ECMAScript Language Specification - ECMA-262 Edition 5.1
Standard ECMA-262
5.1 Edition / June 2011
ECMAScript® Language Specification
This is the HTML rendering of
Ecma-262 Edition 5.1, The ECMAScript Language Specification.
The PDF rendering of this document is located at
The PDF version is the definitive specification. Any discrepancies between this HTML version and the PDF version are unintentional.
Copyright notice
Copyright © 2011 Ecma International
Ecma International
Rue du Rhone 114
CH-1204 Geneva
Tel: +41 22 849 6000
Fax: +41 22 849 6001
Web:
This document and possible translations of it may be copied and furnished to others, and derivative works that comment on or
otherwise explain it or assist in its implementation may be prepared, copied, published, and distributed, in whole or in part,
without restriction of any kind, provided that the above copyright notice and this section are included on all such copies and
derivative works. However, this document itself may not be modified in any way, including by removing the copyright notice or
references to Ecma International, except as needed for the purpose of developing any document or deliverable produced by Ecma
International (in which case the rules applied to copyrights must be followed) or as required to translate it into languages
other than English.
The limited permissions granted above are perpetual and will not be revoked by Ecma International or its successors or
assigns.
This document and the information contained herein is provided on an "AS IS" basis and ECMA INTERNATIONAL DISCLAIMS ALL
WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT
INFRINGE ANY OWNERSHIP RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
Software License
All Software contained in this document ("Software)" is protected by copyright and is being made available under the "BSD
License", included below. This Software may be subject to third party rights (rights from parties other than Ecma
International), including patent rights, and no licenses under such third party rights are granted under this license even if
the third party concerned is a member of Ecma International. SEE THE ECMA CODE OF CONDUCT IN PATENT MATTERS AVAILABLE AT
REQUIRED TO IMPLEMENT ECMA INTERNATIONAL STANDARDS*.
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following
conditions are met:
1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following
disclaimer.
2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following
disclaimer in the documentation and/or other materials provided with the distribution.
3. Neither the name of the authors nor Ecma International may be used to endorse or promote products derived from this
software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE ECMA INTERNATIONAL "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL ECMA
INTERNATIONAL BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
Contents
Copyright notice
Introduction
Scope
Conformance
Normative references
Overview
4.1
Web Scripting
4.2
Language Overview
4.2.1
Objects
4.2.2
The Strict Variant of
ECMAScript
4.3
Terms and definitions
4.3.1
type
4.3.2
primitive value
4.3.3
object
4.3.4
constructor
4.3.5
prototype
4.3.6
native object
4.3.7
built-in object
4.3.8
host object
4.3.9
undefined value
4.3.10
Undefined type
4.3.11
null value
4.3.12
Null type
4.3.13
Boolean value
4.3.14
Boolean type
4.3.15
Boolean object
4.3.16
String value
4.3.17
String type
4.3.18
String object
4.3.19
Number value
4.3.20
Number type
4.3.21
Number object
4.3.22
Infinity
4.3.23
NaN
4.3.24
function
4.3.25
built-in function
4.3.26
property
4.3.27
method
4.3.28
built-in method
4.3.29
attribute
4.3.30
own property
4.3.31
inherited property
Notational Conventions
5.1
Syntactic and Lexical Grammars
5.1.1
Context-Free Grammars
5.1.2
The Lexical and RegExp
Grammars
5.1.3
The Numeric String
Grammar
5.1.4
The Syntactic Grammar
5.1.5
The JSON Grammar
5.1.6
Grammar Notation
5.2
Algorithm Conventions
Source Text
Lexical Conventions
7.1
Unicode Format-Control
Characters
7.2
White Space
7.3
Line Terminators
7.4
Comments
7.5
Tokens
7.6
Identifier Names and Identifiers
7.6.1
Reserved Words
7.7
Punctuators
7.8
Literals
7.8.1
Null Literals
7.8.2
Boolean Literals
7.8.3
Numeric Literals
7.8.4
String Literals
7.8.5
Regular Expression
Literals
7.9
Automatic Semicolon Insertion
7.9.1
Rules of Automatic
Semicolon Insertion
7.9.2
Examples of Automatic
Semicolon Insertion
Types
8.1
The Undefined Type
8.2
The Null Type
8.3
The Boolean Type
8.4
The String Type
8.5
The Number Type
8.6
The Object Type
8.6.1
Property Attributes
8.6.2
Object Internal Properties
and Methods
8.7
The Reference Specification Type
8.7.1
GetValue (V)
8.7.2
PutValue (V, W)
8.8
The List Specification Type
8.9
The Completion Specification
Type
8.10
The Property Descriptor and
Property Identifier Specification Types
8.10.1
IsAccessorDescriptor (
Desc )
8.10.2
IsDataDescriptor ( Desc
8.10.3
IsGenericDescriptor (
Desc )
8.10.4
FromPropertyDescriptor (
Desc )
8.10.5
ToPropertyDescriptor (
Obj )
8.11
The Lexical Environment and
Environment Record Specification Types
8.12
Algorithms for Object Internal
Methods
8.12.1
[[GetOwnProperty]]
(P)
8.12.2
[[GetProperty]] (P)
8.12.3
[[Get]] (P)
8.12.4
[[CanPut]] (P)
8.12.5
[[Put]] ( P, V, Throw
8.12.6
[[HasProperty]] (P)
8.12.7
[[Delete]] (P,
Throw)
8.12.8
[[DefaultValue]]
(hint)
8.12.9
[[DefineOwnProperty]]
(P, Desc, Throw)
Type Conversion and Testing
9.1
ToPrimitive
9.2
ToBoolean
9.3
ToNumber
9.3.1
ToNumber Applied to the
String Type
9.4
ToInteger
9.5
ToInt32: (Signed 32 Bit
Integer)
9.6
ToUint32: (Unsigned 32 Bit
Integer)
9.7
ToUint16: (Unsigned 16 Bit
Integer)
9.8
ToString
9.8.1
ToString Applied to the
Number Type
9.9
ToObject
9.10
CheckObjectCoercible
9.11
IsCallable
9.12
The SameValue Algorithm
10
Executable Code and Execution Contexts
10.1
Types of Executable Code
10.1.1
Strict Mode Code
10.2
Lexical Environments
10.2.1
Environment Records
10.2.2
Lexical Environment
Operations
10.2.3
The Global
Environment
10.3
Execution Contexts
10.3.1
Identifier
Resolution
10.4
Establishing an Execution
Context
10.4.1
Entering Global
Code
10.4.2
Entering Eval Code
10.4.3
Entering Function
Code
10.5
Declaration Binding
Instantiation
10.6
Arguments Object
11
Expressions
11.1
Primary Expressions
11.1.1
The
this
Keyword
11.1.2
Identifier
Reference
11.1.3
Literal Reference
11.1.4
Array Initialiser
11.1.5
Object Initialiser
11.1.6
The Grouping
Operator
11.2
Left-Hand-Side Expressions
11.2.1
Property Accessors
11.2.2
The
new
Operator
11.2.3
Function Calls
11.2.4
Argument Lists
11.2.5
Function
Expressions
11.3
Postfix Expressions
11.3.1
Postfix Increment
Operator
11.3.2
Postfix Decrement
Operator
11.4
Unary Operators
11.4.1
The
delete
Operator
11.4.2
The
void
Operator
11.4.3
The
typeof
Operator
11.4.4
Prefix Increment
Operator
11.4.5
Prefix Decrement
Operator
11.4.6
Unary
Operator
11.4.7
Unary
Operator
11.4.8
Bitwise NOT Operator (
11.4.9
Logical NOT Operator (
11.5
Multiplicative Operators
11.5.1
Applying the
Operator
11.5.2
Applying the
Operator
11.5.3
Applying the
Operator
11.6
Additive Operators
11.6.1
The Addition operator (
11.6.2
The Subtraction Operator
11.6.3
Applying the Additive
Operators to Numbers
11.7
Bitwise Shift Operators
11.7.1
The Left Shift Operator
<<
11.7.2
The Signed Right Shift
Operator (
>>
11.7.3
The Unsigned Right Shift
Operator (
>>>
11.8
Relational Operators
11.8.1
The Less-than Operator (
11.8.2
The Greater-than
Operator (
11.8.3
The Less-than-or-equal
Operator (
<=
11.8.4
The
Greater-than-or-equal Operator (
>=
11.8.5
The Abstract Relational
Comparison Algorithm
11.8.6
The instanceof
operator
11.8.7
The in operator
11.9
Equality Operators
11.9.1
The Equals Operator (
==
11.9.2
The Does-not-equals
Operator (
!=
11.9.3
The Abstract Equality
Comparison Algorithm
11.9.4
The Strict Equals
Operator (
===
11.9.5
The Strict
Does-not-equal Operator (
!==
11.9.6
The Strict Equality
Comparison Algorithm
11.10
Binary Bitwise Operators
11.11
Binary Logical Operators
11.12
Conditional Operator (
? : )
11.13
Assignment Operators
11.13.1
Simple Assignment (
11.13.2
Compound Assignment (
op=
11.14
Comma Operator (
, )
12
Statements
12.1
Block
12.2
Variable Statement
12.2.1
Strict Mode
Restrictions
12.3
Empty Statement
12.4
Expression Statement
12.5
The
if
Statement
12.6
Iteration Statements
12.6.1
The
do
while
Statement
12.6.2
The
while
Statement
12.6.3
The
for
Statement
12.6.4
The
for
in
Statement
12.7
The
continue
Statement
12.8
The
break
Statement
12.9
The
return
Statement
12.10
The
with
Statement
12.10.1
Strict Mode
Restrictions
12.11
The
switch
Statement
12.12
Labelled Statements
12.13
The
throw
Statement
12.14
The
try
Statement
12.14.1
Strict Mode
Restrictions
12.15
The
debugger
statement
13
Function Definition
13.1
Strict Mode Restrictions
13.2
Creating Function Objects
13.2.1
[[Call]]
13.2.2
[[Construct]]
13.2.3
The [[ThrowTypeError]]
Function Object
14
Program
14.1
Directive Prologues and the Use
Strict Directive
15
Standard Built-in ECMAScript Objects
15.1
The Global Object
15.1.1
Value Properties of the
Global Object
15.1.2
Function Properties of
the Global Object
15.1.3
URI Handling Function
Properties
15.1.4
Constructor Properties
of the Global Object
15.1.5
Other Properties of the
Global Object
15.2
Object Objects
15.2.1
The Object Constructor
Called as a Function
15.2.2
The Object
Constructor
15.2.3
Properties of the Object
Constructor
15.2.4
Properties of the Object
Prototype Object
15.2.5
Properties of Object
Instances
15.3
Function Objects
15.3.1
The Function Constructor
Called as a Function
15.3.2
The Function
Constructor
15.3.3
Properties of the
Function Constructor
15.3.4
Properties of the
Function Prototype Object
15.3.5
Properties of Function
Instances
15.4
Array Objects
15.4.1
The Array Constructor
Called as a Function
15.4.2
The Array
Constructor
15.4.3
Properties of the Array
Constructor
15.4.4
Properties of the Array
Prototype Object
15.4.5
Properties of Array
Instances
15.5
String Objects
15.5.1
The String Constructor
Called as a Function
15.5.2
The String
Constructor
15.5.3
Properties of the String
Constructor
15.5.4
Properties of the String
Prototype Object
15.5.5
Properties of String
Instances
15.6
Boolean Objects
15.6.1
The Boolean Constructor
Called as a Function
15.6.2
The Boolean
Constructor
15.6.3
Properties of the
Boolean Constructor
15.6.4
Properties of the
Boolean Prototype Object
15.6.5
Properties of Boolean
Instances
15.7
Number Objects
15.7.1
The Number Constructor
Called as a Function
15.7.2
The Number
Constructor
15.7.3
Properties of the Number
Constructor
15.7.4
Properties of the Number
Prototype Object
15.7.5
Properties of Number
Instances
15.8
The Math Object
15.8.1
Value Properties of the
Math Object
15.8.2
Function Properties of
the Math Object
15.9
Date Objects
15.9.1
Overview of Date Objects
and Definitions of Abstract Operators
15.9.2
The Date Constructor
Called as a Function
15.9.3
The Date
Constructor
15.9.4
Properties of the Date
Constructor
15.9.5
Properties of the Date
Prototype Object
15.9.6
Properties of Date
Instances
15.10
RegExp (Regular Expression)
Objects
15.10.1
Patterns
15.10.2
Pattern Semantics
15.10.3
The RegExp Constructor
Called as a Function
15.10.4
The RegExp
Constructor
15.10.5
Properties of the
RegExp Constructor
15.10.6
Properties of the
RegExp Prototype Object
15.10.7
Properties of RegExp
Instances
15.11
Error Objects
15.11.1
The Error Constructor
Called as a Function
15.11.2
The Error
Constructor
15.11.3
Properties of the
Error Constructor
15.11.4
Properties of the
Error Prototype Object
15.11.5
Properties of Error
Instances
15.11.6
Native Error Types
Used in This Standard
15.11.7
NativeError
Object Structure
15.12
The JSON Object
15.12.1
The JSON Grammar
15.12.2
parse ( text [ ,
reviver ] )
15.12.3
stringify ( value [ ,
replacer [ , space ] ] )
16
Errors
Annex A
(informative)
Grammar Summary
A.1
Lexical Grammar
A.2
Number Conversions
A.3
Expressions
A.4
Statements
A.5
Functions and Programs
A.6
Universal Resource Identifier
Character Classes
A.7
Regular Expressions
A.8
JSON
A.8.1
JSON Lexical Grammar
A.8.2
JSON Syntactic
Grammar
Annex B
(informative)
Compatibility
B.1
Additional Syntax
B.1.1
Numeric Literals
B.1.2
String Literals
B.2
Additional Properties
B.2.1
escape (string)
B.2.2
unescape (string)
B.2.3
String.prototype.substr
(start, length)
B.2.4
Date.prototype.getYear (
B.2.5
Date.prototype.setYear
(year)
B.2.6
Date.prototype.toGMTString
( )
Annex C
(informative)
The Strict Mode of ECMAScript
Annex D
(informative)
Corrections and Clarifications in the 5
th
Edition with Possible
rd
Edition Compatibility Impact
Annex E
(informative)
Additions and Changes in the 5
th
Edition that Introduce
Incompatibilities with the 3
rd
Edition
Annex F
(informative)
Technically Significant Corrections and Clarifications in the 5.1 Edition
Bibliography
Introduction
This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript
(Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0
browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet
Explorer 3.0.
The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma
General Assembly of June 1997.
That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international
standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it
fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.
The third edition of the Standard introduced powerful regular expressions, better string handling, new control statements,
try/catch exception handling, tighter definition of errors, formatting for numeric output and minor changes in anticipation of
forthcoming internationalisation facilities and future language growth. The third edition of the ECMAScript standard was adopted
by the Ecma General Assembly of December 1999 and published as ISO/IEC 16262:2002 in June 2002.
Since publication of the third edition, ECMAScript has achieved massive adoption in conjunction with the World Wide Web where
it has become the programming language that is supported by essentially all web browsers. Significant work was done to develop a
fourth edition of ECMAScript. Although that work was not completed and not published as the fourth edition of ECMAScript, it
informs continuing evolution of the language. The fifth edition of ECMAScript (published as ECMA-262 5
th
edition)
codifies de facto interpretations of the language specification that have become common among browser implementations and adds
support for new features that have emerged since the publication of the third edition. Such features include accessor
properties, reflective creation and inspection of objects, program control of property attributes, additional array manipulation
functions, support for the JSON object encoding format, and a strict mode that provides enhanced error checking and program
security.
This present edition 5.1 of the ECMAScript Standard is fully aligned with third edition of the international standard ISO/IEC
16262:2011.
ECMAScript is a vibrant language and the evolution of the language is not complete. Significant technical enhancement will
continue with future editions of this specification.
This Ecma Standard has been adopted by the General Assembly of June 2011.
ECMAScript Language Specification
Scope
This Standard defines the ECMAScript scripting language.
Conformance
A conforming implementation of ECMAScript must provide and support all the types, values, objects, properties, functions, and
program syntax and semantics described in this specification.
A conforming implementation of this Standard shall interpret characters in conformance with the Unicode Standard, Version 3.0
or later and ISO/IEC 10646-1 with either UCS-2 or UTF-16 as the adopted encoding form, implementation level 3. If the adopted
ISO/IEC 10646-1 subset is not otherwise specified, it is presumed to be the BMP subset, collection 300. If the adopted encoding
form is not otherwise specified, it presumed to be the UTF-16 encoding form.
A conforming implementation of ECMAScript is permitted to provide additional types, values, objects, properties, and
functions beyond those described in this specification. In particular, a conforming implementation of ECMAScript is permitted to
provide properties not described in this specification, and values for those properties, for objects that are described in this
specification.
A conforming implementation of ECMAScript is permitted to support program and regular expression syntax not described in this
specification. In particular, a conforming implementation of ECMAScript is permitted to support program syntax that makes use of
the “future reserved words” listed in
7.6.1.2
of this specification.
Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the
edition cited applies. For undated references, the latest edition of the referenced document (including any amendments)
applies.
ISO/IEC 9899:1996
, Programming Languages – C, including amendment 1 and technical
corrigenda 1 and 2
ISO/IEC 10646-1:1993
, Information Technology – Universal Multiple-Octet Coded
Character Set (UCS) plus its amendments and corrigenda
Overview
This section contains a non-normative overview of the ECMAScript language.
ECMAScript is an object-oriented programming language for performing computations and manipulating computational objects
within a host environment. ECMAScript as defined here is not intended to be computationally self-sufficient; indeed, there are
no provisions in this specification for input of external data or output of computed results. Instead, it is expected that the
computational environment of an ECMAScript program will provide not only the objects and other facilities described in this
specification but also certain environment-specific
host
objects, whose description and behaviour are beyond the scope of
this specification except to indicate that they may provide certain properties that can be accessed and certain functions that
can be called from an ECMAScript program.
scripting language
is a programming language that is used to manipulate, customise, and automate the
facilities of an existing system. In such systems, useful functionality is already available through a user interface, and the
scripting language is a mechanism for exposing that functionality to program control. In this way, the existing system is said
to provide a host environment of objects and facilities, which completes the capabilities of the scripting language. A scripting
language is intended for use by both professional and non-professional programmers.
ECMAScript was originally designed to be a
Web scripting language
, providing a mechanism to enliven Web pages
in browsers and to perform server computation as part of a Web-based client-server architecture. ECMAScript can provide core
scripting capabilities for a variety of host environments, and therefore the core scripting language is specified in this
document apart from any particular host environment.
Some of the facilities of ECMAScript are similar to those used in other programming languages; in particular Java™,
Self, and Scheme as described in:
Gosling, James, Bill Joy and Guy Steele.
The Java™ Language
Specification
. Addison Wesley Publishing Co., 1996.
Ungar, David, and Smith, Randall B.
Self: The Power of Simplicity
OOPSLA '87 Conference Proceedings, pp. 227–241, Orlando, FL, October 1987.
IEEE Standard for the Scheme Programming Language
. IEEE Std
1178-1990.
4.1
Web Scripting
A web browser provides an ECMAScript host environment for client-side computation including, for instance, objects that
represent windows, menus, pop-ups, dialog boxes, text areas, anchors, frames, history, cookies, and input/output. Further, the
host environment provides a means to attach scripting code to events such as change of focus, page and image loading,
unloading, error and abort, selection, form submission, and mouse actions. Scripting code appears within the HTML and the
displayed page is a combination of user interface elements and fixed and computed text and images. The scripting code is
reactive to user interaction and there is no need for a main program.
A web server provides a different host environment for server-side computation including objects representing requests,
clients, and files; and mechanisms to lock and share data. By using browser-side and server-side scripting together, it is
possible to distribute computation between the client and server while providing a customised user interface for a Web-based
application.
Each Web browser and server that supports ECMAScript supplies its own host environment, completing the ECMAScript execution
environment.
4.2
Language Overview
The following is an informal overview of ECMAScript—not all parts of the language are described. This overview is not
part of the standard proper.
ECMAScript is object-based: basic language and host facilities are provided by objects, and an ECMAScript program is a
cluster of communicating objects. An ECMAScript
object
is a collection of
properties
each with
zero or more
attributes
that determine how each property can be used—for example, when the Writable
attribute for a property is set to
false
, any attempt by executed ECMAScript code to change the value of the property
fails. Properties are containers that hold other objects,
primitive values
, or
functions
. A
primitive value is a member of one of the following built-in types:
Undefined
Null
Boolean
Number
, and
String
; an object is a member of the remaining built-in type
Object
; and a function is a
callable object. A function that is associated with an object via a property is a
method
ECMAScript defines a collection of
built-in objects
that round out the definition of ECMAScript entities.
These built-in objects include the global object, the
Object
object, the
Function
object, the
Array
object, the
String
object, the
Boolean
object, the
Number
object, the
Math
object, the
Date
object, the
RegExp
object, the
JSON
object, and the Error objects
Error, EvalError
RangeError,
ReferenceError, SyntaxError, TypeError
and
URIError
ECMAScript also defines a set of built-in
operators
. ECMAScript operators include various unary operations,
multiplicative operators, additive operators, bitwise shift operators, relational operators, equality operators, binary
bitwise operators, binary logical operators, assignment operators, and the comma operator.
ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to serve as an easy-to-use
scripting language. For example, a variable is not required to have its type declared nor are types associated with
properties, and defined functions are not required to have their declarations appear textually before calls to them.
4.2.1
Objects
ECMAScript does not use classes such as those in C++, Smalltalk, or Java. Instead objects may be created in various ways
including via a literal notation or via
constructors
which create objects and then execute code that
initialises all or part of them by assigning initial values to their properties. Each constructor is a function that has a
property named “
prototype
” that is used to implement
prototype-based inheritance
and
shared properties
. Objects are created by using constructors in
new
expressions; for example,
new
Date(2009,11)
creates a new Date object. Invoking a constructor without using
new
has consequences that depend
on the constructor. For example,
Date()
produces a string representation of the current date and time rather
than an object.
Every object created by a constructor has an implicit reference (called the object’s
prototype
) to the value
of its constructor’s “
prototype
” property. Furthermore, a prototype may have a non-null
implicit reference to its prototype, and so on; this is called the
prototype chain
. When a reference is made to a
property in an object, that reference is to the property of that name in the first object in the prototype chain that
contains a property of that name. In other words, first the object mentioned directly is examined for such a property; if
that object contains the named property, that is the property to which the reference refers; if that object does not contain
the named property, the prototype for that object is examined next; and so on.
Figure 1 — Object/Prototype Relationships
In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and
inheritance is only of structure and behaviour. In ECMAScript, the state and methods are carried by objects, and structure,
behaviour, and state are all inherited.
All objects that do not directly contain a particular property that their prototype contains share that property and its
value. Figure 1 illustrates this:
CF
is a constructor (and also an object). Five objects have been created by using
new
expressions:
cf
cf
cf
cf
, and
cf
. Each
of these objects contains properties named
q1
and
q2
. The dashed lines represent the implicit prototype relationship; so, for example,
cf
’s prototype is
CF
. The constructor,
CF
, has two properties itself,
named
P1
and
P2
, which are not
visible to
CF
cf
cf
cf
cf
, or
cf
. The property named
CFP1
in
CF
is shared by
cf
cf
cf
cf
, and
cf
(but not by
CF
), as are any properties found in
CF
’s implicit prototype chain that are not named
q1
q2
, or
CFP1
. Notice that there is no
implicit prototype link between
CF
and
CF
Unlike class-based object languages, properties can be added to objects dynamically by assigning values to them. That is,
constructors are not required to name or assign values to all or any of the constructed object’s properties. In the
above diagram, one could add a new shared property for
cf
cf
cf
cf
, and
cf
by assigning a new value to the property in
CF
4.2.2
The Strict Variant of
ECMAScript
The ECMAScript Language recognises the possibility that some users of the language may wish to restrict their usage of
some features available in the language. They might do so in the interests of security, to avoid what they consider to be
error-prone features, to get enhanced error checking, or for other reasons of their choosing. In support of this
possibility, ECMAScript defines a strict variant of the language. The strict variant of the language excludes some specific
syntactic and semantic features of the regular ECMAScript language and modifies the detailed semantics of some features. The
strict variant also specifies additional error conditions that must be reported by throwing error exceptions in situations
that are not specified as errors by the non-strict form of the language.
The strict variant of ECMAScript is commonly referred to as the
strict mode
of the language. Strict mode selection
and use of the strict mode syntax and semantics of ECMAScript is explicitly made at the level of individual ECMAScript code
units. Because strict mode is selected at the level of a syntactic code unit, strict mode only imposes restrictions that
have local effect within such a code unit. Strict mode does not restrict or modify any aspect of the ECMAScript semantics
that must operate consistently across multiple code units. A complete ECMAScript program may be composed for both strict
mode and non-strict mode ECMAScript code units. In this case, strict mode only applies when actually executing code that is
defined within a
strict mode code
unit.
In order to conform to this specification, an ECMAScript implementation must implement both the full unrestricted
ECMAScript language and the strict mode variant of the ECMAScript language as defined by this specification. In addition, an
implementation must support the combination of unrestricted and
strict mode code
units into a
single composite program.
4.3
Terms and definitions
For the purposes of this document, the following terms and definitions apply.
4.3.1
type
set of data values as defined in
Clause 8
of this specification
4.3.2
primitive value
member of one of the types Undefined, Null, Boolean, Number, or String as defined in
Clause 8
NOTE
A primitive value is a datum that is represented directly at the lowest level of the
language implementation.
4.3.3
object
member of the type Object
NOTE
An object is a collection of properties and has a single prototype object. The prototype
may be the null value.
4.3.4
constructor
function object that creates and initialises objects
NOTE
The value of a constructor’s “
prototype
” property is a prototype object that is used to implement inheritance and shared
properties.
4.3.5
prototype
object that provides shared properties for other objects
NOTE
When a constructor creates an object, that object implicitly references the
constructor’s “
prototype
” property for the purpose of resolving property references. The
constructor’s “
prototype
” property can be referenced by the program expression
constructor
.prototype
, and properties added to an object’s
prototype are shared, through inheritance, by all objects sharing the prototype. Alternatively, a new object may be
created with an explicitly specified prototype by using the
Object.create
built-in function.
4.3.6
native object
object in an ECMAScript implementation whose semantics are fully defined by this specification rather than by the host
environment
NOTE
Standard native objects are defined in this specification. Some native objects are
built-in; others may be constructed during the course of execution of an ECMAScript program.
4.3.7
built-in object
object supplied by an ECMAScript implementation, independent of the host environment, that is present at the start of the
execution of an ECMAScript program
NOTE
Standard built-in objects are defined in this specification, and an ECMAScript
implementation may specify and define others. Every built-in object is a native object. A
built-in constructor
is a
built-in object that is also a constructor.
4.3.8
host object
object supplied by the host environment to complete the execution environment of ECMAScript
NOTE
Any object that is not native is a host object.
4.3.9
undefined value
primitive value used when a variable has not been assigned a value
4.3.10
Undefined type
type whose sole value is the undefined value
4.3.11
null value
primitive value that represents the intentional absence of any object value
4.3.12
Null type
type whose sole value is the null value
4.3.13
Boolean value
member of the Boolean type
NOTE
There are only two Boolean values,
true
and
false
4.3.14
Boolean type
type consisting of the primitive values
true
and
false
4.3.15
Boolean object
member of the Object type that is an instance of the standard built-in
Boolean
constructor
NOTE
A Boolean object is created by using the
Boolean
constructor in a
new
expression, supplying a Boolean value as an argument. The resulting object has an internal property whose
value is the Boolean value. A Boolean object can be coerced to a Boolean value.
4.3.16
String value
primitive value that is a finite ordered sequence of zero or more 16-bit unsigned integer
NOTE
A String value is a member of the String type. Each integer value in the sequence usually
represents a single 16-bit unit of UTF-16 text. However, ECMAScript does not place any restrictions or requirements on the
values except that they must be 16-bit unsigned integers.
4.3.17
String type
set of all possible String values
4.3.18
String object
member of the Object type that is an instance of the standard built-in
String
constructor
NOTE
A String object is created by using the
String
constructor in a
new
expression, supplying a String value as an argument. The resulting object has an internal property whose
value is the String value. A String object can be coerced to a String value by calling the
String
constructor
as a function (
15.5.1
).
4.3.19
Number value
primitive value corresponding to a double-precision 64-bit binary format IEEE 754 value
NOTE
A Number value is a member of the Number type and is a direct representation of a
number.
4.3.20
Number type
set of all possible Number values including the special “Not-a-Number” (NaN) values, positive infinity, and
negative infinity
4.3.21
Number object
member of the Object type that is an instance of the standard built-in
Number
constructor
NOTE
A Number object is created by using the
Number
constructor in a
new
expression, supplying a Number value as an argument. The resulting object has an internal property whose
value is the Number value. A Number object can be coerced to a Number value by calling the
Number
constructor
as a function (
15.7.1
).
4.3.22
Infinity
number value that is the positive infinite Number value
4.3.23
NaN
number value that is a IEEE 754 “Not-a-Number” value
4.3.24
function
member of the Object type that is an instance of the standard built-in
Function
constructor and that may be
invoked as a subroutine
NOTE
In addition to its named properties, a function contains executable code and state that
determine how it behaves when invoked. A function’s code may or may not be written in ECMAScript.
4.3.25
built-in function
built-in object that is a function
NOTE
Examples of built-in functions include
parseInt
and
Math.exp
. An
implementation may provide implementation-dependent built-in functions that are not described in this specification.
4.3.26
property
association between a name and a value that is a part of an object
NOTE
Depending upon the form of the property the value may be represented either directly as a
data value (a primitive value, an object, or a function object) or indirectly by a pair of accessor functions.
4.3.27
method
function that is the value of a property
NOTE
When a function is called as a method of an object, the object is passed to the function
as its
this
value.
4.3.28
built-in method
method that is a built-in function
NOTE
Standard built-in methods are defined in this specification, and an ECMAScript
implementation may specify and provide other additional built-in methods.
4.3.29
attribute
internal value that defines some characteristic of a property
4.3.30
own property
property that is directly contained by its object
4.3.31
inherited property
property of an object that is not an own property but is a property (either own or inherited) of the object’s
prototype
Notational Conventions
5.1
Syntactic and Lexical Grammars
5.1.1
Context-Free Grammars
context-free grammar
consists of a number of
productions
. Each production has an abstract symbol called a
nonterminal
as its
left-hand side
, and a sequence of zero or more nonterminal and
terminal
symbols as
its
right-hand side
. For each grammar, the terminal symbols are drawn from a specified alphabet.
Starting from a sentence consisting of a single distinguished nonterminal, called the
goal symbol
, a given
context-free grammar specifies a
language
, namely, the (perhaps infinite) set of possible sequences of terminal
symbols that can result from repeatedly replacing any nonterminal in the sequence with a right-hand side of a production for
which the nonterminal is the left-hand side.
5.1.2
The Lexical and RegExp
Grammars
lexical grammar
for ECMAScript is given in
clause 7
. This grammar has as its terminal
symbols characters (Unicode code units) that conform to the rules for
SourceCharacter
defined in
Clause 6
. It defines a set of productions, starting from the goal symbol
InputElementDiv
or
InputElementRegExp
, that describe how sequences of such
characters are translated into a sequence of input elements.
Input elements other than white space and comments form the terminal symbols for the syntactic grammar for ECMAScript and
are called ECMAScript
tokens
. These tokens are the reserved words, identifiers, literals, and punctuators of the
ECMAScript language. Moreover, line terminators, although not considered to be tokens, also become part of the stream of
input elements and guide the process of
automatic semicolon insertion (7.9)
. Simple white space and
single-line comments are discarded and do not appear in the stream of input elements for the syntactic grammar. A
MultiLineComment
(that is, a comment of the form “
/*
*/
” regardless of whether it spans more than
one line) is likewise simply discarded if it contains no line terminator; but if a
MultiLineComment
contains one or more line terminators, then it is replaced by a single line terminator, which becomes part of the stream of
input elements for the syntactic grammar.
RegExp grammar
for ECMAScript is given in
15.10
. This grammar also has as its terminal
symbols the characters as defined by
SourceCharacter
. It defines a set of productions, starting from
the goal symbol
Pattern
, that describe how sequences of characters are translated into regular
expression patterns.
Productions of the lexical and RegExp grammars are distinguished by having two colons “
::
” as
separating punctuation. The lexical and RegExp grammars share some productions.
5.1.3
The Numeric String Grammar
Another grammar is used for translating Strings into numeric values. This grammar is similar to the part of the lexical
grammar having to do with numeric literals and has as its terminal symbols
SourceCharacter
. This
grammar appears in
9.3.1
Productions of the numeric string grammar are distinguished by having three colons “
:::
” as
punctuation.
5.1.4
The Syntactic Grammar
The
syntactic grammar
for ECMAScript is given in clauses 11, 12, 13 and 14. This grammar has ECMAScript tokens
defined by the lexical grammar as its terminal symbols (
5.1.2
). It defines a set of productions,
starting from the goal symbol
Program
, that describe how sequences of tokens can form syntactically
correct ECMAScript programs.
When a stream of characters is to be parsed as an ECMAScript program, it is first converted to a stream of input elements
by repeated application of the lexical grammar; this stream of input elements is then parsed by a single application of the
syntactic grammar. The program is syntactically in error if the tokens in the stream of input elements cannot be parsed as a
single instance of the goal nonterminal
Program
, with no tokens left over.
Productions of the syntactic grammar are distinguished by having just one colon “
” as
punctuation.
The syntactic grammar as presented in clauses 11, 12, 13 and 14 is actually not a complete account of which token
sequences are accepted as correct ECMAScript programs. Certain additional token sequences are also accepted, namely, those
that would be described by the grammar if only semicolons were added to the sequence in certain places (such as before line
terminator characters). Furthermore, certain token sequences that are described by the grammar are not considered acceptable
if a terminator character appears in certain “awkward” places.
5.1.5
The JSON Grammar
The JSON grammar is used to translate a String describing a set of ECMAScript objects into actual objects. The JSON
grammar is given in
15.12.1
The JSON grammar consists of the JSON lexical grammar and the JSON syntactic grammar. The JSON lexical grammar is used to
translate character sequences into tokens and is similar to parts of the ECMAScript lexical grammar. The JSON syntactic
grammar describes how sequences of tokens from the JSON lexical grammar can form syntactically correct JSON object
descriptions.
Productions of the JSON lexical grammar are distinguished by having two colons “
::
” as separating
punctuation. The JSON lexical grammar uses some productions from the ECMAScript lexical grammar. The JSON syntactic grammar
is similar to parts of the ECMAScript syntactic grammar. Productions of the JSON syntactic grammar are distinguished by
using one colon “
” as separating punctuation.
5.1.6
Grammar Notation
Terminal symbols of the lexical, RegExp, and numeric string grammars, and some of the terminal symbols of the other
grammars, are shown in
fixed width
font, both in the productions of the grammars and throughout this
specification whenever the text directly refers to such a terminal symbol. These are to appear in a program exactly as
written. All terminal symbol characters specified in this way are to be understood as the appropriate Unicode character from
the ASCII range, as opposed to any similar-looking characters from other Unicode ranges.
Nonterminal symbols are shown in
italic
type. The definition of a nonterminal is introduced by the name of the
nonterminal being defined followed by one or more colons. (The number of colons indicates to which grammar the production
belongs.) One or more alternative right-hand sides for the nonterminal then follow on succeeding lines. For example, the
syntactic definition:
WhileStatement
while
Expression
Statement
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:
ArgumentList
AssignmentExpression
ArgumentList
AssignmentExpression
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:
VariableDeclaration
Identifier
Initialiser
opt
is a convenient abbreviation for:
VariableDeclaration
Identifier
Identifier
Initialiser
and that:
IterationStatement
for
ExpressionNoIn
opt
Expression
opt
Expression
opt
Statement
is a convenient abbreviation for:
IterationStatement
for
Expression
opt
Expression
opt
Statement
for
ExpressionNoIn
Expression
opt
Expression
opt
Statement
which in turn is an abbreviation for:
IterationStatement
for
Expression
opt
Statement
for
Expression
Expression
opt
Statement
for
ExpressionNoIn
Expression
opt
Statement
for
ExpressionNoIn
Expression
Expression
opt
Statement
which in turn is an abbreviation for:
IterationStatement
for
Statement
for
Expression
Statement
for
Expression
Statement
for
Expression
Expression
Statement
for
ExpressionNoIn
Statement
for
ExpressionNoIn
Expression
Statement
for
ExpressionNoIn
Expression
Statement
for
ExpressionNoIn
Expression
Expression
Statement
so the nonterminal
IterationStatement
actually has eight alternative right-hand sides.
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:
NonZeroDigit
::
one of
which is merely a convenient abbreviation for:
NonZeroDigit
::
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 is a member of the given
set
. The
set
can be written as a list of terminals enclosed in curly braces. 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. For example, given the definitions
DecimalDigit
::
one of
DecimalDigits
::
DecimalDigit
DecimalDigits
DecimalDigit
the definition
LookaheadExample
::
[lookahead ∉ {
}]
DecimalDigits
DecimalDigit
[lookahead ∉
DecimalDigit
matches either the letter
followed by one or more decimal digits the first of which is even, or a decimal
digit not followed by another decimal digit.
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:
ThrowStatement
throw
[no
LineTerminator
here]
Expression
indicates that the production may not be used if a
LineTerminator
occurs in the program 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 program.
When an alternative in a production of the lexical grammar or the numeric string grammar appears to be a multi-character
token, it represents the sequence of characters 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:
Identifier
::
IdentifierName
but not
ReservedWord
means that the nonterminal
Identifier
may be replaced by any sequence of characters that could
replace
IdentifierName
provided that the same sequence of characters 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:
SourceCharacter
::
any Unicode code unit
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.
In order to facilitate their use in multiple parts of this specification, some algorithms, called
abstract
operations
, are named and written in parameterised functional form so that they may be referenced by name from within
other algorithms.
When an algorithm is to produce a value as a result, the directive “
return
” is used to indicate that the result of the algorithm is the value of
and that the
algorithm should terminate. The notation
Result(
is used as
shorthand for “the
result of step
”.
For clarity of expression, 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
Substep.
Substep
Subsubstep.
Subsubstep.
Subsubsubstep
Subsubsubsubstep
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 may assert 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.
Mathematical operations such as addition, subtraction, negation, multiplication, division, and the mathematical functions
defined later in this clause should always be understood as computing exact mathematical results on mathematical real numbers,
which do not include infinities and do not include a negative zero that is distinguished from positive zero. Algorithms in
this standard that model floating-point arithmetic include explicit steps, where necessary, to handle infinities and signed
zero and to perform rounding. If a mathematical operation or function is applied to a floating-point number, it should be
understood as being applied to the exact mathematical value represented by that floating-point number; such a floating-point
number must be finite, and if it is
+0
or
−0
then the
corresponding mathematical value is simply
The mathematical function
abs(
yields the absolute value of
, which is
if
is negative (less
than zero) and otherwise is
itself.
The mathematical function
sign(
yields
if
is positive and
−1
if
is negative. The sign function is not used in this standard for cases when
is zero.
The notation “
modulo
” (
must be
finite and nonzero) computes a value
of the same sign as
(or zero) such that
abs(
) < abs(
) and
for some integer
The mathematical function
floor(
yields the largest integer
(closest to positive infinity) that is not larger than
NOTE
floor(
) =
−(
modulo
1)
If an algorithm is defined to “throw an exception”, execution of the algorithm is terminated and no result is
returned. The calling algorithms are also terminated, until an algorithm step is reached that explicitly deals with the
exception, using terminology such as “If an exception was thrown…”. Once such an algorithm step has been
encountered the exception is no longer considered to have occurred.
Source Text
ECMAScript source text is represented as a sequence of characters in the Unicode character encoding, version 3.0 or later.
The text is expected to have been normalised to Unicode Normalization Form C (canonical composition), as described in Unicode
Technical Report #15. Conforming ECMAScript implementations are not required to perform any normalisation of text, or behave as
though they were performing normalisation of text, themselves. ECMAScript source text is assumed to be a sequence of 16-bit
code units for the purposes of this specification. Such a source text may include sequences of 16-bit code units that are not
valid UTF-16 character encodings. If an actual source text is encoded in a form other than 16-bit code units it must be
processed as if it was first converted to UTF-16.
Syntax
SourceCharacter
::
any Unicode code unit
Throughout the rest of this document, the phrase “code unit” and the word “character” will be used to
refer to a 16-bit unsigned value used to represent a single 16-bit unit of text. The phrase “Unicode character” will
be used to refer to the abstract linguistic or typographical unit represented by a single Unicode scalar value (which may be
longer than 16 bits and thus may be represented by more than one code unit). The phrase “code point” refers to such
a Unicode scalar value. “Unicode character” only refers to entities represented by single Unicode scalar values: the
components of a combining character sequence are still individual “Unicode characters,” even though a user might
think of the whole sequence as a single character.
In string literals, regular expression literals, and identifiers, any character (code unit) may also be expressed as a
Unicode escape sequence consisting of six characters, namely
\u
plus four hexadecimal digits. Within a comment,
such an escape sequence is effectively ignored as part of the comment. Within a string literal or regular expression literal,
the Unicode escape sequence contributes one character to the value of the literal. Within an identifier, the escape sequence
contributes one character to the identifier.
NOTE
Although this document sometimes refers to a “transformation” between a
“character” within a “string” and the 16-bit unsigned integer that is the code unit of that character,
there is actually no transformation because a “character” within a “string” is actually represented
using that 16-bit unsigned value.
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 character
000A
is line feed) and therefore the next character 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 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 a character
to the String value of the literal and is never interpreted as a line terminator or as a quote mark that might terminate the
string literal.
Lexical Conventions
The source text of an ECMAScript program 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 characters as the next input element.
There are two goal symbols for the lexical grammar. The
InputElementDiv
symbol is used in those
syntactic grammar contexts where a leading division (
) or division-assignment (
/=
) operator is
permitted. The
InputElementRegExp
symbol is used in other syntactic grammar contexts.
NOTE
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 7.9)
; in examples such as the following:
a = b
/hi/g.exec(c).map(d);
where the first non-whitespace, non-comment character after a
LineTerminator
is slash
) 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
InputElementDiv
::
WhiteSpace
LineTerminator
Comment
Token
DivPunctuator
InputElementRegExp
::
WhiteSpace
LineTerminator
Comment
Token
RegularExpressionLiteral
7.1
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 and regular expression literals.
and
are format-control characters that are used to make necessary distinctions when forming words or
phrases in certain languages. In ECMAScript source text,
and
may also be used in an identifier after the first character.
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.
characters intended for this purpose can sometimes also appear after the start of a text, for
example as a result of concatenating files.
see 7.2
).
The special treatment of certain format-control characters outside of comments, string literals, and regular expression
literals is summarised in Table 1.
Table 1 — Format-Control Character Usage
Code Unit
Value
Name
Formal Name
Usage
\u200C
Zero width non-joiner
IdentifierPart
\u200D
Zero width joiner
IdentifierPart
\uFEFF
Byte Order Mark
Whitespace
7.2
White Space
White space characters are used to improve source text readability and to separate tokens (indivisible lexical units) from
each other, but are otherwise insignificant. White space characters may occur between any two tokens and at the start or end
of input. White space characters may also occur within a
StringLiteral
or a
RegularExpressionLiteral
(where they are considered significant characters forming part of the literal
value) or within a
Comment
, but cannot appear within any other kind of token.
The ECMAScript white space characters are listed in Table 2.
Table 2 — Whitespace Characters
Code Unit Value
Name
Formal Name
\u0009
Tab
\u000B
Vertical Tab
\u000C
Form Feed
\u0020
Space
\u00A0
No-break space
\uFEFF
Other category “Zs”
Byte Order Mark
Any other Unicode “space separator”
ECMAScript implementations must recognise all of the white space characters defined in Unicode 3.0. Later editions of the
Unicode Standard may define other white space characters. ECMAScript implementations may recognise white space characters from
later editions of the Unicode Standard.
Syntax
WhiteSpace
::
7.3
Line Terminators
Like white space characters, line terminator characters are used to improve source text readability and to separate tokens
(indivisible lexical units) from each other. However, unlike white space characters, 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 (7.9)
. A line terminator cannot occur within any token except a
StringLiteral
. Line terminators may only occur within a
StringLiteral
token as part
of a
LineContinuation
A line terminator can occur within a
MultiLineComment
7.4
) but cannot occur
within a
SingleLineComment
Line terminators are included in the set of white space characters that are matched by the
\s
class in regular
expressions.
The ECMAScript line terminator characters are listed in Table 3.
Table 3 — Line Terminator Characters
Code Unit Value
Name
Formal Name
\u000A
Line Feed
\u000D
Carriage Return
\u2028
Line separator
\u2029
Paragraph separator
Only the characters in Table 3 are treated as line terminators. Other new line or line breaking characters are treated as
white space but not as line terminators. The character sequence
should be considered a single character for the purpose of reporting line numbers.
Syntax
LineTerminator
::
LineTerminatorSequence
::
[lookahead ∉
7.4
Comments
Comments can be either single or multi-line. Multi-line comments cannot nest.
Because a single-line comment can contain any character except a
LineTerminator
character, and
because of the general rule that a token is always as long as possible, a single-line comment always consists of all
characters 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 recognised 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 7.9)
Comments behave like white space and are discarded except that, if a
MultiLineComment
contains a
line terminator character, then the entire comment is considered to be a
LineTerminator
for purposes
of parsing by the syntactic grammar.
Syntax
Comment
::
MultiLineComment
SingleLineComment
MultiLineComment
::
/*
MultiLineCommentChars
opt
*/
MultiLineCommentChars
::
MultiLineNotAsteriskChar
MultiLineCommentChars
opt
PostAsteriskCommentChars
opt
PostAsteriskCommentChars
::
MultiLineNotForwardSlashOrAsteriskChar
MultiLineCommentChars
opt
PostAsteriskCommentChars
opt
MultiLineNotAsteriskChar
::
SourceCharacter
but not
MultiLineNotForwardSlashOrAsteriskChar
::
SourceCharacter
but not one of
or
SingleLineComment
::
//
SingleLineCommentChars
opt
SingleLineCommentChars
::
SingleLineCommentChar
SingleLineCommentChars
opt
SingleLineCommentChar
::
SourceCharacter
but not
LineTerminator
7.5
Tokens
Syntax
Token
::
IdentifierName
Punctuator
NumericLiteral
StringLiteral
NOTE
The
DivPunctuator
and
RegularExpressionLiteral
productions define tokens, but are not included in the
Token
production.
7.6
Identifier Names and Identifiers
Identifier Names are tokens that are interpreted according to the grammar given in the “Identifiers” section of
chapter 5 of the Unicode standard, with some small modifications. An
Identifier
is an
IdentifierName
that is not a
ReservedWord
see 7.6.1
). The
Unicode identifier grammar is based on both normative and informative character categories specified by the Unicode Standard.
The characters in the specified categories in version 3.0 of the Unicode standard must be treated as in those categories by
all conforming ECMAScript implementations.
This standard specifies specific character additions: The dollar sign (
) and the underscore (
are permitted anywhere in an
IdentifierName
Unicode escape sequences are also permitted in an
IdentifierName
, where they contribute a single
character to the
IdentifierName
, as computed by the CV of the
UnicodeEscapeSequence
see 7.8.4
). The
preceding the
UnicodeEscapeSequence
does not contribute a character to the
IdentifierName
. A
UnicodeEscapeSequence
cannot be used to put a character into an
IdentifierName
that
would otherwise be illegal. In other words, if a
UnicodeEscapeSequence
sequence were
replaced by its
UnicodeEscapeSequence
's CV, the result must still be a valid
IdentifierName
that has the exact same sequence of characters as the original
IdentifierName
. All interpretations of identifiers within this specification are based upon their actual
characters regardless of whether or not an escape sequence was used to contribute any particular characters.
Two
IdentifierName
that are canonically equivalent according to the Unicode standard are
not
equal unless they are represented by the exact same sequence of code units (in other words, conforming ECMAScript
implementations are only required to do bitwise comparison on IdentifierName values). The intent is that the incoming source
text has been converted to normalised form C before it reaches the compiler.
ECMAScript implementations may recognise identifier characters defined in later editions of the Unicode Standard. If
portability is a concern, programmers should only employ identifier characters defined in Unicode 3.0.
Syntax
Identifier
::
IdentifierName
but not
ReservedWord
IdentifierName
::
IdentifierStart
IdentifierName
IdentifierPart
IdentifierStart
::
UnicodeLetter
UnicodeEscapeSequence
IdentifierPart
::
IdentifierStart
UnicodeCombiningMark
UnicodeDigit
UnicodeConnectorPunctuation
UnicodeLetter
::
any character in the Unicode categories “Uppercase letter (Lu)”, “Lowercase letter (Ll)”, “Titlecase letter (Lt)”, “Modifier letter (Lm)”, “Other letter (Lo)”, or “Letter number (Nl)”.
UnicodeCombiningMark
::
any character in the Unicode categories “Non-spacing mark (Mn)” or “Combining spacing mark (Mc)”
UnicodeDigit
::
any character in the Unicode category “Decimal number (Nd)”
UnicodeConnectorPunctuation
::
any character in the Unicode category “Connector punctuation (Pc)”
The definitions of the nonterminal
UnicodeEscapeSequence
is given in
7.8.4
7.6.1
Reserved Words
A reserved word is an
IdentifierName
that cannot be used as an
Identifier
Syntax
ReservedWord
::
Keyword
FutureReservedWord
NullLiteral
BooleanLiteral
7.6.1.1
Keywords
The following tokens are ECMAScript keywords and may not be used as
Identifiers
in ECMAScript
programs.
Syntax
Keyword
::
one of
break
do
instanceof
typeof
case
else
new
var
catch
finally
return
void
continue
for
switch
while
debugger
function
this
with
default
if
throw
delete
in
try
7.6.1.2
Future Reserved Words
The following words are used as keywords in proposed extensions and are therefore reserved to allow for the possibility
of future adoption of those extensions.
Syntax
FutureReservedWord
::
one of
class
enum
extends
super
const
export
import
The following tokens are also considered to be
FutureReservedWords
when they occur within
strict mode code (see 10.1.1)
. The occurrence of any of these tokens within
strict mode code
in any context where the occurrence of a
FutureReservedWord
would produce an error must also produce an equivalent error:
implements
let
private
public
yield
interface
package
protected
static
7.7
Punctuators
Syntax
Punctuator
::
one of
<=
>=
==
!=
===
!==
++
--
<<
>>
>>>
&&
||
+=
-=
*=
%=
<<=
>>=
>>>=
&=
|=
^=
DivPunctuator
::
one of
/=
7.8
Literals
Syntax
Literal
::
NullLiteral
BooleanLiteral
NumericLiteral
StringLiteral
RegularExpressionLiteral
7.8.1
Null Literals
Syntax
NullLiteral
::
null
Semantics
The value of the null literal
null
is the sole value of the Null type, namely
null
7.8.2
Boolean Literals
Syntax
BooleanLiteral
::
true
false
Semantics
The value of the Boolean literal
true
is a value of the Boolean type, namely
true
The value of the Boolean literal
false
is a value of the Boolean type, namely
false
7.8.3
Numeric Literals
Syntax
NumericLiteral
::
DecimalLiteral
HexIntegerLiteral
DecimalLiteral
::
DecimalIntegerLiteral
DecimalDigits
opt
ExponentPart
opt
DecimalDigits
ExponentPart
opt
DecimalIntegerLiteral
ExponentPart
opt
DecimalIntegerLiteral
::
NonZeroDigit
DecimalDigits
opt
DecimalDigits
::
DecimalDigit
DecimalDigits
DecimalDigit
DecimalDigit
::
one of
NonZeroDigit
::
one of
ExponentPart
::
ExponentIndicator
SignedInteger
ExponentIndicator
::
one of
SignedInteger
::
DecimalDigits
DecimalDigits
DecimalDigits
HexIntegerLiteral
::
0x
HexDigit
0X
HexDigit
HexIntegerLiteral
HexDigit
HexDigit
::
one of
The source character immediately following a
NumericLiteral
must not be an
IdentifierStart
or
DecimalDigit
NOTE
For example:
3in
is an error and not the two input elements
and
in
Semantics
A numeric literal stands for a value of the Number type. This value is determined in two steps: first, a mathematical
value (MV) is derived from the literal; second, this mathematical value is rounded as described below.
The MV of
NumericLiteral
::
DecimalLiteral
is the MV of
DecimalLiteral
The MV of
NumericLiteral
::
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
times 10
), where
is the number of characters in
DecimalDigit
s.
The MV of
DecimalLiteral
::
DecimalIntegerLiteral
ExponentPart
is the MV
of
DecimalIntegerLiteral
times 10
, where
is the MV of
ExponentPart
The MV of
DecimalLiteral
::
DecimalIntegerLiteral
DecimalDigits
ExponentPart
is (the MV of
DecimalIntegerLiteral
plus (the MV of
DecimalDigits
times 10
)) times 10
, where
is the number of characters in
DecimalDigit
and e
is the MV of
ExponentPart
The MV of
DecimalLiteral
::
DecimalDigits
is the MV of
DecimalDigits
times
10
, where
is the number of characters in
DecimalDigit
s.
The MV of
DecimalLiteral
::
DecimalDigits
ExponentPart
is the MV of
DecimalDigits
times 10
, where
is the number of characters in
DecimalDigit
s and
is the MV of
ExponentPart
The MV of
DecimalLiteral
::
DecimalIntegerLiteral
is the MV of
DecimalIntegerLiteral
The MV of
DecimalLiteral
::
DecimalIntegerLiteral
ExponentPart
is the MV of
DecimalIntegerLiteral
times 10
, where
is the MV of
ExponentPart
The MV of
DecimalIntegerLiteral
::
is 0.
The MV of
DecimalIntegerLiteral
::
NonZeroDigit
is the MV of
NonZeroDigit.
The MV of
DecimalIntegerLiteral
::
NonZeroDigit
DecimalDigits
is (the MV of
NonZeroDigit
times
10
) plus the MV of
DecimalDigits
, where
is the number of characters in
DecimalDigits
The MV of
DecimalDigits
::
DecimalDigit
is the MV of
DecimalDigit
The MV of
DecimalDigits
::
DecimalDigits
DecimalDigit
is (the MV of
DecimalDigits
times 10)
plus the MV of
DecimalDigit
The MV of
ExponentPart
::
ExponentIndicator
SignedInteger
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
::
or of
HexDigit
::
is 0.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
is 1.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
is 2.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
is 3.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
is 4.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
is 5.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
is 6.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
is 7.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
is 8.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
is 9.
The MV of
HexDigit
::
or of
HexDigit
::
is 10.
The MV of
HexDigit
::
or of
HexDigit
::
is 11.
The MV of
HexDigit
::
or of
HexDigit
::
is 12.
The MV of
HexDigit
::
or of
HexDigit
::
is 13.
The MV of
HexDigit
::
or of
HexDigit
::
is 14.
The MV of
HexDigit
::
or of
HexDigit
::
is 15.
The MV of
HexIntegerLiteral
::
0x
HexDigit
is the MV of
HexDigit
The MV of
HexIntegerLiteral
::
0X
HexDigit
is the MV of
HexDigit
The MV of
HexIntegerLiteral
::
HexIntegerLiteral
HexDigit
is (the MV of
HexIntegerLiteral
times
16) plus the MV of
HexDigit
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, then the rounded value is
+0
; otherwise, the rounded value must be the Number value for the
MV (as specified in
8.5
), 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
digit or the Number value for the
MV of a literal produced by replacing each significant digit after the 20th with a
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
; or
there is a nonzero digit to its left and there is a nonzero digit, not in the
ExponentPart
, to its right.
A conforming implementation, when processing
strict mode code (see 10.1.1)
, must not extend the
syntax of
NumericLiteral
to include
OctalIntegerLiteral
as described in
B.1.1
7.8.4
String Literals
A string literal is zero or more characters enclosed in single or double quotes. Each character may be represented by an
escape sequence. All characters may appear literally in a string literal except for the closing quote character, backslash,
carriage return, line separator, paragraph separator, and line feed. Any character may appear in the form of an escape
sequence.
Syntax
StringLiteral
::
DoubleStringCharacters
opt
SingleStringCharacters
opt
DoubleStringCharacters
::
DoubleStringCharacter
DoubleStringCharacters
opt
SingleStringCharacters
::
SingleStringCharacter
SingleStringCharacters
opt
DoubleStringCharacter
::
SourceCharacter
but not one of
or
or
LineTerminator
EscapeSequence
LineContinuation
SingleStringCharacter
::
SourceCharacter
but not one of
or
or
LineTerminator
EscapeSequence
LineContinuation
LineContinuation
::
LineTerminatorSequence
EscapeSequence
::
CharacterEscapeSequence
[lookahead ∉
DecimalDigit
HexEscapeSequence
UnicodeEscapeSequence
CharacterEscapeSequence
::
SingleEscapeCharacter
NonEscapeCharacter
SingleEscapeCharacter
::
one of
NonEscapeCharacter
::
SourceCharacter
but not one of
EscapeCharacter
or
LineTerminator
EscapeCharacter
::
SingleEscapeCharacter
DecimalDigit
HexEscapeSequence
::
HexDigit
HexDigit
UnicodeEscapeSequence
::
HexDigit
HexDigit
HexDigit
HexDigit
The definition of the nonterminal
HexDigit
is given in
7.8.3
SourceCharacter
is defined in
clause 6
Semantics
A string literal stands for a value of the String type. The String value (SV) of the literal is described in terms of
character values (CV) contributed by the various parts of the string literal. As part of this process, some characters
within the string literal are interpreted as having a mathematical value (MV), as described below or in
7.8.3
The SV of
StringLiteral
::
""
is the empty character sequence.
The SV of
StringLiteral
::
''
is the empty character sequence.
The SV of
StringLiteral
::
DoubleStringCharacters
is the SV of
DoubleStringCharacters
The SV of
StringLiteral
::
SingleStringCharacters
is the SV of
SingleStringCharacters
The SV of
DoubleStringCharacters
::
DoubleStringCharacter
is a sequence of one character, the CV of
DoubleStringCharacter
The SV of
DoubleStringCharacters
::
DoubleStringCharacter
DoubleStringCharacters
is a sequence of the CV of
DoubleStringCharacter
followed by all the characters in the SV of
DoubleStringCharacters
in order.
The SV of
SingleStringCharacters
::
SingleStringCharacter
is a sequence of one character, the CV of
SingleStringCharacter
The SV of
SingleStringCharacters
::
SingleStringCharacter
SingleStringCharacters is a sequence of the CV of
SingleStringCharacter
followed by all the characters in the SV of
SingleStringCharacters
in order.
The SV of
LineContinuation
::
LineTerminatorSequence
is the empty character sequence.
The CV of
DoubleStringCharacter
::
SourceCharacter
but not one of
or
or
LineTerminator
is the
SourceCharacter
character itself.
The CV of
DoubleStringCharacter
::
EscapeSequence
is the CV of the
EscapeSequence
The CV of
DoubleStringCharacter
::
LineContinuation
is the empty character sequence.
The CV of
SingleStringCharacter
::
SourceCharacter
but not one of
or
or
LineTerminator
is the
SourceCharacter
character itself.
The CV of
SingleStringCharacter
::
EscapeSequence
is the CV of the
EscapeSequence
The CV of
SingleStringCharacter
::
LineContinuation
is the empty character sequence.
The CV of
EscapeSequence
::
CharacterEscapeSequence
is the CV of the
CharacterEscapeSequence
The CV of
EscapeSequence
::
[lookahead ∉
DecimalDigit
is a
The CV of
EscapeSequence
::
HexEscapeSequence
is the CV of the
HexEscapeSequence
The CV of
EscapeSequence
::
UnicodeEscapeSequence
is the CV of the
UnicodeEscapeSequence
The CV of
CharacterEscapeSequence
::
SingleEscapeCharacter
is the character whose code unit value is determined by the
SingleEscapeCharacter
according to Table 4:
Table 4 — String Single Character Escape Sequences
Escape Sequence
Code Unit Value
Name
Symbol
\b
\u0008
backspace
\t
\u0009
horizontal tab
\n
\u000A
line feed (new line)
\v
\u000B
vertical tab
\f
\u000C
form feed
\r
\u000D
carriage return
\"
\u0022
double quote
\'
\u0027
single quote
\\
\u005C
backslash
The CV of
CharacterEscapeSequence
::
NonEscapeCharacter
is the CV of the
NonEscapeCharacter
The CV of
NonEscapeCharacter
::
SourceCharacter
but not one of
EscapeCharacter
or
LineTerminator
is the
SourceCharacter
character
itself.
