
5. Let nextValue be IteratorValue(next).
6. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
7. ReturnIfAbrupt(nextValue).
8. Append undefined to values.
9. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue »).
10. Let steps be the algorithm steps defined in Promise.all Resolve Element Functions.
11. Let resolveElement be ! CreateBuiltinFunction(steps, « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »).
12. Set resolveElement.[[AlreadyCalled]] to the Record { [[Value]]: false }.
13. Set resolveElement.[[Index]] to index.
14. Set resolveElement.[[Values]] to values.
15. Set resolveElement.[[Capability]] to resultCapability.
16. Set resolveElement.[[RemainingElements]] to remainingElementsCount.
17. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1.
18. Perform ? Invoke(nextPromise, "then", « resolveElement, resultCapability.[[Reject]] »).
19. Set index to index + 1.

25.6.4.1.2 `Promise.all` Resolve Element Functions

A Promise.all resolve element function is an anonymous built-in function that is used to resolve a specific Promise.all element. Each Promise.all resolve element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.

When a Promise.all resolve element function is called with argument x, the following steps are taken:

Let F be the active function object.
Let alreadyCalled be F.[[AlreadyCalled]].
If alreadyCalled.[[Value]] is true, return undefined.
Set alreadyCalled.[[Value]] to true.
Let index be F.[[Index]].
Let values be F.[[Values]].
Let promiseCapability be F.[[Capability]].
Let remainingElementsCount be F.[[RemainingElements]].
Set values[index] to x.
Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
If remainingElementsCount.[[Value]] is 0, then
1. Let valuesArray be ! CreateArrayFromList(values).
2. Return ? Call(promiseCapability.[[Resolve]], undefined, « valuesArray »).
Return undefined.

The "length" property of a Promise.all resolve element function is 1.

25.6.4.2 Promise.allSettled ( `iterable` )

The allSettled function returns a promise that is fulfilled with an array of promise state snapshots, but only after all the original promises have settled, i.e. become either fulfilled or rejected. It resolves all elements of the passed iterable to promises as it runs this algorithm.

Let C be the this value.
Let promiseCapability be ? NewPromiseCapability(C).
Let iteratorRecord be GetIterator(iterable).
IfAbruptRejectPromise(iteratorRecord, promiseCapability).
Let result be PerformPromiseAllSettled(iteratorRecord, C, promiseCapability).
If result is an abrupt completion, then
1. If iteratorRecord.[[Done]] is false, set result to IteratorClose(iteratorRecord, result).
2. IfAbruptRejectPromise(result, promiseCapability).
Return Completion(result).

Note

The allSettled function requires its this value to be a constructor function that supports the parameter conventions of the Promise constructor.

25.6.4.2.1 Runtime Semantics: PerformPromiseAllSettled ( `iteratorRecord`, `constructor`, `resultCapability` )

The abstract operation PerformPromiseAllSettled takes arguments iteratorRecord, constructor, and resultCapability. It performs the following steps when called:

Assert: ! IsConstructor(constructor) is true.
Assert: resultCapability is a PromiseCapability Record.
Let values be a new empty List.
Let remainingElementsCount be the Record { [[Value]]: 1 }.
Let index be 0.
Let promiseResolve be ? Get(constructor, "resolve").
If IsCallable(promiseResolve) is false, throw a TypeError exception.
Repeat,
1. Let next be IteratorStep(iteratorRecord).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, then
  1. Set iteratorRecord.[[Done]] to true.
  2. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  3. If remainingElementsCount.[[Value]] is 0, then
    1. Let valuesArray be ! CreateArrayFromList(values).
    2. Perform ? Call(resultCapability.[[Resolve]], undefined, « valuesArray »).
  4. Return resultCapability.[[Promise]].
5. Let nextValue be IteratorValue(next).
6. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
7. ReturnIfAbrupt(nextValue).
8. Append undefined to values.
9. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue »).
10. Let steps be the algorithm steps defined in Promise.allSettled Resolve Element Functions.
11. Let resolveElement be ! CreateBuiltinFunction(steps, « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »).
12. Let alreadyCalled be the Record { [[Value]]: false }.
13. Set resolveElement.[[AlreadyCalled]] to alreadyCalled.
14. Set resolveElement.[[Index]] to index.
15. Set resolveElement.[[Values]] to values.
16. Set resolveElement.[[Capability]] to resultCapability.
17. Set resolveElement.[[RemainingElements]] to remainingElementsCount.
18. Let rejectSteps be the algorithm steps defined in Promise.allSettled Reject Element Functions.
19. Let rejectElement be ! CreateBuiltinFunction(rejectSteps, « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »).
20. Set rejectElement.[[AlreadyCalled]] to alreadyCalled.
21. Set rejectElement.[[Index]] to index.
22. Set rejectElement.[[Values]] to values.
23. Set rejectElement.[[Capability]] to resultCapability.
24. Set rejectElement.[[RemainingElements]] to remainingElementsCount.
25. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1.
26. Perform ? Invoke(nextPromise, "then", « resolveElement, rejectElement »).
27. Set index to index + 1.

25.6.4.2.2 `Promise.allSettled` Resolve Element Functions

A Promise.allSettled resolve element function is an anonymous built-in function that is used to resolve a specific Promise.allSettled element. Each Promise.allSettled resolve element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.

When a Promise.allSettled resolve element function is called with argument x, the following steps are taken:

Let F be the active function object.
Let alreadyCalled be F.[[AlreadyCalled]].
If alreadyCalled.[[Value]] is true, return undefined.
Set alreadyCalled.[[Value]] to true.
Let index be F.[[Index]].
Let values be F.[[Values]].
Let promiseCapability be F.[[Capability]].
Let remainingElementsCount be F.[[RemainingElements]].
Let obj be ! OrdinaryObjectCreate(%Object.prototype%).
Perform ! CreateDataPropertyOrThrow(obj, "status", "fulfilled").
Perform ! CreateDataPropertyOrThrow(obj, "value", x).
Set values[index] to obj.
Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
If remainingElementsCount.[[Value]] is 0, then
1. Let valuesArray be ! CreateArrayFromList(values).
2. Return ? Call(promiseCapability.[[Resolve]], undefined, « valuesArray »).
Return undefined.

The "length" property of a Promise.allSettled resolve element function is 1.

25.6.4.2.3 `Promise.allSettled` Reject Element Functions

A Promise.allSettled reject element function is an anonymous built-in function that is used to reject a specific Promise.allSettled element. Each Promise.allSettled reject element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.

When a Promise.allSettled reject element function is called with argument x, the following steps are taken:

Let F be the active function object.
Let alreadyCalled be F.[[AlreadyCalled]].
If alreadyCalled.[[Value]] is true, return undefined.
Set alreadyCalled.[[Value]] to true.
Let index be F.[[Index]].
Let values be F.[[Values]].
Let promiseCapability be F.[[Capability]].
Let remainingElementsCount be F.[[RemainingElements]].
Let obj be ! OrdinaryObjectCreate(%Object.prototype%).
Perform ! CreateDataPropertyOrThrow(obj, "status", "rejected").
Perform ! CreateDataPropertyOrThrow(obj, "reason", x).
Set values[index] to obj.
Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
If remainingElementsCount.[[Value]] is 0, then
1. Let valuesArray be ! CreateArrayFromList(values).
2. Return ? Call(promiseCapability.[[Resolve]], undefined, « valuesArray »).
Return undefined.

The "length" property of a Promise.allSettled reject element function is 1.

25.6.4.3 Promise.prototype

The initial value of Promise.prototype is %Promise.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

25.6.4.4 Promise.race ( `iterable` )

The race function returns a new promise which is settled in the same way as the first passed promise to settle. It resolves all elements of the passed iterable to promises as it runs this algorithm.

Let C be the this value.
Let promiseCapability be ? NewPromiseCapability(C).
Let iteratorRecord be GetIterator(iterable).
IfAbruptRejectPromise(iteratorRecord, promiseCapability).
Let result be PerformPromiseRace(iteratorRecord, C, promiseCapability).
If result is an abrupt completion, then
1. If iteratorRecord.[[Done]] is false, set result to IteratorClose(iteratorRecord, result).
2. IfAbruptRejectPromise(result, promiseCapability).
Return Completion(result).

Note 1

If the iterable argument is empty or if none of the promises in iterable ever settle then the pending promise returned by this method will never be settled.

Note 2

The race function expects its this value to be a constructor function that supports the parameter conventions of the Promise constructor. It also expects that its this value provides a resolve method.

25.6.4.4.1 Runtime Semantics: PerformPromiseRace ( `iteratorRecord`, `constructor`, `resultCapability` )

The abstract operation PerformPromiseRace takes arguments iteratorRecord, constructor, and resultCapability. It performs the following steps when called:

Assert: IsConstructor(constructor) is true.
Assert: resultCapability is a PromiseCapability Record.
Let promiseResolve be ? Get(constructor, "resolve").
If ! IsCallable(promiseResolve) is false, throw a TypeError exception.
Repeat,
1. Let next be IteratorStep(iteratorRecord).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, then
  1. Set iteratorRecord.[[Done]] to true.
  2. Return resultCapability.[[Promise]].
5. Let nextValue be IteratorValue(next).
6. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
7. ReturnIfAbrupt(nextValue).
8. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue »).
9. Perform ? Invoke(nextPromise, "then", « resultCapability.[[Resolve]], resultCapability.[[Reject]] »).

25.6.4.5 Promise.reject ( `r` )

The reject function returns a new promise rejected with the passed argument.

Let C be the this value.
Let promiseCapability be ? NewPromiseCapability(C).
Perform ? Call(promiseCapability.[[Reject]], undefined, « r »).
Return promiseCapability.[[Promise]].

This function is the %Promise_reject% intrinsic object.

Note

The reject function expects its this value to be a constructor function that supports the parameter conventions of the Promise constructor.

25.6.4.6 Promise.resolve ( `x` )

The resolve function returns either a new promise resolved with the passed argument, or the argument itself if the argument is a promise produced by this constructor.

Let C be the this value.
If Type(C) is not Object, throw a TypeError exception.
Return ? PromiseResolve(C, x).

This function is the %Promise_resolve% intrinsic object.

Note

The resolve function expects its this value to be a constructor function that supports the parameter conventions of the Promise constructor.

25.6.4.6.1 PromiseResolve ( `C`, `x` )

The abstract operation PromiseResolve takes arguments C (a constructor) and x (an ECMAScript language value). It returns a new promise resolved with x. It performs the following steps when called:

Assert: Type(C) is Object.
If IsPromise(x) is true, then
1. Let xConstructor be ? Get(x, "constructor").
2. If SameValue(xConstructor, C) is true, return x.
Let promiseCapability be ? NewPromiseCapability(C).
Perform ? Call(promiseCapability.[[Resolve]], undefined, « x »).
Return promiseCapability.[[Promise]].

25.6.4.7 get Promise [ @@species ]

Promise[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

Note

Promise prototype methods normally use their this object's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.

25.6.5 Properties of the Promise Prototype Object

The Promise prototype object:

is the intrinsic object %PromisePrototype%.
has a [[Prototype]] internal slot whose value is %Object.prototype%.
is an ordinary object.
does not have a [[PromiseState]] internal slot or any of the other internal slots of Promise instances.

25.6.5.1 Promise.prototype.catch ( `onRejected` )

When the catch method is called with argument onRejected, the following steps are taken:

Let promise be the this value.
Return ? Invoke(promise, "then", « undefined, onRejected »).

25.6.5.2 Promise.prototype.constructor

The initial value of Promise.prototype.constructor is %Promise%.

25.6.5.3 Promise.prototype.finally ( `onFinally` )

When the finally method is called with argument onFinally, the following steps are taken:

Let promise be the this value.
If Type(promise) is not Object, throw a TypeError exception.
Let C be ? SpeciesConstructor(promise, %Promise%).
Assert: IsConstructor(C) is true.
If IsCallable(onFinally) is false, then
1. Let thenFinally be onFinally.
2. Let catchFinally be onFinally.
Else,
1. Let stepsThenFinally be the algorithm steps defined in Then Finally Functions.
2. Let thenFinally be ! CreateBuiltinFunction(stepsThenFinally, « [[Constructor]], [[OnFinally]] »).
3. Set thenFinally.[[Constructor]] to C.
4. Set thenFinally.[[OnFinally]] to onFinally.
5. Let stepsCatchFinally be the algorithm steps defined in Catch Finally Functions.
6. Let catchFinally be ! CreateBuiltinFunction(stepsCatchFinally, « [[Constructor]], [[OnFinally]] »).
7. Set catchFinally.[[Constructor]] to C.
8. Set catchFinally.[[OnFinally]] to onFinally.
Return ? Invoke(promise, "then", « thenFinally, catchFinally »).

25.6.5.3.1 Then Finally Functions

A Then Finally function is an anonymous built-in function that has a [[Constructor]] and an [[OnFinally]] internal slot. The value of the [[Constructor]] internal slot is a Promise-like constructor function object, and the value of the [[OnFinally]] internal slot is a function object.

When a Then Finally function is called with argument value, the following steps are taken:

Let F be the active function object.
Let onFinally be F.[[OnFinally]].
Assert: IsCallable(onFinally) is true.
Let result be ? Call(onFinally, undefined).
Let C be F.[[Constructor]].
Assert: IsConstructor(C) is true.
Let promise be ? PromiseResolve(C, result).
Let valueThunk be equivalent to a function that returns value.
Return ? Invoke(promise, "then", « valueThunk »).

The "length" property of a Then Finally function is 1.

25.6.5.3.2 Catch Finally Functions

A Catch Finally function is an anonymous built-in function that has a [[Constructor]] and an [[OnFinally]] internal slot. The value of the [[Constructor]] internal slot is a Promise-like constructor function object, and the value of the [[OnFinally]] internal slot is a function object.

When a Catch Finally function is called with argument reason, the following steps are taken:

Let F be the active function object.
Let onFinally be F.[[OnFinally]].
Assert: IsCallable(onFinally) is true.
Let result be ? Call(onFinally, undefined).
Let C be F.[[Constructor]].
Assert: IsConstructor(C) is true.
Let promise be ? PromiseResolve(C, result).
Let thrower be equivalent to a function that throws reason.
Return ? Invoke(promise, "then", « thrower »).

The "length" property of a Catch Finally function is 1.

25.6.5.4 Promise.prototype.then ( `onFulfilled`, `onRejected` )

When the then method is called with arguments onFulfilled and onRejected, the following steps are taken:

Let promise be the this value.
If IsPromise(promise) is false, throw a TypeError exception.
Let C be ? SpeciesConstructor(promise, %Promise%).
Let resultCapability be ? NewPromiseCapability(C).
Return PerformPromiseThen(promise, onFulfilled, onRejected, resultCapability).

This function is the %PromiseProto_then% intrinsic object.

25.6.5.4.1 PerformPromiseThen ( `promise`, `onFulfilled`, `onRejected` [ , `resultCapability` ] )

The abstract operation PerformPromiseThen takes arguments promise, onFulfilled, and onRejected and optional argument resultCapability. It performs the “then” operation on promise using onFulfilled and onRejected as its settlement actions. If resultCapability is passed, the result is stored by updating resultCapability's promise. If it is not passed, then PerformPromiseThen is being called by a specification-internal operation where the result does not matter. It performs the following steps when called:

Assert: IsPromise(promise) is true.
If resultCapability is present, then
1. Assert: resultCapability is a PromiseCapability Record.
Else,
1. Set resultCapability to undefined.
If IsCallable(onFulfilled) is false, then
1. Set onFulfilled to undefined.
If IsCallable(onRejected) is false, then
1. Set onRejected to undefined.
Let fulfillReaction be the PromiseReaction { [[Capability]]: resultCapability, [[Type]]: Fulfill, [[Handler]]: onFulfilled }.
Let rejectReaction be the PromiseReaction { [[Capability]]: resultCapability, [[Type]]: Reject, [[Handler]]: onRejected }.
If promise.[[PromiseState]] is pending, then
1. Append fulfillReaction as the last element of the List that is promise.[[PromiseFulfillReactions]].
2. Append rejectReaction as the last element of the List that is promise.[[PromiseRejectReactions]].
Else if promise.[[PromiseState]] is fulfilled, then
1. Let value be promise.[[PromiseResult]].
2. Let fulfillJob be NewPromiseReactionJob(fulfillReaction, value).
3. Perform HostEnqueuePromiseJob(fulfillJob.[[Job]], fulfillJob.[[Realm]]).
Else,
1. Assert: The value of promise.[[PromiseState]] is rejected.
2. Let reason be promise.[[PromiseResult]].
3. If promise.[[PromiseIsHandled]] is false, perform HostPromiseRejectionTracker(promise, "handle").
4. Let rejectJob be NewPromiseReactionJob(rejectReaction, reason).
5. Perform HostEnqueuePromiseJob(rejectJob.[[Job]], rejectJob.[[Realm]]).
Set promise.[[PromiseIsHandled]] to true.
If resultCapability is undefined, then
1. Return undefined.
Else,
1. Return resultCapability.[[Promise]].

25.6.5.5 Promise.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Promise".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.6.6 Properties of Promise Instances

Promise instances are ordinary objects that inherit properties from the Promise prototype object (the intrinsic, %Promise.prototype%). Promise instances are initially created with the internal slots described in Table 78.

Table 78: Internal Slots of Promise Instances

Internal Slot	Description
[[PromiseState]]	One of pending, fulfilled, or rejected. Governs how a promise will react to incoming calls to its `then` method.
[[PromiseResult]]	The value with which the promise has been fulfilled or rejected, if any. Only meaningful if [[PromiseState]] is not pending.
[[PromiseFulfillReactions]]	A List of PromiseReaction records to be processed when/if the promise transitions from the pending state to the fulfilled state.
[[PromiseRejectReactions]]	A List of PromiseReaction records to be processed when/if the promise transitions from the pending state to the rejected state.
[[PromiseIsHandled]]	A boolean indicating whether the promise has ever had a fulfillment or rejection handler; used in unhandled rejection tracking.

25.7 AsyncFunction Objects

AsyncFunction objects are functions that are usually created by evaluating AsyncFunctionDeclarations, AsyncFunctionExpressions, AsyncMethods, and AsyncArrowFunctions. They may also be created by calling the %AsyncFunction% intrinsic.

25.7.1 The AsyncFunction Constructor

The AsyncFunction constructor:

is the intrinsic object %AsyncFunction%.
is a subclass of Function.
creates and initializes a new AsyncFunction object when called as a function rather than as a constructor. Thus the function call AsyncFunction(…) is equivalent to the object creation expression new AsyncFunction(…) with the same arguments.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified AsyncFunction behaviour must include a super call to the AsyncFunction constructor to create and initialize a subclass instance with the internal slots necessary for built-in async function behaviour. All ECMAScript syntactic forms for defining async function objects create direct instances of AsyncFunction. There is no syntactic means to create instances of AsyncFunction subclasses.

25.7.1.1 AsyncFunction ( `p1`, `p2`, … , `pn`, `body` )

The last argument specifies the body (executable code) of an async function. Any preceding arguments specify formal parameters.

When the AsyncFunction function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no p arguments, and where body might also not be provided), the following steps are taken:

Let C be the active function object.
Let args be the argumentsList that was passed to this function by [[Call]] or [[Construct]].
Return CreateDynamicFunction(C, NewTarget, async, args).

Note

See NOTE for 19.2.1.1.

25.7.2 Properties of the AsyncFunction Constructor

The AsyncFunction constructor:

is a standard built-in function object that inherits from the Function constructor.
has a [[Prototype]] internal slot whose value is %Function%.
has a "name" property whose value is "AsyncFunction".
has the following properties:

25.7.2.1 AsyncFunction.length

This is a data property with a value of 1. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.7.2.2 AsyncFunction.prototype

The initial value of AsyncFunction.prototype is %AsyncFunction.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

25.7.3 Properties of the AsyncFunction Prototype Object

The AsyncFunction prototype object:

is an ordinary object.
is not a function object and does not have an [[ECMAScriptCode]] internal slot or any other of the internal slots listed in Table 27.
is the value of the "prototype" property of %AsyncFunction%.
is the intrinsic object %AsyncFunctionPrototype%.
has a [[Prototype]] internal slot whose value is %Function.prototype%.

25.7.3.1 AsyncFunction.prototype.constructor

The initial value of AsyncFunction.prototype.constructor is %AsyncFunction%

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.7.3.2 AsyncFunction.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "AsyncFunction".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.7.4 AsyncFunction Instances

Every AsyncFunction instance is an ECMAScript function object and has the internal slots listed in Table 27. The value of the [[IsClassConstructor]] internal slot for all such instances is false. AsyncFunction instances are not constructors and do not have a [[Construct]] internal method. AsyncFunction instances do not have a prototype property as they are not constructible.

Each AsyncFunction instance has the following own properties:

25.7.4.1 length

The specification for the "length" property of Function instances given in 19.2.4.1 also applies to AsyncFunction instances.

25.7.4.2 name

The specification for the "name" property of Function instances given in 19.2.4.2 also applies to AsyncFunction instances.

25.7.5 Async Functions Abstract Operations

25.7.5.1 AsyncFunctionStart ( `promiseCapability`, `asyncFunctionBody` )

The abstract operation AsyncFunctionStart takes arguments promiseCapability and asyncFunctionBody. It performs the following steps when called:

Let runningContext be the running execution context.
Let asyncContext be a copy of runningContext.
NOTE: Copying the execution state is required for the step below to resume its execution. It is ill-defined to resume a currently executing context.
Set the code evaluation state of asyncContext such that when evaluation is resumed for that execution context the following steps will be performed:
1. Let result be the result of evaluating asyncFunctionBody.
2. Assert: If we return here, the async function either threw an exception or performed an implicit or explicit return; all awaiting is done.
3. Remove asyncContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
4. If result.[[Type]] is normal, then
  1. Perform ! Call(promiseCapability.[[Resolve]], undefined, « undefined »).
5. Else if result.[[Type]] is return, then
  1. Perform ! Call(promiseCapability.[[Resolve]], undefined, « result.[[Value]] »).
6. Else,
  1. Assert: result.[[Type]] is throw.
  2. Perform ! Call(promiseCapability.[[Reject]], undefined, « result.[[Value]] »).
7. Return.
Push asyncContext onto the execution context stack; asyncContext is now the running execution context.
Resume the suspended evaluation of asyncContext. Let result be the value returned by the resumed computation.
Assert: When we return here, asyncContext has already been removed from the execution context stack and runningContext is the currently running execution context.
Assert: result is a normal completion with a value of undefined. The possible sources of completion values are Await or, if the async function doesn't await anything, the step 4.g above.
Return.

26 Reflection

26.1 The Reflect Object

The Reflect object:

is the intrinsic object %Reflect%.
is the initial value of the "Reflect" property of the global object.
is an ordinary object.
has a [[Prototype]] internal slot whose value is %Object.prototype%.
is not a function object.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.

26.1.1 Reflect.apply ( `target`, `thisArgument`, `argumentsList` )

When the apply function is called with arguments target, thisArgument, and argumentsList, the following steps are taken:

If IsCallable(target) is false, throw a TypeError exception.
Let args be ? CreateListFromArrayLike(argumentsList).
Perform PrepareForTailCall().
Return ? Call(target, thisArgument, args).

26.1.2 Reflect.construct ( `target`, `argumentsList` [ , `newTarget` ] )

When the construct function is called with arguments target, argumentsList, and newTarget, the following steps are taken:

If IsConstructor(target) is false, throw a TypeError exception.
If newTarget is not present, set newTarget to target.
Else if IsConstructor(newTarget) is false, throw a TypeError exception.
Let args be ? CreateListFromArrayLike(argumentsList).
Return ? Construct(target, args, newTarget).

26.1.3 Reflect.defineProperty ( `target`, `propertyKey`, `attributes` )

When the defineProperty function is called with arguments target, propertyKey, and attributes, the following steps are taken:

If Type(target) is not Object, throw a TypeError exception.
Let key be ? ToPropertyKey(propertyKey).
Let desc be ? ToPropertyDescriptor(attributes).
Return ? target.[[DefineOwnProperty]](key, desc).

26.1.4 Reflect.deleteProperty ( `target`, `propertyKey` )

When the deleteProperty function is called with arguments target and propertyKey, the following steps are taken:

If Type(target) is not Object, throw a TypeError exception.
Let key be ? ToPropertyKey(propertyKey).
Return ? target.[[Delete]](key).

26.1.5 Reflect.get ( `target`, `propertyKey` [ , `receiver` ] )

When the get function is called with arguments target, propertyKey, and receiver, the following steps are taken:

If Type(target) is not Object, throw a TypeError exception.
Let key be ? ToPropertyKey(propertyKey).
If receiver is not present, then
1. Set receiver to target.
Return ? target.[[Get]](key, receiver).

26.1.6 Reflect.getOwnPropertyDescriptor ( `target`, `propertyKey` )

When the getOwnPropertyDescriptor function is called with arguments target and propertyKey, the following steps are taken:

If Type(target) is not Object, throw a TypeError exception.
Let key be ? ToPropertyKey(propertyKey).
Let desc be ? target.[[GetOwnProperty]](key).
Return FromPropertyDescriptor(desc).

26.1.7 Reflect.getPrototypeOf ( `target` )

When the getPrototypeOf function is called with argument target, the following steps are taken:

If Type(target) is not Object, throw a TypeError exception.
Return ? target.[[GetPrototypeOf]]().

26.1.8 Reflect.has ( `target`, `propertyKey` )

When the has function is called with arguments target and propertyKey, the following steps are taken:

If Type(target) is not Object, throw a TypeError exception.
Let key be ? ToPropertyKey(propertyKey).
Return ? target.[[HasProperty]](key).

26.1.9 Reflect.isExtensible ( `target` )

When the isExtensible function is called with argument target, the following steps are taken:

If Type(target) is not Object, throw a TypeError exception.
Return ? target.[[IsExtensible]]().

26.1.10 Reflect.ownKeys ( `target` )

When the ownKeys function is called with argument target, the following steps are taken:

If Type(target) is not Object, throw a TypeError exception.
Let keys be ? target.[[OwnPropertyKeys]]().
Return CreateArrayFromList(keys).

26.1.11 Reflect.preventExtensions ( `target` )

When the preventExtensions function is called with argument target, the following steps are taken:

If Type(target) is not Object, throw a TypeError exception.
Return ? target.[[PreventExtensions]]().

26.1.12 Reflect.set ( `target`, `propertyKey`, `V` [ , `receiver` ] )

When the set function is called with arguments target, V, propertyKey, and receiver, the following steps are taken:

If Type(target) is not Object, throw a TypeError exception.
Let key be ? ToPropertyKey(propertyKey).
If receiver is not present, then
1. Set receiver to target.
Return ? target.[[Set]](key, V, receiver).

26.1.13 Reflect.setPrototypeOf ( `target`, `proto` )

When the setPrototypeOf function is called with arguments target and proto, the following steps are taken:

If Type(target) is not Object, throw a TypeError exception.
If Type(proto) is not Object and proto is not null, throw a TypeError exception.
Return ? target.[[SetPrototypeOf]](proto).

26.2 Proxy Objects

26.2.1 The Proxy Constructor

The Proxy constructor:

is the intrinsic object %Proxy%.
is the initial value of the "Proxy" property of the global object.
creates and initializes a new proxy exotic object when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.

26.2.1.1 Proxy ( `target`, `handler` )

When Proxy is called with arguments target and handler, it performs the following steps:

If NewTarget is undefined, throw a TypeError exception.
Return ? ProxyCreate(target, handler).

26.2.2 Properties of the Proxy Constructor

The Proxy constructor:

has a [[Prototype]] internal slot whose value is %Function.prototype%.
does not have a "prototype" property because proxy exotic objects do not have a [[Prototype]] internal slot that requires initialization.
has the following properties:

26.2.2.1 Proxy.revocable ( `target`, `handler` )

The Proxy.revocable function is used to create a revocable Proxy object. When Proxy.revocable is called with arguments target and handler, the following steps are taken:

Let p be ? ProxyCreate(target, handler).
Let steps be the algorithm steps defined in Proxy Revocation Functions.
Let revoker be ! CreateBuiltinFunction(steps, « [[RevocableProxy]] »).
Set revoker.[[RevocableProxy]] to p.
Let result be OrdinaryObjectCreate(%Object.prototype%).
Perform ! CreateDataPropertyOrThrow(result, "proxy", p).
Perform ! CreateDataPropertyOrThrow(result, "revoke", revoker).
Return result.