The CV of
HexEscapeSequence
::
HexDigit
HexDigit
is the character whose code
unit value is (16 times the MV of the first
HexDigit
) plus the MV of the second
HexDigit
The CV of
UnicodeEscapeSequence
::
HexDigit
HexDigit
HexDigit
HexDigit
is the character whose code unit value is (4096 times the MV of the first
HexDigit
) plus (256 times the MV of the second
HexDigit
) plus (16 times the MV of the third
HexDigit
) plus the MV of the fourth
HexDigit
A conforming implementation, when processing
strict mode code (see 10.1.1)
, may not extend the
syntax of
EscapeSequence
to include
OctalEscapeSequence
as described in
B.1.2
NOTE
A line terminator character cannot appear in a string literal, except as part of a
LineContinuation
to produce the empty character sequence. The correct way to cause a line terminator
character to be part of the String value of a string literal is to use an escape sequence such as
\n
or
\u000A
7.8.5
Regular Expression Literals
A regular expression literal is an input element that is converted to a RegExp object (
see
15.10
) 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
see 15.10.4
) or calling the
RegExp
constructor as a function (
15.10.3
).
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 Strings of characters comprising the
RegularExpressionBody
and the
RegularExpressionFlags
are passed uninterpreted to
the regular expression constructor, which interprets them according to its own, more stringent grammar. An implementation
may extend the regular expression constructor's grammar, but it must not extend the
RegularExpressionBody
and
RegularExpressionFlags
productions or the productions
used by these productions.
Syntax
RegularExpressionLiteral
::
RegularExpressionBody
RegularExpressionFlags
RegularExpressionBody
::
RegularExpressionFirstChar
RegularExpressionChars
RegularExpressionChars
::
[empty]
RegularExpressionChars
RegularExpressionChar
RegularExpressionFirstChar
::
RegularExpressionNonTerminator
but not one of
or
or
or
RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionChar
::
RegularExpressionNonTerminator
but not one of
or
or
RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionBackslashSequence
::
RegularExpressionNonTerminator
RegularExpressionNonTerminator
::
SourceCharacter
but not
LineTerminator
RegularExpressionClass
::
RegularExpressionClassChars
RegularExpressionClassChars
::
[empty]
RegularExpressionClassChars
RegularExpressionClassChar
RegularExpressionClassChar
::
RegularExpressionNonTerminator
but not one of
or
RegularExpressionBackslashSequence
RegularExpressionFlags
::
[empty]
RegularExpressionFlags
IdentifierPart
NOTE
Regular expression literals may not be empty; instead of representing an empty regular
expression literal, the characters
//
start a single-line comment. To specify an empty regular expression,
use:
/(?:)/
Semantics
A regular expression literal evaluates to a value of the Object type that is an instance of the standard built-in
constructor RegExp. This value is determined in two steps: first, the characters comprising the regular expression's
RegularExpressionBody
and
RegularExpressionFlags
production expansions are
collected uninterpreted into two Strings Pattern and Flags, respectively. Then each time the literal is evaluated, a new
object is created as if by the expression
new RegExp(
Pattern,
Flags
where RegExp is the standard built-in constructor with that name. The newly constructed object becomes
the value of the
RegularExpressionLiteral
. If the call to
new RegExp
would generate an
error as specified in
15.10.4.1
, the error must be treated as an early error (
Clause 16
).
7.9
Automatic Semicolon Insertion
Certain ECMAScript statements (empty statement, variable statement, expression statement,
do
while
statement,
continue
statement,
break
statement,
return
statement, and
throw
statement) must be terminated with semicolons. 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.
7.9.1
Rules of Automatic Semicolon
Insertion
There are three basic rules of semicolon insertion:
When, as the program 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
When, as the program 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 complete ECMAScript
Program
, then a
semicolon is automatically inserted at the end of the input stream.
When, as the program 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 12.6.3
).
NOTE
The following are the only restricted productions in the grammar:
PostfixExpression
LeftHandSideExpression
[no
LineTerminator
here]
++
LeftHandSideExpression
[no
LineTerminator
here]
--
ContinueStatement
continue
[no
LineTerminator
here]
Identifier
BreakStatement
break
[no
LineTerminator
here]
Identifier
ReturnStatement
return
[no
LineTerminator
here]
Expression
ThrowStatement
throw
[no
LineTerminator
here]
Expression
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
, or
throw
token is encountered and a
LineTerminator
is encountered before the next token, a semicolon is automatically inserted after the
continue
break
return
, or
throw
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 should start on the same
line as the
return
or
throw
token.
An
Identifier
in a
break
or
continue
statement should be on the same
line as the
break
or
continue
token.
7.9.2
Examples of Automatic Semicolon
Insertion
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
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
The token
++
is not treated as a postfix operator applying to the variable
, because a
LineTerminator
occurs between
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 parenthesised 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.
Types
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.
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, Number, and Object.
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 are
Reference
List
Completion
Property Descriptor
Property Identifier
Lexical Environment
, and
Environment
Record
. 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.
Within this specification, the notation “
Type(
” is
used as shorthand for “
the type of
” where “
type
” refers to the ECMAScript language and specification types defined in
this clause.
8.1
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
8.2
The Null Type
The Null type has exactly one value, called
null
8.3
The Boolean Type
The Boolean type represents a logical entity having two values, called
true
and
false
8.4
The String Type
The String type is the set of all finite ordered sequences of zero or more 16-bit unsigned integer values
(“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 code unit value (
see Clause 6
). 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 position 0, the next element (if any) at position 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.
When a String contains actual textual data, each element is considered to be a single UTF-16 code unit. Whether or not this
is the actual storage format of a String, the characters within a String are numbered by their initial code unit element
position as though they were represented using UTF-16. All operations on Strings (except as otherwise stated) treat them as
sequences of undifferentiated 16-bit unsigned integers; they do not ensure the resulting String is in normalised form, nor do
they ensure language-sensitive results.
NOTE
The rationale behind this design was to keep the implementation of Strings as simple and
high-performing as possible. The intent is that textual data coming into the execution environment from outside (e.g., user
input, text read from a file or received over the network, etc.) be converted to Unicode Normalised Form C before the
running program sees it. Usually this would occur at the same time incoming text is converted from its original character
encoding to Unicode (and would impose no additional overhead). Since it is recommended that ECMAScript source code be in
Normalised Form C, string literals are guaranteed to be normalised (if source text is guaranteed to be normalised), as long
as they do not contain any Unicode escape sequences.
8.5
The Number Type
The Number type has exactly
18437736874454810627
(that is,
64
−2
53
+3
) values, representing the double-precision
64-bit format IEEE 754 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the
9007199254740990
(that is,
53
−2
) 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.
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
(that is,
64
−2
53
) 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
) and
-0
.)
The
18437736874454810622
(that is,
64
−2
53
−2
) finite nonzero values are of two kinds:
18428729675200069632
(that is,
64
−2
54
) of them are normalised, having the form
× 2
where
is
+1
or
−1
is a positive integer less than
53
but not less than
52
, and
is an integer ranging from
−1074
to
971
, inclusive.
The remaining
9007199254740990
(that is,
53
−2
) values are denormalised, having the form
× 2
where
is
+1
or
−1
is a positive integer less than
52
, and
is
−1074
Note that all the positive and negative integers whose magnitude is no greater than
53
are representable in the Number type (indeed, the integer
has two representations,
+0
and
-0
).
A finite number has an
odd significand
if it is nonzero and the integer
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
” where
represents an exact nonzero real mathematical quantity (which might even be an
irrational number such as
) 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
1024
(which is
+1 × 2
53
× 2
971
) and
−2
1024
(which is
−1 ×
53
× 2
971
). Choose the member of this
set that is closest in value to
. 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
1024
and
−2
1024
are considered to
have even significands. Finally, if
1024
was chosen, replace it
with
+∞
; if
−2
1024
was
chosen, replace it with
−∞
; if
+0
was chosen, replace it
with
−0
if and only if
is less than zero; any other chosen value is used
unchanged. The result is the Number value for
. (This procedure corresponds exactly to the behaviour of the IEEE
754 “round to nearest” mode.)
Some ECMAScript operators deal only with integers in the range
−2
31
through
31
−1
inclusive, or in the range
through
32
−1
, inclusive. These operators accept any value of the Number type but first convert each
such value to one of
32
integer values. See the descriptions of
the
ToInt32
and
ToUint32
operators in
9.5
and
9.6
, respectively.
8.6
The Object Type
An Object is a collection of properties. Each property is either a named data property, a named accessor property, or an
internal property:
named data property
associates a name with an ECMAScript language value and a set of Boolean attributes.
named accessor property
associates a name 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.
An
internal property
has no name and is not directly accessible via ECMAScript language operators. Internal
properties exist purely for specification purposes.
There are two kinds of access for named (non-internal) properties:
get
and
put
, corresponding to retrieval
and assignment, respectively.
8.6.1
Property Attributes
Attributes are used in this specification to define and explain the state of named properties. A named data property
associates a name with the attributes listed in Table 5
Table 5 — Attributes of a Named Data Property
Attribute Name
Value Domain
Description
[[Value]]
Any ECMAScript language type
The value retrieved by reading the property.
[[Writable]]
Boolean
If
false
, attempts by ECMAScript code to change the property’s [[Value]] attribute using [[Put]] will not succeed.
[[Enumerable]]
Boolean
If
true
, the property will be enumerated by a for-in enumeration (
see 12.6.4
). 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]]) will fail.
A named accessor property associates a name with the attributes listed in Table 6.
Table 6 — Attributes of a Named Accessor Property
Attribute Name
Value Domain
Description
[[Get]]
Object
or
Undefined
If the value is an Object it must be a function Object. The function’s [[Call]] internal method (
8.6.2
) is called with an empty arguments list to return the property value each time a get access of the property is performed.
[[Set]]
Object
or
Undefined
If the value is an Object it must be a function Object. The function’s [[Call]] internal method (
8.6.2
) 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 12.6.4
). 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 value of an attribute is not explicitly specified by this specification for a named property, the default value
defined in Table 7 is used.
Table 7 — Default Attribute Values
Attribute Name
Default Value
[[Value]]
undefined
[[Get]]
undefined
[[Set]]
undefined
[[Writable]]
false
[[Enumerable]]
false
[[Configurable]]
false
8.6.2
Object Internal Properties and
Methods
This specification uses various internal properties to define the semantics of object values. These internal properties
are not part of the ECMAScript language. They are defined by this specification purely for expository purposes. An
implementation of ECMAScript must behave as if it produced and operated upon internal properties in the manner described
here. The names of internal properties are enclosed in double square brackets [[ ]]. When an algorithm uses an internal
property of an object and the object does not implement the indicated internal property, a
TypeError
exception is
thrown.
The Table 8 summarises the internal properties used by this specification that are applicable to all ECMAScript objects.
The Table 9 summarises the internal properties used by this specification that are only applicable to some ECMAScript
objects. The descriptions in these tables indicate their behaviour for native ECMAScript objects, unless stated otherwise in
this document for particular kinds of native ECMAScript objects. Host objects may support these internal properties with any
implementation-dependent behaviour as long as it is consistent with the specific host object restrictions stated in this
document.
The “Value Type Domain” columns of the following tables define the types of values associated with internal
properties. The type names refer to the types defined in
Clause 8
augmented by the following additional
names. “
any
” means the value may be any ECMAScript language type. “
primitive
” means
Undefined, Null, Boolean, String, or Number. “
SpecOp
” means the internal property is an internal method,
an implementation provided procedure defined by an abstract operation specification. “SpecOp” is followed by a
list of descriptive parameter names. If a parameter name is the same as a type name then the name describes the type of the
parameter. If a “SpecOp” returns a value, its parameter list is followed by the symbol “→” and
the type of the returned value.
Table 8 — Internal Properties Common to All Objects
Internal Property
Value Type Domain
Description
[[Prototype]]
Object
or
Null
The prototype of this object.
[[Class]]
String
A String value indicating a specification defined classification of objects.
[[Extensible]]
Boolean
If
true
, own properties may be added to the object.
[[Get]]
SpecOp(
propertyName
any
Returns the value of the named property.
[[GetOwnProperty]]
SpecOp (
propertyName
Undefined
or
Property Descriptor
Returns the
Property Descriptor
of the named own property of this object, or
undefined
if absent.
[[GetProperty]]
SpecOp (
propertyName
Undefined
or
Property Descriptor
Returns the fully populated
Property Descriptor
of the named property of this object, or
undefined
if absent.
[[Put]]
SpecOp (
propertyName
any, Boolean
Sets the specified named property to the value of the second parameter. The flag controls failure handling.
[[CanPut]]
SpecOp (
propertyName
Boolean
Returns a Boolean value indicating whether a [[Put]] operation with
PropertyName
can be performed.
[[HasProperty]]
SpecOp (
propertyName
Boolean
Returns a Boolean value indicating whether the object already has a property with the given name.
[[Delete]]
SpecOp (
propertyName, Boolean
Boolean
Removes the specified named own property from the object. The flag controls failure handling.
[[DefaultValue]]
SpecOp (
Hint
primitive
Hint is a String. Returns a default value for the object.
[[DefineOwnProperty]]
SpecOp (
propertyName, PropertyDescriptor, Boolean
Boolean
Creates or alters the named own property to have the state described by a
Property Descriptor
. The flag controls failure handling.
Every object (including host objects) must implement all of the internal properties listed in Table 8. However, the
[[DefaultValue]] internal method may, for some objects, simply throw a
TypeError
exception.
All objects have an internal property called [[Prototype]]. The value of this property is either
null
or an object
and is used for implementing inheritance. Whether or not a native object can have a host object as its [[Prototype]] depends
on the implementation. Every [[Prototype]] chain must have finite length (that is, starting from any object, recursively
accessing the [[Prototype]] internal property must eventually lead to a
null
value). Named data properties of the
[[Prototype]] object are inherited (are visible as properties of the child object) for the purposes of get access, but not
for put access. Named accessor properties are inherited for both get access and put access.
Every ECMAScript object has a Boolean-valued [[Extensible]] internal property that controls whether or not named
properties may be added to the object. If the value of the [[Extensible]] internal property is
false
then additional
named properties may not be added to the object. In addition, if [[Extensible]] is
false
the value of the [[Class]]
and [[Prototype]] internal properties of the object may not be modified. Once the value of an [[Extensible]] internal
property has been set to
false
it may not be subsequently changed to
true
NOTE
This specification defines no ECMAScript language operators or built-in functions that
permit a program to modify an object’s [[Class]] or [[Prototype]] internal properties or to change the value of
[[Extensible]] from
false
to
true
. Implementation specific extensions that modify [[Class]], [[Prototype]]
or [[Extensible]] must not violate the invariants defined in the preceding paragraph.
The value of the [[Class]] internal property is defined by this specification for every kind of built-in object. The
value of the [[Class]] internal property of a host object may be any String value except one of
"Arguments"
"Array"
"Boolean"
"Date"
"Error"
"Function"
"JSON"
"Math"
"Number"
"Object"
"RegExp"
, and
"String"
. The value of a [[Class]] internal property is used internally to distinguish different kinds of
objects. Note that this specification does not provide any means for a program to access that value except through
Object.prototype.toString
see 15.2.4.2
).
Unless otherwise specified, the common internal methods of native ECMAScript objects behave as described in
8.12
. Array objects have a slightly different implementation of the [[DefineOwnProperty]] internal
method (
see 15.4.5.1
) and String objects have a slightly different implementation of the
[[GetOwnProperty]] internal method (
see 15.5.5.2
). Arguments objects (
10.6
) have different implementations of [[Get]], [[GetOwnProperty]], [[DefineOwnProperty]], and
[[Delete]]. Function objects (
15.3
) have a different implementation of [[Get]].
Host objects may implement these internal methods in any manner unless specified otherwise; for example, one possibility
is that [[Get]] and [[Put]] for a particular host object indeed fetch and store property values but [[HasProperty]] always
generates
false
. However, if any specified manipulation of a host object's internal properties is not supported by an
implementation, that manipulation must throw a
TypeError
exception when attempted.
The [[GetOwnProperty]] internal method of a host object must conform to the following invariants for each property of the
host object:
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 internal methods.
If a property is described as a data property and its [[Writable]] and [[Configurable]] are both
false
, then
the
SameValue (according to 9.12)
must be returned for the [[Value]] attribute of the property
on all calls to [[GetOwnProperty]].
If the attributes other than [[Writable]] may change over time or if the property might disappear, then the
[[Configurable]] attribute must be
true
If the [[Writable]] attribute may change from
false
to
true
, then the [[Configurable]] attribute must
be
true
If the value of the host object’s [[Extensible]] internal property has been observed by ECMAScript code to be
false
, then if a call to [[GetOwnProperty]] describes a property as non-existent all subsequent calls must also
describe that property as non-existent.
The [[DefineOwnProperty]] internal method of a host object must not permit the addition of a new property to a host
object if the [[Extensible]] internal property of that host object has been observed by ECMAScript code to be
false
If the [[Extensible]] internal property of that host object has been observed by ECMAScript code to be
false
then
it must not subsequently become
true
Table 9 — Internal Properties Only Defined for Some Objects
Internal Property
Value Type Domain
Description
[[PrimitiveValue]]
primitive
Internal state information associated with this object. Of the standard built-in ECMAScript objects, only Boolean, Date, Number, and String objects implement [[PrimitiveValue]].
[[Construct]]
SpecOp(a
List
of
any
Object
Creates an object. Invoked via the
new
operator. The arguments to the SpecOp are the arguments passed to the
new
operator. Objects that implement this internal method are called
constructors
[[Call]]
SpecOp(
any
, a
List
of
any
any
or
Reference
Executes code associated with the object. Invoked via a function call expression. The arguments to the SpecOp are this object and a list containing the arguments passed to the function call expression. Objects that implement this internal method are
callable
. Only callable objects that are host objects may return
Reference
values.
[[HasInstance]]
SpecOp(
any
Boolean
Returns a Boolean value indicating whether the argument is likely an Object that was constructed by this object. Of the standard built-in ECMAScript objects, only Function objects implement [[HasInstance]].
[[Scope]]
Lexical Environment
lexical environment
that defines the environment in which a Function object is executed. Of the standard built-in ECMAScript objects, only Function objects implement [[Scope]].
[[FormalParameters]]
List
of Strings
A possibly empty
List
containing the identifier Strings of a Function’s
FormalParameterList
. Of the standard built-in ECMAScript objects, only Function objects implement [[FormalParameterList]].
[[Code]]
ECMAScript code
The ECMAScript code of a function. Of the standard built-in ECMAScript objects, only Function objects implement [[Code]].
[[TargetFunction]]
Object
The target function of a function object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[TargetFunction]] internal property.
[[BoundThis]]
any
The pre-bound this value of a function Object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[BoundThis]] internal property.
[[BoundArguments]]
List
of
any
The pre-bound argument values of a function Object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[BoundArguments]] internal property.
[[Match]]
SpecOp(
String
index
MatchResult
Tests for a regular expression match and returns a MatchResult value (
see 15.10.2.1
). Of the standard built-in ECMAScript objects, only RegExp objects implement [[Match]].
[[ParameterMap]]
Object
Provides a mapping between the properties of an arguments object (
see 10.6
) and the formal parameters of the associated function. Only ECMAScript objects that are arguments objects have a [[ParameterMap]] internal property.
8.7
The Reference Specification Type
The Reference type is used to explain the behaviour of such operators as
delete
typeof
, and the
assignment operators. For example, the left-hand operand of an assignment is expected to produce a reference. The behaviour of
assignment could, instead, be explained entirely in terms of a case analysis on the syntactic form of the left-hand operand of
an assignment operator, but for one difficulty: function calls are permitted to return references. This possibility is
admitted purely for the sake of host objects. No built-in ECMAScript function defined by this specification returns a
reference and there is no provision for a user-defined function to return a reference. (Another reason not to use a syntactic
case analysis is that it would be lengthy and awkward, affecting many parts of the specification.)
Reference
is a resolved name binding. A Reference consists of three components, the
base
value, the
referenced name
and the Boolean valued
strict reference
flag. The base value is either
undefined
an Object, a Boolean, a String, a Number, or an
environment record (10.2.1)
. A base value of
undefined
indicates that the reference could not be resolved to a binding. The referenced name is a String.
The following abstract operations are used in this specification to access the components of references:
GetBase(V). Returns the base value component of the reference V.
GetReferencedName(V). Returns the referenced name component of the reference V.
IsStrictReference(V). Returns the strict reference component of the reference V.
HasPrimitiveBase(V). Returns
true
if the base value is a Boolean, String, or Number.
IsPropertyReference(V). Returns
true
if either the base value is an object or HasPrimitiveBase(V) is
true
; otherwise returns
false
IsUnresolvableReference(V). Returns
true
if the base value is
undefined
and
false
otherwise.
The following abstract operations are used in this specification to operate on references:
8.7.1
GetValue (V)
If
Type
) is not
Reference
, return
Let
base
be the result of calling
GetBase
).
If
IsUnresolvableReference
), throw a
ReferenceError
exception.
If
IsPropertyReference
), then
If
HasPrimitiveBase
) is
false
, then let
get
be the [[Get]] internal
method of
base
, otherwise let
get
be the special [[Get]] internal method defined below.
Return the result of calling the
get
internal method using
base
as its
this
value, and
passing
GetReferencedName
) for the argument.
Else,
base
must be an environment record.
Return the result of calling the GetBindingValue (
see 10.2.1
) concrete method of
base
passing
GetReferencedName
) and
IsStrictReference
) as arguments.
The following [[Get]] internal method is used by GetValue when
is a property reference with a primitive base
value. It is called using
base
as its
this
value and with property
as its argument. The
following steps are taken:
Let
be
ToObject
base
).
Let
desc
be the result of calling the [[GetProperty]] internal method of
with property name
If
desc
is
undefined
, return
undefined
If
IsDataDescriptor
desc
) is
true
, return
desc
.[[Value]].
Otherwise,
IsAccessorDescriptor
desc
) must be
true
so, let
getter
be
desc
.[[Get]] (
see 8.10
).
If
getter
is
undefined
, return
undefined
Return the result calling the [[Call]] internal method of
getter
providing
base
as the
this
value
and providing no arguments.
NOTE
The object that may be created in step 1 is not accessible outside of the above method. An
implementation might choose to avoid the actual creation of the object. The only situation where such an actual property
access that uses this internal method can have visible effect is when it invokes an accessor function.
8.7.2
PutValue (V, W)
If
Type
) is not
Reference
, throw a
ReferenceError
exception.
Let
base
be the result of calling
GetBase
).
If
IsUnresolvableReference
), then
If
IsStrictReference
) is
true
, then
Throw
ReferenceError
exception.
Call the [[Put]] internal method of the global object, passing
GetReferencedName
for the property name,
for the value, and
false
for the
Throw
flag.
Else if
IsPropertyReference
), then
If
HasPrimitiveBase
) is
false
, then let
put
be the [[Put]] internal
method of
base
, otherwise let
put
be the special [[Put]] internal method defined below.
Call the
put
internal method using
base
as its
this
value, and passing
GetReferencedName
) for the property name,
for the value, and
IsStrictReference
) for the
Throw
flag.
Else
base
must be a reference whose base is an environment record. So,
Call the SetMutableBinding (
10.2.1
) concrete method of
base
, passing
GetReferencedName
),
, and
IsStrictReference
as arguments.
Return.
The following [[Put]] internal method is used by PutValue when
is a property reference with a primitive base
value. It is called using
base
as its
this
value and with property
, value
, and
Boolean flag
Throw
as arguments. The following steps are taken:
Let
be
ToObject
base
).
If the result of calling the [[CanPut]] internal method of
with argument
is
false
, then
If
Throw
is
true
, then throw a
TypeError
exception.
Else return.
Let
ownDesc
be the result of calling the [[GetOwnProperty]] internal method of
with argument
If
IsDataDescriptor
ownDesc
) is
true
, then
If
Throw
is
true
, then throw a
TypeError
exception.
Else return.
Let
desc
be the result of calling the [[GetProperty]] internal method of
with argument
. This
may be either an own or inherited accessor property descriptor or an inherited data property descriptor.
If
IsAccessorDescriptor
desc
) is
true
, then
Let
setter
be
desc
.[[Set]] (
see 8.10
) which cannot be
undefined
Call the [[Call]] internal method of
setter
providing
base
as the
this
value and an argument
list containing only
Else, this is a request to create an own property on the transient object
If
Throw
is
true
, then throw a
TypeError
exception.
Return.
NOTE
The object that may be created in step 1 is not accessible outside of the above method.
An implementation might choose to avoid the actual creation of that transient object. The only situations where such an
actual property assignment that uses this internal method can have visible effect are when it either invokes an accessor
function or is in violation of a
Throw
predicated error check. When
Throw
is
true
any property assignment that would create a new property on the transient object throws an error.
8.8
The List Specification Type
The List type is used to explain the evaluation of argument lists (
see 11.2.4
) in
new
expressions, in function calls, and in other algorithms where a simple list of values is needed. Values of
the List type are simply ordered sequences of values. These sequences may be of any length.
8.9
The Completion Specification Type
The Completion type is used to explain the behaviour of statements (
break
continue
return
and
throw
) that perform nonlocal transfers of control. Values of the Completion type are
triples of the form (
type
value
target
), where
type
is one of
normal
break
continue
return
, or
throw
value
is any ECMAScript language value or
empty
, and
target
is any ECMAScript identifier or
empty
. If
cv
is a completion value then
cv
.type
cv
.value
and
cv
.target
may be used to directly refer to its constituent
values.
The term “abrupt completion” refers to any completion with a
type
other than
normal
8.10
The Property Descriptor and Property
Identifier Specification Types
The Property Descriptor type is used to explain the manipulation and reification of named property attributes. Values of
the Property Descriptor type are records composed of named fields where each field’s name is an attribute name and its
value is a corresponding attribute value as specified in
8.6.1
. In addition, any field may be present
or absent.
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 8.6.1 Table 5 or Table 6.
For notational convenience within this specification, an object literal-like syntax can be used to define a property
descriptor value. For example, Property Descriptor {[[Value]]: 42, [[Writable]]:
false
, [[Configurable]]:
true
defines a data property descriptor. 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 Property Descriptor. For
example, if D is a property descriptor then D.[[Value]] is shorthand for “the field of D named [[Value]]”.
The Property Identifier type is used to associate a property name with a Property Descriptor. Values of the Property
Identifier type are pairs of the form (name, descriptor), where name is a String and descriptor is a Property Descriptor
value.
The following abstract operations are used in this specification to operate upon Property Descriptor values:
8.10.1
IsAccessorDescriptor ( Desc
When the abstract operation IsAccessorDescriptor is called with property descriptor
Desc
, the
following steps are taken:
If
Desc
is
undefined
, then return
false
If both
Desc
.[[Get]] and
Desc
.[[Set]] are absent, then return
false
Return
true
8.10.2
IsDataDescriptor ( Desc )
When the abstract operation IsDataDescriptor is called with property descriptor
Desc
, the
following steps are taken:
If
Desc
is
undefined
, then return
false
If both
Desc
.[[Value]] and
Desc
.[[Writable]] are absent, then return
false
Return
true
8.10.3
IsGenericDescriptor ( Desc
When the abstract operation IsGenericDescriptor is called with property descriptor
Desc
, the
following steps are taken:
If
Desc
is
undefined
, then return
false
If
IsAccessorDescriptor
Desc
) and
IsDataDescriptor
Desc
) are both
false
, then return
true
Return
false
8.10.4
FromPropertyDescriptor ( Desc
When the abstract operation FromPropertyDescriptor is called with property descriptor
Desc
, the
following steps are taken:
The following algorithm assumes that
Desc
is a fully populated
Property
Descriptor
, such as that returned from [[GetOwnProperty]] (
see 8.12.1
).
If
Desc
is
undefined
, then return
undefined
Let
obj
be the result of creating a new object as if by the expression
new Object()
where
Object
is the standard built-in constructor with that name.
If
IsDataDescriptor
Desc
) is
true
, then
Call the [[DefineOwnProperty]] internal method of
obj
with arguments "
value
",
Property Descriptor
{[[Value]]:
Desc
.[[Value]], [[Writable]]:
true
[[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Call the [[DefineOwnProperty]] internal method of
obj
with arguments "
writable
",
Property Descriptor
{[[Value]]:
Desc
.[[Writable]], [[Writable]]:
true
[[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Else,
IsAccessorDescriptor
Desc
) must be
true
, so
Call the [[DefineOwnProperty]] internal method of
obj
with arguments "
get"
Property Descriptor
{[[Value]]:
Desc
.[[Get]], [[Writable]]:
true
[[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Call the [[DefineOwnProperty]] internal method of
obj
with arguments "
set
",
Property Descriptor
{[[Value]]:
Desc
.[[Set]], [[Writable]]:
true
[[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Call the [[DefineOwnProperty]] internal method of
obj
with arguments "
enumerable
",
Property Descriptor
{[[Value]]:
Desc
.[[Enumerable]], [[Writable]]:
true
[[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Call the [[DefineOwnProperty]] internal method of
obj
with arguments "
configurable
",
Property Descriptor
{[[Value]]:
Desc
.[[Configurable]], [[Writable]]:
true
[[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Return
obj
8.10.5
ToPropertyDescriptor ( Obj
When the abstract operation ToPropertyDescriptor is called with object
Obj
, the following steps
are taken:
If
Type
Obj
) is not Object throw a
TypeError
exception.
Let
desc
be the result of creating a new
Property Descriptor
that initially has no
fields.
If the result of calling the [[HasProperty]] internal method of
Obj
with argument "
enumerable
" is
true
, then
Let
enum
be the result of calling the [[Get]] internal method of
Obj
with
enumerable
".
Set the [[Enumerable]] field of
desc
to
ToBoolean
enum
).
If the result of calling the [[HasProperty]] internal method of
Obj
with argument "
configurable
is
true
, then
Let
conf
be the result of calling the [[Get]] internal method of
Obj
with argument
configurable
".
Set the [[Configurable]] field of
desc
to
ToBoolean
conf
).
If the result of calling the [[HasProperty]] internal method of
Obj
with argument "
value
" is
true
, then
Let
value
be the result of calling the [[Get]] internal method of
Obj
with argument
value
”.
Set the [[Value]] field of
desc
to
value
If the result of calling the [[HasProperty]] internal method of
Obj
with argument "
writable
" is
true
, then
Let
writable
be the result of calling the [[Get]] internal method of
Obj
with argument
writable
".
Set the [[Writable]] field of
desc
to
ToBoolean
writable
).
If the result of calling the [[HasProperty]] internal method of
Obj
with argument "
get
" is
true
, then
Let
getter
be the result of calling the [[Get]] internal method of
Obj
with argument
get
".
If
IsCallable
getter
) is
false
and
getter
is not
undefined
then throw a
TypeError
exception.
Set the [[Get]] field of
desc
to
getter
If the result of calling the [[HasProperty]] internal method of
Obj
with argument "
set
" is
true
, then
Let
setter
be the result of calling the [[Get]] internal method of
Obj
with argument
set
".
If
IsCallable
setter
) is
false
and
setter
is not
undefined
then throw a
TypeError
exception.
Set the [[Set]] field of
desc
to
setter
If either
desc
.[[Get]] or
desc
.[[Set]] are present, then
If either
desc
.[[Value]] or
desc
.[[Writable]] are present, then throw a
TypeError
exception.
Return
desc
8.11
The Lexical Environment and
Environment Record Specification Types
The
Lexical Environment
and
Environment Record
types are used to explain
the behaviour of name resolution in nested functions and blocks. These types and the operations upon them are defined in
Clause 10
8.12
Algorithms for Object Internal
Methods
In the following algorithm descriptions, assume
is a native ECMAScript object,
is a String,
Desc
is a Property Description record, and
Throw
is a Boolean flag.
8.12.1
[[GetOwnProperty]] (P)
When the [[GetOwnProperty]] internal method of
is called with property name
, the following
steps are taken:
If
doesn’t have an own property with name
, return
undefined
Let
be a newly created
Property Descriptor
with no fields.
Let
be
’s own property named
If
is a data property, then
Set
.[[Value]] to the value of
’s [[Value]] attribute.
Set
.[[Writable]] to the value of
’s [[Writable]] attribute
Else
is an accessor property, so
Set
.[[Get]] to the value of
’s [[Get]] attribute.
Set
.[[Set]] to the value of
’s [[Set]] attribute.
Set
.[[Enumerable]] to the value of
’s [[Enumerable]] attribute.
Set
.[[Configurable]] to the value of
’s [[Configurable]] attribute.
Return
However, if
is a String object it has a more elaborate [[GetOwnProperty]] internal method defined in
15.5.5.2
8.12.2
[[GetProperty]] (P)
When the [[GetProperty]] internal method of
is called with property name
, the following steps
are taken:
Let
prop
be the result of calling the [[GetOwnProperty]] internal method of
with property name
If
prop
is not
undefined
, return
prop
Let
proto
be the value of the [[Prototype]] internal property of
O.
If
proto
is
null
, return
undefined
Return the result of calling the [[GetProperty]] internal method of
proto
with argument
8.12.3
[[Get]] (P)
When the [[Get]] internal method of
is called with property name
, the following steps are
taken:
Let
desc
be the result of calling the [[GetProperty]] internal method of
with property name
If
desc
is
undefined
, return
undefined
If
IsDataDescriptor
desc
) is
true
, return
desc
.[[Value]].
Otherwise,
IsAccessorDescriptor
desc
) must be true so, let
getter
be
desc
.[[Get]].
If
getter
is
undefined
, return
undefined
Return the result calling the [[Call]] internal method of
getter
providing
as the
this
value
and providing no arguments.
8.12.4
[[CanPut]] (P)
When the [[CanPut]] internal method of
is called with property name
, the following steps are
taken:
Let
desc
be the result of calling the [[GetOwnProperty]] internal method of
with argument
If
desc
is not
undefined
, then
If
IsAccessorDescriptor
desc
) is
true
, then
If
desc
.[[Set]] is
undefined
, then return
false
Else return
true
Else,
desc
must be a DataDescriptor so return the value of
desc
.[[Writable]].
Let
proto
be the [[Prototype]] internal property of
If
proto
is
null
, then return the value of the [[Extensible]] internal property of
Let
inherited
be the result of calling the [[GetProperty]] internal method of
proto
with property name
If
inherited
is
undefined
, return the value of the [[Extensible]] internal property of
If
IsAccessorDescriptor
inherited
) is
true
, then
If
inherited
.[[Set]] is
undefined
, then return
false
Else return
true
Else,
inherited
must be a DataDescriptor
If the [[Extensible]] internal property of
is
false
, return
false
Else return the value of
inherited
.[[Writable]].
Host objects may define additional constraints upon [[Put]] operations. If possible, host objects should not
allow [[Put]] operations in situations where this definition of [[CanPut]] returns false.
8.12.5
[[Put]] ( P, V, Throw )
When the [[Put]] internal method of
is called with property
, value
, and Boolean
flag
Throw
, the following steps are taken:
If the result of calling the [[CanPut]] internal method of
with argument
is
false
, then
If
Throw
is
true
, then throw a
TypeError
exception.
Else return.
Let
ownDesc
be the result of calling the [[GetOwnProperty]] internal method of
with argument
If
IsDataDescriptor
ownDesc
) is
true
, then
Let
valueDesc
be the
Property Descriptor
{[[Value]]:
}.
Call the [[DefineOwnProperty]] internal method of
passing
valueDesc
, and
Throw
as
arguments.
Return.
Let
desc
be the result of calling the [[GetProperty]] internal method of
with argument
. This
may be either an own or inherited accessor property descriptor or an inherited data property descriptor.
If
IsAccessorDescriptor
desc
) is
true
, then
Let
setter
be
desc
.[[Set]] which cannot be
undefined
Call the [[Call]] internal method of
setter
providing
as the
this
value and providing
as the sole argument.
Else, create a named data property named
on object
as follows
Let
newDesc
be the
Property Descriptor
{[[Value]]:
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}.
Call the [[DefineOwnProperty]] internal method of
passing
newDesc
, and
Throw
as
arguments.
Return.
8.12.6
[[HasProperty]] (P)
When the [[HasProperty]] internal method of
is called with property name
, the following steps
are taken:
Let
desc
be the result of calling the [[GetProperty]] internal method of
with property name
If
desc
is
undefined
, then return
false
Else return
true
8.12.7
[[Delete]] (P, Throw)
When the [[Delete]] internal method of
is called with property name
and the Boolean flag
Throw
, the following steps are taken:
Let
desc
be the result of calling the [[GetOwnProperty]] internal method of
with property name
If
desc
is
undefined
, then return
true
If
desc
.[[Configurable]] is
true
, then
Remove the own property with name
from
Return
true
Else if
Throw
, then throw a
TypeError
exception.
Return
false
8.12.8
[[DefaultValue]] (hint)
When the [[DefaultValue]] internal method of
is called with hint String, the following steps are taken:
Let
toString
be the result of calling the [[Get]] internal method of object
with argument
toString
".
If
IsCallable
toString)
is
true
then,
Let
str
be the result of calling the [[Call]] internal method of
toString
, with
as the
this
value and an empty argument list.
If
str
is a primitive value, return
str
Let
valueOf
be the result of calling the [[Get]] internal method of object
with argument
valueOf
".
If
IsCallable
valueOf)
is
true
then,
Let
val
be the result of calling the [[Call]] internal method of
valueOf
, with
as the this
value and an empty argument list.
If
val
is a primitive value, return
val
Throw a
TypeError
exception.
When the [[DefaultValue]] internal method of
is called with hint Number, the following steps are taken:
Let
valueOf
be the result of calling the [[Get]] internal method of object
with argument
valueOf
".
If
IsCallable
valueOf)
is
true
then,
Let
val
be the result of calling the [[Call]] internal method of
valueOf
, with
as the
this
value and an empty argument list.
If
val
is a primitive value, return
val
Let
toString
be the result of calling the [[Get]] internal method of object
with argument
toString
".
If
IsCallable
toString)
is
true
then,
Let
str
be the result of calling the [[Call]] internal method of
toString
, with
as the this
value and an empty argument list.
If
str
is a primitive value, return
str
Throw a
TypeError
exception.
When the [[DefaultValue]] internal method of
is called with no hint, then it behaves as if the hint were
Number, unless
is a Date object (
see 15.9.6
), in which case it behaves as if the hint
were String.
The above specification of [[DefaultValue]] for native objects can return only primitive values. If a host object
implements its own [[DefaultValue]] internal method, it must ensure that its [[DefaultValue]] internal method can return
only primitive values.
8.12.9
[[DefineOwnProperty]] (P,
Desc, Throw)
In the following algorithm, the term “Reject” means “If
Throw
is
true
then throw a
TypeError
exception, otherwise return
false
”. The algorithm contains steps that test
various fields of the
Property Descriptor
Desc
for specific values. The
fields that are tested in this manner need not actually exist in
Desc
. If a field is absent then
its value is considered to be
false
When the [[DefineOwnProperty]] internal method of
is called with property name
, property
descriptor
Desc
, and Boolean flag
Throw
, the following steps are taken:
Let
current
be the result of calling the [[GetOwnProperty]] internal method of
with property name
Let
extensible
be the value of the [[Extensible]] internal property of
If
current
is
undefined
and
extensible
is
false
, then Reject.
If
current
is
undefined
and
extensible
is
true
, then
If
IsGenericDescriptor
Desc
) or
IsDataDescriptor
Desc
) is
true
, then
Create an own data property named
of object
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.
Else,
Desc
must be an accessor
Property Descriptor
so,
Create an own accessor property named
of object
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.
Return
true
Return
true
, if every field in
Desc
is absent.
Return
true
, if every field in
Desc
also occurs in
current
and the value of every field in
Desc
is the same value as the corresponding field in
current
when compared using
the
SameValue algorithm (9.12)
If the [[Configurable]] field of
current
is
false
then
Reject, if the [[Configurable]] field of
Desc
is
true
Reject, if the [[Enumerable]] field of
Desc
is present and the [[Enumerable]] fields of
current
and
Desc
are the Boolean negation of each other.
If
IsGenericDescriptor
Desc
) is
true
, then no further validation is
required.
Else, if
IsDataDescriptor
current
) and
IsDataDescriptor
Desc
) have different results, then
Reject, if the [[Configurable]] field of
current
is
false
If
IsDataDescriptor
current
) is
true
, then
Convert the property named
of object
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.
Else,
Convert the property named
of object
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
If the [[Configurable]] field of
current
is
false
, then
Reject, if the [[Writable]] field of
current
is
false
and the [[Writable]] field of
Desc
is
true
If the [[Writable]] field of
current
is
false
, then
Reject, if the [[Value]] field of
Desc
is present and
SameValue
Desc
.[[Value]],
current
.[[Value]]) is
false
else, the [[Configurable]] field of
current
is
true
, so any change is acceptable.
Else,
IsAccessorDescriptor
current
) and
IsAccessorDescriptor
Desc
) are both
true
so,
If the [[Configurable]] field of
current
is
false
, then
Reject, if the [[Set]] field of
Desc
is present and
SameValue
Desc
.[[Set]],
current
.[[Set]]) is
false
Reject, if the [[Get]] field of
Desc
is present and
SameValue
Desc
.[[Get]],
current
.[[Get]]) is
false
For each attribute field of
Desc
that is present, set the correspondingly named attribute of the property named
of object
to the value of the field.
Return
true
However, if
is an Array object, it has a more elaborate [[DefineOwnProperty]] internal method defined in
15.4.5.1
NOTE
Step 10.b allows any field of Desc to be different from the corresponding field of current
if current’s [[Configurable]] field is
true
. This even permits changing the [[Value]] of a property whose
[[Writable]] attribute is
false
. This is allowed because a
true
[[Configurable]] attribute would permit an
equivalent sequence of calls where [[Writable]] is first set to
true
, a new [[Value]] is set, and then [[Writable]]
is set to
false
Type Conversion and Testing
The ECMAScript runtime system performs automatic type conversion as needed. To clarify the semantics of certain constructs it
is useful to define a set of conversion abstract operations. These abstract operations are not a part of the language; they are
defined here to aid the specification of the semantics of the language. The conversion abstract operations are polymorphic; that
is, they can accept a value of any ECMAScript language type, but not of specification types.
9.1
ToPrimitive
The abstract operation ToPrimitive takes an
input
argument and an optional argument
PreferredType
. The abstract operation ToPrimitive 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. Conversion occurs according to Table 10:
Table 10 — ToPrimitive Conversions
Input Type
Result
Undefined
The result equals the
input
argument (no conversion).
Null
The result equals the
input
argument (no conversion).
Boolean
The result equals the
input
argument (no conversion).
Number
The result equals the
input
argument (no conversion).
String
The result equals the
input
argument (no conversion).
Object
Return a default value for the Object. The default value of an object is retrieved by calling the [[DefaultValue]] internal method of the object, passing the optional hint
PreferredType
. The behaviour of the [[DefaultValue]] internal method is defined by this specification for all native ECMAScript objects in
8.12.8
9.2
ToBoolean
The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 11:
Table 11 — ToBoolean Conversions
Argument Type
Result
Undefined
false
Null
false
Boolean
The result equals the input argument (no conversion).
Number
The result is
false
if the argument is
+0
−0
, or
NaN
; otherwise the result is
true
String
The result is
false
if the argument is the empty String (its length is zero); otherwise the result is
true
Object
true
9.3
ToNumber
The abstract operation ToNumber converts its argument to a value of type Number according to Table 12:
Table 12 — To Number Conversions
Argument Type
Result
Undefined
NaN
Null
+0
Boolean
The result is
if the argument is
true
. The result is
+0
if the argument is
false
Number
The result equals the input argument (no conversion).
String
See grammar and note below.
Object
Apply the following steps:
Let
primValue
be
ToPrimitive
input argument
, hint Number).
Return ToNumber(
primValue
).
9.3.1
ToNumber Applied to the String
Type
ToNumber
applied to Strings applies the following grammar to the input String. If the grammar
cannot interpret the String as an expansion of
StringNumericLiteral
, then the result of
ToNumber
is
NaN
Syntax
StringNumericLiteral
:::
StrWhiteSpace
opt
StrWhiteSpace
opt
StrNumericLiteral
StrWhiteSpace
opt
StrWhiteSpace
:::
StrWhiteSpaceChar
StrWhiteSpace
opt
StrWhiteSpaceChar
:::
WhiteSpace
LineTerminator
StrNumericLiteral
:::
StrDecimalLiteral
HexIntegerLiteral
StrDecimalLiteral
:::
StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral
:::
Infinity
DecimalDigits
DecimalDigits
opt
ExponentPart
opt
DecimalDigits
ExponentPart
opt
DecimalDigits
ExponentPart
opt
DecimalDigits
:::
DecimalDigit
DecimalDigits
DecimalDigit
DecimalDigit
:::
one of
ExponentPart
:::
ExponentIndicator
SignedInteger
ExponentIndicator
:::
one of
SignedInteger
:::
DecimalDigits
DecimalDigits
DecimalDigits
HexIntegerLiteral
:::
0x
HexDigit
0X
HexDigit
HexIntegerLiteral
HexDigit
HexDigit
:::
one of
Some differences should be noted between the syntax of a
StringNumericLiteral
and a
NumericLiteral
see 7.8.3
):
StringNumericLiteral
may be preceded and/or followed by white space and/or line
terminators.
StringNumericLiteral
that is decimal may have any number of leading
digits.
StringNumericLiteral
that is decimal may be preceded by
or
to
indicate its sign.
StringNumericLiteral
that is empty or contains only white space is converted to
+0
The conversion of a String to a Number value is similar overall to the determination of the Number value for a numeric
literal (
see 7.8.3
), 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 in full. 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 of
StringNumericLiteral
:::
[empty]
is 0.
The MV of
StringNumericLiteral
:::
StrWhiteSpace
is 0.
The MV of
StringNumericLiteral
:::
StrWhiteSpace
opt
StrNumericLiteral
StrWhiteSpace
opt
is the MV of
StrNumericLiteral
, no matter
whether white space is present or not.
The MV of
StrNumericLiteral
:::
StrDecimalLiteral
is the MV of
StrDecimalLiteral
The MV of
StrNumericLiteral
:::
HexIntegerLiteral
is the MV of
HexIntegerLiteral
The MV of
StrDecimalLiteral
:::
StrUnsignedDecimalLiteral
is the MV of
StrUnsignedDecimalLiteral
The MV of
StrDecimalLiteral
:::
StrUnsignedDecimalLiteral
is the MV of
StrUnsignedDecimalLiteral
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
10000
(a value so
large that it will round to
+∞
).
The MV of
StrUnsignedDecimalLiteral
:::
DecimalDigits
is the MV of
DecimalDigits
The MV of
StrUnsignedDecimalLiteral
:::
DecimalDigits
DecimalDigits
is the MV of the
first
DecimalDigits
plus (the MV of the second
DecimalDigits
times
10
), where
is the number of
characters in the second
DecimalDigits
The MV of
StrUnsignedDecimalLiteral
:::
DecimalDigits
ExponentPart
is the MV of
DecimalDigits
times 10
, where
is the MV of
ExponentPart
The MV of
StrUnsignedDecimalLiteral
:::
DecimalDigits
DecimalDigits
ExponentPart
is (the MV of the first
DecimalDigits
plus (the MV of the second
DecimalDigits
times 10
)) times 10
, where
is the number of
characters in the second
DecimalDigit
s and
is the MV of
ExponentPart
The MV of
StrUnsignedDecimalLiteral
:::
DecimalDigits
is the MV of
DecimalDigits
times
10
, where
is the number of characters in
DecimalDigit
s.
The MV of
StrUnsignedDecimalLiteral
:::
DecimalDigits
ExponentPart
is the MV of
DecimalDigits
times 10
, where
is the number of characters in
DecimalDigit
s and
is the MV of
ExponentPart
The MV of
StrUnsignedDecimalLiteral
:::
DecimalDigits
is the MV of
DecimalDigits
The MV of
StrUnsignedDecimalLiteral
:::
DecimalDigits
ExponentPart
is the MV of
DecimalDigits
times
10
, where
is the MV of
ExponentPart
The MV of
DecimalDigits
:::
DecimalDigit
is the MV of
DecimalDigit
The MV of
DecimalDigits
:::
DecimalDigits
DecimalDigit
is (the MV of
DecimalDigits
times 10)
plus the MV of
DecimalDigit
The MV of
ExponentPart
:::
ExponentIndicator
SignedInteger
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
:::
or of
HexDigit
:::
is 0.
The MV of
DecimalDigit
:::
or of
HexDigit
:::
is 1.
The MV of
DecimalDigit
:::
or of
HexDigit
:::
is 2.
The MV of
DecimalDigit
:::
or of
HexDigit
:::
is 3.
The MV of
DecimalDigit
:::
or of
HexDigit
:::
is 4.
The MV of
DecimalDigit
:::
or of
HexDigit
:::
is 5.
The MV of
DecimalDigit
:::
or of
HexDigit
:::
is 6.
The MV of
DecimalDigit
:::
or of
HexDigit
:::
is 7.
The MV of
DecimalDigit
:::
or of
HexDigit
:::
is 8.
The MV of
DecimalDigit
:::
or of
HexDigit
:::
is 9.
The MV of
HexDigit
:::
or of
HexDigit
:::
is 10.
The MV of
HexDigit
:::
or of
HexDigit
:::
is 11.
The MV of
HexDigit
:::
or of
HexDigit
:::
is 12.
The MV of
HexDigit
:::
or of
HexDigit
:::
is 13.
The MV of
HexDigit
:::
or of
HexDigit
:::
is 14.
The MV of
HexDigit
:::
or of
HexDigit
:::
is 15.
The MV of
HexIntegerLiteral
:::
0x
HexDigit
is the MV of
HexDigit
The MV of
HexIntegerLiteral
:::
0X
HexDigit
is the MV of
HexDigit
The MV of
HexIntegerLiteral
:::
HexIntegerLiteral
HexDigit
is (the MV of
HexIntegerLiteral
times
16) plus the MV of
HexDigit
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 character 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
8.5
), 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
; or
there is a nonzero digit to its left and there is a nonzero digit, not in the
ExponentPart
, to
its right.
9.4
ToInteger
The abstract operation ToInteger converts its argument to an integral numeric value. This abstract operation functions as
follows:
Let
number
be the result of calling
ToNumber
on the input argument.
If
number
is
NaN
, return
+0
If
number
is
+0
−0
+∞,
or
−∞
, return
number
Return the result of computing
sign
number
) ×
floor
abs
number
)).
9.5
ToInt32: (Signed 32 Bit Integer)
The abstract operation ToInt32 converts its argument to one of
32
integer values in the range
−2
31
through
31
−1
inclusive. This abstract operation functions as follows:
Let
number
be the result of calling
ToNumber
on the input argument.
If
number
is
NaN
+0
−0
+∞
, or
−∞
, return
+0
Let
posInt
be
sign
number
) *
floor
abs
number
)).
Let
int32bit
be
posInt
modulo
32
; that is, a finite integer value k of
Number type with positive sign and less than 2
32
in magnitude such that the mathematical difference of
posInt
and k is mathematically an integer multiple of 2
32
If
int32bit
is greater than or equal to 2
31
, return
int32bit
− 2
32
, 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(
for all values of
. (It is to preserve this latter property that
and −
are mapped to
+0
.)
ToInt32 maps
−0
to
+0
9.6
ToUint32: (Unsigned 32 Bit
Integer)
The abstract operation ToUint32 converts its argument to one of
32
integer values in the range
through
32
−1
, inclusive. This abstraction operation functions as
follows:
Let
number
be the result of calling
ToNumber
on the input argument.
If
number
is
NaN
, +0, −0, +
, or −
, return
+0
Let
posInt
be
sign
number
) ×
floor
abs
number
)).
Let
int32bit
be
posInt
modulo
32
; that is, a finite integer value k of
Number type with positive sign and less than 2
32
in magnitude such that the mathematical difference of
posInt
and k is mathematically an integer multiple of 2
32
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
))
is equal to
ToUint32(
for all values of
. (It is to preserve
this latter property that
+∞
and
−∞
are mapped to
+0
.)
ToUint32 maps
−0
to
+0
9.7
ToUint16: (Unsigned 16 Bit
Integer)
The abstract operation ToUint16 converts its argument to one of
16
integer values in the range
through
16
−1
, inclusive. This abstract operation functions as
follows:
Let
number
be the result of calling
ToNumber
on the input argument.
If
number
is
NaN
, +0, −0, +
, or −
, return
+0
Let
posInt
be
sign
number
) ×
floor
abs
number
)).
Let
int16bit
be
posInt
modulo
16
; that is, a finite integer value
of Number type with positive sign and less than 2
16
in magnitude such that the mathematical
difference of
posInt
and
is mathematically an integer multiple of 2
16
Return
int16bit
NOTE
Given the above definition of ToUint16:
The substitution of
16
for
32
in step 4 is the only difference between
ToUint32
and
ToUint16.
ToUint16 maps
−0
to
+0
9.8
ToString
The abstract operation ToString converts its argument to a value of type String according to Table 13:
Table 13 — ToString Conversions
Argument Type
Result
Undefined
"undefined"
Null
"null"
Boolean
If the argument is
true
, then the result is
"true"
If the argument is
false
, then the result is
"false"
Number
See
9.8.1
String
Return the input argument (no conversion)
Object
Apply the following steps:
1. Let
primValue
be
ToPrimitive
(input argument, hint String).
2. Return ToString(
primValue
).
9.8.1
ToString Applied to the Number
Type
The abstract operation
ToString
converts a Number
to String format as follows:
If
is
NaN
, return the String
"NaN"
If
is
+0
or
−0
, return the String
"0"
If
is less than zero, return the String concatenation of the String
"-"
and
ToString
(−
).
If
is infinity, return the String
"Infinity"
Otherwise, let
, and
be integers such that
≥ 1, 10
−1
< 10
, the Number value for
× 10
n−k
is
, and
is as small as possible. Note that
is the number of digits in the decimal
representation of
, that
is not divisible by 10, and that the least significant digit of
is
not necessarily uniquely determined by these criteria.
If
≤ 21, return the String consisting of the
digits of the decimal representation
of s (in order, with no leading zeroes), followed by
n−k
occurrences of the character
’.
If 0 < n ≤ 21, return the String consisting of the most significant
digits of the decimal
representation of
, followed by a decimal point ‘
’, followed by the remaining
k−n
digits of the decimal representation of
If −6 < n ≤ 0, return the String consisting of the character ‘
’, followed by a
decimal point ‘
’, followed by −
occurrences of the character
’, followed by the
digits of the decimal representation of
Otherwise, if
= 1, return the String consisting of the single digit of
, followed by lowercase
character ‘
’, followed by a plus sign ‘
’ or minus sign
’ according to whether
−1 is positive or negative, followed by the
decimal representation of the integer
abs
−1) (with no leading zeroes).
Return the String consisting of the most significant digit of the decimal representation of
, followed by a
decimal point ‘.’, followed by the remaining
−1 digits of the decimal representation of
, followed by the lowercase character ‘
’, followed by a plus sign
’ or minus sign ‘
’ according to whether
−1 is
positive or negative, followed by the decimal representation of the integer
abs
−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
, and
be integers such that
≥ 1, 10
−1
< 10
, the Number value for
× 10
is
, and
is as small as possible. If there are multiple possibilities for
, choose the value of
for which
× 10
is closest in value to
. If there are two
such possible values of
, choose the one that is even. Note that
is the number of digits in the decimal
representation of
and that
is not divisible by 10.
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
z. Associated code
available as
and as
and may also be found
at the various
netlib
mirror sites.
9.9
ToObject
The abstract operation ToObject converts its argument to a value of type Object according to Table 14:
Table 14 — ToObject
Argument Type
Result
Undefined
Throw a
TypeError
exception.
Null
Throw a
TypeError
exception.
Boolean
Create a new Boolean object whose [[PrimitiveValue]] internal property is set to the value of the argument. See
15.6
for a description of Boolean objects.
Number
Create a new Number object whose [[PrimitiveValue]] internal property is set to the value of the argument. See
15.7
for a description of Number objects.
String
Create a new String object whose [[PrimitiveValue]] internal property is set to the value of the argument. See
15.5
for a description of String objects.
Object
The result is the input argument (no conversion).
9.10
CheckObjectCoercible
The abstract operation CheckObjectCoercible throws an error if its argument is a value that cannot be converted to an
Object using
ToObject
. It is defined by Table 15:
Table 15 — CheckObjectCoercible Results
Argument Type
Result
Undefined
Throw a
TypeError
exception.
Null
Throw a
TypeError
exception.
Boolean
Return
Number
Return
String
Return
Object
Return
9.11
IsCallable
The abstract operation IsCallable determines if its argument, which must be an ECMAScript language value, is a callable
function Object according to Table 16:
Table 16 — IsCallable Results
Argument Type
Result
Undefined
Return
false
Null
Return
false
Boolean
Return
false
Number
Return
false
String
Return
false
Object
If the argument object has a [[Call]] internal method, then return
true
, otherwise return
false
9.12
The SameValue Algorithm
The internal comparison abstract operation SameValue(
), where
and
are
ECMAScript language values, produces
true
or
false
. Such a comparison is performed as follows:
If
Type
) is different from
Type
), return
false
If
Type
) is Undefined, return
true
If
Type
) is Null, return
true
If
Type
) is Number, then.
If
is NaN and
is NaN, return
true
If
is +0 and
is -0, return
false
If
is -0 and
is +0, return
false
If
is the same Number value as
, return
true
Return
false
If
Type
) is String, then return
true
if
and
are exactly the same
sequence of characters (same length and same characters in corresponding positions); otherwise, return
false
If
Type
) is Boolean, return
true
if
and
are both
true
or
both
false
; otherwise, return
false
Return true if
and
refer to the same object. Otherwise, return
false
10
Executable Code and Execution
Contexts
10.1
Types of Executable Code
There are three types of ECMAScript executable code:
Global code
is source text that is treated as an ECMAScript
Program
. The global code of a particular
Program
does not include any source text that is parsed as part of a
FunctionBody
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
Program
. The eval
code for a particular invocation of
eval
is the global code portion of that
Program
Function code
is source text that is parsed as part of a
FunctionBody
. The
function code
of a
particular
FunctionBody
does not include any source text that is parsed as part of a nested
FunctionBody
Function code
also denotes the source text supplied when using the built-in
Function
object as a
constructor. More precisely, the last parameter provided to the
Function
constructor is converted to a String
and treated as the
FunctionBody
. If more than one parameter is provided to the
Function
constructor,
all parameters except the last one are converted to Strings and concatenated together, separated by commas. The resulting
String is interpreted as the
FormalParameterList
for the
FunctionBody
defined by the last parameter. The
function code for a particular instantiation of a
Function
does not include any source text that is parsed as
part of a nested
FunctionBody
10.1.1
Strict Mode Code
An ECMAScript
Program
syntactic unit may be processed using either unrestricted or strict mode
syntax and semantics. When processed using strict mode the three types of ECMAScript code are referred to as strict global
code, strict eval code, and strict function code. Code is interpreted as strict mode code in the following situations:
Global code is strict global code if it begins with a
Directive Prologue
that contains a
Use Strict Directive
see 14.1
).
Eval code is strict eval code if it begins with a
Directive Prologue
that contains a
Use Strict Directive
or if the call to eval is a
direct call (see
15.1.2.1.1) to the eval function
that is contained in strict mode code.
Function code that is part of a
FunctionDeclaration
FunctionExpression
, or accessor
PropertyAssignment
is strict function code if
its
FunctionDeclaration
FunctionExpression
, or
PropertyAssignment
is contained in strict mode code or if the function code begins with a
Directive Prologue
that contains a
Use Strict Directive
Function code that is supplied as the last argument to the built-in Function constructor is strict function code if
the last argument is a String that when processed as a
FunctionBody
begins with a
Directive Prologue
that contains a
Use Strict Directive
10.2
Lexical Environments
Lexical Environment
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. A Lexical Environment
consists of an
Environment Record
and a possibly null reference to an
outer
Lexical
Environment. Usually a Lexical Environment is associated with some specific syntactic structure of ECMAScript code such as a
FunctionDeclaration
, a
WithStatement
, or a
Catch
clause of a
TryStatement
and a new Lexical Environment is created each time such code is
evaluated.
An
Environment Record
records the identifier bindings that are created within the scope of
its associated Lexical Environment.