26.2.2.1.1 Proxy Revocation Functions

A Proxy revocation function is an anonymous built-in function that has the ability to invalidate a specific Proxy object.

Each Proxy revocation function has a [[RevocableProxy]] internal slot.

When a Proxy revocation function is called, the following steps are taken:

Let F be the active function object.
Let p be F.[[RevocableProxy]].
If p is null, return undefined.
Set F.[[RevocableProxy]] to null.
Assert: p is a Proxy object.
Set p.[[ProxyTarget]] to null.
Set p.[[ProxyHandler]] to null.
Return undefined.

The "length" property of a Proxy revocation function is 0.

26.3 Module Namespace Objects

A Module Namespace Object is a module namespace exotic object that provides runtime property-based access to a module's exported bindings. There is no constructor function for Module Namespace Objects. Instead, such an object is created for each module that is imported by an ImportDeclaration that includes a NameSpaceImport.

In addition to the properties specified in 9.4.6 each Module Namespace Object has the following own property:

26.3.1 @@toStringTag

The initial value of the @@toStringTag property is the String value "Module".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

27 Memory Model

The memory consistency model, or memory model, specifies the possible orderings of Shared Data Block events, arising via accessing TypedArray instances backed by a SharedArrayBuffer and via methods on the Atomics object. When the program has no data races (defined below), the ordering of events appears as sequentially consistent, i.e., as an interleaving of actions from each agent. When the program has data races, shared memory operations may appear sequentially inconsistent. For example, programs may exhibit causality-violating behaviour and other astonishments. These astonishments arise from compiler transforms and the design of CPUs (e.g., out-of-order execution and speculation). The memory model defines both the precise conditions under which a program exhibits sequentially consistent behaviour as well as the possible values read from data races. To wit, there is no undefined behaviour.

The memory model is defined as relational constraints on events introduced by abstract operations on SharedArrayBuffer or by methods on the Atomics object during an evaluation.

Note

This section provides an axiomatic model on events introduced by the abstract operations on SharedArrayBuffers. It bears stressing that the model is not expressible algorithmically, unlike the rest of this specification. The nondeterministic introduction of events by abstract operations is the interface between the operational semantics of ECMAScript evaluation and the axiomatic semantics of the memory model. The semantics of these events is defined by considering graphs of all events in an evaluation. These are neither Static Semantics nor Runtime Semantics. There is no demonstrated algorithmic implementation, but instead a set of constraints that determine if a particular event graph is allowed or disallowed.

27.1 Memory Model Fundamentals

Shared memory accesses (reads and writes) are divided into two groups, atomic accesses and data accesses, defined below. Atomic accesses are sequentially consistent, i.e., there is a strict total ordering of events agreed upon by all agents in an agent cluster. Non-atomic accesses do not have a strict total ordering agreed upon by all agents, i.e., unordered.

Note 1

No orderings weaker than sequentially consistent and stronger than unordered, such as release-acquire, are supported.

A Shared Data Block event is either a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory Record.

Table 79: ReadSharedMemory Event Fields

Field Name	Value	Meaning
[[Order]]	SeqCst \| Unordered	The weakest ordering guaranteed by the memory model for the event.
[[NoTear]]	A Boolean	Whether this event is allowed to read from multiple write events on equal range as this event.
[[Block]]	A Shared Data Block	The block the event operates on.
[[ByteIndex]]	A nonnegative integer	The byte address of the read in [[Block]].
[[ElementSize]]	A nonnegative integer	The size of the read.

Table 80: WriteSharedMemory Event Fields

Field Name	Value	Meaning
[[Order]]	SeqCst \| Unordered \| Init	The weakest ordering guaranteed by the memory model for the event.
[[NoTear]]	A Boolean	Whether this event is allowed to be read from multiple read events with equal range as this event.
[[Block]]	A Shared Data Block	The block the event operates on.
[[ByteIndex]]	A nonnegative integer	The byte address of the write in [[Block]].
[[ElementSize]]	A nonnegative integer	The size of the write.
[[Payload]]	A List	The List of byte values to be read by other events.

Table 81: ReadModifyWriteSharedMemory Event Fields

Field Name	Value	Meaning
[[Order]]	SeqCst	Read-modify-write events are always sequentially consistent.
[[NoTear]]	true	Read-modify-write events cannot tear.
[[Block]]	A Shared Data Block	The block the event operates on.
[[ByteIndex]]	A nonnegative integer	The byte address of the read-modify-write in [[Block]].
[[ElementSize]]	A nonnegative integer	The size of the read-modify-write.
[[Payload]]	A List	The List of byte values to be passed to [[ModifyOp]].
[[ModifyOp]]	A semantic function	A pure semantic function that returns a modified List of byte values from a read List of byte values and [[Payload]].

These events are introduced by abstract operations or by methods on the Atomics object.

Some operations may also introduce Synchronize events. A Synchronize event has no fields, and exists purely to directly constrain the permitted orderings of other events.

In addition to Shared Data Block and Synchronize events, there are host-specific events.

Let the range of a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory event be the Set of contiguous integers from its [[ByteIndex]] to [[ByteIndex]] + [[ElementSize]] - 1. Two events' ranges are equal when the events have the same [[Block]], and the ranges are element-wise equal. Two events' ranges are overlapping when the events have the same [[Block]], the ranges are not equal and their intersection is non-empty. Two events' ranges are disjoint when the events do not have the same [[Block]] or their ranges are neither equal nor overlapping.

Note 2

Examples of host-specific synchronizing events that should be accounted for are: sending a SharedArrayBuffer from one agent to another (e.g., by postMessage in a browser), starting and stopping agents, and communicating within the agent cluster via channels other than shared memory. It is assumed those events are appended to agent-order during evaluation like the other SharedArrayBuffer events.

Events are ordered within candidate executions by the relations defined below.

27.2 Agent Events Records

An Agent Events Record is a Record with the following fields.

Table 82: Agent Events Record Fields

Field Name	Value	Meaning
[[AgentSignifier]]	A value that admits equality testing	The agent whose evaluation resulted in this ordering.
[[EventList]]	A List of events	Events are appended to the list during evaluation.
[[AgentSynchronizesWith]]	A List of pairs of Synchronize events	Synchronize relationships introduced by the operational semantics.

27.3 Chosen Value Records

A Chosen Value Record is a Record with the following fields.

Table 83: Chosen Value Record Fields

Field Name	Value	Meaning
[[Event]]	A Shared Data Block event	The ReadSharedMemory or ReadModifyWriteSharedMemory event that was introduced for this chosen value.
[[ChosenValue]]	A List of byte values	The bytes that were nondeterministically chosen during evaluation.

27.4 Candidate Executions

A candidate execution of the evaluation of an agent cluster is a Record with the following fields.

Table 84: Candidate Execution Record Fields

Field Name	Value	Meaning
[[EventsRecords]]	A List of Agent Events Records.	Maps an agent to Lists of events appended during the evaluation.
[[ChosenValues]]	A List of Chosen Value Records.	Maps ReadSharedMemory or ReadModifyWriteSharedMemory events to the List of byte values chosen during the evaluation.
[[AgentOrder]]	An agent-order Relation.	Defined below.
[[ReadsBytesFrom]]	A reads-bytes-from semantic function.	Defined below.
[[ReadsFrom]]	A reads-from Relation.	Defined below.
[[HostSynchronizesWith]]	A host-synchronizes-with Relation.	Defined below.
[[SynchronizesWith]]	A synchronizes-with Relation.	Defined below.
[[HappensBefore]]	A happens-before Relation.	Defined below.

An empty candidate execution is a candidate execution Record whose fields are empty Lists and Relations.

27.5 Abstract Operations for the Memory Model

27.5.1 EventSet ( `execution` )

The abstract operation EventSet takes argument execution (a candidate execution). It performs the following steps when called:

Let events be an empty Set.
For each Agent Events Record aer in execution.[[EventsRecords]], do
1. For each event E in aer.[[EventList]], do
  1. Add E to events.
Return events.

27.5.2 SharedDataBlockEventSet ( `execution` )

The abstract operation SharedDataBlockEventSet takes argument execution (a candidate execution). It performs the following steps when called:

Let events be an empty Set.
For each event E in EventSet(execution), do
1. If E is a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory event, add E to events.
Return events.

27.5.3 HostEventSet ( `execution` )

The abstract operation HostEventSet takes argument execution (a candidate execution). It performs the following steps when called:

Let events be an empty Set.
For each event E in EventSet(execution), do
1. If E is not in SharedDataBlockEventSet(execution), add E to events.
Return events.

27.5.4 ComposeWriteEventBytes ( `execution`, `byteIndex`, `Ws` )

The abstract operation ComposeWriteEventBytes takes arguments execution (a candidate execution), byteIndex (a non-negative integer), and Ws (a List of WriteSharedMemory or ReadModifyWriteSharedMemory events). It performs the following steps when called:

Let byteLocation be byteIndex.
Let bytesRead be a new empty List.
For each element W of Ws in List order, do
1. Assert: W has byteLocation in its range.
2. Let payloadIndex be byteLocation - W.[[ByteIndex]].
3. If W is a WriteSharedMemory event, then
  1. Let byte be W.[[Payload]][payloadIndex].
4. Else,
  1. Assert: W is a ReadModifyWriteSharedMemory event.
  2. Let bytes be ValueOfReadEvent(execution, W).
  3. Let bytesModified be W.[[ModifyOp]](bytes, W.[[Payload]]).
  4. Let byte be bytesModified[payloadIndex].
5. Append byte to bytesRead.
6. Set byteLocation to byteLocation + 1.
Return bytesRead.

Note 1

The semantic function [[ModifyOp]] is given by the function properties on the Atomics object that introduce ReadModifyWriteSharedMemory events.

Note 2

This abstract operation composes a List of write events into a List of byte values. It is used in the event semantics of ReadSharedMemory and ReadModifyWriteSharedMemory events.

27.5.5 ValueOfReadEvent ( `execution`, `R` )

The abstract operation ValueOfReadEvent takes arguments execution (a candidate execution) and R (a ReadSharedMemory or ReadModifyWriteSharedMemory event). It performs the following steps when called:

Assert: R is a ReadSharedMemory or ReadModifyWriteSharedMemory event.
Let Ws be execution.[[ReadsBytesFrom]](R).
Assert: Ws is a List of WriteSharedMemory or ReadModifyWriteSharedMemory events with length equal to R.[[ElementSize]].
Return ComposeWriteEventBytes(execution, R.[[ByteIndex]], Ws).

27.6 Relations of Candidate Executions

27.6.1 agent-order

For a candidate execution execution, execution.[[AgentOrder]] is a Relation on events that satisfies the following.

For each pair (E, D) in EventSet(execution), (E, D) is in execution.[[AgentOrder]] if there is some Agent Events Record aer in execution.[[EventsRecords]] such that E and D are in aer.[[EventList]] and E is before D in List order of aer.[[EventList]].

Note

Each agent introduces events in a per-agent strict total order during the evaluation. This is the union of those strict total orders.

27.6.2 reads-bytes-from

For a candidate execution execution, execution.[[ReadsBytesFrom]] is a semantic function from events in SharedDataBlockEventSet(execution) to Lists of events in SharedDataBlockEventSet(execution) that satisfies the following conditions.

For each ReadSharedMemory or ReadModifyWriteSharedMemory event R in SharedDataBlockEventSet(execution), execution.[[ReadsBytesFrom]](R) is a List of length equal to R.[[ElementSize]] of WriteSharedMemory or ReadModifyWriteSharedMemory events Ws such that all of the following are true.
- Each event W with index i in Ws has R.[[ByteIndex]] + i in its range.
- R is not in Ws.

27.6.3 reads-from

For a candidate execution execution, execution.[[ReadsFrom]] is the least Relation on events that satisfies the following.

For each pair (R, W) in SharedDataBlockEventSet(execution), (R, W) is in execution.[[ReadsFrom]] if W is in execution.[[ReadsBytesFrom]](R).

27.6.4 host-synchronizes-with

For a candidate execution execution, execution.[[HostSynchronizesWith]] is a host-provided strict partial order on host-specific events that satisfies at least the following.

If (E, D) is in execution.[[HostSynchronizesWith]], E and D are in HostEventSet(execution).
There is no cycle in the union of execution.[[HostSynchronizesWith]] and execution.[[AgentOrder]].

Note 1

For two host-specific events E and D, E host-synchronizes-with D implies E happens-before D.

Note 2

The host-synchronizes-with relation allows the host to provide additional synchronization mechanisms, such as postMessage between HTML workers.

27.6.5 synchronizes-with

For a candidate execution execution, execution.[[SynchronizesWith]] is the least Relation on events that satisfies the following.

For each pair (R, W) in execution.[[ReadsFrom]], (W, R) is in execution.[[SynchronizesWith]] if R.[[Order]] is SeqCst, W.[[Order]] is SeqCst, and R and W have equal ranges.
For each element eventsRecord of execution.[[EventsRecords]], the following is true.
- For each pair (S, Sw) in eventsRecord.[[AgentSynchronizesWith]], (S, Sw) is in execution.[[SynchronizesWith]].
For each pair (E, D) in execution.[[HostSynchronizesWith]], (E, D) is in execution.[[SynchronizesWith]].

Note 1

Owing to convention, write events synchronizes-with read events, instead of read events synchronizes-with write events.

Note 2

Init events do not participate in synchronizes-with, and are instead constrained directly by happens-before.

Note 3

Not all SeqCst events related by reads-from are related by synchronizes-with. Only events that also have equal ranges are related by synchronizes-with.

Note 4

For Shared Data Block events R and W such that W synchronizes-with R, R may reads-from other writes than W.

27.6.6 happens-before

For a candidate execution execution, execution.[[HappensBefore]] is the least Relation on events that satisfies the following.

For each pair (E, D) in execution.[[AgentOrder]], (E, D) is in execution.[[HappensBefore]].
For each pair (E, D) in execution.[[SynchronizesWith]], (E, D) is in execution.[[HappensBefore]].
For each pair (E, D) in SharedDataBlockEventSet(execution), (E, D) is in execution.[[HappensBefore]] if E.[[Order]] is Init and E and D have overlapping ranges.
For each pair (E, D) in EventSet(execution), (E, D) is in execution.[[HappensBefore]] if there is an event F such that the pairs (E, F) and (F, D) are in execution.[[HappensBefore]].

Note

Because happens-before is a superset of agent-order, candidate executions are consistent with the single-thread evaluation semantics of ECMAScript.

27.7 Properties of Valid Executions

27.7.1 Valid Chosen Reads

A candidate execution execution has valid chosen reads if the following abstract operation returns true.

For each ReadSharedMemory or ReadModifyWriteSharedMemory event R in SharedDataBlockEventSet(execution), do
1. Let chosenValueRecord be the element of execution.[[ChosenValues]] whose [[Event]] field is R.
2. Let chosenValue be chosenValueRecord.[[ChosenValue]].
3. Let readValue be ValueOfReadEvent(execution, R).
4. Let chosenLen be the number of elements of chosenValue.
5. Let readLen be the number of elements of readValue.
6. If chosenLen is not equal to readLen, then
  1. Return false.
7. If chosenValue[i] is not equal to readValue[i] for any integer value i in the range 0 through chosenLen, exclusive, then
  1. Return false.
Return true.

27.7.2 Coherent Reads

A candidate execution execution has coherent reads if the following abstract operation returns true.

For each ReadSharedMemory or ReadModifyWriteSharedMemory event R in SharedDataBlockEventSet(execution), do
1. Let Ws be execution.[[ReadsBytesFrom]](R).
2. Let byteLocation be R.[[ByteIndex]].
3. For each element W of Ws in List order, do
  1. If (R, W) is in execution.[[HappensBefore]], then
    1. Return false.
  2. If there is a WriteSharedMemory or ReadModifyWriteSharedMemory event V that has byteLocation in its range such that the pairs (W, V) and (V, R) are in execution.[[HappensBefore]], then
    1. Return false.
  3. Set byteLocation to byteLocation + 1.
Return true.

27.7.3 Tear Free Reads

A candidate execution execution has tear free reads if the following abstract operation returns true.

For each ReadSharedMemory or ReadModifyWriteSharedMemory event R in SharedDataBlockEventSet(execution), do
1. If R.[[NoTear]] is true, then
  1. Assert: The remainder of dividing R.[[ByteIndex]] by R.[[ElementSize]] is 0.
  2. For each event W such that (R, W) is in execution.[[ReadsFrom]] and W.[[NoTear]] is true, do
    1. If R and W have equal ranges, and there is an event V such that V and W have equal ranges, V.[[NoTear]] is true, W is not V, and (R, V) is in execution.[[ReadsFrom]], then
      1. Return false.
Return true.

Note

An event's [[NoTear]] field is true when that event was introduced via accessing an integer TypedArray, and false when introduced via accessing a floating point TypedArray or DataView.

Intuitively, this requirement says when a memory range is accessed in an aligned fashion via an integer TypedArray, a single write event on that range must "win" when in a data race with other write events with equal ranges. More precisely, this requirement says an aligned read event cannot read a value composed of bytes from multiple, different write events all with equal ranges. It is possible, however, for an aligned read event to read from multiple write events with overlapping ranges.

27.7.4 Sequentially Consistent Atomics

For a candidate execution execution, memory-order is a strict total order of all events in EventSet(execution) that satisfies the following.

For each pair (E, D) in execution.[[HappensBefore]], (E, D) is in memory-order.
For each pair (R, W) in execution.[[ReadsFrom]], there is no WriteSharedMemory or ReadModifyWriteSharedMemory event V in SharedDataBlockEventSet(execution) such that V.[[Order]] is SeqCst, the pairs (W, V) and (V, R) are in memory-order, and any of the following conditions are true.
- The pair (W, R) is in execution.[[SynchronizesWith]], and V and R have equal ranges.
- The pairs (W, R) and (V, R) are in execution.[[HappensBefore]], W.[[Order]] is SeqCst, and W and V have equal ranges.
- The pairs (W, R) and (W, V) are in execution.[[HappensBefore]], R.[[Order]] is SeqCst, and V and R have equal ranges.
Note 1

This clause additionally constrains SeqCst events on equal ranges.
For each WriteSharedMemory or ReadModifyWriteSharedMemory event W in SharedDataBlockEventSet(execution), if W.[[Order]] is SeqCst, then it is not the case that there is an infinite number of ReadSharedMemory or ReadModifyWriteSharedMemory events in SharedDataBlockEventSet(execution) with equal range that is memory-order before W.
Note 2

This clause together with the forward progress guarantee on agents ensure the liveness condition that SeqCst writes become visible to SeqCst reads with equal range in finite time.

A candidate execution has sequentially consistent atomics if a memory-order exists.

Note 3

While memory-order includes all events in EventSet(execution), those that are not constrained by happens-before or synchronizes-with are allowed to occur anywhere in the order.

27.7.5 Valid Executions

A candidate execution execution is a valid execution (or simply an execution) if all of the following are true.

The host provides a host-synchronizes-with Relation for execution.[[HostSynchronizesWith]].
execution.[[HappensBefore]] is a strict partial order.
execution has valid chosen reads.
execution has coherent reads.
execution has tear free reads.
execution has sequentially consistent atomics.

All programs have at least one valid execution.

27.8 Races

For an execution execution, two events E and D in SharedDataBlockEventSet(execution) are in a race if the following abstract operation returns true.

If E is not D, then
1. If the pairs (E, D) and (D, E) are not in execution.[[HappensBefore]], then
  1. If E and D are both WriteSharedMemory or ReadModifyWriteSharedMemory events and E and D do not have disjoint ranges, then
    1. Return true.
  2. If either (E, D) or (D, E) is in execution.[[ReadsFrom]], then
    1. Return true.
Return false.

27.9 Data Races

For an execution execution, two events E and D in SharedDataBlockEventSet(execution) are in a data race if the following abstract operation returns true.

If E and D are in a race in execution, then
1. If E.[[Order]] is not SeqCst or D.[[Order]] is not SeqCst, then
  1. Return true.
2. If E and D have overlapping ranges, then
  1. Return true.
Return false.

27.10 Data Race Freedom

An execution execution is data race free if there are no two events in SharedDataBlockEventSet(execution) that are in a data race.

A program is data race free if all its executions are data race free.

The memory model guarantees sequential consistency of all events for data race free programs.

27.11 Shared Memory Guidelines

Note 1

The following are guidelines for ECMAScript programmers working with shared memory.

We recommend programs be kept data race free, i.e., make it so that it is impossible for there to be concurrent non-atomic operations on the same memory location. Data race free programs have interleaving semantics where each step in the evaluation semantics of each agent are interleaved with each other. For data race free programs, it is not necessary to understand the details of the memory model. The details are unlikely to build intuition that will help one to better write ECMAScript.

More generally, even if a program is not data race free it may have predictable behaviour, so long as atomic operations are not involved in any data races and the operations that race all have the same access size. The simplest way to arrange for atomics not to be involved in races is to ensure that different memory cells are used by atomic and non-atomic operations and that atomic accesses of different sizes are not used to access the same cells at the same time. Effectively, the program should treat shared memory as strongly typed as much as possible. One still cannot depend on the ordering and timing of non-atomic accesses that race, but if memory is treated as strongly typed the racing accesses will not "tear" (bits of their values will not be mixed).

Note 2

The following are guidelines for ECMAScript implementers writing compiler transformations for programs using shared memory.

It is desirable to allow most program transformations that are valid in a single-agent setting in a multi-agent setting, to ensure that the performance of each agent in a multi-agent program is as good as it would be in a single-agent setting. Frequently these transformations are hard to judge. We outline some rules about program transformations that are intended to be taken as normative (in that they are implied by the memory model or stronger than what the memory model implies) but which are likely not exhaustive. These rules are intended to apply to program transformations that precede the introductions of the events that make up the agent-order.

Let an agent-order slice be the subset of the agent-order pertaining to a single agent.

Let possible read values of a read event be the set of all values of ValueOfReadEvent for that event across all valid executions.

Any transformation of an agent-order slice that is valid in the absence of shared memory is valid in the presence of shared memory, with the following exceptions.

Atomics are carved in stone: Program transformations must not cause the SeqCst events in an agent-order slice to be reordered with its Unordered operations, nor its SeqCst operations to be reordered with each other, nor may a program transformation remove a SeqCst operation from the agent-order.

(In practice, the prohibition on reorderings forces a compiler to assume that every SeqCst operation is a synchronization and included in the final memory-order, which it would usually have to assume anyway in the absence of inter-agent program analysis. It also forces the compiler to assume that every call where the callee's effects on the memory-order are unknown may contain SeqCst operations.)
Reads must be stable: Any given shared memory read must only observe a single value in an execution.

(For example, if what is semantically a single read in the program is executed multiple times then the program is subsequently allowed to observe only one of the values read. A transformation known as rematerialization can violate this rule.)
Writes must be stable: All observable writes to shared memory must follow from program semantics in an execution.

(For example, a transformation may not introduce certain observable writes, such as by using read-modify-write operations on a larger location to write a smaller datum, writing a value to memory that the program could not have written, or writing a just-read value back to the location it was read from, if that location could have been overwritten by another agent after the read.)
Possible read values must be nonempty: Program transformations cannot cause the possible read values of a shared memory read to become empty.

(Counterintuitively, this rule in effect restricts transformations on writes, because writes have force in memory model insofar as to be read by read events. For example, writes may be moved and coalesced and sometimes reordered between two SeqCst operations, but the transformation may not remove every write that updates a location; some write must be preserved.)

Examples of transformations that remain valid are: merging multiple non-atomic reads from the same location, reordering non-atomic reads, introducing speculative non-atomic reads, merging multiple non-atomic writes to the same location, reordering non-atomic writes to different locations, and hoisting non-atomic reads out of loops even if that affects termination. Note in general that aliased TypedArrays make it hard to prove that locations are different.

Note 3

The following are guidelines for ECMAScript implementers generating machine code for shared memory accesses.

For architectures with memory models no weaker than those of ARM or Power, non-atomic stores and loads may be compiled to bare stores and loads on the target architecture. Atomic stores and loads may be compiled down to instructions that guarantee sequential consistency. If no such instructions exist, memory barriers are to be employed, such as placing barriers on both sides of a bare store or load. Read-modify-write operations may be compiled to read-modify-write instructions on the target architecture, such as LOCK-prefixed instructions on x86, load-exclusive/store-exclusive instructions on ARM, and load-link/store-conditional instructions on Power.

Specifically, the memory model is intended to allow code generation as follows.

Every atomic operation in the program is assumed to be necessary.
Atomic operations are never rearranged with each other or with non-atomic operations.
Functions are always assumed to perform atomic operations.
Atomic operations are never implemented as read-modify-write operations on larger data, but as non-lock-free atomics if the platform does not have atomic operations of the appropriate size. (We already assume that every platform has normal memory access operations of every interesting size.)

Naive code generation uses these patterns:

Regular loads and stores compile to single load and store instructions.
Lock-free atomic loads and stores compile to a full (sequentially consistent) fence, a regular load or store, and a full fence.
Lock-free atomic read-modify-write accesses compile to a full fence, an atomic read-modify-write instruction sequence, and a full fence.
Non-lock-free atomics compile to a spinlock acquire, a full fence, a series of non-atomic load and store instructions, a full fence, and a spinlock release.

That mapping is correct so long as an atomic operation on an address range does not race with a non-atomic write or with an atomic operation of different size. However, that is all we need: the memory model effectively demotes the atomic operations involved in a race to non-atomic status. On the other hand, the naive mapping is quite strong: it allows atomic operations to be used as sequentially consistent fences, which the memory model does not actually guarantee.

A number of local improvements to those basic patterns are also intended to be legal:

There are obvious platform-dependent improvements that remove redundant fences. For example, on x86 the fences around lock-free atomic loads and stores can always be omitted except for the fence following a store, and no fence is needed for lock-free read-modify-write instructions, as these all use LOCK-prefixed instructions. On many platforms there are fences of several strengths, and weaker fences can be used in certain contexts without destroying sequential consistency.
Most modern platforms support lock-free atomics for all the data sizes required by ECMAScript atomics. Should non-lock-free atomics be needed, the fences surrounding the body of the atomic operation can usually be folded into the lock and unlock steps. The simplest solution for non-lock-free atomics is to have a single lock word per SharedArrayBuffer.
There are also more complicated platform-dependent local improvements, requiring some code analysis. For example, two back-to-back fences often have the same effect as a single fence, so if code is generated for two atomic operations in sequence, only a single fence need separate them. On x86, even a single fence separating atomic stores can be omitted, as the fence following a store is only needed to separate the store from a subsequent load.

A Grammar Summary

A.1 Lexical Grammar

any Unicode code point

InputElementRegExpOrTemplateTail

InputElementTemplateTail

WhiteSpace

<TAB>

<VT>

<FF>

<SP>

<NBSP>

<USP>

LineTerminator

<LF>

<CR>

<LS>

<PS>

<LF>

<CR>

[lookahead ≠ <LF>]

<LS>

<PS>

<CR>

<LF>

opt

opt

opt

opt

opt

but not *

but not one of / or *

SingleLineComment

opt

opt

but not LineTerminator

<ZWNJ>

<ZWJ>

any Unicode code point with the Unicode property “ID_Start”

any Unicode code point with the Unicode property “ID_Continue”

ReservedWord

one of

await

break

case

catch

class

const

continue

debugger

default

delete

else

enum

export

extends

false

finally

for

function

import

instanceof

new

null

return

super

switch

this

throw

true

try

typeof

var

void

while

with

yield

Punctuator

[lookahead ∉ DecimalDigit]

one of

{

(

)

[

]

...

;

===

!==

>>>

**=

<<=

>>=

>>>=

}

null

true

false

opt

opt

opt

opt

ExponentPart

opt

opt

one of

one of

one of

one of

one of

one of

opt

opt

opt

opt

but not one of " or \ or LineTerminator

<LS>

<PS>

but not one of ' or \ or LineTerminator

<LS>

<PS>

[lookahead ∉ DecimalDigit]

one of

but not one of EscapeCharacter or LineTerminator

EscapeCharacter

}

[empty]

but not one of * or \ or / or [

but not one of \ or / or [

but not LineTerminator

[

]

[empty]

RegularExpressionClassChar

but not one of ] or \

[empty]

Template

opt

opt

}

opt

}

opt

opt

[lookahead ≠ {]

but not one of ` or \ or $ or LineTerminator

NotEscapeSequence

but not 0

[lookahead ∉ HexDigit]

[lookahead ∉ HexDigit]

[lookahead ≠ {]

[lookahead ∉ HexDigit]

[lookahead ∉ HexDigit]

[lookahead ∉ HexDigit]

{

[lookahead ∉ HexDigit]

{

[lookahead ∉ HexDigit]

{

[lookahead ∉ HexDigit]

[lookahead ≠ }]

but only if MV of HexDigits > 0x10FFFF

but only if MV of HexDigits ≤ 0x10FFFF

A.2 Expressions

[Yield, Await]

[~Yield]

yield

[~Await]

await

[Yield, Await]

yield

await

[Yield, Await]

[~Yield]

yield

[~Await]

await

but not ReservedWord

[Yield, Await]

this

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, ~Tagged]

[?Yield, ?Await]

[Yield, Await]

(

[+In, ?Yield, ?Await]

)

(

[+In, ?Yield, ?Await]

)

(

)

(

...