The outer environment reference is used to model the logical nesting of Lexical Environment values. The outer reference of
a (inner) Lexical Environment is a reference to the Lexical Environment that logically surrounds the inner Lexical
Environment. An outer Lexical Environment may, of course, have its own outer Lexical Environment. A Lexical Environment may
serve as the outer environment for multiple inner Lexical Environments. For example, if a
FunctionDeclaration
contains two nested
FunctionDeclarations
then the Lexical
Environments of each of the nested functions will have as their outer Lexical Environment the Lexical Environment of the
current execution of the surrounding function.
Lexical Environments and
Environment Record
values 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.
10.2.1
Environment Records
There are two kinds of Environment Record values used in this specification:
declarative environment records
and
object environment records
. Declarative environment records are 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. Object
environment records are used to define the effect of ECMAScript elements such as
Program
and
WithStatement
that associate identifier bindings with the properties of some object.
For specification purposes Environment Record values can be thought of as existing in a simple object-oriented hierarchy
where Environment Record is an abstract class with two concrete subclasses, declarative environment record and object
environment record. The abstract class includes the abstract specification methods defined in Table 17. These abstract
methods have distinct concrete algorithms for each of the concrete subclasses.
Table 17 — Abstract Methods of Environment Records
Method
Purpose
HasBinding(N)
Determine if an environment record has a binding for an identifier. Return
true
if it does and
false
if it does not. The String value
is the text of the identifier.
CreateMutableBinding(N, D)
Create a new mutable binding in an environment record. The String value
is the text of the bound name. If the optional Boolean argument
is
true
the binding is may be subsequently deleted.
SetMutableBinding(N,V, S)
Set the value of an already existing mutable binding in an environment record. The String value
is the text of the bound name.
is the value for the binding and may be a value of any ECMAScript language type.
is a Boolean flag. If
is
true
and the binding cannot be set throw a
TypeError
exception.
is used to identify strict mode references.
GetBindingValue(N,S)
Returns the value of an already existing binding from an environment record. The String value
is the text of the bound name.
is used to identify strict mode references. If
is
true
and the binding does not exist or is uninitialised throw a
ReferenceError
exception.
DeleteBinding(N)
Delete a binding from an environment record. The String value
is the text of the bound name If a binding for
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
ImplicitThisValue()
Returns the value to use as the
this
value on calls to function objects that are obtained as binding values from this environment record.
10.2.1.1
Declarative Environment
Records
Each
declarative environment record
is associated with an ECMAScript program scope containing
variable and/or function declarations. A
declarative environment record
binds the set of
identifiers defined by the declarations contained within its scope.
In addition to the mutable bindings supported by all Environment Records, declarative environment records also provide
for immutable bindings. An immutable binding is one where the association between an identifier and a value may not be
modified once it has been established. Creation and initialisation of immutable binding are distinct steps so it is
possible for such bindings to exist in either an initialised or uninitialised state. Declarative environment records
support the methods listed in Table 18 in addition to the Environment Record abstract specification methods:
Table 18 — Additional Methods of Declarative Environment Records
Method
Purpose
CreateImmutableBinding(N)
Create a new but uninitialised immutable binding in an environment record. The String value
is the text of the bound name.
InitializeImmutableBinding(N,V)
Set the value of an already existing but uninitialised immutable binding in an environment record. The String value
is the text of the bound name.
is the value for the binding and is a value of any ECMAScript language type.
The behaviour of the concrete specification methods for Declarative Environment Records is defined by the following
algorithms.
10.2.1.1.1
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
, return
true
If it does not have such a binding, return
false
10.2.1.1.2
CreateMutableBinding (N, D)
The concrete
Environment Record
method CreateMutableBinding for declarative environment
records creates a new mutable binding for the name
that is initialised to the value
undefined
. A
binding must not already exist in this
Environment Record
for
. If Boolean
argument
is provided and 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
Create a mutable binding in
envRec
for
and set its bound value to
undefined
. If
is
true record that the newly created binding may be deleted by a subsequent DeleteBinding call.
10.2.1.1.3
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
to the value of argument
. A binding for
must already exist. If the
binding is an immutable binding, a
TypeError
is thrown if
is
true
Let
envRec
be the
declarative environment record
for which the method was
invoked.
Assert:
envRec
must have a binding for
If the binding for
in
envRec
is a mutable binding, change its bound value to
Else this must be an attempt to change the value of an immutable binding so if
if
true
throw a
TypeError
exception.
10.2.1.1.4
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
. The binding must
already exist. If
is
true
and the binding is an uninitialised immutable binding throw a
ReferenceError
exception.
Let
envRec
be the
declarative environment record
for which the method was
invoked.
Assert:
envRec
has a binding for
If the binding for
in
envRec
is an uninitialised immutable binding, then
If
is
false
, return the value
undefined
, otherwise throw a
ReferenceError
exception.
Else, return the value currently bound to
in
envRec
10.2.1.1.5
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.
If
envRec
does not have a binding for the name that is the value of
, return
true
If the binding for
in
envRec
is cannot be deleted, return
false
Remove the binding for
from
envRec
Return
true
10.2.1.1.6
ImplicitThisValue()
Declarative Environment Records always return
undefined
as their ImplicitThisValue.
Return
undefined
10.2.1.1.7
CreateImmutableBinding (N)
The concrete
Environment Record
method CreateImmutableBinding for declarative environment
records creates a new immutable binding for the name
that is initialised to the value
undefined
. A
binding must not already exist in this environment record for
Let
envRec
be the
declarative environment record
for which the method was
invoked.
Assert:
envRec
does not already have a binding for
Create an immutable binding in
envRec
for
and record that it is uninitialised.
10.2.1.1.8
InitializeImmutableBinding (N,V)
The concrete
Environment Record
method InitializeImmutableBinding 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
to the value of argument
. An uninitialised immutable binding for
must already exist.
Let
envRec
be the
declarative environment record
for which the method was
invoked.
Assert:
envRec
must have an uninitialised immutable binding for N.
Set the bound value for
in
envRec
to
Record that the immutable binding for
in
envRec
has been initialised.
10.2.1.2
Object Environment
Records
Each
object environment record
is associated with an object called its
binding object
An
object environment record
binds the set of identifier names that directly correspond to the
property names of its binding object. Property names that are not 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 can be configured to provide their binding object as an implicit this value for use in
function calls. This capability is used to specify the behaviour of With Statement (
12.10
induced bindings. The capability is controlled by a
provideThis
Boolean value that is associated with each
object environment record
. By default, the value of
provideThis
is
false
for any
object environment record
The behaviour of the concrete specification methods for Object Environment Records is defined by the following
algorithms.
10.2.1.2.1
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
Let
envRec
be the
object environment record
for which the method was
invoked.
Let
bindings
be the binding object for
envRec
Return the result of calling the [[HasProperty]] internal method of
bindings
, passing
as the
property name.
10.2.1.2.2
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
initialises it to the value
undefined
. A property named
must not already exist in the binding
object. If Boolean argument
is provided and 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
Assert: The result of calling the [[HasProperty]] internal method of
bindings
, passing
as the
property name, is
false
If D is
true
then let
configValue
be
true
otherwise let
configValue
be
false
Call the [[DefineOwnProperty]] internal method of
bindings
, passing
Property
Descriptor
{[[Value]]:
undefined
, [[Writable]]:
true
, [[Enumerable]]:
true
[[Configurable]]:
configValue
}, and
true
as arguments.
10.2.1.2.3
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
to the value of argument
. A property named
should already
exist but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument
Let
envRec
be the
object environment record
for which the method was
invoked.
Let
bindings
be the binding object for
envRec
Call the [[Put]] internal method of
bindings
with arguments
, and
10.2.1.2.4
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
. The property should already exist but if it does not the result depends upon the value of the
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 the result of calling the [[HasProperty]] internal method of
bindings
, passing
as the property name.
If
value
is
false
, then
If
is
false
, return the value
undefined
, otherwise throw a
ReferenceError
exception.
Return the result of calling the [[Get]] internal method of
bindings
, passing
for the
argument.
10.2.1.2.5
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 the result of calling the [[Delete]] internal method of
bindings
, passing
and
false
as arguments.
10.2.1.2.6
ImplicitThisValue()
Object Environment Records return
undefined
as their ImplicitThisValue unless their
provideThis
flag is
true
Let
envRec
be the
object environment record
for which the method was
invoked.
If the
provideThis
flag of
envRec
is
true
, return the binding object for
envRec
Otherwise, return
undefined
10.2.2
Lexical Environment
Operations
The following abstract operations are used in this specification to operate upon lexical environments:
10.2.2.1
GetIdentifierReference
(lex, name, strict)
The abstract operation GetIdentifierReference is called with a
Lexical Environment
lex
, an identifier String
name
, and a Boolean flag
strict.
The value of
lex
may be
null
. When called, the following steps are performed:
If
lex
is the value
null
, then
Return a value of type
Reference
whose base value is
undefined
, whose referenced
name is
name
, and whose strict mode flag is
strict
Let
envRec
be
lex
’s environment record.
Let
exists
be the result of calling the HasBinding(
) concrete method of
envRec
passing
name
as the argument
If
exists
is
true
, then
Return a value of type
Reference
whose base value is
envRec
, whose referenced name
is
name
, and whose strict mode flag is
strict.
Else
Let
outer
be the value of
lex’s
outer environment reference
Return the result of calling GetIdentifierReference passing
outer
name
, and
strict
as
arguments.
10.2.2.2
NewDeclarativeEnvironment (E)
When the abstract operation NewDeclarativeEnvironment is called with either a
Lexical
Environment
or
null
as argument
the following steps are performed:
Let
env
be a new
Lexical Environment
Let
envRec
be a new
declarative environment record
containing no bindings.
Set
env’s
environment record to be
envRec
Set the
outer lexical environment reference
of
env
to
Return
env
10.2.2.3
NewObjectEnvironment (O,
E)
When the abstract operation NewObjectEnvironment is called with an Object
and a
Lexical Environment
(or
null)
as arguments, the following steps are
performed:
Let
env
be a new
Lexical Environment
Let
envRec
be a new
object environment record
containing
as the binding
object.
Set
env’s
environment record to be
envRec
Set the
outer lexical environment reference
of
env
to
Return
env
10.2.3
The Global Environment
The
global environment
is a unique
Lexical Environment
which is created before any
ECMAScript code is executed. The global environment’s
Environment Record
is an
object environment record
whose binding object is the global object (
15.1
).
The global environment’s
outer environment reference
is
null
As ECMAScript code is executed, additional properties may be added to the global object and the initial properties may be
modified.
10.3
Execution Contexts
When control is transferred to ECMAScript executable code, control is entering an
execution context
. Active
execution contexts logically form a stack. The top execution context on this logical stack is the running execution context. 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 state is necessary to track the execution progress of its associated code. In
addition, each execution context has the state components listed in Table 19.
Table 19 —Execution Context State Components
Component
Purpose
LexicalEnvironment
Identifies the
Lexical Environment
used to resolve identifier references made by code within this execution context.
VariableEnvironment
Identifies the
Lexical Environment
whose environment record holds bindings created by
VariableStatements
and
FunctionDeclarations
within this execution context.
ThisBinding
The value associated with the
this
keyword within ECMAScript code associated with this execution context.
The LexicalEnvironment and VariableEnvironment components of an execution context are always Lexical Environments. When an
execution context is created its LexicalEnvironment and VariableEnvironment components initially have the same value. The
value of the VariableEnvironment component never changes while the value of the LexicalEnvironment component may change during
execution of code within an execution context.
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”,
“VariableEnvironment” and “ThisBinding” 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 an ECMAScript program to access an execution context.
10.3.1
Identifier Resolution
Identifier resolution is the process of determining the binding of an
Identifier
using the
LexicalEnvironment
of the running execution context. During execution of ECMAScript code, the syntactic
production
PrimaryExpression
Identifier
is evaluated using the following algorithm:
Let
env
be the running execution context’s
LexicalEnvironment
If the syntactic production that is being evaluated is contained in a
strict mode code
, then
let
strict
be
true
, else let
strict
be
false
Return the result of calling
GetIdentifierReference
function passing
env
Identifier
, and
strict
as arguments.
The result of evaluating an identifier is always a value of type
Reference
with its referenced
name component equal to the
Identifier
String.
10.4
Establishing an Execution
Context
Evaluation of global code or code using the eval function (
15.1.2.1
) establishes and enters a
new execution context. Every invocation of an ECMAScript code function (
13.2.1
) also establishes and
enters a new execution context, even if a function is calling itself recursively. Every return exits an execution context. A
thrown exception may also exit one or more execution contexts.
When control enters an execution context, the execution context’s
ThisBinding
is set, its
VariableEnvironment
and initial
LexicalEnvironment
are defined, and
declaration binding instantiation (10.5)
is performed. The exact manner in which these actions occur
depend on the type of code being entered.
10.4.1
Entering Global Code
The following steps are performed when control enters the execution context for global code:
Initialise the execution context using the global code as described in
10.4.1.1
Perform
Declaration Binding Instantiation
as described in
10.5
using
the global code.
10.4.1.1
Initial Global Execution
Context
The following steps are performed to initialise a global execution context for ECMAScript code
Set the
VariableEnvironment
to
the Global Environment
Set the
LexicalEnvironment
to
the Global Environment
Set the
ThisBinding
to the global object.
10.4.2
Entering Eval Code
The following steps are performed when control enters the execution context for eval code:
If there is no calling context or if the eval code is not being evaluated by a direct call (
15.1.2.1.1
) to the eval function then,
Initialise the execution context as if it was a global execution context using the eval code as
as
described in
10.4.1.1
Else,
Set the
ThisBinding
to the same value as the
ThisBinding
of the
calling execution context.
Set the
LexicalEnvironment
to the same value as the
LexicalEnvironment
of the calling execution context.
Set the
VariableEnvironment
to the same value as the
VariableEnvironment
of the calling execution context.
If the eval code is
strict code
, then
Let
strictVarEnv
be the result of calling
NewDeclarativeEnvironment
passing
the
LexicalEnvironment
as the argument.
Set the
LexicalEnvironment
to
strictVarEnv
Set the
VariableEnvironment
to
strictVarEnv
Perform
Declaration Binding Instantiation
as described in
10.5
using
the eval code.
10.4.2.1
Strict Mode
Restrictions
The eval code cannot instantiate variable or function bindings in the variable environment of the calling context that
invoked the eval if either the code of the calling context or the eval code is
strict code
Instead such bindings are instantiated in a new
VariableEnvironment
that is only accessible to the
eval code.
10.4.3
Entering Function Code
The following steps are performed when control enters the execution context for function code contained in function
object
, a caller provided
thisArg
, and a caller provided
argumentsList
If the function code is
strict code
, set the
ThisBinding
to
thisArg
Else if
thisArg
is
null
or
undefined
, set the
ThisBinding
to the global
object.
Else if
Type
thisArg
) is not Object, set the
ThisBinding
to
ToObject
thisArg
).
Else set the
ThisBinding
to
thisArg
Let
localEnv
be the result of calling
NewDeclarativeEnvironment
passing the value
of the [[Scope]] internal property of
as the argument.
Set the
LexicalEnvironment
to
localEnv
Set the
VariableEnvironment
to
localEnv
Let
code
be the value of
’s [[Code]] internal property.
Perform
Declaration Binding Instantiation
using the function code
code
and
argumentsList
as described in
10.5
10.5
Declaration Binding
Instantiation
Every execution context has an associated
VariableEnvironment
. Variables and functions declared in
ECMAScript code evaluated in an execution context are added as bindings in that
VariableEnvironment
’s
Environment Record
. For function code, parameters
are also added as bindings to that
Environment Record
Which
Environment Record
is used to bind a declaration and its kind depends upon the type of
ECMAScript code executed by the execution context, but the remainder of the behaviour is generic. On entering an execution
context, bindings are created in the
VariableEnvironment
as follows using the caller provided
code
and, if it is function code, argument
List
args
Let
env
be the environment record component of the running execution context’s
VariableEnvironment
If
code
is eval code, then let
configurableBindings
be
true
else let
configurableBindings
be
false
If
code
is
strict mode code
, then let
strict
be
true
else let
strict
be
false
If
code
is function code, then
Let
func
be the function whose [[Call]] internal method initiated execution of
code
. Let
names
be the value of
func
’s [[FormalParameters]] internal property.
Let
argCount
be the number of elements in
args
Let
be the number 0.
For each String
argName
in
names
, in list order do
Let
be the current value of
plus 1.
If
is greater than
argCount
, let
be
undefined
otherwise let
be the
value of the
’th element of
args
Let
argAlreadyDeclared
be the result of calling
env’s
HasBinding concrete method passing
argName
as the argument.
If
argAlreadyDeclared
is
false
, call
env’s
CreateMutableBinding concrete method
passing
argName
as the argument.
Call
env’s
SetMutableBinding concrete method passing
argName
, and
strict
as
the arguments.
For each
FunctionDeclaration
in
code
, in source text order do
Let
fn
be the
Identifier
in
FunctionDeclaration
f.
Let
fo
be the result of instantiating
FunctionDeclaration f
as described in
Clause
13
Let
funcAlreadyDeclared
be the result of calling
env’s
HasBinding concrete method passing
fn
as the argument.
If
funcAlreadyDeclared
is
false
, call
env’s
CreateMutableBinding concrete method passing
fn
and
configurableBindings
as the arguments.
Else if
env
is the environment record component of
the global environment
then
Let
go
be the global object.
Let
existingProp
be the resulting of calling the [[GetProperty]] internal method of
go
with
argument
fn
If
existingProp
.[[Configurable]] is
true
, then
Call the [[DefineOwnProperty]] internal method of
go
, passing
fn
Property
Descriptor
{[[Value]]:
undefined
, [[Writable]]:
true
, [[Enumerable]]:
true
[[Configurable]]:
configurableBindings
}, and
true
as arguments.
Else if
IsAccessorDescriptor
existingProp
) or
existingProp
does not have
attribute values {[[Writable]]:
true
, [[Enumerable]]:
true
}, then
Throw a TypeError exception.
Call
env’s
SetMutableBinding concrete method passing
fn
fo
, and
strict
as the
arguments.
Let
argumentsAlreadyDeclared
be the result of calling
env’s
HasBinding concrete method passing
"arguments"
as the argument.
If
code
is function code and
argumentsAlreadyDeclared
is
false
, then
Let
argsObj
be the result of calling the abstract operation CreateArgumentsObject (
10.6
) passing
func, names, args,
env
and
strict
as arguments.
If
strict
is
true
, then
Call
env
’s CreateImmutableBinding concrete method passing the String "
arguments
" as
the argument.
Call
env
’s InitializeImmutableBinding concrete method passing "
arguments
" and
argsObj
as arguments.
Else,
Call
env
’s CreateMutableBinding concrete method passing the String "
arguments
" as the
argument.
Call
env
’s SetMutableBinding concrete method passing "
arguments
",
argsObj
, and
false
as arguments.
For each
VariableDeclaration
and
VariableDeclarationNoIn
in
code
, in source text order do
Let
dn
be the
Identifier
in
d.
Let
varAlreadyDeclared
be the result of calling
env’s
HasBinding concrete method passing
dn
as the argument.
If
varAlreadyDeclared
is
false
, then
Call
env
’s CreateMutableBinding concrete method passing
dn
and
configurableBindings
as the arguments.
Call
env
’s SetMutableBinding concrete method passing
dn
undefined
, and
strict
as the arguments.
10.6
Arguments Object
When control enters an execution context for function code, an arguments object is created unless (as specified in
10.5
) the identifier
arguments
occurs as an
Identifier
in the
function’s
FormalParameterList
or occurs as the
Identifier
of a
VariableDeclaration
or
FunctionDeclaration
contained in the function code.
The arguments object is created by calling the abstract operation CreateArgumentsObject with arguments
func
the
function object whose code is to be evaluated
, names
List
containing the function’s
formal parameter names
, args
the actual arguments passed to the [[Call]] internal method,
env
the
variable environment for the function code, and
strict
a Boolean that indicates whether or not the function code is
strict code
. When CreateArgumentsObject is called the following steps are performed:
Let
len
be the number of elements in
args
Let
obj
be the result of creating a new ECMAScript object.
Set all the internal methods of
obj
as specified in
8.12
Set the [[Class]] internal property of
obj
to
Arguments
".
Let
Object
be the standard built-in Object constructor (
15.2.2
).
Set the [[Prototype]] internal property of
obj
to the standard built-in Object prototype object (
15.2.4
).
Call the [[DefineOwnProperty]] internal method on
obj
passing
length
, the
Property Descriptor
{[[Value]]:
len
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
true
}, and
false
as arguments.
Let
map
be the result of creating a new object as if by the expression
new Object()
where
Object
is the standard built-in constructor with that name
Let
mappedNames
be an empty
List
Let
indx
len
- 1.
Repeat while
indx
>= 0,
Let
val
be the element of
args
at 0-origined list position
indx
Call the [[DefineOwnProperty]] internal method on
obj
passing
ToString
indx
),
the property descriptor {[[Value]]:
val
, [[Writable]]:
true
, [[Enumerable]]:
true
[[Configurable]]:
true
}, and
false
as arguments.
If
indx
is less than the number of elements in
names
, then
Let
name
be the element of
names
at 0-origined list position
indx
If
strict
is
false
and
name
is not an element of
mappedNames
, then
Add
name
as an element of the list
mappedNames
Let
be the result of calling the
MakeArgGetter
abstract operation with arguments
name
and
env
Let
be the result of calling the
MakeArgSetter
abstract operation with arguments
name
and
env
Call the [[DefineOwnProperty]] internal method of
map
passing
ToString
indx
), the
Property Descriptor
{[[Set]]:
, [[Get]]:
g,
[[Configurable]]:
true
}, and
false
as arguments.
Let
indx
indx
- 1
If
mappedNames
is not empty, then
Set the [[ParameterMap]] internal property of
obj
to
map
Set the [[Get]], [[GetOwnProperty]], [[DefineOwnProperty]], and [[Delete]] internal methods of
obj
to the
definitions provided below.
If
strict
is
false
, then
Call the [[DefineOwnProperty]] internal method on
obj
passing "
callee
", the property descriptor
{[[Value]]:
func
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
true
},
and
false
as arguments.
Else,
strict
is
true
so
Let
thrower
be the [[ThrowTypeError]] function Object (
13.2.3
).
Call the [[DefineOwnProperty]] internal method of
obj
with arguments
"caller"
PropertyDescriptor {[[Get]]:
thrower
, [[Set]]:
thrower
, [[Enumerable]]:
false
[[Configurable]]:
false
}, and
false
Call the [[DefineOwnProperty]] internal method of
obj
with arguments
"callee"
PropertyDescriptor {[[Get]]:
thrower
, [[Set]]:
thrower
, [[Enumerable]]:
false
[[Configurable]]:
false
}, and
false
Return
obj
The abstract operation
MakeArgGetter
called with String
name
and environment record
env
creates a function object that when executed returns the value bound for
name
in
env
. It performs the
following steps:
Let
body
be the result of concatenating the Strings "
return
",
name
, and
".
Return the result of creating a function object as described in
13.2
using no
FormalParameterList
body
for
FunctionBody
env
as
Scope
, and
true
for
Strict
The abstract operation
MakeArgSetter
called with String
name
and environment record
env
creates a function object that when executed sets the value bound for
name
in
env
. It performs the
following steps:
Let
param
be the String
name
concatenated with the String "
_arg
".
Let
body
be the String
"
with
replaced by the value of
name
and
replaced by the value of
param
Return the result of creating a function object as described in
13.2
using a
List
containing the single String
param
as
FormalParameterList
body
for
FunctionBody
env
as
Scope
, and
true
for
Strict
The [[Get]] internal method of an arguments object for a non-strict mode function with formal parameters when called with a
property name
performs the following steps:
Let
map
be the value of the [[ParameterMap]] internal property of the arguments object.
Let
isMapped
be the result of calling the [[GetOwnProperty]] internal method of
map
passing
as
the argument.
If the value of
isMapped
is
undefined
, then
Let
be the result of calling the default [[Get]] internal method (
8.12.3
) on the
arguments object passing
as the argument.
If
is
"caller"
and
is a strict mode Function object, throw a
TypeError
exception.
Return
Else,
map
contains a formal parameter mapping for
so,
Return the result of calling the [[Get]] internal method of
map
passing
as the argument.
The [[GetOwnProperty]] internal method of an arguments object for a non-strict mode function with formal parameters when
called with a property name
performs the following steps:
Let
desc
be the result of calling the default [[GetOwnProperty]] internal method (
8.12.1
) on the arguments object passing
as the argument.
If
desc
is
undefined
then return
desc
Let
map
be the value of the [[ParameterMap]] internal property of the arguments object.
Let
isMapped
be the result of calling the [[GetOwnProperty]] internal method of
map
passing
as
the argument.
If the value of
isMapped
is not
undefined
, then
Set
desc
.[[Value]] to the result of calling the [[Get]] internal method of
map
passing
as the
argument.
Return
desc
The [[DefineOwnProperty]] internal method of an arguments object for a non-strict mode function with formal parameters
when called with a property name
Property Descriptor
Desc
, and
Boolean flag
Throw
performs the following steps:
Let
map
be the value of the [[ParameterMap]] internal property of the arguments object.
Let
isMapped
be the result of calling the [[GetOwnProperty]] internal method of
map
passing
as
the argument.
Let
allowed
be the result of calling the default [[DefineOwnProperty]] internal method (
8.12.9
) on the arguments object passing
Desc
, and
false
as the
arguments.
If
allowed
is
false
, then
If
Throw
is
true
then throw a
TypeError
exception, otherwise return
false
If the value of
isMapped
is not
undefined
, then
If
IsAccessorDescriptor
Desc
) is
true
, then
Call the [[Delete]] internal method of
map
passing
, and
false
as the arguments.
Else
If
Desc
.[[Value]] is present, then
Call the [[Put]] internal method of
map
passing
Desc
.[[Value]], and
Throw
as
the arguments.
If
Desc
.[[Writable]] is present and its value is
false
, then
Call the [[Delete]] internal method of
map
passing
and
false
as arguments.
Return
true
The [[Delete]] internal method of an arguments object for a non-strict mode function with formal parameters when called
with a property name
and Boolean flag
Throw
performs the following steps:
Let
map
be the value of the [[ParameterMap]] internal property of the arguments object.
Let
isMapped
be the result of calling the [[GetOwnProperty]] internal method of
map
passing
as
the argument.
Let
result
be the result of calling the default [[Delete]] internal method (
8.12.7
) on
the arguments object passing
and
Throw
as the arguments.
If
result
is
true
and the value of
isMapped
is not
undefined
, then
Call the [[Delete]] internal method of
map
passing
, and
false
as the arguments.
Return
result
NOTE 1
For non-strict mode functions the array index (defined in
15.4
named data properties of an arguments 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. For strict mode functions, the values of the arguments object’s 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 2
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 accessible from ECMAScript code. An ECMAScript implementation does not need to actually create
or use such objects to implement the specified semantics.
NOTE 3
Arguments objects for strict mode functions define non-configurable accessor properties
named "
caller
" and "
callee
" which throw a
TypeError
exception on access. The
callee
" property has a more specific meaning for non-strict mode functions and a "
caller
property has historically been provided as an implementation-defined extension by some ECMAScript implementations. The
strict mode definition of these properties exists to ensure that neither of them is defined in any other manner by
conforming ECMAScript implementations.
11
Expressions
11.1
Primary Expressions
Syntax
PrimaryExpression
this
Identifier
Literal
ArrayLiteral
ObjectLiteral
Expression
11.1.1
The
this
Keyword
The
this
keyword evaluates to the value of the
ThisBinding
of the current execution
context.
11.1.2
Identifier Reference
An
Identifier
is evaluated by performing
Identifier Resolution as specified
in 10.3.1
. The result of evaluating an
Identifier
is always a value of type
Reference
11.1.3
Literal Reference
Literal
is evaluated as described in
7.8
11.1.4
Array Initialiser
An array initialiser is an expression describing the initialisation of an Array object, written in a form of a literal.
It is 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 initialiser 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
ArrayLiteral
Elision
opt
ElementList
ElementList
Elision
opt
ElementList
Elision
opt
AssignmentExpression
ElementList
Elision
opt
AssignmentExpression
Elision
Elision
Semantics
The production
ArrayLiteral
Elision
opt
is evaluated as
follows:
Let
array
be the result of creating a new object as if by the expression
new Array()
where
Array
is the standard built-in constructor with that name.
Let
pad
be the result of evaluating
Elision
; if not present, use the numeric value zero.
Call the [[Put]] internal method of
array
with arguments
length
pad
, and
false
Return
array
The production
ArrayLiteral
ElementList
is evaluated as follows:
Return the result of evaluating
ElementList
The production
ArrayLiteral
ElementList
Elision
opt
is evaluated as follows:
Let
array
be the result of evaluating
ElementList
Let
pad
be the result of evaluating
Elision
; if not present, use the numeric value zero.
Let
len
be the result of calling the [[Get]] internal method of
array
with argument
length
Call the [[Put]] internal method of
array
with arguments
length
ToUint32
pad
len
), and
false
Return
array
The production
ElementList
Elision
opt
AssignmentExpression
is evaluated as follows:
Let
array
be the result of creating a new object as if by the expression
new Array()
where
Array
is the standard built-in constructor with that name.
Let
firstIndex
be the result of evaluating
Elision
; if not present, use the numeric value zero.
Let
initResult
be the result of evaluating
AssignmentExpression
Let
initValue
be
GetValue
initResult
).
Call the [[DefineOwnProperty]] internal method of
array
with arguments
ToString
firstIndex)
, the
Property Descriptor
{ [[Value]]:
initValue
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Return
array
The production
ElementList
ElementList
Elision
opt
AssignmentExpression
is evaluated as follows:
Let
array
be the result of evaluating
ElementList
Let
pad
be the result of evaluating
Elision
; if not present, use the numeric value zero.
Let
initResult
be the result of evaluating
AssignmentExpression
Let
initValue
be
GetValue
initResult
).
Let
len
be the result of calling the [[Get]] internal method of
array
with argument
length
Call the [[DefineOwnProperty]] internal method of
array
with arguments
ToString
ToUint32
((
pad
len
)) and the
Property Descriptor
{ [[Value]]:
initValue
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Return
array
The production
Elision
is evaluated as follows:
Return the numeric value 1.
The production
Elision
Elision
is evaluated as follows:
Let
preceding
be the result of evaluating
Elision
Return
preceding
+1.
NOTE
[[DefineOwnProperty]] 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 [[Put]].
11.1.5
Object Initialiser
An object initialiser is an expression describing the initialisation of an Object, written in a form resembling a
literal. It is a list of zero or more pairs of property names and associated values, enclosed in curly braces. The values
need not be literals; they are evaluated each time the object initialiser is evaluated.
Syntax
ObjectLiteral
PropertyNameAndValueList
PropertyNameAndValueList
PropertyNameAndValueList
PropertyAssignment
PropertyNameAndValueList
PropertyAssignment
PropertyAssignment
PropertyName
AssignmentExpression
get
PropertyName
FunctionBody
set
PropertyName
PropertySetParameterList
FunctionBody
PropertyName
IdentifierName
StringLiteral
NumericLiteral
PropertySetParameterList
Identifier
Semantics
The production
ObjectLiteral
is evaluated as follows:
Return a new object created as if by the expression
new Object()
where
Object
is the
standard built-in constructor with that name.
The productions
ObjectLiteral
PropertyNameAndValueList
and
ObjectLiteral
PropertyNameAndValueList
are evaluated as
follows:
Return the result of evaluating
PropertyNameAndValueList
The production
PropertyNameAndValueList
PropertyAssignment
is evaluated as follows:
Let
obj
be the result of creating a new object as if by the expression
new Object()
where
Object
is the standard built-in constructor with that name.
Let
propId
be the result of evaluating
PropertyAssignment
Call the [[DefineOwnProperty]] internal method of
obj
with arguments
propId
.name,
propId
.descriptor, and
false
Return
obj
The production
PropertyNameAndValueList
PropertyNameAndValueList
PropertyAssignment
is evaluated as follows:
Let
obj
be the result of evaluating
PropertyNameAndValueList
Let
propId
be the result of evaluating
PropertyAssignment
Let
previous
be the result of calling the [[GetOwnProperty]] internal method of
obj
with argument
propId
.name.
If
previous
is not
undefined
then throw a
SyntaxError
exception if any of the following
conditions are true
This production is contained in
strict code
and
IsDataDescriptor
previous
) is
true
and
IsDataDescriptor
propId
.descriptor) is
true
IsDataDescriptor
previous
) is
true
and
IsAccessorDescriptor
propId
.descriptor) is
true.
IsAccessorDescriptor
previous
) is
true
and
IsDataDescriptor
propId
.descriptor) is
true
IsAccessorDescriptor
previous
) is
true
and
IsAccessorDescriptor
propId
.descriptor) is
true
and either both
previous
and
propId
.descriptor have [[Get]] fields or both
previous
and
propId
.descriptor have [[Set]] fields
Call the [[DefineOwnProperty]] internal method of
obj
with arguments
propId
.name,
propId
.descriptor, and
false
Return
obj
If the above steps would throw a
SyntaxError
then an implementation must treat the error as an early error (
Clause 16
).
The production
PropertyAssignment
PropertyName
AssignmentExpression
is evaluated as
follows:
Let
propName
be the result of evaluating
PropertyName
Let
exprValue
be the result of evaluating
AssignmentExpression
Let
propValue
be
GetValue
exprValue
).
Let
desc
be the
Property Descriptor
{[[Value]]:
propValue
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
Return
Property Identifier
propName
desc
).
The production
PropertyAssignment
get
PropertyName
FunctionBody
is evaluated as follows:
Let
propName
be the result of evaluating
PropertyName
Let
closure
be the result of creating a new Function object as specified in
13.2
with
an empty parameter list and body specified by
FunctionBody
. Pass in the
LexicalEnvironment
of the running execution context as the
Scope
. Pass in
true
as
the
Strict
flag if the
PropertyAssignment
is contained in
strict code
or if
its
FunctionBody
is
strict code
Let
desc
be the
Property Descriptor
{[[Get]]:
closure
, [[Enumerable]]:
true
, [[Configurable]]:
true
Return
Property Identifier
propName
desc
).
The production
PropertyAssignment
set
PropertyName
PropertySetParameterList
FunctionBody
is evaluated as follows:
Let
propName
be the result of evaluating
PropertyName
Let
closure
be the result of creating a new Function object as specified in
13.2
with
parameters specified by
PropertySetParameterList
and body specified by
FunctionBody
. Pass in the
LexicalEnvironment
of the running execution context as the
Scope
. Pass in
true
as
the
Strict
flag if the
PropertyAssignment
is contained in
strict code
or if
its
FunctionBody
is
strict code
Let
desc
be the
Property Descriptor
{[[Set]]:
closure
, [[Enumerable]]:
true
, [[Configurable]]:
true
Return
Property Identifier
propName
desc
).
It is a
SyntaxError
if the
Identifier
"eval"
or the
Identifier
"arguments"
occurs as the
Identifier
in a
PropertySetParameterList
of a
PropertyAssignment
that
is contained in
strict code
or if its
FunctionBody
is
strict code
The production
PropertyName
IdentifierName
is evaluated as follows:
Return the String value containing the same sequence of characters as the
IdentifierName
The production
PropertyName
StringLiteral
is evaluated as follows:
Return the SV of the
StringLiteral
The production
PropertyName
NumericLiteral
is evaluated as follows:
Let
nbr
be the result of forming the value of the
NumericLiteral
Return
ToString
nbr
).
11.1.6
The Grouping Operator
The production
PrimaryExpression
Expression
is evaluated as follows:
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 parenthesised expressions.
11.2
Left-Hand-Side Expressions
Syntax
MemberExpression
PrimaryExpression
FunctionExpression
MemberExpression
Expression
MemberExpression
IdentifierName
new
MemberExpression
Arguments
NewExpression
MemberExpression
new
NewExpression
CallExpression
MemberExpression
Arguments
CallExpression
Arguments
CallExpression
Expression
CallExpression
IdentifierName
Arguments
ArgumentList
ArgumentList
AssignmentExpression
ArgumentList
AssignmentExpression
LeftHandSideExpression
NewExpression
CallExpression
11.2.1
Property Accessors
Properties are accessed by name, using either the dot notation:
MemberExpression
IdentifierName
CallExpression
IdentifierName
or the bracket notation:
MemberExpression
Expression
CallExpression
Expression
The dot notation is explained by the following syntactic conversion:
MemberExpression
IdentifierName
is identical in its behaviour to
MemberExpression
and similarly
CallExpression
IdentifierName
is identical in its behaviour to
CallExpression
where
is a string literal containing the same sequence of characters after
processing of Unicode escape sequences as the
IdentifierName
The production
MemberExpression
MemberExpression
Expression
is evaluated as follows:
Let
baseReference
be the result of evaluating
MemberExpression
Let
baseValue
be
GetValue
baseReference
).
Let
propertyNameReference
be the result of evaluating
Expression
Let
propertyNameValue
be
GetValue
propertyNameReference
).
Call
CheckObjectCoercible
baseValue
).
Let
propertyNameString
be
ToString
propertyNameValue
).
If the syntactic production that is being evaluated is contained in
strict mode code
, let
strict
be
true
, else let
strict
be
false
Return a value of type
Reference
whose base value is
baseValue
and whose referenced name
is
propertyNameString
, and whose strict mode flag is
strict
The production
CallExpression
CallExpression
Expression
is evaluated in exactly the same manner, except that the contained
CallExpression
is evaluated in
step 1.
11.2.2
The
new
Operator
The production
NewExpression
new
NewExpression
is evaluated as follows:
Let
ref
be the result of evaluating
NewExpression
Let
constructor
be
GetValue
ref
).
If
Type
constructor
) is not Object, throw a
TypeError
exception.
If
constructor
does not implement the [[Construct]] internal method, throw a
TypeError
exception.
Return the result of calling the [[Construct]] internal method on
constructor
, providing no arguments (that is,
an empty list of arguments).
The production
MemberExpression
new
MemberExpression
Arguments
is evaluated as
follows:
Let
ref
be the result of evaluating
MemberExpression
Let
constructor
be
GetValue
ref
).
Let
argList
be the result of evaluating
Arguments
, producing an internal list of argument values (
11.2.4
).
If
Type
constructor
) is not Object, throw a
TypeError
exception.
If
constructor
does not implement the [[Construct]] internal method, throw a
TypeError
exception.
Return the result of calling the [[Construct]] internal method on
constructor
, providing the list
argList
as the argument values.
11.2.3
Function Calls
The production
CallExpression
MemberExpression
Arguments
is evaluated as follows:
Let
ref
be the result of evaluating
MemberExpression
Let
func
be
GetValue
ref
).
Let
argList
be the result of evaluating
Arguments
, producing an internal list of argument values (
see 11.2.4
).
If
Type
func
) is not Object, throw a
TypeError
exception.
If
IsCallable
func
) is
false
, throw a
TypeError
exception.
If
Type
ref
) is
Reference
, then
If
IsPropertyReference
ref
) is
true
, then
Let
thisValue
be
GetBase
ref
).
Else, the base of
ref
is an
Environment Record
Let
thisValue
be the result of calling the ImplicitThisValue concrete method of
GetBase
ref
).
Else,
Type
ref
) is not
Reference
Let
thisValue
be
undefined
Return the result of calling the [[Call]] internal method on
func
, providing
thisValue
as the
this
value and providing the list
argList
as the argument values.
The production
CallExpression
CallExpression
Arguments
is evaluated in exactly the same manner, except
that the contained
CallExpression
is evaluated in step 1.
NOTE
The returned result will never be of type
Reference
if
func
is a native ECMAScript object. Whether calling a host object can return a value of type
Reference
is implementation-dependent. If a value of type
Reference
is
returned, it must be a non-strict Property
Reference
11.2.4
Argument Lists
The evaluation of an argument list produces a
List
of values (
see 8.8
).
The production
Arguments
is evaluated as follows:
Return an empty
List
The production
Arguments
ArgumentList
is evaluated as follows:
Return the result of evaluating
ArgumentList
The production
ArgumentList
AssignmentExpression
is evaluated as follows:
Let
ref
be the result of evaluating
AssignmentExpression
Let
arg
be
GetValue
ref
).
Return a
List
whose sole item is
arg
The production
ArgumentList
ArgumentList
AssignmentExpression
is evaluated as
follows:
Let
precedingArgs
be the result of evaluating
ArgumentList
Let
ref
be the result of evaluating
AssignmentExpression
Let
arg
be
GetValue
ref
).
Return a
List
whose length is one greater than the length of
precedingArgs
and whose
items are the items of
precedingArgs
, in order, followed at the end by
arg
which is the last item of the
new list.
11.2.5
Function Expressions
The production
MemberExpression
FunctionExpression
is evaluated as follows:
Return the result of evaluating
FunctionExpression
11.3
Postfix Expressions
Syntax
PostfixExpression
LeftHandSideExpression
LeftHandSideExpression
[no
LineTerminator
here]
++
LeftHandSideExpression
[no
LineTerminator
here]
--
11.3.1
Postfix Increment
Operator
The production
PostfixExpression
LeftHandSideExpression
[no
LineTerminator
here]
++
is evaluated as follows:
Let
lhs
be the result of evaluating
LeftHandSideExpression
Throw a
SyntaxError
exception if the following conditions are all true:
Type
lhs
) is
Reference
is
true
IsStrictReference
lhs
) is
true
Type
GetBase
lhs
)) is
Environment
Record
GetReferencedName
lhs
) is either
"eval"
or
"arguments"
Let
oldValue
be
ToNumber
GetValue
lhs
)).
Let
newValue
be the result of adding the value
to
oldValue
, using the same rules as for
the
operator (
see 11.6.3
).
Call
PutValue
lhs
newValue
).
Return
oldValue
11.3.2
Postfix Decrement
Operator
The production
PostfixExpression
LeftHandSideExpression
[no
LineTerminator
here]
--
is evaluated as follows:
Let
lhs
be the result of evaluating
LeftHandSideExpression
Throw a
SyntaxError
exception if the following conditions are all true:
Type
lhs
) is
Reference
is
true
IsStrictReference
lhs
) is
true
Type
GetBase
lhs
)) is
Environment
Record
GetReferencedName
lhs
) is either
"eval"
or
"arguments"
Let
oldValue
be
ToNumber
GetValue
lhs
)).
Let
newValue
be the result of subtracting the value
from
oldValue
, using the same rules
as for the
operator (
11.6.3
).
Call
PutValue
lhs
newValue
).
Return
oldValue
11.4
Unary Operators
Syntax
UnaryExpression
PostfixExpression
delete
UnaryExpression
void
UnaryExpression
typeof
UnaryExpression
++
UnaryExpression
--
UnaryExpression
UnaryExpression
UnaryExpression
UnaryExpression
UnaryExpression
11.4.1
The
delete
Operator
The production
UnaryExpression
delete
UnaryExpression
is evaluated as follows:
Let
ref
be the result of evaluating
UnaryExpression
If
Type
ref
) is not
Reference
, return
true
If
IsUnresolvableReference
ref
) then,
If
IsStrictReference
ref
) is
true
, throw a
SyntaxError
exception.
Else, return
true
If
IsPropertyReference
ref
) is
true
, then
Return the result of calling the [[Delete]] internal method on
ToObject
GetBase
ref)
) providing
GetReferencedName
ref
) and
IsStrictReference
ref
) as the arguments.
Else,
ref
is a
Reference
to an
Environment Record
binding, so
If
IsStrictReference
ref
) is
true
, throw a
SyntaxError
exception.
Let
bindings
be
GetBase
ref
).
Return the result of calling the DeleteBinding concrete method of
bindings
, providing
GetReferencedName
ref
) as the argument.
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.
11.4.2
The
void
Operator
The production
UnaryExpression
void
UnaryExpression
is evaluated as follows:
Let
expr
be the result of evaluating
UnaryExpression
Call
GetValue
expr
).
Return
undefined
NOTE
GetValue
must be called even though its value is not used because
it may have observable side-effects.
11.4.3
The
typeof
Operator
The production
UnaryExpression
typeof
UnaryExpression
is evaluated as follows:
Let
val
be the result of evaluating
UnaryExpression
If
Type
val
) is
Reference
, then
If
IsUnresolvableReference
val
) is
true
, return
"undefined"
Let
val
be
GetValue
val
).
Return a String determined by
Type
val
) according to Table 20.
Table 20 — typeof Operator Results
Type of
val
Result
Undefined
"undefined"
Null
"object"
Boolean
"boolean"
Number
"number"
String
"string"
Object (native and does not implement [[Call]])
"object"
Object (native or host and does implement [[Call]])
"function"
Object (host and does not implement [[Call]])
Implementation-defined except may not be
"undefined"
"boolean"
"number
", or
"string".
11.4.4
Prefix Increment Operator
The production
UnaryExpression
++
UnaryExpression
is evaluated as follows:
Let
expr
be the result of evaluating UnaryExpression.
Throw a
SyntaxError
exception if the following conditions are all true:
Type
expr
) is
Reference
is
true
IsStrictReference
expr
) is
true
Type
GetBase
expr
)) is
Environment
Record
GetReferencedName
expr
) is either
"eval"
or
"arguments"
Let
oldValue
be
ToNumber
GetValue
expr
)).
Let
newValue
be the result of adding the value
to
oldValue
, using the same rules as for
the
operator (
see 11.6.3
).
Call
PutValue
expr
newValue
).
Return
newValue
11.4.5
Prefix Decrement Operator
The production
UnaryExpression
--
UnaryExpression
is evaluated as follows:
Let
expr
be the result of evaluating UnaryExpression.
Throw a
SyntaxError
exception if the following conditions are all true:
Type
expr
) is
Reference
is
true
IsStrictReference
expr
) is
true
Type
GetBase
expr
)) is
Environment
Record
GetReferencedName
expr
) is either
"eval"
or
"arguments"
Let
oldValue
be
ToNumber
GetValue
expr
)).
Let
newValue
be the result of subtracting the value
from
oldValue
, using the same rules
as for the
operator (
see 11.6.3
).
Call
PutValue
expr
newValue
).
Return
newValue
11.4.6
Unary
Operator
The unary + operator converts its operand to Number type.
The production
UnaryExpression
UnaryExpression
is evaluated as follows:
Let
expr
be the result of evaluating UnaryExpression.
Return
ToNumber
GetValue
expr
)).
11.4.7
Unary
Operator
The unary
operator converts its operand to Number type and then negates it. Note that negating
+0
produces
−0
, and negating
−0
produces
+0
The production
UnaryExpression
UnaryExpression
is evaluated as follows:
Let
expr
be the result of evaluating UnaryExpression.
Let
oldValue
be
ToNumber
GetValue
expr
)).
If
oldValue
is
NaN
, return
NaN
Return the result of negating
oldValue
; that is, compute a Number with the same magnitude but opposite
sign.
11.4.8
Bitwise NOT Operator (
The production
UnaryExpression
UnaryExpression
is evaluated as follows:
Let
expr
be the result of evaluating
UnaryExpression
Let
oldValue
be
ToInt32
GetValue
expr
)).
Return the result of applying bitwise complement to
oldValue
. The result is a signed 32-bit integer.
11.4.9
Logical NOT Operator (
The production
UnaryExpression
UnaryExpression
is evaluated as follows:
Let
expr
be the result of evaluating
UnaryExpression
Let
oldValue
be
ToBoolean
GetValue
expr
)).
If
oldValue
is
true
, return
false
Return
true
11.5
Multiplicative Operators
Syntax
MultiplicativeExpression
UnaryExpression
MultiplicativeExpression
UnaryExpression
MultiplicativeExpression
UnaryExpression
MultiplicativeExpression
UnaryExpression
Semantics
The production
MultiplicativeExpression
MultiplicativeExpression
UnaryExpression
, where @
stands for one of the operators in the above definitions, is evaluated as follows:
Let
left
be the result of evaluating MultiplicativeExpression.
Let
leftValue
be
GetValue
left
).
Let
right
be the result of evaluating UnaryExpression.
Let
rightValue
be
GetValue
right
).
Let
leftNum
be
ToNumber
leftValue
).
Let
rightNum
be
ToNumber
rightValue
).
Return the result of applying the specified operation (*, /, or %) to
leftNum
and
rightNum
. See the Notes
below
11.5.1
11.5.2
, 11.5.3.
11.5.1
Applying the
Operator
The
operator performs multiplication, producing the product of its operands. 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 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 or NaN is involved, the product is computed and rounded to the
nearest representable value using IEEE 754 round-to-nearest 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.
11.5.2
Applying the
Operator
The
operator performs division, producing the quotient of its operands. The left operand is the dividend
and the right operand 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 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 round-to-nearest 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.
11.5.3
Applying the
Operator
The
operator yields the remainder of its operands from an implied division; the left operand is the
dividend and the right operand 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. The IEEE 754 “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 round-to-nearest mode.
11.6
Additive Operators
Syntax
AdditiveExpression
MultiplicativeExpression
AdditiveExpression
MultiplicativeExpression
AdditiveExpression
MultiplicativeExpression
11.6.1
The Addition operator (
The addition operator either performs string concatenation or numeric addition.
The production
AdditiveExpression
AdditiveExpression
MultiplicativeExpression
is
evaluated as follows:
Let
lref
be the result of evaluating AdditiveExpression.
Let
lval
be
GetValue
lref
).
Let
rref
be the result of evaluating MultiplicativeExpression.
Let
rval
be
GetValue
rref
).
Let
lprim
be
ToPrimitive
lval
).
Let
rprim
be
ToPrimitive
rval
).
If
Type
lprim
) is String or
Type
rprim
) is String, then
Return the String that is the result of concatenating
ToString
lprim
) followed by
ToString
rprim
Return the result of applying the addition operation to
ToNumber
lprim
) and
ToNumber
rprim
). See the Note below
11.6.3
NOTE 1
No hint is provided in the calls to
ToPrimitive
in steps 5 and 6.
All native ECMAScript 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. Host objects may handle the absence of a hint in
some other manner.
NOTE 2
Step 7 differs from step 3 of the comparison algorithm for the relational operators (
11.8.5
), by using the logical-or operation instead of the logical-and operation.
11.6.2
The Subtraction Operator (
The production
AdditiveExpression
AdditiveExpression
MultiplicativeExpression
is
evaluated as follows:
Let
lref
be the result of evaluating AdditiveExpression.
Let
lval
be
GetValue
lref
).
Let
rref
be the result of evaluating MultiplicativeExpression.
Let
rval
be
GetValue
rref
).
Let
lnum
be
ToNumber
lval
).
Let
rnum
be
ToNumber
rval
).
Return the result of applying the subtraction operation to
lnum
and
rnum
. See the note below
11.6.3
11.6.3
Applying the Additive
Operators to Numbers
The
operator performs addition when applied to two operands of numeric type, producing the sum of the
operands. The
operator performs subtraction, producing the difference of two numeric operands.
Addition is a commutative operation, but not always associative.
The result of an addition is determined using the rules of IEEE 754 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
round-to-nearest 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.
The
operator performs subtraction when applied to two operands of numeric type, producing the difference
of its operands; the left operand is the minuend and the right operand is the subtrahend. Given numeric operands
and
, it is always the case that
produces the same
result as
+(–
11.7
Bitwise Shift Operators
Syntax
ShiftExpression
AdditiveExpression
ShiftExpression
<<
AdditiveExpression
ShiftExpression
>>
AdditiveExpression
ShiftExpression
>>>
AdditiveExpression
11.7.1
The Left Shift Operator (
<<
Performs a bitwise left shift operation on the left operand by the amount specified by the right operand.
The production
ShiftExpression
ShiftExpression
<<
AdditiveExpression
is
evaluated as follows:
Let
lref
be the result of evaluating
ShiftExpression
Let
lval
be
GetValue
lref
).
Let
rref
be the result of evaluating
AdditiveExpression
Let
rval
be
GetValue
rref
).
Let
lnum
be
ToInt32
lval
).
Let
rnum
be
ToUint32
rval
).
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.
11.7.2
The Signed Right Shift
Operator (
>>
Performs a sign-filling bitwise right shift operation on the left operand by the amount specified by the right
operand.
The production
ShiftExpression
ShiftExpression
>>
AdditiveExpression
is
evaluated as follows:
Let
lref
be the result of evaluating
ShiftExpression
Let
lval
be
GetValue
lref
).
Let
rref
be the result of evaluating
AdditiveExpression
Let
rval
be
GetValue
rref
).
Let
lnum
be
ToInt32
lval
).
Let
rnum
be
ToUint32
rval
).
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.
11.7.3
The Unsigned Right Shift
Operator (
>>>
Performs a zero-filling bitwise right shift operation on the left operand by the amount specified by the right
operand.
The production
ShiftExpression
ShiftExpression
>>>
AdditiveExpression
is
evaluated as follows:
Let
lref
be the result of evaluating
ShiftExpression
Let
lval
be
GetValue
lref
).
Let
rref
be the result of evaluating
AdditiveExpression
Let
rval
be
GetValue
rref
).
Let
lnum
be
ToUint32
lval
).
Let
rnum
be
ToUint32
rval
).
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.
11.8
Relational Operators
Syntax
RelationalExpression
ShiftExpression
RelationalExpression
ShiftExpression
RelationalExpression
ShiftExpression
RelationalExpression
<=
ShiftExpression
RelationalExpression
>=
ShiftExpression
RelationalExpression
instanceof
ShiftExpression
RelationalExpression
in
ShiftExpression
RelationalExpressionNoIn
ShiftExpression
RelationalExpressionNoIn
ShiftExpression
RelationalExpressionNoIn
ShiftExpression
RelationalExpressionNoIn
<=
ShiftExpression
RelationalExpressionNoIn
>=
ShiftExpression
RelationalExpressionNoIn
instanceof
ShiftExpression
NOTE
The “NoIn” variants are needed to avoid confusing the
in
operator in a relational expression with the
in
operator in
for
statement.
Semantics
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.
The
RelationalExpressionNoIn
productions are evaluated in the same manner as the
RelationalExpression
productions except that the contained
RelationalExpressionNoIn
is evaluated instead of the contained
RelationalExpression
11.8.1
The Less-than Operator (
The production
RelationalExpression
RelationalExpression
ShiftExpression
is
evaluated as follows:
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
be the result of performing abstract relational comparison
lval
rval
. (
see 11.8.5
If
is
undefined
, return
false
. Otherwise, return
11.8.2
The Greater-than Operator (
The production RelationalExpression
RelationalExpression
ShiftExpression
is evaluated as follows:
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
be the result of performing abstract relational comparison
rval
lval
with
LeftFirst
equal to
false
. (
see 11.8.5
).
If
is
undefined
, return
false
. Otherwise, return
11.8.3
The Less-than-or-equal
Operator (
<=
The production RelationalExpression
RelationalExpression
<=
ShiftExpression
is evaluated as follows:
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
be the result of performing abstract relational comparison
rval
lval
with
LeftFirst
equal to
false
. (
see 11.8.5
).
If
is
true
or
undefined
, return
false
. Otherwise, return
true
11.8.4
The Greater-than-or-equal
Operator (
>=
The production RelationalExpression
RelationalExpression
>=
ShiftExpression
is evaluated as follows:
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
be the result of performing abstract relational comparison
lval
rval
. (
see 11.8.5
If
is
true
or
undefined
, return
false
. Otherwise, return
true
11.8.5
The Abstract Relational
Comparison Algorithm
The comparison
, where
and
are values, produces
true
false
, or
undefined
(which indicates that at least one operand is
NaN
). In addition to
and
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
and
. It is necessary because ECMAScript specifies left to right evaluation of expressions. The default value of
LeftFirst
is
true
and indicates that the
parameter corresponds to an expression
that occurs to the left of the
parameter’s corresponding expression. If
LeftFirst
is
false
, the reverse is the case and operations must be performed upon
before
. Such a
comparison is performed as follows:
If the
LeftFirst
flag is
true
, then
Let
px
be the result of calling
ToPrimitive
, hint Number).
Let
py
be the result of calling
ToPrimitive
, hint Number).
Else the order of evaluation needs to be reversed to preserve left to right evaluation
Let
py
be the result of calling
ToPrimitive
, hint Number).
Let
px
be the result of calling
ToPrimitive
, hint Number).
If it is not the case that both
Type
px
) is String and
Type
py
) is String, then
Let
nx
be the result of calling
ToNumber
px
). Because
px
and
py
are primitive values evaluation order is not important.
Let
ny
be the result of calling
ToNumber
py
).
If
nx
is
NaN
, return
undefined
If
ny
is
NaN
, return
undefined
If
nx
and
ny
are the same Number value, return
false
If
nx
is
+0
and
ny
is
−0
, return
false
If
nx
is
−0
and
ny
is
+0
, return
false
If
nx
is
+∞
, return
false
If
ny
is
+∞
, return
true
If
ny
is
−∞
, return
false
If
nx
is
−∞
, return
true
If the mathematical value of
nx
is less than the mathematical value of
ny
—note that these
mathematical values are both finite and not both zero—return
true
. Otherwise, return
false
Else, both
px
and
py
are Strings
If
py
is a prefix of
px
, return
false
. (A String value
is a prefix of String value
if
can be the result of concatenating
and some other String
. Note that any
String is a prefix of itself, because
may be the empty String.)
If
px
is a prefix of
py
, return
true
Let
be the smallest nonnegative integer such that the character at position
within
px
is
different from the character at position
within
py
. (There must be such a
, for neither
String is a prefix of the other.)
Let
be the integer that is the code unit value for the character at position
within
px
Let
be the integer that is the code unit value for the character at position
within
py
If
, return
true
. Otherwise, return
false
NOTE 1
Step 3 differs from step 7 in the algorithm for the addition operator
11.6.1
) in using and instead of or.
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 normalised 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.
11.8.6
The instanceof operator
The production
RelationalExpression
RelationalExpression
instanceof
ShiftExpression
is
evaluated as follows:
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.
If
rval
does not have a [[HasInstance]] internal method, throw a
TypeError
exception.
Return the result of calling the [[HasInstance]] internal method of
rval
with argument
lval
11.8.7
The in operator
The production
RelationalExpression
RelationalExpression
in
ShiftExpression
is evaluated
as follows:
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 the result of calling the [[HasProperty]] internal method of
rval
with argument
ToString
lval
).
11.9
Equality Operators
Syntax
EqualityExpression
RelationalExpression
EqualityExpression
==
RelationalExpression
EqualityExpression
!=
RelationalExpression
EqualityExpression
===
RelationalExpression
EqualityExpression
!==
RelationalExpression
EqualityExpressionNoIn
RelationalExpressionNoIn
EqualityExpressionNoIn
==
RelationalExpressionNoIn
EqualityExpressionNoIn
!=
RelationalExpressionNoIn
EqualityExpressionNoIn
===
RelationalExpressionNoIn
EqualityExpressionNoIn
!==
RelationalExpressionNoIn
Semantics
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.
The
EqualityExpressionNoIn
productions are evaluated in the same manner as the
EqualityExpression
productions except that the contained
EqualityExpressionNoIn
and
RelationalExpressionNoIn
are evaluated instead of the contained
EqualityExpression
and
RelationalExpression
, respectively.
11.9.1
The Equals Operator (
==
The production EqualityExpression :
EqualityExpression
==
RelationalExpression
is evaluated as follows:
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
. (
see
11.9.3
).
11.9.2
The Does-not-equals Operator (
!=
The production EqualityExpression
EqualityExpression
!=
RelationalExpression
is evaluated as follows:
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
be the result of performing abstract equality comparison
rval
==
lval
. (
see 11.9.3
).
If
is
true
, return
false
. Otherwise, return
true
11.9.3
The Abstract Equality
Comparison Algorithm
The comparison
==
, where
and
are values, produces
true
or
false
. Such a comparison is performed as follows:
If
Type
) is the same as
Type
), then
If
Type
) is Undefined, return
true
If
Type
) is Null, return
true
If
Type
) is Number, then
If
is
NaN
, return
false
If
is
NaN
, return
false
If
is the same Number value as
, return
true
If
is
+0
and
is
−0
, return
true
If
is
−0
and
is
+0
, return
true
Return
false
If
Type
) is String, then return
true
if
and
are exactly the
same sequence of characters (same length and same characters in corresponding positions). Otherwise, return
false
If
Type
) is Boolean, return
true
if
and
are both
true
or both
false
. Otherwise, return
false
Return
true
if
and
refer to the same object. Otherwise, return
false
If
is
null
and
is
undefined
, return
true
If
is
undefined
and
is
null
, return
true
If
Type
) is Number and
Type
) is String,
return the
result of the comparison
==
ToNumber
).
If
Type
) is String and
Type
) is Number,
return the
result of the comparison
ToNumber
) ==
If
Type
) is Boolean, return the result of the comparison
ToNumber
) ==
If
Type
) is Boolean, return the result of the comparison
==
ToNumber
).