[?Yield, ?Await]

)

(

...

[?Yield, ?Await]

)

(

[+In, ?Yield, ?Await]

...

[?Yield, ?Await]

)

(

[+In, ?Yield, ?Await]

...

[?Yield, ?Await]

)

When processing an instance of the production PrimaryExpression[Yield, Await]:CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await] the interpretation of CoverParenthesizedExpressionAndArrowParameterList is refined using the following grammar:

[Yield, Await]

(

[+In, ?Yield, ?Await]

)

[Yield, Await]

[

opt

]

[

[?Yield, ?Await]

]

[

[?Yield, ?Await]

opt

]

[Yield, Await]

opt

[+In, ?Yield, ?Await]

opt

[?Yield, ?Await]

[?Yield, ?Await]

opt

[+In, ?Yield, ?Await]

[?Yield, ?Await]

opt

[?Yield, ?Await]

[Yield, Await]

...

[+In, ?Yield, ?Await]

[Yield, Await]

{

}

{

[?Yield, ?Await]

}

{

[?Yield, ?Await]

}

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

[?Yield, ?Await]

...

[+In, ?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[

[+In, ?Yield, ?Await]

]

CoverInitializedName

[Yield, Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[Yield, Await, Tagged]

[?Yield, ?Await, ?Tagged]

[Yield, Await, Tagged]

[+In, ?Yield, ?Await]

[?Yield, ?Await, ?Tagged]

[Yield, Await, Tagged]

[?Yield, ?Await, ?Tagged]

[Yield, Await, Tagged]

[+In, ?Yield, ?Await]

[?Yield, ?Await, ?Tagged]

[+In, ?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[

[+In, ?Yield, ?Await]

]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, +Tagged]

[?Yield, ?Await]

new

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

super

[

[+In, ?Yield, ?Await]

]

super

new

target

import

meta

[Yield, Await]

[?Yield, ?Await]

new

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[

[+In, ?Yield, ?Await]

]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, +Tagged]

When processing an instance of the production CallExpression[Yield, Await]:CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await] the interpretation of CoverCallExpressionAndAsyncArrowHead is refined using the following grammar:

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

super

[?Yield, ?Await]

ImportCall

[Yield, Await]

import

(

[+In, ?Yield, ?Await]

)

[Yield, Await]

(

)

(

[?Yield, ?Await]

)

(

[?Yield, ?Await]

)

[Yield, Await]

[+In, ?Yield, ?Await]

...

[+In, ?Yield, ?Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

[?Yield, ?Await]

...

[+In, ?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[

[+In, ?Yield, ?Await]

]

[?Yield, ?Await, +Tagged]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[

[+In, ?Yield, ?Await]

]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, +Tagged]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[no LineTerminator here]

[?Yield, ?Await]

[no LineTerminator here]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

delete

[?Yield, ?Await]

void

[?Yield, ?Await]

typeof

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[+Await]

AwaitExpression

[?Yield]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

one of

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

>>>

[?Yield, ?Await]

[In, Yield, Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

instanceof

[?Yield, ?Await]

[+In]

[+In, ?Yield, ?Await]

[?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

===

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

!==

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[+In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[+Yield]

[?In, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

one of

<<=

>>=

>>>=

**=

In certain circumstances when processing an instance of the production AssignmentExpression[In, Yield, Await]:LeftHandSideExpression[?Yield, ?Await]=AssignmentExpression[?In, ?Yield, ?Await] the following grammar is used to refine the interpretation of LeftHandSideExpression:

AssignmentPattern

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

{

}

{

[?Yield, ?Await]

}

{

[?Yield, ?Await]

}

{

[?Yield, ?Await]

[?Yield, ?Await]

opt

}

[Yield, Await]

[

opt

[?Yield, ?Await]

opt

]

[

[?Yield, ?Await]

]

[

[?Yield, ?Await]

opt

[?Yield, ?Await]

opt

]

[Yield, Await]

...

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

opt

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

[Yield, Await]

...

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

A.3 Statements

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await]

[?Yield, ?Await]

[+Return]

[?Yield, ?Await]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await]

[?Yield, ?Await, ?Return]

[Yield, Await]

[?Yield, ?Await, ~Default]

[?Yield, ?Await, ~Default]

[+In, ?Yield, ?Await]

[Yield, Await, Default]

[?Yield, ?Await, ?Default]

[?Yield, ?Await, ?Default]

[?Yield, ?Await, ?Default]

[?Yield, ?Await, ?Default]

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

{

[?Yield, ?Await, ?Return]

opt

}

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

;

let

const

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

opt

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[Yield, Await]

var

[+In, ?Yield, ?Await]

;

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

opt

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

{

}

{

[?Yield, ?Await]

}

{

[?Yield, ?Await]

}

{

[?Yield, ?Await]

[?Yield, ?Await]

opt

}

[Yield, Await]

[

opt

[?Yield, ?Await]

opt

]

[

[?Yield, ?Await]

]

[

[?Yield, ?Await]

opt

[?Yield, ?Await]

opt

]

[Yield, Await]

...

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

opt

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

[Yield, Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

[Yield, Await]

...

[?Yield, ?Await]

...

[?Yield, ?Await]

EmptyStatement

;

[Yield, Await]

[lookahead ∉ { {, function, async

[no LineTerminator here]

function, class, let [ }]

[+In, ?Yield, ?Await]

;

[Yield, Await, Return]

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

else

[?Yield, ?Await, ?Return]

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

while

(

[+In, ?Yield, ?Await]

)

;

while

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

[lookahead ≠ let []

[~In, ?Yield, ?Await]

opt

;

[+In, ?Yield, ?Await]

opt

;

[+In, ?Yield, ?Await]

opt

)

[?Yield, ?Await, ?Return]

for

(

var

[~In, ?Yield, ?Await]

;

[+In, ?Yield, ?Await]

opt

;

[+In, ?Yield, ?Await]

opt

)

[?Yield, ?Await, ?Return]

for

(

[~In, ?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

;

[+In, ?Yield, ?Await]

opt

)

[?Yield, ?Await, ?Return]

for

(

[lookahead ≠ let []

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

var

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

[lookahead ≠ let]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

var

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[+Await]

for

await

(

[lookahead ≠ let]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[+Await]

for

await

(

var

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[+Await]

for

await

(

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

continue

;

continue

[no LineTerminator here]

[?Yield, ?Await]

;

[Yield, Await]

break

;

break

[no LineTerminator here]

[?Yield, ?Await]

;

[Yield, Await]

return

;

return

[no LineTerminator here]

[+In, ?Yield, ?Await]

;

[Yield, Await, Return]

with

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

switch

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

{

[?Yield, ?Await, ?Return]

opt

}

{

[?Yield, ?Await, ?Return]

opt

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

opt

}

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

case

[+In, ?Yield, ?Await]

[?Yield, ?Await, ?Return]

opt

[Yield, Await, Return]

default

[?Yield, ?Await, ?Return]

opt

[Yield, Await, Return]

[?Yield, ?Await]

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ~Default]

[Yield, Await]

throw

[no LineTerminator here]

[+In, ?Yield, ?Await]

;

[Yield, Await, Return]

try

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

try

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

try

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

catch

(

[?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

catch

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

finally

[?Yield, ?Await, ?Return]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

debugger

;

A.4 Functions and Classes

[Yield, Await, Default]

function

[?Yield, ?Await]

(

[~Yield, ~Await]

)

{

[~Yield, ~Await]

}

[+Default]

function

(

[~Yield, ~Await]

)

{

[~Yield, ~Await]

}

function

[~Yield, ~Await]

opt

(

[~Yield, ~Await]

)

{

[~Yield, ~Await]

}

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[empty]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await, +Return]

opt

[In, Yield, Await]

[?Yield, ?Await]

[no LineTerminator here]

[?In]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[In]

[lookahead ≠ {]

[?In, ~Await]

{

[~Yield, ~Await]

}

[In, Await]

[?In, ~Yield, ?Await]

When the production ArrowParameters[Yield, Await]:CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await] is recognized the following grammar is used to refine the interpretation of CoverParenthesizedExpressionAndArrowParameterList:

[Yield, Await]

(

[?Yield, ?Await]

)

[In, Yield, Await]

async

[no LineTerminator here]

[?Yield]

[no LineTerminator here]

[?In]

[?Yield, ?Await]

[no LineTerminator here]

[?In]

[In]

[lookahead ≠ {]

[?In, +Await]

{

}

[Yield]

[?Yield, +Await]

When the production AsyncArrowFunction[In, Yield, Await]:CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await][no LineTerminator here]=>AsyncConciseBody[?In] is recognized the following grammar is used to refine the interpretation of CoverCallExpressionAndAsyncArrowHead:

AsyncArrowHead

async

[no LineTerminator here]

[~Yield, +Await]

[Yield, Await]

[?Yield, ?Await]

(

[~Yield, ~Await]

)

{

[~Yield, ~Await]

}

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

get

[?Yield, ?Await]

(

)

{

[~Yield, ~Await]

}

set

[?Yield, ?Await]

(

)

{

[~Yield, ~Await]

}

[~Yield, ~Await]

[Yield, Await]

[?Yield, ?Await]

(

[+Yield, ~Await]

)

{

}

[Yield, Await, Default]

function

[?Yield, ?Await]

(

[+Yield, ~Await]

)

{

}

[+Default]

function

(

[+Yield, ~Await]

)

{

}

function

[+Yield, ~Await]

opt

(

[+Yield, ~Await]

)

{

}

[+Yield, ~Await]

[In, Await]

yield

[no LineTerminator here]

[?In, +Yield, ?Await]

yield

[no LineTerminator here]

[?In, +Yield, ?Await]

[Yield, Await]

async

[no LineTerminator here]

[?Yield, ?Await]

(

[+Yield, +Await]

)

{

}

[Yield, Await, Default]

async

[no LineTerminator here]

function

[?Yield, ?Await]

(

[+Yield, +Await]

)

{

}

[+Default]

async

[no LineTerminator here]

function

(

[+Yield, +Await]

)

{

}

async

[no LineTerminator here]

function

[+Yield, +Await]

opt

(

[+Yield, +Await]

)

{

}

[+Yield, +Await]

[Yield, Await, Default]

async

[no LineTerminator here]

function

[?Yield, ?Await]

(

[~Yield, +Await]

)

{

}

[+Default]

async

[no LineTerminator here]

function

(

[~Yield, +Await]

)

{

}

async

[no LineTerminator here]

function

(

[~Yield, +Await]

)

{

}

async

[no LineTerminator here]

function

[~Yield, +Await]

(

[~Yield, +Await]

)

{

}

[Yield, Await]

async

[no LineTerminator here]

[?Yield, ?Await]

(

[~Yield, +Await]

)

{

}

[~Yield, +Await]

[Yield]

await

[?Yield, +Await]

[Yield, Await, Default]

class

[?Yield, ?Await]

[?Yield, ?Await]

[+Default]

class

[?Yield, ?Await]

[Yield, Await]

class

[?Yield, ?Await]

opt

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

opt

{

[?Yield, ?Await]

opt

}

[Yield, Await]

extends

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

static

[?Yield, ?Await]

;

A.5 Scripts and Modules

opt

[~Yield, ~Await, ~Return]

opt

[~Yield, ~Await, ~Return]

import

;

import

;

{

}

{

}

{

}

from

[~Yield, ~Await]

export

;

export

;

export

[~Yield, ~Await]

export

[~Yield, ~Await]

export

default

[~Yield, ~Await, +Default]

export

default

[~Yield, ~Await, +Default]

export

default

[lookahead ∉ { function, async

[no LineTerminator here]

function, class }]

[+In, ~Yield, ~Await]

;

{

}

{

}

{

}

A.6 Number Conversions

:::

opt

opt

opt

:::

opt

:::

:::

StrUnsignedDecimalLiteral

StrDecimalLiteral

:::

:::

Infinity

opt

opt

opt

opt

All grammar symbols not explicitly defined by the StringNumericLiteral grammar have the definitions used in the Lexical Grammar for numeric literals.

A.7 Universal Resource Identifier Character Classes

:::

opt

:::

opt

:::

:::

one of

;

:::

:::

:::

one of

uriMark

:::

one of

(

)

A.8 Regular Expressions

[U, N]

[?U, ?N]

[U, N]

[?U, ?N]

[?U, ?N]

[?U, ?N]

[U, N]

[empty]

[?U, ?N]

[?U, ?N]

[U, N]

[?U, ?N]

[?U, ?N]

[?U, ?N]

[U, N]

(

[?U, ?N]

)

(

[?U, ?N]

)

(

[?U, ?N]

)

(

[?U, ?N]

)

QuantifierPrefix

{

}

{

}

{

}

[U, N]

[?U, ?N]

[?U]

(

GroupSpecifier

[?U]

[?U, ?N]

)

(

[?U, ?N]

)

one of

(

)

[

]

{

}

PatternCharacter

but not SyntaxCharacter

[U, N]

[?U]

[?U]

[+N]

[?U]

[U]

[lookahead ∉ DecimalDigit]

[?U]

[?U]

one of

one of

[U]

[empty]

[?U]

[U]

[?U]

[U]

[?U]

[?U]

[?U]

[U]

[+U]

[~U]

[U]

[+U]

[~U]

<ZWNJ>

<ZWJ>

[U]

[+U]

[+U]

[+U]

[+U]

[~U]

[+U]

}

any Unicode code point in the inclusive range 0xD800 to 0xDBFF

any Unicode code point in the inclusive range 0xDC00 to 0xDFFF

LeadSurrogate

but only if the SV of Hex4Digits is in the inclusive range 0xD800 to 0xDBFF

TrailSurrogate

but only if the SV of Hex4Digits is in the inclusive range 0xDC00 to 0xDFFF

NonSurrogate

but only if the SV of Hex4Digits is not in the inclusive range 0xD800 to 0xDFFF

[U]

[+U]

[+U]

[~U]

but not UnicodeIDContinue

opt

[lookahead ∉ DecimalDigit]

[U]

[+U]

}

[+U]

}

opt

opt

CharacterClass

[U]

[

[lookahead ≠ ^]

[?U]

]

[

[?U]

]

[U]

[empty]

[?U]

[U]

[?U]

[?U]

[?U]

[?U]

[?U]

[?U]

[U]

[?U]

[?U]

[?U]

[?U]

[?U]

[?U]

[U]

[?U]

[U]

but not one of \ or ] or -

[?U]

[U]

[+U]

[?U]

[?U]

B Additional ECMAScript Features for Web Browsers

The ECMAScript language syntax and semantics defined in this annex are required when the ECMAScript host is a web browser. The content of this annex is normative but optional if the ECMAScript host is not a web browser.

Note

This annex describes various legacy features and other characteristics of web browser based ECMAScript implementations. All of the language features and behaviours specified in this annex have one or more undesirable characteristics and in the absence of legacy usage would be removed from this specification. However, the usage of these features by large numbers of existing web pages means that web browsers must continue to support them. The specifications in this annex define the requirements for interoperable implementations of these legacy features.

These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code. ECMAScript implementations are discouraged from implementing these features unless the implementation is part of a web browser or is required to run the same legacy ECMAScript code that web browsers encounter.

B.1 Additional Syntax

B.1.1 Numeric Literals

The syntax and semantics of 11.8.3 is extended as follows except that this extension is not allowed for strict mode code:

Syntax

LegacyOctalIntegerLiteral

LegacyOctalIntegerLiteral

NonOctalDecimalIntegerLiteral

opt

NonOctalDigit

LegacyOctalLikeDecimalIntegerLiteral

NonOctalDigit

NonOctalDecimalIntegerLiteral

LegacyOctalLikeDecimalIntegerLiteral

LegacyOctalLikeDecimalIntegerLiteral

NonOctalDigit

one of

B.1.1.1 Static Semantics

The MV of LegacyOctalIntegerLiteral::0OctalDigit is the MV of OctalDigit.
The MV of LegacyOctalIntegerLiteral::LegacyOctalIntegerLiteralOctalDigit is (the MV of LegacyOctalIntegerLiteral times 8) plus the MV of OctalDigit.
The MV of DecimalIntegerLiteral::NonOctalDecimalIntegerLiteral is the MV of NonOctalDecimalIntegerLiteral.
The MV of NonOctalDecimalIntegerLiteral::0NonOctalDigit is the MV of NonOctalDigit.
The MV of NonOctalDecimalIntegerLiteral::LegacyOctalLikeDecimalIntegerLiteralNonOctalDigit is (the MV of LegacyOctalLikeDecimalIntegerLiteral times 10) plus the MV of NonOctalDigit.
The MV of NonOctalDecimalIntegerLiteral::NonOctalDecimalIntegerLiteralDecimalDigit is (the MV of NonOctalDecimalIntegerLiteral times 10) plus the MV of DecimalDigit.
The MV of LegacyOctalLikeDecimalIntegerLiteral::0OctalDigit is the MV of OctalDigit.
The MV of LegacyOctalLikeDecimalIntegerLiteral::LegacyOctalLikeDecimalIntegerLiteralOctalDigit is (the MV of LegacyOctalLikeDecimalIntegerLiteral times 10) plus the MV of OctalDigit.
The MV of NonOctalDigit::8 is 8.
The MV of NonOctalDigit::9 is 9.

B.1.2 String Literals

The syntax and semantics of 11.8.4 is extended as follows except that this extension is not allowed for strict mode code:

Syntax

[lookahead ∉ OctalDigit]

ZeroToThree

SingleLineHTMLOpenComment

[lookahead ∉ OctalDigit]

one of

one of

This definition of EscapeSequence is not used in strict mode or when parsing TemplateCharacter.

B.1.2.1 Static Semantics

The SV of EscapeSequence::LegacyOctalEscapeSequence is the SV of LegacyOctalEscapeSequence.
The SV of LegacyOctalEscapeSequence::OctalDigit is the code unit whose value is the MV of OctalDigit.
The SV of LegacyOctalEscapeSequence::ZeroToThreeOctalDigit is the code unit whose value is (8 times the MV of ZeroToThree) plus the MV of OctalDigit.
The SV of LegacyOctalEscapeSequence::FourToSevenOctalDigit is the code unit whose value is (8 times the MV of FourToSeven) plus the MV of OctalDigit.
The SV of LegacyOctalEscapeSequence::ZeroToThreeOctalDigitOctalDigit is the code unit whose value is (64 (that is, 8²) times the MV of ZeroToThree) plus (8 times the MV of the first OctalDigit) plus the MV of the second OctalDigit.
The MV of ZeroToThree::0 is 0.
The MV of ZeroToThree::1 is 1.
The MV of ZeroToThree::2 is 2.
The MV of ZeroToThree::3 is 3.
The MV of FourToSeven::4 is 4.
The MV of FourToSeven::5 is 5.
The MV of FourToSeven::6 is 6.
The MV of FourToSeven::7 is 7.

B.1.3 HTML-like Comments

The syntax and semantics of 11.4 is extended as follows except that this extension is not allowed when parsing source code using the goal symbol Module:

Syntax

Comment

MultiLineComment

SingleLineComment

SingleLineHTMLCloseComment

SingleLineDelimitedComment

MultiLineComment

FirstCommentLine

opt

LineTerminator

opt

HTMLCloseComment

opt

FirstCommentLine

SingleLineHTMLOpenComment

<!--

SingleLineHTMLCloseComment

opt

SingleLineDelimitedComment

HTMLCloseComment

opt

HTMLCloseComment

SingleLineDelimitedCommentSequence

opt

opt

-->

opt

SingleLineNotAsteriskChar

SingleLinePostAsteriskCommentChars

opt

opt

SingleLineNotAsteriskChar

SingleLinePostAsteriskCommentChars

but not one of * or LineTerminator

SingleLineNotForwardSlashOrAsteriskChar

SingleLinePostAsteriskCommentChars

opt

opt

SingleLineNotForwardSlashOrAsteriskChar

but not one of / or * or LineTerminator

WhiteSpace

SingleLineDelimitedCommentSequence

opt

SingleLineDelimitedComment

SingleLineDelimitedCommentSequence

opt

opt

Similar to a MultiLineComment that contains a line terminator code point, a SingleLineHTMLCloseComment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.

B.1.4 Regular Expressions Patterns

The syntax of 21.2.1 is modified and extended as follows. These changes introduce ambiguities that are broken by the ordering of grammar productions and by contextual information. When parsing using the following grammar, each alternative is considered only if previous production alternatives do not match.

This alternative pattern grammar and semantics only changes the syntax and semantics of BMP patterns. The following grammar extensions include productions parameterized with the [U] parameter. However, none of these extensions change the syntax of Unicode patterns recognized when parsing with the [U] parameter present on the goal symbol.

Syntax

Term

[U, N]

[+U]

Assertion

[+U, ?N]

[+U]

Atom

[+U, ?N]

[+U]

Atom

[+U, ?N]

[~U]

QuantifiableAssertion

[?N]

[~U]

Assertion

[~U, ?N]

[~U]

[?N]

[~U]

[?N]

[U, N]

[+U]

(

[+U, ?N]

)

[+U]

(

[+U, ?N]

)

[~U]

QuantifiableAssertion

[?N]

(

[?U, ?N]

)

(

[?U, ?N]

)

QuantifiableAssertion

[N]

(

[~U, ?N]

)

(

[~U, ?N]

)

ExtendedAtom

[N]

[~U, ?N]

[lookahead = c]

CharacterClass

[~U]

(

[~U, ?N]

)

(

[~U, ?N]

)

InvalidBracedQuantifier

ExtendedPatternCharacter

InvalidBracedQuantifier

{

}

{

}

{

}

ExtendedPatternCharacter

but not one of ^

(

)

[

[U, N]

[+U]

[~U]

but only if the CapturingGroupNumber of DecimalEscape is <= _NcapturingParens_

[?U]

[~U, ?N]

[+N]

[?U]

[U, N]

[lookahead ∉ DecimalDigit]

[?U]

[~U]

[?U, ?N]

[U, N]

[+U]

SourceCharacterIdentityEscape

[+U]

[~U]

[?N]

SourceCharacterIdentityEscape

[N]

[~N]

but not c

[+N]

but not one of c or k

[U, N]

but not one of \ or ] or -

[?U, ?N]

[lookahead = c]

[U, N]

[+U]

[~U]

[?U]

[?U, ?N]

Note

When the same left hand sides occurs with both [+U] and [~U] guards it is to control the disambiguation priority.

B.1.4.1 Static Semantics: Early Errors

The semantics of 21.2.1.1 is extended as follows:

ExtendedAtom

InvalidBracedQuantifier

It is a Syntax Error if any source text matches this rule.

NonemptyClassRanges

It is a Syntax Error if IsCharacterClass of the first ClassAtom is true or IsCharacterClass of the second ClassAtom is true and this production has a _[U] parameter.

It is a Syntax Error if IsCharacterClass of ClassAtomNoDash is true or IsCharacterClass of ClassAtom is true and this production has a _[U] parameter.

B.1.4.2 Static Semantics: IsCharacterClass

The semantics of 21.2.1.3 is extended as follows:

[lookahead = c]

Return false.

B.1.4.3 Static Semantics: CharacterValue

The semantics of 21.2.1.4 is extended as follows:

[lookahead = c]

Return the code point value of U+005C (REVERSE SOLIDUS).

ClassControlLetter

Let ch be the code point matched by ClassControlLetter.
Let i be ch's code point value.
Return the remainder of dividing i by 32.

Evaluate the SV of LegacyOctalEscapeSequence (see B.1.2) to obtain a code unit cu.
Return the numeric value of cu.

B.1.4.4 Pattern Semantics

The semantics of 21.2.2 is extended as follows:

Within 21.2.2.5 reference to “ Atom::(GroupSpecifierDisjunction) ” are to be interpreted as meaning “ Atom::(GroupSpecifierDisjunction) ” or “ ExtendedAtom::(Disjunction) ”.

Term (21.2.2.5) includes the following additional evaluation rules:

The production Term::QuantifiableAssertionQuantifier evaluates the same as the production Term::AtomQuantifier but with QuantifiableAssertion substituted for Atom.

The production Term::ExtendedAtomQuantifier evaluates the same as the production Term::AtomQuantifier but with ExtendedAtom substituted for Atom.

The production Term::ExtendedAtom evaluates the same as the production Term::Atom but with ExtendedAtom substituted for Atom.

Assertion (21.2.2.6) includes the following additional evaluation rule:

The production Assertion::QuantifiableAssertion evaluates as follows:

Evaluate QuantifiableAssertion to obtain a Matcher m.
Return m.

Assertion (21.2.2.6) evaluation rules for the Assertion::(?=Disjunction) and Assertion::(?!Disjunction) productions are also used for the QuantifiableAssertion productions, but with QuantifiableAssertion substituted for Assertion.

Atom (21.2.2.8) evaluation rules for the Atom productions except for Atom::PatternCharacter are also used for the ExtendedAtom productions, but with ExtendedAtom substituted for Atom. The following evaluation rules, with parameter direction, are also added:

The production ExtendedAtom::\[lookahead = c] evaluates as follows:

Let A be the CharSet containing the single character \ U+005C (REVERSE SOLIDUS).
Call CharacterSetMatcher(A, false, direction) and return its Matcher result.

The production ExtendedAtom::ExtendedPatternCharacter evaluates as follows:

Let ch be the character represented by ExtendedPatternCharacter.
Let A be a one-element CharSet containing the character ch.
Call CharacterSetMatcher(A, false, direction) and return its Matcher result.

CharacterEscape (21.2.2.10) includes the following additional evaluation rule:

The production CharacterEscape::LegacyOctalEscapeSequence evaluates as follows:

Let cv be the CharacterValue of this CharacterEscape.
Return the character whose character value is cv.

NonemptyClassRanges (21.2.2.15) modifies the following evaluation rule:

The production NonemptyClassRanges::ClassAtom-ClassAtomClassRanges evaluates as follows:

Evaluate the first ClassAtom to obtain a CharSet A.
Evaluate the second ClassAtom to obtain a CharSet B.
Evaluate ClassRanges to obtain a CharSet C.
Call CharacterRangeOrUnion(A, B) and let D be the resulting CharSet.
Return the union of CharSets D and C.

NonemptyClassRangesNoDash (21.2.2.16) modifies the following evaluation rule:

The production NonemptyClassRangesNoDash::ClassAtomNoDash-ClassAtomClassRanges evaluates as follows:

Evaluate ClassAtomNoDash to obtain a CharSet A.
Evaluate ClassAtom to obtain a CharSet B.
Evaluate ClassRanges to obtain a CharSet C.
Call CharacterRangeOrUnion(A, B) and let D be the resulting CharSet.
Return the union of CharSets D and C.

ClassEscape (21.2.2.19) includes the following additional evaluation rule:

The production ClassEscape::cClassControlLetter evaluates as follows:

Let cv be the CharacterValue of this ClassEscape.
Let c be the character whose character value is cv.
Return the CharSet containing the single character c.

ClassAtomNoDash (21.2.2.18) includes the following additional evaluation rule:

The production ClassAtomNoDash::\[lookahead = c] evaluates as follows:

Return the CharSet containing the single character \ U+005C (REVERSE SOLIDUS).

Note

This production can only be reached from the sequence \c within a character class where it is not followed by an acceptable control character.

B.1.4.4.1 Runtime Semantics: CharacterRangeOrUnion ( `A`, `B` )

The abstract operation CharacterRangeOrUnion takes arguments A (a CharSet) and B (a CharSet). It performs the following steps when called:

If Unicode is false, then
1. If A does not contain exactly one character or B does not contain exactly one character, then
  1. Let C be the CharSet containing the single character - U+002D (HYPHEN-MINUS).
  2. Return the union of CharSets A, B and C.
Return CharacterRange(A, B).