If
Type
) is either String or Number and
Type
) is
Object,
return the result of the comparison
==
ToPrimitive
).
If
Type
) is Object and
Type
) is either String or
Number,
return the result of the comparison
ToPrimitive
) ==
Return
false
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:
!=
is equivalent to
!(A
==
B)
==
is equivalent to
==
, except
in the order of evaluation of
and
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 normalised
form.
11.9.4
The Strict Equals Operator (
===
The production EqualityExpression
EqualityExpression
===
RelationalExpression
is evaluated as follows:
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 the strict equality comparison
rval
===
lval
. (See
11.9.6
11.9.5
The Strict Does-not-equal
Operator (
!==
The production EqualityExpression
EqualityExpression
!==
RelationalExpression
is evaluated as follows:
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
be the result of performing strict equality comparison
rval
===
lval
. (See
11.9.6
If
is
true
, return
false
. Otherwise, return
true
11.9.6
The Strict Equality Comparison
Algorithm
The comparison
===
, where
and
are values, produces
true
or
false
. Such a comparison is performed as follows:
If
Type
) is different from
Type
), return
false
If
Type
) is Undefined, return
true
If
Type
) is Null, return
true
If
Type
) is Number, then
If
is
NaN
, return
false
If
is
NaN
, return
false
If
is the same Number value as
, return
true
If
is
+0
and
is
−0
, return
true
If
is
−0
and
is
+0
, return
true
Return
false
If
Type
) is String, then return
true
if
and
are exactly the
same sequence of characters (same length and same characters in corresponding positions); otherwise, return
false
If
Type
) is Boolean, return
true
if
and
are both
true
or
both
false
; otherwise, return
false
Return
true
if
and
refer to the same object. Otherwise, return
false
NOTE
This algorithm differs from
the SameValue Algorithm (9.12)
in its
treatment of signed zeroes and NaNs.
11.10
Binary Bitwise Operators
Syntax
BitwiseANDExpression
EqualityExpression
BitwiseANDExpression
EqualityExpression
BitwiseANDExpressionNoIn
EqualityExpressionNoIn
BitwiseANDExpressionNoIn
EqualityExpressionNoIn
BitwiseXORExpression
BitwiseANDExpression
BitwiseXORExpression
BitwiseANDExpression
BitwiseXORExpressionNoIn
BitwiseANDExpressionNoIn
BitwiseXORExpressionNoIn
BitwiseANDExpressionNoIn
BitwiseORExpression
BitwiseXORExpression
BitwiseORExpression
BitwiseXORExpression
BitwiseORExpressionNoIn
BitwiseXORExpressionNoIn
BitwiseORExpressionNoIn
BitwiseXORExpressionNoIn
Semantics
The production
, where @ is one of the bitwise operators in the productions above, is evaluated as follows:
Let
lref
be the result of evaluating
Let
lval
be
GetValue
lref
).
Let
rref
be the result of evaluating
Let
rval
be
GetValue
rref
).
Let
lnum
be
ToInt32
lval
).
Let
rnum
be
ToInt32
rval
).
Return the result of applying the bitwise operator @ to
lnum
and
rnum
. The result is a signed 32 bit
integer.
11.11
Binary Logical Operators
Syntax
LogicalANDExpression
BitwiseORExpression
LogicalANDExpression
&&
BitwiseORExpression
LogicalANDExpressionNoIn
BitwiseORExpressionNoIn
LogicalANDExpressionNoIn
&&
BitwiseORExpressionNoIn
LogicalORExpression
LogicalANDExpression
LogicalORExpression
||
LogicalANDExpression
LogicalORExpressionNoIn
LogicalANDExpressionNoIn
LogicalORExpressionNoIn
||
LogicalANDExpressionNoIn
Semantics
The production
LogicalANDExpression
LogicalANDExpression
&&
BitwiseORExpression
is
evaluated as follows:
Let
lref
be the result of evaluating
LogicalANDExpression
Let
lval
be
GetValue
lref
).
If
ToBoolean
lval
) is
false
, return
lval
Let
rref
be the result of evaluating
BitwiseORExpression
Return
GetValue
rref
).
The production
LogicalORExpression
LogicalORExpression
||
LogicalANDExpression
is
evaluated as follows:
Let
lref
be the result of evaluating
LogicalORExpression
Let
lval
be
GetValue
lref
).
If
ToBoolean
lval
) is
true
, return
lval
Let
rref
be the result of evaluating
LogicalANDExpression
Return
GetValue
rref
).
The
LogicalANDExpressionNoIn
and
LogicalORExpressionNoIn
productions are
evaluated in the same manner as the
LogicalANDExpression
and
LogicalORExpression
productions except that the contained
LogicalANDExpressionNoIn
BitwiseORExpressionNoIn
and
LogicalORExpressionNoIn
are evaluated instead of
the contained
LogicalANDExpression
BitwiseORExpression
and
LogicalORExpression
, respectively.
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.
11.12
Conditional Operator (
? : )
Syntax
ConditionalExpression
LogicalORExpression
LogicalORExpression
AssignmentExpression
AssignmentExpression
ConditionalExpressionNoIn
LogicalORExpressionNoIn
LogicalORExpressionNoIn
AssignmentExpression
AssignmentExpressionNoIn
Semantics
The production
ConditionalExpression
LogicalORExpression
AssignmentExpression
AssignmentExpression
is evaluated as follows:
Let
lref
be the result of evaluating
LogicalORExpression
If
ToBoolean
GetValue
lref
)) is
true
, then
Let
trueRef
be the result of evaluating the first
AssignmentExpression
Return
GetValue
trueRef
).
Else
Let
falseRef
be the result of evaluating the second
AssignmentExpression
Return
GetValue
falseRef
).
The
ConditionalExpressionNoIn
production is evaluated in the same manner as the
ConditionalExpression
production except that the contained
LogicalORExpressionNoIn
AssignmentExpression
and
AssignmentExpressionNoIn
are evaluated instead of the
contained
LogicalORExpression
, first
AssignmentExpression
and second
AssignmentExpression
, respectively.
NOTE
The grammar for a ConditionalExpression in ECMAScript is a little bit 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.
11.13
Assignment Operators
Syntax
AssignmentExpression
ConditionalExpression
LeftHandSideExpression
AssignmentExpression
LeftHandSideExpression
AssignmentOperator
AssignmentExpression
AssignmentExpressionNoIn
ConditionalExpressionNoIn
LeftHandSideExpression
AssignmentExpressionNoIn
LeftHandSideExpression
AssignmentOperator
AssignmentExpressionNoIn
AssignmentOperator
one of
*=
/=
%=
+=
-=
<<=
>>=
>>>=
&=
^=
|=
Semantics
The
AssignmentExpressionNoIn
productions are evaluated in the same manner as the
AssignmentExpression
productions except that the contained
ConditionalExpressionNoIn
and
AssignmentExpressionNoIn
are evaluated instead of the contained
ConditionalExpression
and
AssignmentExpression
, respectively.
11.13.1
Simple Assignment (
The production
AssignmentExpression
LeftHandSideExpression
AssignmentExpression
is
evaluated as follows:
Let
lref
be the result of evaluating
LeftHandSideExpression
Let
rref
be the result of evaluating
AssignmentExpression
Let
rval
be
GetValue
rref
).
Throw a
SyntaxError
exception if the following conditions are all true:
Type
lref
) is
Reference
is
true
IsStrictReference
lref
) is
true
Type
GetBase
lref
)) is
Environment
Record
GetReferencedName
lref
) is either
"eval"
or
"arguments"
Call
PutValue
lref
rval
).
Return
rval
NOTE
When an assignment occurs within
strict mode code
, its
LeftHandSide
must not evaluate to an unresolvable reference. If it does a
ReferenceError
exception is thrown upon assignment. The
LeftHandSide
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 whose [[Extensible]] internal property has the value
false
. In these
cases a
TypeError
exception is thrown.
11.13.2
Compound Assignment (
op=
The production
AssignmentExpression
LeftHandSideExpression
AssignmentOperator
AssignmentExpression
, where
AssignmentOperator
is @
and @
represents one of the operators indicated above, is evaluated as follows:
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
be the result of applying operator @ to
lval
and
rval
Throw a
SyntaxError
exception if the following conditions are all true:
Type
lref
) is
Reference
is
true
IsStrictReference
lref
) is
true
Type
GetBase
lref
)) is
Environment
Record
GetReferencedName
(lref) is either
"eval"
or
"arguments"
Call
PutValue
lref
).
Return
NOTE
See NOTE 11.13.1.
11.14
Comma Operator (
, )
Syntax
Expression
AssignmentExpression
Expression
AssignmentExpression
ExpressionNoIn
AssignmentExpressionNoIn
ExpressionNoIn
AssignmentExpressionNoIn
Semantics
The production
Expression
Expression
AssignmentExpression
is evaluated as
follows:
Let
lref
be the result of evaluating
Expression
Call
GetValue
lref
).
Let
rref
be the result of evaluating
AssignmentExpression
Return
GetValue
rref
).
The
ExpressionNoIn
production is evaluated in the same manner as the
Expression
production except that the contained
ExpressionNoIn
and
AssignmentExpressionNoIn
are evaluated instead of the contained
Expression
and
AssignmentExpression
, respectively.
NOTE
GetValue
must be called even though its value is not used because
it may have observable side-effects.
12
Statements
Syntax
Statement
Block
VariableStatement
EmptyStatement
ExpressionStatement
IfStatement
IterationStatement
ContinueStatement
BreakStatement
ReturnStatement
WithStatement
LabelledStatement
SwitchStatement
ThrowStatement
TryStatement
DebuggerStatement
Semantics
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. A
LabelledStatement
has no semantic meaning other than the introduction of a label to a
label set
. The
label set of an
IterationStatement
or a
SwitchStatement
initially contains the
single element
empty
. The label set of any other statement is initially empty.
The result of evaluating a
Statement
is always a
Completion
value.
NOTE
Several widely used implementations of ECMAScript are known to support the use of
FunctionDeclaration
as a
Statement
. However there are significant and irreconcilable variations among the implementations in the
semantics applied to such
FunctionDeclarations
. Because of these irreconcilable differences, the use
of a
FunctionDeclaration
as a
Statement
results in code that is not reliably
portable among implementations. It is recommended that ECMAScript implementations either disallow this usage of
FunctionDeclaration
or issue a warning when such a usage is encountered. Future editions of ECMAScript may
define alternative portable means for declaring functions in a
Statement
context.
12.1
Block
Syntax
Block
StatementList
opt
StatementList
Statement
StatementList
Statement
Semantics
The production
Block
is evaluated as follows:
Return (
normal
empty
empty
).
The production
Block
StatementList
is evaluated as follows:
Return the result of evaluating
StatementList
The production
StatementList
Statement
is evaluated as follows:
Let
be the result of evaluating
Statement
If an exception was thrown, return (
throw
empty
) where
is the exception. (Execution now proceeds as if no
exception were thrown.)
Return
The production
StatementList
StatementList
Statement
is evaluated as follows:
Let
sl
be the result of evaluating
StatementList
If
sl
is an
abrupt completion
, return
sl
Let
be the result of evaluating
Statement
If an exception was thrown, return (
throw
empty
) where
is the exception. (Execution now proceeds as if no
exception were thrown.)
If
.value is
empty
, let
sl
.value, otherwise let
.value.
Return (
.type,
.target).
NOTE
Steps 5 and 6 of the above algoritm ensure that the value of a
StatementList
is the value of the last value producing
Statement
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;")
12.2
Variable Statement
Syntax
VariableStatement
var
VariableDeclarationList
VariableDeclarationList
VariableDeclaration
VariableDeclarationList
VariableDeclaration
VariableDeclarationListNoIn
VariableDeclarationNoIn
VariableDeclarationListNoIn
VariableDeclarationNoIn
VariableDeclaration
Identifier
Initialiser
opt
VariableDeclarationNoIn
Identifier
InitialiserNoIn
opt
Initialiser
AssignmentExpression
InitialiserNoIn
AssignmentExpressionNoIn
A variable statement declares variables that are created as defined in
10.5
. Variables are
initialised to
undefined
when created. A variable with an
Initialiser
is assigned the value of
its
AssignmentExpression
when the
VariableStatement
is executed, not when the
variable is created.
Semantics
The production
VariableStatement
var
VariableDeclarationList
is evaluated as
follows:
Evaluate
VariableDeclarationList
Return (
normal
empty
empty
).
The production
VariableDeclarationList
:VariableDeclaration
is evaluated as follows:
Evaluate
VariableDeclaration
The production
VariableDeclarationList
VariableDeclarationList
VariableDeclaration
is
evaluated as follows:
Evaluate
VariableDeclarationList
Evaluate
VariableDeclaration
The production
VariableDeclaration
Identifier
is evaluated as follows:
Return a String value containing the same sequence of characters as in the
Identifier
The production
VariableDeclaration
Identifier
Initialiser
is evaluated as follows:
Let
lhs
be the result of evaluating
Identifier
as described in
11.1.2
Let
rhs
be the result of evaluating
Initialiser
Let
value
be
GetValue
rhs
).
Call
PutValue
lhs
value
).
Return a String value containing the same sequence of characters as in the
Identifier
NOTE
The String value of a
VariableDeclaration
is used in the evaluation
of for-in statements (
12.6.4
).
If a
VariableDeclaration
is nested within a with statement and the Identifier in the
VariableDeclaration
is the same as a property name of the binding object of the with statement’s
object environment record
, then step 4 will assign value to the property instead of to the
VariableEnvironment
binding of the
Identifier
The production
Initialiser
AssignmentExpression
is evaluated as follows:
Return the result of evaluating
AssignmentExpression
The
VariableDeclarationListNoIn
VariableDeclarationNoIn
and
InitialiserNoIn
productions are evaluated in the same manner as the
VariableDeclarationList
VariableDeclaration
and
Initialiser
productions except that the contained
VariableDeclarationListNoIn
VariableDeclarationNoIn
InitialiserNoIn
and
AssignmentExpressionNoIn
are evaluated instead of the contained
VariableDeclarationList
VariableDeclaration
Initialiser
and
AssignmentExpression
, respectively.
12.2.1
Strict Mode Restrictions
It is a
SyntaxError
if a
VariableDeclaration
or
VariableDeclarationNoIn
occurs within
strict code
and its
Identifier
is either
"eval"
or
"arguments"
12.3
Empty Statement
Syntax
EmptyStatement
Semantics
The production
EmptyStatement
is evaluated as follows:
Return (
normal
empty
empty
12.4
Expression Statement
Syntax
ExpressionStatement
[lookahead ∉ {
function
}]
Expression
NOTE
An
ExpressionStatement
cannot start with an opening curly brace
because that might make it ambiguous with a
Block
. Also, an
ExpressionStatement
cannot start with the
function
keyword because that might make it
ambiguous with a
FunctionDeclaration
Semantics
The production
ExpressionStatement
[lookahead ∉ {
function
}]
Expression
is evaluated as follows:
Let
exprRef
be the result of evaluating
Expression
Return (
normal
GetValue
exprRef
),
empty
).
12.5
The
if
Statement
Syntax
IfStatement
if
Expression
Statement
else
Statement
if
Expression
Statement
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
Semantics
The production
IfStatement
if
Expression
Statement
else
Statement
is evaluated as follows:
Let
exprRef
be the result of evaluating
Expression
If
ToBoolean
GetValue
exprRef
)) is
true
, then
Return the result of evaluating the first
Statement
Else,
Return the result of evaluating the second
Statement
The production
IfStatement
if
Expression
Statement
is evaluated as follows:
Let
exprRef
be the result of evaluating
Expression
If
ToBoolean
GetValue
exprRef
)) is
false
, return (
normal
empty
empty
).
Return the result of evaluating
Statement
12.6
Iteration Statements
Syntax
IterationStatement
do
Statement
while
Expression
);
while
Expression
Statement
for
ExpressionNoIn
opt
Expression
opt
Expression
opt
Statement
for
var
VariableDeclarationListNoIn
Expression
opt
Expression
opt
Statement
for
LeftHandSideExpression
in
Expression
Statement
for
var
VariableDeclarationNoIn
in
Expression
Statement
12.6.1
The
do
while
Statement
The production
do
Statement
while
Expression
);
is evaluated as follows:
Let
empty
Let
iterating
be
true
Repeat, while
iterating
is
true
Let
stmt
be the result of evaluating
Statement
If
stmt
.value is not
empty
, let
stmt
.value
If
stmt
.type is not
continue
||
stmt
.target is not in
the current label set, then
If
stmt
.type is
break
and
stmt
.target is in the
current label set, return (
normal
empty
).
If
stmt
is an
abrupt completion
, return
stmt
Let
exprRef
be the result of evaluating
Expression
If
ToBoolean
GetValue
exprRef
)) is
false
, set
iterating
to
false
Return (
normal
empty
);
12.6.2
The
while
Statement
The production
IterationStatement
while
Expression
Statement
is evaluated as follows:
Let
empty
Repeat
Let
exprRef
be the result of evaluating
Expression
If
ToBoolean
GetValue
exprRef
)) is
false
, return
normal
empty
).
Let
stmt
be the result of evaluating
Statement
If
stmt
.value is not
empty
, let
stmt
.value.
If
stmt
.type is not
continue
||
stmt
.target is not in
the current label set, then
If
stmt
.type is
break
and
stmt
.target is in the
current label set, then
Return (
normal
empty
).
If
stmt
is an
abrupt completion
, return
stmt
12.6.3
The
for
Statement
The production
IterationStatement
for
ExpressionNoIn
opt
Expression
opt
Expression
opt
Statement
is evaluated as follows:
If
ExpressionNoIn
is present, then.
Let
exprRef
be the result of evaluating
ExpressionNoIn
Call
GetValue
exprRef
). (This value is not used but the call may have
side-effects.)
Let
empty
Repeat
If the first
Expression
is present, then
Let
testExprRef
be the result of evaluating the first
Expression
If
ToBoolean
GetValue
testExprRef
)) is
false
return (
normal
empty
).
Let
stmt
be the result of evaluating
Statement
If
stmt
.value is not
empty
, let
stmt
.value
If
stmt
.type is
break
and
stmt
.target is in the current
label set, return (
normal
empty
).
If
stmt
.type is not
continue
||
stmt
.target is not in
the current label set, then
If
stmt
is an
abrupt completion
, return
stmt
If the second
Expression
is present, then
Let
incExprRef
be the result of evaluating the second
Expressio
n.
Call
GetValue
incExprRef
). (This value is not used.)
The production
IterationStatement
for
var
VariableDeclarationListNoIn
Expression
opt
Expression
opt
Statement
is evaluated
as follows:
Evaluate
VariableDeclarationListNoIn
Let V =
empty
Repeat
If the first
Expression
is present, then
Let
testExprRef
be the result of evaluating the first
Expression
If
ToBoolean
GetValue
testExprRef
)) is
false
then return (
normal
, V,
empty
).
Let
stmt
be the result of evaluating
Statement
If
stmt
.value is not
empty
, let V =
stmt
.value.
If
stmt
.type is
break
and
stmt
.target is in the current
label set, return (
normal
, V,
empty
).
If
stmt
.type is not
continue
||
stmt
.target is not in
the current label set, then
If
stmt
is an
abrupt completion
, return
stmt
If the second
Expression
is present, then.
Let
incExprRef
be the result of evaluating the second
Expression
Call
GetValue
incExprRef
). (This value is not used.)
12.6.4
The
for
in
Statement
The production
IterationStatement
for
LeftHandSideExpression
in
Expression
Statement
is evaluated as follows:
Let
exprRef
be the result of evaluating the
Expression
Let
experValue
be
GetValue
exprRef
).
If
experValue
is
null
or
undefined
, return (
normal
empty
empty
).
Let
obj
be
ToObject
experValue
).
Let
empty
Repeat
Let
be the name of the next property of
obj
whose [[Enumerable]] attribute is
true
. If
there is no such property, return (
normal
empty
).
Let
lhsRef
be the result of evaluating the
LeftHandSideExpression
( it may be evaluated
repeatedly).
Call
PutValue
lhsRef
).
Let
stmt
be the result of evaluating
Statement
If
stmt
.value is not
empty
, let
stmt
.value.
If
stmt
.type is
break
and
stmt
.target is in the current
label set, return (
normal
empty
).
If
stmt
.type is not
continue
||
stmt
.target is not in the current label set, then
If
stmt
is an
abrupt completion
, return
stmt
The production
IterationStatement
for
var
VariableDeclarationNoIn
in
Expression
Statement
is evaluated as follows:
Let
varName
be the result of evaluating
VariableDeclarationNoIn
Let
exprRef
be the result of evaluating the
Expression
Let
experValue
be
GetValue
exprRef
).
If
experValue
is
null
or
undefined
, return (
normal
empty
empty
).
Let
obj
be
ToObject
experValue
).
Let
empty
Repeat
Let
be the name of the next property of
obj
whose [[Enumerable]] attribute is
true
. If
there is no such property, return (
normal
empty
).
Let
varRef
be the result of evaluating
varName
as if it were an Identifier
Reference
11.1.2
); it may be evaluated repeatedly.
Call
PutValue
varRef
).
Let
stmt
be the result of evaluating
Statement
If
stmt
.value is not
empty
, let
stmt
.value.
If
stmt
.type is
break
and
stmt
.target is in the current
label set, return (
normal
empty
).
If
stmt
.type is not
continue
||
stmt
.target is not in
the current label set, then
If
stmt
is an
abrupt completion
, return
stmt
The mechanics and order of enumerating the properties (step 6.a in the first algorithm, step 7.a in the second) is not
specified. Properties of the object being enumerated may be deleted during enumeration. If a property that has not yet been
visited during enumeration is deleted, then it will not be visited. If new properties are added to the object being
enumerated during enumeration, the newly added properties are not guaranteed to be visited in the active enumeration. A
property name must not be visited more than once in any enumeration.
Enumerating the properties of an 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 enumerated if it is “shadowed” because
some previous object in the prototype chain has a property with the same name. The values of [[Enumerable]] attributes are
not considered when determining if a property of a prototype object is shadowed by a previous object on the prototype
chain.
NOTE
See NOTE 11.13.1.
12.7
The
continue
Statement
Syntax
ContinueStatement
continue
continue
[no
LineTerminator
here]
Identifier
Semantics
A program is considered syntactically incorrect if either of the following is true:
The program contains a
continue
statement without the optional
Identifier
, which is not nested,
directly or indirectly (but not crossing function boundaries), within an
IterationStatement
The program contains a
continue
statement with the optional
Identifier
, where
Identifier
does not appear in the label set of an enclosing (but not crossing function boundaries)
IterationStatement
ContinueStatement
without an
Identifier
is evaluated as follows:
Return (
continue
empty
empty
).
ContinueStatement
with the optional
Identifier
is evaluated as
follows:
Return (
continue
empty
Identifier
).
12.8
The
break
Statement
Syntax
BreakStatement
break
break
[no
LineTerminator
here]
Identifier
Semantics
A program is considered syntactically incorrect if either of the following is true:
The program contains a
break
statement without the optional
Identifier
, which is not nested,
directly or indirectly (but not crossing function boundaries), within an
IterationStatement
or a
SwitchStatement
The program contains a
break
statement with the optional
Identifier
, where
Identifier
does
not appear in the label set of an enclosing (but not crossing function boundaries)
Statement
BreakStatement
without an
Identifier
is evaluated as follows:
Return (
break
empty
empty
).
BreakStatement
with an
Identifier
is evaluated as follows:
Return (
break
empty
Identifier
).
12.9
The
return
Statement
Syntax
ReturnStatement
return
return
[no
LineTerminator
here]
Expression
Semantics
An ECMAScript program is considered syntactically incorrect if it contains a
return
statement that is not
within a
FunctionBody
. A
return
statement causes a function to cease execution and return
a value to the caller. If
Expression
is omitted, the return value is
undefined
. Otherwise, the
return value is the value of
Expression
ReturnStatement
is evaluated as follows:
If the
Expression
is not present, return (
return
undefined
empty
).
Let
exprRef
be the result of evaluating
Expression
Return (
return
GetValue
exprRef
),
empty
).
12.10
The
with
Statement
Syntax
WithStatement
with
Expression
Statement
The
with
statement adds an
object environment record
for a computed object to the
lexical environment
of the current execution context. It then executes a statement using this augmented
lexical environment
. Finally, it restores the original
lexical
environment
Semantics
The production
WithStatement
with
Expression
Statement
is evaluated as follows:
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 the result of calling
NewObjectEnvironment
passing
obj
and
oldEnv
as the arguments.
Set the
provideThis
flag of
newEnv
to
true
Set the running execution context’s
LexicalEnvironment
to
newEnv
Let
be the result of evaluating
Statement
but if an exception is thrown during the evaluation, let
be (
throw
empty
), where
is the exception. (Execution now proceeds as if no exception were
thrown.)
Set the running execution context’s
Lexical Environment
to
oldEnv
Return
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.
12.10.1
Strict Mode
Restrictions
Strict mode code may not include a
WithStatement
. The occurrence of a
WithStatement
in such a context is treated as a
SyntaxError
12.11
The
switch
Statement
Syntax
SwitchStatement
switch
Expression
CaseBlock
CaseBlock
CaseClauses
opt
CaseClauses
opt
DefaultClause
CaseClauses
opt
CaseClauses
CaseClause
CaseClauses
CaseClause
CaseClause
case
Expression
StatementList
opt
DefaultClause
default
StatementList
opt
Semantics
The production
SwitchStatement
switch
Expression
CaseBlock
is evaluated as follows:
Let
exprRef
be the result of evaluating
Expression
Let
be the result of evaluating
CaseBlock
, passing it
GetValue
exprRef
as a parameter.
If
.type is
break
and
.target is in the current label set,
return (
normal
.value,
empty
).
Return
The production
CaseBlock
CaseClauses
opt
is given an input parameter,
input
, and is evaluated as follows:
Let
empty
Let
be the list of
CaseClause
items in source text order.
Let
searching
be
true
Repeat, while
searching
is
true
Let
be the next
CaseClause
in
. If there is no such
CaseClause
, return (
normal
empty
).
Let
clauseSelector
be the result of evaluating
If
input
is equal to
clauseSelector
as defined by the
===
operator, then
Set
searching
to
false
If
has a
StatementList
, then
Evaluate
’s
StatementList
and let
be the result.
If
is an
abrupt completion
, then return
Let
.value.
Repeat
Let
be the next
CaseClause
in
. If there is no such
CaseClause
, return
normal
empty
).
If
has a
StatementList
, then
Evaluate
’s
StatementList
and let
be the result.
If
.value is not
empty
, then let
.value.
If
is an
abrupt completion
, then return
(R
.type,
.target).
The production
CaseBlock
CaseClauses
opt
DefaultClause
CaseClauses
opt
is given an input parameter,
input
, and
is evaluated as follows:
Let
empty
Let
be the list of
CaseClause
items in the first
CaseClauses
, in source text order.
Let
be the list of
CaseClause
items in the second
CaseClauses
, in source text order.
Let
found
be
false
Repeat letting
be in order each
CaseClause
in
If
found
is
false
, then
Let
clauseSelector
be the result of evaluating
If
input
is equal to
clauseSelector
as defined by the
===
operator, then set
found
to
true
If
found
is
true
, then
If
has a
StatementList
, then
Evaluate
’s
StatementList
and let
be the result.
If
.value is not
empty
, then let
.value.
is an
abrupt completion
, then return
(R
.type,
.target).
Let
foundInB
be
false
If
found
is
false
, then
Repeat, while
foundInB
is
false
and all elements of
have not been processed
Let
be the next
CaseClause
in
Let
clauseSelector
be the result of evaluating
If
input
is equal to
clauseSelector
as defined by the
===
operator, then
Set
foundInB
to
true
If
has a
StatementList
, then
Evaluate
’s
StatementList
and let
be the result.
If
.value is not
empty
, then let
.value.
is an
abrupt completion
, then return
(R
.type,
.target).
If
foundInB
is
false
and the
DefaultClause
has a
StatementList
, then
Evaluate the
DefaultClause’s
StatementList
and let
be the result.
If
.value is not
empty
, then let
.value.
If
is an
abrupt completion
, then return (
.type,
.target).
Repeat (Note that if step 7.a.i has been performed this loop does not start at the beginning of
Let
be the next
CaseClause
in
. If there is no such
CaseClause
, return (
normal
empty
).
If
has a
StatementList
, then
Evaluate
’s
StatementList
and let
be the result.
If
.value is not
empty
, then let
.value.
If
is an
abrupt completion
, then return (
.type,
.target).
The production
CaseClause
case
Expression
StatementList
opt
is
evaluated as follows:
Let
exprRef
be the result of evaluating
Expression
Return
GetValue
exprRef
).
NOTE
Evaluating
CaseClause
does not execute the associated
StatementList
. It simply evaluates the
Expression
and returns the value, which the
CaseBlock
algorithm uses to determine which
StatementList
to start
executing.
12.12
Labelled Statements
Syntax
LabelledStatement
Identifier
Statement
Semantics
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.
An ECMAScript program is considered syntactically incorrect if it contains a
LabelledStatement
that
is enclosed by a
LabelledStatement
with the same
Identifier
as label. This
does not apply to labels appearing within the body of a
FunctionDeclaration
that is nested, directly
or indirectly, within a labelled statement.
The production
Identifier
Statement
is evaluated by adding
Identifier
to the label set of
Statement
and then evaluating
Statement
. If the
LabelledStatement
itself has a non-empty label set, these labels are also added to the label set of
Statement
before evaluating it. If the result of evaluating
Statement
is
break
) where
is equal to
Identifier
, the production
results in (
normal
empty
).
Prior to the evaluation of a
LabelledStatement
, the contained
Statement
is
regarded as possessing an empty label set, unless it is an
IterationStatement
or a
SwitchStatement
, in which case it is regarded as possessing a label set consisting of the single element,
empty
12.13
The
throw
Statement
Syntax
ThrowStatement
throw
[no
LineTerminator
here]
Expression
Semantics
The production
ThrowStatement
throw
[no
LineTerminator
here]
Expression
is evaluated as follows:
Let
exprRef
be the result of evaluating
Expression
Return (
throw
GetValue
exprRef
),
empty
).
12.14
The
try
Statement
Syntax
TryStatement
try
Block
Catch
try
Block
Finally
try
Block
Catch
Finally
Catch
catch
Identifier
Block
Finally
finally
Block
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
Identifier
is bound to that exception.
Semantics
The production
TryStatement
try
Block
Catch
is evaluated as follows:
Let
be the result of evaluating
Block
If
.type is not
throw
, return
Return the result of evaluating
Catch
with parameter
.value.
The production
TryStatement
try
Block
Finally
is evaluated as follows:
Let
be the result of evaluating
Block
Let
be the result of evaluating
Finally
If
.type is
normal
, return
Return
The production
TryStatement
try
Block
Catch
Finally
is
evaluated as follows:
Let
be the result of evaluating
Block
If
.type is
throw
, then
Let
be the result of evaluating
Catch
with parameter
.value.
Else,
.type is not
throw
Let
be
Let
be the result of evaluating
Finally
If
.type is
normal
, return
Return
The production
Catch
catch
Identifier
Block
is
evaluated as follows:
Let
be the parameter that has been passed to this production.
Let
oldEnv
be the running execution context’s
LexicalEnvironment
Let
catchEnv
be the result of calling
NewDeclarativeEnvironment
passing
oldEnv
as the argument.
Call the CreateMutableBinding concrete method of
catchEnv
passing the
Identifier
String value as the
argument.
Call the SetMutableBinding concrete method of
catchEnv
passing the
Identifier
, and
false
as arguments. Note that the last argument is immaterial in this situation.
Set the running execution context’s
LexicalEnvironment
to
catchEnv
Let
be the result of evaluating
Block
Set the running execution context’s
LexicalEnvironment
to
oldEnv
Return
NOTE
No matter how control leaves the
Block
the
LexicalEnvironment
is always restored to its former state.
The production
Finally
finally
Block
is evaluated as follows:
Return the result of evaluating
Block
12.14.1
Strict Mode
Restrictions
It is a
SyntaxError
if a
TryStatement
with a
Catch
occurs within
strict code
and the
Identifier
of the
Catch
production is either
"eval"
or
"arguments"
12.15
The
debugger
statement
Syntax
DebuggerStatement
debugger
Semantics
Evaluating the
DebuggerStatement
production 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.
The production
DebuggerStatement
debugger
is evaluated as follows:
If an implementation defined debugging facility is available and enabled, then
Perform an implementation defined debugging action.
Let
result
be an implementation defined
Completion
value.
Else
Let
result
be (
normal
empty
empty
).
Return
result
13
Function Definition
Syntax
FunctionDeclaration
function
Identifier
FormalParameterList
opt
FunctionBody
FunctionExpression
function
Identifier
opt
FormalParameterList
opt
FunctionBody
FormalParameterList
Identifier
FormalParameterList
Identifier
FunctionBody
SourceElements
opt
Semantics
The production
FunctionDeclaration
function
Identifier
FormalParameterList
opt
FunctionBody
is instantiated as follows during Declaration Binding
instantiation (
10.5
):
Return the result of creating a new Function object as specified in
13.2
with parameters specified
by
FormalParameterList
opt
, and body specified by
FunctionBody
. Pass in the
VariableEnvironment
of the running execution context as the
Scope
. Pass in
true
as the
Strict
flag if the
FunctionDeclaration
is contained in
strict code
or if its
FunctionBody
is
strict code
The production
FunctionExpression
function
FormalParameterList
opt
FunctionBody
is evaluated
as follows:
Return the result of creating a new Function object as specified in
13.2
with parameters specified
by
FormalParameterList
opt
and body specified by
FunctionBody
. Pass in the
LexicalEnvironment
of the running execution context as the
Scope
. Pass in
true
as the
Strict
flag if the
FunctionExpression
is contained in
strict code
or if its
FunctionBody
is
strict code
The production
FunctionExpression
function
Identifier
FormalParameterList
opt
FunctionBody
is evaluated as follows:
Let
funcEnv
be the result of calling
NewDeclarativeEnvironment
passing the running
execution context’s
Lexical Environment
as the argument
Let
envRec
be
funcEnv’s
environment record.
Call the CreateImmutableBinding concrete method of
envRec
passing the String value of
Identifier
as the
argument.
Let
closure
be the result of creating a new Function object as specified in
13.2
with
parameters specified by
FormalParameterList
opt
and body specified by
FunctionBody
. Pass in
funcEnv
as the
Scope
. Pass in
true
as the
Strict
flag if the
FunctionExpression
is
contained in
strict code
or if its
FunctionBody
is
strict
code
Call the InitializeImmutableBinding concrete method of
envRec
passing the String value of
Identifier
and
closure
as the arguments.
Return
closure
NOTE
The Identifier 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
Identifier
in a
FunctionExpression
cannot be referenced from and does not affect the scope enclosing the
FunctionExpression
The production
FunctionBody
SourceElements
opt
is evaluated as follows:
The code of this
FunctionBody
is
strict mode code
if it is part of a
FunctionDeclaration
or
FunctionExpression
that is contained in
strict mode code
or
if the
Directive Prologue
14.1
) of its
SourceElements
contains a
Use Strict Directive
or if any of the conditions in
10.1.1
apply. If
the code of this
FunctionBody
is
strict mode code
SourceElements
is evaluated in
the following steps as
strict mode code
. Otherwise,
SourceElements
is evaluated in the
following steps as non-
strict mode code
If
SourceElements
is present return the result of evaluating
SourceElements
Else return (
normal
undefined
empty
).
13.1
Strict Mode Restrictions
It is a
SyntaxError
if any
Identifier
value occurs more than once within a
FormalParameterList
of a strict mode
FunctionDeclaration
or
FunctionExpression
It is a
SyntaxError
if the
Identifier
"eval"
or the
Identifier
"arguments"
occurs within a
FormalParameterList
of a strict
mode
FunctionDeclaration
or
FunctionExpression
It is a
SyntaxError
if the
Identifier
"eval"
or the
Identifier
"arguments"
occurs as the
Identifier
of a strict mode
FunctionDeclaration
or
FunctionExpression
13.2
Creating Function Objects
Given an optional parameter list specified by
FormalParameterList
, a body specified by
FunctionBody
, a
Lexical Environment
specified by
Scope
, and
a Boolean flag
Strict
, a Function object is constructed as follows:
Create a new native ECMAScript object and let
be that object.
Set all the internal methods, except for [[Get]], of
as described in
8.12
Set the [[Class]] internal property of
to
"Function"
Set the [[Prototype]] internal property of
to the standard built-in Function prototype object as specified in
15.3.3.1
Set the [[Get]] internal property of
as described in
15.3.5.4
Set the [[Call]] internal property of
as described in
13.2.1
Set the [[Construct]] internal property of
as described in
13.2.2
Set the [[HasInstance]] internal property of
as described in
15.3.5.3
Set the [[Scope]] internal property of
to the value of
Scope
Let
names
be a
List
containing, in left to right textual order, the Strings corresponding
to the identifiers of
FormalParameterList
. If no parameters are specified, let
names
be the empty
list.
Set the [[FormalParameters]] internal property of
to
names
Set the [[Code]] internal property of
to
FunctionBody
Set the [[Extensible]] internal property of
to
true
Let
len
be the number of formal parameters specified in
FormalParameterList
. If no parameters are
specified, let
len
be 0.
Call the [[DefineOwnProperty]] internal method of
with arguments
"length"
Property Descriptor
{[[Value]]:
len
, [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}, and
false
Let
proto
be the result of creating a new object as would be constructed by the expression
new
Object()
where
Object
is the standard built-in constructor with that name.
Call the [[DefineOwnProperty]] internal method of
proto
with arguments
"constructor"
Property Descriptor
{[[Value]]:
, { [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
true
}, and
false
Call the [[DefineOwnProperty]] internal method of
with arguments
"prototype"
Property Descriptor
{[[Value]]:
proto
, { [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}, and
false
If
Strict
is
true
, then
Let
thrower
be the [[ThrowTypeError]] function Object (
13.2.3
).
Call the [[DefineOwnProperty]] internal method of
with arguments
"caller"
, PropertyDescriptor
{[[Get]]:
thrower
, [[Set]]:
thrower
, [[Enumerable]]:
false
, [[Configurable]]:
false
},
and
false
Call the [[DefineOwnProperty]] internal method of
with arguments
"arguments"
PropertyDescriptor {[[Get]]:
thrower
, [[Set]]:
thrower
, [[Enumerable]]:
false
[[Configurable]]:
false
}, and
false
Return
NOTE
prototype
property is automatically created for every function, to allow for
the possibility that the function will be used as a constructor.
13.2.1
[[Call]]
When the [[Call]] internal method for a Function object
is called with a this value and a list of arguments,
the following steps are taken:
Let
funcCtx
be the result of establishing a new execution context for function code using the value of
's [[FormalParameters]] internal property, the passed arguments
List
args
, and
the
this
value as described in
10.4.3
Let
result
be the result of evaluating the
FunctionBody
that is the value of
's [[Code]]
internal property. If
does not have a [[Code]] internal property or if its value is an empty
FunctionBody
, then
result
is (
normal
undefined
empty
).
Exit the execution context
funcCtx
, restoring the previous execution context.
If
result
.type is
throw
then throw
result
.value.
If
result
.type is
return
then return
result
.value.
Otherwise
result
.type must be
normal
. Return
undefined
13.2.2
[[Construct]]
When the [[Construct]] internal method for a Function object
is called with a possibly empty list of
arguments, the following steps are taken:
Let
obj
be a newly created native ECMAScript object.
Set all the internal methods of
obj
as specified in
8.12
Set the [[Class]] internal property of
obj
to
"Object"
Set the [[Extensible]] internal property of
obj
to
true
Let
proto
be the value of calling the [[Get]] internal property of
with argument
"prototype"
If
Type
proto
) is Object, set the [[Prototype]] internal property of
obj
to
proto
If
Type
proto
) is not Object, set the [[Prototype]] internal property of
obj
to the
standard built-in Object prototype object as described in
15.2.4
Let
result
be the result of calling the [[Call]] internal property of
, providing
obj
as the
this
value and providing the argument list passed into [[Construct]] as
args
If
Type
result
) is Object then return
result
Return
obj
13.2.3
The [[ThrowTypeError]]
Function Object
The [[ThrowTypeError]] object is a unique function object that is defined once as follows:
Create a new native ECMAScript object and let
be that object.
Set all the internal methods of
as described in
8.12
Set the [[Class]] internal property of
to
"Function"
Set the [[Prototype]] internal property of
to the standard built-in Function prototype object as specified in
15.3.3.1
Set the [[Call]] internal property of
as described in
13.2.1
Set the [[Scope]] internal property of
to
the Global Environment
Set the [[FormalParameters]] internal property of
to an empty
List
Set the [[Code]] internal property of
to be a
FunctionBody
that unconditionally throws a
TypeError
exception and performs no other action.
Call the [[DefineOwnProperty]] internal method of
with arguments
"length"
Property Descriptor
{[[Value]]:
, [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}, and
false
Set the [[Extensible]] internal property of
to
false
Let [[ThrowTypeError]] be
14
Program
Syntax
Program
SourceElements
opt
SourceElements
SourceElement
SourceElements
SourceElement
SourceElement
Statement
FunctionDeclaration
Semantics
The production
Program
SourceElements
opt
is evaluated as follows:
The code of this
Program
is
strict mode code
if the
Directive
Prologue
14.1
) of its
SourceElements
contains a
Use Strict
Directive
or if any of the conditions of
10.1.1
apply. If the code of this
Program
is
strict mode code
SourceElements
is evaluated in the following steps as
strict mode code
. Otherwise
SourceElements
is evaluated in the following steps as non-
strict mode code
If
SourceElements
is not present, return (
normal
empty
empty
).
Let
progCxt
be a new execution context for global code as described in
10.4.1
Let
result
be the result of evaluating
SourceElements
Exit the execution context
progCxt
Return
result
NOTE
The processes for initiating the evaluation of a
Program
and for
dealing with the result of such an evaluation are defined by an ECMAScript implementation and not by this specification.
The production
SourceElements
SourceElements
SourceElement
is evaluated as follows:
Let
headResult
be the result of evaluating
SourceElements
If
headResult
is an
abrupt completion
, return
headResult.
Let
tailResult
be result of evaluating
SourceElement
If
tailResult
.value is
empty
, let
headResult
.value,
otherwise let
tailResult
.value.
Return (tailResult.type, V, tailResult.target)
The production
SourceElement
Statement
is evaluated as follows:
Return the result of evaluating
Statement
The production
SourceElement
FunctionDeclaration
is evaluated as follows:
Return (
normal
empty
empty
).
14.1
Directive Prologues and the Use
Strict Directive
A Directive Prologue is the longest sequence of
ExpressionStatement
productions occurring as the
initial
SourceElement
productions of a
Program
or
FunctionBody
and where each
ExpressionStatement
in the sequence consists entirely of
StringLiteral
token followed 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 the exact character 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
ExpressionStatement
productions of a Directive Prologue are
evaluated normally during evaluation of the containing
SourceElements
production. Implementations
may define implementation specific meanings for
ExpressionStatement
productions 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 or which does not have a meaning defined by the
implementation.
15
Standard Built-in ECMAScript Objects
There are certain built-in objects available whenever an ECMAScript program begins execution. One, the global object, is part
of the
lexical environment
of the executing program. Others are accessible as initial properties of the
global object.
Unless specified otherwise, the [[Class]] internal property of a built-in object is
"Function"
if that built-in
object has a [[Call]] internal property, or
"Object"
if that built-in object does not have a [[Call]] internal
property. Unless specified otherwise, the [[Extensible]] internal property of a built-in object initially has the value
true
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 properties of the 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 function or constructor described in this clause
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.
Unless otherwise specified in the description of a particular function, if a function or constructor described in this clause
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
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.
Every built-in function and every built-in constructor has the Function prototype object, which is the initial value of the
expression
Function.prototype
15.3.4
), as the value of its [[Prototype]] internal
property.
Unless otherwise specified every built-in prototype object has the Object prototype object, which is the initial value of the
expression
Object.prototype
15.2.4
), as the value of its [[Prototype]] internal
property, except the Object prototype object itself.
None of the built-in functions described in this clause that are not constructors shall implement the [[Construct]] internal
method unless otherwise specified in the description of a particular function. None of the built-in functions described in this
clause shall have a
prototype
property unless otherwise specified in the description of a particular function.
This clause generally describes distinct behaviours for when a constructor is “called as a function” and for when
it is “called as part of a
new
expression”. The “called as a function” behaviour corresponds to
the invocation of the constructor’s [[Call]] internal method and the “called as part of a new expression”
behaviour corresponds to the invocation of the constructor’s [[Construct]] internal method.
Every built-in Function object described in this clause—whether as a constructor, an ordinary function, or
both—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, including optional
parameters.
NOTE
For example, the Function object that is the initial value of the
slice
property of the String prototype object is described under the subclause heading
“String.prototype.slice (start, end)” which shows the two named arguments start and end; therefore the value of
the
length
property of that Function object is
In every case, the
length
property of a built-in Function object described in this clause has the attributes
{ [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}. Every other property
described in this clause has the attributes { [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
true
} unless otherwise specified.
15.1
The Global Object
The unique
global object
is created before control enters any execution context.
Unless otherwise specified, the standard built-in properties of the global object have attributes {[[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
The global object does not have a [[Construct]] internal property; it is not possible to use the global object as a
constructor with the
new
operator.
The global object does not have a [[Call]] internal property; it is not possible to invoke the global object as a
function.
The values of the [[Prototype]] and [[Class]] internal properties of the global object are implementation-dependent.
In addition to the properties defined in this specification the global object may have additional host defined properties.
This may include a property whose value is the global object itself; for example, in the HTML document object model the
window
property of the global object is the global object itself.
15.1.1
Value Properties of the Global
Object
15.1.1.1
NaN
The value of
NaN
is
NaN
see 8.5
). This property has the attributes {
[[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.1.1.2
Infinity
The value of
Infinity
is
+∞
see 8.5
). This property has the
attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.1.1.3
undefined
The value of
undefined
is
undefined
see 8.1
). This property has the
attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.1.2
Function Properties of the
Global Object
15.1.2.1
eval (x)
When the
eval
function is called with one argument
, the following steps are taken:
If
Type
) is not String, return
Let
prog
be the ECMAScript code that is the result of parsing
as a
Program
. If the parse
fails, throw a
SyntaxError
exception (but
see also clause 16
).
Let
evalCtx
be the result of establishing a new execution context (
10.4.2
) for the
eval code
prog
Let
result
be the result of evaluating the program
prog
Exit the running execution context
evalCtx
, restoring the previous execution context.
If
result
.type is
normal
and its completion value is a value
, then return the value
If
result
.type is
normal
and its completion value is
empty
, then return the value
undefined
Otherwise,
result
.type must be
throw
. Throw
result
.value
as an exception.
15.1.2.1.1
Direct Call to
Eval
A direct call to the eval function is one that is expressed as a
CallExpression
that meets
the following two conditions:
The
Reference
that is the result of evaluating the
MemberExpression
in
the
CallExpression
has an environment record as its base value and its reference name is
eval
The result of calling the abstract operation
GetValue
with that
Reference
as the argument is the standard built-in function defined in
15.1.2.1
15.1.2.2
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 character 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 character pairs
0x
or
0X
When the
parseInt
function is called, the following steps are taken:
Let
inputString
be
ToString
string
).
Let
be a newly created substring of
inputString
consisting of the first character that is not a
StrWhiteSpaceChar
and all characters following that character. (In other words, remove leading white space.)
If
inputString
does not contain any such characters, let
be the empty string.
Let
sign
be 1.
If
is not empty and the first character of
is a minus sign
, let
sign
be
−1.
If
is not empty and the first character of
is a plus sign
or a minus sign
, then
remove the first character from
Let
ToInt32
radix
).
Let
stripPrefix
be
true
If
≠ 0, then
If
< 2 or
> 36, then return
NaN
If
≠ 16, let
stripPrefix
be
false
Else,
= 0
Let
= 10.
If
stripPrefix
is
true
, then
If the length of
is at least 2 and the first two characters of
are either
0x
” or “
0X
”, then remove the first two characters from
and let
= 16.
If
contains any character that is not a radix-
digit, then let
be the substring of
consisting of all characters before the first such character; otherwise, let
be
If
is empty, return
NaN
Let
mathInt
be the mathematical integer value that is represented by
in radix-
notation,
using the letters
and
for digits with values 10 through 35. (However, if
is 10 and
contains more than 20 significant digits, every significant digit after the 20th may be
replaced by a
digit, at the option of the implementation; and if
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
in radix-
notation.)
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 characters that cannot be interpreted as part of the notation of an integer, and no
indication is given that any such characters were ignored.
15.1.2.3
parseFloat (string)
The
parseFloat
function produces a Number value dictated by interpretation of the contents of the
string
argument as a decimal literal.
When the
parseFloat
function is called, the following steps are taken:
Let
inputString
be
ToString
string
).
Let
trimmedString
be a substring of
inputString
consisting of the leftmost character that is not a
StrWhiteSpaceChar
and all characters to the right of that character. (In other words, remove leading white
space.) If
inputString
does not contain any such characters, let
trimmedString
be the empty
string.
If neither
trimmedString
nor any prefix of
trimmedString
satisfies the syntax of a
StrDecimalLiteral
see 9.3.1
), return
NaN
Let
numberString
be the longest prefix of
trimmedString
, which might be
trimmedString
itself,
that satisfies the syntax of a
StrDecimalLiteral
Return the Number value for the MV of
numberString
NOTE
parseFloat
may interpret only a leading portion of
string
as a
Number value; it ignores any characters that cannot be interpreted as part of the notation of an decimal literal, and no
indication is given that any such characters were ignored.
15.1.2.4
isNaN (number)
Returns
true
if the argument coerces to
NaN
, and otherwise returns
false
If
ToNumber
number
) is
NaN
, return
true
Otherwise, return
false
NOTE
A reliable way for ECMAScript code to test if a value
is a
NaN
is an expression of the form
X !== X
The result will be
true
if and only if
is a
NaN
15.1.2.5
isFinite (number)
Returns
false
if the argument coerces to
NaN
+∞
, or
−∞
, and otherwise
returns
true
If
ToNumber
number
) is
NaN
+∞
, or
−∞
, return
false
Otherwise, return
true
15.1.3
URI Handling Function
Properties
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
15.1.3.1
15.1.3.2
15.1.3.3
and
15.1.3.4
NOTE
Many implementations of ECMAScript provide additional functions and methods that
manipulate web pages; these functions are beyond the scope of this standard.
A URI is composed of a sequence of components separated by component separators. The general form is:
Scheme
First
Second
Third
Fourth
where the italicised names represent components and “
”, “
”,
” and “
” are reserved characters used as separators. The
encodeURI
and
decodeURI
functions are intended to work with complete URIs; they assume that any
reserved characters 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 characters represent text and so must be encoded so that they are
not interpreted as reserved characters when the component is part of a complete URI.
The following lexical grammar specifies the form of encoded URIs.
Syntax
uri
:::
uriCharacters
opt
uriCharacters
:::
uriCharacter
uriCharacters
opt
uriCharacter
:::
uriReserved
uriUnescaped
uriEscaped
uriReserved
:::
one of
uriUnescaped
:::
uriAlpha
DecimalDigit
uriMark
uriEscaped
:::
HexDigit
HexDigit
uriAlpha
:::
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.
When a character to be included in a URI is not listed above or is not intended to have the special meaning sometimes
given to the reserved characters, that character must be encoded. The character 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
”.
The encoding and escaping process is described by the abstract operation Encode taking two String arguments
string
and
unescapedSet
Let
strLen
be the number of characters in
string
Let
be the empty String.
Let
be 0.
Repeat
If
equals
strLen
, return
Let
be the character at position
within
string
If
is in
unescapedSet
, then
Let
be a String containing only the character
Let
be a new String value computed by concatenating the previous value of
and
Else,
is not in
unescapedSet
If the code unit value of
is not less than 0xDC00 and not greater than 0xDFFF, throw a
URIError
exception.
If the code unit value of
is less than 0xD800 or greater than 0xDBFF, then
Let
be the code unit value of
Else,
Increase
by 1.
If
equals
strLen
, throw a
URIError
exception.
Let
kChar
be the code unit value of the character at position
within
string
If
kChar
is less than 0xDC00 or greater than 0xDFFF, throw a
URIError
exception.
Let
be (((the code unit value of
) – 0xD800) × 0x400 + (
kChar
0xDC00) + 0x10000).
Let
Octets
be the array of octets resulting by applying the UTF-8 transformation to
, and let
be the array size.
Let
be 0.
Repeat, while
Let
jOctet
be the value at position
within
Octets
Let
be a String containing three characters “
XY
” where
XY
are
two uppercase hexadecimal digits encoding the value of
jOctet
Let
be a new String value computed by concatenating the previous value of
and
Increase
by 1.
Increase
by 1.
The unescaping and decoding process is described by the abstract operation Decode taking two String arguments
string
and
reservedSet
Let
strLen
be the number of characters in
string
Let
be the empty String.
Let
be 0.
Repeat
If
equals
strLen
, return
Let
be the character at position
within
string
If
is not ‘
’, then
Let
be the String containing only the character
Else,
is ‘
Let
start
be
If
+ 2 is greater than or equal to
strLen
, throw a
URIError
exception.
If the characters at position (
+1) and (
+ 2) within
string
do not represent
hexadecimal digits, throw a
URIError
exception.
Let
be the 8-bit value represented by the two hexadecimal digits at position (
+ 1) and
+ 2).
Increment
by 2.
If the most significant bit in
is 0, then
Let
be the character with code unit value
If
is not in
reservedSet
, then
Let
be the String containing only the character
Else,
is in
reservedSet
Let
be the substring of
string
from position
start
to position
included.
Else, the most significant bit in
is 1
Let
be the smallest non-negative number such that (
<<
) & 0x80 is
equal to 0.
If
equals 1 or
is greater than 4, throw a
URIError
exception.
Let
Octets
be an array of 8-bit integers of size
Put
into
Octets
at position 0.
If
+ (3 × (
– 1)) is greater than or equal to
strLen
, throw a
URIError
exception.
Let
be 1.
Repeat, while
Increment
by 1.
If the character at position
is not ‘%’, throw a
URIError
exception.
If the characters at position (
+1) and (
+ 2) within
string
do not represent
hexadecimal digits, throw a
URIError
exception.
Let
be the 8-bit value represented by the two hexadecimal digits at position (
+ 1)
and (
+ 2).
If the two most significant bits in
are not 10, throw a
URIError
exception.
Increment
by 2.
Put
into
Octets
at position
Increment
by 1.
Let
be the value obtained by applying the UTF-8 transformation to
Octets
, that is, from an
array of octets into a 21-bit value. If
Octets
does not contain a valid UTF-8 encoding of a Unicode
code point throw an
URIError
exception.
If
is less than 0x10000, then
Let
be the character with code unit value
If
is not in
reservedSet
, then
Let
be the String containing only the character
Else,
is in
reservedSet
Let
be the substring of
string
from position
start
to position
included.
Else,
is ≥ 0x10000
Let
be (((
– 0x10000) & 0x3FF) + 0xDC00).
Let
be ((((
– 0x10000) >> 10) & 0x3FF) + 0xD800).
Let
be the String containing the two characters with code unit values
and
Let
be a new String value computed by concatenating the previous value of
and
Increase
by 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 21.
Table 21 — UTF-8 Encodings
Code Unit Value
Representation
st
Octet
nd
Octet
rd
Octet
th
Octet
0x0000 - 0x007F
00000000
zzzzzzz
zzzzzzz
0x0080 - 0x07FF
00000
yyy yyzzzzzz
110
yyyyy
10
zzzzzz
0x0800 - 0xD7FF
xxxxyyyy yyzzzzzz
1110
xxxx
10
yyyyyy
10
zzzzzz
0xD800 - 0xDBFF
followed by
0xDC00 – 0xDFFF
110110
vv vvwwwwxx
followed by
110111
yy yyzzzzzz
11110
uuu
10
uuwwww
10
xxyyyy
10
zzzzzz
0xD800 - 0xDBFF
not followed by
0xDC00 – 0xDFFF
causes
URIError
0xDC00 – 0xDFFF
causes
URIError
0xE000 - 0xFFFF
xxxxyyyy yyzzzzzz
1110
xxxx
10
yyyyyy
10
zzzzzz
Where
uuuuu
vvvv
+ 1
to account for the addition of 0x10000 as in Surrogates, section 3.7, of the Unicode Standard.
The range of code unit values 0xD800-0xDFFF is used to encode surrogate pairs; the above transformation combines a UTF-16
surrogate pair into a UTF-32 representation and encodes the resulting 21-bit value in 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 character U+0000. Implementations of the Decode algorithm are required to throw a
URIError
when encountering
such invalid sequences.
15.1.3.1
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 character that it
represents. Escape sequences that could not have been introduced by
encodeURI
are not replaced.
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 character valid in
uriReserved
plus
”.
Return the result of calling Decode(
uriString
reservedURISet
NOTE
The character “
” is not decoded from escape sequences even
though it is not a reserved URI character.
15.1.3.2
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
character that it represents.
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 the result of calling Decode(
componentString
reservedURIComponentSet
15.1.3.3
encodeURI (uri)
The
encodeURI
function computes a new version of a URI in which each instance of certain characters is
replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the character.
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 character valid in
uriReserved
and
uriUnescaped
plus “
”.
Return the result of calling Encode(
uriString
unescapedURISet
NOTE
The character “
” is not encoded to an escape sequence even
though it is not a reserved or unescaped URI character.
15.1.3.4
encodeURIComponent
(uriComponent)
The
encodeURIComponent
function computes a new version of a URI in which each instance of certain
characters is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the character.
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 character valid in
uriUnescaped
Return the result of calling Encode(
componentString
unescapedURIComponentSet
15.1.4
Constructor Properties of the
Global Object
15.1.4.1
Object ( . . . )
See
15.2.1
and
15.2.2
15.1.4.2
Function ( . . . )
See
15.3.1
and
15.3.2
15.1.4.3
Array ( . . . )
See
15.4.1
and
15.4.2
15.1.4.4
String ( . . . )
See
15.5.1
and
15.5.2
15.1.4.5
Boolean ( . . . )
See
15.6.1
and
15.6.2
15.1.4.6
Number ( . . . )
See
15.7.1
and
15.7.2
15.1.4.7
Date ( . . . )
See
15.9.2
15.1.4.8
RegExp ( . . . )
See
15.10.3
and
15.10.4
15.1.4.9
Error ( . . . )
See
15.11.1
and
15.11.2
15.1.4.10
EvalError ( . . .
See
15.11.6.1
15.1.4.11
RangeError ( . . .
See
15.11.6.2
15.1.4.12
ReferenceError ( . . .
See
15.11.6.3
15.1.4.13
SyntaxError ( . . .
See
15.11.6.4
15.1.4.14
TypeError ( . . .
See
15.11.6.5
15.1.4.15
URIError ( . . .
See
15.11.6.6
15.1.5
Other Properties of the Global
Object
15.1.5.1
Math
See
15.8
15.1.5.2
JSON
See
15.12
15.2
Object Objects
15.2.1
The Object Constructor Called
as a Function
When
Object
is called as a function rather than as a constructor, it performs a type conversion.
15.2.1.1
Object ( [ value ]
When the
Object
function is called with no arguments or with one argument
value
, the following
steps are taken:
If
value
is
null
undefined
or not supplied, create and return a new Object object exactly as
if the standard built-in Object constructor had been called with the same arguments (
15.2.2.1
).
Return
ToObject
value
).
15.2.2
The Object Constructor
When
Object
is called as part of a
new
expression, it is a constructor that may create an
object.
15.2.2.1
new Object ( [ value ]
When the
Object
constructor is called with no arguments or with one argument
value
, the
following steps are taken:
If
value
is supplied, then
If
Type
value
) is Object, then
If the
value
is a native ECMAScript object, do not create a new object but simply return
value
If the
value
is a host object, then actions are taken and a result is returned in an
implementation-dependent manner that may depend on the host object.
If
Type
value
) is String, return
ToObject
value
).
If
Type
value
) is Boolean, return
ToObject
value
).
If
Type
value
) is Number, return
ToObject
value
).
Assert: The argument
value
was not supplied or its type was Null or Undefined.
Let
obj
be a newly created native ECMAScript object.
Set the [[Prototype]] internal property of
obj
to the standard built-in Object prototype object (
15.2.4
).
Set the [[Class]] internal property of
obj
to
"Object"
Set the [[Extensible]] internal property of
obj
to
true
Set all the internal methods of
obj
as specified in
8.12
Return
obj
15.2.3
Properties of the Object
Constructor
The value of the [[Prototype]] internal property of the Object constructor is the standard built-in Function prototype
object.
Besides the internal properties and the
length
property (whose value is
), the Object constructor
has the following properties:
15.2.3.1
Object.prototype
The initial value of
Object.prototype
is the standard built-in Object prototype object (
15.2.4
).
This property has the attributes {[[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.2.3.2
Object.getPrototypeOf (
O )
When the
getPrototypeOf
function is called with argument
, the following steps are taken:
If
Type
) is not Object throw a
TypeError
exception.
Return the value of the [[Prototype]] internal property of
15.2.3.3
Object.getOwnPropertyDescriptor ( O, P )
When the
getOwnPropertyDescriptor
function is called, the following steps are taken:
If
Type
) is not Object throw a
TypeError
exception.
Let
name
be
ToString
).
Let
desc
be the result of calling the [[GetOwnProperty]] internal method of
with argument
name
Return the result of calling
FromPropertyDescriptor
desc
) (
8.10.4
).
15.2.3.4
Object.getOwnPropertyNames ( O )
When the
getOwnPropertyNames
function is called, the following steps are taken:
If
Type
) is not Object throw a
TypeError
exception.
Let
array
be the result of creating a new object as if by the expression
new Array ()
where
Array
is the standard built-in constructor with that name.
Let
be 0.
For each named own property
of
Let
name
be the String value that is the name of
Call the [[DefineOwnProperty]] internal method of
array
with arguments
ToString
), the PropertyDescriptor {[[Value]]:
name
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Increment
by 1.
Return
array
NOTE
If
is a String instance, the set of own properties processed in step 4
includes the implicit properties defined in
15.5.5.2
that correspond to character positions
within the object’s [[PrimitiveValue]] String.
15.2.3.5
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
) is not Object or Null throw a
TypeError
exception.
Let
obj
be the result of creating a new object as if by the expression new Object() where Object is the
standard built-in constructor with that name
Set the [[Prototype]] internal property of
obj
to
If the argument
Properties
is present and not
undefined
, add own properties to
obj
as if by
calling the standard built-in function
Object.defineProperties
with arguments
obj
and
Properties
Return
obj
15.2.3.6
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
) is not Object throw a
TypeError
exception.
Let
name
be
ToString
).
Let
desc
be the result of calling
ToPropertyDescriptor
with
Attributes
as
the argument.
Call the [[DefineOwnProperty]] internal method of
with arguments
name
desc
, and
true
Return
15.2.3.7
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:
If
Type
) is not Object throw a
TypeError
exception.
Let
props
be
ToObject
Properties
).
Let
names
be an internal list containing the names of each enumerable own property of
props.
Let
descriptors
be an empty internal
List
For each element
of
names
in list order,
Let
descObj
be the result of calling the [[Get]] internal method of
props
with
as the
argument.
Let
desc
be the result of calling
ToPropertyDescriptor
with
descObj
as
the argument.
Append the pair (a two element
List
) consisting of
and
desc
to the end of
descriptors
For each
pair
from
descriptors
in list order,
Let
be the first element of
pair
Let
desc
be the second element of
pair
Call the [[DefineOwnProperty]] internal method of
with arguments
desc
, and
true
Return
If an implementation defines a specific order of enumeration for the for-in statement, that same enumeration order must
be used to order the list elements in step 3 of this algorithm.
15.2.3.8
Object.seal ( O )
When the
seal
function is called, the following steps are taken:
If
Type
) is not Object throw a
TypeError
exception.
For each named own property name
of
Let
desc
be the result of calling the [[GetOwnProperty]] internal method of
with
If
desc
.[[Configurable]] is
true
, set
desc
.[[Configurable]] to
false
Call the [[DefineOwnProperty]] internal method of
with
desc
, and
true
as
arguments.
Set the [[Extensible]] internal property of
to
false
Return
15.2.3.9
Object.freeze ( O )
When the
freeze
function is called, the following steps are taken:
If
Type
) is not Object throw a
TypeError
exception.
For each named own property name
of
Let
desc
be the result of calling the [[GetOwnProperty]] internal method of
with
If
IsDataDescriptor
desc
) is
true
, then
If
desc
.[[Writable]] is
true
, set
desc
.[[Writable]] to
false
If
desc
.[[Configurable]] is
true
, set
desc
.[[Configurable]] to
false
Call the [[DefineOwnProperty]] internal method of
with
desc
, and
true
as
arguments.
Set the [[Extensible]] internal property of
to
false
Return
15.2.3.10
Object.preventExtensions ( O )
When the
preventExtensions
function is called, the following steps are taken:
If
Type
) is not Object throw a
TypeError
exception.
Set the [[Extensible]] internal property of
to
false
Return
15.2.3.11
Object.isSealed ( O
When the
isSealed
function is called with argument
, the following steps are taken:
If
Type
) is not Object throw a
TypeError
exception.
For each named own property name
of
Let
desc
be the result of calling the [[GetOwnProperty]] internal method of
with
If
desc
.[[Configurable]] is
true
, then return
false
If the [[Extensible]] internal property of
is
false
, then return
true
Otherwise, return
false
15.2.3.12
Object.isFrozen ( O
When the
isFrozen
function is called with argument
, the following steps are taken:
If
Type
) is not Object throw a
TypeError
exception.
For each named own property name
of
Let
desc
be the result of calling the [[GetOwnProperty]] internal method of
with
If
IsDataDescriptor
desc
) is
true
then
If
desc
.[[Writable]] is
true
, return
false
If
desc
.[[Configurable]] is
true
, then return
false
If the [[Extensible]] internal property of
is
false
, then return
true
Otherwise, return
false
15.2.3.13
Object.isExtensible (
O )
When the
isExtensible
function is called with argument
, the following steps are taken:
If
Type
) is not Object throw a
TypeError
exception.
Return the Boolean value of the [[Extensible]] internal property of
15.2.3.14
Object.keys ( O )
When the
keys
function is called with argument
, the following steps are taken:
If the
Type
) is not Object, throw a
TypeError
exception.
Let
be the number of own enumerable properties of
Let
array
be the result of creating a new Object as if by the expression
new Array(n)
where
Array
is the standard built-in constructor with that name.
Let
index
be 0.
For each own enumerable property of
whose name String is
Call the [[DefineOwnProperty]] internal method of
array
with arguments
ToString
index
), the PropertyDescriptor {[[Value]]:
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Increment
index
by 1.
Return
array
If an implementation defines a specific order of enumeration for the for-in statement, that same enumeration order must
be used in step 5 of this algorithm.
15.2.4
Properties of the Object
Prototype Object
The value of the [[Prototype]] internal property of the Object prototype object is
null
, the value of the
[[Class]] internal property is
"Object"
, and the initial value of the [[Extensible]] internal property is
true
15.2.4.1
Object.prototype.constructor
The initial value of
Object.prototype.constructor
is the standard built-in
Object
constructor.
15.2.4.2
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
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
class
be the value of the [[Class]] internal property of
Return the String value that is the result of concatenating the three Strings
"[object "
class
and
"]"
15.2.4.3
Object.prototype.toLocaleString ( )
When the
toLocaleString
method is called, the following steps are taken:
Let
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
toString
be the result of calling the [[Get]] internal method of
passing
"toString"
as
the argument.
If
IsCallable
toString
) is
false
, throw a
TypeError
exception.
Return the result of calling the [[Call]] internal method of
toString
passing
as the
this
value and no arguments.
NOTE 1
This function is provided to give all Objects a generic
toLocaleString
interface, even though not all may use it. Currently,
Array
Number
, and
Date
provide their own locale-sensitive
toLocaleString
methods.
NOTE 2
The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything else.
15.2.4.4
Object.prototype.valueOf
( )
When the
valueOf
method is called, the following steps are taken:
Let
be the result of calling
ToObject
passing the
this
value as the
argument.
If
is the result of calling the Object constructor with a host object (
15.2.2.1
), then
Return either
or another value such as the host object originally passed to the constructor. The
specific result that is returned is implementation-defined.
Return
15.2.4.5
Object.prototype.hasOwnProperty (V)
When the
hasOwnProperty
method is called with argument
, the following steps are taken:
Let
be
ToString
).
Let
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
desc
be the result of calling the [[GetOwnProperty]] internal method of
passing
as the
argument.
If
desc
is
undefined
, return
false
Return
true
NOTE 1
Unlike [[HasProperty]] (
8.12.6
), this method does not
consider objects in the prototype chain.
NOTE 2
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
15.2.4.6
Object.prototype.isPrototypeOf (V)
When the
isPrototypeOf
method is called with argument
, the following steps are taken:
If
is not an object, return
false
Let
be the result of calling
ToObject
passing the
this
value as the
argument.
Repeat
Let
be the value of the [[Prototype]] internal property of
if
is
null
, return
false
If
and
refer to the same object, return
true
NOTE
The ordering of steps 1 and 2 is chosen to preserve 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.
15.2.4.7
Object.prototype.propertyIsEnumerable (V)
When the
propertyIsEnumerable
method is called with argument
, the following steps are
taken:
Let
be
ToString
).
Let
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
desc
be the result of calling the [[GetOwnProperty]] internal method of
passing
as the
argument.
If
desc
is
undefined
, return
false
Return the value of
desc
.[[Enumerable]].
NOTE 1
This method does not consider objects in the prototype chain.
NOTE 2
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
15.2.5
Properties of Object
Instances
Object instances have no special properties beyond those inherited from the Object prototype object.
15.3
Function Objects
15.3.1
The Function Constructor
Called as a Function
When
Function
is called as a function rather than as a constructor, it creates and initialises a new
Function object. Thus the function call
Function(
is
equivalent to the object creation expression
new
Function(
with the same arguments.
15.3.1.1
Function (p1, p2,
… , pn, body)
When the
Function
function is called with some arguments
p1
p2
, … ,
pn
body
(where
might be
, that is,
there are no “
” arguments, and where
body
might also not be provided), the following
steps are taken:
Create and return a new Function object as if the standard built-in constructor Function was used in a
new
expression with the same arguments (
15.3.2.1
).
15.3.2
The Function Constructor
When
Function
is called as part of a
new
expression, it is a constructor: it initialises the
newly created object.
15.3.2.1
new 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
constructor is called with some arguments
p1
p2
, … ,
pn
body
(where
might be
, that is,
there are no “
” arguments, and where
body
might also not be provided), the following
steps are taken:
Let
argCount
be the total number of arguments passed to this function invocation.
Let
be the empty String.
If
argCount
= 0, let
body
be the empty String.
Else if
argCount
= 1, let
body
be that argument.
Else,
argCount
> 1
Let
firstArg
be the first argument.
Let
be
ToString
firstArg
).
Let
be 2.
Repeat, while
argCount
Let
nextArg
be the
’th argument.
Let
be the result of concatenating the previous value of
, the String
","
(a
comma), and
ToString
nextArg
).
Increase
by 1.
Let
body
be the
’th argument.
Let
body
be
ToString
body
).
If
is not parsable as a
FormalParameterList
opt
then throw a
SyntaxError
exception.
If
body
is not parsable as
FunctionBody
then throw a
SyntaxError
exception.
If
body
is
strict mode code (see 10.1.1)
then let
strict
be
true
else let
strict
be
false
If
strict
is
true
, throw any exceptions specified in
13.1
that apply.
Return a new Function object created as specified in
13.2
passing
as the
FormalParameterList
opt
and
body
as the
FunctionBody
. Pass in
the Global Environment
as the
Scope
parameter and
strict
as the
Strict
flag.
prototype
property is automatically created for every function, to provide for the possibility that the
function will be used as a constructor.
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")
15.3.3
Properties of the Function
Constructor
The Function constructor is itself a Function object and its [[Class]] is
"Function"
. The value of the
[[Prototype]] internal property of the Function constructor is the standard built-in Function prototype object (
15.3.4
).
The value of the [[Extensible]] internal property of the Function constructor is
true
The Function constructor has the following properties:
15.3.3.1
Function.prototype
The initial value of
Function.prototype
is the standard built-in Function prototype object (
15.3.4
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.3.3.2
Function.length
This is a data property with a value of 1. This property has the attributes { [[Writable]]:
false
[[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.3.4
Properties of the Function
Prototype Object
The Function prototype object is itself a Function object (its [[Class]] is
"Function"
) that, when invoked,
accepts any arguments and returns
undefined
The value of the [[Prototype]] internal property of the Function prototype object is the standard built-in Object
prototype object (
15.2.4
). The initial value of the [[Extensible]] internal property of the
Function prototype object is
true
The Function prototype object does not have a
valueOf
property of its own; however, it inherits the
valueOf
property from the Object prototype Object.
The
length
property of the Function prototype object is
15.3.4.1
Function.prototype.constructor
The initial value of
Function.prototype.constructor
is the built-in
Function
constructor.
15.3.4.2
Function.prototype.toString ( )
An implementation-dependent representation of the function is returned. This representation has the syntax of a
FunctionDeclaration
. Note in particular that the use and placement of white space, line terminators, and
semicolons within the representation String is implementation-dependent.
The
toString
function is not generic; it throws a
TypeError
exception if its
this
value is
not a Function object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
15.3.4.3
Function.prototype.apply
(thisArg, argArray)
When the
apply
method is called on an object
func
with arguments
thisArg
and
argArray
, the following steps are taken:
If
IsCallable
func
) is
false
, then throw a
TypeError
exception.
If
argArray
is
null
or
undefined
, then
Return the result of calling the [[Call]] internal method of
func
, providing
thisArg
as the
this
value and an empty list of arguments.
If
Type
argArray
) is not Object, then throw a
TypeError
exception.
Let
len
be the result of calling the [[Get]] internal method of
argArray
with argument
"length"
Let
be
ToUint32
len
).
Let
argList
be an empty
List
Let
index
be 0.
Repeat while
index
Let
indexName
be
ToString
index
).
Let
nextArg
be the result of calling the [[Get]] internal method of
argArray
with
indexName
as the argument.
Append
nextArg
as the last element of
argList
Set
index
to
index
+ 1.
Return the result of calling the [[Call]] internal method of
func
, providing
thisArg
as the
this
value and
argList
as the list of arguments.
The
length
property of the
apply
method is
NOTE
The thisArg value is passed without modification as the
this
value. This is a
change from Edition 3, where a
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.
15.3.4.4
Function.prototype.call
(thisArg [ , arg1 [ , arg2, … ] ] )
When the
call
method is called on an object
func
with argument
thisArg
and optional
arguments
arg1
arg2
etc, the following steps are taken:
If
IsCallable
func
) is
false
, then throw a
TypeError
exception.
Let
argList
be an empty
List
If this method was called with more than one argument then in left to right order starting with
arg1
append
each argument as the last element of
argList
Return the result of calling the [[Call]] internal method of
func
, providing
thisArg
as the
this
value and
argList
as the list of arguments.
The
length
property of the
call
method is
NOTE
The thisArg value is passed without modification as the
this
value. This is a
change from Edition 3, where a
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.
15.3.4.5
Function.prototype.bind
(thisArg [, arg1 [, arg2, …]])
The bind method takes one or more arguments,
thisArg
and (optionally)
arg1
arg2
, etc,
and returns a new function object by performing the following steps:
Let
Target
be the
this
value.
If
IsCallable
Target
) is
false
, throw a
TypeError
exception.
Let
be a new (possibly empty) internal list of all of the argument values provided after
thisArg
arg1
arg2
etc), in order.
Let
be a new native ECMAScript object .
Set all the internal methods, except for [[Get]], of
as specified in
8.12
Set the [[Get]] internal property of
as specified in
15.3.5.4
Set the [[TargetFunction]] internal property of
to
Target
Set the [[BoundThis]] internal property of
to the value of
thisArg
Set the [[BoundArgs]] internal property of
to
Set the [[Class]] internal property of
to
"Function"
Set the [[Prototype]] internal property of
to the standard built-in Function prototype object as specified
in
15.3.3.1
Set the [[Call]] internal property of
as described in
15.3.4.5.1
Set the [[Construct]] internal property of
as described in
15.3.4.5.2
Set the [[HasInstance]] internal property of
as described in
15.3.4.5.3
If the [[Class]] internal property of
Target
is
"Function"
, then
Let
be the
length
property of
Target
minus the length of
Set the
length
own property of
to either 0 or
, whichever is larger.
Else set the
length
own property of
to 0.
Set the attributes of the
length
own property of
to the values specified in
15.3.5.1
Set the [[Extensible]] internal property of
to
true
Let
thrower
be the [[ThrowTypeError]] function Object (
13.2.3
).
Call the [[DefineOwnProperty]] internal method of
with arguments
"caller"
, PropertyDescriptor
{[[Get]]:
thrower
, [[Set]]:
thrower
, [[Enumerable]]:
false
, [[Configurable]]:
false
},
and
false
Call the [[DefineOwnProperty]] internal method of
with arguments
"arguments"
PropertyDescriptor {[[Get]]:
thrower
, [[Set]]:
thrower
, [[Enumerable]]:
false
[[Configurable]]:
false
}, and
false
Return
The
length
property of the
bind
method is
NOTE
Function objects created using
Function.prototype.bind
do not have a
prototype
property or the [[Code]], [[FormalParameters]], and [[Scope]] internal properties.
15.3.4.5.1
[[Call]]
When the [[Call]] internal method of a function object,
, which was created using the bind function is
called with a
this
value and a list of arguments
ExtraArgs
, the following steps are
taken:
Let
boundArgs
be the value of
F’s
[[BoundArgs]] internal property.
Let
boundThis
be the value of
F’s
[[BoundThis]] internal property.
Let
target
be the value of
F’s
[[TargetFunction]] internal property.
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
ExtraArgs
in the same order.
Return the result of calling the [[Call]] internal method of
target
providing
boundThis
as the
this
value and providing
args
as the arguments.
15.3.4.5.2
[[Construct]]
When the [[Construct]] internal method of a function object,
that was created using the bind function is
called with a list of arguments
ExtraArgs
, the following steps are taken:
Let
target
be the value of
F’s
[[TargetFunction]] internal property.
If
target
has no [[Construct]] internal method, a
TypeError
exception is thrown.
Let
boundArgs
be the value of
F’s
[[BoundArgs]] internal property.
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
ExtraArgs
in the same order.
Return the result of calling the [[Construct]] internal method of
target
providing
args
as the
arguments.
15.3.4.5.3
[[HasInstance]]
(V)
When the [[HasInstance]] internal method of a function object
, that was created using the bind function
is called with argument
, the following steps are taken:
Let
target
be the value of
F’s
[[TargetFunction]] internal property.
If
target
has no [[HasInstance]] internal method, a
TypeError
exception is thrown.
Return the result of calling the [[HasInstance]] internal method of
target
providing
as the
argument.
15.3.5
Properties of Function
Instances
In addition to the required internal properties, every function instance has a [[Call]] internal property and in most
cases uses a different version of the [[Get]] internal property. Depending on how they are created (see
8.6.2
13.2
, 15, and
15.3.4.5
), function instances
may have a [[HasInstance]] internal property, a [[Scope]] internal property, a [[Construct]] internal property, a
[[FormalParameters]] internal property, a [[Code]] internal property, a [[TargetFunction]] internal property, a
[[BoundThis]] internal property, and a [[BoundArgs]] internal property.
The value of the [[Class]] internal property is
"Function"
Function instances that correspond to strict mode functions (
13.2
) and function instances created
using the
Function.prototype.bind method
15.3.4.5
) have properties named
“caller” and “arguments” that throw a
TypeError
exception. An ECMAScript implementation must
not associate any implementation specific behaviour with accesses of these properties from strict mode function code.
15.3.5.1
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]]:
false
}.
15.3.5.2
prototype
The value of the
prototype
property is used to initialise the [[Prototype]] internal property of a newly
created object before the Function object is invoked as a constructor for that newly created object. This property has the
attribute { [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
NOTE
Function objects created using
Function.prototype.bind
do not have a
prototype
property.
15.3.5.3
[[HasInstance]] (V)
Assume
is a Function object.
When the [[HasInstance]] internal method of
is called with value
, the following steps are
taken:
If
is not an object, return
false
Let
be the result of calling the [[Get]] internal method of
with property name
"prototype"
If
Type
) is not Object, throw a
TypeError
exception.
Repeat
Let
be the value of the [[Prototype]] internal property of
If
is
null
, return
false
If
and
refer to the same object, return
true
NOTE
Function objects created using
Function.prototype.bind
have a different
implementation of [[HasInstance]] defined in
15.3.4.5.3
15.3.5.4
[[Get]] (P)
Function objects use a variation of the [[Get]] internal method used for other native ECMAScript objects (
8.12.3
).
Assume
is a Function object. When the [[Get]] internal method of
is called with property name
, the following steps are taken:
Let
be the result of calling the default [[Get]] internal method (
8.12.3
) on
passing
as the property name argument.
If
is
"caller"
and
is a strict mode Function object, throw a
TypeError
exception.
Return
NOTE
Function objects created using
Function.prototype.bind
use the default
[[Get]] internal method.
15.4
Array Objects
Array objects give special treatment to a certain class of property names. A property name
(in the form of a
String value) is an
array index
if and only if
ToString
ToUint32
))
is equal to
and
ToUint32
is not equal to
32
−1
. A property whose property name is an array index is also
called an
element
. Every Array object has a
length
property whose value is always a nonnegative integer
less than
32
. The value of the
length
property is
numerically greater than the name of every property whose name is an array index; whenever a property of an Array object is
created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever a property
is added whose name is an array index, the
length
property is changed, if necessary, to be one more than the
numeric value of that array index; and whenever the
length
property is changed, every property whose name is an
array index whose value is not smaller than the new length is automatically 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.
An object,
, is said to be
sparse
if the following algorithm returns
true
Let
len
be the result of calling the [[Get]] internal method of
with argument
"length"
For each integer
in the range 0≤
ToUint32
len
Let
elem
be the result of calling the [[GetOwnProperty]] internal method of
with argument
ToString
).
If
elem
is
undefined
, return
true
Return
false
15.4.1
The Array Constructor Called
as a Function
When
Array
is called as a function rather than as a constructor, it creates and initialises a new Array
object. Thus the function call
Array(
is equivalent to
the object creation expression
new Array(
with the
same arguments.
15.4.1.1
Array ( [ item1 [ ,
item2 [ , … ] ] ] )
When the
Array
function is called the following steps are taken:
Create and return a new Array object exactly as if the standard built-in constructor
Array
was used in
new
expression with the same arguments (
15.4.2
).
15.4.2
The Array Constructor
When
Array
is called as part of a
new
expression, it is a constructor: it initialises the newly
created object.
15.4.2.1
new Array ( [ item0 [ ,
item1 [ , … ] ] ] )
This description applies if and only if the Array constructor is given no arguments or at least two arguments.
The [[Prototype]] internal property of the newly constructed object is set to the original Array prototype object, the
one that is the initial value of
Array.prototype
15.4.3.1
).
The [[Class]] internal property of the newly constructed object is set to
"Array"
The [[Extensible]] internal property of the newly constructed object is set to
true
The
length
property of the newly constructed object is set to the number of arguments.
The
property of the newly constructed object is set to
item0
(if supplied); the
property of the newly constructed object is set to
item1
(if supplied); and, in general, for as
many arguments as there are, the
property of the newly constructed object is set to argument
where the first argument is considered to be argument number
. These properties all have the attributes
{[[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}.
15.4.2.2
new Array (len)
The [[Prototype]] internal property of the newly constructed object is set to the original Array prototype object, the
one that is the initial value of
Array.prototype
15.4.3.1
). The [[Class]]
internal property of the newly constructed object is set to
"Array"
. The [[Extensible]] internal property of
the newly constructed object is set to
true
If the argument
len
is a Number and
ToUint32
len
is equal to
len
, then the
length
property of the
newly constructed object is set to
ToUint32
len
. If the argument
len
is a Number and
ToUint32
len
is not equal to
len
, a
RangeError
exception is thrown.
If the argument
len
is not a Number, then the
length
property of the newly constructed object
is set to
and the
property of the newly constructed object is set to
len
with
attributes {[[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}.
15.4.3
Properties of the Array
Constructor
The value of the [[Prototype]] internal property of the Array constructor is the Function prototype object (
15.3.4
).
Besides the internal properties and the
length
property (whose value is
), the Array constructor has
the following properties:
15.4.3.1
Array.prototype
The initial value of
Array.prototype
is the Array prototype object (
15.4.4
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.4.3.2
Array.isArray ( arg
The isArray function takes one argument
arg
, and returns the Boolean value
true
if the argument is an
object whose class internal property is
"Array"
; otherwise it returns
false
. The following steps are
taken:
If
Type
arg
) is not Object, return
false
If the value of the [[Class]] internal property of
arg
is
"Array"
, then return
true
Return
false
15.4.4
Properties of the Array
Prototype Object
The value of the [[Prototype]] internal property of the Array prototype object is the standard built-in Object prototype
object (
15.2.4
).
The Array prototype object is itself an array; its [[Class]] is
"Array"
, and it has a
length
property (whose initial value is
+0
) and the special [[DefineOwnProperty]] internal method described in
15.4.5.1
In following descriptions of functions that are properties of the Array prototype object, the phrase “this
object” refers to the object that is the
this
value for the invocation of the function. It is permitted for the
this
to be an object for which the value of the [[Class]] internal property is not
"Array"
NOTE
The Array prototype object does not have a
valueOf
property of its own;
however, it inherits the
valueOf
property from the standard built-in Object prototype Object.
15.4.4.1
Array.prototype.constructor
The initial value of
Array.prototype.constructor
is the standard built-in
Array
constructor.
15.4.4.2
Array.prototype.toString
( )
When the
toString
method is called, the following steps are taken:
Let
array
be the result of calling
ToObject
on the
this
value.
Let
func
be the result of calling the [[Get]] internal method of
array
with argument
"join"
If
IsCallable
func
) is
false
, then let
func
be the standard built-in
method Object.prototype.toString (
15.2.4.2
).
Return the result of calling the [[Call]] internal method of
func
providing
array
as the
this
value and an empty arguments list.
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.
Whether the
toString
function can be applied successfully to a host object is implementation-dependent.
15.4.4.3
Array.prototype.toLocaleString ( )
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.
The result is calculated as follows:
Let
array
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
arrayLen
be the result of calling the [[Get]] internal method of
array
with argument
"length"
Let
len
be
ToUint32
arrayLen
).
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).
If
len
is zero, return the empty String.
Let
firstElement
be the result of calling the [[Get]] internal method of
array
with argument
"0"
If
firstElement
is
undefined
or
null
, then
Let
be the empty String.
Else
Let
elementObj
be
ToObject
firstElement
).
Let
func
be the result of calling the [[Get]] internal method of
elementObj
with argument
"toLocaleString"
If
IsCallable
func
) is
false
, throw a
TypeError
exception.
Let
be the result of calling the [[Call]] internal method of
func
providing
elementObj
as
the
this
value and an empty arguments list.
Let
be
Repeat, while
len
Let
be a String value produced by concatenating
and
separator
Let
nextElement
be the result of calling the [[Get]] internal method of
array
with argument
ToString
).
If
nextElement
is
undefined
or
null
, then
Let
be the empty String.
Else
Let
elementObj
be
ToObject
nextElement
).
Let
func
be the result of calling the [[Get]] internal method of
elementObj
with argument
"toLocaleString"
If
IsCallable
func
) is
false
, throw a
TypeError
exception.
Let
be the result of calling the [[Call]] internal method of
func
providing
elementObj
as the
this
value and an empty arguments list.
Let
be a String value produced by concatenating
and
Increase
by 1.
Return
NOTE 1
The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything else.
NOTE 2
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. Whether the
toLocaleString
function can be applied successfully to a host object is
implementation-dependent.
15.4.4.4
Array.prototype.concat (
[ item1 [ , item2 [ , … ] ] ] )
When the
concat
method is called with zero or more arguments
item1
item2
, etc., 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
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
be a new array created as if by the expression
new Array()
where
Array
is the
standard built-in constructor with that name.
Let
be 0.
Let
items
be an internal
List
whose first element is
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
Remove the first element from
items
and let
be the value of the element.
If the value of the [[Class]] internal property of
is
"Array"
, then
Let
be 0.
Let
len
be the result of calling the [[Get]] internal method of
with argument
"length"
Repeat, while
len
Let
be
ToString
).
Let
exists
be the result of calling the [[HasProperty]] internal method of
with
If
exists
is
true
, then
Let
subElement
be the result of calling the [[Get]] internal method of
with argument
Call the [[DefineOwnProperty]] internal method of
with arguments
ToString
),
Property Descriptor
{[[Value]]:
subElement
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Increase
by 1.
Increase
by 1.
Else,
is not an Array
Call the [[DefineOwnProperty]] internal method of
with arguments
ToString
),
Property Descriptor
{[[Value]]:
[[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Increase
by 1.
Return
The
length
property of the
concat
method is
NOTE
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.
Whether the
concat
function can be applied successfully to a host object is implementation-dependent.
15.4.4.5
Array.prototype.join
(separator)
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
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
lenVal
be the result of calling the [[Get]] internal method of
with argument
"length"
Let
len
be
ToUint32
lenVal
).
If
separator
is
undefined
, let
separator
be the single-character String
","
Let
sep
be
ToString
separator
).
If
len
is zero, return the empty String.
Let
element0
be the result of calling the [[Get]] internal method of
with argument
"0"
If
element0
is
undefined
or
null
, let
be the empty String; otherwise, Let
be
ToString
element0
).
Let
be
Repeat, while
len
Let
be the String value produced by concatenating
and
sep
Let
element
be the result of calling the [[Get]] internal method of
with argument
ToString
).
If
element
is
undefined
or
null
, Let
next
be the empty String; otherwise, let
next
be
ToString
element
).
Let
be a String value produced by concatenating
and
next
Increase
by 1.
Return
The
length
property of the
join
method is
NOTE
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.
Whether the
join
function can be applied successfully to a host object is implementation-dependent.
15.4.4.6
Array.prototype.pop (
The last element of the array is removed from the array and returned.
Let
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
lenVal
be the result of calling the [[Get]] internal method of
with argument
length
Let
len
be
ToUint32
lenVal
).
If
len
is zero,
Call the [[Put]] internal method of
with arguments
length
, 0, and
true
Return
undefined
Else,
len
> 0
Let
indx
be
ToString
len
–1).
Let
element
be the result of calling the [[Get]] internal method of
with argument
indx
Call the [[Delete]] internal method of
with arguments
indx
and
true
Call the [[Put]] internal method of
with arguments
length
indx
, and
true
Return
element
NOTE
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.
Whether the
pop
function can be applied successfully to a host object is implementation-dependent.
15.4.4.7
Array.prototype.push ( [
item1 [ , item2 [ , … ] ] ] )
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
item1
item2
, etc., the
following steps are taken:
Let
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
lenVal
be the result of calling the [[Get]] internal method of
with argument
length
Let
be
ToUint32
lenVal
).
Let
items
be an internal
List
whose elements are, in left to right order, the
arguments that were passed to this function invocation.
Repeat, while
items
is not empty
Remove the first element from
items
and let
be the value of the element.
Call the [[Put]] internal method of
with arguments
ToString
),
and
true
Increase
by 1.
Call the [[Put]] internal method of
with arguments
length
, and
true
Return
The
length
property of the
push
method is
NOTE
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.
Whether the
push
function can be applied successfully to a host object is implementation-dependent.
15.4.4.8
Array.prototype.reverse
( )
The elements of the array are rearranged so as to reverse their order. The object is returned as the result of the
call.
Let
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
lenVal
be the result of calling the [[Get]] internal method of
with argument
"length"
Let
len
be
ToUint32
lenVal
).
Let
middle
be
floor
len
/2).
Let
lower
be
Repeat, while
lower
middle
Let
upper
be
len
lower
−1.
Let
upperP
be
ToString
upper
).
Let
lowerP
be
ToString
lower
).
Let
lowerValue
be the result of calling the [[Get]] internal method of
with argument
lowerP
Let
upperValue
be the result of calling the [[Get]] internal method of
with argument
upperP
Let
lowerExists
be the result of calling the [[HasProperty]] internal method of
with argument
lowerP
Let
upperExists
be the result of calling the [[HasProperty]] internal method of
with argument
upperP
If
lowerExists
is
true
and
upperExists
is
true
, then
Call the [[Put]] internal method of
with arguments
lowerP
upperValue
, and
true
Call the [[Put]] internal method of
with arguments
upperP
lowerValue
, and
true
Else if
lowerExists
is
false
and
upperExists
is
true
, then
Call the [[Put]] internal method of
with arguments
lowerP
upperValue
, and
true
Call the [[Delete]] internal method of
, with arguments
upperP
and
true
Else if
lowerExists
is
true
and
upperExists
is
false
, then
Call the [[Delete]] internal method of
, with arguments
lowerP
and
true
Call the [[Put]] internal method of
with arguments
upperP
lowerValue
, and
true
Else, both
lowerExists
and
upperExists
are
false
No action is required.
Increase
lower
by 1.
Return
NOTE
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.
Whether the
reverse
function can be applied successfully to a host object is implementation-dependent.
15.4.4.9
Array.prototype.shift (
The first element of the array is removed from the array and returned.
Let
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
lenVal
be the result of calling the [[Get]] internal method of
with argument
length
Let
len
be
ToUint32
lenVal
).
If
len
is zero, then
Call the [[Put]] internal method of
with arguments
length
, 0, and
true
Return
undefined
Let
first
be the result of calling the [[Get]] internal method of
with argument
Let
be 1.
Repeat, while
len
Let
from
be
ToString
).
Let
to
be
ToString
–1).
Let
fromPresent
be the result of calling the [[HasProperty]] internal method of
with argument
from
If
fromPresent
is
true
, then
Let
fromVal
be the result of calling the [[Get]] internal method of
with argument
from
Call the [[Put]] internal method of
with arguments
to
fromVal
, and
true
Else,
fromPresent
is
false
Call the [[Delete]] internal method of
with arguments
to
and
true
Increase
by 1.
Call the [[Delete]] internal method of
with arguments
ToString
len
–1)
and
true
Call the [[Put]] internal method of
with arguments
length
, (
len
–1) , and
true
Return
first
NOTE
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.
Whether the
shift
function can be applied successfully to a host object is implementation-dependent.
15.4.4.10
Array.prototype.slice
(start, end)
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
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
be a new array created as if by the expression
new Array()
where
Array
is the
standard built-in constructor with that name.
Let
lenVal
be the result of calling the [[Get]] internal method of
with argument
length
Let
len
be
ToUint32
lenVal
).
Let
relativeStart
be
ToInteger
start
).
If
relativeStart
is negative, let
be max((
len
relativeStart
),0); else let
be min(
relativeStart
len
).
If
end
is
undefined
, let
relativeEnd
be
len
; else let
relativeEnd
be
ToInteger
end
).
If
relativeEnd
is negative, let
final
be max((
len
relativeEnd
),0); else let
final
be min(
relativeEnd
len
).
Let
be 0.
Repeat, while
final
Let
Pk
be
ToString
).
Let
kPresent
be the result of calling the [[HasProperty]] internal method of
with argument
Pk
If
kPresent
is
true
, then
Let
kValue
be the result of calling the [[Get]] internal method of
with argument
Pk
Call the [[DefineOwnProperty]] internal method of
with arguments
ToString
),
Property Descriptor
{[[Value]]:
kValue
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Increase
by 1.
Increase
by 1.
Return
The
length
property of the
slice
method is
NOTE
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.
Whether the
slice
function can be applied successfully to a host object is implementation-dependent.
15.4.4.11
Array.prototype.sort
(comparefn)
The elements of this array are sorted. The sort is not necessarily stable (that is, elements that compare equal do not
necessarily remain in their original order). If
comparefn
is not
undefined
, it should be a function that
accepts two arguments
and
and returns a negative value if
, zero if
, or a positive value if
Let
obj
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
len
be the result of applying Uint32 to the result of calling the [[Get]] internal method of
obj
with argument "
length
".
If
comparefn
is not
undefined
and is not a consistent comparison function for the elements of this
array (see below), the behaviour of
sort
is implementation-defined.
Let
proto
be the value of the [[Prototype]] internal property of
obj
. If
proto
is not
null
and there exists an integer
such that all of the conditions below are satisfied then the
behaviour of
sort
is implementation-defined:
obj
is sparse (
15.4
0 ≤
len
The result of calling the [[HasProperty]] internal method of
proto
with argument
ToString
is
true
The behaviour of
sort
is also implementation defined if
obj
is sparse and any of the following
conditions are true:
The [[Extensible]] internal property of
obj
is
false
Any array index property of
obj
whose name is a nonnegative integer less than
len
is a data
property whose [[Configurable]] attribute is
false
The behaviour of
sort
is also implementation defined if any array 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
Otherwise, the following steps are taken.
Perform an implementation-dependent sequence of calls to the [[Get]] , [[Put]], and [[Delete]] internal methods of
obj
and to SortCompare (described below), where the first argument for each call to [[Get]], [[Put]], or
[[Delete]] is a nonnegative integer less than
len
and where the arguments for calls to SortCompare are
results of previous calls to the [[Get]] internal method. The throw argument to the [[Put]] and [[Delete]] internal
methods will be the value
true
. If
obj
is not sparse then [[Delete]] must not be called.
Return
obj
The returned object must have the following two properties.
There must be some mathematical permutation
of the
nonnegative integers less than
len
, such that for every nonnegative integer
less than
len
, if property
old[
existed, then
new[π(
)]
is exactly the same value as
old[
. But if property
old[
did not exist, then
new[π(
)]
does not exist.
Then for all nonnegative integers
and
, each less than
len
, if
SortCompare(
) < 0
(see SortCompare below), then
) <
Here the notation
old[
is used to refer to the hypothetical
result of calling the [[Get]] internal method of
obj
with argument
before this function is
executed, and the notation
new[
to refer to the hypothetical
result of calling the [[Get]] internal method of
obj
with argument
after this function has been
executed.
A function
comparefn
is a consistent comparison function for a set of values
if all of the
requirements below are met for all values
, and
(possibly the same value) in the
set
: The notation
CF
means
comparefn
) < 0
CF
means
comparefn
) = 0
(of either sign); and
CF
means
comparefn
) > 0
Calling
comparefn
) always returns the same value
when given a specific pair of
values
and
as its two arguments. Furthermore,
Type
) is Number, and
is not NaN. Note that this implies that exactly one of
CF
CF
, and
CF
will be true for a given
pair of
and
Calling
comparefn
) does not modify the
this
object.
CF
(reflexivity)
If
CF
, then
CF
(symmetry)
If
CF
and
CF
, then
CF
(transitivity of =
CF
If
CF
and
CF
, then
CF
(transitivity of <
CF
If
CF
and
CF
, then
CF
(transitivity of >
CF
NOTE
The above conditions are necessary and sufficient to ensure that
comparefn
divides the set
into equivalence classes and that these equivalence classes are totally ordered.
When the SortCompare abstract operation is called with two arguments
and
, the following steps
are taken:
Let
jString
be
ToString
).
Let
kString
be
ToString
).
Let
hasj
be the result of calling the [[HasProperty]] internal method of
obj
with argument
jString
Let
hask
be the result of calling the [[HasProperty]] internal method of
obj
with argument
kString
If
hasj
and
hask
are both
false
, then return
+0
If
hasj
is
false
, then return 1.
If
hask
is
false
, then return –1.
Let
be the result of calling the [[Get]] internal method of
obj
with argument
jString
Let
be the result of calling the [[Get]] internal method of
obj
with argument
kString
If
and
are both
undefined
, return
+0
If
is
undefined
, return 1.
If
is
undefined
, return −1.
If the argument
comparefn
is not
undefined
, then
If
IsCallable
comparefn
) is
false
, throw a
TypeError
exception.
Return the result of calling the [[Call]] internal method of
comparefn
passing
undefined
as the
this
value and with arguments
and
Let
xString
be
ToString
).
Let
yString
be
ToString
).
If
xString
yString
, return −1.
If
xString
yString
, 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
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.
Whether the
sort
function can be applied successfully to a host object is implementation-dependent.
15.4.4.12
Array.prototype.splice
(start, deleteCount [ , item1 [ , item2 [ , … ] ] ] )
When the
splice
method is called with two or more arguments
start
deleteCount
and
(optionally)
item1
item2
, etc., the
deleteCount
elements of the array starting at array
index
start
are replaced by the arguments
item1
item2
, etc. An Array object containing
the deleted elements (if any) is returned. The following steps are taken:
Let
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
be a new array created as if by the expression
new Array()
where
Array
is the
standard built-in constructor with that name.
Let
lenVal
be the result of calling the [[Get]] internal method of
with argument
length
Let
len
be
ToUint32
lenVal
).
Let
relativeStart
be
ToInteger
start
).
If
relativeStart
is negative, let
actualStart
be max((
len
relativeStart
),0); else let
actualStart
be min(
relativeStart
len
).
Let
actualDeleteCount
be min(max(
ToInteger
deleteCount
),0),
len
actualStart
).
Let
be 0.
Repeat, while
actualDeleteCount
Let
from
be
ToString
actualStart
).
Let
fromPresent
be the result of calling the [[HasProperty]] internal method of
with argument
from
If
fromPresent
is
true
, then
Let
fromValue
be the result of calling the [[Get]] internal method of
with argument
from
Call the [[DefineOwnProperty]] internal method of
with arguments
ToString
),
Property Descriptor
{[[Value]]:
fromValue
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
},
and
false
Increment
by 1.
Let
items
be an internal
List
whose elements are, in left to right order, the portion
of the actual argument list starting with
item1
. The list will be empty if no such items are present.
Let
itemCount
be the number of elements in
items
If
itemCount
actualDeleteCount
, then
Let
be
actualStart
Repeat, while
< (
len
actualDeleteCount
Let
from
be
ToString
actualDeleteCount
).
Let
to
be
ToString
itemCount
).
Let
fromPresent
be the result of calling the [[HasProperty]] internal method of
with
argument
from
If
fromPresent
is
true
, then
Let
fromValue
be the result of calling the [[Get]] internal method of
with argument
from
Call the [[Put]] internal method of
with arguments
to
fromValue
, and
true
Else,
fromPresent
is
false
Call the [[Delete]] internal method of
with arguments
to
and
true
Increase k by 1.
Let
be
len
Repeat, while
> (
len
actualDeleteCount
itemCount
Call the [[Delete]] internal method of
with arguments
ToString
–1) and
true
Decrease
by 1.
Else if
itemCount
actualDeleteCount
, then
Let
be (
len
actualDeleteCount
).
Repeat, while
actualStart
Let
from
be
ToString
actualDeleteCount
– 1).
Let
to
be
ToString
itemCount
– 1)
Let
fromPresent
be the result of calling the [[HasProperty]] internal method of
with
argument
from
If
fromPresent
is
true
, then
Let
fromValue
be the result of calling the [[Get]] internal method of
with argument
from
Call the [[Put]] internal method of
with arguments
to
fromValue
, and
true
Else,
fromPresent
is
false
Call the [[Delete]] internal method of
with argument
to
and
true
Decrease
by 1.
Let
be
actualStart
Repeat, while
items
is not empty
Remove the first element from
items
and let
be the value of that element.
Call the [[Put]] internal method of
with arguments
ToString
),
and
true
Increase
by 1.
Call the [[Put]] internal method of
with arguments
length
, (
len
actualDeleteCount
itemCount
), and
true
Return
The
length
property of the
splice
method is
NOTE
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.
Whether the
splice
function can be applied successfully to a host object is implementation-dependent.
15.4.4.13
Array.prototype.unshift ( [ item1 [ , item2 [ , … ] ] ] )
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
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
lenVal
be the result of calling the [[Get]] internal method of
with argument
length
Let
len
be
ToUint32
lenVal
).
Let
argCount
be the number of actual arguments.
Let
be
len
Repeat, while
> 0,
Let
from
be
ToString
–1).
Let
to
be
ToString
argCount
–1).
Let
fromPresent
be the result of calling the [[HasProperty]] internal method of
with argument
from
If
fromPresent
is
true
, then
Let
fromValue
be the result of calling the [[Get]] internal method of
with argument
from
Call the [[Put]] internal method of
with arguments
to
fromValue
, and
true
Else,
fromPresent
is
false
Call the [[Delete]] internal method of
with arguments
to
, and
true
Decrease
by 1.
Let
be 0.
Let
items
be an internal
List
whose elements are, in left to right order, the
arguments that were passed to this function invocation.
Repeat, while
items
is not empty
Remove the first element from
items
and let
be the value of that element.
Call the [[Put]] internal method of
with arguments
ToString
),
and
true
Increase
by 1.
Call the [[Put]] internal method of
with arguments
length
len
argCount
, and
true
Return
len
argCount
The
length
property of the
unshift
method is
NOTE
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.
Whether the
unshift
function can be applied successfully to a host object is implementation-dependent.
15.4.4.14
Array.prototype.indexOf ( searchElement [ , fromIndex ] )
indexOf
compares
searchElement
to the elements of the array, in ascending order, using the
internal Strict Equality Comparison Algorithm (
11.9.6
), and if found at one or more positions,
returns the index of the first such position; 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
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
lenValue
be the result of calling the [[Get]] internal method of
with the argument
length
Let
len
be
ToUint32
lenValue
).
If
len
is 0, return -1.
If argument
fromIndex
was passed let
be
ToInteger
fromIndex
); else let
be 0.
If
len
, return -1.
If
≥ 0, then
Let
be
Else,
<0
Let
be
len
abs
).
If
is less than 0, then let
be 0.
Repeat, while
len
Let
kPresent
be the result of calling the [[HasProperty]] internal method of
with argument
ToString
).
If
kPresent
is
true
, then
Let
elementK
be the result of calling the [[Get]] internal method of
with the argument
ToString
).
Let
same
be the result of applying the Strict Equality Comparison Algorithm to
searchElement
and
elementK
If
same
is
true,
return
Increase
by 1.
Return -1.
The
length
property of the
indexOf
method is
NOTE
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.
Whether the
indexOf
function can be applied successfully to a host object is implementation-dependent.
15.4.4.15
Array.prototype.lastIndexOf ( searchElement [ , fromIndex ] )
lastIndexOf
compares
searchElement
to the elements of the array in descending order using the
internal Strict Equality Comparison Algorithm (
11.9.6
), and if found at one or more positions,
returns the index of the last such position; 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
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
lenValue
be the result of calling the [[Get]] internal method of
with the argument
length
Let
len
be
ToUint32
lenValue
).
If
len
is 0, return -1.
If argument
fromIndex
was passed let
be
ToInteger
fromIndex
); else let
be
len
-1.
If
, then let
be min(
len
– 1).
Else,
< 0
Let
be
len
abs
).
Repeat, while
Let
kPresent
be the result of calling the [[HasProperty]] internal method of
with argument
ToString
).
If
kPresent
is
true
, then
Let
elementK
be the result of calling the [[Get]] internal method of
with the argument
ToString
).
Let
same
be the result of applying the Strict Equality Comparison Algorithm to
searchElement
and
elementK
If
same
is
true,
return
Decrease
by 1.
Return -1.
The
length
property of the
lastIndexOf
method is
NOTE
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.
Whether the
lastIndexOf
function can be applied successfully to a host object is
implementation-dependent.
15.4.4.16
Array.prototype.every
( callbackfn [ , thisArg ] )
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
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
lenValue
be the result of calling the [[Get]] internal method of
with the argument
"length"
Let
len
be
ToUint32
lenValue
).
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If
thisArg
was supplied, let
be
thisArg
; else let
be
undefined
Let
be 0.
Repeat, while
len
Let
Pk
be
ToString
).
Let
kPresent
be the result of calling the [[HasProperty]] internal method of
with argument
Pk
If
kPresent
is
true
, then
Let
kValue
be the result of calling the [[Get]] internal method of
with argument
Pk
Let
testResult
be the result of calling the [[Call]] internal method of
callbackfn
with
as the
this
value and argument list containing
kValue
, and
If
ToBoolean
testResult)
is
false
, return
false
Increase
by 1.
Return
true
The
length
property of the
every
method is
NOTE
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.
Whether the
every
function can be applied successfully to a host object is implementation-dependent.
15.4.4.17
Array.prototype.some (
callbackfn [ , thisArg ] )
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
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
lenValue
be the result of calling the [[Get]] internal method of
with the argument
"length"
Let
len
be
ToUint32
lenValue
).
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If
thisArg
was supplied, let
be
thisArg
; else let
be
undefined
Let
be 0.
Repeat, while
len
Let
Pk
be
ToString
).
Let
kPresent
be the result of calling the [[HasProperty]] internal method of
with argument
Pk
If
kPresent
is
true
, then
Let
kValue
be the result of calling the [[Get]] internal method of
with argument
Pk
Let
testResult
be the result of calling the [[Call]] internal method of
callbackfn
with
as the
this
value and argument list containing
kValue
, and
If
ToBoolean
testResult)
is
true
, return
true
Increase
by 1.
Return
false
The
length
property of the
some
method is
NOTE
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.
Whether the
some
function can be applied successfully to a host object is implementation-dependent.
15.4.4.18
Array.prototype.forEach ( callbackfn [ , thisArg ] )
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
The range of elements processed by
forEach
is set before the first call to
callbackfn
. Elements
which are appended to the array after the call to
forEach
begins will not be visited by
callbackfn
. If existing elements of the array are changed, their value as passed to callback will be the value
at the time
forEach
visits them; elements that are deleted after the call to
forEach
begins and
before being visited are not visited.
When the
forEach
method is called with one or two arguments, the following steps are taken:
Let
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
lenValue
be the result of calling the [[Get]] internal method of
with the argument
"length"
Let
len
be
ToUint32
lenValue
).
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If
thisArg
was supplied, let
be
thisArg
; else let
be
undefined
Let
be 0.
Repeat, while
len
Let
Pk
be
ToString
).
Let
kPresent
be the result of calling the [[HasProperty]] internal method of
with argument
Pk
If
kPresent
is
true
, then
Let
kValue
be the result of calling the [[Get]] internal method of
with argument
Pk
Call the [[Call]] internal method of
callbackfn
with
as the
this
value and argument
list containing
kValue
, and
Increase
by 1.
Return
undefined
The
length
property of the
forEach
method is
NOTE
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.
Whether the
forEach
function can be applied successfully to a host object is implementation-dependent.
15.4.4.19
Array.prototype.map (
callbackfn [ , thisArg ] )
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
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
lenValue
be the result of calling the [[Get]] internal method of
with the argument
"length"
Let
len
be
ToUint32
lenValue
).
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If
thisArg
was supplied, let
be
thisArg
; else let
be
undefined
Let
be a new array created as if by the expression
new Array(
len
where
Array
is the standard built-in constructor with that name and
len
is the value of
len
Let
be 0.
Repeat, while
len
Let
Pk
be
ToString
).
Let
kPresent
be the result of calling the [[HasProperty]] internal method of
with argument
Pk
If
kPresent
is
true
, then
Let
kValue
be the result of calling the [[Get]] internal method of
with argument
Pk
Let
mappedValue
be the result of calling the [[Call]] internal method of
callbackfn
with
as the
this
value and argument list containing
kValue
, and
Call the [[DefineOwnProperty]] internal method of
with arguments
Pk
Property Descriptor
{[[Value]]:
mappedValue
, [[Writable]]:
true
[[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Increase
by 1.
Return
The
length
property of the
map
method is
NOTE
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.
Whether the
map
function can be applied successfully to a host object is implementation-dependent.
15.4.4.20
Array.prototype.filter
( callbackfn [ , thisArg ] )
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
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
lenValue
be the result of calling the [[Get]] internal method of
with the argument
"length"
Let
len
be
ToUint32
lenValue
).
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If
thisArg
was supplied, let
be
thisArg
; else let
be
undefined
Let
be a new array created as if by the expression
new Array()
where
Array
is the
standard built-in constructor with that name.
Let
be 0.
Let
to
be 0.
Repeat, while
len
Let
Pk
be
ToString
).
Let
kPresent
be the result of calling the [[HasProperty]] internal method of
with argument
Pk
If
kPresent
is
true
, then
Let
kValue
be the result of calling the [[Get]] internal method of
with argument
Pk
Let
selected
be the result of calling the [[Call]] internal method of
callbackfn
with
as the
this
value and argument list containing
kValue
, and
If
ToBoolean
selected
) is
true
, then
Call the [[DefineOwnProperty]] internal method of
with arguments
ToString
to
),
Property Descriptor
{[[Value]]:
kValue
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
},
and
false
Increase
to
by 1.
Increase
by 1.
Return
The
length
property of the
filter
method is
NOTE
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.
Whether the
filter
function can be applied successfully to a host object is implementation-dependent.
15.4.4.21
Array.prototype.reduce
( callbackfn [ , initialValue ] )
callbackfn
should be a function that takes four arguments.
reduce
calls the callback, as a
function, once for each element present in the array, in ascending order.
callbackfn
is called with four arguments: the
previousValue
(or 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 provided 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 provided, 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
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
lenValue
be the result of calling the [[Get]] internal method of
with the argument
"length"
Let
len
be
ToUint32
lenValue
).
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If len is 0 and
initialValue
is not present, throw a
TypeError
exception.
Let
be 0.
If
initialValue
is present, then
Set
accumulator
to
initialValue
Else,
initialValue
is not present
Let
kPresent
be
false
Repeat, while
kPresent
is
false
and
len
Let
Pk
be
ToString
).
Let
kPresent
be the result of calling the [[HasProperty]] internal method of
with argument
Pk
If
kPresent
is
true
, then
Let
accumulator
be the result of calling the [[Get]] internal method of
with argument
Pk
Increase
by 1.
If
kPresent
is
false
, throw a
TypeError
exception.
Repeat, while
len
Let
Pk
be
ToString
).
Let
kPresent
be the result of calling the [[HasProperty]] internal method of
with argument
Pk
If
kPresent
is
true
, then
Let
kValue
be the result of calling the [[Get]] internal method of
with argument
Pk
Let
accumulator
be the result of calling the [[Call]] internal method of
callbackfn
with
undefined
as the
this
value and argument list containing
accumulator
kValue
, and
Increase
by 1.
Return
accumulator
The
length
property of the
reduce
method is
NOTE
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.
Whether the
reduce
function can be applied successfully to a host object is implementation-dependent.
15.4.4.22
Array.prototype.reduceRight ( callbackfn [ , initialValue ] )
callbackfn
should be a function that takes four arguments.
reduceRight
calls the callback, as a
function, once for each element present in the array, in descending order.
callbackfn
is called with four arguments: the
previousValue
(or 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 provided 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 provided, 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
be the result of calling
ToObject
passing the
this
value as the
argument.
Let
lenValue
be the result of calling the [[Get]] internal method of
with the argument
"length"
Let
len
be
ToUint32
lenValue
).
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If len is 0 and
initialValue
is not present, throw a
TypeError
exception.
Let
be
len
-1.
If
initialValue
is present, then
Set
accumulator
to
initialValue
Else,
initialValue
is not present
Let
kPresent
be
false
Repeat, while
kPresent
is
false
and
≥ 0
Let
Pk
be
ToString
).
Let
kPresent
be the result of calling the [[HasProperty]] internal method of
with argument
Pk
If
kPresent
is
true
, then
Let
accumulator
be the result of calling the [[Get]] internal method of
with argument
Pk
Decrease
by 1.
If
kPresent
is
false
, throw a
TypeError
exception.
Repeat, while
≥ 0
Let
Pk
be
ToString
).
Let
kPresent
be the result of calling the [[HasProperty]] internal method of
with argument
Pk
If
kPresent
is
true
, then
Let
kValue
be the result of calling the [[Get]] internal method of
with argument
Pk
Let
accumulator
be the result of calling the [[Call]] internal method of
callbackfn
with
undefined
as the
this
value and argument list containing
accumulator
kValue
, and
Decrease
by 1.
Return
accumulator
The
length
property of the
reduceRight
method is
NOTE
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.
Whether the
reduceRight
function can be applied successfully to a host object is
implementation-dependent.
15.4.5
Properties of Array
Instances
Array instances inherit properties from the Array prototype object and their [[Class]] internal property value is
"Array"
. Array instances also have the following properties.
15.4.5.1
[[DefineOwnProperty]] (
P, Desc, Throw )
Array objects use a variation of the [[DefineOwnProperty]] internal method used for other native ECMAScript objects (
8.12.9
).
Assume
is an Array object,
Desc
is a
Property Descriptor
and
Throw
is a Boolean flag.
In the following algorithm, the term “
Reject
” means
If
Throw
is
true
, then throw a
TypeError
exception,
otherwise return
false
When the [[DefineOwnProperty]] internal method of
is called with property
Property Descriptor
Desc
, and Boolean flag
Throw
, the
following steps are taken:
Let
oldLenDesc
be the result of calling the [[GetOwnProperty]] internal method of
passing
length
as the argument. The result will never be
undefined
or an
accessor descriptor because Array objects are created with a length data property that cannot be deleted or
reconfigured.
Let
oldLen
be
oldLenDesc
.[[Value]].
If
is
length
, then
If the [[Value]] field of
Desc
is absent, then
Return the result of calling the default [[DefineOwnProperty]] internal method (
8.12.9
) on
passing
length
Desc
, and
Throw
as arguments.
Let
newLenDesc
be a copy of
Desc
Let
newLen
be
ToUint32
Desc
.[[Value]]).
If
newLen
is not equal to
ToNumber
Desc
.[[Value]]), throw a
RangeError
exception.
Set
newLenDesc
.[[Value] to
newLen
If
newLen
oldLen
, then
Return the result of calling the default [[DefineOwnProperty]] internal method (
8.12.9
) on
passing
length
newLenDesc
, and
Throw
as arguments.
Reject if
oldLenDesc
.[[Writable]] is
false
If
newLenDesc
.[[Writable]] is absent or has the value
true
, let
newWritable
be
true
Else,
Need to defer setting the [[Writable]] attribute to
false
in case any elements cannot be
deleted.
Let
newWritable
be
false
Set
newLenDesc
.[[Writable] to
true
Let
succeeded
be the result of calling the default [[DefineOwnProperty]] internal method (
8.12.9
) on
passing
length
newLenDesc
, and
Throw
as arguments.
If
succeeded
is
false
, return
false
While
newLen
oldLen
repeat,
Set
oldLen
to
oldLen
– 1.
Let
deleteSucceeded
be the result of calling the [[Delete]] internal method of
passing
ToString
oldLen
) and
false
as arguments.
If
deleteSucceeded
is
false
, then
Set
newLenDesc
.[[Value] to
oldLen+1
If
newWritable
is
false
, set
newLenDesc
.[[Writable] to
false
Call the default [[DefineOwnProperty]] internal method (
8.12.9
) on
passing
length
newLenDesc
, and
false
as arguments.
Reject.
If
newWritable
is
false
, then
Call the default [[DefineOwnProperty]] internal method (
8.12.9
) on
passing
length
Property
Descriptor
{[[Writable]]:
false
}, and
false
as arguments. This call will always return
true
Return
true
Else if
is an array index (
15.4
), then
Let
index
be
ToUint32
).
Reject if
index
oldLen
and
oldLenDesc
.[[Writable]] is
false
Let
succeeded
be the result of calling the default [[DefineOwnProperty]] internal method (
8.12.9
) on
passing
Desc
, and
false
as arguments.
Reject if
succeeded
is
false
If
index
oldLen
Set
oldLenDesc
.[[Value]] to
index
+ 1.
Call the default [[DefineOwnProperty]] internal method (
8.12.9
) on
passing
length
oldLenDesc
, and
false
as
arguments. This call will always return
true
Return
true
Return the result of calling the default [[DefineOwnProperty]] internal method (
8.12.9
) on
passing
Desc
, and
Throw
as arguments.
15.4.5.2
length
The
length
property of this Array object is a data property whose value is always numerically greater than
the name of every deletable property whose name is an array index.
The
length
property initially has the attributes
[[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
NOTE
Attempting to set the length property of an Array object to a value that is numerically
less than or equal to the largest numeric property name of an existing array indexed non-deletable property of the array
will result in the length being set to a numeric value that is one greater than that largest numeric property name. See
15.4.5.1
15.5
String Objects
15.5.1
The String Constructor Called
as a Function
When
String
is called as a function rather than as a constructor, it performs a type conversion.
15.5.1.1
String ( [ value ]
Returns a String value (not a String object) computed by
ToString
value
. If
value
is not supplied, the empty String
""
is returned.
15.5.2
The String Constructor
When
String
is called as part of a
new
expression, it is a constructor: it initialises the
newly created object.
15.5.2.1
new String ( [ value ]
The
[[Prototype]]
internal property of the newly constructed object
is set to the standard built-in String prototype object that is the initial value of
String.prototype
15.5.3.1
).
The
[[Class]]
internal property of the newly constructed object is
set to
"String"
The
[[Extensible]]
internal property of the newly constructed object
is set to
true
The
[[PrimitiveValue]]
internal property of the newly constructed
object is set to
ToString
value
, or to
the empty String if
value
is not supplied.
15.5.3
Properties of the String
Constructor
The value of the [[Prototype]] internal property of the String constructor is the standard built-in Function prototype
object (
15.3.4
).
Besides the internal properties and the
length
property (whose value is
), the String constructor
has the following properties:
15.5.3.1
String.prototype
The initial value of
String.prototype
is the standard built-in String prototype object (
15.5.4
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.5.3.2
String.fromCharCode ( [
char0 [ , char1 [ , … ] ] ] )
Returns a String value containing as many characters as the number of arguments. Each argument specifies one character
of the resulting String, with the first argument specifying the first character, and so on, from left to right. An
argument is converted to a character by applying the operation
ToUint16
9.7
) and regarding the resulting 16-bit integer as the code unit value of a character. If no arguments
are supplied, the result is the empty String.
The
length
property of the
fromCharCode
function is
15.5.4
Properties of the String
Prototype Object
The String prototype object is itself a String object (its [[Class]] is
"String"
) whose value is an empty
String.
The value of the [[Prototype]] internal property of the String prototype object is the standard built-in Object prototype
object (
15.2.4
).
15.5.4.1
String.prototype.constructor
The initial value of
String.prototype.constructor
is the built-in
String
constructor.
15.5.4.2
String.prototype.toString ( )
Returns this String value. (Note that, for a String object, the
toString
method happens to return the same
thing as the
valueOf
method.)
The
toString
function is not generic; it throws a
TypeError
exception if its
this
value is
not a String or a String object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
15.5.4.3
String.prototype.valueOf
( )
Returns this String value.
The
valueOf
function is not generic; it throws a
TypeError
exception if its
this
value is
not a String or String object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
15.5.4.4
String.prototype.charAt
(pos)
Returns a String containing the character at position
pos
in the String resulting from converting this
object to a String. If there is no character at that position, 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:
Call
CheckObjectCoercible
passing the
this
value as its argument.
Let
be the result of calling
ToString
, giving it the
this
value as its
argument.
Let
position
be
ToInteger
pos
).
Let
size
be the number of characters in
If
position
< 0 or
position
size
, return the empty String.
Return a String of length 1, containing one character from
, namely the character at position
position
, where the first (leftmost) character in
is considered to be at position 0, the next one at
position 1, and so on.
NOTE
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.
15.5.4.5
String.prototype.charCodeAt (pos)
Returns a Number (a nonnegative integer less than
16
representing the code unit value of the character at position
pos
in the String resulting from converting this
object to a String. If there is no character at that position, the result is
NaN
When the
charCodeAt
method is called with one argument
pos
, the following steps are taken:
Call
CheckObjectCoercible
passing the
this
value as its argument.
Let
be the result of calling
ToString
, giving it the
this
value as its
argument.
Let
position
be
ToInteger
pos
).
Let
size
be the number of characters in
If
position
< 0 or
position
size
, return
NaN
Return a value of Number type, whose value is the code unit value of the character at position
position
in
the String
, where the first (leftmost) character in
is considered to be at position 0, the next
one at position 1, and so on.
NOTE
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.
15.5.4.6
String.prototype.concat
( [ string1 [ , string2 [ , … ] ] ] )
When the
concat
method is called with zero or more arguments
string1
string2
, etc.,
it returns a String consisting of the characters of this object (converted to a String) followed by the characters of each
of
string1
string2
, etc. (where each argument is converted to a String). The result is a String
value, not a String object. The following steps are taken:
Call
CheckObjectCoercible
passing the
this
value as its argument.
Let
be the result of calling
ToString
, giving it the
this
value as its
argument.
Let
args
be an internal list that is a copy of the argument list passed to this function.
Let
be
Repeat, while
args
is not empty
Remove the first element from
args
and let
next
be the value of that element.
Let
be the String value consisting of the characters in the previous value of
followed by the
characters of
ToString
next
).
Return
The
length
property of the
concat
method is
NOTE
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.
15.5.4.7
String.prototype.indexOf
(searchString, position)
If
searchString
appears as a substring of the result of converting this object to a String, at one or more
positions that are greater than or equal to
position
, then the index of the smallest such position 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:
Call
CheckObjectCoercible
passing the
this
value as its argument.
Let
be the result of calling
ToString
, giving it the
this
value as its
argument.
Let
searchStr
be
ToString
searchString
).
Let
pos
be
ToInteger
position
). (If
position
is
undefined
, this
step produces the value
).
Let
len
be the number of characters in
Let
start
be min(max(
pos
, 0),
len
).
Let
searchLen
be the number of characters in
searchStr
Return the smallest possible integer
not smaller than
start
such that
searchLen
is
not greater than
len
, and for all nonnegative integers
less than
searchLen
, the character at
position
of
is the same as the character at position
of
searchStr
; but if
there is no such integer
, then return the value
-1
The
length
property of the
indexOf
method is
NOTE
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.
15.5.4.8
String.prototype.lastIndexOf (searchString, position)
If
searchString
appears as a substring of the result of converting this object to a String at one or more
positions that are smaller than or equal to
position
, then the index of the greatest such position 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:
Call
CheckObjectCoercible
passing the
this
value as its argument.
Let
be the result of calling
ToString
, giving it the
this
value as its
argument.
Let
searchStr
be
ToString
searchString
).
Let
numPos
be
ToNumber
position
). (If
position
is
undefined
, this
step produces the value
NaN
).
If
numPos
is
NaN
, let
pos
be
+∞
; otherwise, let
pos
be
ToInteger
numPos
).
Let
len
be the number of characters in
Let
start
min(max(
pos
, 0),
len
).
Let
searchLen
be the number of characters in
searchStr
Return the largest possible nonnegative integer
not larger than
start
such that
searchLen
is not greater than
len
, and for all nonnegative integers
less than
searchLen
, the character at position
of
is the same as the character at position
of
searchStr
; but if there is no such integer
, then return the value
-1
The
length
property of the
lastIndexOf
method is
NOTE
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.
15.5.4.9
String.prototype.localeCompare (that)
When the
localeCompare
method is called with one 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
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 the system default locale, and will be negative, zero, or positive, depending on whether
comes
before
That
in the sort order, the Strings are equal, or
comes after
That
in the sort order, respectively.
Before perform the comparisons the following steps are performed to prepare the Strings:
Call
CheckObjectCoercible
passing the
this
value as its argument.
Let
be the result of calling
ToString
, giving it the
this
value as its
argument.
Let
That
be
ToString
that
).
The
localeCompare
method, if considered as a function of two arguments
this
and
that
, is
a consistent comparison function (as defined in
15.4.4.11
) 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 and to return
when comparing
Strings that are considered canonically equivalent by the Unicode standard.
If no language-sensitive comparison at all is available from the host environment, this function may perform a bitwise
comparison.
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. It is strongly recommended that this function treat Strings that
are canonically equivalent according to the Unicode standard as identical (in other words, compare the Strings as if
they had both been converted to Normalised Form C or D first). It is also recommended that this function not honour
Unicode compatibility equivalences or decompositions.
NOTE 3
The second parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything else.
NOTE 4
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.
15.5.4.10
String.prototype.match
(regexp)
When the
match
method is called with argument
regexp
, the following steps are taken:
Call
CheckObjectCoercible
passing the
this
value as its argument.
Let
be the result of calling
ToString
, giving it the
this
value as its
argument.
If
Type
regexp
) is Object and the value of the [[Class]] internal property of
regexp
is
"RegExp"
, then let
rx
be
regexp
Else, let
rx
be a new RegExp object created as if by the expression
new
RegExp(
regexp
where
RegExp
is the standard built-in constructor with that
name.
Let
global
be the result of calling the [[Get]] internal method of
rx
with argument
"global"
Let
exec
be the standard built-in function
RegExp.prototype.exec
see
15.10.6.2
If
global
is not
true
, then
Return the result of calling the [[Call]] internal method of
exec
with
rx
as the
this
value
and argument list containing
Else,
global
is
true
Call the [[Put]] internal method of
rx
with arguments
"lastIndex"
and 0.
Let
be a new array created as if by the expression
new Array()
where
Array
is
the standard built-in constructor with that name.
Let
previousLastIndex
be 0.
Let
be 0.
Let
lastMatch
be
true
Repeat, while
lastMatch
is
true
Let
result
be the result of calling the [[Call]] internal method of
exec
with
rx
as the
this
value and argument list containing
If
result
is
null
, then set
lastMatch
to
false
Else,
result
is not
null
Let
thisIndex
be the result of calling the [[Get]] internal method of
rx
with argument
"lastIndex"
If
thisIndex
previousLastIndex
then
Call the [[Put]] internal method of
rx
with arguments
"lastIndex"
and
thisIndex
+1.
Set
previousLastIndex
to
thisIndex
+1.
Else, set
previousLastIndex
to
thisIndex
Let
matchStr
be the result of calling the [[Get]] internal method of
result
with argument
"0"
Call the [[DefineOwnProperty]] internal method of
with arguments
ToString
), the
Property Descriptor
{[[Value]]:
matchStr
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[configurable]]:
true
},
and
false
Increment
If
= 0, then return
null
Return
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.
15.5.4.11
String.prototype.replace (searchValue, replaceValue)
First set
string
according to the following steps:
Call
CheckObjectCoercible
passing the
this
value as its argument.
Let
string
be the result of calling
ToString
, giving it the
this
value as its
argument.
If
searchValue
is a regular expression (an object whose [[Class]] internal property is
"RegExp"
), do the following: If
searchValue
.global is
false
, then search
string
for the first match of the regular expression
searchValue
. If
searchValue
.global is
true
then search
string
for all matches of the regular expression
searchValue
. Do the search in the same
manner as in
String.prototype.match
, including the update of
searchValue
lastIndex
Let
be the number of left capturing parentheses in
searchValue
(using
NcapturingParens
as specified in
15.10.2.1
).
If
searchValue
is not a regular expression, let
searchString
be
ToString
searchValue
and search
string
for the first
occurrence of
searchString
. Let
be 0.
If
replaceValue
is a function, then for each matched substring, call the function with the following
+ 3 arguments. Argument 1 is the substring that matched. If
searchValue
is a regular expression,
the next
arguments are all of the captures in the MatchResult (
see 15.10.2.1
).
Argument
+ 2 is the offset within
string
where the match occurred, and argument
+ 3
is
string
. The result is a String value derived from the original input by replacing each matched substring
with the corresponding return value of the function call, converted to a String if need be.
Otherwise, let
newstring
denote the result of converting
replaceValue
to a String. The result is
a String value derived from the original input String by replacing each matched substring with a String derived from
newstring
by replacing characters in
newstring
by replacement text as specified in Table 22. These
replacements are done left-to-right, and, once such a replacement is performed, the new replacement text is
not subject to further replacements. For example,
"$1,$2".replace(/(\$(\d))/g, "$$1-$1$2")
returns
"$1-$11,$1-$22"
. A
in
newstring
that does not match any of the forms below is left
as is.
Table 22 — Replacement Text Symbol Substitutions
Characters
Replacement text
$$
$&
The matched substring.
$‘
The portion of
string
that precedes the matched substring.
$’
The portion of
string
that follows the matched substring.
$n
The
th
capture, where
is a single digit in the range
to
and
is not followed by a decimal digit. If
and the
th capture is
undefined
, use the empty String instead. If
, the result is implementation-defined.
$nn
The
nn
th
capture, where
nn
is a two-digit decimal number in the range
01
to
99
. If
nn
and the
nn
th
capture is
undefined
, use the empty String instead. If
nn
, the result is implementation-defined.
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.
15.5.4.12
String.prototype.search (regexp)
When the search method is called with argument
regexp
, the following steps are taken:
Call
CheckObjectCoercible
passing the
this
value as its argument.
Let
string
be the result of calling
ToString
, giving it the
this
value as its
argument.
If
Type
regexp
) is Object and the value of the [[Class]] internal property of
regexp
is
"RegExp"
, then let
rx
be
regexp
Else, let
rx
be a new RegExp object created as if by the expression
new
RegExp(
regexp
where
RegExp
is the standard built-in constructor with that
name.
Search the value
string
from its beginning for an occurrence of the regular expression pattern
rx
. Let
result
be a Number indicating the offset within
string
where the pattern matched, or –1 if there
was no match. The
lastIndex
and
global
properties of
regexp
are ignored when
performing the search. The
lastIndex
property of
regexp
is left unchanged.
Return
result
NOTE
The
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.
15.5.4.13
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 character position
start
and running to, but not
including, character position
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:
Call
CheckObjectCoercible
passing the
this
value as its argument.
Let
be the result of calling
ToString
, giving it the
this
value as its
argument.
Let
len
be the number of characters in
Let
intStart
be
ToInteger
start
).
If
end
is
undefined
, let
intEnd
be
len
; else let
intEnd
be
ToInteger
end
).
If
intStart
is negative, let
from
be max(
len
intStart
,0); else let
from
be
min(
intStart
len
).
If
intEnd
is negative, let
to
be max(
len
intEnd
,0); else let
to
be
min(
intEnd
len
).
Let
span
be max(
to
from
,0).
Return a String containing
span
consecutive characters from
beginning with the character at position
from
The
length
property of the
slice
method is
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.
15.5.4.14
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 a RegExp object (i.e., an object whose [[Class]] internal
property is
"RegExp"
; see
15.10
).
The value of
separator
may be an empty String, an empty regular expression, or a regular expression that can
match 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. (For example, if
separator
is the empty String, the String is split up into individual characters; the length of the result
array equals the length of the String, and each substring contains one character.) If
separator
is a regular
expression, only the first match at a given position of the
this
String is considered, even if backtracking could
yield a non-empty-substring match at that position. (For example,
"ab".split(/a*?/)
evaluates to the array
["a","b"]
, while
"ab".split(/a*/)
evaluates to the array
["","b"]
.)
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 a regular expression that 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,
"Aboldandcoded".split(/<(\/)?([^<>]+)>/)
evaluates to the array
["A", undefined, "B", "bold", "/", "B", "and", undefined,
"CODE", "coded", "/", "CODE", ""]
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.
When the
split
method is called, the following steps are taken:
Call
CheckObjectCoercible
passing the
this
value as its argument.
Let
be the result of calling
ToString
, giving it the
this
value as its
argument.
Let
be a new array created as if by the expression
new Array()
where
Array
is the
standard built-in constructor with that name.
Let
lengthA
be 0.
If
limit
is
undefined
, let
lim
= 2
32
–1; else let
lim
ToUint32
limit
).
Let
be the number of characters in
Let
= 0.
If
separator
is a RegExp object (its [[Class]] is
"RegExp"
), let
separator
otherwise let
ToString
separator
).
If
lim
= 0, return
If
separator
is
undefined
, then
Call the [[DefineOwnProperty]] internal method of
with arguments
Property Descriptor
{[[Value]]:
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Return
If
= 0, then
Call
SplitMatch
, 0,
) and let
be its MatchResult result.
If
is not
failure
, return
Call the [[DefineOwnProperty]] internal method of
with arguments
Property Descriptor
{[[Value]]:
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Return
Let
Repeat, while
Call
SplitMatch
S, q, R
) and let
be its MatchResult result.
If
is
failure
, then let
+1.
Else,
is not
failure
must be a State. Let
be
's
endIndex
and let
cap
be
's
captures
array.
If
, then let
+1.
Else,
Let
be a String value equal to the substring of
consisting of the characters at
positions
(inclusive) through
(exclusive).
Call the [[DefineOwnProperty]] internal method of
with arguments
ToString
lengthA
),
Property Descriptor
{[[Value]]:
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Increment
lengthA
by 1.
If
lengthA
lim
, return
Let p = e.
Let
= 0.
Repeat, while
is not equal to the number of elements in
cap
Let
+1.
Call the [[DefineOwnProperty]] internal method of
with arguments
ToString
lengthA
),
Property Descriptor
{[[Value]]:
cap
], [[Writable]]:
true
, [[Enumerable]]:
true
[[Configurable]]:
true
}, and
false
Increment
lengthA
by 1.
If
lengthA
lim
, return
Let
Let
be a String value equal to the substring of
consisting of the characters at positions
(inclusive) through
(exclusive).
Call the [[DefineOwnProperty]] internal method of
with arguments
ToString
lengthA
),
Property Descriptor
{[[Value]]:
[[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Return
The abstract operation
SplitMatch
takes three parameters, a String
, an integer
, and a String or RegExp
, and performs the following in order to return a MatchResult (
see 15.10.2.1
):
If
is a RegExp object (its [[Class]] is
"RegExp"
), then
Call the [[Match]] internal method of
giving it the arguments
and
, and return the
MatchResult result.
Type
) must be String. Let
be the number of characters in
Let
be the number of characters in
If
then return the MatchResult
failure
If there exists an integer
between 0 (inclusive) and
(exclusive) such that the character at
position
of
is different from the character at position
of
, then return
failure
Let
cap
be an empty array of captures (
see 15.10.2.1
).
Return the State (
cap
). (
see 15.10.2.1
The
length
property of the
split
method is
NOTE 1
The
split
method ignores the value of separator
.global
for
separators that are RegExp objects.
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.
15.5.4.15
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 character position
start
and running to, but not
including, character position
end
of the String (or through the end of the String is
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:
Call
CheckObjectCoercible
passing the
this
value as its argument.
Let
be the result of calling
ToString
, giving it the
this
value as its
argument.
Let
len
be the number of characters in
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 a String whose length is
to
from
, containing characters from
, namely the characters
with indices
from
through
to
−1, in ascending order.
The
length
property of the
substring
method is
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.
15.5.4.16
String.prototype.toLowerCase ( )
The following steps are taken:
Call
CheckObjectCoercible
passing the
this
value as its argument.
Let
be the result of calling
ToString
, giving it the
this
value as its
argument.
Let
be a String where each character of
is either the Unicode lowercase equivalent of the
corresponding character of
or the actual corresponding character of
if no Unicode lowercase
equivalent exists.
Return
For the purposes of this operation, the 16-bit code units of the Strings are treated as code points in the Unicode
Basic Multilingual Plane. Surrogate code points are directly transferred from
to
without any
mapping.
The result must be derived according to the case mappings in the Unicode character database (this
explicitly includes not only the UnicodeData.txt file, but also the SpecialCasings.txt file that accompanies it in Unicode
2.1.8 and later).
NOTE 1
The case mapping of some characters may produce multiple characters. 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.
15.5.4.17
String.prototype.toLocaleLowerCase ( )
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 1
The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything else.
NOTE 2
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.
15.5.4.18
String.prototype.toUpperCase ( )
This function behaves in exactly the same way as
String.prototype.toLowerCase
, except that characters are
mapped to their
uppercase
equivalents as specified in the Unicode Character Database.
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.
15.5.4.19
String.prototype.toLocaleUpperCase ( )
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 1
The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything else.
NOTE 2
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.
15.5.4.20
String.prototype.trim
( )
The following steps are taken:
Call
CheckObjectCoercible
passing the
this
value as its argument.
Let
be the result of calling
ToString
, giving it the
this
value as its
argument.
Let
be a String value that is a copy of
with both leading and trailing white space removed. The
definition of white space is the union of
WhiteSpace
and
LineTerminator
Return
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.
15.5.5
Properties of String
Instances
String instances inherit properties from the String prototype object and their [[Class]] internal property value is
"String"
. String instances also have a [[PrimitiveValue]] internal property, a
length
property,
and a set of enumerable properties with array index names.
The [[PrimitiveValue]] internal property is the String value represented by this String object. The array index named
properties correspond to the individual characters of the String value. A special [[GetOwnProperty]] internal method is used
to specify the number, values, and attributes of the array index named properties.
15.5.5.1
length
The number of characters in the String value represented by this String object.
Once a String object is created, this property is unchanging. It has the attributes { [[Writable]]:
false
[[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.5.5.2
[[GetOwnProperty]] ( P
String objects use a variation of the [[GetOwnProperty]] internal method used for other native ECMAScript objects (
8.12.1
). This special internal method provides access to named properties corresponding to the
individual characters of String objects.
Assume
is a String object and
is a String.
When the [[GetOwnProperty]] internal method of
is called with property name
, the following
steps are taken:
Let
desc
be the result of calling the default [[GetOwnProperty]] internal method (
8.12.1
) on
with argument
If
desc
is not
undefined
return
desc
If
ToString
abs
ToInteger
))) is not
the same value as
, return
undefined
Let
str
be the String value of the [[PrimitiveValue]] internal property of
Let
index
be
ToInteger
).
Let
len
be the number of characters in
str
If
len
index
, return
undefined
Let
resultStr
be a String of length 1, containing one character from
str
, specifically the character
at position
index
, where the first (leftmost) character in
str
is considered to be at position 0, the
next one at position 1, and so on.
Return a
Property Descriptor
{ [[Value]]:
resultStr
, [[Enumerable]]:
true
[[Writable]]:
false
, [[Configurable]]:
false
15.6
Boolean Objects
15.6.1
The Boolean Constructor Called
as a Function
When
Boolean
is called as a function rather than as a constructor, it performs a type conversion.
15.6.1.1
Boolean (value)
Returns a Boolean value (not a Boolean object) computed by
ToBoolean
value
15.6.2
The Boolean Constructor
When
Boolean
is called as part of a
new
expression it is a constructor: it initialises the
newly created object.
15.6.2.1
new Boolean (value)
The [[Prototype]] internal property of the newly constructed object is set to the original Boolean prototype object,
the one that is the initial value of
Boolean.prototype
15.6.3.1
).
The [[Class]] internal property of the newly constructed Boolean object is set to
"Boolean"
The [[PrimitiveValue]] internal property of the newly constructed Boolean object is set to
ToBoolean
value
The [[Extensible]] internal property of the newly constructed object is set to
true
15.6.3
Properties of the Boolean
Constructor
The value of the [[Prototype]] internal property of the Boolean constructor is the Function prototype object (
15.3.4
).
Besides the internal properties and the
length
property (whose value is
), the Boolean constructor
has the following property:
15.6.3.1
Boolean.prototype
The initial value of
Boolean.prototype
is the Boolean prototype object (
15.6.4
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.6.4
Properties of the Boolean
Prototype Object
The Boolean prototype object is itself a Boolean object (its [[Class]] is
"Boolean"
) whose value is
false
The value of the [[Prototype]] internal property of the Boolean prototype object is the standard built-in Object
prototype object (
15.2.4
).
15.6.4.1
Boolean.prototype.constructor
The initial value of
Boolean.prototype.constructor
is the built-in
Boolean
constructor.
15.6.4.2
Boolean.prototype.toString ( )
The following steps are taken:
Let
be the
this
value.
If
Type
) is Boolean, then let
be
Else if
Type
) is Object and the value of the [[Class]] internal property of
is
"Boolean"
, then let
be the value of the [[PrimitiveValue]] internal property of
Else throw a
TypeError
exception.
If
is
true
, then return
"true"
; else return
"false"
15.6.4.3
Boolean.prototype.valueOf ( )
The following steps are taken:
Let
be the
this
value.
If
Type
) is Boolean, then let
be
Else if
Type
) is Object and the value of the [[Class]] internal property of
is
"Boolean"
, then let
be the value of the [[PrimitiveValue]] internal property of
Else throw a
TypeError
exception.
Return
15.6.5
Properties of Boolean
Instances
Boolean instances inherit properties from the Boolean prototype object and their [[Class]] internal property value is
"Boolean"
. Boolean instances also have a [[PrimitiveValue]] internal property.
The [[PrimitiveValue]] internal property is the Boolean value represented by this Boolean object.
15.7
Number Objects
15.7.1
The Number Constructor Called
as a Function
When
Number
is called as a function rather than as a constructor, it performs a type conversion.
15.7.1.1
Number ( [ value ]
Returns a Number value (not a Number object) computed by
ToNumber
value
if
value
was supplied, else returns
+0
15.7.2
The Number Constructor
When
Number
is called as part of a
new
expression it is a constructor: it initialises the newly
created object.
15.7.2.1
new Number ( [ value ]
The [[Prototype]] internal property of the newly constructed object is set to the original Number prototype object, the
one that is the initial value of
Number.prototype
15.7.3.1
).
The [[Class]] internal property of the newly constructed object is set to
"Number"
The [[PrimitiveValue]] internal property of the newly constructed object is set to
ToNumber
value
if
value
was supplied, else to +0.
The [[Extensible]] internal property of the newly constructed object is set to
true
15.7.3
Properties of the Number
Constructor
The value of the [[Prototype]] internal property of the Number constructor is the Function prototype object (
15.3.4
).
Besides the internal properties and the
length
property (whose value is
), the Number constructor
has the following properties:
15.7.3.1
Number.prototype
The initial value of
Number.prototype
is the Number prototype object (
15.7.4
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.7.3.2
Number.MAX_VALUE
The value of
Number.MAX_VALUE
is the largest positive finite value of the Number type, which is
approximately
1.7976931348623157 × 10
308
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.7.3.3
Number.MIN_VALUE
The value of
Number.MIN_VALUE
is the smallest positive value of the Number type, which is approximately
5 × 10
‑324
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.7.3.4
Number.NaN
The value of
Number.NaN
is
NaN
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.7.3.5
Number.NEGATIVE_INFINITY
The value of Number.NEGATIVE_INFINITY is −∞.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.7.3.6
Number.POSITIVE_INFINITY
The value of Number.POSITIVE_INFINITY is +∞.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.7.4
Properties of the Number
Prototype Object
The Number prototype object is itself a Number object (its [[Class]] is
"Number"
) whose value is +0.
The value of the [[Prototype]] internal property of the Number prototype object is the standard built-in Object prototype
object (
15.2.4
).
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 for which the value of the [[Class]] internal property is
"Number"
In the following descriptions of functions that are properties of the Number prototype object, the phrase “this
Number object” refers to either the object that is the
this
value for the invocation of the function or, if
Type
this
value) is Number, an object that is created as if by the expression
new Number(
this
value
where
Number
is the standard
built-in constructor with that name. Also, the phrase “this Number value” refers to either the Number value
represented by this Number object, that is, the value of the [[PrimitiveValue]] internal property of this Number object or
the
this
value if its type is Number. A
TypeError
exception is thrown if the
this
value is neither an
object for which the value of the [[Class]] internal property is
"Number"
or a value whose type is Number.
15.7.4.1
Number.prototype.constructor
The initial value of
Number.prototype.constructor
is the built-in
Number
constructor.
15.7.4.2
Number.prototype.toString ( [ radix ] )
The optional
radix
should be an integer value in the inclusive range
to
36
. If
radix
not present or is
undefined
the Number
10
is used as the value of
radix
If
ToInteger
radix
is the Number
10
then this Number value is given as an argument to the
ToString
abstract operation; the resulting String value is returned.
If
ToInteger
radix
is not an integer
between 2 and 36 inclusive throw a
RangeError
exception. If
ToInteger
radix
is an integer from 2 to 36, but not 10, the result is a String
representation of this Number value using the specified radix. Letters
are used for digits
with values 10 through 35. The precise algorithm is implementation-dependent if the radix is not 10, however the algorithm
should be a generalisation of that specified in
9.8.1
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.
15.7.4.3
Number.prototype.toLocaleString()
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
NOTE
The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything else.
15.7.4.4
Number.prototype.valueOf
( )
Returns this Number value.
The
valueOf
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.
15.7.4.5
Number.prototype.toFixed
(fractionDigits)
Return 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. Specifically, perform the
following steps:
Let
be
ToInteger
fractionDigits
). (If
fractionDigits
is
undefined
, this step produces the value
).
If
< 0 or
> 20, throw a
RangeError
exception.
Let
be this Number value.
If
is
NaN
, return the String
"NaN"
Let
be the empty String.
If
< 0, then
Let s be "
".
Let x = –x.
If
≥ 10
21
, then
Let
ToString
).
Else,
< 10
21
Let
be an integer for which the exact mathematical value of
÷ 10
is as close to zero as possible. If there are two such
, pick the larger
If
= 0, let
be the String
"0"
. Otherwise, let
be the String consisting
of the digits of the decimal representation of
(in order, with no leading zeroes).
If
≠ 0, then
Let
be the number of characters in
If
, then
Let
be the String consisting of
+1–
occurrences of the character
’.
Let
be the concatenation of Strings
and
Let
+ 1.
Let
be the first
characters of
, and let
be the remaining
characters of
Let
be the concatenation of the three Strings
"."
, and
Return the concatenation of the Strings
and
The
length
property of the
toFixed
method is
If the
toFixed
method is called with more than one argument, then the behaviour is undefined (
see clause 15
).
An implementation is permitted to extend the behaviour of
toFixed
for values of
fractionDigits
less than 0 or greater than 20. In this case
toFixed
would not necessarily throw
RangeError
for such
values.
NOTE
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"
15.7.4.6
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
be this Number value.
Let
be
ToInteger
fractionDigits
).
If
is
NaN
, return the String
"NaN"
Let
be the empty String.
If
0, then
Let
be
"-"
Let
= –
If
= +∞, then
Return the concatenation of the Strings
and
"Infinity"
If
fractionDigits
is not
undefined
and (
< 0 or
> 20), throw a
RangeError
exception.
If
= 0, then
Let
= 0.
Let
be the String consisting of
+1 occurrences of the character ‘
’.
Let
= 0.
Else,
≠ 0
If fractionDigits is not
undefined
, then
Let
and
be integers such that 10
10
+1
and for which the exact mathematical value of
10
is as close to zero as possible. If there are two such
sets of
and
, pick the
and
for which
10
is larger.
Else,
fractionDigits
is
undefined
Let
, and
be integers such that
≥ 0, 10
< 10
+1
, the number value for n × 10
is
, and
is as small as possible. Note that the decimal representation of
has
+1 digits,
is not divisible by 10, and the least significant digit of
is not
necessarily uniquely determined by these criteria.
Let
be the String consisting of the digits of the decimal representation of
(in order, with no
leading zeroes).
If
≠ 0, then
Let
be the first character of
, and let
be the remaining
characters of
Let
be the concatenation of the three Strings
"."
, and
If
= 0, then
Let
"+".
Let
"0".
Else
If
> 0, then let
"+".
Else,
≤ 0
Let
"-"
Let
= –
Let
be the String consisting of the digits of the decimal representation of
(in order, with no
leading zeroes).
Let
be the concatenation of the four Strings
"e"
, and
Return the concatenation of the Strings
and
The
length
property of the
toExponential
method is
If the
toExponential
method is called with more than one argument, then the behaviour is undefined (
see clause 15
).
An implementation is permitted to extend the behaviour of
toExponential
for values of
fractionDigits
less than 0 or greater than 20. In this case
toExponential
would not necessarily
throw
RangeError
for such values.
NOTE
For implementations that provide more accurate conversions than required by the rules
above, it is recommended that the following alternative version of step 9.b.i be used as a guideline:
Let
, and
be integers such that
≥ 0, 10
≤ n <
10
+1
, the number value for n × 10
is x, and
is
as small as possible. If there are multiple possibilities for
, choose the value of
for which
× 10
is closest in value to
. If there are two such
possible values of
, choose the one that is even.
15.7.4.7
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
9.8.1
) instead.
Specifically, perform the following steps:
Let
be this Number value.
If
precision
is
undefined
, return
ToString
).
Let
be
ToInteger
precision
).
If
is
NaN
, return the String
"NaN"
Let
be the empty String.
If
< 0, then
Let
be
"-"
Let
= –
If
= +∞, then
Return the concatenation of the Strings
and
"Infinity"
If
< 1 or
> 21, throw a
RangeError
exception.
If
= 0, then
Let
be the String consisting of
occurrences of the character ‘
’.
Let
= 0.
Else
≠ 0,
Let
and
be integers such that 10
–1
10
and for which the exact mathematical value of
10
+1
is as close to zero as possible. If there are two such
sets of
and
, pick the
and
for which
10
+1
is larger.
Let
be the String consisting of the digits of the decimal representation of
(in order, with no
leading zeroes).
If
< –6 or
, then
Let
be the first character of
, and let
be the remaining
–1
characters of
Let
be the concatenation of the three Strings
"."
, and
If
= 0, then
Let
"+"
and
"0"
Else
≠ 0,
If
> 0, then
Let
"+"
Else
< 0,
Let
"-"
Let
= –
Let
be the String consisting of the digits of the decimal representation of
(in order,
with no leading zeroes).
Let
be the concatenation of the five Strings
"e"
, and
If
–1, then return the concatenation of the Strings
and
If
≥ 0, then
Let
be the concatenation of the first
+1 characters of
, the character ‘
’, and the remaining
– (
+1) characters of
Else
< 0,
Let
be the concatenation of the String
"0."
, –(
+1) occurrences of the
character ‘
’, and the String
Return the concatenation of the Strings
and
The
length
property of the
toPrecision
method is
If the
toPrecision
method is called with more than one argument, then the behaviour is undefined (
see clause 15
).
An implementation is permitted to extend the behaviour of
toPrecision
for values of
precision
less than 1 or greater than 21. In this case
toPrecision
would not necessarily throw
RangeError
for
such values.
15.7.5
Properties of Number
Instances
Number instances inherit properties from the Number prototype object and their [[Class]] internal property value is
"Number"
. Number instances also have a [[PrimitiveValue]] internal property.
The [[PrimitiveValue]] internal property is the Number value represented by this Number object.
15.8
The Math Object
The Math object is a single object that has some named properties, some of which are functions.
The value of the [[Prototype]] internal property of the Math object is the standard built-in Object prototype object (
15.2.4
). The value of the [[Class]] internal property of the Math object is
"Math"
The Math object does not have a [[Construct]] internal property; it is not possible to use the Math object as a constructor
with the
new
operator.
The Math object does not have a [[Call]] internal property; it is not possible to invoke the Math object as a function.
NOTE
In this specification, the phrase “the Number value for
” has a
technical meaning defined in
8.5
15.8.1
Value Properties of the Math
Object
15.8.1.1
The Number value for
, the base of the natural logarithms, which is approximately
2.7182818284590452354.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.8.1.2
LN10
The Number value for the natural logarithm of 10, which is approximately 2.302585092994046.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.8.1.3
LN2
The Number value for the natural logarithm of 2, which is approximately 0.6931471805599453.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.8.1.4
LOG2E
The Number value for the base-
logarithm of
, 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
15.8.1.5
LOG10E
The Number value for the base-
10
logarithm of
, 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
15.8.1.6
PI
The Number value for
, 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
}.
15.8.1.7
SQRT1_2
The Number value for the square root of
, 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
15.8.1.8
SQRT2
The Number value for the square root of
, which is approximately
1.4142135623730951
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.8.2
Function Properties of the
Math Object
Each of the following
Math
object functions applies the
ToNumber
abstract operator to
each of its arguments (in left-to-right order if there is more than one) and then performs a computation on the resulting
Number value(s).
In the function descriptions below, the symbols NaN, −0, +0, −∞ and +∞ refer to the Number values
described in
8.5
NOTE
The behaviour of the functions
acos
asin
atan
atan2
cos
exp
log
pow
sin
sqrt
, and
tan
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 arithmetic contained in
fdlibm
, the freely
distributable mathematical library from Sun Microsystems (
15.8.2.1
abs (x)
Returns the absolute value of
; the result has the same magnitude as
but has positive
sign.
If
is NaN, the result is NaN.
If
is −0, the result is +0.
If
is −∞, the result is +∞.
15.8.2.2
acos (x)
Returns an implementation-dependent approximation to the arc cosine of
. The result is expressed in radians
and ranges from
+0
to
+π
If
is NaN, the result is NaN.
If
is greater than 1, the result is NaN.
If
is less than
1, the result is NaN.
If
is exactly 1, the result is +0.
15.8.2.3
asin (x)
Returns an implementation-dependent approximation to the arc sine of
. The result is expressed in radians
and ranges from
−π/2
to
+π/2
If
is NaN, the result is NaN.
If
is greater than 1, the result is NaN.
If
is less than –1, the result is NaN.
If
is +0, the result is +0.
If
is −0, the result is −0.
15.8.2.4
atan (x)
Returns an implementation-dependent approximation to the arc tangent of
. The result is expressed in
radians and ranges from
−π/2
to
+π/2
If
is NaN, the result is NaN.
If
is +0, the result is +0.
If
is −0, the result is −0.
If
is +∞, the result is an implementation-dependent approximation to +π/2.
If
is −∞, the result is an implementation-dependent approximation to −π/2.
15.8.2.5
atan2 (y, x)
Returns an implementation-dependent approximation to the arc tangent of the quotient
of the arguments
and
, where the signs of
and
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
be first and the argument named
be
second. The result is expressed in radians and ranges from
−π
to
+π
If either
or
is NaN, the result is NaN.
If
>0 and
is +0, the result is an implementation-dependent approximation to +π/2.
If
>0 and
is −0, the result is an implementation-dependent approximation to +π/2.
If
is +0 and
>0, the result is +0.
If
is +0 and
is +0, the result is +0.
If
is +0 and
is −0, the result is an implementation-dependent approximation to +π.
If
is +0 and
<0, the result is an implementation-dependent approximation to +π.
If
is −0 and
>0, the result is −0.
If
is −0 and
is +0, the result is −0.
If
is −0 and
is −0, the result is an implementation-dependent approximation to
−π.
If
is −0 and
<0, the result is an implementation-dependent approximation to
−π.
If
<0 and
is +0, the result is an implementation-dependent approximation to −π/2.
If
<0 and
is −0, the result is an implementation-dependent approximation to
−π/2.
If
>0 and
is finite and
is +∞, the result is +0.
If
>0 and
is finite and
is −∞, the result if an implementation-dependent
approximation to +π.
If
<0 and
is finite and
is +∞, the result is −0.
If
<0 and
is finite and
is −∞, the result is an implementation-dependent
approximation to −π.
If
is +∞ and
is finite, the result is an implementation-dependent approximation to
+π/2.
If
is −∞ and
is finite, the result is an implementation-dependent approximation to
−π/2.
If
is +∞ and
is +∞, the result is an implementation-dependent approximation to
+π/4.
If
is +∞ and
is −∞, the result is an implementation-dependent approximation to
+3π/4.
If
is −∞ and
is +∞, the result is an implementation-dependent approximation to
−π/4.
If
is −∞ and
is −∞, the result is an implementation-dependent
approximation to −3π/4.
15.8.2.6
ceil (x)
Returns the smallest (closest to
−∞
) Number value that is not less than
and is equal to
a mathematical integer. If
is already an integer, the result is
If
is NaN, the result is NaN.
If
is +0, the result is +0.
If
is −0, the result is −0.
If
is +∞, the result is +∞.
If
is −∞, the result is −∞.
If
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)
15.8.2.7
cos (x)
Returns an implementation-dependent approximation to the cosine of
. The argument is expressed in
radians.
If
is NaN, the result is NaN.
If
is +0, the result is 1.
If
is −0, the result is 1.
If
is +∞, the result is NaN.
If
is −∞, the result is NaN.
15.8.2.8
exp (x)
Returns an implementation-dependent approximation to the exponential function of
raised to
the power of
, where
is the base of the natural logarithms).
If
is NaN, the result is NaN.
If
is +0, the result is 1.
If
is −0, the result is 1.
If
is +∞, the result is +∞.
If
is −∞, the result is +0.
15.8.2.9
floor (x)
Returns the greatest (closest to
+∞
) Number value that is not greater than
and is equal to a
mathematical integer. If
is already an integer, the result is
If
is NaN, the result is NaN.
If
is +0, the result is +0.
If
is −0, the result is −0.
If
is +∞, the result is +∞.
If
is −∞, the result is −∞.
If
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)
15.8.2.10
log (x)
Returns an implementation-dependent approximation to the natural logarithm of
If
is NaN, the result is NaN.
If
is less than 0, the result is NaN.
If
is +0 or −0, the result is −∞.
If
is 1, the result is +0.
If
is +∞, the result is +∞.
15.8.2.11
max ( [ value1 [ ,
value2 [ , … ] ] ] )
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 as in
11.8.5
except that
+0 is considered to be larger than −0.
The
length
property of the
max
method is
15.8.2.12
min ( [ value1 [ ,
value2 [ , … ] ] ] )
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 as in
11.8.5
except that
+0 is considered to be larger than −0.
The
length
property of the
min
method is
15.8.2.13
pow (x, y)
Returns an implementation-dependent approximation to the result of raising
to the power
If
is NaN, the result is NaN.
If
is +0, the result is 1, even if
is NaN.
If
is −0, the result is 1, even if
is NaN.
If
is NaN and
is nonzero, the result is NaN.
If
abs
)>1 and
is +∞, the result is +∞.
If
abs
)>1 and
is −∞, the result is +0.
If
abs
)==1 and
is +∞, the result is NaN.
If
abs
)==1 and
is −∞, the result is NaN.
If
abs
)<1 and
is +∞, the result is +0.
If
abs
)<1 and
is −∞, the result is +∞.
If
is +∞ and
>0, the result is +∞.
If
is +∞ and
<0, the result is +0.
If
is −∞ and
>0 and
is an odd integer, the result is −∞.
If
is −∞ and
>0 and
is not an odd integer, the result is +∞.
If
is −∞ and
<0 and
is an odd integer, the result is −0.
If
is −∞ and
<0 and
is not an odd integer, the result is +0.
If
is +0 and
>0, the result is +0.
If
is +0 and
<0, the result is +∞.
If
is −0 and
>0 and
is an odd integer, the result is −0.
If
is −0 and
>0 and
is not an odd integer, the result is +0.
If
is −0 and
<0 and
is an odd integer, the result is −∞.
If
is −0 and
<0 and
is not an odd integer, the result is +∞.
If
<0 and
is finite and
is finite and
is not an integer, the result is
NaN.
15.8.2.14
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.
15.8.2.15
round (x)
Returns the Number value that is closest to
and is equal to a mathematical integer. If two integer Number
values are equally close to
, then the result is the Number value that is closer to
+∞
. If
is already an integer, the result is
If
is NaN, the result is NaN.
If
is +0, the result is +0.
If
is −0, the result is −0.
If
is +∞, the result is +∞.
If
is −∞, the result is −∞.
If
is greater than 0 but less than 0.5, the result is +0.
If
is less than 0 but greater than or equal to -0.5, the result is −0.
NOTE 1
Math.round(3.5)
returns
, but
Math.round(–3.5)
returns
–3
NOTE 2
The value of
Math.round(x)
is the same as the value of
Math.
floor
(x+0.5)
, except when
is
−0
or is less than
but greater than or equal to
-0.5
; for these cases
Math.round(x)
returns
−0
, but
Math.
floor
(x+0.5)
returns
+0
15.8.2.16
sin (x)
Returns an implementation-dependent approximation to the sine of
. The argument is expressed in
radians.
If
is NaN, the result is NaN.
If
is +0, the result is +0.
If
is −0, the result is −0.
If
is +∞ or −∞, the result is NaN.
15.8.2.17
sqrt (x)
Returns an implementation-dependent approximation to the square root of
If
is NaN, the result is NaN.
If
is less than 0, the result is NaN.
If
is +0, the result is +0.
If
is −0, the result is −0.
If
is +∞, the result is +∞.
15.8.2.18
tan (x)
Returns an implementation-dependent approximation to the tangent of
. The argument is expressed in
radians.
If
is NaN, the result is NaN.
If
is +0, the result is +0.
If
is −0, the result is −0.
If
is +∞ or −∞, the result is NaN.
15.9
Date Objects
15.9.1
Overview of Date Objects and
Definitions of Abstract Operators
The following functions are abstract operations that operate on time values (defined in
15.9.1.1
). Note that, in every case, if any argument to one of these functions is
NaN
, the
result will be
NaN
15.9.1.1
Time Values and Time
Range
A Date object contains a Number indicating a particular instant in time to within a millisecond. Such a Number is
called a
time value
. A time value may also be
NaN
, indicating that the Date object does not represent a
specific instant of time.
Time is measured in ECMAScript in milliseconds since 01 January, 1970 UTC. In time values leap seconds are ignored. It
is assumed that there are exactly 86,400,000 milliseconds per day. ECMAScript Number values can represent all integers
from –9,007,199,254,740,992 to 9,007,199,254,740,992; this range suffices to measure times to millisecond precision
for any instant that is within approximately 285,616 years, either forward or backward, from 01 January, 1970 UTC.
The actual range of times supported by ECMAScript Date objects is slightly smaller: exactly –100,000,000 days to
100,000,000 days measured relative to midnight at the beginning of 01 January, 1970 UTC. This gives a range of
8,640,000,000,000,000 milliseconds to either side of 01 January, 1970 UTC.
The exact moment of midnight at the beginning of 01 January, 1970 UTC is represented by the value
+0
15.9.1.2
Day Number and Time
within Day
A given
time value
belongs to day number
Day(
) =
floor
/ msPerDay)
where the number of milliseconds per day is
msPerDay = 86400000
The remainder is called the time within the day:
TimeWithinDay(
) =
modulo
msPerDay
15.9.1.3
Year Number
ECMAScript uses an extrapolated Gregorian system to map a day number to a year number and to determine the month and
date within that year. In this system, leap years are precisely those which are (divisible by
) and ((not divisible by
100
) or (divisible by
400
)). The number of days in year number
is therefore
defined by
DaysInYear(
) = 365 if (
modulo
4) ≠ 0
= 366 if (
modulo
4) = 0 and (y
modulo
100) ≠ 0
= 365 if (
modulo
100) = 0 and (y
modulo
400) ≠ 0
= 366 if (
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
is given by:
DayFromYear(
) = 365 × (
−1970) +
floor
((
−1969)/4) −
floor
((
−1901)/100) +
floor
((
−1601)/400)
The
time value
of the start of a year is:
TimeFromYear(
) =
msPerDay
× DayFromYear(
time value
determines a year by:
YearFromTime(
) = the largest integer
(closest to positive infinity) such that TimeFromYear(
) ≤
The leap-year function is 1 for a time within a leap year and otherwise is zero:
InLeapYear(
) = 0 if DaysInYear(YearFromTime(
)) = 365
= 1 if DaysInYear(YearFromTime
(t
)) = 366
15.9.1.4
Month Number
Months are identified by an integer in the range
to
11
, inclusive. The mapping MonthFromTime(
) from a
time value
to a month number is defined by:
MonthFromTime(
) = 0 if 0 ≤ DayWithinYear(
) < 31
= 1 if 31 ≤ DayWithinYear (
) < 59+
InLeapYear
= 2 if 59+
InLeapYear
) ≤ DayWithinYear (
) < 90+
InLeapYear
= 3 if 90+
InLeapYear
) ≤ DayWithinYear (
) < 120+
InLeapYear
= 4 if 120+
InLeapYear
) ≤ DayWithinYear (
) < 151+
InLeapYear
= 5 if 151+
InLeapYear
) ≤ DayWithinYear (
) < 181+
InLeapYear
= 6 if 181+
InLeapYear
) ≤ DayWithinYear (
) < 212+
InLeapYear
= 7 if 212+
InLeapYear
) ≤ DayWithinYear (
) < 243+
InLeapYear
= 8 if 243+
InLeapYear
) ≤ DayWithinYear (
) < 273+
InLeapYear
= 9 if 273+
InLeapYear
) ≤ DayWithinYear (
) < 304+
InLeapYear
= 10 if 304+
InLeapYear
) ≤ DayWithinYear (
) < 334+
InLeapYear
= 11 if 334+
InLeapYear
) ≤ DayWithinYear (
) < 365+
InLeapYear
where
DayWithinYear(
) =
Day
)−DayFromYear(
YearFromTime
t)
A month value of
specifies January;
specifies February;
specifies March;
specifies April;
specifies May;
specifies June;
specifies July;
specifies August;
specifies September;
specifies October;
10
specifies November; and
11
specifies December. Note that
MonthFromTime(0) = 0
, corresponding to Thursday, 01 January, 1970.
15.9.1.5
Date Number
A date number is identified by an integer in the range
through
31
, inclusive. The mapping DateFromTime(
) from a
time value
to a month number is defined by:
DateFromTime(
) =
DayWithinYear
)+1 if
MonthFromTime
)=0
DayWithinYear
)−30 if
MonthFromTime
)=1
DayWithinYear
)−58−
InLeapYear
) if
MonthFromTime
)=2
DayWithinYear
)−89−
InLeapYear
) if
MonthFromTime
)=3
DayWithinYear
)−119−
InLeapYear
) if
MonthFromTime
)=4
DayWithinYear
)−150−
InLeapYear
) if
MonthFromTime
)=5
DayWithinYear
)−180−
InLeapYear
) if
MonthFromTime
)=6
DayWithinYear
)−211−
InLeapYear
) if
MonthFromTime
)=7
DayWithinYear
)−242−
InLeapYear
) if
MonthFromTime
)=8
DayWithinYear
)−272−
InLeapYear
) if
MonthFromTime
)=9
DayWithinYear
)−303−
InLeapYear
) if
MonthFromTime
)=10
DayWithinYear
)−333−
InLeapYear
) if
MonthFromTime
)=11
15.9.1.6
Week Day
The weekday for a particular
time value
is defined as
WeekDay(
) = (
Day
) + 4)
modulo
A weekday value of
specifies Sunday;
specifies Monday;
specifies Tuesday;
specifies Wednesday;
specifies Thursday;
specifies Friday; and
specifies Saturday. Note that
WeekDay(0) = 4
, corresponding to Thursday, 01 January, 1970.
15.9.1.7
Local Time Zone
Adjustment
An implementation of ECMAScript is expected to determine the local time zone adjustment. The local time zone adjustment
is a value LocalTZA measured in milliseconds which when added to UTC represents the local
standard
time. Daylight
saving time is
not
reflected by LocalTZA. The value LocalTZA does not vary with time but depends only on the
geographic location.
15.9.1.8
Daylight Saving Time
Adjustment
An implementation of ECMAScript is expected to determine the daylight saving time algorithm. The algorithm to determine
the daylight saving time adjustment DaylightSavingTA(
), measured in milliseconds, must depend only on four
things:
(1) the time since the beginning of the year
TimeFromYear
YearFromTime
))
(2) whether
is in a leap year
InLeapYear
(3) the week day of the beginning of the year
WeekDay
TimeFromYear
YearFromTime
)))
and (4) the geographic location.
The implementation of ECMAScript should not try to determine whether the exact time was subject to daylight saving
time, but just whether daylight saving time would have been in effect if the current daylight saving time algorithm had
been used at the time. This avoids complications such as taking into account the years that the locale observed daylight
saving time year round.
If the host environment provides functionality for determining daylight saving time, the implementation of ECMAScript
is free to map the year in question to an equivalent year (same leap-year-ness and same starting week day for the year)
for which the host environment provides daylight saving time information. The only restriction is that all equivalent
years should produce the same result.
15.9.1.9
Local Time
Conversion from UTC to local time is defined by
LocalTime(
) =
LocalTZA
DaylightSavingTA
Conversion from local time to UTC is defined by
UTC(
) =
LocalTZA
DaylightSavingTA
LocalTZA
Note that
UTC(LocalTime(
))
is not necessarily always equal to
15.9.1.10
Hours, Minutes,
Second, and Milliseconds
The following functions are useful in decomposing time values:
HourFromTime(
) =
floor
/ msPerHour)
modulo
HoursPerDay
MinFromTime(
) =
floor
/ msPerMinute)
modulo
MinutesPerHour
SecFromTime(
) =
floor
/ msPerSecond)
modulo
SecondsPerMinute
msFromTime(
) = t
modulo
msPerSecond
where
HoursPerDay = 24
MinutesPerHour = 60
SecondsPerMinute = 60
msPerSecond = 1000
msPerMinute = 60000 = msPerSecond × SecondsPerMinute
msPerHour = 3600000 = msPerMinute × MinutesPerHour
15.9.1.11
MakeTime (hour, min,
sec, ms)
The operator MakeTime calculates a number of milliseconds from its four arguments, which must be ECMAScript Number
values. This operator functions as follows:
If
hour
is not finite or
min
is not finite or
sec
is not finite or
ms
is not finite,
return
NaN
Let
be
ToInteger
hour
).
Let
be
ToInteger
min
).
Let
be
ToInteger
sec
).
Let
milli
be
ToInteger
ms
).
Let
be
msPerHour
msPerMinute
msPerSecond
milli
, performing the arithmetic according to IEEE 754
rules (that is, as if using the ECMAScript operators
and
).
Return
15.9.1.12
MakeDay (year, month,
date)
The operator MakeDay calculates a number of days from its three arguments, which must be ECMAScript Number values. This
operator functions as follows:
If
year
is not finite or
month
is not finite or
date
is not finite, return
NaN
Let
be
ToInteger
year
).
Let
be
ToInteger
month
).
Let
dt
be
ToInteger
date
).
Let
ym
be
floor
/12).
Let
mn
be
modulo
12.
Find a value
such that
YearFromTime
==
ym
and
MonthFromTime
==
mn
and
DateFromTime
==
1; but if this is not possible (because some
argument is out of range), return
NaN
Return
Day
) +
dt
− 1.
15.9.1.13
MakeDate (day,
time)
The operator MakeDate calculates a number of milliseconds from its two arguments, which must be ECMAScript Number
values. This operator functions as follows:
If
day
is not finite or
time
is not finite, return
NaN
Return
day
msPerDay
time
15.9.1.14
TimeClip (time)
The operator TimeClip calculates a number of milliseconds from its argument, which must be an ECMAScript Number value.
This operator functions as follows:
If
time
is not finite, return
NaN
If
abs
time
) > 8.64
10
15
, return
NaN
Return an implementation-dependent choice of either
ToInteger
time
) or
ToInteger
time
) + (
+0
). (Adding a positive zero converts
−0
to
+0
.)
NOTE
The point of step 3 is that an implementation is permitted a choice of internal
representations of time values, for example as a 64-bit signed integer or as a 64-bit floating-point value. Depending on
the implementation, this internal representation may or may not distinguish
−0
and
+0
15.9.1.15
Date Time String
Format
ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 Extended
Format. The format is as follows:
YYYY-MM-DDTHH:mm:ss.sss
Where the fields are as follows:
YYYY
is the decimal digits of the year 0000 to 9999 in the
Gregorian calendar.
” (hyphen) appears literally twice in the string.
MM
is the month of the year from 01 (January) to 12
(December).
DD
is the day of the month from 01 to 31.
” 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.
is the time zone offset specified as
” (for UTC) or either
” or “
” followed by a time expression
HH:mm
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 time zone offset appended:
THH:mm
THH:mm:ss
THH:mm:ss.sss
All numbers must be base
10
. If the
MM
or
DD
fields are absent “
01
” is used as the value. If the
HH
mm
, or
ss
fields are absent “
00
” is used as the value and the value of
an absent
sss
field is “
000
”. The value of an absent time zone offset is
”.
Illegal values (out-of-bounds as well as syntax errors) in a format string means that the format string 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
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, ISO 8601 and this format specifies numeric representations of date and time.
15.9.1.15.1
Extended
years
ECMAScript requires the ability to specify
digit years (extended
years); approximately
285,426
years, either forward or backward, from
01 January, 1970 UTC. To represent years before
or after
9999
, ISO 8601 permits the expansion of the year representation, but only by
prior agreement between the sender and the receiver. In the simplified ECMAScript format such an expanded year
representation shall have
extra year digits and is always prefixed
with a + or – sign. The year
is considered positive and hence
prefixed with a + sign.
NOTE
Examples of extended years:
-283457-03-21T15:00:59.008Z 283458 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.
+287396-10-12T08:59:00.992Z 287396 A.D.
15.9.2
The Date Constructor Called as
a Function
When
Date
is called as a function rather than as a constructor, it returns a String representing the current
time (UTC).
NOTE
The function call
Date(
is not equivalent to the object creation expression
new Date(
with the same arguments.
15.9.2.1
Date ( [ year [, month
[, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] ] ] )
All of the arguments are optional; any arguments supplied are accepted but are completely ignored. A String is created
and returned as if by the expression
(new Date()).toString()
where
Date
is the standard built-in
constructor with that name and
toString
is the standard built-in method
Date.prototype.toString
15.9.3
The Date Constructor
When
Date
is called as part of a
new
expression, it is a constructor: it initialises the newly
created object.
15.9.3.1
new Date (year, month [,
date [, hours [, minutes [, seconds [, ms ] ] ] ] ] )
When
Date
is called with two to seven arguments, it computes the date from
year
month
, and
(optionally)
date
hours
minutes
seconds
and
ms
The [[Prototype]] internal property of the newly constructed object is set to the original Date prototype object, the
one that is the initial value of
Date.prototype
15.9.4.1
).
The [[Class]] internal property of the newly constructed object is set to
"Date"
The [[Extensible]] internal property of the newly constructed object is set to
true
The [[PrimitiveValue]] internal property of the newly constructed object is set as follows:
Let
be
ToNumber
year
).
Let
be
ToNumber
month
).
If
date
is supplied then let
dt
be
ToNumber
date
); else let
dt
be
If
hours
is supplied then let
be
ToNumber
hours
); else let
be
If
minutes
is supplied then let
min
be
ToNumber
minutes
); else let
min
be
If
seconds
is supplied then let
be
ToNumber
seconds
); else let
be
If
ms
is supplied then let
milli
be
ToNumber
ms
); else let
milli
be
If
is not
NaN
and 0 ≤
ToInteger
) ≤ 99, then let
yr
be 1900+
ToInteger
); otherwise, let
yr
be
Let
finalDate
be
MakeDate
MakeDay
yr
dt
),
MakeTime
min
milli
)).
Set the [[PrimitiveValue]] internal property of the newly constructed object to
TimeClip
UTC
finalDate
)).
15.9.3.2
new Date (value)
The [[Prototype]] internal property of the newly constructed object is set to the original Date prototype object, the
one that is the initial value of
Date.prototype
15.9.4.1
).
The [[Class]] internal property of the newly constructed object is set to
"Date"
The [[Extensible]] internal property of the newly constructed object is set to
true
The [[PrimitiveValue]] internal property of the newly constructed object is set as follows:
Let
be
ToPrimitive
value
).
If
Type
) is String, then
Parse
as a date, in exactly the same manner as for the
parse
method (
15.9.4.2
); let
be the
time value
for this
date.
Else, let
be
ToNumber
).
Set the [[PrimitiveValue]] internal property of the newly constructed object to
TimeClip
) and return.
15.9.3.3
new Date ( )
The [[Prototype]] internal property of the newly constructed object is set to the original Date prototype object, the
one that is the initial value of
Date.prototype
15.9.4.1
).
The [[Class]] internal property of the newly constructed object is set to
"Date"
The [[Extensible]] internal property of the newly constructed object is set to
true
The [[PrimitiveValue]] internal property of the newly constructed object is set to the
time
value
(UTC) identifying the current time.
15.9.4
Properties of the Date
Constructor
The value of the [[Prototype]] internal property of the Date constructor is the Function prototype object (
15.3.4
).
Besides the internal properties and the
length
property (whose value is
), the Date
constructor has the following properties:
15.9.4.1
Date.prototype
The initial value of
Date.prototype
is the built-in Date prototype object (
15.9.5
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.9.4.2
Date.parse (string)
The
parse
function applies the
ToString
operator to its argument and 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 format of the String according to the rules called
out in Date Time String Format (
15.9.1.15
). If the String does not conform to that format the
function may fall back to any implementation-specific heuristics or implementation-specific date formats. Unrecognisable
Strings or dates containing illegal element values in the format String shall cause
Date.parse
to return
NaN
If
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:
.valueOf()
Date.parse(
.toString())
Date.parse(
.toUTCString())
Date.parse(
.toISOString())
However, the expression
Date.parse(
.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 (
15.9.1.15
) and that could not be produced in that implementation by the
toString
or
toUTCString
method.
15.9.4.3
Date.UTC (year, month [,
date [, hours [, minutes [, seconds [, ms ] ] ] ] ] )
When the
UTC
function is called with fewer than two arguments, the behaviour is implementation-dependent.
When the
UTC
function is called with two to seven arguments, it computes the date from
year
month
and (optionally)
date
hours
minutes
seconds
and
ms
. The following steps are taken:
Let
be
ToNumber
year
).
Let
be
ToNumber
month
).
If
date
is supplied then let
dt
be
ToNumber
date
); else let
dt
be
If
hours
is supplied then let
be
ToNumber
hours
); else let
be
If
minutes
is supplied then let
min
be
ToNumber
minutes
); else let
min
be
If
seconds
is supplied then let
be
ToNumber
seconds
); else let
be
If
ms
is supplied then let
milli
be
ToNumber
ms
); else let
milli
be
If
is not
NaN
and 0 ≤
ToInteger
) ≤ 99, then let
yr
be 1900+
ToInteger
); otherwise, let
yr
be
Return
TimeClip
MakeDate
MakeDay
yr
dt
),
MakeTime
min
milli
))).
The
length
property of the
UTC
function is
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.
15.9.4.4
Date.now ( )
The
now
function return a Number value that is the
time value
designating the
UTC date and time of the occurrence of the call to
now
15.9.5
Properties of the Date
Prototype Object
The Date prototype object is itself a Date object (its [[Class]] is
"Date"
) whose [[PrimitiveValue]] is
NaN
The value of the [[Prototype]] internal property of the Date prototype object is the standard built-in Object prototype
object (
15.2.4
).
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. Unless explicitly noted
otherwise, none of these functions are generic; a
TypeError
exception is thrown if the
this
value is not an
object for which the value of the [[Class]] internal property is
"Date"
. Also, the phrase “this
time value
” refers to the Number value for the time represented by this Date object, that is,
the value of the [[PrimitiveValue]] internal property of this Date object.
15.9.5.1
Date.prototype.constructor
The initial value of
Date.prototype.constructor
is the built-in
Date
constructor.
15.9.5.2
Date.prototype.toString
( )
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.
NOTE
For any Date value
whose
milliseconds amount is zero, the result of
Date.parse(
.toString())
is equal to
.valueOf()
. See
15.9.4.2
15.9.5.3
Date.prototype.toDateString ( )
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.
15.9.5.4
Date.prototype.toTimeString ( )
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.
15.9.5.5
Date.prototype.toLocaleString ( )
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.
NOTE
The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything else.
15.9.5.6
Date.prototype.toLocaleDateString ( )
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.
NOTE
The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything else.
15.9.5.7
Date.prototype.toLocaleTimeString ( )
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.
NOTE
The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything else.
15.9.5.8
Date.prototype.valueOf (
The
valueOf
function returns a Number, which is
this time value
15.9.5.9
Date.prototype.getTime (
Return
this time value
15.9.5.10
Date.prototype.getFullYear ( )
Let
be
this time value
If
is
NaN
, return
NaN
Return
YearFromTime
LocalTime
)).
15.9.5.11
Date.prototype.getUTCFullYear ( )
Let
be
this time value
If
is
NaN
, return
NaN
Return
YearFromTime
).
15.9.5.12
Date.prototype.getMonth ( )
Let
be
this time value
If
is
NaN
, return
NaN
Return
MonthFromTime
LocalTime
)).
15.9.5.13
Date.prototype.getUTCMonth ( )
Let
be
this time value
If
is
NaN
, return
NaN
Return
MonthFromTime
).
15.9.5.14
Date.prototype.getDate
( )
Let
be
this time value
If
is
NaN
, return
NaN
Return
DateFromTime
LocalTime
)).
15.9.5.15
Date.prototype.getUTCDate ( )
Let
be
this time value
If
is
NaN
, return
NaN
Return
DateFromTime
).
15.9.5.16
Date.prototype.getDay
( )
Let
be
this time value
If
is
NaN
, return
NaN
Return
WeekDay
LocalTime
)).
15.9.5.17
Date.prototype.getUTCDay ( )
Let
be
this time value
If
is
NaN
, return
NaN
Return
WeekDay
).
15.9.5.18
Date.prototype.getHours ( )
Let
be
this time value
If
is
NaN
, return
NaN
Return
HourFromTime
LocalTime
)).
15.9.5.19
Date.prototype.getUTCHours ( )
Let
be
this time value
If
is
NaN
, return
NaN
Return
HourFromTime
).
15.9.5.20
Date.prototype.getMinutes ( )
Let
be
this time value
If
is
NaN
, return
NaN
Return
MinFromTime
LocalTime
)).
15.9.5.21
Date.prototype.getUTCMinutes ( )
Let
be
this time value
If
is
NaN
, return
NaN
Return
MinFromTime
).
15.9.5.22
Date.prototype.getSeconds ( )
Let
be
this time value
If
is
NaN
, return
NaN
Return
SecFromTime
LocalTime
)).
15.9.5.23
Date.prototype.getUTCSeconds ( )
Let
be
this time value
If
is
NaN
, return
NaN
Return
SecFromTime
).
15.9.5.24
Date.prototype.getMilliseconds ( )
Let
be
this time value
If
is
NaN
, return
NaN
Return
msFromTime
LocalTime
)).
15.9.5.25
Date.prototype.getUTCMilliseconds ( )
Let
be
this time value
If
is
NaN
, return
NaN
Return
msFromTime
).
15.9.5.26
Date.prototype.getTimezoneOffset ( )
Returns the difference between local time and UTC time in minutes.
Let
be
this time value
If
is
NaN
, return
NaN
Return (
LocalTime
)) /
msPerMinute
15.9.5.27
Date.prototype.setTime
(time)
Let
be
TimeClip
ToNumber
time
)).
Set the [[PrimitiveValue]] internal property of this Date object to
Return
15.9.5.28
Date.prototype.setMilliseconds (ms)
Let
be the result of
LocalTime
this time value
).
Let
time
be
MakeTime
HourFromTime
),
MinFromTime
),
SecFromTime
),
ToNumber
ms
)).
Let
be
TimeClip
UTC
MakeDate
Day
),
time
))).
Set the [[PrimitiveValue]] internal property of this Date object to
Return
15.9.5.29
Date.prototype.setUTCMilliseconds (ms)
Let
be
this time value
Let
time
be
MakeTime
HourFromTime
),
MinFromTime
),
SecFromTime
),
ToNumber
ms
)).
Let
be
TimeClip
MakeDate
Day
),
time
)).
Set the [[PrimitiveValue]] internal property of this Date object to
Return
15.9.5.30
Date.prototype.setSeconds (sec [, ms ] )
If
ms
is not specified, this behaves as if
ms
were specified with the value
getMilliseconds()
Let
be the result of
LocalTime
this time value
).
Let
be
ToNumber
sec
).
If
ms
is not specified, then let
milli
be
msFromTime
);
otherwise, let
milli
be
ToNumber
ms
).
Let
date
be
MakeDate
Day
),
MakeTime
HourFromTime
),
MinFromTime
),
milli
)).
Let
be
TimeClip
UTC
date
)).
Set the [[PrimitiveValue]] internal property of this Date object to
Return
The
length
property of the
setSeconds
method is
15.9.5.31
Date.prototype.setUTCSeconds (sec [, ms ] )
If
ms
is not specified, this behaves as if
ms
were specified with the value
getUTCMilliseconds()
Let
be
this time value
Let
be
ToNumber
sec
).
If
ms
is not specified, then let
milli
be
msFromTime
);
otherwise, let
milli
be
ToNumber
ms
).
Let
date
be
MakeDate
Day
),
MakeTime
HourFromTime
),
MinFromTime
),
milli
)).
Let
be
TimeClip
date
).
Set the [[PrimitiveValue]] internal property of this Date object to
Return
The
length
property of the
setUTCSeconds
method is
15.9.5.32
Date.prototype.setMinutes (min [, sec [, ms ] ] )
If
sec
is not specified, this behaves as if
sec
were specified with the value
getSeconds()
If
ms
is not specified, this behaves as if
ms
were specified with the value
getMilliseconds()
Let
be the result of
LocalTime
this time value
).
Let
be
ToNumber
min
).
If
sec
is not specified, then let
be
SecFromTime
); otherwise,
let
be
ToNumber
sec
).
If
ms
is not specified, then let
milli
be
msFromTime
);
otherwise, let
milli
be
ToNumber
ms
).
Let
date
be
MakeDate
Day
),
MakeTime
HourFromTime
),
milli
)).
Let
be
TimeClip
UTC
date
)).
Set the [[PrimitiveValue]] internal property of this Date object to
Return
The
length
property of the
setMinutes
method is
15.9.5.33
Date.prototype.setUTCMinutes (min [, sec [, ms ] ] )
If
sec
is not specified, this behaves as if
sec
were specified with the value
getUTCSeconds()
If
ms
is not specified, this function behaves as if
ms
were specified with the value return by
getUTCMilliseconds()
Let
be
this time value
Let
be
ToNumber
min
).
If
sec
is not specified, then let
be
SecFromTime
); otherwise,
let
be
ToNumber
sec
).
If
ms
is not specified, then let
milli
be
msFromTime
);
otherwise, let
milli
be
ToNumber
ms
).
Let
date
be
MakeDate
Day
),
MakeTime
HourFromTime
),
milli
)).
Let
be
TimeClip
date
).
Set the [[PrimitiveValue]] internal property of this Date object to
Return
The
length
property of the
setUTCMinutes
method is
15.9.5.34
Date.prototype.setHours (hour [, min [, sec [, ms ] ] ] )
If
min
is not specified, this behaves as if
min
were specified with the value
getMinutes()
If
sec
is not specified, this behaves as if
sec
were specified with the value
getSeconds()
If
ms
is not specified, this behaves as if
ms
were specified with the value
getMilliseconds()
Let
be the result of
LocalTime
this time value
).
Let
be
ToNumber
hour
).
If
min
is not specified, then let
be
MinFromTime
); otherwise,
let
be
ToNumber
min
).
If If
sec
is not specified, then let
be
SecFromTime
);
otherwise, let
be
ToNumber
sec
).
If
ms
is not specified, then let
milli
be
msFromTime
);
otherwise, let
milli
be
ToNumber
ms
).
Let
date
be
MakeDate
Day
),
MakeTime
milli
)).
Let
be
TimeClip
UTC
date
)).
Set the [[PrimitiveValue]] internal property of this Date object to
Return
The
length
property of the
setHours
method is
15.9.5.35
Date.prototype.setUTCHours (hour [, min [, sec [, ms ] ] ] )
If
min
is not specified, this behaves as if
min
were specified with the value
getUTCMinutes()
If
sec
is not specified, this behaves as if
sec
were specified with the value
getUTCSeconds()
If
ms
is not specified, this behaves as if
ms
were specified with the value
getUTCMilliseconds()
Let
be
this time value
Let
be
ToNumber
hour
).
If
min
is not specified, then let
be
MinFromTime
); otherwise,
let
be
ToNumber
min
).
If
sec
is not specified, then let
be
SecFromTime
); otherwise,
let
be
ToNumber
sec
).
If
ms
is not specified, then let
milli
be
msFromTime
);
otherwise, let
milli
be
ToNumber
ms
).
Let
newDate
be
MakeDate
Day
),
MakeTime
milli
)).
Let
be
TimeClip
newDate
).
Set the [[PrimitiveValue]] internal property of this Date object to
Return
The
length
property of the
setUTCHours
method is
15.9.5.36
Date.prototype.setDate
(date)
Let
be the result of
LocalTime
this time value
).
Let
dt
be
ToNumber
date
).
Let
newDate
be
MakeDate
MakeDay
YearFromTime
),
MonthFromTime
),
dt
),
TimeWithinDay
)).
Let
be
TimeClip
UTC
newDate
)).
Set the [[PrimitiveValue]] internal property of this Date object to
Return
15.9.5.37
Date.prototype.setUTCDate (date)
Let
be
this time value
Let
dt
be
ToNumber
date
).
Let
newDate
be
MakeDate
MakeDay
YearFromTime
),
MonthFromTime
),
dt
),
TimeWithinDay
)).
Let
be
TimeClip
newDate
).
Set the [[PrimitiveValue]] internal property of this Date object to
Return
15.9.5.38
Date.prototype.setMonth (month [, date ] )
If
date
is not specified, this behaves as if
date
were specified with the value
getDate()
Let
be the result of
LocalTime
this time value
).
Let
be
ToNumber
month
).
If
date
is not specified, then let
dt
be
DateFromTime
);
otherwise, let
dt
be
ToNumber
date
).
Let
newDate
be
MakeDate
MakeDay
YearFromTime
),
dt
),
TimeWithinDay
)).
Let
be
TimeClip
UTC
newDate
)).
Set the [[PrimitiveValue]] internal property of this Date object to
Return
The
length
property of the
setMonth
method is
15.9.5.39
Date.prototype.setUTCMonth (month [, date ] )
If
date
is not specified, this behaves as if
date
were specified with the value
getUTCDate()
Let
be
this time value
Let
be
ToNumber
month
).
If
date
is not specified, then let
dt
be
DateFromTime
);
otherwise, let
dt
be
ToNumber
date
).
Let
newDate
be
MakeDate
MakeDay
YearFromTime
),
dt
),
TimeWithinDay
)).
Let
be
TimeClip
newDate
).
Set the [[PrimitiveValue]] internal property of this Date object to
Return
The
length
property of the
setUTCMonth
method is
15.9.5.40
Date.prototype.setFullYear (year [, month [, date ] ] )
If
month
is not specified, this behaves as if
month
were specified with the value
getMonth()
If
date
is not specified, this behaves as if
date
were specified with the value
getDate()
Let
be the result of
LocalTime
this time value
); but
if
this time value
is
NaN
, let
be
+0
Let
be
ToNumber
year
).
If
month
is not specified, then let
be
MonthFromTime
);
otherwise, let
be
ToNumber
month
).
If
date
is not specified, then let
dt
be
DateFromTime
);
otherwise, let
dt
be
ToNumber
date
).
Let
newDate
be
MakeDate
MakeDay
dt
),
TimeWithinDay
)).
Let
be
TimeClip
UTC
newDate
)).
Set the [[PrimitiveValue]] internal property of this Date object to
Return
The
length
property of the
setFullYear
method is
15.9.5.41
Date.prototype.setUTCFullYear (year [, month [, date ] ] )
If
month
is not specified, this behaves as if
month
were specified with the value
getUTCMonth()
If
date
is not specified, this behaves as if
date
were specified with the value
getUTCDate()
Let
be
this time value
; but if
this time value
is
NaN
, let
be
+0
Let
be
ToNumber
year
).
If
month
is not specified, then let
be
MonthFromTime
);
otherwise, let
be
ToNumber
month
).
If
date
is not specified, then let
dt
be
DateFromTime
);
otherwise, let
dt
be
ToNumber
date
).
Let
newDate
be
MakeDate
MakeDay
dt
),
TimeWithinDay
)).
Let
be
TimeClip
newDate
).
Set the [[PrimitiveValue]] internal property of this Date object to
Return
The
length
property of the
setUTCFullYear
method is
15.9.5.42
Date.prototype.toUTCString ( )
This function returns a String value. The contents of the String are implementation-dependent, but are intended to
represent the Date in a convenient, human-readable form in UTC.
NOTE
The intent is to produce a String representation of a date that is more readable than
the format specified in
15.9.1.15
. It is not essential that the chosen format be
unambiguous or easily machine parsable. If an implementation does not have a preferred human-readable format it is
recommended to use the format defined in
15.9.1.15
but with a space rather than a
” used to separate the date and time elements.
15.9.5.43
Date.prototype.toISOString ( )
This function returns a String value represent the instance in time represented by this Date object. The format of the
String is the Date Time string format defined in
15.9.1.15
. All fields are present in the
String. The time zone is always UTC, denoted by the suffix Z. If the
time value
of this object
is not a finite Number a
RangeError
exception is thrown.
15.9.5.44
Date.prototype.toJSON
( key )
This function provides a String representation of a Date object for use by
JSON.stringify
15.12.3
).
When the
toJSON
method is called with argument
key
, the following steps are taken:
Let
be the result of calling
ToObject
, giving it the
this
value as its
argument.
Let
tv
be
ToPrimitive
, hint Number).
If
tv
is a Number and is not finite, return
null
Let
toISO
be the result of calling the [[Get]] internal method of
with argument
toISOString
".
If
IsCallable
toISO
) is
false
, throw a
TypeError
exception.
Return the result of calling the [[Call]] internal method of
toISO
with
as the
this
value and
an empty argument list.
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. An object is free to use the
argument
key
to filter its stringification.
15.9.6
Properties of Date
Instances
Date instances inherit properties from the Date prototype object and their [[Class]] internal property value is
"Date"
. Date instances also have a [[PrimitiveValue]] internal property.
The [[PrimitiveValue]] internal property is
time value
represented by this Date object.
15.10
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.
15.10.1
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
Pattern
::
Disjunction
Disjunction
::
Alternative
Alternative
Disjunction
Alternative
::
[empty]
Alternative
Term
Term
::
Assertion
Atom
Atom
Quantifier
Assertion
::
Disjunction
Disjunction
Quantifier
::
QuantifierPrefix
QuantifierPrefix
QuantifierPrefix
::
DecimalDigits
DecimalDigits
DecimalDigits
DecimalDigits
Atom
::
PatternCharacter
AtomEscape
CharacterClass
Disjunction
Disjunction
PatternCharacter
::
SourceCharacter
but not one of
AtomEscape
::
DecimalEscape
CharacterEscape
CharacterClassEscape
CharacterEscape
::
ControlEscape
ControlLetter
HexEscapeSequence
UnicodeEscapeSequence
IdentityEscape
ControlEscape
::
one of
ControlLetter
::
one of
IdentityEscape
::
SourceCharacter
but not
IdentifierPart
DecimalEscape
::
DecimalIntegerLiteral
[lookahead ∉
DecimalDigit
CharacterClassEscape
::
one of
CharacterClass
::
[lookahead ∉ {
}]
ClassRanges
ClassRanges
ClassRanges
::
[empty]
NonemptyClassRanges
NonemptyClassRanges
::
ClassAtom
ClassAtom
NonemptyClassRangesNoDash
ClassAtom
ClassAtom
ClassRanges
NonemptyClassRangesNoDash
::
ClassAtom
ClassAtomNoDash
NonemptyClassRangesNoDash
ClassAtomNoDash
ClassAtom
ClassRanges
ClassAtom
::
ClassAtomNoDash
ClassAtomNoDash
::
SourceCharacter
but not one of
or
or
ClassEscape
ClassEscape
::
DecimalEscape
CharacterEscape
CharacterClassEscape
15.10.2
Pattern Semantics
A regular expression pattern is converted into an internal procedure 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 internal
procedure is used as the value of a RegExp object’s [[Match]] internal property.
15.10.2.1
Notation
The descriptions below use the following variables:
Input
is the String being matched by the regular expression pattern. The notation
input
means the
th
character of
input
, where
can range between 0 (inclusive)
and
InputLength
(exclusive).
InputLength
is the number of characters in the
Input
String.
NcapturingParens
is the total number of left capturing parentheses (i.e. the total number
of times the
Atom
::
Disjunction
production is expanded) in the pattern. A left
capturing parenthesis is any
pattern character that is matched by the
terminal of the
Atom
::
Disjunction
production.
IgnoreCase
is the setting of the RegExp object's
ignoreCase
property.
Multiline
is the setting of the RegExp object’s
multiline
property.
Furthermore, the descriptions below use the following internal data structures:
CharSet
is a mathematical set of characters.
State
is an ordered pair
endIndex
captures
where
endIndex
is an integer and
captures
is an internal array 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
th
element of
captures
is either a
String that represents the value obtained by the
th
set of capturing parentheses or
undefined
if the
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.
MatchResult
is either a
State
or the special token
failure
that indicates that the match failed.
Continuation
procedure is an internal closure (i.e. an internal procedure with some
arguments already bound to values) that takes one
State
argument and returns a
MatchResult
result. If an internal closure references variables bound in the function that creates
the closure, the closure uses the values that these variables had at the time the closure was created. The
Continuation
attempts to match the remaining portion (specified by the closure's already-bound
arguments) of the pattern against the input String, 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
Matcher
procedure is an internal 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 already-bound arguments) of the pattern against the input String, 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.
An
AssertionTester
procedure is an internal closure that takes a
State
argument and returns a Boolean result. The assertion tester tests a specific condition
(specified by the closure's already-bound arguments) against the current place in the input String and returns
true
if the condition matched or
false
if not.
An
EscapeValue
is either a character or an integer. An
EscapeValue
is used to denote the interpretation of a
DecimalEscape
escape sequence: a character
ch
means that the escape sequence is interpreted as the character
ch
, while an integer
means that the escape sequence is interpreted as a backreference to the
th
set of capturing parentheses.
15.10.2.2
Pattern
The production
Pattern
::
Disjunction
evaluates as follows:
Evaluate
Disjunction
to obtain a Matcher
Return an internal closure that takes two arguments, a String
str
and an integer
index
, and performs
the following:
Let
Input
be the given String
str
. This variable will be used throughout the algorithms in
15.10.2
Let
InputLength
be the length of
Input
. This variable will be used throughout the algorithms in
15.10.2
Let
be a Continuation that always returns its State argument as a successful MatchResult.
Let
cap
be an internal array of
NcapturingParens
undefined
values, indexed 1 through
NcapturingParens
Let
be the State (
index
cap
).
Call
) and return its result.
NOTE
A Pattern evaluates ("compiles") to an internal procedure value.
RegExp.prototype.exec
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
15.10.2
are designed so
that compiling a pattern may throw a
SyntaxError
exception; on the other hand, once the pattern is successfully
compiled, applying its result internal procedure 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).
15.10.2.3
Disjunction
The production
Disjunction
::
Alternative
evaluates by evaluating
Alternative
to obtain a
Matcher
and returning that
Matcher
The production
Disjunction
::
Alternative
Disjunction
evaluates as
follows:
Evaluate
Alternative
to obtain a Matcher
m1
Evaluate
Disjunction
to obtain a Matcher
m2
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and
performs the following:
Call
m1
) and let
be its result.
If
isn't
failure
, return
Call
m2
) 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]
15.10.2.4
Alternative
The production
Alternative
::
[empty]
evaluates by returning a Matcher that takes two arguments, a State
and a Continuation
, and returns the result of calling
).
The production
Alternative
::
Alternative
Term
evaluates as follows:
Evaluate
Alternative
to obtain a Matcher
m1
Evaluate
Term
to obtain a Matcher
m2
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and
performs the following:
Create a Continuation
that takes a State argument
and returns the result of calling
m2
).
Call
m1
) and return its result.
NOTE
Consecutive
try to simultaneously match consecutive
portions of the input String. 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
15.10.2.5
Term
The production
Term
::
Assertion
evaluates by returning an internal Matcher closure that takes two arguments, a State
and a Continuation
, and performs the following:
Evaluate
Assertion
to obtain an AssertionTester
Call
) and let
be the resulting Boolean value.
If
is
false
, return
failure
Call
) and return its result.
The production
Term
::
Atom
evaluates by evaluating
Atom
to obtain a Matcher and returning that
Matcher.
The production
Term
::
Atom
Quantifier
evaluates as follows:
Evaluate
Atom
to obtain a Matcher
Evaluate
Quantifier
to obtain the three results: an integer
min
, an integer (or ∞)
max
and Boolean
greedy
If
max
is finite and less than
min
, then throw a
SyntaxError
exception.
Let
parenIndex
be the number of left capturing parentheses in the entire regular expression that occur to the
left of this production expansion's
Term
. This is the total number of times the
Atom
::
Disjunction
production is expanded prior to this production's
Term
plus the total number
of
Atom
::
Disjunction
productions enclosing this
Term
Let
parenCount
be the number of left capturing parentheses in the expansion of this production's
Atom
This is the total number of
Atom
::
Disjunction
productions enclosed by this
production's
Atom
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and
performs the following:
Call RepeatMatcher(
min
max
greedy
parenIndex
parenCount
) and return its result.
The abstract operation
RepeatMatcher
takes eight parameters, a Matcher
, an integer
min
an integer (or ∞)
max
, a Boolean
greedy
, a State
, a Continuation
, an
integer
parenIndex
, and an integer
parenCount
, and performs the following:
If
max
is zero, then call
) and return its result.
Create an internal Continuation closure
that takes one State argument
and performs the following:
If
min
is zero and
's
endIndex
is equal to
's
endIndex
, then return
failure
If
min
is zero then let
min2
be zero; otherwise let
min2
be
min
–1.
If
max
is ∞, then let
max2
be ∞; otherwise let
max2
be
max
–1.
Call RepeatMatcher(
min2
max2
greedy
parenIndex
parenCount
) and return its result.
Let
cap
be a fresh copy of
's
captures
internal array.
For every integer
that satisfies
parenIndex
and
parenIndex
parenCount
, set
cap
] to
undefined
Let
be
's
endIndex
Let
xr
be the State (
cap
).
If
min
is not zero, then call
xr
) and return its result.
If
greedy
is
false
, then
Call
) and let
be its result.
If
is not
failure
, return
Call
xr
) and return its result.
Call
xr
) and let
be its result.
If
is not
failure
, return
Call
) 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 String 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 1 of the RepeatMatcher's
closure states that, once the minimum
number of repetitions has been satisfied, any more expansions of
Atom
that match the empty
String 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", ""]
15.10.2.6
Assertion
The production
Assertion
::
evaluates by returning an internal AssertionTester closure that takes a State argument
and performs the following:
Let
be
's
endIndex
If
is zero, return
true
If
Multiline
is
false
, return
false
If the character
Input
–1] is one of
LineTerminator
, return
true
Return
false
The production
Assertion
::
evaluates by returning an internal AssertionTester closure that takes a State argument
and performs the following:
Let
be
's
endIndex
If
is equal to
InputLength
, return
true
If
multiline
is
false
, return
false
If the character
Input
] is one of
LineTerminator
, return
true
Return
false
The production
Assertion
::
evaluates by returning an internal AssertionTester closure that takes a
State argument
and performs the following:
Let
be
's
endIndex
Call
IsWordChar
–1) and let
be the Boolean result.
Call
IsWordChar
) and let
be the Boolean result.
If
is
true
and
is
false
, return
true
If
is
false
and
is
true
, return
true
Return
false
The production
Assertion
::
evaluates by returning an internal AssertionTester closure that takes a
State argument
and performs the following:
Let
be
's
endIndex
Call
IsWordChar
–1) and let
be the Boolean result.
Call
IsWordChar
) and let b be the Boolean result.
If a is
true
and b is
false
, return
false
If a is
false
and b is
true
, return
false
Return
true
The production
Assertion
::
Disjunction
evaluates as follows:
Evaluate
Disjunction
to obtain a Matcher
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and
performs the following steps:
Let
be a Continuation that always returns its State argument as a successful MatchResult.
Call
) and let
be its result.
If
is
failure
, return
failure
Let
be
's State.
Let
cap
be
's
captures
internal array.
Let
xe
be
's
endIndex
Let
be the State (
xe
cap
).
Call
) and return its result.
The production
Assertion
::
Disjunction
evaluates as follows:
Evaluate
Disjunction
to obtain a Matcher
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and
performs the following steps:
Let
be a Continuation that always returns its State argument as a successful MatchResult.
Call
) and let
be its result.
If
isn't
failure
, return
failure
Call
) and return its result.
The abstract operation
IsWordChar
takes an integer parameter
and performs the
following:
If
== –1 or
==
InputLength
, return
false
Let
be the character
Input
].
If
is one of the sixty-three characters below, return
true
Return
false
15.10.2.7
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 by returning the two results 0 and ∞.
The production
QuantifierPrefix
::
evaluates by returning the two results 1 and ∞.
The production
QuantifierPrefix
::
evaluates by returning the two results 0 and 1.
The production
QuantifierPrefix
::
DecimalDigits
evaluates as follows:
Let
be the MV of
DecimalDigits
see 7.8.3
).
Return the two results
and
The production
QuantifierPrefix
::
DecimalDigits
evaluates as follows:
Let
be the MV of
DecimalDigits
Return the two results
and ∞.
The production
QuantifierPrefix
::
DecimalDigits
DecimalDigits
evaluates as follows:
Let
be the MV of the first
DecimalDigits
Let
be the MV of the second
DecimalDigits
Return the two results
and
15.10.2.8
Atom
The production
Atom
::
PatternCharacter
evaluates as follows:
Let
ch
be the character represented by
PatternCharacter
Let
be a one-element CharSet containing the character
ch
Call
CharacterSetMatcher
false
) and return its Matcher result.
The production
Atom
::
evaluates as follows:
Let
be the set of all characters except
LineTerminator
Call
CharacterSetMatcher
false
) and return its Matcher result.
The production
Atom
::
AtomEscape
evaluates by evaluating
AtomEscape
to obtain a Matcher
and returning that Matcher.
The production
Atom
::
CharacterClass
evaluates as follows:
Evaluate
CharacterClass
to obtain a CharSet
and a Boolean
invert
Call
CharacterSetMatcher
invert
) and return its Matcher result.
The production
Atom
::
Disjunction
evaluates as follows:
Evaluate
Disjunction
to obtain a Matcher
Let
parenIndex
be the number of left capturing parentheses in the entire regular expression that occur to the
left of this production expansion's initial left parenthesis. This is the total number of times the
Atom
::
Disjunction
production is expanded prior to this production's
Atom
plus the total number of
Atom
::
Disjunction
productions enclosing
this
Atom
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and
performs the following steps:
Create an internal Continuation closure
that takes one State argument
and performs the
following steps:
Let
cap
be a fresh copy of
's
captures
internal array.
Let
xe
be
's
endIndex
Let
ye
be
's
endIndex
Let
be a fresh String whose characters are the characters of
Input
at positions
xe
(inclusive) through
ye
(exclusive).
Set
cap
parenIndex
+1] to
Let
be the State (
ye
cap
).
Call
) and return its result.
Call
) and return its result.
The production
Atom
::
Disjunction
evaluates by evaluating
Disjunction
to obtain a Matcher and returning that Matcher.
The abstract operation
CharacterSetMatcher
takes two arguments, a CharSet
and a
Boolean flag
invert
, and performs the following:
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and
performs the following steps:
Let
be
's
endIndex
If
==
InputLength
, return
failure
Let
ch
be the character
Input
].
Let
cc
be the result of
Canonicalize
ch
).
If
invert
is
false
, then
If there does not exist a member
of set
such that
Canonicalize
) ==
cc
, return
failure
Else
invert
is
true
If there exists a member
of set
such that
Canonicalize
) ==
cc
return
failure.
Let
cap
be
's
captures
internal array.
Let
be the State (
+1,
cap
).
Call
) and return its result.
The abstract operation
Canonicalize
takes a character parameter
ch
and performs the
following steps:
If
IgnoreCase
is
false
, return
ch
Let
be
ch
converted to upper case as if by calling the standard built-in method
String.prototype.toUpperCase
on the one-character String
ch
If
does not consist of a single character, return
ch
Let
cu
be
's character.
If
ch
's code unit value is greater than or equal to decimal 128 and
cu
's code unit value is less than
decimal 128, then return
ch
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
internal procedure. 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
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
not immediately followed by some positive number n of
's, a
another n
's (specified by the first
\2
) and a
. 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"]
In case-insignificant matches all characters are implicitly converted to upper case immediately before they are
compared. However, if converting a character to upper case would expand that character into more than one character (such
as converting
"ß"
\u00DF
) into
"SS"
), then
the character is left as-is instead. The character is also left as-is if it is not an ASCII character but converting it to
upper case would make it into an ASCII character. This prevents Unicode characters such as
\u0131
and
\u017F
from matching regular expressions such as
/[a‑z]/i
, which are only intended to match ASCII letters. Furthermore, if these conversions were
allowed, then
/[^\W]/i
would match each of
, …,
, but not
or
15.10.2.9
AtomEscape
The production
AtomEscape
::
DecimalEscape
evaluates as follows:
Evaluate
DecimalEscape
to obtain an EscapeValue
If
is a character, then
Let
ch
be
's character.
Let
be a one-element CharSet containing the character
ch
Call
CharacterSetMatcher
false
) and return its Matcher result.
must be an integer. Let
be that integer.
If
=0 or
NCapturingParens
then throw a
SyntaxError
exception.
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and
performs the following:
Let
cap
be
's
captures
internal array.
Let
be
cap
].
If
is
undefined
, then call
) and return its result.
Let
be
's
endIndex
Let
len
be
's length.
Let
be
len
If
InputLength
, return
failure
If there exists an integer
between 0 (inclusive) and
len
(exclusive) such that
Canonicalize
]) is not the same character as
Canonicalize
Input
]), then return
failure
Let
be the State (
cap
).
Call
) and return its result.
The production
AtomEscape
::
CharacterEscape
evaluates as follows:
Evaluate
CharacterEscape
to obtain a character
ch
Let
be a one-element CharSet containing the character
ch
Call
CharacterSetMatcher
false
) and return its Matcher result.
The production
AtomEscape
::
CharacterClassEscape
evaluates as follows:
Evaluate
CharacterClassEscape
to obtain a CharSet
Call
CharacterSetMatcher
false
) and return its Matcher result.
NOTE
An escape sequence of the form
followed by a nonzero decimal number
matches the result of the
th set of capturing parentheses (
see
15.10.2.11
). It is an error if the regular expression has fewer than
capturing parentheses. If the
regular expression has
or more capturing parentheses but the
th one is
undefined
because
it has not captured anything, then the backreference always succeeds.
15.10.2.10
CharacterEscape
The production
CharacterEscape
::
ControlEscape
evaluates by returning the character according to Table 23.
Table 23 — ControlEscape Character Values
ControlEscape
Code Unit
Name
Symbol
\u0009
horizontal tab
\u000A
line feed (new line)
\u000B
vertical tab
\u000C
form feed
\u000D
carriage return
The production
CharacterEscape
::
ControlLetter
evaluates as follows:
Let
ch
be the character represented by
ControlLetter
Let
be
ch
's code unit value.
Let
be the remainder of dividing
by 32.
Return the character whose code unit value is
The production
CharacterEscape
::
HexEscapeSequence
evaluates by evaluating the CV of the
HexEscapeSequence
see 7.8.4
) and returning its character result.
The production
CharacterEscape
::
UnicodeEscapeSequence
evaluates by evaluating the CV of the
UnicodeEscapeSequence
see 7.8.4
) and returning its character result.
The production
CharacterEscape
::
IdentityEscape
evaluates by returning the character represented by
IdentityEscape
15.10.2.11
DecimalEscape
The production
DecimalEscape
::
DecimalIntegerLiteral
[lookahead ∉
DecimalDigit
evaluates as follows:
Let
be the MV of
DecimalIntegerLiteral
If
is zero, return the EscapeValue consisting of a
Return the EscapeValue consisting of the integer
The definition of “the MV of
DecimalIntegerLiteral
” is in
7.8.3
NOTE
If
is followed by a decimal number
whose first digit is not
, then the escape sequence is considered to be a backreference. It is an error if
is greater
than the total number of left capturing parentheses in the entire regular expression.
\0
represents the
15.10.2.12
CharacterClassEscape
The production
CharacterClassEscape
::
evaluates by returning the ten-element set of characters containing the characters
through
inclusive.
The production
CharacterClassEscape
::
evaluates by returning the set of all characters not included in the set returned by
CharacterClassEscape
::
The production
CharacterClassEscape
::
evaluates by returning the set of characters containing the characters that are on the
right-hand side of the
WhiteSpace
7.2
) or
LineTerminator
7.3
) productions.
The production
CharacterClassEscape
::
evaluates by returning the set of all characters not included in the set returned by
CharacterClassEscape
::
The production
CharacterClassEscape
::
evaluates by returning the set of characters containing the sixty-three characters:
The production
CharacterClassEscape
::
evaluates by returning the set of all characters not included in the set returned by
CharacterClassEscape
::
15.10.2.13
CharacterClass
The production
CharacterClass
::
[lookahead ∉ {
}]
ClassRanges
evaluates by evaluating
ClassRanges
to obtain a CharSet and returning that CharSet and the Boolean
false
The production
CharacterClass
::
ClassRanges
evaluates
by evaluating
ClassRanges
to obtain a CharSet and returning that CharSet and the Boolean
true
15.10.2.14
ClassRanges
The production
ClassRanges
::
[empty]
evaluates by returning the empty CharSet.
The production
ClassRanges
::
NonemptyClassRanges
evaluates by evaluating
NonemptyClassRanges
to obtain
a CharSet and returning that CharSet.
15.10.2.15
NonemptyClassRanges
The production
NonemptyClassRanges
::
ClassAtom
evaluates by evaluating
ClassAtom
to obtain a CharSet and
returning that CharSet.
The production
NonemptyClassRanges
::
ClassAtom
NonemptyClassRangesNoDash
evaluates as follows:
Evaluate
ClassAtom
to obtain a CharSet
Evaluate
NonemptyClassRangesNoDash
to obtain a CharSet
Return the union of CharSets
and
The production
NonemptyClassRanges
::
ClassAtom
ClassAtom
ClassRanges
evaluates as follows:
Evaluate the first
ClassAtom
to obtain a CharSet
Evaluate the second
ClassAtom
to obtain a CharSet
Evaluate
ClassRanges
to obtain a CharSet
Call
CharacterRange
) and let
be the resulting CharSet.
Return the union of CharSets
and
The abstract operation
CharacterRange
takes two CharSet parameters
and
and performs the following:
If
does not contain exactly one character or
does not contain exactly one character then throw a
SyntaxError
exception.
Let
be the one character in CharSet
Let
be the one character in CharSet
Let
be the code unit value of character
Let
be the code unit value of character
If
then throw a
SyntaxError
exception.
Return the set containing all characters numbered
through
, inclusive.
15.10.2.16
NonemptyClassRangesNoDash
The production
NonemptyClassRangesNoDash
::
ClassAtom
evaluates by evaluating
ClassAtom
to obtain a CharSet and
returning that CharSet.
The production
NonemptyClassRangesNoDash
::
ClassAtomNoDash
NonemptyClassRangesNoDash
evaluates as follows:
Evaluate
ClassAtomNoDash
to obtain a CharSet
Evaluate
NonemptyClassRangesNoDash
to obtain a CharSet
Return the union of CharSets
and
The production
NonemptyClassRangesNoDash
::
ClassAtomNoDash
ClassAtom
ClassRanges
evaluates as follows:
Evaluate
ClassAtomNoDash
to obtain a CharSet
Evaluate
ClassAtom
to obtain a CharSet
Evaluate
ClassRanges
to obtain a CharSet
Call
CharacterRange
) and let
be the resulting CharSet.
Return the union of CharSets
and
NOTE 1
ClassRanges
can expand into single
ClassAtoms
and/or ranges of two
ClassAtoms
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
code unit value is greater than the second
ClassAtom's
code unit 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
, and
, while the pattern
/[E-f]/i
matches all upper and lower-case ASCII letters as well as the symbols
, and
NOTE 3
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.
15.10.2.17
ClassAtom
The production
ClassAtom
::
evaluates by returning the CharSet containing the one character
The production
ClassAtom
::
ClassAtomNoDash
evaluates by evaluating
ClassAtomNoDash
to obtain a
CharSet and returning that CharSet.
15.10.2.18
ClassAtomNoDash
The production
ClassAtomNoDash
::
SourceCharacter
but not one of
or
or
evaluates by returning a one-element CharSet containing the character represented by
SourceCharacter
The production
ClassAtomNoDash
::
ClassEscape
evaluates by evaluating
ClassEscape
to obtain a CharSet and returning that CharSet.
15.10.2.19
ClassEscape
The production
ClassEscape
::
DecimalEscape
evaluates as follows:
Evaluate
DecimalEscape
to obtain an EscapeValue
If
is not a character then throw a
SyntaxError
exception.
Let
ch
be
's character.
Return the one-element CharSet containing the character
ch
The production
ClassEscape
::
evaluates by returning the CharSet containing the one character
0008).
The production
ClassEscape
::
CharacterEscape
evaluates by evaluating
CharacterEscape
to obtain a
character and returning a one-element CharSet containing that character.
The production
ClassEscape
::
CharacterClassEscape
evaluates by evaluating
CharacterClassEscape
to
obtain a CharSet and returning that CharSet.
NOTE
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.
15.10.3
The RegExp Constructor
Called as a Function
15.10.3.1
RegExp(pattern,
flags)
If
pattern
is an object
whose [[Class]] internal property is
"RegExp"
and
flags
is
undefined
, then return
unchanged. Otherwise call the standard built-in
RegExp
constructor (
15.10.4.1
) as if by the expression
new
RegExp(
pattern
flags
and return the object constructed by that
constructor.
15.10.4
The RegExp Constructor
When
RegExp
is called as part of a
new
expression, it is a constructor: it initialises the
newly created object.
15.10.4.1
new RegExp(pattern,
flags)
If
pattern
is an object
whose [[Class]] internal property is
"RegExp"
and
flags
is
undefined
, then let
be the
pattern
used to construct
and
let
be the flags used to construct
. If
pattern
is an object
whose
[[Class]] internal property is
"RegExp"
and
flags
is not
undefined
, then throw a
TypeError
exception. Otherwise, let
be the empty String if
pattern
is
undefined
and
ToString
pattern
otherwise, and let
be the empty String if
flags
is
undefined
and
ToString
flags
otherwise.
If the characters of
do not have the syntactic form
Pattern
, then throw a
SyntaxError
exception. Otherwise let the newly constructed object have a [[Match]] internal property obtained by
evaluating ("compiling") the characters of
as a
Pattern
as described in
15.10.2
If
contains any character other than
"g"
"i"
, or
"m"
, or if it
contains the same character more than once, then throw a
SyntaxError
exception.
If a
SyntaxError
exception is not thrown, then:
Let
be a String in the form of a
Pattern
equivalent to
, in which
certain characters are escaped as described below.
may or may not be identical to
or
pattern
; however, the internal procedure that would result from evaluating
as a
Pattern
must behave identically to the internal procedure given by the constructed object's [[Match]]
internal property.
The characters
occurring in the pattern shall be escaped in
as necessary to ensure that the
String value formed by concatenating the Strings
"/"
"/"
, and
can be
parsed (in an appropriate lexical context) as a
RegularExpressionLiteral
that behaves identically
to the constructed regular expression. For example, if
is
"/"
, then
could be
"\/"
or
"\u002F"
, among other possibilities, but not
"/"
, because
///
followed by
would be parsed as a
SingleLineComment
rather than a
RegularExpressionLiteral
. If
is the empty String, this specification can be met by letting
be
"(?:)"
The following properties of the newly constructed object are data properties with the attributes that are specified in
15.10.7
. The [[Value]] of each property is set as follows:
The
source
property of the newly constructed object is set to
The
global
property of the newly constructed object is set to a Boolean value that is
true
if
contains the character
"g"
and
false
otherwise.
The
ignoreCase
property of the newly constructed object is set to a Boolean value that is
true
if
contains the character
"i"
and
false
otherwise.
The
multiline
property of the newly constructed object is set to a Boolean value that is
true
if
contains the character
"m"
and
false
otherwise.
The
lastIndex
property of the newly constructed object is set to
The [[Prototype]] internal property of the newly constructed object is set to the standard built-in RegExp prototype
object as specified in
15.10.6
The [[Class]] internal property of the newly constructed object is set to
"RegExp"
NOTE
If pattern is 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
recognised by RegExp, any backslash
characters must be escaped within the
StringLiteral
to prevent them being removed when the contents of the
StringLiteral
are formed.
15.10.5
Properties of the RegExp
Constructor
The value of the [[Prototype]] internal property of the RegExp constructor is the standard built-in Function prototype
object (
15.3.4
).
Besides the internal properties and the
length
property (whose value is
), the RegExp constructor
has the following properties:
15.10.5.1
RegExp.prototype
The initial value of
RegExp.prototype
is the RegExp prototype object (
15.10.6
).
This property shall have the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
[[Configurable]]:
false
}.
15.10.6
Properties of the RegExp
Prototype Object
The value of the [[Prototype]] internal property of the RegExp prototype object is the standard built-in Object prototype
object (
15.2.4
). The RegExp prototype object is itself a regular expression object; its [[Class]]
is
RegExp
. The initial values of the RegExp prototype object’s
data properties (
15.10.7
) are set as if the object was created by the expression
new
RegExp()
where
RegExp
is that standard built-in constructor with that name.
The RegExp prototype object does not have a
valueOf
property of its own; however, it inherits the
valueOf
property from the Object prototype object.
In the following descriptions of functions that are properties of the RegExp prototype object, the phrase “this
RegExp object” refers to the object that is the
this
value for the invocation of the function; a
TypeError
exception is thrown if the
this
value is not an object or an object for which the value of the
[[Class]] internal property is not
"RegExp"
15.10.6.1
RegExp.prototype.constructor
The initial value of
RegExp.prototype.constructor
is the standard built-in
RegExp
constructor.
15.10.6.2
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
be this RegExp object.
Let
be the value of
ToString
string
).
Let
length
be the length of
Let
lastIndex
be the result of calling the [[Get]] internal method of
with argument
lastIndex
".
Let
be the value of
ToInteger
lastIndex
).
Let
global
be the result of calling the [[Get]] internal method of
with argument
global"
If
global
is
false
, then let
= 0.
Let
matchSucceeded
be
false
Repeat, while
matchSucceeded
is
false
If
< 0 or
length
, then
Call the [[Put]] internal method of
with arguments "
lastIndex"
, 0, and
true
Return
null
Call the [[Match]] internal method of
with arguments
and
If [[Match]] returned
failure
, then
Let i = i+1.
else
Let
be the State result of the call to [[Match]].
Set
matchSucceeded
to
true
Let
be
's
endIndex
value.
If
global
is
true
Call the [[Put]] internal method of
with arguments "
lastIndex"
, and
true
Let
be the length of
's
captures
array. (This is the same value as 15.10.2.1's
NCapturingParens
.)
Let
be a new array created as if by the expression
new Array()
where
Array
is the
standard built-in constructor with that name.
Let
matchIndex
be
Call the [[DefineOwnProperty]] internal method of
with arguments "
index"
Property Descriptor
{[[Value]]:
matchIndex
, [[Writable]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
true
Call the [[DefineOwnProperty]] internal method of
with arguments "
input"
Property Descriptor
{[[Value]]:
, [[Writable]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
true
Call the [[DefineOwnProperty]] internal method of
with arguments "
length"
Property Descriptor
{[[Value]]:
+ 1}, and
true
Let
matchedSubstr
be the matched substring (i.e. the portion of
between offset
inclusive
and offset
exclusive).
Call the [[DefineOwnProperty]] internal method of
with arguments "
0"
Property Descriptor
{[[Value]]:
matchedSubstr
, [[Writable]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
true
For each integer
such that
> 0 and
Let
captureI
be
th
element of
's
captures
array.
Call the [[DefineOwnProperty]] internal method of
with arguments
ToString
),
Property Descriptor
{[[Value]]:
captureI
[[Writable]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
true
Return
15.10.6.3
RegExp.prototype.test(string)
The following steps are taken:
Let
match
be the result of evaluating the
RegExp.prototype.exec
15.10.6.2
) algorithm upon this RegExp object using
string
as the argument.
If
match
is not null, then return
true
; else return
false
15.10.6.4
RegExp.prototype.toString()
Return the String value formed by concatenating the Strings
, the
String value of the
source
property of this RegExp object, and
; plus
if the
global
property is
true
if the
ignoreCase
property is
true
, and
if the
multiline
property is
true
NOTE
The returned String has the form of a
RegularExpressionLiteral
that evaluates to another RegExp object with the same behaviour as this object.
15.10.7
Properties of RegExp
Instances
RegExp instances inherit properties from the RegExp prototype object and their [[Class]] internal property value is
"RegExp"
. RegExp instances also have a [[Match]] internal property and a
length
property.
The value of the [[Match]] internal property is an implementation dependent representation of the
Pattern
of the RegExp object.
RegExp instances also have the following properties.
15.10.7.1
source
The value of the
source
property is a String in the form of a
Pattern
representing
the current regular expression. This property shall have the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.10.7.2
global
The value of the
global
property is a Boolean value indicating whether the flags contained the character
“g”
. This property shall have the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.10.7.3
ignoreCase
The value of the
ignoreCase
property is a Boolean value indicating whether the flags contained the
character
“i”
. This property shall have the attributes { [[Writable]]:
false
[[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.10.7.4
multiline
The value of the
multiline
property is a Boolean value indicating whether the flags contained the
character
“m”
. This property shall have the attributes { [[Writable]]:
false
[[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.10.7.5
lastIndex
The value of the
lastIndex
property specifies the String position at which to start the next match. It is
coerced to an integer when used (
see 15.10.6.2
). This property shall have the attributes
{ [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
NOTE
Unlike the other standard built-in properties of RegExp instances,
lastIndex
is writable.
15.11
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.
15.11.1
The Error Constructor Called
as a Function
When
Error
is called as a function rather than as a constructor, it creates and initialises a new Error
object. Thus the function call
Error(
is equivalent to
the object creation expression
new Error(
with the same
arguments.
15.11.1.1
Error (message)
The [[Prototype]] internal property of the newly constructed object is set to the original Error prototype object, the
one that is the initial value of
Error.prototype
15.11.3.1
).
The [[Class]] internal property of the newly constructed object is set to
"Error"
The [[Extensible]] internal property of the newly constructed object is set to
true
If the argument
message
is not
undefined
, the
message
own property of the newly
constructed object is set to
ToString
message
).
15.11.2
The Error Constructor
When
Error
is called as part of a
new
expression, it is a constructor: it initialises the newly
created object.
15.11.2.1
new Error
(message)
The [[Prototype]] internal property of the newly constructed object is set to the original Error prototype object, the
one that is the initial value of
Error.prototype
15.11.3.1
).
The [[Class]] internal property of the newly constructed Error object is set to
"Error"
The [[Extensible]] internal property of the newly constructed object is set to
true
If the argument
message
is not
undefined
, the
message
own property of the newly
constructed object is set to
ToString
message
).
15.11.3
Properties of the Error
Constructor
The value of the [[Prototype]] internal property of the Error constructor is the Function prototype object (
15.3.4
).
Besides the internal properties and the
length
property (whose value is
), the Error constructor has
the following property:
15.11.3.1
Error.prototype
The initial value of
Error.prototype
is the Error prototype object (
15.11.4
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.11.4
Properties of the Error
Prototype Object
The Error prototype object is itself an Error object (its [[Class]] is
"Error"
).
The value of the [[Prototype]] internal property of the Error prototype object is the standard built-in Object prototype
object (
15.2.4
).
15.11.4.1
Error.prototype.constructor
The initial value of
Error.prototype.constructor
is the built-in
Error
constructor.
15.11.4.2
Error.prototype.name
The initial value of
Error.prototype.name
is
Error
15.11.4.3
Error.prototype.message
The initial value of
Error.prototype.message
is the empty String.
15.11.4.4
Error.prototype.toString ( )
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
Let
name
be the result of calling the [[Get]] internal method of
with argument
name
If
name
is
undefined
, then let
name
be
Error
; else let
name
be
ToString
name
).
Let
msg
be the result of calling the [[Get]] internal method of
with argument
message
If
msg
is undefined, then let
msg
be the empty String; else let
msg
be
ToString
msg
).
If
msg
is undefined, then let
msg
be the empty String; else let
msg
be
ToString
msg
).
If
name
is the empty String, return
msg
If
msg
is the empty String, return
name
Return the result of concatenating
name
":"
, a single space character, and
msg
15.11.5
Properties of Error
Instances
Error instances inherit properties from the Error prototype object and their [[Class]] internal property value is
"Error"
. Error instances have no special properties.
15.11.6
Native Error Types Used in
This Standard
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
15.11.7
15.11.6.1
EvalError
This exception is not currently used within this specification. This object remains for compatibility with previous
editions of this specification.
15.11.6.2
RangeError
Indicates a numeric value has exceeded the allowable range. See
15.4.2.2
15.4.5.1
15.7.4.2
15.7.4.5
15.7.4.6
15.7.4.7
, and
15.9.5.43
15.11.6.3
ReferenceError
Indicate that an invalid reference value has been detected. See
8.7.1
8.7.2
10.2.1
10.2.1.1.4
10.2.1.2.4
, and
11.13.1
15.11.6.4
SyntaxError
Indicates that a parsing error has occurred. See
11.1.5
11.3.1
11.3.2
11.4.1
11.4.4
11.4.5
11.13.1
11.13.2
12.2.1
12.10.1
12.14.1
13.1
15.1.2.1
15.3.2.1
15.10.2.2
15.10.2.5
15.10.2.9
15.10.2.15
15.10.2.19
15.10.4.1
and
15.12.2
15.11.6.5
TypeError
Indicates the actual type of an operand is different than the expected type. See
8.6.2
8.7.2
8.10.5
8.12.5
8.12.7
8.12.8
8.12.9
9.9
9.10
10.2.1
10.2.1.1.3
10.6
11.2.2
11.2.3
11.4.1
11.8.6
11.8.7
11.3.1
13.2
13.2.3
, 15,
15.2.3.2
15.2.3.3
15.2.3.4
15.2.3.5
15.2.3.6
15.2.3.7
15.2.3.8
15.2.3.9
15.2.3.10
15.2.3.11
15.2.3.12
15.2.3.13
15.2.3.14
15.2.4.3
15.3.4.2
15.3.4.3
15.3.4.4
15.3.4.5
15.3.4.5.2
15.3.4.5.3
15.3.5
15.3.5.3
15.3.5.4
15.4.4.3
15.4.4.11
15.4.4.16
15.4.4.17
15.4.4.18
15.4.4.19
15.4.4.20
15.4.4.21
15.4.4.22
15.4.5.1
15.5.4.2
15.5.4.3
15.6.4.2
15.6.4.3
15.7.4
15.7.4.2
15.7.4.4
15.9.5
15.9.5.44
15.10.4.1
15.10.6
15.11.4.4
and
15.12.3
15.11.6.6
URIError
Indicates that one of the global URI handling functions was used in a way that is incompatible with its definition. See
15.1.3
15.11.7
NativeError
Object
Structure
When an ECMAScript implementation detects a runtime error, it throws an instance of one of the
NativeError
objects
defined in
15.11.6
. 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
15.11.6
15.11.7.1
NativeError
Constructors Called as Functions
When a
NativeError
constructor is called as a function rather than as a constructor, it creates and initialises
a new object. A call of the object as a function is equivalent to calling it as a constructor with the same
arguments.
15.11.7.2
NativeError
(message)
The [[Prototype]] internal property of the newly constructed object is set to the prototype object for this error
constructor. The [[Class]] internal property of the newly constructed object is set to
Error
. The [[Extensible]] internal property of the newly constructed object is set to
true
If the argument
message
is not
undefined
, the
message
own property of the newly
constructed object is set to
ToString
message
).
15.11.7.3
The
NativeError
Constructors
When a
NativeError
constructor is called as part of a
new
expression, it is a constructor: it
initialises the newly created object.
15.11.7.4
new
NativeError
(message)
The [[Prototype]] internal property of the newly constructed object is set to the prototype object for this
NativeError
constructor. The [[Class]] internal property of the newly constructed object is set to
Error
. The [[Extensible]] internal property of the newly constructed object
is set to
true
If the argument
message
is not
undefined
, the
message
own property of the newly
constructed object is set to
ToString
message
).
15.11.7.5
Properties of the
NativeError
Constructors
The value of the [[Prototype]] internal property of a
NativeError
constructor is the Function prototype object
15.3.4
).
Besides the internal properties and the
length
property (whose value is
), each
NativeError
constructor has the following property:
15.11.7.6
NativeError
.prototype
The initial value of
NativeError
.prototype
is a
NativeError
prototype object (
15.11.7.7
). Each
NativeError
constructor has a
separate prototype object.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
15.11.7.7
Properties of the
NativeError
Prototype Objects
Each
NativeError
prototype object is an Error object (its [[Class]] is
Error
).
The value of the [[Prototype]] internal property of each
NativeError
prototype object is the standard built-in
Error prototype object (
15.11.4
).
15.11.7.8
NativeError
.prototype.constructor
The initial value of the
constructor
property of the prototype for a given
NativeError
constructor
is the
NativeError
constructor function itself (
15.11.7
).
15.11.7.9
NativeError
.prototype.name
The initial value of the
name
property of the prototype for a given
NativeError
constructor is the
name of the constructor (the name used instead of
NativeError
).
15.11.7.10
NativeError
.prototype.message
The initial value of the
message
property of the prototype for a given
NativeError
constructor is
the empty String.
NOTE
The prototypes for the NativeError constructors do not themselves provide a
toString
function, but instances of errors will inherit it from the Error prototype object.
15.11.7.11
Properties of
NativeError
Instances
NativeError
instances inherit properties from their
NativeError
prototype object and their [[Class]]
internal property value is
"Error"
NativeError
instances have no special properties.
15.12
The JSON Object
The
JSON
object is a single object that contains two functions,
parse
and
stringify
, that are used to
parse and construct JSON texts. The JSON Data Interchange Format is described in RFC 4627 <
>. The JSON interchange format used in
this specification is exactly that described by RFC 4627 with two exceptions:
The top level
JSONText
production of the ECMAScript JSON grammar may consist of any
JSONValue
rather than being restricted to being a
JSONObject
or a
JSONArray
as specified by RFC 4627.
Conforming implementations of
JSON.parse
and
JSON.stringify
must support the exact interchange format
described in this specification without any deletions or extensions to the format. This differs from RFC 4627 which
permits a JSON parser to accept non-JSON forms and extensions.
The value of the [[Prototype]] internal property of the JSON object is the standard built-in Object prototype object (
15.2.4
). The value of the [[Class]] internal property of the JSON object is
"JSON"
. The
value of the [[Extensible]] internal property of the JSON object is set to
true
The JSON object does not have a [[Construct]] internal property; it is not possible to use the JSON object as a constructor
with the
new
operator.
The JSON object does not have a [[Call]] internal property; it is not possible to invoke the JSON object as a function.
15.12.1
The JSON Grammar
JSON.stringify produces a String that conforms to the following JSON grammar. JSON.parse accepts a String that conforms
to the JSON grammar.
15.12.1.1
The JSON Lexical
Grammar
JSON is similar to ECMAScript source text in that it consists of a sequence of characters conforming to the rules of
SourceCharacter
. The JSON Lexical Grammar defines the tokens that make up a JSON text similar to
the manner that the ECMAScript lexical grammar defines the tokens of an ECMAScript source text. The JSON Lexical grammar
only recognises the white space character specified by the production
JSONWhiteSpace
. The JSON
lexical grammar shares some productions with the ECMAScript lexical grammar. All nonterminal symbols of the grammar that
do not begin with the characters “JSON” are defined by productions of the ECMAScript lexical grammar.
Syntax
JSONWhiteSpace
::
JSONString
::
JSONStringCharacters
opt
JSONStringCharacters
::
JSONStringCharacter
JSONStringCharacters
opt
JSONStringCharacter
::
SourceCharacter
but not one of
or
or
U+0000
through
U+001F
JSONEscapeSequence
JSONEscapeSequence
::
JSONEscapeCharacter
UnicodeEscapeSequence
JSONEscapeCharacter
::
one of
JSONNumber
::
opt
DecimalIntegerLiteral
JSONFraction
opt
ExponentPart
opt
JSONFraction
::
DecimalDigits
JSONNullLiteral
::
NullLiteral
JSONBooleanLiteral
::
BooleanLiteral
15.12.1.2
The JSON Syntactic
Grammar
The JSON Syntactic Grammar defines a valid JSON text in terms of tokens defined by the JSON lexical grammar. The goal
symbol of the grammar is
JSONText
Syntax
JSONText
JSONValue
JSONValue
JSONNullLiteral
JSONBooleanLiteral
JSONObject
JSONArray
JSONString
JSONNumber
JSONObject
JSONMemberList
JSONMember
JSONString
JSONValue
JSONMemberList
JSONMember
JSONMemberList
JSONMember
JSONArray
JSONElementList
JSONElementList
JSONValue
JSONElementList
JSONValue
15.12.2
parse ( text [ , reviver ]
The
parse
function parses a JSON text (a JSON-formatted String) and produces an ECMAScript value. The JSON
format is a restricted form of ECMAScript literal. JSON objects are realized as ECMAScript objects. JSON arrays are realized
as ECMAScript arrays. JSON strings, numbers, booleans, and null are realized as ECMAScript Strings, Numbers, Booleans, and
null
. JSON uses a more limited set of white space characters than
WhiteSpace
and allows
Unicode code points U+2028 and U+2029 to directly appear in
JSONString
literals without using an
escape sequence. The process of parsing is similar to
11.1.4
and
11.1.5
as constrained by the JSON grammar.
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
JText
be
ToString
text
).
Parse
JText
using the grammars in
15.12.1
. Throw a
SyntaxError
exception if
JText
did not conform to the JSON grammar for the goal symbol
JSONText
Let
unfiltered
be the result of parsing and evaluating
JText
as if it was the source text of an
ECMAScript
Program
but using
JSONString
in place of
StringLiteral
. Note that since
JText
conforms to the JSON grammar this result will be either a primitive value or an object that is defined by either an
ArrayLiteral
or an
ObjectLiteral
If
IsCallable
reviver
) is
true
, then
Let
root
be a new object created as if by the expression
new Object()
, where
Object
is the standard built-in constructor with that name.
Call the [[DefineOwnProperty]] internal method of
root
with the empty String, the PropertyDescriptor
{[[Value]]:
unfiltered
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
as arguments.
Return the result of calling the abstract operation Walk, passing
root
and the empty String. The abstract
operation Walk is described below.
Else
Return
unfiltered
The abstract operation Walk is a recursive abstract operation that takes two parameters: a
holder
object and
the String
name
of a property in that object. Walk uses the value of
reviver
that was originally
passed to the above parse function.
Let
val
be the result of calling the [[Get]] internal method of
holder
with argument
name
If
val
is an object, then
If the [[Class]] internal property of
val
is
"Array"
Set
to 0.
Let
len
be the result of calling the [[Get]] internal method of
val
with argument
"length"
Repeat while
len
Let
newElement
be the result of calling the abstract operation Walk, passing
val
and
ToString
).
If
newElement
is
undefined
, then
Call the [[Delete]] internal method of
val
with
ToString
) and
false
as arguments.
Else
Call the [[DefineOwnProperty]] internal method of
val
with arguments
ToString
), the
Property Descriptor
{[[Value]]:
newElement
, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and
false
Add 1 to
Else
Let
keys
be an internal
List
of String values consisting of the names of all the
own properties of
val
whose [[Enumerable]] attribute is
true
. The ordering of the Strings should
be the same as that used by the
Object.keys
standard built-in function.
For each String
in
keys
do,
Let
newElement
be the result of calling the abstract operation Walk, passing
val
and
If
newElement
is
undefined
, then
Call the [[Delete]] internal method of
val
with
and
false
as arguments.
Else
Call the [[DefineOwnProperty]] internal method of
val
with arguments
, the
Property Descriptor
{[[Value]]:
newElement
, [[Writable]]:
true
[[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Return the result of calling the [[Call]] internal method of
reviver
passing
holder
as the
this
value and with an argument list consisting of
name
and
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
In the case where there are duplicate name Strings within an object, lexically preceding
values for the same key shall be overwritten.
15.12.3
stringify ( value [ ,
replacer [ , space ] ] )
The
stringify
function returns a String in JSON format representing an ECMAScript value. It can take three
parameters. The first parameter is required. 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 a white 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 an empty
List
Let
indent
be the empty String.
Let
PropertyList
and
ReplacerFunction
be
undefined
If
Type
replacer
) is Object, then
If
IsCallable
replacer
) is
true
, then
Let
ReplacerFunction
be
replacer
Else if the [[Class]] internal property of
replacer
is
"Array"
, then
Let
PropertyList
be an empty internal
List
For each value
of a property of
replacer
that has an array index property name. The properties
are enumerated in the ascending array index order of their names.
Let
item
be
undefined
If
Type
) is String then let
item
be
v.
Else if
Type
) is Number then let
item
be
ToString
).
Else if
Type
) is Object then,
If the [[Class]] internal property of
is
"String"
or
"Number"
then
let
item
be
ToString
).
If
item
is not undefined and
item
is not currently an element of
PropertyList
then,
Append
item
to the end of
PropertyList
If
Type
space
) is Object then,
If the [[Class]] internal property of
space
is
"Number"
then,
Let
space
be
ToNumber
space
).
Else if the [[Class]] internal property of
space
is
"String"
then,
Let
space
be
ToString
space
).
If
Type
space
) is Number
Let
space
be min(10,
ToInteger
space
)).
Set
gap
to a String containing
space
space characters. This will be the empty String if
space
is less than 1.
Else if
Type
space)
is String
If the number of characters in
space
is 10 or less, set
gap
to
space
otherwise set
gap
to a String consisting of the first 10 characters of
space
Else
Set
gap
to the empty String.
Let
wrapper
be a new object created as if by the expression
new Object()
, where
Object
is the standard built-in constructor with that name.
Call the [[DefineOwnProperty]] internal method of
wrapper
with arguments the empty String, the
Property Descriptor
{[[Value]]:
value
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}, and
false
Return the result of calling the abstract operation
Str
with the empty String and
wrapper
The abstract operation
Str
key
holder
has access
to
ReplacerFunction
from the invocation of the
stringify
method. Its algorithm is as
follows:
Let
value
be the result of calling the [[Get]] internal method of
holder
with argument
key
If
Type
value
) is Object, then
Let
toJSON
be the result of calling the [[Get]] internal method of
value
with argument
"toJSON"
If
IsCallable
toJSON
) is
true
Let
value
be the result of calling the [[Call]] internal method of
toJSON
passing
value
as the
this
value and with an argument list consisting of
key
If
ReplacerFunction
is not
undefined
, then
Let
value
be the result of calling the [[Call]] internal method of
ReplacerFunction
passing
holder
as the
this
value and with an argument list consisting of
key
and
value
If
Type
value
) is Object then,
If the [[Class]] internal property of
value
is
"Number"
then,
Let
value
be
ToNumber
value
).
Else if the [[Class]] internal property of
value
is
"String"
then,
Let
value
be
ToString
value
).
Else if the [[Class]] internal property of
value
is
"Boolean"
then,
Let
value
be the value of the [[PrimitiveValue]] internal property of
value
If
value
is
null
then return
"null"
If
value
is
true
then return
"true"
If
value
is
false
then return
"false"
If
Type
value
) is String, then return the result of calling the abstract operation
Quote
with argument
value
If
Type
value
) is Number
If
value
is finite then return
ToString
value
).
Else, return
"null"
If
Type
value
) is Object, and
IsCallable
value
) is
false
If the [[Class]] internal property of
value
is
"Array"
then
Return the result of calling the abstract operation
JA
with argument
value
Else, return the result of calling the abstract operation
JO
with argument
value
Return
undefined
The abstract operation
Quote
value
wraps a String value
in double quotes and escapes characters within it.
Let
product
be the double quote character.
For each character
in
value
If
is the double quote character or the backslash character
Let
product
be the concatenation of
product
and the backslash character.
Let
product
be the concatenation of
product
and
Else if
is backspace, formfeed, newline, carriage return, or tab
Let
product
be the concatenation of
product
and the backslash character.
Let
abbrev
be the character corresponding to the value of
as follows:
backspace
"b"
formfeed
"f"
newline
"n"
carriage return
"r"
tab
"t"
Let
product
be the concatenation of
product
and
abbrev
Else if
is a control character having a code unit value less than the space character
Let
product
be the concatenation of
product
and the backslash character.
Let
product
be the concatenation of
product
and
"u"
Let
hex
be the result of converting the numeric code unit value of
to a String of four
hexadecimal digits.
Let
product
be the concatenation of
product
and
hex
Else
Let
product
be the concatenation of
product
and
Let
product
be the concatenation of
product
and the double quote character.
Return
product
The abstract operation
JO
value
serializes an object. It
has access to the
stack
indent
gap
PropertyList
ReplacerFunction
, and
space
of the invocation of the stringify method.
If
stack
contains
value
then throw a
TypeError
exception because the structure is cyclical.
Append
value
to
stack
Let
stepback
be
indent
Let
indent
be the concatenation of
indent
and
gap
If
PropertyList
is not
undefined
, then
Let
be
PropertyList
Else
Let
be an internal
List
of Strings consisting of the names of all the own
properties of
value
whose [[Enumerable]] attribute is
true
. The ordering of the Strings should be
the same as that used by the
Object.keys
standard built-in function.
Let
partial
be an empty
List
For each element
of
Let
strP
be the result of calling the abstract operation
Str
with arguments
and
value
If
strP
is not
undefined
Let
member
be the result of calling the abstract operation
Quote
with argument
Let
member
be the concatenation of
member
and the colon character.
If
gap
is not the empty String
Let
member
be the concatenation of
member
and the
space
character.
Let
member
be the concatenation of
member
and
strP
Append
member
to
partial
If
partial
is empty, then
Let
final
be
"{}"
Else
If
gap
is the empty String
Let
properties
be a String formed by concatenating all the element Strings of
partial
with each
adjacent pair of Strings separated with the comma character. A comma is not inserted either before the first
String or after the last String.
Let
final
be the result of concatenating
"{"
properties
, and
"}"
Else
gap
is not the empty String
Let
separator
be the result of concatenating the comma character, the line feed character, and
indent
Let
properties
be a String 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.
Let
final
be the result of concatenating
"{"
, the line feed character,
indent
properties
, the line feed character,
stepback
, and
"}
".
Remove the last element of
stack
Let
indent
be
stepback
Return
final
The abstract operation
JA
value
serializes an array. It
has access to the
stack
indent
gap
, and
space
of the invocation of the
stringify method. The representation of arrays includes only the elements between zero and
array.length
– 1
inclusive. Named properties are excluded from the stringification. An
array is stringified as an open left bracket, elements separated by comma, and a closing right bracket.
If
stack
contains
value
then throw a
TypeError
exception because the structure is cyclical.
Append
value
to
stack
Let
stepback
be
indent
Let
indent
be the concatenation of
indent
and
gap
Let
partial
be an empty
List
Let
len
be the result of calling the [[Get]] internal method of value with argument
"length"
Let
index
be 0.
Repeat while
index
len
Let
strP
be the result of calling the abstract operation
Str
with arguments
ToString
index
) and
value
If
strP
is
undefined
Append
"null"
to
partial
Else
Append
strP
to
partial
Increment
index
by 1.
If
partial
is empty ,then
Let
final
be
"[]"
Else
If
gap
is the empty String
Let
properties
be a String formed by concatenating all the element Strings of
partial
with each
adjacent pair of Strings separated with the comma character. A comma is not inserted either before the first
String or after the last String.
Let
final
be the result of concatenating
"["
properties
, and
"]"
Else
Let
separator
be the result of concatenating the comma character, the line feed character, and
indent
Let
properties
be a String 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.
Let
final
be the result of concatenating
"["
, the line feed character,
indent
properties
, the line feed character,
stepback
, and
"]
".
Remove the last element of
stack
Let
indent
be
stepback
Return
final
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 an 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 double quotes. The characters
and
are escaped with
prefixes. Control characters are replaced with escape sequences
\u
HHHH, or with the shorter forms,
\b
(backspace),
\f
(formfeed),
\n
(newline),
\r
(carriage return),
\t
(tab).
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 an opening left brace followed by zero or more properties,
separated with commas, closed with a right brace. A property is a quoted String representing the key or property name, a
colon, and then the stringified property value. An array is rendered as an opening left bracket followed by zero or more
values, separated with commas, closed with a right bracket.
16
Errors
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
Program
containing the error. An implementation must report early errors in a
Program
prior to the first evaluation of that
Program
. Early errors in
eval
code are reported at the time
eval
is called but prior to evaluation of any construct within the
eval
code. All errors that are not early
errors are runtime errors.
An implementation must treat any instance of the following kinds of errors as an early error:
Any syntax error.
Attempts to define an
ObjectLiteral
that has multiple
get
property assignments with the
same name or multiple
set
property assignments with the same name.
Attempts to define an
ObjectLiteral
that has both a data property assignment and a
get
or
set
property assignment with the same name.
Errors in regular expression literals that are not implementation-defined syntax extensions.
Attempts in
strict mode code
to define an
ObjectLiteral
that has
multiple data property assignments with the same name.
The occurrence of a
WithStatement
in
strict mode code
The occurrence of an
Identifier
value appearing more than once within a
FormalParameterList
of an individual strict mode
FunctionDeclaration
or
FunctionExpression
Improper uses of
return
break
, and
continue
Attempts to call
PutValue
on any value for which an early determination can be made that the
value is not a
Reference
(for example, executing the assignment statement
3=4
).
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:
An implementation may extend program 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 program syntax or regular expression pattern or flag syntax.
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
).
An implementation may define behaviour other than throwing
RangeError
for
toFixed
toExponential
, and
toPrecision
when the
fractionDigits
or
precision
argument
is outside the specified range.
Annex A
(informative)
Grammar Summary
A.1
Lexical Grammar
See clause 6
SourceCharacter
::
any Unicode code unit
See clause 7
InputElementDiv
::
WhiteSpace
LineTerminator
Comment
Token
DivPunctuator
See clause 7
InputElementRegExp
::
WhiteSpace
LineTerminator
Comment
Token
RegularExpressionLiteral
See 7.2
WhiteSpace
::
See 7.3
LineTerminator
::
See 7.3
LineTerminatorSequence
::
[lookahead ∉
See 7.4
Comment
::
MultiLineComment
SingleLineComment
See 7.4
MultiLineComment
::
/*
MultiLineCommentChars
opt
*/
See 7.4
MultiLineCommentChars
::
MultiLineNotAsteriskChar
MultiLineCommentChars
opt
PostAsteriskCommentChars
opt
See 7.4
PostAsteriskCommentChars
::
MultiLineNotForwardSlashOrAsteriskChar
MultiLineCommentChars
opt
PostAsteriskCommentChars
opt
See 7.4
MultiLineNotAsteriskChar
::
SourceCharacter
but not
See 7.4
MultiLineNotForwardSlashOrAsteriskChar
::
SourceCharacter
but not one of
or
See 7.4
SingleLineComment
::
//
SingleLineCommentChars
opt
See 7.4
SingleLineCommentChars
::
SingleLineCommentChar
SingleLineCommentChars
opt
See 7.4
SingleLineCommentChar
::
SourceCharacter
but not
LineTerminator
See 7.5
Token
::
IdentifierName
Punctuator
NumericLiteral
StringLiteral
See 7.6
Identifier
::
IdentifierName
but not
ReservedWord
See 7.6
IdentifierName
::
IdentifierStart
IdentifierName
IdentifierPart
See 7.6
IdentifierStart
::
UnicodeLetter
UnicodeEscapeSequence
See 7.6
IdentifierPart
::
IdentifierStart
UnicodeCombiningMark
UnicodeDigit
UnicodeConnectorPunctuation
See 7.6
UnicodeLetter
::
any character in the Unicode categories “Uppercase letter (Lu)”, “Lowercase letter (Ll)”, “Titlecase letter (Lt)”, “Modifier letter (Lm)”, “Other letter (Lo)”, or “Letter number (Nl)”.
See 7.6
UnicodeCombiningMark
::
any character in the Unicode categories “Non-spacing mark (Mn)” or “Combining spacing mark (Mc)”
See 7.6
UnicodeDigit
::
any character in the Unicode category “Decimal number (Nd)”
See 7.6
UnicodeConnectorPunctuation
::
any character in the Unicode category “Connector punctuation (Pc)”
See 7.6.1
ReservedWord
::
Keyword
FutureReservedWord
NullLiteral
BooleanLiteral
See 7.6.1.1
Keyword
::
one of
break
do
instanceof
typeof
case
else
new
var
catch
finally
return
void
continue
for
switch
while
debugger
function
this
with
default
if
throw
delete
in
try
See 7.6.1.2
FutureReservedWord
::
one of
class
enum
extends
super
const
export
import
The following tokens are also considered to be
FutureReservedWords
when parsing
strict mode code (see 10.1.1)
implements
let
private
public
interface
package
protected
static
yield
See 7.7
Punctuator
::
one of
<=
>=
==
!=
===
!==
++
--
<<
>>
>>>
&&
||
+=
-=
*=
%=
<<=
>>=
>>>=
&=
|=
^=
See 7.7
DivPunctuator
::
one of
/=
See 7.8
Literal
::
NullLiteral
BooleanLiteral
NumericLiteral
StringLiteral
RegularExpressionLiteral
See 7.8.1
NullLiteral
::
null
See 7.8.2
BooleanLiteral
::
true
false
See 7.8.3
NumericLiteral
::
DecimalLiteral
HexIntegerLiteral
See 7.8.3
DecimalLiteral
::
DecimalIntegerLiteral
DecimalDigits
opt
ExponentPart
opt
DecimalDigits
ExponentPart
opt
DecimalIntegerLiteral
ExponentPart
opt
See 7.8.3
DecimalIntegerLiteral
::
NonZeroDigit
DecimalDigits
opt
See 7.8.3
DecimalDigits
::
DecimalDigit
DecimalDigits
DecimalDigit
See 7.8.3
DecimalDigit
::
one of
See 7.8.3
NonZeroDigit
::
one of
See 7.8.3
ExponentPart
::
ExponentIndicator
SignedInteger
See 7.8.3
ExponentIndicator
::
one of
See 7.8.3
SignedInteger
::
DecimalDigits
DecimalDigits
DecimalDigits
See 7.8.3
HexIntegerLiteral
::
0x
HexDigit
0X
HexDigit
HexIntegerLiteral
HexDigit
See 7.8.3
HexDigit
::
one of
See 7.8.4
StringLiteral
::
DoubleStringCharacters
opt
SingleStringCharacters
opt
See 7.8.4
DoubleStringCharacters
::
DoubleStringCharacter
DoubleStringCharacters
opt
See 7.8.4
SingleStringCharacters
::
SingleStringCharacter
SingleStringCharacters
opt
See 7.8.4
DoubleStringCharacter
::
SourceCharacter
but not one of
or
or
LineTerminator
EscapeSequence
LineContinuation
See 7.8.4
SingleStringCharacter
::
SourceCharacter
but not one of
or
or
LineTerminator
EscapeSequence
LineContinuation
See 7.8.4
LineContinuation
::
LineTerminatorSequence
See 7.8.4
EscapeSequence
::
CharacterEscapeSequence
[lookahead ∉
DecimalDigit
HexEscapeSequence
UnicodeEscapeSequence
See 7.8.4
CharacterEscapeSequence
::
SingleEscapeCharacter
NonEscapeCharacter
See 7.8.4
SingleEscapeCharacter
::
one of
See 7.8.4
NonEscapeCharacter
::
SourceCharacter
but not one of
EscapeCharacter
or
LineTerminator
See 7.8.4
EscapeCharacter
::
SingleEscapeCharacter
DecimalDigit
See 7.8.4
HexEscapeSequence
::
HexDigit
HexDigit
See 7.8.4
UnicodeEscapeSequence
::
HexDigit
HexDigit
HexDigit
HexDigit
See 7.8.5
RegularExpressionLiteral
::
RegularExpressionBody
RegularExpressionFlags
See 7.8.5
RegularExpressionBody
::
RegularExpressionFirstChar
RegularExpressionChars
See 7.8.5
RegularExpressionChars
::
[empty]
RegularExpressionChars
RegularExpressionChar
See 7.8.5
RegularExpressionFirstChar
::
RegularExpressionNonTerminator
but not one of
or
or
or
RegularExpressionBackslashSequence
RegularExpressionClass
See 7.8.5
RegularExpressionChar
::
RegularExpressionNonTerminator
but not
or
or
RegularExpressionBackslashSequence
RegularExpressionClass
See 7.8.5
RegularExpressionBackslashSequence
::
RegularExpressionNonTerminator
See 7.8.5
RegularExpressionNonTerminator
::
SourceCharacter
but not
LineTerminator
See 7.8.5
RegularExpressionClass
::
RegularExpressionClassChars
See 7.8.5
RegularExpressionClassChars
::
[empty]
RegularExpressionClassChars
RegularExpressionClassChar
See 7.8.5
RegularExpressionClassChar
::
RegularExpressionNonTerminator
but not
or
RegularExpressionBackslashSequence
See 7.8.5
RegularExpressionFlags
::
[empty]
RegularExpressionFlags
IdentifierPart
A.2
Number Conversions
See 9.3.1
StringNumericLiteral
:::
StrWhiteSpace
opt
StrWhiteSpace
opt
StrNumericLiteral
StrWhiteSpace
opt
See 9.3.1
StrWhiteSpace
:::
StrWhiteSpaceChar
StrWhiteSpace
opt
See 9.3.1
StrWhiteSpaceChar
:::
WhiteSpace
LineTerminator
See 9.3.1
StrNumericLiteral
:::
StrDecimalLiteral
HexIntegerLiteral
See 9.3.1
StrDecimalLiteral
:::
StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral
See 9.3.1
StrUnsignedDecimalLiteral
:::
Infinity
DecimalDigits
DecimalDigits
opt
ExponentPart
opt
DecimalDigits
ExponentPart
opt
DecimalDigits
ExponentPart
opt
See 9.3.1
DecimalDigits
:::
DecimalDigit
DecimalDigits
DecimalDigit
See 9.3.1
DecimalDigit
:::
one of
See 9.3.1
ExponentPart
:::
ExponentIndicator
SignedInteger
See 9.3.1
ExponentIndicator
:::
one of
See 9.3.1
SignedInteger
:::
DecimalDigits
DecimalDigits
DecimalDigits
See 9.3.1
HexIntegerLiteral
:::
0x
HexDigit
0X
HexDigit
HexIntegerLiteral
HexDigit
See 9.3.1
HexDigit
:::
one of
A.3
Expressions
See 11.1
PrimaryExpression
this
Identifier
Literal
ArrayLiteral
ObjectLiteral
Expression
See 11.1.4
ArrayLiteral
Elision
opt
ElementList
ElementList
Elision
opt
See 11.1.4
ElementList
Elision
opt
AssignmentExpression
ElementList
Elision
opt
AssignmentExpression
See 11.1.4
Elision
Elision
See 11.1.5
ObjectLiteral
PropertyNameAndValueList
PropertyNameAndValueList
See 11.1.5
PropertyNameAndValueList
PropertyAssignment
PropertyNameAndValueList
PropertyAssignment
See 11.1.5
PropertyAssignment
PropertyName
AssignmentExpression
get
PropertyName
FunctionBody
set
PropertyName
PropertySetParameterList
FunctionBody
See 11.1.5
PropertyName
IdentifierName
StringLiteral
NumericLiteral
See 11.1.5
PropertySetParameterList
Identifier
See 11.2
MemberExpression
PrimaryExpression
FunctionExpression
MemberExpression
Expression
MemberExpression
IdentifierName
new
MemberExpression
Arguments
See 11.2
NewExpression
MemberExpression
new
NewExpression
See 11.2
CallExpression
MemberExpression
Arguments
CallExpression
Arguments
CallExpression
Expression
CallExpression
IdentifierName
See 11.2
Arguments
ArgumentList
See 11.2
ArgumentList
AssignmentExpression
ArgumentList
AssignmentExpression
See 11.2
LeftHandSideExpression
NewExpression
CallExpression
See 11.3
PostfixExpression
LeftHandSideExpression
LeftHandSideExpression
[no
LineTerminator
here]
++
LeftHandSideExpression
[no
LineTerminator
here]
--
See 11.4
UnaryExpression
PostfixExpression
delete
UnaryExpression
void
UnaryExpression
typeof
UnaryExpression
++
UnaryExpression
--
UnaryExpression
UnaryExpression
UnaryExpression
UnaryExpression
UnaryExpression
See 11.5
MultiplicativeExpression
UnaryExpression
MultiplicativeExpression
UnaryExpression
MultiplicativeExpression
UnaryExpression
MultiplicativeExpression
UnaryExpression
See 11.6
AdditiveExpression
MultiplicativeExpression
AdditiveExpression
MultiplicativeExpression
AdditiveExpression
MultiplicativeExpression
See 11.7
ShiftExpression
AdditiveExpression
ShiftExpression
<<
AdditiveExpression
ShiftExpression
>>
AdditiveExpression
ShiftExpression
>>>
AdditiveExpression
See 11.8
RelationalExpression
ShiftExpression
RelationalExpression
ShiftExpression
RelationalExpression
ShiftExpression
RelationalExpression
<=
ShiftExpression
RelationalExpression
>=
ShiftExpression
RelationalExpression
instanceof
ShiftExpression
RelationalExpression
in
ShiftExpression
See 11.8
RelationalExpressionNoIn
ShiftExpression
RelationalExpressionNoIn
ShiftExpression
RelationalExpressionNoIn
ShiftExpression
RelationalExpressionNoIn
<=
ShiftExpression
RelationalExpressionNoIn
>=
ShiftExpression
RelationalExpressionNoIn
instanceof
ShiftExpression
See 11.9
EqualityExpression
RelationalExpression
EqualityExpression
==
RelationalExpression
EqualityExpression
!=
RelationalExpression
EqualityExpression
===
RelationalExpression
EqualityExpression
!==
RelationalExpression
See 11.9
EqualityExpressionNoIn
RelationalExpressionNoIn
EqualityExpressionNoIn
==
RelationalExpressionNoIn
EqualityExpressionNoIn
!=
RelationalExpressionNoIn
EqualityExpressionNoIn
===
RelationalExpressionNoIn
EqualityExpressionNoIn
!==
RelationalExpressionNoIn
See 11.10
BitwiseANDExpression
EqualityExpression
BitwiseANDExpression
EqualityExpression
See 11.10
BitwiseANDExpressionNoIn
EqualityExpressionNoIn
BitwiseANDExpressionNoIn
EqualityExpressionNoIn
See 11.10
BitwiseXORExpression
BitwiseANDExpression
BitwiseXORExpression
BitwiseANDExpression
See 11.10
BitwiseXORExpressionNoIn
BitwiseANDExpressionNoIn
BitwiseXORExpressionNoIn
BitwiseANDExpressionNoIn
See 11.10
BitwiseORExpression
BitwiseXORExpression
BitwiseORExpression
BitwiseXORExpression
See 11.10
BitwiseORExpressionNoIn
BitwiseXORExpressionNoIn
BitwiseORExpressionNoIn
BitwiseXORExpressionNoIn
See 11.11
LogicalANDExpression
BitwiseORExpression
LogicalANDExpression
&&
BitwiseORExpression
See 11.11
LogicalANDExpressionNoIn
BitwiseORExpressionNoIn
LogicalANDExpressionNoIn
&&
BitwiseORExpressionNoIn
See 11.11
LogicalORExpression
LogicalANDExpression
LogicalORExpression
||
LogicalANDExpression
See 11.11
LogicalORExpressionNoIn
LogicalANDExpressionNoIn
LogicalORExpressionNoIn
||
LogicalANDExpressionNoIn
See 11.12
ConditionalExpression
LogicalORExpression
LogicalORExpression
AssignmentExpression
AssignmentExpression
See 11.12
ConditionalExpressionNoIn
LogicalORExpressionNoIn
LogicalORExpressionNoIn
AssignmentExpression
AssignmentExpressionNoIn
See 11.13
AssignmentExpression
ConditionalExpression
LeftHandSideExpression
AssignmentExpression
LeftHandSideExpression
AssignmentOperator
AssignmentExpression
See 11.13
AssignmentExpressionNoIn
ConditionalExpressionNoIn
LeftHandSideExpression
AssignmentExpressionNoIn
LeftHandSideExpression
AssignmentOperator
AssignmentExpressionNoIn
See 11.13
AssignmentOperator
one of
*=
/=
%=
+=
-=
<<=
>>=
>>>=
&=
^=
|=
See 11.14
Expression
AssignmentExpression
Expression
AssignmentExpression
See 11.14
ExpressionNoIn
AssignmentExpressionNoIn
ExpressionNoIn
AssignmentExpressionNoIn
A.4
Statements
See clause 12
Statement
Block
VariableStatement
EmptyStatement
ExpressionStatement
IfStatement
IterationStatement
ContinueStatement
BreakStatement
ReturnStatement
WithStatement
LabelledStatement
SwitchStatement
ThrowStatement
TryStatement
DebuggerStatement
See 12.1
Block
StatementList
opt
See 12.1
StatementList
Statement
StatementList
Statement
See 12.2
VariableStatement
var
VariableDeclarationList
See 12.2
VariableDeclarationList
VariableDeclaration
VariableDeclarationList
VariableDeclaration
See 12.2
VariableDeclarationListNoIn
VariableDeclarationNoIn
VariableDeclarationListNoIn
VariableDeclarationNoIn
See 12.2
VariableDeclaration
Identifier
Initialiser
opt
See 12.2
VariableDeclarationNoIn
Identifier
InitialiserNoIn
opt
See 12.2
Initialiser
AssignmentExpression
See 12.2
InitialiserNoIn
AssignmentExpressionNoIn
See 12.3
EmptyStatement
See 12.4
ExpressionStatement
[lookahead ∉ {
function
}]
Expression
See 12.5
IfStatement
if
Expression
Statement
else
Statement
if
Expression
Statement
See 12.6
IterationStatement
do
Statement
while
Expression
);
while
Expression
Statement
for
ExpressionNoIn
opt
Expression
opt
Expression
opt
Statement
for
var
VariableDeclarationListNoIn
Expression
opt
Expression
opt
Statement
for
LeftHandSideExpression
in
Expression
Statement
for
var
VariableDeclarationNoIn
in
Expression
Statement
See 12.7
ContinueStatement
continue
continue
[no
LineTerminator
here]
Identifier
See 12.8
BreakStatement
break
break
[no
LineTerminator
here]
Identifier
See 12.9
ReturnStatement
return
return
[no
LineTerminator
here]
Expression
See 12.10
WithStatement
with
Expression
Statement
See 12.11
SwitchStatement
switch
Expression
CaseBlock
See 12.11
CaseBlock
CaseClauses
opt
CaseClauses
opt
DefaultClause
CaseClauses
opt
See 12.11
CaseClauses
CaseClause
CaseClauses
CaseClause
See 12.11
CaseClause
case
Expression
StatementList
opt
See 12.11
DefaultClause
default
StatementList
opt
See 12.12
LabelledStatement
Identifier
Statement
See 12.13
ThrowStatement
throw
[no
LineTerminator
here]
Expression
See 12.14
TryStatement
try
Block
Catch
try
Block
Finally
try
Block
Catch
Finally
See 12.14
Catch
catch
Identifier
Block
See 12.14
Finally
finally
Block
See 12.15
DebuggerStatement
debugger
A.5
Functions and Programs
See clause 13
FunctionDeclaration
function
Identifier
FormalParameterList
opt
FunctionBody
See clause 13
FunctionExpression
function
Identifier
opt
FormalParameterList
opt
FunctionBody
See clause 13
FormalParameterList
Identifier
FormalParameterList
Identifier
See clause 13
FunctionBody
SourceElements
opt
See clause 14
Program
SourceElements
opt
See clause 14
SourceElements
SourceElement
SourceElements
SourceElement
See clause 14
SourceElement
Statement
FunctionDeclaration
A.6
Universal Resource Identifier
Character Classes
See 15.1.3
uri
:::
uriCharacters
opt
See 15.1.3
uriCharacters
:::
uriCharacter
uriCharacters
opt
See 15.1.3
uriCharacter
:::
uriReserved
uriUnescaped
uriEscaped
See 15.1.3
uriReserved
:::
one of
See 15.1.3
uriUnescaped
:::
uriAlpha
DecimalDigit
uriMark
See 15.1.3
uriEscaped
:::
HexDigit
HexDigit
See 15.1.3
uriAlpha
:::
one of
See 15.1.3
uriMark
:::
one of
A.7
Regular Expressions
See 15.10.1
Pattern
::
Disjunction
See 15.10.1
Disjunction
::
Alternative
Alternative
Disjunction
See 15.10.1
Alternative
::
[empty]
Alternative
Term
See 15.10.1
Term
::
Assertion
Atom
Atom
Quantifier
See 15.10.1
Assertion
::
Disjunction
Disjunction
See 15.10.1
Quantifier
::
QuantifierPrefix
QuantifierPrefix
See 15.10.1
QuantifierPrefix
::
DecimalDigits
DecimalDigits
DecimalDigits
DecimalDigits
See 15.10.1
Atom
::
PatternCharacter
AtomEscape
CharacterClass
Disjunction
Disjunction
See 15.10.1
PatternCharacter
::
SourceCharacter
but not one of
See 15.10.1
AtomEscape
::
DecimalEscape
CharacterEscape
CharacterClassEscape
See 15.10.1
CharacterEscape
::
ControlEscape
ControlLetter
HexEscapeSequence
UnicodeEscapeSequence
IdentityEscape
See 15.10.1
ControlEscape
::
one of
See 15.10.1
ControlLetter
::
one of
See 15.10.1
IdentityEscape
::
SourceCharacter
but not
IdentifierPart
See 15.10.1
DecimalEscape
::
DecimalIntegerLiteral
[lookahead ∉
DecimalDigit
See 15.10.1
CharacterClassEscape
::
one of
See 15.10.1
CharacterClass
::
[lookahead ∉ {
}]
ClassRanges
ClassRanges
See 15.10.1
ClassRanges
::
[empty]
NonemptyClassRanges
See 15.10.1
NonemptyClassRanges
::
ClassAtom
ClassAtom
NonemptyClassRangesNoDash
ClassAtom
ClassAtom
ClassRanges
See 15.10.1
NonemptyClassRangesNoDash
::
ClassAtom
ClassAtomNoDash
NonemptyClassRangesNoDash
ClassAtomNoDash
ClassAtom
ClassRanges
See 15.10.1
ClassAtom
::
ClassAtomNoDash
See 15.10.1
ClassAtomNoDash
::
SourceCharacter
but not one of
or
or
ClassEscape
See 15.10.1
ClassEscape
::
DecimalEscape
CharacterEscape
CharacterClassEscape
A.8
JSON
A.8.1
JSON Lexical Grammar
See 15.12.1.1
JSONWhiteSpace
::
See 15.12.1.1
JSONString
::
JSONStringCharacters
opt
See 15.12.1.1
JSONStringCharacters
::
JSONStringCharacter
JSONStringCharacters
opt
See 15.12.1.1
JSONStringCharacter
::
SourceCharacter
but not one of
or
or
U+0000 through U+001F
JSONEscapeSequence
See 15.12.1.1
JSONEscapeSequence
::
JSONEscapeCharacter
UnicodeEscapeSequence
See 15.12.1.1
JSONEscapeCharacter
::
one of
See 15.12.1.1
JSONNumber
::
opt
DecimalIntegerLiteral
JSONFraction
opt
ExponentPart
opt
See 15.12.1.1
JSONFraction
::
DecimalDigits
See 15.12.1.1
JSONNullLiteral
::
NullLiteral
See 15.12.1.1
JSONBooleanLiteral
::
BooleanLiteral
A.8.2
JSON Syntactic Grammar
See 15.12.1.2
JSONText
JSONValue
See 15.12.1.2
JSONValue
JSONNullLiteral
JSONBooleanLiteral
JSONObject
JSONArray
JSONString
JSONNumber
See 15.12.1.2
JSONObject
JSONMemberList
See 15.12.1.2
JSONMember
JSONString
JSONValue
See 15.12.1.2
JSONMemberList
JSONMember
JSONMemberList
JSONMember
See 15.12.1.2
JSONArray
JSONElementList
See 15.12.1.2
JSONElementList
JSONValue
JSONElementList
JSONValue
Annex B
(informative)
Compatibility
B.1
Additional Syntax
Past editions of ECMAScript have included additional syntax and semantics for specifying octal literals and octal escape
sequences. These have been removed from this edition of ECMAScript. This non-normative annex presents uniform syntax and
semantics for octal literals and octal escape sequences for compatibility with some older ECMAScript programs.
B.1.1
Numeric Literals
The syntax and semantics of
7.8.3
can be extended as follows except that this extension is not
allowed for
strict mode code
Syntax
NumericLiteral
::
DecimalLiteral
HexIntegerLiteral
OctalIntegerLiteral
OctalIntegerLiteral
::
OctalDigit
OctalIntegerLiteral
OctalDigit
OctalDigit
::
one of
Semantics
The MV of
NumericLiteral
::
OctalIntegerLiteral
is the MV of
OctalIntegerLiteral
The MV of
OctalDigit
::
is 0.
The MV of
OctalDigit
::
is 1.
The MV of
OctalDigit
::
is 2.
The MV of
OctalDigit
::
is 3.
The MV of
OctalDigit
::
is 4.
The MV of
OctalDigit
::
is 5.
The MV of
OctalDigit
::
is 6.
The MV of
OctalDigit
::
is 7.
The MV of
OctalIntegerLiteral
::
OctalDigit
is the MV of
OctalDigit
The MV of
OctalIntegerLiteral
::
OctalIntegerLiteral
OctalDigit
is (the MV of
OctalIntegerLiteral
times 8) plus the MV of
OctalDigit
B.1.2
String Literals
The syntax and semantics of
7.8.4
can be extended as follows except that this extension is not
allowed for
strict mode code
Syntax
EscapeSequence
::
CharacterEscapeSequence
OctalEscapeSequence
HexEscapeSequence
UnicodeEscapeSequence
OctalEscapeSequence
::
OctalDigit
[lookahead ∉
DecimalDigit
ZeroToThree
OctalDigit
[lookahead ∉
DecimalDigit
FourToSeven
OctalDigit
ZeroToThree
OctalDigit
OctalDigit
ZeroToThree
::
one of
FourToSeven
::
one of
Semantics
The CV of
EscapeSequence
::
OctalEscapeSequence
is the CV of the
OctalEscapeSequence
The CV of
OctalEscapeSequence
::
OctalDigit
[lookahead ∉
DecimalDigit
] is the character whose code unit value is
the MV of the
OctalDigit
The CV of
OctalEscapeSequence
::
ZeroToThree
OctalDigit
[lookahead ∉
DecimalDigit
] is the
character whose code unit value is (8 times the MV of the
ZeroToThree
) plus the MV of the
OctalDigit
The CV of
OctalEscapeSequence
::
FourToSeven
OctalDigit
is the character whose code unit value is (8
times the MV of the
FourToSeven
) plus the MV of the
OctalDigit
The CV of
OctalEscapeSequence
::
ZeroToThree
OctalDigit
OctalDigit
is the
character whose code unit value is (64 (that is, 8
) times the MV of the
ZeroToThree
) plus (8 times
the MV of the first
OctalDigit
) plus the MV of the second
OctalDigit
The MV of
ZeroToThree
::
is 0.
The MV of
ZeroToThree
::
is 1.
The MV of
ZeroToThree
::
is 2.
The MV of
ZeroToThree
::
is 3.
The MV of
FourToSeven
::
is 4.
The MV of
FourToSeven
::
is 5.
The MV of
FourToSeven
::
is 6.
The MV of
FourToSeven
::
is 7.
B.2
Additional Properties
Some implementations of ECMAScript have included additional properties for some of the standard native objects. This
non-normative annex suggests uniform semantics for such properties without making the properties or their semantics part of
this standard.
B.2.1
escape (string)
The
escape
function is a property of the global object. It computes a new version of a String value in which
certain characters have been replaced by a hexadecimal escape sequence.
For those characters being replaced whose code unit value is
0xFF
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
0xFF
, a four-digit escape sequence of the form
%u
xxxx
is used.
When the
escape
function is called with one argument
string
, the following steps are taken:
Call
ToString
string
).
Compute the number of characters in Result(1).
Let
be the empty string.
Let
be 0.
If
equals Result(2), return
Get the character (represented as a 16-bit unsigned integer) at position
within Result(1).
If Result(6) is one of the 69 nonblank
characters
“ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@*_+-./”
then
go to step 13.
If Result(6), is less than 256, go to step 11.
Let
be a String containing six characters
“%u
wxyz
where
wxyz
are four hexadecimal digits encoding the value of Result(6).
Go to step 14.
Let
be a String containing three characters
“%
xy
where
xy
are two hexadecimal digits encoding the value of Result(6).
Go to step 14.
Let
be a String containing the single character Result(6).
Let
be a new String value computed by concatenating the previous value of
and
Increase
by 1.
Go to step 5.
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.2.2
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
character that it represents.
When the
unescape
function is called with one argument
string
, the following steps are taken:
Call
ToString
string
).
Compute the number of characters in Result(1).
Let
be the empty String.
Let
be 0.
If
equals Result(2), return
Let
be the character at position
within Result(1).
If
is not
, go to step 18.
If
is greater than Result(2)−6, go to step 14.
If the character at position
+1 within Result(1) is not
, go to step 14.
If the four characters at positions
+2,
+3,
+4, and
+5 within Result(1) are not all
hexadecimal digits, go to step 14.
Let
be the character whose code unit value is the integer represented by the four hexadecimal digits at
positions
+2,
+3,
+4, and
+5 within Result(1).
Increase k by 5.
Go to step 18.
If
is greater than Result(2)−3, go to step 18.
If the two characters at positions
+1 and
+2 within Result(1) are not both hexadecimal digits, go to
step 18.
Let
be the character whose code unit value is the integer represented by two zeroes plus the two hexadecimal
digits at positions
+1 and
+2 within Result(1).
Increase
by 2.
Let
be a new String value computed by concatenating the previous value of
and
Increase
by 1.
Go to step 5.
B.2.3
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 character position
start
and running for
length
characters (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:
Call
ToString
, giving it the
this
value as its argument.
Call
ToInteger
start
).
If
length
is
undefined
, use
+∞
; otherwise call
ToInteger
length
).
Compute the number of characters in Result(1).
If Result(2) is positive or zero, use Result(2); else use max(Result(4)+Result(2),0).
Compute min(max(Result(3),0), Result(4)–Result(5)).
If Result(6) ≤ 0, return the empty String “”.
Return a String containing Result(6) consecutive characters from Result(1) beginning with the character at position
Result(5).
The
length
property of the
substr
method is
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.2.4
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
be
this time value
If
is
NaN
, return
NaN
Return
YearFromTime
LocalTime
)) − 1900.
B.2.5
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
be the result of
LocalTime
this time value
); but if
this time value
is
NaN
, let
be
+0
Call
ToNumber
year
).
If Result(2) is
NaN
, set the [[PrimitiveValue]] internal property of the
this
value to
NaN
and
return
NaN
If Result(2) is not
NaN
and 0 ≤
ToInteger
(Result(2)) ≤ 99 then Result(4) is
ToInteger
(Result(2)) + 1900. Otherwise, Result(4) is Result(2).
Compute
MakeDay
(Result(4),
MonthFromTime
),
DateFromTime
)).
Compute
UTC
MakeDate
(Result(5),
TimeWithinDay
))).
Set the [[PrimitiveValue]] internal property of the
this
value to
TimeClip
(Result(6)).
Return the value of the [[PrimitiveValue]] internal property of the
this
value.
B.2.6
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
Annex C
(informative)
The Strict Mode of ECMAScript
The strict mode restriction and exceptions
The identifiers "
implements
", "
interface
", "
let
", "
package
",
private
", "
protected
", "
public
", "
static
", and "
yield
" are
classified as
FutureReservedWord
tokens within
strict mode code
. (7.6.12).
A conforming implementation, when processing
strict mode code
, may not extend the syntax of
NumericLiteral
7.8.3
) to include
OctalIntegerLiteral
as described in
B.1.1
A conforming implementation, when processing
strict mode code (see 10.1.1)
, may not extend the
syntax of
EscapeSequence
to include
OctalEscapeSequence
as described in
B.1.2
Assignment to an undeclared identifier or otherwise unresolvable reference does not create a property in the global
object. When a simple assignment occurs within
strict mode code
, its
LeftHandSide
must not
evaluate to an
unresolvable Reference
. If it does a
ReferenceError
exception is thrown (
8.7.2
). The
LeftHandSide
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 whose [[Extensible]] internal property has the value
false
. In these cases a
TypeError
exception is thrown (
11.13.1
).
The identifier
eval
or
arguments
may not appear as the
LeftHandSideExpression
of an
Assignment operator (
11.13
) or of a
PostfixExpression
11.3
) or as
the
UnaryExpression
operated upon by a Prefix Increment (
11.4.4
) or a Prefix Decrement (
11.4.5
) operator.
Arguments objects for strict mode functions define non-configurable accessor properties named "
caller
" and
callee
" which throw a
TypeError
exception on access (
10.6
).
Arguments objects for strict mode functions do not dynamically share their array indexed property values with the
corresponding formal parameter bindings of their functions. (
10.6
).
For strict mode functions, if an arguments object is created the binding of the local identifier
arguments
to the arguments object is immutable and hence may not be the target of an assignment expression. (
10.5
).
It is a
SyntaxError
if
strict mode code
contains an
ObjectLiteral
with more than
one definition of any data property (
11.1.5
).
It is a
SyntaxError
if the
Identifier
"eval"
or the
Identifier
"arguments"
occurs as the
Identifier
in a
PropertySetParameterList
of a
PropertyAssignment
that is contained in
strict code
or if its
FunctionBody
is
strict code
11.1.5
).
Strict mode eval code cannot instantiate variables or functions in the variable environment of the caller to eval.
Instead, a new variable environment is created and that environment is used for declaration binding instantiation for the
eval code (
10.4.2
).
If
this
is evaluated within
strict mode code
, then the
this
value is not coerced
to an object. A
this
value of
null
or
undefined
is not converted to the global object and primitive
values are not converted to wrapper objects. The
this
value passed via a function call (including calls made using
Function.prototype.apply
and
Function.prototype.call
do not
coerce the passed this value to an object (
10.4.3
11.1.1
15.3.4.3
15.3.4.4
).
When a
delete
operator occurs within
strict mode code
, a
SyntaxError
is
thrown if its
UnaryExpression
is a direct reference to a variable, function argument, or function name(11.4.1).
When a
delete
operator occurs within
strict mode code
, a
TypeError
is
thrown if the property to be deleted has the attribute { [[Configurable]]:
false
} (
11.4.1
).
It is a
SyntaxError
if a
VariableDeclaration
or
VariableDeclarationNoIn
occurs within
strict code
and its
Identifier
is
eval
or
arguments
12.2.1
).
Strict mode code may not include a
WithStatement
. The occurrence of a
WithStatement
in such a context is an
SyntaxError
12.10
).
It is a
SyntaxError
if a
TryStatement
with a
Catch
occurs within
strict code
and the
Identifier
of the
Catch
production is
eval
or
arguments
12.14.1
It is a
SyntaxError
if the identifier
eval
or
arguments
appears within a
FormalParameterList
of a strict mode
FunctionDeclaration
or
FunctionExpression
13.1
A strict mode function may not have two or more formal parameters that have the same name. An attempt to create such a
function using a
FunctionDeclaration
FunctionExpression
, or
Function
constructor is a
SyntaxError
13.1
15.3.2
).
An implementation may not extend, beyond that defined in this specification, the meanings within strict mode functions of
properties named
caller
or
arguments
of function instances. ECMAScript code may not create or
modify properties with these names on function objects that correspond to strict mode functions (
10.6
, 13.2, 15.3.4.5.3).
It is a
SyntaxError
to use within
strict mode code
the identifiers
eval
or
arguments
as the
Identifier
of a
FunctionDeclaration
or
FunctionExpression
or as a formal
parameter name (
13.1
). Attempting to dynamically define such a strict mode function using the
Function
constructor (
15.3.2
) will throw a
SyntaxError
exception.
Annex D
(informative)
Corrections and Clarifications in the 5
th
Edition with Possible
rd
Edition Compatibility Impact
Throughout: In the Edition 3 specification the meaning of phrases such as “as if by the expression
new
Array()
” are subject to misinterpretation. In the Edition 5 specification text for all internal references and
invocations of standard built-in objects and methods has been clarified by making it explicit that the intent is that the actual
built-in object is to be used rather than the current dynamic value of the correspondingly named property.
11.8.2
11.8.3
11.8.5
: ECMAScript generally
uses a left to right evaluation order, however the Edition 3 specification language for the > and <= operators resulted in
a partial right to left order. The specification has been corrected for these operators such that it now specifies a full left
to right evaluation order. However, this change of order is potentially observable if side-effects occur during the evaluation
process.
11.1.4
: Edition 5 clarifies the fact that a trailing comma at the end of an
ArrayInitialiser
does not add to the length of the array. This is not a semantic change from Edition 3 but
some implementations may have previously misinterpreted this.
11.2.3
: Edition 5 reverses the order of steps 2 and 3 of the algorithm. The original order as
specified in Editions 1 through 3 was incorrectly specified such that side-effects of evaluating
Arguments
could affect the result of evaluating
MemberExpression
12.4
: In Edition 3, an object is created, as if by
new Object()
to serve as the scope for
resolving the name of the exception parameter passed to a
catch
clause of a
try
statement. If the
actual exception object is a function and it is called from within the
catch
clause, the scope object will be
passed as the
this
value of the call. The body of the function can then define new properties on its
this
value
and those property names become visible identifiers bindings within the scope of the
catch
clause after the function
returns. In Edition 5, when an exception parameter is called as a function,
undefined
is passed as the
this
value.
13: In Edition 3, the algorithm for the production
FunctionExpression
with an
Identifier
adds an object created as if by
new Object()
to the scope chain to serve as a scope
for looking up the name of the function. The identifier resolution rules (10.1.4 in Edition 3) when applied to such an object
will, if necessary, follow the object’s prototype chain when attempting to resolve an identifier. This means all the
properties of Object.prototype are visible as identifiers within that scope. In practice most implementations of Edition 3 have
not implemented this semantics. Edition 5 changes the specified semantics by using a
Declarative
Environment Record
to bind the name of the function.
14: In Edition 3, the algorithm for the production
SourceElements
SourceElements
SourceElement
did not correctly
propagate statement result values in the same manner as
Block
. This could result in the
eval
function producing an incorrect result when evaluating a
Program
text. In practice
most implementations of Edition 3 have implemented the correct propagation rather than what was specified in Edition 5.
15.10.6
: RegExp.prototype is now a RegExp object rather than an instance of Object. The value of
its [[Class]] internal property which is observable using Object.prototype.toString is now “RegExp” rather than
“Object”.
Annex E
(informative)
Additions and Changes in the 5
th
Edition that Introduce
Incompatibilities with the 3
rd
Edition
7.1
: Unicode format control characters are no longer stripped from ECMAScript source text before
processing. In Edition 5, if such a character appears in a
StringLiteral
or
RegularExpressionLiteral
the character will be incorporated into the literal where in Edition 3 the character
would not be incorporated into the literal.
7.2
: Unicode character
what appears to be an identifier could result in a syntax error which would not have occurred in Edition 3
7.3
: Line terminator characters that are preceded by an escape sequence are now allowed within a
string literal token. In Edition 3 a syntax error would have been produced.
7.8.5
: Regular expression literals now return a unique object each time the literal is evaluated.
This change is detectable by any programs that test the object identity of such literal values or that are sensitive to the
shared side effects.
7.8.5
: Edition 5 requires early reporting of any possible RegExp constructor errors that would be
produced when converting a
RegularExpressionLiteral
to a RegExp object. Prior to Edition 5
implementations were permitted to defer the reporting of such errors until the actual execution time creation of the object.
7.8.5
: In Edition 5 unescaped “/” characters may appear as a
CharacterClass
in a regular expression literal. In Edition 3 such a character would have been interpreted as
the final character of the literal.
10.4.2
: In Edition 5, indirect calls to the
eval
function use
the global environment
as both the variable environment and
lexical environment
for the eval code. In Edition 3, the variable and lexical environments of the caller of an indirect
eval
was used
as the environments for the eval code.
15.4.4
: In Edition 5 all methods of
Array.prototype
are intentionally generic. In
Edition 3
toString
and
toLocaleString
were not generic and would throw a
TypeError
exception if applied to objects that were not instances of Array.
10.6
: In Edition 5 the array indexed properties of argument objects that correspond to actual formal
parameters are enumerable. In Edition 3, such properties were not enumerable.
10.6
: In Edition 5 the value of the [[Class]] internal property of an arguments object is
"Arguments"
. In Edition 3, it was
"Object"
. This is observable if
toString
is called as
a method of an arguments object.
12.6.4
: for-in statements no longer throw a
TypeError
if the
in
expression
evaluates to
null
or
undefined
. Instead, the statement behaves as if the value of the expression was an object
with no enumerable properties.
15: In Edition 5, the following new properties are defined on built-in objects that exist in Edition 3:
Object.getPrototypeOf
Object.getOwnPropertyDescriptor
Object.getOwnPropertyNames
Object.create
Object.defineProperty
Object.defineProperties
Object.seal
Object.freeze
Object.preventExtensions
Object.isSealed
Object.isFrozen
Object.isExtensible
Object.keys
Function.prototype.bind
Array.prototype.indexOf
Array.prototype.lastIndexOf
Array.prototype.every
Array.prototype.some
Array.prototype.forEach
Array.prototype.map
Array.prototype.filter
Array.prototype.reduce
Array.prototype.reduceRight
String.prototype.trim
Date.now
Date.prototype.toISOString, Date.prototype.toJSON
15: Implementations are now required to ignore extra arguments to standard built-in methods unless otherwise explicitly
specified. In Edition 3 the handling of extra arguments was unspecified and implementations were explicitly allowed to throw a
TypeError
exception.
15.1.1
: The value properties
NaN
Infinity
, and
undefined
of the Global Object
have been changed to be read-only properties.
15.1.2.1. Implementations are no longer permitted to restrict the use of eval in ways that are not a direct call. In
addition, any invocation of eval that is not a direct call uses
the global environment
as its variable
environment rather than the caller’s variable environment.
15.1.2.2
: The specification of the function
parseInt
no longer allows
implementations to treat Strings beginning with a
character as octal values.
15.3.4.3
: In Edition 3, a
TypeError
is thrown if the second argument passed to
Function.prototype.apply
is neither an array object nor an arguments object. In Edition 5, the second argument may
be any kind of generic array-like object that has a valid
length
property.
15.3.4.3
, 15.3.4.4: In Edition 3 passing
undefined
or
null
as the first
argument to either
Function.prototype.apply
or
Function.prototype.call
causes the global object to be
passed to the indirectly invoked target function as the
this
value. If the first argument is a primitive value the result
of calling
ToObject
on the primitive value is passed as the
this
value. In Edition 5, these
transformations are not performed and the actual first argument value is passed as the
this
value. This difference will
normally be unobservable to existing ECMAScript Edition 3 code because a corresponding transformation takes place upon
activation of the target function. However, depending upon the implementation, this difference may be observable by host object
functions called using
apply
or
call
. In addition, invoking a standard built-in function in this
manner with
null
or
undefined
passed as the this value will in many cases cause behaviour in Edition 5
implementations that differ from Edition 3 behaviour. In particular, in Edition 5 built-in functions that are specified to
actually use the passed
this
value as an object typically throw a
TypeError
exception if passed
null
or
undefined
as the
this
value.
15.3.5.2
: In Edition 5, the
prototype
property of Function instances is not
enumerable. In Edition 3, this property was enumerable.
15.5.5.2
: In Edition 5, the individual characters of a String object’s [[PrimitiveValue]
may be accessed as array indexed properties of the String object. These properties are non-writable and non-configurable and
shadow any inherited properties with the same names. In Edition 3, these properties did not exist and ECMAScript code could
dynamically add and remove writable properties with such names and could access inherited properties with such names.
15.9.4.2
Date.parse
is now required to first attempt to parse its argument as an
ISO format string. Programs that use this format but depended upon implementation specific behaviour (including failure) may
behave differently.
15.10.2.12
: In Edition 5,
\s
now additionally matches
15.10.4.1
: In Edition 3, the exact form of the String value of the
source
property
of an object created by the
RegExp
constructor is implementation defined. In Edition 5, the String must conform to
certain specified requirements and hence may be different from that produced by an Edition 3 implementation.
15.10.6.4
: In Edition 3, the result of
RegExp.prototype.toString
need not be
derived from the value of the RegExp object’s
source
property. In Edition 5 the result must be derived from
the
source
property in a specified manner and hence may be different from the result produced by an Edition 3
implementation.
15.11.2.1
15.11.4.3
: In Edition 5, if an initial value for the
message
property of an Error object is not specified via the
Error
constructor the initial value of
the property is the empty String. In Edition 3, such an initial value is implementation defined.
15.11.4.4
: In Edition 3, the result of
Error.prototype.toString
is implementation
defined. In Edition 5, the result is fully specified and hence may differ from some Edition 3 implementations.
15.12
: In Edition 5, the name
JSON
is defined in
the global
environment
. In Edition 3, testing for the presence of that name will show it to be undefined unless it is defined by the
program or implementation.
Annex F
(informative)
Technically Significant Corrections and Clarifications in the 5.1 Edition
7.8.4
: CV definitions added for
DoubleStringCharacter
::
LineContinuation
and
SingleStringCharacter
::
LineContinuation
10.2.1.1.3
: The argument S is not ignored. It controls whether an exception is thrown when
attempting to set an immutable binding.
10.2.1.2.2
: In algorithm step 5,
true
is passed as the last argument to
[[DefineOwnProperty]].
10.5
: Former algorithm step 5.e is now 5.f and a new step 5.e was added to restore compatibility with
rd
Edition when redefining global functions.
11.5.3
: In the final bullet item, use of IEEE 754 round-to-nearest mode is specified.
12.6.3
: Missing
ToBoolean
restored in step 3.a.ii of both algorithms.
12.6.4
: Additional final sentences in each of the last two paragraphs clarify certain property
enumeration requirements.
12.7
12.8
12.9
: BNF modified to clarify that a
continue
or
break
statement without an
Identifier
or a
return
statement without an
Expression
may have a
LineTerminator
before the
semi-colon.
12.14
: Step 3 of algorithm 1 and step 2.a of algorithm 3 are corrected such that the
value
field of
is passed as a parameter rather than
itself.
15.1.2.2
: In step 2 of algorithm, clarify that
may be the empty string.
15.1.2.3
: In step 2 of algorithm clarify that
trimmedString
may be the empty
string.
15.1.3
: Added notes clarifying that ECMAScript’s URI syntax is based upon RFC 2396 and not
the newer RFC 3986. In the algorithm for Decode, a step was removed that immediately preceded the current step 4.d.vii.10.a
because it tested for a condition that cannot occur.
15.2.3.7
: Corrected use of variable
in steps 5 and 6 of algorithm.
15.2.4.2
: Edition 5 handling of
undefined
and
null
as
this
value caused existing code to fail. Specification modified to
maintain compatibility with such code. New steps 1 and 2 added to the algorithm.
15.3.4.3
: Steps 5 and 7 of Edition 5 algorithm have been deleted because they imposed
requirements upon the
argArray
argument that are inconsistent with other uses of generic array-like objects.
15.4.4.12
: In step 9.a, incorrect reference to
relativeStart
was replaced with a
reference to
actualStart
15.4.4.15
: Clarified that the default value for
fromIndex
is the length minus 1 of
the array.
15.4.4.18
: In step 9 of the algorithm,
undefined
is now the specified
return value.
15.4.4.22
: In step 9.c.ii the first argument to the [[Call]] internal method has been changed to
undefined
for consistency with the definition of
Array.prototype.reduce
15.4.5.1
: In Algorithm steps 3.l.ii and 3.l.iii the variable name was inverted resulting in an
incorrectly inverted test.
15.5.4.9
: Normative requirement concerning canonically equivalent strings deleted from paragraph
following algorithm because it is listed as a recommendation in NOTE 2.
15.5.4.14
: In
split
algorithm step 11.a and 13.a,
the positional order of the arguments to
SplitMatch
was corrected to match the actual parameter
signature of
SplitMatch
. In step 13.a.iii.7.d,
lengthA
replaces
.l
ength
15.5.5.2
: In first paragraph, removed the implication that the individual character property
access had “array index” semantics. Modified algorithm steps 3 and 5 such that they do not enforce “array
index” requirement.
15.9.1.15
: Specified legal value ranges for fields that lacked them. Eliminated
“time-only” formats. Specified default values for all optional fields.
15.10.2.2
: The step numbers of the algorithm for the internal closure produced by step 2 were
incorrectly numbered in a manner that implied that they were steps of the outer algorithm.
15.10.2.6
: In the abstract operation
IsWordChar
the first character in
the list in step 3 is “
” rather than “
”.
15.10.2.8
: In the algorithm for the closure returned by the abstract operation
CharacterSetMatcher
, the variable defined by step 3 and passed as an argument in step 4 was renamed to
ch
in order to avoid a name conflict with a formal parameter of the closure.
15.10.6.2
: Step 9.e was deleted because It performed an extra increment of
15.11.1.1
: Removed requirement that the
message
own
property is set to the empty String when the
message
argument is
undefined
15.11.1.2: Removed requirement that the
message
own property is set to the empty
String when the
message
argument is
undefined
15.11.4.4
: Steps 6-10 modified/added to correctly deal with missing or empty
message
property value.
15.11.1.2: Removed requirement that the
message
own property is set to the empty
String when the
message
argument is
undefined
15.12.3
: In step 10.b.iii of the
JA
internal operation, the last element of the
concatenation is “
”.
B.2.1
: Added to NOTE that the encoding is based upon RFC 1738 rather than the newer RFC 3986.
Annex C
: An item was added corresponding to 7.6.12 regarding
FutureReservedWords
in strict mode.
Bibliography
IEEE Std 754-2008: IEEE Standard for Floating-Point Arithmetic. Institute of Electrical and
Electronic Engineers, New York (2008)
The Unicode Consortium. The Unicode Standard, Version 3.0, defined by: The Unicode Standard,
Version 3.0 (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5)
Unicode Inc. (2010), Unicode Technical Report #15: Unicode Normalization Forms
ISO 8601:2004(E)
Data elements and interchange formats – Information
interchange --
Representation of dates and times
RFC 1738 "Uniform Resource Locators (URL)", available at
RFC 2396 "Uniform Resource Identifiers (URI): Generic Syntax", available at
RFC 3629 "UTF-8, a transformation format of ISO 10646", available at
RFC 4627 "The application/json Media Type for JavaScript Object Notation (JSON)" , available at
US