B.2 Additional Built-in Properties

When the ECMAScript host is a web browser the following additional properties of the standard built-in objects are defined.

B.2.1 Additional Properties of the Global Object

The entries in Table 85 are added to Table 8.

Table 85: Additional Well-known Intrinsic Objects

Intrinsic Name	Global Name	ECMAScript Language Association
%escape%	`escape`	The `escape` function (B.2.1.1)
%unescape%	`unescape`	The `unescape` function (B.2.1.2)

B.2.1.1 escape ( `string` )

The escape function is a property of the global object. It computes a new version of a String value in which certain code units have been replaced by a hexadecimal escape sequence.

For those code units being replaced whose value is 0x00FF or less, a two-digit escape sequence of the form %xx is used. For those characters being replaced whose code unit value is greater than 0x00FF, a four-digit escape sequence of the form %uxxxx is used.

The escape function is the %escape% intrinsic object. When the escape function is called with one argument string, the following steps are taken:

Set string to ? ToString(string).
Let length be the number of code units in string.
Let R be the empty String.
Let k be 0.
Repeat, while k < length,
1. Let char be the code unit (represented as a 16-bit unsigned integer) at index k within string.
2. If char is one of the code units in "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@*_+-./", then
  1. Let S be the String value containing the single code unit char.
3. Else if char ≥ 256, then
  1. Let n be the numeric value of char.
  2. Let S be the string-concatenation of:
    - "%u"
    - the String representation of n, formatted as a four-digit uppercase hexadecimal number, padded to the left with zeroes if necessary
4. Else,
  1. Assert: char < 256.
  2. Let n be the numeric value of char.
  3. Let S be the string-concatenation of:
    - "%"
    - the String representation of n, formatted as a two-digit uppercase hexadecimal number, padded to the left with a zero if necessary
5. Set R to the string-concatenation of the previous value of R and S.
6. Set k to k + 1.
Return R.

Note

The encoding is partly based on the encoding described in RFC 1738, but the entire encoding specified in this standard is described above without regard to the contents of RFC 1738. This encoding does not reflect changes to RFC 1738 made by RFC 3986.

B.2.1.2 unescape ( `string` )

The unescape function is a property of the global object. It computes a new version of a String value in which each escape sequence of the sort that might be introduced by the escape function is replaced with the code unit that it represents.

The unescape function is the %unescape% intrinsic object. When the unescape function is called with one argument string, the following steps are taken:

Set string to ? ToString(string).
Let length be the number of code units in string.
Let R be the empty String.
Let k be 0.
Repeat, while k ≠ length
1. Let c be the code unit at index k within string.
2. If c is the code unit 0x0025 (PERCENT SIGN), then
  1. If k ≤ length - 6 and the code unit at index k + 1 within string is the code unit 0x0075 (LATIN SMALL LETTER U) and the four code units at indices k + 2, k + 3, k + 4, and k + 5 within string are all hexadecimal digits, then
    1. Set c to the code unit whose value is the integer represented by the four hexadecimal digits at indices k + 2, k + 3, k + 4, and k + 5 within string.
    2. Set k to k + 5.
  2. Else if k ≤ length - 3 and the two code units at indices k + 1 and k + 2 within string are both hexadecimal digits, then
    1. Set c to the code unit whose value is the integer represented by two zeroes plus the two hexadecimal digits at indices k + 1 and k + 2 within string.
    2. Set k to k + 2.
3. Set R to the string-concatenation of the previous value of R and c.
4. Set k to k + 1.
Return R.

B.2.2 Additional Properties of the Object.prototype Object

B.2.2.1 Object.prototype.proto

Object.prototype.__proto__ is an accessor property with attributes { [[Enumerable]]: false, [[Configurable]]: true }. The [[Get]] and [[Set]] attributes are defined as follows:

B.2.2.1.1 get Object.prototype.proto

The value of the [[Get]] attribute is a built-in function that requires no arguments. It performs the following steps when called:

Let O be ? ToObject(this value).
Return ? O.[[GetPrototypeOf]]().

B.2.2.1.2 set Object.prototype.proto

The value of the [[Set]] attribute is a built-in function that takes an argument proto. It performs the following steps when called:

Let O be ? RequireObjectCoercible(this value).
If Type(proto) is neither Object nor Null, return undefined.
If Type(O) is not Object, return undefined.
Let status be ? O.[[SetPrototypeOf]](proto).
If status is false, throw a TypeError exception.
Return undefined.

B.2.2.2 Object.prototype.defineGetter ( `P`, `getter` )

When the __defineGetter__ method is called with arguments P and getter, the following steps are taken:

Let O be ? ToObject(this value).
If IsCallable(getter) is false, throw a TypeError exception.
Let desc be PropertyDescriptor { [[Get]]: getter, [[Enumerable]]: true, [[Configurable]]: true }.
Let key be ? ToPropertyKey(P).
Perform ? DefinePropertyOrThrow(O, key, desc).
Return undefined.

B.2.2.3 Object.prototype.defineSetter ( `P`, `setter` )

When the __defineSetter__ method is called with arguments P and setter, the following steps are taken:

Let O be ? ToObject(this value).
If IsCallable(setter) is false, throw a TypeError exception.
Let desc be PropertyDescriptor { [[Set]]: setter, [[Enumerable]]: true, [[Configurable]]: true }.
Let key be ? ToPropertyKey(P).
Perform ? DefinePropertyOrThrow(O, key, desc).
Return undefined.

B.2.2.4 Object.prototype.lookupGetter ( `P` )

When the __lookupGetter__ method is called with argument P, the following steps are taken:

Let O be ? ToObject(this value).
Let key be ? ToPropertyKey(P).
Repeat,
1. Let desc be ? O.[[GetOwnProperty]](key).
2. If desc is not undefined, then
  1. If IsAccessorDescriptor(desc) is true, return desc.[[Get]].
  2. Return undefined.
3. Set O to ? O.[[GetPrototypeOf]]().
4. If O is null, return undefined.

B.2.2.5 Object.prototype.lookupSetter ( `P` )

When the __lookupSetter__ method is called with argument P, the following steps are taken:

Let O be ? ToObject(this value).
Let key be ? ToPropertyKey(P).
Repeat,
1. Let desc be ? O.[[GetOwnProperty]](key).
2. If desc is not undefined, then
  1. If IsAccessorDescriptor(desc) is true, return desc.[[Set]].
  2. Return undefined.
3. Set O to ? O.[[GetPrototypeOf]]().
4. If O is null, return undefined.

B.2.3 Additional Properties of the String.prototype Object

B.2.3.1 String.prototype.substr ( `start`, `length` )

The substr method takes two arguments, start and length, and returns a substring of the result of converting the this object to a String, starting from index start and running for length code units (or through the end of the String if length is undefined). If start is negative, it is treated as sourceLength + start where sourceLength is the length of the String. The result is a String value, not a String object. The following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let intStart be ? ToInteger(start).
If length is undefined, let end be +∞; otherwise let end be ? ToInteger(length).
Let size be the number of code units in S.
If intStart < 0, set intStart to max(size + intStart, 0).
Let resultLength be min(max(end, 0), size - intStart).
If resultLength ≤ 0, return the empty String.
Return the String value containing resultLength consecutive code units from S beginning with the code unit at index intStart.

Note

The substr function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

B.2.3.2 String.prototype.anchor ( `name` )

When the anchor method is called with argument name, the following steps are taken:

Let S be the this value.
Return ? CreateHTML(S, "a", "name", name).

B.2.3.2.1 Runtime Semantics: CreateHTML ( `string`, `tag`, `attribute`, `value` )

The abstract operation CreateHTML takes arguments string, tag (a String), attribute (a String), and value. It performs the following steps when called:

Let str be ? RequireObjectCoercible(string).
Let S be ? ToString(str).
Let p1 be the string-concatenation of "<" and tag.
If attribute is not the empty String, then
1. Let V be ? ToString(value).
2. Let escapedV be the String value that is the same as V except that each occurrence of the code unit 0x0022 (QUOTATION MARK) in V has been replaced with the six code unit sequence """.
3. Set p1 to the string-concatenation of:
  - p1
  - the code unit 0x0020 (SPACE)
  - attribute
  - the code unit 0x003D (EQUALS SIGN)
  - the code unit 0x0022 (QUOTATION MARK)
  - escapedV
  - the code unit 0x0022 (QUOTATION MARK)
Let p2 be the string-concatenation of p1 and ">".
Let p3 be the string-concatenation of p2 and S.
Let p4 be the string-concatenation of p3, "</", tag, and ">".
Return p4.

B.2.3.3 String.prototype.big ( )

When the big method is called with no arguments, the following steps are taken:

Let S be the this value.
Return ? CreateHTML(S, "big", "", "").

B.2.3.4 String.prototype.blink ( )

When the blink method is called with no arguments, the following steps are taken:

Let S be the this value.
Return ? CreateHTML(S, "blink", "", "").

B.2.3.5 String.prototype.bold ( )

When the bold method is called with no arguments, the following steps are taken:

Let S be the this value.
Return ? CreateHTML(S, "b", "", "").

B.2.3.6 String.prototype.fixed ( )

When the fixed method is called with no arguments, the following steps are taken:

Let S be the this value.
Return ? CreateHTML(S, "tt", "", "").

B.2.3.7 String.prototype.fontcolor ( `color` )

When the fontcolor method is called with argument color, the following steps are taken:

Let S be the this value.
Return ? CreateHTML(S, "font", "color", color).

B.2.3.8 String.prototype.fontsize ( `size` )

When the fontsize method is called with argument size, the following steps are taken:

Let S be the this value.
Return ? CreateHTML(S, "font", "size", size).

B.2.3.9 String.prototype.italics ( )

When the italics method is called with no arguments, the following steps are taken:

Let S be the this value.
Return ? CreateHTML(S, "i", "", "").

B.2.3.10 String.prototype.link ( `url` )

When the link method is called with argument url, the following steps are taken:

Let S be the this value.
Return ? CreateHTML(S, "a", "href", url).

B.2.3.11 String.prototype.small ( )

When the small method is called with no arguments, the following steps are taken:

Let S be the this value.
Return ? CreateHTML(S, "small", "", "").

B.2.3.12 String.prototype.strike ( )

When the strike method is called with no arguments, the following steps are taken:

Let S be the this value.
Return ? CreateHTML(S, "strike", "", "").

B.2.3.13 String.prototype.sub ( )

When the sub method is called with no arguments, the following steps are taken:

Let S be the this value.
Return ? CreateHTML(S, "sub", "", "").

B.2.3.14 String.prototype.sup ( )

When the sup method is called with no arguments, the following steps are taken:

Let S be the this value.
Return ? CreateHTML(S, "sup", "", "").

B.2.3.15 String.prototype.trimLeft ( )

Note

The property "trimStart" is preferred. The "trimLeft" property is provided principally for compatibility with old code. It is recommended that the "trimStart" property be used in new ECMAScript code.

The initial value of the "trimLeft" property is the same function object as the initial value of the String.prototype.trimStart property.

B.2.3.16 String.prototype.trimRight ( )

Note

The property "trimEnd" is preferred. The "trimRight" property is provided principally for compatibility with old code. It is recommended that the "trimEnd" property be used in new ECMAScript code.

The initial value of the "trimRight" property is the same function object as the initial value of the String.prototype.trimEnd property.

B.2.4 Additional Properties of the Date.prototype Object

B.2.4.1 Date.prototype.getYear ( )

Note

The getFullYear method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”

When the getYear method is called with no arguments, the following steps are taken:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return YearFromTime(LocalTime(t)) - 1900.

B.2.4.2 Date.prototype.setYear ( `year` )

Note

The setFullYear method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”

When the setYear method is called with one argument year, the following steps are taken:

Let t be ? thisTimeValue(this value).
If t is NaN, set t to +0; otherwise, set t to LocalTime(t).
Let y be ? ToNumber(year).
If y is NaN, then
1. Set the [[DateValue]] internal slot of this Date object to NaN.
2. Return NaN.
Let yi be ! ToInteger(y).
If 0 ≤ yi ≤ 99, let yyyy be yi + 1900.
Else, let yyyy be y.
Let d be MakeDay(yyyy, MonthFromTime(t), DateFromTime(t)).
Let date be UTC(MakeDate(d, TimeWithinDay(t))).
Set the [[DateValue]] internal slot of this Date object to TimeClip(date).
Return the value of the [[DateValue]] internal slot of this Date object.

B.2.4.3 Date.prototype.toGMTString ( )

Note

The property "toUTCString" is preferred. The "toGMTString" property is provided principally for compatibility with old code. It is recommended that the "toUTCString" property be used in new ECMAScript code.

The function object that is the initial value of Date.prototype.toGMTString is the same function object that is the initial value of Date.prototype.toUTCString.

B.2.5 Additional Properties of the RegExp.prototype Object

B.2.5.1 RegExp.prototype.compile ( `pattern`, `flags` )

When the compile method is called with arguments pattern and flags, the following steps are taken:

Let O be the this value.
If Type(O) is not Object or Type(O) is Object and O does not have a [[RegExpMatcher]] internal slot, then
1. Throw a TypeError exception.
If Type(pattern) is Object and pattern has a [[RegExpMatcher]] internal slot, then
1. If flags is not undefined, throw a TypeError exception.
2. Let P be pattern.[[OriginalSource]].
3. Let F be pattern.[[OriginalFlags]].
Else,
1. Let P be pattern.
2. Let F be flags.
Return ? RegExpInitialize(O, P, F).

Note

The compile method completely reinitializes the this object RegExp with a new pattern and flags. An implementation may interpret use of this method as an assertion that the resulting RegExp object will be used multiple times and hence is a candidate for extra optimization.

B.3 Other Additional Features

B.3.1 proto Property Names in Object Initializers

The following Early Error rule is added to those in 12.2.6.1. When ObjectLiteral appears in a context where ObjectAssignmentPattern is required the Early Error rule is not applied. In addition, it is not applied when initially parsing a CoverParenthesizedExpressionAndArrowParameterList or a CoverCallExpressionAndAsyncArrowHead.

{

}

{

}

It is a Syntax Error if PropertyNameList of PropertyDefinitionList contains any duplicate entries for "__proto__" and at least two of those entries were obtained from productions of the form PropertyDefinition:PropertyName:AssignmentExpression .

Note

The List returned by PropertyNameList does not include string literal property names defined as using a ComputedPropertyName.

In 12.2.6.8 the PropertyDefinitionEvaluation algorithm for the production
PropertyDefinition:PropertyName:AssignmentExpression
is replaced with the following definition:

Let propKey be the result of evaluating PropertyName.
ReturnIfAbrupt(propKey).
If propKey is the String value "__proto__" and if IsComputedPropertyKey(PropertyName) is false, then
1. Let isProtoSetter be true.
Else,
1. Let isProtoSetter be false.
If IsAnonymousFunctionDefinition(AssignmentExpression) is true and isProtoSetter is false, then
1. Let propValue be NamedEvaluation of AssignmentExpression with argument propKey.
Else,
1. Let exprValueRef be the result of evaluating AssignmentExpression.
2. Let propValue be ? GetValue(exprValueRef).
If isProtoSetter is true, then
1. If Type(propValue) is either Object or Null, then
  1. Return object.[[SetPrototypeOf]](propValue).
2. Return NormalCompletion(empty).
Assert: enumerable is true.
Assert: object is an ordinary, extensible object with no non-configurable properties.
Return ! CreateDataPropertyOrThrow(object, propKey, propValue).

B.3.2 Labelled Function Declarations

Prior to ECMAScript 2015, the specification of LabelledStatement did not allow for the association of a statement label with a FunctionDeclaration. However, a labelled FunctionDeclaration was an allowable extension for non-strict code and most browser-hosted ECMAScript implementations supported that extension. In ECMAScript 2015, the grammar productions for LabelledStatement permits use of FunctionDeclaration as a LabelledItem but 13.13.1 includes an Early Error rule that produces a Syntax Error if that occurs. For web browser compatibility, that rule is modified with the addition of the highlighted text:

It is a Syntax Error if any strict mode source code matches this rule.

Note

The early error rules for WithStatement, IfStatement, and IterationStatement prevent these statements from containing a labelled FunctionDeclaration in non-strict code.

B.3.3 Block-Level Function Declarations Web Legacy Compatibility Semantics

Prior to ECMAScript 2015, the ECMAScript specification did not define the occurrence of a FunctionDeclaration as an element of a Block statement's StatementList. However, support for that form of FunctionDeclaration was an allowable extension and most browser-hosted ECMAScript implementations permitted them. Unfortunately, the semantics of such declarations differ among those implementations. Because of these semantic differences, existing web ECMAScript code that uses Block level function declarations is only portable among browser implementation if the usage only depends upon the semantic intersection of all of the browser implementations for such declarations. The following are the use cases that fall within that intersection semantics:

A function is declared and only referenced within a single block
- One or more FunctionDeclarations whose BindingIdentifier is the name f occur within the function code of an enclosing function g and that declaration is nested within a Block.
- No other declaration of f that is not a var declaration occurs within the function code of g
- All occurrences of f as an IdentifierReference are within the StatementList of the Block containing the declaration of f.
A function is declared and possibly used within a single Block but also referenced by an inner function definition that is not contained within that same Block.
- One or more FunctionDeclarations whose BindingIdentifier is the name f occur within the function code of an enclosing function g and that declaration is nested within a Block.
- No other declaration of f that is not a var declaration occurs within the function code of g
- There may be occurrences of f as an IdentifierReference within the StatementList of the Block containing the declaration of f.
- There is at least one occurrence of f as an IdentifierReference within another function h that is nested within g and no other declaration of f shadows the references to f from within h.
- All invocations of h occur after the declaration of f has been evaluated.
A function is declared and possibly used within a single block but also referenced within subsequent blocks.
- One or more FunctionDeclaration whose BindingIdentifier is the name f occur within the function code of an enclosing function g and that declaration is nested within a Block.
- No other declaration of f that is not a var declaration occurs within the function code of g
- There may be occurrences of f as an IdentifierReference within the StatementList of the Block containing the declaration of f.
- There is at least one occurrence of f as an IdentifierReference within the function code of g that lexically follows the Block containing the declaration of f.

The first use case is interoperable with the semantics of Block level function declarations provided by ECMAScript 2015. Any pre-existing ECMAScript code that employs that use case will operate using the Block level function declarations semantics defined by clauses 9, 13, and 14 of this specification.

ECMAScript 2015 interoperability for the second and third use cases requires the following extensions to the clause 9, clause 14, clause 18.2.1 and clause 15.1.11 semantics.

If an ECMAScript implementation has a mechanism for reporting diagnostic warning messages, a warning should be produced when code contains a FunctionDeclaration for which these compatibility semantics are applied and introduce observable differences from non-compatibility semantics. For example, if a var binding is not introduced because its introduction would create an early error, a warning message should not be produced.

B.3.3.1 Changes to FunctionDeclarationInstantiation

During FunctionDeclarationInstantiation the following steps are performed in place of step 29:

If strict is false, then
1. For each FunctionDeclaration f that is directly contained in the StatementList of a Block, CaseClause, or DefaultClause, do
  1. Let F be StringValue of the BindingIdentifier of f.
  2. If replacing the FunctionDeclaration f with a VariableStatement that has F as a BindingIdentifier would not produce any Early Errors for func and F is not an element of parameterNames, then
    1. NOTE: A var binding for F is only instantiated here if it is neither a VarDeclaredName, the name of a formal parameter, or another FunctionDeclaration.
    2. If initializedBindings does not contain F and F is not "arguments", then
      1. Perform ! varEnv.CreateMutableBinding(F, false).
      2. Perform varEnv.InitializeBinding(F, undefined).
      3. Append F to instantiatedVarNames.
    3. When the FunctionDeclaration f is evaluated, perform the following steps in place of the FunctionDeclaration Evaluation algorithm provided in 14.1.25:
      1. Let fenv be the running execution context's VariableEnvironment.
      2. Let benv be the running execution context's LexicalEnvironment.
      3. Let fobj be ! benv.GetBindingValue(F, false).
      4. Perform ! fenv.SetMutableBinding(F, fobj, false).
      5. Return NormalCompletion(empty).

B.3.3.2 Changes to GlobalDeclarationInstantiation

During GlobalDeclarationInstantiation the following steps are performed in place of step 13:

Let strict be IsStrict of script.
If strict is false, then
1. Let declaredFunctionOrVarNames be a new empty List.
2. Append to declaredFunctionOrVarNames the elements of declaredFunctionNames.
3. Append to declaredFunctionOrVarNames the elements of declaredVarNames.
4. For each FunctionDeclaration f that is directly contained in the StatementList of a Block, CaseClause, or DefaultClause Contained within script, do
  1. Let F be StringValue of the BindingIdentifier of f.
  2. If replacing the FunctionDeclaration f with a VariableStatement that has F as a BindingIdentifier would not produce any Early Errors for script, then
    1. If env.HasLexicalDeclaration(F) is false, then
      1. Let fnDefinable be ? env.CanDeclareGlobalVar(F).
      2. If fnDefinable is true, then
        NOTE: A var binding for F is only instantiated here if it is neither a VarDeclaredName nor the name of another FunctionDeclaration.
        If declaredFunctionOrVarNames does not contain F, then
        Perform ? env.CreateGlobalVarBinding(F, false).
        Append F to declaredFunctionOrVarNames.
        When the FunctionDeclaration f is evaluated, perform the following steps in place of the FunctionDeclaration Evaluation algorithm provided in 14.1.25:
        Let genv be the running execution context's VariableEnvironment.
        Let benv be the running execution context's LexicalEnvironment.
        Let fobj be ! benv.GetBindingValue(F, false).
        Perform ? genv.SetMutableBinding(F, fobj, false).
        Return NormalCompletion(empty).

B.3.3.3 Changes to EvalDeclarationInstantiation

During EvalDeclarationInstantiation the following steps are performed in place of step 7:

If strict is false, then
1. Let declaredFunctionOrVarNames be a new empty List.
2. Append to declaredFunctionOrVarNames the elements of declaredFunctionNames.
3. Append to declaredFunctionOrVarNames the elements of declaredVarNames.
4. For each FunctionDeclaration f that is directly contained in the StatementList of a Block, CaseClause, or DefaultClause Contained within body, do
  1. Let F be StringValue of the BindingIdentifier of f.
  2. If replacing the FunctionDeclaration f with a VariableStatement that has F as a BindingIdentifier would not produce any Early Errors for body, then
    1. Let bindingExists be false.
    2. Let thisEnv be lexEnv.
    3. Assert: The following loop will terminate.
    4. Repeat, while thisEnv is not the same as varEnv,
      1. If thisEnv is not an object Environment Record, then
        If thisEnv.HasBinding(F) is true, then
        Let bindingExists be true.
      2. Set thisEnv to thisEnv.[[OuterEnv]].
    5. If bindingExists is false and varEnv is a global Environment Record, then
      1. If varEnv.HasLexicalDeclaration(F) is false, then
        Let fnDefinable be ? varEnv.CanDeclareGlobalVar(F).
      2. Else,
        Let fnDefinable be false.
    6. Else,
      1. Let fnDefinable be true.
    7. If bindingExists is false and fnDefinable is true, then
      1. If declaredFunctionOrVarNames does not contain F, then
        If varEnv is a global Environment Record, then
        Perform ? varEnv.CreateGlobalVarBinding(F, true).
        Else,
        Let bindingExists be varEnv.HasBinding(F).
        If bindingExists is false, then
        Perform ! varEnv.CreateMutableBinding(F, true).
        Perform ! varEnv.InitializeBinding(F, undefined).
        Append F to declaredFunctionOrVarNames.
      2. When the FunctionDeclaration f is evaluated, perform the following steps in place of the FunctionDeclaration Evaluation algorithm provided in 14.1.25:
        Let genv be the running execution context's VariableEnvironment.
        Let benv be the running execution context's LexicalEnvironment.
        Let fobj be ! benv.GetBindingValue(F, false).
        Perform ? genv.SetMutableBinding(F, fobj, false).
        Return NormalCompletion(empty).

B.3.3.4 Changes to Block Static Semantics: Early Errors

For web browser compatibility, that rule is modified with the addition of the highlighted text:

{

}

It is a Syntax Error if the LexicallyDeclaredNames of StatementList contains any duplicate entries, unless the source code matching this production is not strict mode code and the duplicate entries are only bound by FunctionDeclarations.

B.3.3.5 Changes to `switch` Statement Static Semantics: Early Errors

For web browser compatibility, that rule is modified with the addition of the highlighted text:

switch

(

)

It is a Syntax Error if the LexicallyDeclaredNames of CaseBlock contains any duplicate entries, unless the source code matching this production is not strict mode code and the duplicate entries are only bound by FunctionDeclarations.

B.3.3.6 Changes to BlockDeclarationInstantiation

During BlockDeclarationInstantiation the following steps are performed in place of step 3.a.ii.1:

If env.HasBinding(dn) is false, then
1. Perform ! env.CreateMutableBinding(dn, false).

During BlockDeclarationInstantiation the following steps are performed in place of step 3.b.iii:

If the binding for fn in env is an uninitialized binding, then
1. Perform env.InitializeBinding(fn, fo).
Else,
1. Assert: d is a FunctionDeclaration.
2. Perform env.SetMutableBinding(fn, fo, false).

B.3.4 FunctionDeclarations in IfStatement Statement Clauses

The following augments the IfStatement production in 13.6:

[Yield, Await, Return]

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ~Default]

else

[?Yield, ?Await, ?Return]

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

else

[?Yield, ?Await, ~Default]

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ~Default]

else

[?Yield, ?Await, ~Default]

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ~Default]

This production only applies when parsing non-strict code. Code matching this production is processed as if each matching occurrence of FunctionDeclaration[?Yield, ?Await, ~Default] was the sole StatementListItem of a BlockStatement occupying that position in the source code. The semantics of such a synthetic BlockStatement includes the web legacy compatibility semantics specified in B.3.3.

B.3.5 VariableStatements in Catch Blocks

The content of subclause 13.15.1 is replaced with the following:

catch

(

)

It is a Syntax Error if BoundNames of CatchParameter contains any duplicate elements.
It is a Syntax Error if any element of the BoundNames of CatchParameter also occurs in the LexicallyDeclaredNames of Block.
It is a Syntax Error if any element of the BoundNames of CatchParameter also occurs in the VarDeclaredNames of Block unless CatchParameter is CatchParameter:BindingIdentifier .

Note

The Block of a Catch clause may contain var declarations that bind a name that is also bound by the CatchParameter. At runtime, such bindings are instantiated in the VariableDeclarationEnvironment. They do not shadow the same-named bindings introduced by the CatchParameter and hence the Initializer for such var declarations will assign to the corresponding catch parameter rather than the var binding.

This modified behaviour also applies to var and function declarations introduced by direct eval calls contained within the Block of a Catch clause. This change is accomplished by modifying the algorithm of 18.2.1.3 as follows:

Step 3.d.i.2.a.i is replaced by:

If thisEnv is not the Environment Record for a Catch clause, throw a SyntaxError exception.

Step 7.d.ii.4.a.i.i is replaced by:

If thisEnv is not the Environment Record for a Catch clause, let bindingExists be true.

B.3.6 Initializers in ForIn Statement Heads

The following augments the IterationStatement production in 13.7:

[Yield, Await, Return]

for

(

var

[?Yield, ?Await]

[~In, ?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

This production only applies when parsing non-strict code.

The static semantics of ContainsDuplicateLabels in 13.7.5.3 are augmented with the following:

for

(

var

)

Return ContainsDuplicateLabels of Statement with argument labelSet.

The static semantics of ContainsUndefinedBreakTarget in 13.7.5.4 are augmented with the following:

for

(

var

)

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

The static semantics of ContainsUndefinedContinueTarget in 13.7.5.5 are augmented with the following:

for

(

var

)

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

The static semantics of IsDestructuring in 13.7.5.6 are augmented with the following:

yield

await

Return false.

The static semantics of VarDeclaredNames in 13.7.5.7 are augmented with the following:

for

(

var

)

Let names be the BoundNames of BindingIdentifier.
Append to names the elements of the VarDeclaredNames of Statement.
Return names.

The static semantics of VarScopedDeclarations in 13.7.5.8 are augmented with the following:

for

(

var

)

Let declarations be a List containing BindingIdentifier.
Append to declarations the elements of the VarScopedDeclarations of Statement.
Return declarations.

The runtime semantics of LabelledEvaluation in 13.7.5.11 are augmented with the following: