‟Ex igne vita”
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This is not the normative ECMAScript Language specification. The normative spec (ECMA 262) is a PDF file maintained by ECMA TC39 and is available from http://www.ecmascript.org/. An auto-generated HTML version is available, too: http://ecma-international.org/ecma-262/5.1/
This is an annotated, hyperlinked, HTML version of Edition 5.1 of the ECMAScript Specification, the source for which is maintained at https://github.com/es5/es5.github.io. No copyright is asserted on its front matter (everything up through the Table of Contents), but any reuse of its body text (everything following the Table of Contents) must include the normative spec’s copyright notice and license statement.
To view annotations, follow the Ⓐ, Ⓑ, Ⓓ, Ⓡ, Ⓖ, Ⓔ, and ① hyperlinks in the headings. A key to the markers explains the different types of annotations. Also included are a variety of hyperlinked cross-references, following the example of Jason Orendorff’s version at http://people.mozilla.org/~jorendorff/es5.html
© Ecma International 2010
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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 [1] as the fourth edition of ECMAScript, it informs continuing evolution of the language. The present fifth edition of ECMAScript (published as ECMA-262 5th 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.
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.
1 Note: Please note that for ECMAScript Edition 4 the Ecma standard number “ECMA-262 Edition 4” was reserved but not used in the Ecma publication process. Therefore “ECMA-262 Edition 4” as an Ecma International publication does not exist.
This Standard defines the ECMAScript scripting language.
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 International 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 is 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.
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
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.
A 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.
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.
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.
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.
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: cf1, cf2,
cf3, cf4, and cf5.
Each of these objects contains properties named q1
and q2
. The dashed lines
represent the implicit prototype relationship; so, for example,
cf3’s prototype is CFp.
The constructor, CF, has two properties itself, named P1
and P2
, which are not
visible to CFp, cf1,
cf2, cf3, cf4,
or cf5. The property named CFP1
in CFp is shared by cf1,
cf2, cf3, cf4,
and cf5 (but not by CF), as are any
properties found in CFp’s implicit
prototype chain that are not named q1
,
q2
, or CFP1
.
Notice that there is no implicit prototype link between CF
and CFp.
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 cf1, cf2, cf3, cf4, and cf5 by assigning a new value to the property in CFp.
The ECMAScript Language recognizes 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.
For the purposes of this document, the following terms and definitions apply.
set of data values as defined in Clause 8 of this specification.
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.
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.
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.
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.
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.
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.
object supplied by the host environment to complete the execution environment of ECMAScript.
NOTE Any object that is not native is a host object.
primitive value used when a variable has not been assigned a value.
type whose sole value is the undefined value.
primitive value that represents the intentional absence of any object value.
type whose sole value is the null value.
member of the Boolean type.
NOTE There are only two Boolean values, true and false.
type consisting of the primitive values true and false.
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.
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.
set of all possible String values.
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).
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.
set of all possible Number values including the special “Not-a-Number” (NaN) values, positive infinity, and negative infinity.
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).
Number value that is the positive infinite Number value.
Number value that is a IEEE 754 “Not-a-Number” value.
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.
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.
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.
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.
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.
internal value that defines some characteristic of a property.
property that is directly contained by its object.
property of an object that is not an own property but is a property (either own or inherited) of the object’s prototype.
A 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.
A 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.
A 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.
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.
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.
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.
Terminal
symbols of the lexical and string grammars, and some of the terminal
symbols of the syntactic grammar, 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 Initialiseropt
is a convenient abbreviation for:
VariableDeclaration :
Identifier
Identifier
Initialiser
and that:
IterationStatement :
for
(
ExpressionNoInopt
;
Expressionopt
;
Expressionopt
)
Statement
is a convenient abbreviation for:
IterationStatement :
for
( ;
Expressionopt
;
Expressionopt
)
Statement
ExpressionNoIn
for
(
;
Expressionopt
;
Expressionopt
)
Statement
which in turn is an abbreviation for:
IterationStatement :
for
( ; ;
Expressionopt
)
Statement
Expression
for
( ;
;
Expressionopt
)
Statement
ExpressionNoIn
for
(
; ;
Expressionopt
)
Statement
ExpressionNoIn
for
(
;
Expression
;
Expressionopt
)
Statement
which in turn is an abbreviation for:
IterationStatement :
for
( ; ; )
Statementfor
( ; ;
Expression
)
Statement
Expression
for
( ;
; )
Statement
Expression
for
( ;
;
Expression
)
Statement
ExpressionNoIn
for
(;
; )
Statement
ExpressionNoIn
for
(;
;
Expression
)
Statement
ExpressionNoIn
for
(;
Expression
; )
Statement
ExpressionNoIn
for
(;
Expression
;
Expression
)
Statement
so the nonterminal IterationStatement actually has eight alternative right-hand sides.
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
0
1 2 3 4 5 6 7 8 9
DecimalDigits ::
DecimalDigit
DecimalDigits
DecimalDigit
the definition
LookaheadExample ::
n
[lookahead
∉
{1
, 3
, 5
, 7
, 9
}]DecimalDigits
DecimalDigit [lookahead
∉
DecimalDigit ]
matches
either the letter n
followed by one or more decimal digits the first of which is even,
or a decimal digit not followed by another decimal digit.
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:
ReturnStatement :
return
[no LineTerminator here]
Expressionopt ;
indicates
that the production may not be used if a LineTerminator
occurs in the program between the return
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 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
1
2 3 4 5 6 7 8 9
which is merely a convenient abbreviation for:
NonZeroDigit ::
1
2
3
4
5
6
7
8
9
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
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 parameterized 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 x” is used to indicate that the result of the algorithm is the value of x and that the algorithm should terminate. The notation Result(n) is used as shorthand for “the result of step n”.
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.
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 0.
The mathematical function abs(x) yields the absolute value of x, which is −x if x is negative (less than zero) and otherwise is x itself.
The mathematical function sign(x) yields 1 if x is positive and −1 if x is negative. The sign function is not used in this standard for cases when x is zero.
The notation “x modulo y” (y must be finite and nonzero) computes a value k of the same sign as y (or zero) such that abs(k) < abs(y) and x−k = q × y for some integer q.
The mathematical function floor(x) yields the largest integer (closest to positive infinity) that is not larger than x.
NOTE floor(x) = x−(x 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.
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 Normalised 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 convert to UTF-16.
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.
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
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.
<ZWNJ> and <ZWJ> are format-control characters that are used to make necessary distinctions when forming words or phrases in certain languages. In ECMAScript source text, <ZWNJ> and <ZWJ> may also be used in an identifier after the first character.
<BOM> 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. <BOM> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. <BOM> characters are treated as white space characters (see 7.2).
The special treatment of certain format-control characters outside of comments, string literals, and regular expression literals is summarized in Table 1.
Code Unit Value |
Name |
Formal Name |
Usage |
|
Zero width non-joiner |
<ZWNJ> |
IdentifierPart |
|
Zero width joiner |
<ZWJ> |
IdentifierPart |
|
Byte Order Mark |
<BOM> |
Whitespace |
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.
Code Unit Value |
Name |
Formal Name |
|
Tab |
<TAB> |
|
Vertical Tab |
<VT> |
|
Form Feed |
<FF> |
|
Space |
<SP> |
|
No-break space |
<#x0a> |
Other category “Zs” |
Byte Order Mark Any other Unicode “space separator” |
<BOM> <USP> |
ECMAScript implementations must recognize 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 recognize white space characters from later editions of the Unicode Standard.
Syntax
WhiteSpace ::
<TAB>
<VT>
<FF>
<SP>
<#x0a>
<BOM>
<USP>
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.
Code Unit Value |
Name |
Formal Name |
|
Line Feed |
<LF> |
|
Carriage Return |
<CR> |
|
Line separator |
<LS> |
|
Paragraph separator |
<PS> |
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 <CR><LF> is commonly used as a line terminator. It should be considered a single character for the purpose of reporting line numbers.
Syntax
LineTerminator ::
<LF>
<CR>
<LS>
<PS>
LineTerminatorSequence ::
<LF>
<CR>
[lookahead
∉
<LF>
]
<LS>
<PS>
<CR>
<LF>
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 ::
/*
MultiLineCommentCharsopt */
MultiLineCommentChars ::
MultiLineNotAsteriskChar
MultiLineCommentCharsopt*
PostAsteriskCommentCharsopt
PostAsteriskCommentChars ::
MultiLineNotForwardSlashOrAsteriskChar
MultiLineCommentCharsopt*
PostAsteriskCommentCharsopt
MultiLineNotAsteriskChar ::
SourceCharacter but
not asterisk *
MultiLineNotForwardSlashOrAsteriskChar ::
SourceCharacter
but
not forward-slash /
orasterisk *
SingleLineComment ::
//
SingleLineCommentCharsopt
SingleLineCommentChars ::
SingleLineCommentChar SingleLineCommentCharsopt
SingleLineCommentChar ::
SourceCharacter but not LineTerminator
Syntax
Token ::
IdentifierName
Punctuator
NumericLiteral
StringLiteral
NOTE The DivPunctuator and RegularExpressionLiteral productions define tokens, but are not included in the Token production.
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 recognize 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
<ZWNJ>
<ZWJ>
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)”
UnicodeEscapeSequence
see 7.8.4.
A reserved word is an IdentifierName that cannot be used as an Identifier.
Syntax
ReservedWord ::
Keyword
FutureReservedWord
NullLiteral
BooleanLiteral
The following tokens are ECMAScript keywords and may not be used as Identifiers in ECMAScript programs.
Syntax
Keyword :: one of
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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
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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:
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Syntax
Punctuator :: one of
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DivPunctuator :: one of
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Syntax
Literal ::
NullLiteral
BooleanLiteral
NumericLiteral
StringLiteral
RegularExpressionLiteral
Syntax
NullLiteral ::
null
Semantics
The
value of the null literal null
is the sole value of the Null type, namely null.
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.
Syntax
NumericLiteral ::
DecimalLiteral
HexIntegerLiteral
DecimalLiteral ::
DecimalIntegerLiteral .
DecimalDigitsopt
ExponentPartopt.
DecimalDigits ExponentPartopt
DecimalIntegerLiteral
ExponentPartopt
DecimalIntegerLiteral ::
0
NonZeroDigit
DecimalDigitsopt
DecimalDigits ::
DecimalDigit
DecimalDigits
DecimalDigit
DecimalDigit :: one of
0
1 2 3 4 5 6 7 8 9
NonZeroDigit :: one of
1
2 3 4 5 6 7 8 9
ExponentPart ::
ExponentIndicator SignedInteger
ExponentIndicator :: one of
e
E
SignedInteger ::
DecimalDigits+
DecimalDigits-
DecimalDigits
HexIntegerLiteral ::
0x
HexDigit0X
HexDigit
HexIntegerLiteral HexDigit
HexDigit :: one of
0
1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F
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 3
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–n),
where n is the
number of characters in DecimalDigits.
The
MV of DecimalLiteral ::
DecimalIntegerLiteral .
ExponentPart is the MV of
DecimalIntegerLiteral times
10e,
where e is
the MV of ExponentPart.
The
MV of DecimalLiteral ::
DecimalIntegerLiteral .
DecimalDigits ExponentPart is
(the MV of DecimalIntegerLiteral plus
(the MV of DecimalDigits times
10–n))
times 10e,
where n is the
number of characters in DecimalDigits and
e
is the MV of
ExponentPart.
The
MV of DecimalLiteral ::.
DecimalDigits is the MV of
DecimalDigits times
10–n,
where n is the
number of characters in DecimalDigits.
The
MV
of DecimalLiteral ::.
DecimalDigits ExponentPart is
the MV of DecimalDigits times
10e–n,
where n is the
number of characters in DecimalDigits and
e 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 10e, where e is the MV of ExponentPart.
The
MV of DecimalIntegerLiteral ::
0
is 0.
The MV of DecimalIntegerLiteral :: NonZeroDigit DecimalDigits is (the MV of NonZeroDigit times 10n) plus the MV of DecimalDigits, where n 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 ::
0
or
of HexDigit ::
0
is
0.
The
MV of DecimalDigit ::
1
or
of NonZeroDigit ::
1
or
of HexDigit ::
1
is
1.
The
MV of DecimalDigit ::
2
or
of NonZeroDigit ::
2
or
of HexDigit ::
2
is
2.
The
MV of DecimalDigit ::
3
or
of NonZeroDigit ::
3
or
of HexDigit ::
3
is
3.
The
MV of DecimalDigit ::
4
or
of NonZeroDigit ::
4
or
of HexDigit ::
4
is
4.
The
MV of DecimalDigit ::
5
or
of NonZeroDigit ::
5
or
of HexDigit ::
5
is
5.
The
MV of DecimalDigit ::
6
or
of NonZeroDigit ::
6
or
of HexDigit ::
6
is
6.
The
MV of DecimalDigit ::
7
or
of NonZeroDigit ::
7
or
of HexDigit ::
7
is
7.
The
MV of DecimalDigit ::
8
or
of NonZeroDigit ::
8
or
of HexDigit ::
8
is
8.
The
MV of DecimalDigit ::
9
or
of NonZeroDigit ::
9
or
of HexDigit ::
9
is
9.
The
MV of HexDigit ::
a
or
of HexDigit ::
A
is
10.
The
MV of HexDigit ::
b
or
of HexDigit ::
B
is
11.
The
MV of HexDigit ::
c
or
of HexDigit ::
C
is
12.
The
MV of HexDigit ::
d
or
of HexDigit ::
D
is
13.
The
MV of HexDigit ::
e
or
of HexDigit ::
E
is
14.
The
MV of HexDigit ::
f
or
of HexDigit ::
F
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 0
digit or
the Number value for the MV of a literal produced by replacing each
significant digit after the 20th with a 0
digit and then incrementing the literal at the 20th significant
digit position. A digit is significant if it is not part of
an ExponentPart
and
it
is not 0
;
or
there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.
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.
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 ::
"
DoubleStringCharactersopt
"
SingleStringCharactersopt
''
DoubleStringCharacters ::
DoubleStringCharacter DoubleStringCharactersopt
SingleStringCharacters ::
SingleStringCharacter SingleStringCharactersopt
DoubleStringCharacter ::
SourceCharacter but
not double-quote "
or
backslash \
or
LineTerminator\
EscapeSequence
LineContinuation
SingleStringCharacter ::
SourceCharacter but
not single-quote '
orbackslash \
or
LineTerminator\
EscapeSequence
LineContinuation
LineContinuation ::
\
LineTerminatorSequence
EscapeSequence ::
CharacterEscapeSequence0
[lookahead
∉
DecimalDigit]
HexEscapeSequence
UnicodeEscapeSequence
CharacterEscapeSequence ::
SingleEscapeCharacter
NonEscapeCharacter
SingleEscapeCharacter :: one of
'
" \ b f n r t v
NonEscapeCharacter ::
SourceCharacter but not EscapeCharacter or LineTerminator
EscapeCharacter ::
SingleEscapeCharacter
DecimalDigitx
u
HexEscapeSequence ::
x
HexDigit HexDigit
UnicodeEscapeSequence ::
u
HexDigit HexDigit HexDigit HexDigit
The definitions of the nonterminal HexDigit is given in 7.6. 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
double-quote "
or backslash
\
or
LineTerminator is theSourceCharacter 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
single-quote '
or backslash
\
or
LineTerminator is theSourceCharacter 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 ::
0
[lookahead
∉
DecimalDigit]
is a <NUL>
character (Unicode value 0000).
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 theSingleEscapeCharacter according to Table 4:
Escape Sequence |
Code Unit Value |
Name |
Symbol |
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backspace |
<BS> |
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horizontal tab |
<HT> |
|
|
line feed (new line) |
<LF> |
|
|
vertical tab |
<VT> |
|
|
form feed |
<FF> |
|
|
carriage return |
<CR> |
|
|
double quote |
|
|
|
single quote |
|
|
|
backslash |
|
The CV of CharacterEscapeSequence :: NonEscapeCharacter is the CV of the NonEscapeCharacter.
The CV of NonEscapeCharacter :: SourceCharacter but not EscapeCharacter or LineTerminator is the SourceCharacter character itself.
The
CV of HexEscapeSequence ::
x
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 ::
u
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
.
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 *
or
\
or
/
or
[
RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionChar ::
RegularExpressionNonTerminator
but
not \
or
/
or
[
RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionBackslashSequence ::
\
RegularExpressionNonTerminator
RegularExpressionNonTerminator ::
SourceCharacter but not LineTerminator
RegularExpressionClass ::
[
RegularExpressionClassChars
]
RegularExpressionClassChars
::
[empty]
RegularExpressionClassChars
RegularExpressionClassChar
RegularExpressionClassChar
::
RegularExpressionNonTerminator
but
not ]
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).
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.
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.
A
Identifier
in a break
or continue
statement should be on the same line as the break
or continue
token.
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 b
,
because a LineTerminator
occurs between b
and ++
.
The source
if
(a > b)
else c = d
is
not a valid ECMAScript sentence and is not altered by automatic
semicolon insertion before the else
token, even though no production of the grammar applies at that
point, because an automatically inserted semicolon would then be
parsed as an empty statement.
The source
a
= b + c
(d + e).print()
is not transformed by automatic semicolon insertion, because the 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.
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(x)” is used as shorthand for “the type of x” where “type” refers to the ECMAScript language and specification types defined in this clause.
The Undefined type has exactly one value, called undefined. Any variable that has not been assigned a value has the value undefined.
The Null type has exactly one value, called null.
The Boolean type represents a logical entity having two values, called true and false.
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.
The
Number type has exactly 18437736874454810627
(that is, 264−253+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, 253−2)
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, 264−253) values are called the finite numbers. Half of these are positive numbers and half are negative numbers; for every finite positive Number value there is a corresponding negative value having the same magnitude.
Note
that there is both a positive zero and a negative zero.
For brevity, these values are also referred to for expository
purposes by the symbols +0
and −0,
respectively. (Note that these two different zero Number values are
produced by the program expressions +0
(or simply 0
) and
-0
.)
The 18437736874454810622 (that is, 264−253−2) finite nonzero values are of two kinds:
18428729675200069632 (that is, 264−254) of them are normalised, having the form
s × m × 2e
where s is +1 or −1, m is a positive integer less than 253 but not less than 252, and e is an integer ranging from −1074 to 971, inclusive.
The remaining 9007199254740990 (that is, 253−2) values are denormalised, having the form
s × m × 2e
where s is +1 or −1, m is a positive integer less than 252, and e is −1074.
Note
that all the positive and negative integers whose magnitude is no
greater than 253
are representable in the Number type (indeed, the integer 0
has two representations, +0
and -0
).
A finite number has an odd significand if it is nonzero and the integer m used to express it (in one of the two forms shown above) is odd. Otherwise, it has an even significand.
In this specification, the phrase “the Number value for x” where x represents an exact nonzero real mathematical quantity (which might even be an irrational number such as π) means a Number value chosen in the following manner. Consider the set of all finite values of the Number type, with −0 removed and with two additional values added to it that are not representable in the Number type, namely 21024 (which is +1 × 253 × 2971) and −21024 (which is −1 × 253 × 2971). Choose the member of this set that is closest in value to x. If two values of the set are equally close, then the one with an even significand is chosen; for this purpose, the two extra values 21024 and −21024 are considered to have even significands. Finally, if 21024 was chosen, replace it with +∞; if −21024 was chosen, replace it with −∞; if +0 was chosen, replace it with −0 if and only if x is less than zero; any other chosen value is used unchanged. The result is the Number value for x. (This procedure corresponds exactly to the behaviour of the IEEE 754 “round to nearest” mode.)
Some ECMAScript operators deal only with integers in the range −231 through 231−1, inclusive, or in the range 0 through 232−1, inclusive. These operators accept any value of the Number type but first convert each such value to one of 232 integer values. See the descriptions of the ToInt32 and ToUint32 operators in 9.5 and 9.6, respectively.
An Object is a collection of properties. Each property is either a named data property, a named accessor property, or an internal property:
A named data property associates a name with an ECMAScript language value and a set of Boolean attributes.
A 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.
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
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.
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.
Attribute Name |
Default Value |
[[Value]] |
undefined |
[[Get]] |
undefined |
[[Set]] |
undefined |
[[Writable]] |
false |
[[Enumerable]] |
false |
[[Configurable]] |
false |
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 indicates 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.
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) → Undefinedor Property Descriptor |
Returns the Property Descriptor of the named own property of this object, or undefined if absent. |
[[GetProperty]] |
SpecOp (propertyName) → Undefinedor 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 is 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.
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]] |
Creates
an object. Invoked via the |
|
[[Call]] |
Executes code associated with the object. Invoked via a function call expression. The arguments to the SpecOp are a 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]] |
A 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. |
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.)
A 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:
Let base be the result of calling GetBase(V).
If IsUnresolvableReference(V), throw a ReferenceError exception.
If IsPropertyReference(V), then
If HasPrimitiveBase(V) 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(V) 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(V) and IsStrictReference(V) as arguments.
The following [[Get]] internal method is used by GetValue when V is a property reference with a primitive base value. It is called using base as its this value and with property P as its argument. The following steps are taken:
Let O be ToObject(base).
Let desc be the result of calling the [[GetProperty]] internal method of O with property name P.
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 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.
If Type(V) is not Reference, throw a ReferenceError exception.
Let base be the result of calling GetBase(V).
If IsUnresolvableReference(V), then
If IsStrictReference(V) is true, then
Throw ReferenceError exception.
Call the [[Put]] internal method of the global object, passing GetReferencedName(V) for the property name, W for the value, and false for the Throw flag.
Else if IsPropertyReference(V), then
If HasPrimitiveBase(V) 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(V) for the property name, W for the value, and IsStrictReference(V) 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(V), W, and IsStrictReference(V) as arguments.
Return.
The following [[Put]] internal method is used by PutValue when V is a property reference with a primitive base value. It is called using base as its this value and with property P, value W, and Boolean flag Throw as arguments. The following steps are taken:
Let O be ToObject(base).
If the result of calling the [[CanPut]] internal method of O with argument P 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 O with argument P.
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 O with argument P. 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 base as the this value and an argument list containing only W.
Else, this is a request to create an own property on the transient object O
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.
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.
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.
The term “abrupt completion” refers to any completion with a type other than normal.
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:
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.
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.
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.
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.
When the abstract operation ToPropertyDescriptor is called with object Desc, 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.
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.
In the following algorithm descriptions, assume O is a native ECMAScript object, P is a String, Desc is a Property Description record, and Throw is a Boolean flag.
When the [[GetOwnProperty]] internal method of O is called with property name P, the following steps are taken:
If O doesn’t have an own property with name P, return undefined.
Let D be a newly created Property Descriptor with no fields.
Let X be O’s own property named P.
If X is a data property, then
Set D.[[Value]] to the value of X’s [[Value]] attribute.
Set D.[[Writable]] to the value of X’s [[Writable]] attribute
Else X is an accessor property, so
Set D.[[Get]] to the value of X’s [[Get]] attribute.
Set D.[[Set]] to the value of X’s [[Set]] attribute.
Set D.[[Enumerable]] to the value of X’s [[Enumerable]] attribute.
Set D.[[Configurable]] to the value of X’s [[Configurable]] attribute.
Return D.
However, if O is a String object it has a more elaborate [[GetOwnProperty]] internal method defined in 15.5.5.2.
When the [[GetProperty]] internal method of O is called with property name P, the following steps are taken:
Let prop be the result of calling the [[GetOwnProperty]] internal method of O with property name P.
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 P.
When the [[Get]] internal method of O is called with property name P, the following steps are taken:
Let desc be the result of calling the [[GetProperty]] internal method of O with property name P.
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 O as the this value and providing no arguments.
When the [[CanPut]] internal method of O is called with property name P, the following steps are taken:
Let desc be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
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 O.
If proto is null, then return the value of the [[Extensible]] internal property of O.
Let inherited be the result of calling the [[GetProperty]] internal method of proto with property name P.
If inherited is undefined, return the value of the [[Extensible]] internal property of O.
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 O 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.
When the [[Put]] internal method of O is called with property P, value V, and Boolean flag Throw, the following steps are taken:
If the result of calling the [[CanPut]] internal method of O with argument P 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 O with argument P.
If IsDataDescriptor(ownDesc) is true, then
Let valueDesc be the Property Descriptor {[[Value]]: V}.
Call the [[DefineOwnProperty]] internal method of O passing P, valueDesc, and Throw as arguments.
Return.
Let desc be the result of calling the [[GetProperty]] internal method of O with argument P. 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 O as the this value and providing V as the sole argument.
Else, create a named data property named P on object O as follows
Let
newDesc be the Property Descriptor
{[[Value]]: V,
[[Writable]]: true, [[Enumerable]]: true,
[[Configurable]]: true}.
Call the [[DefineOwnProperty]] internal method of O passing P, newDesc, and Throw as arguments.
Return.
When the [[HasProperty]] internal method of O is called with property name P, the following steps are taken:
Let desc be the result of calling the [[GetProperty]] internal method of O with property name P.
If desc is undefined, then return false.
Else return true.
When the [[Delete]] internal method of O is called with property name P and the Boolean flag Throw, the following steps are taken:
Let desc be the result of calling the [[GetOwnProperty]] internal method of O with property name P.
If desc is undefined, then return true.
If desc.[[Configurable]] is true, then
Remove the own property with name P from O.
Return true.
Else if Throw, then throw a TypeError exception.
Return false.
When the [[DefaultValue]] internal method of O is called with hint String, the following steps are taken:
Let
toString be the result of calling the [[Get]] internal
method of object O with argument "toString
".
If IsCallable(toString) is true then,
Let str be the result of calling the [[Call]] internal method of toString, with O 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 O with argument "valueOf
".
If IsCallable(valueOf) is true then,
Let val be the result of calling the [[Call]] internal method of valueOf, with O 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 O is called with hint Number, the following steps are taken:
Let
valueOf be the result of calling the [[Get]] internal method
of object O with argument "valueOf
".
If IsCallable(valueOf) is true then,
Let val be the result of calling the [[Call]] internal method of valueOf, with O 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 O with argument "toString
".
If IsCallable(toString) is true then,
Let str be the result of calling the [[Call]] internal method of toString, with O 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 O is called with no hint, then it behaves as if the hint were Number, unless O 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.
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 O is called with property name P, property descriptor Desc, and Boolean flag Throw, the following steps are taken:
Let current be the result of calling the [[GetOwnProperty]] internal method of O with property name P.
Let extensible be the value of the [[Extensible]] internal property of O.
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 P of object O whose [[Value]], [[Writable]], [[Enumerable]] and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to its default value.
Else, Desc must be an accessor Property Descriptor so,
Create an own accessor property named P of object O whose [[Get]], [[Set]], [[Enumerable]] and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to its default value.
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
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 P of object O from a data property to an accessor property. Preserve the existing values of the converted property’s [[Configurable]] and [[Enumerable]] attributes and set the rest of the property’s attributes to their default values.
Else,
Convert the property named P of object O from an accessor property to a data property. Preserve the existing values of the converted property’s [[Configurable]] and [[Enumerable]] attributes and set the rest of the property’s attributes to their default values.
Else, if IsDataDescriptor(current) and IsDataDescriptor(Desc) are both true, then
If the [[Configurable]] field of current is false, then
else, the [[Configurable]] field of current is true, so any change is acceptable.
Else, IsAccessorDescriptor(current) and IsAccessorDescriptor(Desc) are both true so,
For each attribute field of Desc that is present, set the correspondingly named attribute of the property named P of object O to the value of the field.
Return true.
However, if O 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.
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.
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:
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. |
The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 11:
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 |
The abstract operation ToNumber converts its argument to a value of type Number according to Table 12:
Argument Type |
Result |
Undefined |
NaN |
Null |
+0 |
Boolean |
The result is 1 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:
|
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.
StringNumericLiteral :::
StrWhiteSpaceopt
StrWhiteSpaceoptStrNumericLiteral StrWhiteSpaceopt
StrWhiteSpace :::
StrWhiteSpaceChar StrWhiteSpaceopt
StrWhiteSpaceChar :::
WhiteSpace
LineTerminator
StrNumericLiteral :::
StrDecimalLiteral
HexIntegerLiteral
StrDecimalLiteral :::
StrUnsignedDecimalLiteral+
StrUnsignedDecimalLiteral-
StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral :::
Infinity
DecimalDigits .
DecimalDigitsopt
ExponentPartopt.
DecimalDigits ExponentPartopt
DecimalDigits
ExponentPartopt
DecimalDigits :::
DecimalDigit
DecimalDigits
DecimalDigit
DecimalDigit ::: one of
0
1 2 3 4 5 6 7 8 9
ExponentPart :::
ExponentIndicator SignedInteger
ExponentIndicator ::: one of
e
E
SignedInteger :::
DecimalDigits+
DecimalDigits-
DecimalDigits
HexIntegerLiteral :::
0x
HexDigit0X
HexDigit
HexIntegerLiteral HexDigit
HexDigit ::: one of
0
1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F
Some differences should be noted between the syntax of a StringNumericLiteral and a NumericLiteral (see 7.8.3):
A StringNumericLiteral may be preceded and/or followed by white space and/or line terminators.
A
StringNumericLiteral
that is decimal may have any number of leading 0
digits.
A
StringNumericLiteral
that is decimal may be preceded by +
or -
to indicate
its sign.
A 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 ::: StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt 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 1010000
(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−n),
where n is the
number of characters in the second DecimalDigits.
The
MV of StrUnsignedDecimalLiteral::: DecimalDigits.
ExponentPart is the MV of
DecimalDigits times 10e, where e
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−n))
times 10e, where n is the number of
characters in the second DecimalDigits and e is
the MV of ExponentPart.
The
MV of StrUnsignedDecimalLiteral:::.
DecimalDigits is the MV of
DecimalDigits times 10−n,
where n is the number of characters in DecimalDigits.
The
MV of StrUnsignedDecimalLiteral:::.
DecimalDigits ExponentPart is
the MV of DecimalDigits times 10e−n,
where n is the number of characters in DecimalDigits
and e 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 10e, where e 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 ::: 0
or of HexDigit :::
0
is 0.
The
MV of DecimalDigit ::: 1
or of HexDigit :::
1
is 1.
The
MV of DecimalDigit ::: 2
or of HexDigit :::
2
is 2.
The
MV of DecimalDigit ::: 3
or of HexDigit :::
3
is 3.
The
MV of DecimalDigit ::: 4
or of HexDigit :::
4
is 4.
The
MV of DecimalDigit ::: 5
or of HexDigit :::
5
is 5.
The
MV of DecimalDigit ::: 6
or of HexDigit :::
6
is 6.
The
MV of DecimalDigit ::: 7
or of HexDigit :::
7
is 7.
The
MV of DecimalDigit ::: 8
or of HexDigit :::
8
is 8.
The
MV of DecimalDigit ::: 9
or of HexDigit :::
9
is 9.
The
MV of HexDigit ::: a
or of HexDigit :::
A
is 10.
The
MV of HexDigit ::: b
or of HexDigit :::
B
is 11.
The
MV of HexDigit ::: c
or of HexDigit :::
C
is 12.
The
MV of HexDigit ::: d
or of HexDigit :::
D
is 13.
The
MV of HexDigit ::: e
or of HexDigit :::
E
is 14.
The
MV of HexDigit ::: f
or of HexDigit :::
F
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 0; or
there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.
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)).
The abstract operation ToInt32 converts its argument to one of 232 integer values in the range −231 through 231−1, 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 int32bit be posInt modulo 232; that is, a finite integer value k of Number type with positive sign and less than 232 in magnitude such that the mathematical difference of posInt and k is mathematically an integer multiple of 232.
If int32bit is greater than or equal to 231, return int32bit − 232, otherwise return int32bit.
NOTE Given the above definition of ToInt32:
The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
ToInt32(ToUint32(x)) is equal to ToInt32(x) for all values of x. (It is to preserve this latter property that +∞ and −∞ are mapped to +0.)
ToInt32 maps −0 to +0.
The abstract operation ToUint32 converts its argument to one of 232 integer values in the range 0 through 232−1, 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 int32bit be posInt modulo 232; that is, a finite integer value k of Number type with positive sign and less than 232 in magnitude such that the mathematical difference of posInt and k is mathematically an integer multiple of 232.
Return int32bit.
NOTE Given the above definition of ToUInt32:
Step 5 is the only difference between ToUint32 and ToInt32.
The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
ToUint32(ToInt32(x)) is equal to ToUint32(x) for all values of x. (It is to preserve this latter property that +∞ and −∞ are mapped to +0.)
ToUint32 maps −0 to +0.
The abstract operation ToUint16 converts its argument to one of 216 integer values in the range 0 through 216−1, 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 int16bit be posInt modulo 216; that is, a finite integer value k of Number type with positive sign and less than 216 in magnitude such that the mathematical difference of posInt and k is mathematically an integer multiple of 216.
Return int16bit.
NOTE Given the above definition of ToUint16:
The substitution of 216 for 232 in step 4 is the only difference between ToUint32 and ToUint16.
ToUint16 maps −0 to +0.
The abstract operation ToString converts its argument to a value of type String according to Table 13:
Argument Type |
Result |
Undefined |
|
Null |
|
Boolean |
If
the argument is true, then the result is If
the argument is false, then the result is |
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). |
The abstract operation ToString converts a Number m to String format as follows:
If
m is NaN, return the String "NaN"
.
If
m is +0 or −0,
return the String "0"
.
If
m is less than zero, return the String concatenation of the
String "-"
and ToString(−m).
If
m is infinity, return the String "Infinity"
.
Otherwise, let n, k, and s be integers such that k ≥ 1, 10k−1 ≤ s < 10k, the Number value for s × 10n−k is m, and k is as small as possible. Note that k is the number of digits in the decimal representation of s, that s is not divisible by 10, and that the least significant digit of s is not necessarily uniquely determined by these criteria.
If
k ≤ n ≤
21, return the String consisting of the k digits of the
decimal representation of s (in order, with no leading zeroes),
followed by n−k
occurrences of the character ‘0
’.
If
0 < n ≤ 21, return the
String consisting of the most significant n digits of the
decimal representation of s, followed by a decimal point
‘.
’, followed
by the remaining k−n
digits of the decimal representation of s.
If
−6 < n ≤
0, return the String consisting of the character ‘0
’,
followed by a decimal point ‘.
’,
followed by −n
occurrences of the character ‘0
’,
followed by the k digits of the decimal representation of s.
Otherwise,
if k = 1, return the String consisting of the single digit
of s, followed by lowercase character ‘e
’,
followed by a plus sign ‘+
’
or minus sign ‘−’
according to whether n−1
is positive or negative, followed by the decimal representation of
the integer abs(n−1)
(with no leading zeros).
Return the String consisting of the most significant digit of the decimal representation of s, followed by a decimal point ‘.’, followed by the remaining k−1 digits of the decimal representation of s, followed by the lowercase character ‘e’, followed by a plus sign ‘+’ or minus sign ‘−’ according to whether n−1 is positive or negative, followed by the decimal representation of the integer abs(n−1) (with no leading zeros).
NOTE 1 The following observations may be useful as guidelines for implementations, but are not part of the normative requirements of this Standard:
If x is any Number value other than −0, then ToNumber(ToString(x)) is exactly the same Number value as x.
The least significant digit of s is not always uniquely determined by the requirements listed in step 5.
NOTE 2 For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 5 be used as a guideline:
Otherwise, let n, k, and s be integers such that k ≥ 1, 10k−1 ≤ s < 10k, the Number value for s × 10n−k is m, and k is as small as possible. If there are multiple possibilities for s, choose the value of s for which s × 10n−k is closest in value to m. If there are two such possible values of s, choose the one that is even. Note that k is the number of digits in the decimal representation of s and that s is not divisible by 10.
NOTE 3 Implementers of ECMAScript may find useful the paper and code written by David M. Gay for binary-to-decimal conversion of floating-point numbers:
Gay, David M.
Correctly Rounded Binary-Decimal and Decimal-Binary Conversions.
Numerical Analysis, Manuscript 90-10. AT&T Bell Laboratories
(Murray Hill, New Jersey). November 30, 1990. Available
as
http://cm.bell-labs.com/cm/cs/doc/90/4-10.ps.gz.
Associated code available
as
http://cm.bell-labs.com/netlib/fp/dtoa.c.gz
and as
http://cm.bell-labs.com/netlib/fp/g_fmt.c.gz
and may also be found at the various netlib
mirror sites.
The abstract operation ToObject converts its argument to a value of type Object according to Table 14:
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). |
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:
Argument Type |
Result |
Undefined |
Throw a TypeError exception. |
Null |
Throw a TypeError exception. |
Boolean |
Return |
Number |
Return |
String |
Return |
Object |
Return |
The abstract operation IsCallable determines if its argument, which must be an ECMAScript language value, is a callable function Object according to Table 16:
Argument Type |
Result |
Undefined |
Return false. |
Null |
Return false. |
Boolean |
Return false. |
Number |
Return false. |
String |
Return false. |
Object |
If the argument object has an [[Call]] internal method, then return true, otherwise return false. |
The internal comparison abstract operation SameValue(x, y), where x and y are ECMAScript language values, produces true or false. Such a comparison is performed as follows:
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
If Type(x) is Number, then.
If x is NaN and y is NaN, return true.
If x is +0 and y is -0, return false.
If x is -0 and y is +0, return false.
If x is the same Number value as y, return true.
Return false.
If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, return false.
If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
Return true if x and y refer to the same object. Otherwise, return false.
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.
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.
A 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.
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.
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 N is the text of the identifier. |
CreateMutableBinding(N, D) |
Create a new mutable binding in an environment record. The String value N is the text of the bound name. If the optional Boolean argument D 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 N is the text of the bound name. V is the value for the binding and may be a value of any ECMAScript language type. S is a Boolean flag. If S is true and the binding cannot be set throw a TypeError exception. S 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 N is the text of the bound name. S is used to identify strict mode references. If S is true and the binding does not exist or is uninitialized throw a ReferenceError exception. |
DeleteBinding(N) |
Delete a binding from an environment record. The String value N is the text of the bound name If a binding for N exists, remove the binding and return true. If the binding exists but cannot be removed return false. If the binding does not exist return true. |
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. |
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 initialization of immutable binding are distinct steps so it is possible for such bindings to exist in either an initialized or uninitialized state. Declarative environment records support the methods listed in Table 18 in addition to the Environment Record abstract specification methods:
Method |
Purpose |
Create a new but uninitialized immutable binding in an environment record. The String value N is the text of the bound name. |
|
Set the value of an already existing but uninitialized immutable binding in an environment record. The String value N is the text of the bound name. V is the value for the binding and is a value of any ECMAScript language type. |
The behaviour of the concrete specification methods for Declarative Environment Records are defined by the following algorithms.
The concrete environment record method HasBinding for declarative environment records simply determines if the argument identifier is one of the identifiers bound by the record:
Let envRec be the declarative environment record for which the method was invoked.
If envRec has a binding for the name that is the value of N, return true.
If it does not have such a binding, return false
The concrete Environment Record method CreateMutableBinding for declarative environment records creates a new mutable binding for the name N that is initialized to the value undefined. A binding must not already exist in this Environment Record for N. If Boolean argument D 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 N.
Create a mutable binding in envRec for N and set its bound value to undefined. If D is true record that the newly created binding may be deleted by a subsequent DeleteBinding call.
The concrete Environment Record method SetMutableBinding for declarative environment records attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. A binding for N must already exist. If the binding is an immutable binding, a TypeError is thrown if S is true.
Let envRec be the declarative environment record for which the method was invoked.
Assert: envRec must have a binding for N.
If the binding for N in envRec is a mutable binding, change its bound value to V.
Else this must be an attempt to change the value of an immutable binding so if S is true throw a TypeError exception.
The concrete Environment Record method GetBindingValue for declarative environment records simply returns the value of its bound identifier whose name is the value of the argument N. The binding must already exist. If S is true and the binding is an uninitialized 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 N.
If the binding for N in envRec is an uninitialized immutable binding, then
If S is false, return the value undefined, otherwise throw a ReferenceError exception.
Else, return the value currently bound to N in envRec.
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 N, return true.
If the binding for N in envRec is cannot be deleted, return false.
Remove the binding for N from envRec.
Return true.
Declarative Environment Records always return undefined as their ImplicitThisValue.
Return undefined.
The concrete Environment Record method CreateImmutableBinding for declarative environment records creates a new immutable binding for the name N that is initialized to the value undefined. A binding must not already exist in this environment record for N.
Let envRec be the declarative environment record for which the method was invoked.
Assert: envRec does not already have a binding for N.
Create an immutable binding in envRec for N and record that it is uninitialized.
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 N to the value of argument V. An uninitialized immutable binding for N must already exist.
Let envRec be the declarative environment record for which the method was invoked.
Assert: envRec must have an uninitialized immutable binding for N.
Set the bound value for N in envRec to V.
Record that the immutable binding for N in envRec has been initialized.
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.
The concrete Environment Record method HasBinding for object environment records determines if its associated binding object has a property whose name is the value of the argument N:
Let envRec be the object environment record for which the method was invoked.
Let bindings be the binding object for envRec.
Return the result of calling the [[HasProperty]] internal method of bindings, passing N as the property name.
The concrete Environment Record method CreateMutableBinding for object environment records creates in an environment record’s associated binding object a property whose name is the String value and initializes it to the value undefined. A property named N must not already exist in the binding object. If Boolean argument D 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 N 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 N, Property Descriptor {[[Value]]:undefined, [[Writable]]: true, [[Enumerable]]: true , [[Configurable]]: configValue}, and true as arguments.
The concrete Environment Record method SetMutableBinding for object environment records attempts to set the value of the environment record’s associated binding object’s property whose name is the value of the argument N to the value of argument V. A property named N should already exist but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.
Let envRec be the object environment record for which the method was invoked.
Let bindings be the binding object for envRec.
Call the [[Put]] internal method of bindings with arguments N, V, and S.
The concrete Environment Record method GetBindingValue for object environment records returns the value of its associated binding object’s property whose name is the String value of the argument identifier N. The property should already exist but if it does not the result depends upon the value of the S argument:
Let envRec be the object environment record for which the method was invoked.
Let bindings be the binding object for envRec.
Let value be the result of calling the [[HasProperty]] internal method of bindings, passing N as the property name.
If value is false, then
If S is false, return the value undefined, otherwise throw a ReferenceError exception.
Return the result of calling the [[Get]] internal method of bindings, passing N for the argument.
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 N and false as arguments.
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.
The following abstract operations are used in this specification to operate upon lexical environments:
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(N) concrete method of envRec passing name as the argument N.
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.
When the abstract operation NewDeclarativeEnvironment is called with either a Lexical Environment or null as argument E 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 E.
Return env.
When the abstract operation NewObjectEnvironmentis called with an Object O and a Lexical Environment E (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 O as the binding object.
Set env’s environment record to be envRec.
Set the outer lexical environment reference of env to E.
Return env.
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.
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.
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 |
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.
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.
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.
The following steps are performed when control enters the execution context for global code:
Initialize 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.
The following steps are performed to initialize a global execution context for ECMAScript code C:
Set the VariableEnvironment to the Global Environment.
Set the LexicalEnvironment to the Global Environment.
Set the ThisBinding to the global object.
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,
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.
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.
The following steps are performed when control enters the execution context for function code contained in function object F, 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 F as the argument.
Set the LexicalEnvironment to localEnv.
Set the VariableEnvironment to localEnv.
Let code be the value of F’s [[Code]] internal property.
Perform Declaration Binding Instantiation using the function code code and argumentList as described in 10.5.
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 n be the number 0.
For each String argName in names, in list order do
Let n be the current value of n plus 1.
If n is greater than argCount, let v be undefined otherwise let v be the value of the n’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, v, and strict as the arguments.
For each FunctionDeclaration f 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 IsAccessorDescrptor(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 d 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.
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 a 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 g be the result of calling the MakeArgGetter abstract operation with arguments name and env.
Let p 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]]: p, [[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 "<name> =
<param>;
"
with <name> replaced by the value of name and
<param> 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 P 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 P as the argument.
If the value of isMapped is undefined, then
Else, map contains a formal parameter mapping for P so,
Return the result of calling the [[Get]] internal method of map passing P 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 P performs the following steps:
Let desc be the result of calling the default [[GetOwnProperty]] internal method (8.12.1) on the arguments object passing P 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 P 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 P 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 P, 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 P as the argument.
Let allowed be the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on the arguments object passing P, 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 P, and false as the arguments.
Else
If Desc.[[Value]] is present, then
Call the [[Put]] internal method of map passing P, 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 P 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 P 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 P as the argument.
Let result be the result of calling the default [[Delete]] internal method (8.12.7) on the arguments object passing P 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 P, 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.
Syntax
PrimaryExpression :
this
Identifier
Literal
ArrayLiteral
ObjectLiteral(
Expression )
The
this
keyword
evaluates to the value of the ThisBinding of the current execution
context.
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.
A Literal is evaluated as described in 7.8.
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 :
[
Elisionopt
]
ElementList
[
]
ElementList , Elisionopt
[
]
ElementList :
Elisionopt
AssignmentExpression
ElementList ,
Elisionopt
AssignmentExpression
Elision :
,
Elision ,
Semantics
The
production ArrayLiteral
: [
Elisionopt
]
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
,
Elisionopt
]
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
: Elisionopt
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
, Elisionopt
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]].
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 :
AssignmentExpressionget
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).
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.
Syntax
MemberExpression :
PrimaryExpression
FunctionExpression
MemberExpression [
Expression ]
MemberExpression .
IdentifierNamenew
MemberExpression
Arguments
NewExpression :
MemberExpression
NewExpression
new
CallExpression :
MemberExpression
Arguments
CallExpression
Arguments
CallExpression [
Expression ]
CallExpression .
IdentifierName
Arguments :
(
)
ArgumentList
()
ArgumentList :
AssignmentExpression
ArgumentList ,
AssignmentExpression
LeftHandSideExpression :
NewExpression
CallExpression
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 [
<identifier-name-string> ]
and similarly
CallExpression .
IdentifierName
is identical in its behaviour to
CallExpression [
<identifier-name-string> ]
where <identifier-name-string> 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.
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.
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 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.
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.
The production MemberExpression : FunctionExpression is evaluated as follows:
Return the result of evaluating FunctionExpression.
Syntax
PostfixExpression :
LeftHandSideExpression
LeftHandSideExpression
[no LineTerminator here]
++
LeftHandSideExpression
[no LineTerminator here]
--
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:
IsStrictReference(lhs) is true
Type(GetBase(lhs)) is Environment Record
GetReferencedName(lhs)
is either "eval"
or "arguments"
Let
newValue be the result of adding the value 1
to oldValue, using the same rules as for the +
operator (see 11.6.3).
Call PutValue(lhs, newValue).
Return oldValue.
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:
IsStrictReference(lhs) is true
Type(GetBase(lhs)) is Environment Record
GetReferencedName(lhs)
is either "eval"
or "arguments"
Let
newValue be the result of subtracting the value 1
from oldValue, using the same rules as for the -
operator (11.6.3).
Call PutValue(lhs, newValue).
Return oldValue.
Syntax
UnaryExpression :
PostfixExpression
UnaryExpression
delete
void
UnaryExpressiontypeof
UnaryExpression
UnaryExpression
++--
UnaryExpression+
UnaryExpression-
UnaryExpression~
UnaryExpression!
UnaryExpression
The
production UnaryExpression
: delete
UnaryExpression is evaluated as follows:
Let ref be the result of evaluating UnaryExpression.
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.
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.
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.
Type of val |
Result |
Undefined |
|
Null |
|
Boolean |
|
Number |
|
String |
|
Object (native and does not implement [[Call]]) |
|
Object (native or host and does implement [[Call]]) |
|
Object (host and does not implement [[Call]]) |
Implementation-defined
except may not be |
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:
IsStrictReference(expr) is true
Type(GetBase(expr)) is Environment Record
GetReferencedName(expr)
is either "eval"
or "arguments"
Let
newValue be the result of adding the value 1
to oldValue, using the same rules as for the +
operator (see 11.6.3).
Call PutValue(expr, newValue).
Return newValue.
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:
IsStrictReference(expr) is true
Type(GetBase(expr)) is Environment Record
GetReferencedName(expr)
is either "eval"
or "arguments"
Let
newValue be the result of subtracting the value 1
from oldValue, using the same rules as for the -
operator (see 11.6.3).
Call PutValue(expr, newValue).
Return newValue.
The unary + operator converts its operand to Number type.
The
production UnaryExpression : +
UnaryExpression is
evaluated as follows:
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.
If oldValue is NaN, return NaN.
Return the result of negating oldValue; that is, compute a Number with the same magnitude but opposite sign.
The
production UnaryExpression : ~
UnaryExpression is
evaluated as follows:
Let expr be the result of evaluating UnaryExpression.
Return the result of applying bitwise complement to oldValue. The result is a signed 32-bit integer.
The
production UnaryExpression : !
UnaryExpression is
evaluated as follows:
Let expr be the result of evaluating UnaryExpression.
If oldValue is true, return false.
Return true.
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.
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 non-zero 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.
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 non-zero 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 non-zero 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.
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 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.
Syntax
AdditiveExpression :
MultiplicativeExpression
AdditiveExpression +
MultiplicativeExpression
AdditiveExpression -
MultiplicativeExpression
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).
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.
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.
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 zeros is −0. The sum of two positive zeros, or of two zeros 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 a
and b, it is
always the case that a–
b
produces the same result as a +(–
b)
.
Syntax
ShiftExpression :
AdditiveExpression
ShiftExpression <<
AdditiveExpression
ShiftExpression >>
AdditiveExpression
ShiftExpression >>>
AdditiveExpression
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.
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.
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.
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 a 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.
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 r be the result of performing abstract relational comparison lval < rval. (see 11.8.5)
If r is undefined, return false. Otherwise, return r.
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 r be the result of performing abstract relational comparison rval < lval with LeftFirst equal to false. (see 11.8.5).
If r is undefined, return false. Otherwise, return r.
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 r be the result of performing abstract relational comparison rval < lval with LeftFirst equal to false. (see 11.8.5).
If r is true or undefined, return false. Otherwise, return true.
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 r be the result of performing abstract relational comparison lval < rval. (see 11.8.5)
If r is true or undefined, return false. Otherwise, return true.
The comparison x < y, where x and y are values, produces true, false, or undefined (which indicates that at least one operand is NaN). In addition to x and y the algorithm takes a Boolean flag named LeftFirst as a parameter. The flag is used to control the order in which operations with potentially visible side-effects are performed upon x and y. It is necessary because ECMAScript specifies left to right evaluation of expressions. The default value of LeftFirst is true and indicates that the x parameter corresponds to an expression that occurs to the left of the y parameter’s corresponding expression. If LeftFirst is false, the reverse is the case and operations must be performed upon y before x. Such a comparison is performed as follows:
If the LeftFirst flag is true, then
Let px be the result of calling ToPrimitive(x, hint Number).
Let py be the result of calling ToPrimitive(y, 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(y, hint Number).
Let px be the result of calling ToPrimitive(x, 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 p is a prefix of String value q if q can be the result of concatenating p and some other String r. Note that any String is a prefix of itself, because r may be the empty String.)
If px is a prefix of py, return true.
Let k be the smallest nonnegative integer such that the character at position k within px is different from the character at position k within py. (There must be such a k, for neither String is a prefix of the other.)
Let m be the integer that is the code unit value for the character at position k within px.
Let n be the integer that is the code unit value for the character at position k within py.
If m < n, 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.
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 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.
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).
Return the result of calling the [[HasProperty]] internal method of rval with argument ToString(lval).
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.
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).
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 r be the result of performing abstract equality comparison rval == lval. (see 11.9.3).
If r is true, return false. Otherwise, return true.
The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed as follows:
If Type(x) is the same as Type(y), then
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
If Type(x) is Number, then
If x is NaN, return false.
If y is NaN, return false.
If x is the same Number value as y, return true.
If x is +0 and y is −0, return true.
If x is −0 and y is +0, return true.
Return false.
If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions). Otherwise, return false.
If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false.
Return true if x and y refer to the same object. Otherwise, return false.
If x is null and y is undefined, return true.
If x is undefined and y is null, return true.
If
Type(x) is Number and Type(y) is String,
return
the result of the comparison x == ToNumber(y).
If
Type(x) is String and Type(y) is Number,
return
the result of the comparison ToNumber(x) == y.
If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
If
Type(x) is either String or Number and Type(y) is
Object,
return the result of the comparison x ==
ToPrimitive(y).
If
Type(x) is Object and Type(y) is either String or
Number,
return the result of the comparison ToPrimitive(x)
== y.
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:
A
!=
B
is
equivalent to
!(A
==
B)
.
A
==
B
is
equivalent to B
==
A
,
except in the order of evaluation of
A
and
B
.
NOTE 3 The
equality operator is not always transitive. For example, there might
be two distinct String objects, each representing the same String
value; each String object would be considered equal to the String
value by the ==
operator, but the two String objects would not be equal to each
other.
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.
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)
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 r be the result of performing strict equality comparison rval === lval. (See 11.9.6)
If r is true, return false. Otherwise, return true.
The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
If Type(x) is Number, then
If x is NaN, return false.
If y is NaN, return false.
If x is the same Number value as y, return true.
If x is +0 and y is −0, return true.
If x is −0 and y is +0, return true.
Return false.
If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, return false.
If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
Return true if x and y 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.
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 A : A @ B, where @ is one of the bitwise operators in the productions above, is evaluated as follows:
Let lref be the result of evaluating A.
Let lval be GetValue(lref).
Let rref be the result of evaluating B.
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.
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.
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.
Syntax
AssignmentExpression :
ConditionalExpression
LeftHandSideExpression
AssignmentOperator AssignmentExpression
AssignmentExpressionNoIn :
ConditionalExpressionNoIn
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 andAssignmentExpressionNoIn are evaluated instead of the contained ConditionalExpression and AssignmentExpression, respectively.
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:
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.
The
production AssignmentExpression : LeftHandSideExpression@ =
AssignmentExpression,
where @ 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 r be the result of applying operator @ to lval and rval.
Throw a SyntaxError exception if the following conditions are all true:
IsStrictReference(lref) is true
Type(GetBase(lref)) is Environment Record
GetReferencedName(lref)
is either "eval"
or "arguments"
Call PutValue(lref, r).
Return r.
NOTE See NOTE 11.13.1.
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.
Syntax
Statement :
Block
VariableStatement
EmptyStatement
ExpressionStatement
IfStatement
IterationStatement
ContinueStatement
BreakStatement
ReturnStatement
WithStatement
LabelledStatement
SwitchStatement
ThrowStatement
TryStatement
DebuggerStatement
Semantics
A Statement can be part of a LabelledStatement, which itself can be part of a LabelledStatement, and so on. The labels introduced this way are collectively referred to as the “current label set” when describing the semantics of individual statements. 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.
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 difference, 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.
Syntax
Block :
{
StatementListopt }
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 s be the result of evaluating Statement.
If an exception was thrown, return (throw, V, empty) where V is the exception. (Execution now proceeds as if no exception were thrown.)
Return s.
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 s be the result of evaluating Statement.
If an exception was thrown, return (throw, V, empty) where V is the exception. (Execution now proceeds as if no exception were thrown.)
If s.value is empty, let V = sl.value, otherwise let V = s.value.
Return (s.type, V, s.target).
Syntax
VariableStatement :
var
VariableDeclarationList ;
VariableDeclarationList :
VariableDeclaration
VariableDeclarationList ,
VariableDeclaration
VariableDeclarationListNoIn :
VariableDeclarationNoIn
VariableDeclarationListNoIn ,
VariableDeclarationNoIn
VariableDeclaration :
Identifier Initialiseropt
VariableDeclarationNoIn :
Identifier InitialiserNoInopt
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.
It is
a SyntaxError if a VariableDeclaration
or VariableDeclarationNoIn
occurs within strict code and its Identifier
is either "eval"
or "arguments"
.
Syntax
EmptyStatement :
;
Semantics
The
production EmptyStatement : ;
is
evaluated as follows:
Return (normal, empty, empty).
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).
Syntax
IfStatement :
if
(
Expression )
Statement else
Statementif
(
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.
Syntax
IterationStatement :
do
Statement
while
(
Expression );
while
(
Expression )
Statementfor
(
ExpressionNoInopt;
Expressionopt ;
Expressionopt )
Statementfor
(
var
VariableDeclarationListNoIn;
Expressionopt ;
Expressionopt )
Statementfor
(
LeftHandSideExpression in
Expression )
Statementfor
(
var
VariableDeclarationNoIn in
Expression )
Statement
The
production do
Statement while
(
Expression );
is evaluated as follows:
Let V = 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 V = 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, V, 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, V, empty);
The
production IterationStatement : while
(
Expression )
Statement is evaluated as follows:
Let V = empty.
Repeat
Let exprRef be the result of evaluating Expression.
If ToBoolean(GetValue(exprRef)) is false, 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 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, V, empty).
If stmt is an abrupt completion, return stmt.
The
production
IterationStatement
: for
(
ExpressionNoInopt
;
Expressionopt
;
Expressionopt)
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.)
Let V = empty.
Repeat
If the first Expression is present, then
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.)
The
production
IterationStatement
: for
(
var
VariableDeclarationListNoIn
;
Expressionopt ;
Expressionopt )
Statement
is
evaluated as follows:
Evaluate VariableDeclarationListNoIn.
Let V = empty.
Repeat
If the first Expression is present, then
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.)
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 V = empty.
Repeat
Let P be the name of the next property of obj whose [[Enumerable]] attribute is true. If there is no such property, return (normal, V, empty).
Let lhsRef be the result of evaluating the LeftHandSideExpression ( it may be evaluated repeatedly).
Call PutValue(lhsRef, P).
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.
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 V = empty.
Repeat
Let P be the name of the next property of obj whose [[Enumerable]] attribute is true. If there is no such property, return (normal, V, 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, P).
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.
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.
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.
A ContinueStatement without an Identifier is evaluated as follows:
Return (continue, empty, empty).
A ContinueStatement with the optional Identifier is evaluated as follows:
Return (continue, empty, Identifier).
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.
A BreakStatement without an Identifier is evaluated as follows:
Return (break, empty, empty).
A BreakStatement with an Identifier is evaluated as follows:
Return (break, empty,Identifier).
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.
The
production ReturnStatement
: return
[no LineTerminator here]
Expressionopt
;
is
evaluated as:
If the Expression is not present, return (return, undefined, empty).
Let exprRef be the result of evaluating Expression.
Return (return, GetValue(exprRef), empty).
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 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 C be the result of evaluating Statement but if an exception is thrown during the evaluation, let C be (throw, V, empty), where V is the exception. (Execution now proceeds as if no exception were thrown.)
Set the running execution context’s Lexical Environment to oldEnv.
Return C.
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.
Strict mode code may not include a WithStatement. The occurrence of a WithStatement in such a context is treated as a SyntaxError.
Syntax
SwitchStatement :
switch
(
Expression )
CaseBlock
CaseBlock :
{
CaseClausesopt }
{
CaseClausesoptDefaultClause CaseClausesopt }
CaseClauses :
CaseClause
CaseClauses
CaseClause
CaseClause :
case
Expression :
StatementListopt
DefaultClause :
default
:
StatementListopt
Semantics
The
production SwitchStatement : switch
(
Expression )
CaseBlock is evaluated as follows:
Let exprRef be the result of evaluating Expression.
Let R be the result of evaluating CaseBlock, passing it GetValue(exprRef) as a parameter.
If R.type is break and R.target is in the current label set, return (normal, R.value, empty).
Return R.
The
production CaseBlock
: {
CaseClausesopt
}
is given an input parameter, input,
and is evaluated as follows:
Let V = empty.
Let A be the list of CaseClause items in source text order.
Let searching be true.
Repeat, while searching is true
Let C be the next CaseClause in A. If there is no such CaseClause, return (normal, V, empty).
Let clauseSelector be the result of evaluating C.
If
input
is equal to clauseSelector
as defined by the ===
operator, then
Set searching to false.
IfC has a StatementList, then
EvaluateC’s StatementList and letR be the result.
IfR is an abrupt completion, then returnR.
LetV =R.value.
Repeat
LetC be the next CaseClause inA. If there is no such CaseClause, return (normal,V, empty).
IfC has a StatementList, then
EvaluateC’s StatementList and letR be the result.
IfR.value is not empty, then letV =R.value.
IfR is an abrupt completion, then return (R.type,V,R.target).
The
production CaseBlock
: {
CaseClausesoptDefaultClause CaseClausesopt
}
is given an input parameter, input,
and is evaluated as follows:
Let V = empty.
Let A be the list of CaseClause items in the first CaseClauses, in source text order.
Let B be the list of CaseClause items in the second CaseClauses, in source text order.
Let found be false.
Repeat letting C be in order each CaseClause in A
If found is false, then
Let clauseSelector be the result of evaluating C.
If
input
is equal to clauseSelector
as defined by the ===
operator, then set found
to true.
If found is true, then
If C has a StatementList, then
Evaluate C’s StatementList and let R be the result.
IfR.value is not empty, then letV =R.value.
R is an abrupt completion, then return (R.type,V,R.target).
Let foundInB be false.
If found is false, then
Repeat, while foundInB is false and all elements of B have not been processed
Let C be the next CaseClause in B.
Let clauseSelector be the result of evaluating C.
If
input
is equal to clauseSelector
as defined by the ===
operator, then
Set foundInB to true.
IfC has a StatementList, then
Evaluate C’s StatementList and let R be the result.
IfR.value is not empty, then letV =R.value.
R is an abrupt completion, then return (R.type,V,R.target).
If foundInB is false and the DefaultClause has a StatementList, then
Evaluate the DefaultClause’s StatementList and let R be the result.
If R.value is not empty, then let V = R.value.
If R is an abrupt completion, then return (R.type, V, R.target).
Repeat (Note that if step 7.a.i has been performed this loop does not start at the beginning of B)
Let C be the next CaseClause in B. If there is no such CaseClause, return (normal, V, empty).
If C has a StatementList, then
Evaluate C’s StatementList and let R be the result.
If R.value is not empty, then let V = R.value.
If R is an abrupt completion, then return (R.type, V, R.target).
The
production CaseClause
: case
Expression :
StatementListopt 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.
Syntax
LabelledStatement :
Identifier :
Statement
Semantics
A
Statement may be
prefixed by a label. Labelled statements are only used in
conjunction with labelled break
and continue
statements. ECMAScript has no goto
statement.
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, V,
L) where L
is equal to Identifier,
the production results in (normal, V,
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.
Syntax
ThrowStatement :
throw
[no LineTerminator here] Expression ;
Semantics
The
production ThrowStatement
: throw
[no LineTerminator here]
Expression
;
is evaluated as:
Let exprRef be the result of evaluating Expression.
Return (throw, GetValue(exprRef), empty).
Syntax
TryStatement :
try
Block Catchtry
Block Finallytry
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 B be the result of evaluating Block.
If B.type is not throw, return B.
Return the result of evaluating Catch with parameter B.
The
production TryStatement
: try
Block
Finally is
evaluated as follows:
Let B be the result of evaluating Block.
Let F be the result of evaluating Finally.
If F.type is normal, return B.
Return F.
The
production TryStatement
: try
Block
Catch Finally
is evaluated as follows:
Let B be the result of evaluating Block.
If B.type is throw, then
Let C be the result of evaluating Catch with parameter B.
Else, B.type is not throw,
Let C be B.
Let F be the result of evaluating Finally.
If F.type is normal, return C.
Return F.
The
production Catch :
catch
(
Identifier
)
Block
is evaluated as follows:
Let C 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, C, and false as arguments. Note that the last argument is immaterial in this situation.
Set the running execution context’s LexicalEnvironment to catchEnv.
Let B be the result of evaluating Block.
Set the running execution context’s LexicalEnvironment to oldEnv.
Return B.
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.
It is
an SyntaxError if a TryStatement
with a Catch
occurs within strict code and the Identifier
of the Catch
production is either "eval"
or "arguments"
.
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.
Syntax
FunctionDeclaration :
function
Identifier (
FormalParameterListopt )
{
FunctionBody }
FunctionExpression :
function
Identifieropt (
FormalParameterListopt )
{
FunctionBody }
FormalParameterList :
Identifier
FormalParameterList ,
Identifier
FunctionBody :
Semantics
The
production
FunctionDeclaration : function
Identifier (
FormalParameterListopt )
{
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 FormalParameterListopt, 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
(
FormalParameterListopt )
{
FunctionBody }
is evaluated as follows:
Return the result of creating a new Function object as specified in 13.2 with parameters specified by FormalParameterListopt 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 (
FormalParameterListopt
)
{
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(N) 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 FormalParameterListopt 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(N,V) 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 :SourceElementsopt 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).
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.
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 F be that object.
Set all the internal methods, except for [[Get]], of F as described in 8.12.
Set
the [[Class]] internal property of F
to "Function"
.
Set the [[Prototype]] internal property of F to the standard built-in Function prototype object as specified in 15.3.3.1.
Set the [[Get]] internal property of F as described in 15.3.5.4.
Set the [[Call]] internal property of F as described in 13.2.1.
Set the [[Construct]] internal property of F as described in 13.2.2.
Set the [[HasInstance]] internal property of F as described in 15.3.5.3.
Set the [[Scope]] internal property of F 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.
Set the [[FormalParameters]] internal property of F to names.
Set the [[Code]] internal property of F to FunctionBody.
Set the [[Extensible]] internal property of F 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 F
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]]: F,
{ [[Writable]]: true,
[[Enumerable]]: false,
[[Configurable]]: true},
and false.
Call
the [[DefineOwnProperty]] internal method of F
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 F
with arguments "caller"
,
PropertyDescriptor {[[Get]]: thrower,
[[Set]]: thrower,
[[Enumerable]]: false,
[[Configurable]]: false},
and false.
Call
the [[DefineOwnProperty]] internal method of F
with arguments "arguments"
,
PropertyDescriptor {[[Get]]: thrower,
[[Set]]: thrower,
[[Enumerable]]: false,
[[Configurable]]: false},
and false.
Return F.
NOTE A
prototype
property is automatically created for every function, to allow for
the possibility that the function will be used as a constructor.
When the [[Call]] internal method for a Function object F 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 F'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 F's [[Code]] internal property. If F 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.
When the [[Construct]] internal method for a Function object F 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 F
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 F, 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.
The [[ThrowTypeError]] object is a unique function object that is defined once as follows:
Create a new native ECMAScript object and let F be that object.
Set all the internal methods of F as described in 8.12.
Set
the [[Class]] internal property of F
to "Function"
.
Set the [[Prototype]] internal property of F to the standard built-in Function prototype object as specified in 15.3.3.1.
Set the [[Call]] internal property of F as described in 13.2.1.
Set the [[Scope]] internal property of F to the Global Environment.
Set the [[FormalParameters]] internal property of F to an empty List.
Set the [[Code]] internal property of F to be a FunctionBody that unconditionally throws a TypeError exception and performs no other action.
Call
the [[DefineOwnProperty]] internal method of F
with arguments "
length
"
,
Property Descriptor {[[Value]]: 0,
[[Writable]]: false,
[[Enumerable]]: false,
[[Configurable]]: false},
and false.
Set the [[Extensible]] internal property of F to false.
Let [[ThrowTypeError]] be F.
Syntax
Program :
SourceElementsopt
SourceElements :
SourceElement
SourceElements
SourceElement
SourceElement :
Semantics
The
production Program :SourceElementsopt
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 V = headResult.value, otherwise let V = 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).
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 a 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.
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 2
.
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.
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.
The
value of NaN
is
NaN (see 8.5). This property has the attributes {
[[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
The
value of Infinity
is +∞ (see 8.5).
This property has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: false }.
The
value of undefined
is undefined (see 8.1). This property has the attributes {
[[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
When
the eval
function
is called with one argument x,
the following steps are taken:
If Type(x) is not String, return x.
Let prog be the ECMAScript code that is the result of parsing x 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 V, then return the value V.
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.
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.
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, 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 S 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 S be the empty string.
Let sign be 1.
If
S is not empty and the first character of S is a
minus sign -
, let
sign be −1.
If
S is not empty and the first character of S is a plus
sign +
or a minus
sign -
, then
remove the first character from S.
Let R = ToInt32(radix).
Let stripPrefix be true.
If R ≠ 0, then
If R < 2 or R > 36, then return NaN.
If R ≠ 16, let stripPrefix be false.
Else, R = 0
Let R = 10.
If stripPrefix is true, then
If
the length of S is at least 2 and the first two characters
of S are either “0x
”
or “0X
”,
then remove the first two characters from S and let R
= 16.
If S contains any character that is not a radix-R digit, then let Z be the substring of S consisting of all characters before the first such character; otherwise, let Z be S.
If Z is empty, return NaN.
Let
mathInt be the mathematical integer value that is
represented by Z in radix-R notation, using the
letters A
-Z
and a
-z
for digits with values 10 through 35. (However, if R is 10
and Z contains more than 20 significant digits, every
significant digit after the 20th may be replaced by a 0
digit, at the option of the implementation; and if R
is not 2, 4, 8, 10, 16, or 32, then mathInt may be an
implementation-dependent approximation to the mathematical integer
value that is represented by Z in radix-R notation.)
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.
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.
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 X
is a NaN is an expression of the form X
!== X
. The result will be true if and only if X
is a NaN.
Returns false if the argument coerces to NaN, +∞, or −∞, and otherwise returns true.
If ToNumber(number) is NaN, +∞, or −∞, return false.
Otherwise, return true.
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 the “:
”,
“/
”, “;
”
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.
uri :::
uriCharactersopt
uriCharacters :::
uriCharacter uriCharactersopt
uriCharacter :::
uriReserved
uriUnescaped
uriEscaped
uriReserved ::: one of
;
/ ? : @ & = + $ ,
uriUnescaped :::
uriAlpha
DecimalDigit
uriMark
uriEscaped :::
%
HexDigit HexDigit
uriAlpha ::: one of
a
b c d e f g h i j k l m n o p q r s t u v w x
y z
A B C D E F G H I J K L M N O P Q R S T
U V W X Y Z
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 R be the empty String.
Let k be 0.
Repeat
If k equals strLen, return R.
Let C be the character at position k within string.
If C is in unescapedSet, then
Let S be a String containing only the character C.
Let R be a new String value computed by concatenating the previous value of R and S.
Else, C is not in unescapedSet
If the code unit value of C is not less than 0xDC00 and not greater than 0xDFFF, throw a URIError exception.
If the code unit value of C is less than 0xD800 or greater than 0xDBFF, then
Let V be the code unit value of C.
Else,
Increase k by 1.
If k equals strLen, throw a URIError exception.
Let kChar be the code unit value of the character at position k within string.
If kChar is less than 0xDC00 or greater than 0xDFFF, throw a URIError exception.
Let V be (((the code unit value of C) – 0xD800) * 0x400 + (kChar – 0xDC00) + 0x10000).
Let Octets be the array of octets resulting by applying the UTF-8 transformation to V, and let L be the array size.
Let j be 0.
Repeat, while j < L
Let jOctet be the value at position j within Octets.
Let
S be a String containing three characters “%
XY
”
where XY are two uppercase hexadecimal digits encoding
the value of jOctet.
Let R be a new String value computed by concatenating the previous value of R and S.
Increase j by 1.
Increase k 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 R be the empty String.
Let k be 0.
Repeat
If k equals strLen, return R.
Let C be the character at position k within string.
If
C is not ‘%
’,
then
Let S be the String containing only the character C.
Else,
C is ‘%
’
Let start be k.
If k + 2 is greater than or equal to strLen, throw a URIError exception.
If the characters at position (k+1) and (k + 2) within string do not represent hexadecimal digits, throw a URIError exception.
Let B be the 8-bit value represented by the two hexadecimal digits at position (k + 1) and (k + 2).
Increment k by 2.
If the most significant bit in B is 0, then
Let C be the character with code unit value B.
If C is not in reservedSet, then
Let S be the String containing only the character C.
Else, C is in reservedSet
Let S be the substring of string from position start to position k included.
Else, the most significant bit in B is 1
Let n be the smallest non-negative number such that (B << n) & 0x80 is equal to 0.
If n equals 1 or n is greater than 4, throw a URIError exception.
Let Octets be an array of 8-bit integers of size n.
Put B into Octets at position 0.
If k + (3 * (n – 1)) is greater than or equal to strLen, throw a URIError exception.
Let j be 1.
Repeat, while j < n
Increment k by 1.
If the character at position k is not ‘%’, throw a URIError exception.
If the characters at position (k +1) and (k + 2) within string do not represent hexadecimal digits, throw a URIError exception.
Let B be the 8-bit value represented by the two hexadecimal digits at position (k + 1) and (k + 2).
If the two most significant bits in B are not 10, throw a URIError exception.
Increment k by 2.
Put B into Octets at position j.
Increment j by 1.
Let V be the value obtained by applying the UTF-8 transformation to Octets, that is, from an array of octets into a 32-bit value. If Octets does not contain a valid UTF-8 encoding of a Unicode code point throw a URIError exception.
If V is less than 0x10000, then
Let C be the character with code unit value V.
If C is not in reservedSet, then
Let S be the String containing only the character C.
Else, C is in reservedSet
Let S be the substring of string from position start to position k included.
Else, V is ≥ 0x10000
Let L be (((V – 0x10000) & 0x3FF) + 0xDC00).
Let H be ((((V – 0x10000) >> 10) & 0x3FF) + 0xD800).
Let S be the String containing the two characters with code unit values H and L.
Let R be a new String value computed by concatenating the previous value of R and S.
Increase k by 1.
NOTE The syntax of Uniform Resource Identifiers is given in 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.
Code Unit Value |
Representation |
1st Octet |
2nd Octet |
3rd Octet |
4th Octet |
|
|
|
|||
|
|
|
|
||
|
|
|
|
|
|
followed by
|
followed by
|
|
|
|
|
not followed by
|
|
||||
|
|
||||
|
|
|
|
|
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.
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.
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)
The
encodeURI
function
computes a new version of a URI in which each instance of certain
characters is replaced by one, two or three 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.
The
encodeURIComponent
function computes a new version of a URI in which each instance of
certain characters is replaced by one, two or three 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)
See 15.9.2.
See 15.11.6.1.
See 15.11.6.2.
See 15.11.6.3.
See 15.11.6.4.
See 15.11.6.5.
See 15.11.6.6.
See 15.8.
See 15.12.
When
Object
is called
as a function rather than as a constructor, it performs a type
conversion.
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).
When
Object
is called
as part of a new
expression, it is a constructor that may create an object.
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.
Asset: 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 t 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 the all the internal methods of obj as specified in 8.12
Return obj.
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 1), the Object constructor has the
following properties:
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 }.
When
the getPrototypeOf
function is called with argument O,
the following steps are taken:
If Type(O) is not Object throw a TypeError exception.
Return the value of the [[Prototype]] internal property of O.
When the getOwnPropertyDescriptor function is called, the following steps are taken:
If Type(O) is not Object throw a TypeError exception.
Let name be ToString(P).
Let desc be the result of calling the [[GetOwnProperty]] internal method of O with argument name.
Return the result of calling FromPropertyDescriptor(desc) (8.10.4).
When the getOwnPropertyNames function is called, the following steps are taken:
If Type(O) 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 n be 0.
For each named own property P of O
Let name be the String value that is the name of P.
Call the [[DefineOwnProperty]] internal method of array with arguments ToString(n), the PropertyDescriptor {[[Value]]: name, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
Increment n by 1.
Return array.
NOTE If O 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.
The create function creates a new object with a specified prototype. When the create function is called, the following steps are taken:
If Type(O) is 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 O.
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.
The defineProperty function is used to add an own property and/or update the attributes of an existing own property of an object. When the defineProperty function is called, the following steps are taken:
If Type(O) is not Object throw a TypeError exception.
Let name be ToString(P).
Let desc be the result of calling ToPropertyDescriptor with Attributes as the argument.
Call the [[DefineOwnProperty]] internal method of O with arguments name, desc, and true.
Return O.
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:
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 P of names in list order,
Let descObj be the result of calling the [[Get]] internal method of props with P as the argument.
Let desc be the result of calling ToPropertyDescriptor with descObj as the argument.
Append desc to the end of descriptors.
For each element desc of descriptors in list order,
Call the [[DefineOwnProperty]] internal method of O with arguments P, desc, and true.
Return O
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.
When the seal function is called, the following steps are taken:
If Type(O) is not Object throw a TypeError exception.
For each named own property name P of O,
Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
If desc.[[Configurable]] is true, set desc.[[Configurable]] to false.
Call the [[DefineOwnProperty]] internal method of O with P, desc, and true as arguments.
Set the [[Extensible]] internal property of O to false.
Return O.
When the freeze function is called, the following steps are taken:
If Type(O) is not Object throw a TypeError exception.
For each named own property name P of O,
Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
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 O with P, desc, and true as arguments.
Set the [[Extensible]] internal property of O to false.
Return O.
When the preventExtensions function is called, the following steps are taken:
If Type(O) is not Object throw a TypeError exception.
Set the [[Extensible]] internal property of O to false.
Return O.
When the isSealed function is called with argument O, the following steps are taken:
If Type(O) is not Object throw a TypeError exception.
For each named own property name P of O,
Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
If desc.[[Configurable]] is true, then return false.
If the [[Extensible]] internal property of O is false, then return true.
Otherwise, return false.
When the isFrozen function is called with argument O, the following steps are taken:
If Type(O) is not Object throw a TypeError exception.
For each named own property name P of O,
Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
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 O is false, then return true.
Otherwise, return false.
When the isExtensible function is called with argument O, the following steps are taken:
If Type(O) is not Object throw a TypeError exception.
Return the Boolean value of the [[Extensible]] internal property of O.
When the keys function is called with argument O, the following steps are taken:
Let n be the number of own enumerable properties of O
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 O whose name String is P
Call the [[DefineOwnProperty]] internal method of array with arguments ToString(index), the PropertyDescriptor {[[Value]]: P, [[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.
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.
The
initial value of Object.prototype.constructor
is the standard built-in Object
constructor.
When
the toString
method is called, the following steps are taken:
If the this value is undefined, return "[object Undefined]".
If the this value is null, return "[object Null]".
Let O be the result of calling ToObject passing the this value as the argument.
Let class be the value of the [[Class]] internal property of O.
Return the String value that is the result of concatenating the three Strings "[object ", class, and "]".
When the toLocaleString method is called, the following steps are taken:
Let O 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 O 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 O 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.
When the valueOf method is called, the following steps are taken:
Let O be the result of calling ToObject passing the this value as the argument.
If O is the result of calling the Object constructor with a host object (15.2.2.1), then
Return either O or another value such as the host object originally passed to the constructor. The specific result that is returned is implementation-defined.
Return O.
When
the hasOwnProperty
method is called with argument V,
the following steps are taken:
Let P be ToString(V).
Let O 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 O passing P 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.
When
the isPrototypeOf
method is called with argument V,
the following steps are taken:
If V is not an object, return false.
Let O be the result of calling ToObject passing the this value as the argument.
Repeat
Let V be the value of the [[Prototype]] internal property of V.
if V is null, return false
If O and V 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.
When
the propertyIsEnumerable
method is called with argument V,
the following steps are taken:
Let P be ToString(V).
Let O 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 O passing P 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.
Object instances have no special properties beyond those inherited from the Object prototype object.
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.
When
the Function
function is called with some arguments p1,
p2, … , pn,
body (where n
might be 0, that is,
there are no “p”
arguments, and where body
might also not be provided), the following steps are taken:
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).
When
Function
is called
as part of a new
expression, it is a constructor: it initialises the newly created
object.
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 n
might be 0, that is,
there are no “p”
arguments, and where body
might also not be provided), the following steps are taken:
Let argCount be the total number of arguments passed to this function invocation.
Let P 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 body be ToString(body).
If P is not parsable as a FormalParameterListopt then throw a SyntaxError exception.
If body is not parsable as FunctionBody then throw a SyntaxError exception.
Ifbody 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 P as the FormalParameterList and body as the FunctionBody. Pass in the Global Environment as the Scope parameter and strict as the Strict flag.
A
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")
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:
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 }.
This is a data property with a value of 1. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
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 0.
The
initial value of Function.prototype.constructor
is the built-in Function
constructor.
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.
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 n be ToUint32(len).
Let argList be an empty List.
Let index be 0.
Repeat while index < n
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 2.
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.
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
1.
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.
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 A be a new (possibly empty) internal list of all of the argument values provided after thisArg (arg1, arg2 etc), in order.
Let F be a new native ECMAScript object .
Set all the internal methods, except for [[Get]], of F as specified in 8.12.
Set the [[Get]] internal property of F as specified in 15.3.5.4.
Set the [[TargetFunction]] internal property of F to Target.
Set the [[BoundThis]] internal property of F to the value of thisArg.
Set the [[BoundArgs]] internal property of F to A.
Set the [[Class]] internal property of F to "Function".
Set the [[Prototype]] internal property of F to the standard built-in Function prototype object as specified in 15.3.3.1.
Set the [[Call]] internal property of F as described in 15.3.4.5.1.
Set the [[Construct]] internal property of F as described in 15.3.4.5.2.
Set the [[HasInstance]] internal property of F as described in 15.3.4.5.3.
If the [[Class]] internal property of Target is "Function", then
Let L be the length property of Target minus the length of A.
Set the length own property of F to either 0 or L, whichever is larger.
Else set the length own property of F to 0.
Set the attributes of the length own property of F to the values specified in 15.3.5.1.
Set the [[Extensible]] internal property of F to true.
Let thrower be the [[ThrowTypeError]] function Object (13.2.3).
Call
the [[DefineOwnProperty]] internal method of F with
arguments "caller"
,
PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower,
[[Enumerable]]: false, [[Configurable]]: false}, and
false.
Call
the [[DefineOwnProperty]] internal method of F with
arguments "arguments"
,
PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower,
[[Enumerable]]: false, [[Configurable]]: false}, and
false.
Return F.
The
length
property of
the bind
method is
1.
NOTE Function
objects created using Function.prototype.bind
do not have a prototype
property or the [[Code]], [[FormalParameters]], and [[Scope]]
internal properties.
When the [[Call]] internal method of a function object, F, 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.
When the [[Construct]] internal method of a function object, F 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 oftarget providing args as the arguments.
When the [[HasInstance]] internal method of a function object F, that was created using the bind function is called with argument V, 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 oftarget providing V as the argument.
In addition to the required internal properties, every function instance has a [[Call]] internal property and in most cases use 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.
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 }.
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.
Assume F is a Function object.
When the [[HasInstance]] internal method of F is called with value V, the following steps are taken:
If V is not an object, return false.
Let
O be the result of calling the [[Get]] internal method of F
with property name "prototype"
.
Repeat
Let V be the value of the [[Prototype]] internal property of V.
If
V is null
,
return false.
If O and V 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.
Function objects use a variation of the [[Get]] internal method used for other native ECMAScript objects (8.12.3).
Assume F is a Function object. When the [[Get]] internal method of F is called with property name P, the following steps are taken:
Let v be the result of calling the default [[Get]] internal method (8.12.3) on F passing P as the property name argument.
If
P is "caller"
and v is a strict mode Function object, throw a
TypeError exception.
Return v.
NOTE Function
objects created using Function.prototype.bind
use the default [[Get]] internal method.
Array
objects give special treatment to a certain class of property names.
A property name P
(in the form of a String value) is an array index if and only
if ToString(ToUint32(P))
is equal to P and
ToUint32(P)
is not equal to 232−1.
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 232.
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, O, is said to be sparse if the following algorithm returns true:
Let len be the result of calling the [[Get]] internal method of O with argument "length".
For each integer i in the range 0≤i<ToUint32(len)
Let elem be the result of calling the [[GetOwnProperty]] internal method of O with argument ToString(i).
If elem is undefined, return true.
Return false.
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.
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 a new
expression with the same arguments (15.4.2).
When
Array
is called as
part of a new
expression, it is a constructor: it initialises the newly created
object.
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 0
property of the newly constructed object is set to item0
(if supplied); the 1
property of the newly constructed object is set to item1
(if supplied); and, in general, for as many arguments as there are,
the k property of
the newly constructed object is set to argument k,
where the first argument is considered to be argument number 0
.
These properties all have the attributes {[[Writable]]: true,
[[Enumerable]]: true, [[Configurable]]: true}.
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 1
and the 0
property
of the newly constructed object is set to len
with attributes {[[Writable]]: true, [[Enumerable]]: true,
[[Configurable]]: true}..
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 1), the Array constructor has the
following properties:
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 }.
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.
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.
The
initial value of Array.prototype.constructor
is the standard built-in Array
constructor.
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.
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 O 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 R 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 R be the result of calling the [[Call]] internal method of func providing elementObj as the this value and an empty arguments list.
Let
k be 1
.
Repeat, while k < len
Let S be a String value produced by concatenating R and separator.
Let nextElement be the result of calling the [[Get]] internal method of array with argument ToString(k).
If nextElement is undefined or null, then
Let R 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 R be the result of calling the [[Call]] internal method of func providing elementObj as the this value and an empty arguments list.
Let R be a String value produced by concatenating S and R.
Increase k by 1.
Return R.
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.
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 O be the result of calling ToObject passing the this value as the argument.
Let
A be a new array created as if by the expression new
Array()
where Array
is the standard built-in constructor with that name.
Let n be 0.
Let items be an internal List whose first element is O and whose subsequent elements are, in left to right order, the arguments that were passed to this function invocation.
Repeat, while items is not empty
Remove the first element from items and let E be the value of the element.
If
the value of the [[Class]] internal property of E is
"Array"
,
then
Let k be 0.
Let
len be the result of calling the [[Get]] internal method
of E with argument "length"
.
Repeat, while k < len
Let P be ToString(k).
Let exists be the result of calling the [[HasProperty]] internal method of E with P.
If exists is true, then
Let subElement be the result of calling the [[Get]] internal method of E with argument P.
Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n), Property Descriptor {[[Value]]: subElement, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
Increase n by 1.
Increase k by 1.
Else, E is not an Array
Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n), Property Descriptor {[[Value]]: E, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
Increase n by 1.
Return A.
The
length
property of
the concat
method
is 1.
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.
The elements of the array are converted to Strings, and these Strings are then concatenated, separated by occurrences of the separator. If no separator is provided, a single comma is used as the separator.
The
join
method takes
one argument, separator,
and performs the following steps:
Let O be the result of calling ToObject passing the this value as the argument.
Let
lenVal be the result of calling the [[Get]] internal method
of O 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 O with argument "0"
.
If element0 is undefined or null, let R be the empty String; otherwise, Let R be ToString(element0).
Let
k be 1
.
Repeat, while k < len
Let S be the String value produced by concatenating R and sep.
Let element be the result of calling the [[Get]] internal method of O with argument ToString(k).
If element is undefined or null, Let next be the empty String; otherwise, let next be ToString(element).
Let R be a String value produced by concatenating S and next.
Increase k by 1.
Return R.
The
length
property of
the join
method is
1.
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.
The last element of the array is removed from the array and returned.
Let O 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 O with argument "
length
"
.
Let len be ToUint32(lenVal).
If len is zero,
Call
the [[Put]] internal method of O 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 O with argument indx.
Call the [[Delete]] internal method of O with arguments indx and true.
Call
the [[Put]] internal method of O 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.
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 O 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 O with argument "
length
"
.
Let n 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 E be the value of the element.
Call the [[Put]] internal method of O with arguments ToString(n), E, and true.
Increase n by 1.
Call
the [[Put]] internal method of O with arguments "
length
"
,
n, and true.
Return n.
The
length
property of
the push
method is
1.
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.
The elements of the array are rearranged so as to reverse their order. The object is returned as the result of the call.
Let O 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 O with argument "length"
.
Let len be ToUint32(lenVal).
Let middle be floor(len/2).
Letlower be 0.
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 O with argument lowerP.
Let upperValue be the result of calling the [[Get]] internal method of O with argument upperP .
Let lowerExists be the result of calling the [[HasProperty]] internal method of O with argument lowerP.
Let upperExists be the result of calling the [[HasProperty]] internal method of O with argument upperP.
If lowerExists is true and upperExists is true, then
Call the [[Put]] internal method of O with arguments lowerP, upperValue, and true .
Call the [[Put]] internal method of O with arguments upperP, lowerValue, and true .
Else if lowerExists is false and upperExists is true, then
Call the [[Put]] internal method of O with arguments lowerP, upperValue, and true .
Call the [[Delete]] internal method of O, with arguments upperP and true.
Else if lowerExists is true and upperExists is false, then
Call the [[Delete]] internal method of O, with arguments lowerP and true .
Call the [[Put]] internal method of O with arguments upperP, lowerValue, and true .
Else, both lowerExists and upperExists are false
No action is required.
Increase lower by 1.
Return O .
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.
The first element of the array is removed from the array and returned.
Let O 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 O with argument "
length
"
.
Let len be ToUint32(lenVal).
If len is zero, then
Call
the [[Put]] internal method of O with arguments "
length
"
,
0, and true.
Return undefined.
Let
first be the result of calling the [[Get]] internal method
of O with argument "
0
"
.
Let k be 1.
Repeat, while k < len
Let from be ToString(k).
Let to be ToString(k–1).
Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument from.
If fromPresent is true, then
Let fromVal be the result of calling the [[Get]] internal method of O with argument from.
Call the [[Put]] internal method of O with arguments to, fromVal, and true.
Else, fromPresent is false
Call the [[Delete]] internal method of O with arguments to and true.
Increase k by 1.
Call the [[Delete]] internal method of O with arguments ToString(len–1) and true.
Call
the [[Put]] internal method of O 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.
The
slice
method takes
two arguments, start
and end, and
returns an array containing the elements of the array from element
start up to, but
not including, element end
(or through the end of the array if end
is undefined). If start
is negative, it is treated as length+start
where length is
the length of the array. If end
is negative, it is treated as length+end
where length
is the length of the array. The following steps are taken:
Let O be the result of calling ToObject passing the this value as the argument.
Let
A 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 O with argument "
length
"
.
Let len be ToUint32(lenVal).
Let relativeStart be ToInteger(start).
If relativeStart is negative, let k be max((len +relativeStart),0); else let k be min(relativeStart,len).
If end is undefined, let relativeEnd be len; else let relativeEnd be ToInteger(end).
If relativeEnd is negative, let final be max((len + relativeEnd),0); else let final be min(relativeEnd,len).
Let n be 0.
Repeat, while k < final
Let Pk be ToString(k).
Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
If kPresent is true, then
Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n), Property Descriptor {[[Value]]: kValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
Increase k by 1.
Increase n by 1.
Return A.
The
length
property of
the slice
method
is 2.
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.
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 x and y and returns a negative value if x < y, zero if x = y, or a positive value if x > y.
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 j
such that all of the conditions below are satisfied then the
behaviour of sort
is implementation-defined:
0 ≤ j < len
The result of calling the [[HasProperty]] internal method of proto with argument ToString(j) 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 π
of the nonnegative integers less than len,
such that for every nonnegative integer j
less than len, if
property old[j]
existed, then new[π(j)]
is exactly the same value as old[j],.
But if property old[j]
did not exist, then new[π(j)]
does not exist.
Then
for all nonnegative integers j
and k, each less
than len, if
SortCompare(j,k)
< 0
(see
SortCompare below), then π(j)
< π(k).
Here the notation old[j] is used to refer to the hypothetical result of calling the [[Get]] internal method of obj with argument j before this function is executed, and the notation new[j] to refer to the hypothetical result of calling the [[Get]] internal method of obj with argument j after this function has been executed.
A function comparefn is a consistent comparison function for a set of values S if all of the requirements below are met for all values a, b, and c (possibly the same value) in the set S: The notation a <CF b means comparefn(a,b) < 0; a =CF b means comparefn(a,b) = 0 (of either sign); and a >CF b means comparefn(a,b) > 0.
Calling comparefn(a,b) always returns the same value v when given a specific pair of values a and b as its two arguments. Furthermore, Type(v) is Number, and v is not NaN. Note that this implies that exactly one of a <CF b, a =CF b, and a >CF b will be true for a given pair of a and b.
Calling comparefn(a,b) does not modify the this object.
a =CF a (reflexivity)
If a =CF b, then b =CF a (symmetry)
If a =CF b and b =CF c, then a =CF c (transitivity of =CF)
If a <CF b and b <CF c, then a <CF c (transitivity of <CF)
If a >CF b and b >CF c, then a >CF c (transitivity of >CF)
NOTE The above conditions are necessary and sufficient to ensure that comparefn divides the set S into equivalence classes and that these equivalence classes are totally ordered.
When the SortCompare abstract operation is called with two arguments j and k, the following steps are taken:
Let jString be ToString(j).
Let kString be ToString(k).
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 x be the result of calling the [[Get]] internal method of obj with argument jString.
Let y be the result of calling the [[Get]] internal method of obj with argument kString.
If x and y are both undefined, return +0.
If x is undefined, return 1.
If y 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 x and y.
Let xString be ToString(x).
Let yString be ToString(y).
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.
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 O be the result of calling ToObject passing the this value as the argument.
Let
A 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 O 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 k be 0.
Repeat, while k < actualDeleteCount
Let from be ToString(actualStart+k).
Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument from.
If fromPresent is true, then
Let fromValue be the result of calling the [[Get]] internal method of O with argument from.
Call the [[DefineOwnProperty]] internal method of A with arguments ToString(k), Property Descriptor {[[Value]]: fromValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
Increment k 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 k be actualStart.
Repeat, while k < (len – actualDeleteCount)
Let from be ToString(k+actualDeleteCount).
Let to be ToString(k+itemCount).
Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument from.
If fromPresent is true, then
Let fromValue be the result of calling the [[Get]] internal method of O with argument from.
Call the [[Put]] internal method of O with arguments to, fromValue, and true.
Else, fromPresent is false
Call the [[Delete]] internal method of O with arguments to and true.
Increase k by 1.
Let k be len.
Repeat, while k > (len – actualDeleteCount +itemCount)
Call the [[Delete]] internal method of O with arguments ToString(k–1) and true.
Decrease k by 1.
Else if itemCount > actualDeleteCount, then
Let k be (len – actualDeleteCount).
Repeat, while k > actualStart
Let from be ToString(k + actualDeleteCount – 1).
Let to be ToString(k + itemCount – 1)
Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument from.
If fromPresent is true, then
Let fromValue be the result of calling the [[Get]] internal method of O with argument from.
Call the [[Put]] internal method of O with arguments to, fromValue, and true.
Else, fromPresent is false
Call the [[Delete]] internal method of O with argument to and true.
Decrease k by 1.
Let k be actualStart.
Repeat, while items is not empty
Remove the first element from items and let E be the value of that element.
Call the [[Put]] internal method of O with arguments ToString(k), E, and true.
Increase k by 1.
Call
the [[Put]] internal method of O with arguments "
length
"
,
(len – actualDeleteCount + itemCount), and
true.
Return A.
The
length
property of
the splice
method
is 2.
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.
The arguments are prepended to the start of the array, such that their order within the array is the same as the order in which they appear in the argument list.
When
the unshift
method
is called with zero or more arguments item1,item2, etc., the following steps are taken:
Let O be the result of calling ToObject passing the this value as the argument.
Let
lenVal be the result of calling the [[Get]] internal method
of O with argument "
length
"
.
Let len be ToUint32(lenVal).
Let argCount be the number of actual arguments.
Let k be len.
Repeat, while k > 0,
Let from be ToString(k–1).
Let to be ToString(k+argCount –1).
Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument from.
If fromPresent is true, then
Let fromValue be the result of calling the [[Get]] internal method of O with argument from.
Call the [[Put]] internal method of O with arguments to, fromValue, and true.
Else, fromPresent is false
Call the [[Delete]] internal method of O with arguments to, and true.
Decrease k by 1.
Let j 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 E be the value of that element.
Call the [[Put]] internal method of O with arguments ToString(j), E, and true.
Increase j by 1.
Call
the [[Put]] internal method of O with arguments "
length
"
,
len+argCount, and true.
Return len+argCount.
The
length
property of
the unshift
method
is 1.
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.
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 O 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 O with the argument "
length
"
.
Let len be ToUint32(lenValue).
If len is 0, return -1.
If argument fromIndex was passed let n be ToInteger(fromIndex); else let n be 0.
If n ≥ len, return -1.
If n ≥ 0, then
Let k be n.
Else, n<0
Let k be len - abs(n).
If k is less than 0, then let k be 0.
Repeat, while k<len
Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument ToString(k).
If kPresent is true, then
Let elementK be the result of calling the [[Get]] internal method of O with the argument ToString(k).
Let same be the result of applying the Strict Equality Comparison Algorithm to searchElement and elementK.
If same is true, return k.
Increase k by 1.
Return -1.
The
length
property of
the indexOf
method
is 1.
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.
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 O 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 O with the argument "
length
"
.
Let len be ToUint32(lenValue).
If len is 0, return -1.
If argument fromIndex was passed let n be ToInteger(fromIndex); else let n be len.
If n ≥ 0, then let k be min(n, len – 1).
Else, n < 0
Let k be len - abs(n).
Repeat, while k≥ 0
Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument ToString(k).
If kPresent is true, then
Let elementK be the result of calling the [[Get]] internal method of O with the argument ToString(k).
Let same be the result of applying the Strict Equality Comparision Algorithm to searchElement and elementK.
If same is true, return k.
Decrease k by 1.
Return -1.
The
length
property of
the lastIndexOf
method is 1.
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.
callbackfn should be a function that accepts three arguments and
returns a value that is coercible to the Boolean value true
or false. every
calls callbackfn
once for each element present in the array, in ascending order,
until it finds one where callbackfn
returns false. If such an element is found, every
immediately returns false. Otherwise, if callbackfn
returned true for all elements, every
will return true. callbackfn
is called only for elements of the array which actually exist; it is
not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
every
does not directly mutate the object on which it is called but the
object may be mutated by the calls to callbackfn.
The
range of elements processed by every
is set before the first call to callbackfn.
Elements which are appended to the array after the call to every
begins will not be visited by callbackfn.
If existing elements of the array are changed, their value as passed
to callbackfn will
be the value at the time every
visits them; elements that are deleted after the call to every
begins and before being visited are not visited. every
acts like the "for all" quantifier in mathematics. In
particular, for an empty array, it returns true.
When
the every
method
is called with one or two arguments, the following steps are taken:
Let O be the result of calling ToObject passing the this value as the argument.
Let
lenValue be the result of calling the [[Get]] internal
method of O with the argument "length"
.
Let len be ToUint32(lenValue).
If IsCallable(callbackfn) is false, throw a TypeError exception.
If thisArg was supplied, let T be thisArg; else let T be undefined.
Let k be 0.
Repeat, while k < len
Let Pk be ToString(k).
Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
If kPresent is true, then
Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
Let testResult be the result of calling the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.
If ToBoolean(testResult) is false, return false.
Increase k by 1.
Return true.
The
length
property of
the every
method
is 1.
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.
callbackfn should be a function that accepts three arguments and
returns a value that is coercible to the Boolean value true
or false. some
calls callbackfn
once for each element present in the array, in ascending order,
until it finds one where callbackfn
returns true. If such an element is found, some
immediately returns true. Otherwise, some
returns false. callbackfn
is called only for elements of the array which actually exist; it is
not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
some
does not directly mutate the object on which it is called but the
object may be mutated by the calls to callbackfn.
The
range of elements processed by some
is set before the first call to callbackfn.
Elements that are appended to the array after the call to some
begins will not be visited by callbackfn.
If existing elements of the array are changed, their value as passed
to callbackfn will
be the value at the time that some
visits them; elements that are deleted after the call to some
begins and before being visited are not visited.
some
acts like the "exists" quantifier in
mathematics. In particular, for an empty array, it returns false.
When
the some
method is
called with one or two arguments, the following steps are taken:
Let O be the result of calling ToObject passing the this value as the argument.
Let
lenValue be the result of calling the [[Get]] internal
method of O with the argument "length"
.
Let len be ToUint32(lenValue).
If IsCallable(callbackfn) is false, throw a TypeError exception.
If thisArg was supplied, let T be thisArg; else let T be undefined.
Let k be 0.
Repeat, while k < len
Let Pk be ToString(k).
Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
If kPresent is true, then
Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
Let testResult be the result of calling the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.
If ToBoolean(testResult) is true, return true.
Increase k by 1.
Return false.
The
length
property of
the some
method is
1.
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.
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 O 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 O with the argument "length"
.
Let len be ToUint32(lenValue).
If IsCallable(callbackfn) is false, throw a TypeError exception.
If thisArg was supplied, let T be thisArg; else let T be undefined.
Let k be 0.
Repeat, while k < len
Let Pk be ToString(k).
Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
If kPresent is true, then
Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
Call the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.
Increase k by 1.
Return undefined.
The
length
property of
the forEach
method
is 1.
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.
callbackfn should be a function that accepts three arguments. map
calls callbackfn
once for each element in the array, in ascending order, and
constructs a new Array from the results. callbackfn
is called only for elements of the array which actually exist; it is
not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
map
does not directly mutate the object on which it is called but the
object may be mutated by the calls to callbackfn.
The
range of elements processed by map
is set before the first call to callbackfn.
Elements which are appended to the array after the call to map
begins will not be visited by callbackfn.
If existing elements of the array are changed, their value as passed
to callbackfn will
be the value at the time map
visits them; elements that are deleted after the call to map
begins and before being visited are not visited.
When
the map
method is
called with one or two arguments, the following steps are taken:
Let O be the result of calling ToObject passing the this value as the argument.
Let
lenValue be the result of calling the [[Get]] internal
method of O with the argument "length"
.
Let len be ToUint32(lenValue).
If IsCallable(callbackfn) is false, throw a TypeError exception.
If thisArg was supplied, let T be thisArg; else let T be undefined.
Let
A 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 k be 0.
Repeat, while k < len
Let Pk be ToString(k).
Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
If kPresent is true, then
Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
Let mappedValue be the result of calling the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.
Call the [[DefineOwnProperty]] internal method of A with arguments Pk, Property Descriptor {[[Value]]: mappedValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
Increase k by 1.
Return A.
The
length
property of
the map
method is
1.
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.
callbackfn should be a function that accepts three arguments and
returns a value that is coercible to the Boolean value true
or false. filter
calls callbackfn
once for each element in the array, in ascending order, and
constructs a new array of all the values for which callbackfn
returns true. callbackfn
is called only for elements of the array which actually exist; it is
not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
filter
does not directly mutate the object on which it is called but the
object may be mutated by the calls to callbackfn.
The
range of elements processed by filter
is set before the first call to callbackfn.
Elements which are appended to the array after the call to filter
begins will not be visited by callbackfn.
If existing elements of the array are changed their value as passed
to callbackfn will
be the value at the time filter
visits them; elements that are deleted after the call to filter
begins and before being visited are not visited.
When
the filter
method
is called with one or two arguments, the following steps are taken:
Let O be the result of calling ToObject passing the this value as the argument.
Let
lenValue be the result of calling the [[Get]] internal
method of O with the argument "length"
.
Let len be ToUint32(lenValue).
If IsCallable(callbackfn) is false, throw a TypeError exception.
If thisArg was supplied, let T be thisArg; else let T be undefined.
Let
A be a new array created as if by the expression new
Array()
where Array
is the standard built-in constructor with that name.
Let k be 0.
Let to be 0.
Repeat, while k < len
Let Pk be ToString(k).
Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
If kPresent is true, then
Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
Let selected be the result of calling the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.
If ToBoolean(selected) is true, then
Call the [[DefineOwnProperty]] internal method of A with arguments ToString(to), Property Descriptor {[[Value]]: kValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
Increase to by 1.
Increase k by 1.
Return A.
The
length
property of
the filter
method
is 1.
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.
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 O 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 O 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 k 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 k < len
Let Pk be ToString(k).
Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
If kPresent is true, then
Let accumulator be the result of calling the [[Get]] internal method of O with argument Pk.
Increase k by 1.
If kPresent is false, throw a TypeError exception.
Repeat, while k < len
Let Pk be ToString(k).
Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
If kPresent is true, then
Let kValue be the result of calling the [[Get]] internal method of O 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, k, and O.
Increase k by 1.
Return accumulator.
The
length
property of
the reduce
method
is 1.
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.
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 O 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 O 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 k 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 k ≥ 0
Let Pk be ToString(k).
Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
If kPresent is true, then
Let accumulator be the result of calling the [[Get]] internal method of O with argument Pk.
Decrease k by 1.
If kPresent is false, throw a TypeError exception.
Repeat, while k ≥ 0
Let Pk be ToString(k).
Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
If kPresent is true, then
Let kValue be the result of calling the [[Get]] internal method of O 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, k, and O.
Decrease k by 1.
Return accumulator.
The
length
property of
the reduceRight
method is 1.
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.
Array
instances inherit properties from the Array prototype object and
their [[Class]] internal property value is "Array"
.
Array instances also have the following properties.
Array objects use a variation of the [[DefineOwnProperty]] internal method used for other native ECMAScript objects (8.12.9).
Assume A 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 A is called with property P, Property Descriptor Desc, and Boolean flag Throw, the following steps are taken:
Let
oldLenDesc be the result of calling the [[GetOwnProperty]]
internal method of A 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
P 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 A 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 A 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 A 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 A passing ToString(oldLen) and false as arguments.
If deleteSucceeded is false, then
If newWritable is false, then
Call
the default [[DefineOwnProperty]] internal method (8.12.9) on A
passing "length
",
Property Descriptor{[[Writable]]: false}, and false
as arguments. This call will always return true.
Return true.
Else if P is an array index (15.4), then
Let index be ToUint32(P).
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 A passing P, 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 A
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 A passing P, Desc, and Throw as arguments.
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.
When
String
is called
as a function rather than as a constructor, it performs a type
conversion.
Returns
a String value (not a String object) computed by ToString(value).
If value is not
supplied, the empty String ""
is returned.
When
String
is called
as part of a new
expression, it is a constructor: it initialises the newly created
object.
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.
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 1), the String constructor has the
following properties:
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 }.
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 1.
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).
The
initial value of String.prototype.constructor
is the built-in String
constructor.
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.
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.
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 S 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 S.
If position < 0 or position ≥ size, return the empty String.
Return a String of length 1, containing one character from S, namely the character at position position, where the first (leftmost) character in S 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.
Returns a Number (a nonnegative integer less than 216) 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 S 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 S.
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 S, where the first (leftmost) character in S 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.
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 S 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 R be S.
Repeat, while args is not empty
Remove the first element from args and let next be the value of that element.
Let R be the String value consisting of the characters in the previous value of R followed by the characters of ToString(next).
Return R.
The
length
property of
the concat
method
is 1.
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.
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 S 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 0
).
Let len be the number of characters in S.
Let start be min(max(pos, 0), len).
Let searchLen be the number of characters in searchStr.
Return
the smallest possible integer k not smaller than start
such that k+ searchLen is not greater than len,
and for all nonnegative integers j less than searchLen,
the character at position k+j of S is the same
as the character at position j of searchStr); but if
there is no such integer k, then return the value -1
.
The
length
property of
the indexOf
method
is 1.
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.
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 S 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 S.
Let start min(max(pos, 0), len).
Let searchLen be the number of characters in searchStr.
Return
the largest possible nonnegative integer k not larger than
start such that k+ searchLen is not greater
than len, and for all nonnegative integers j less
than searchLen, the character at position k+j
of S is the same as the character at position j of
searchStr; but if there is no such integer k, then
return the value -1
.
The
length
property of
the lastIndexOf
method is 1.
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.
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 S
and That. The two
Strings are compared in an implementation-defined fashion. The
result is intended to order String values in the sort order
specified by the system default locale, and will be negative, zero,
or positive, depending on whether S
comes before That
in the sort order, the Strings are equal, or S
comes after That
in the sort order, respectively.
Before perform the comparisons the following steps are performed to prepare the Strings:
Call CheckObjectCoercible passing the this value as its argument.
Let S 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 0
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.
When
the match
method
is called with argument regexp,
the following steps are taken:
Call CheckObjectCoercible passing the this value as its argument.
Let S 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 S.
Else, global is true
Call
the [[Put]] internal method of rx with arguments
"lastIndex"
and 0.
Let
A 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 n 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 S.
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 A with arguments ToString(n), the Property Descriptor {[[Value]]: matchStr, [[Writable]]: true, [[Enumerable]]: true, [[configurable]]: true}, and false.
Increment n.
If n = 0, then return null.
Return A.
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.
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 m 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 m be 0.
If replaceValue is a function, then for each matched substring, call the function with the following m + 3 arguments. Argument 1 is the substring that matched. If searchValue is a regular expression, the next m arguments are all of the captures in the MatchResult (see 15.10.2.1). Argument m + 2 is the offset within string where the match occurred, and argument m + 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.
Characters |
Replacement text |
|
|
|
The matched substring. |
|
The portion of string that precedes the matched substring. |
|
The portion of string that follows the matched substring. |
|
The
nth
capture, where n
is a single digit in the range 1
to 9 and |
|
The nnth capture, where nn is a two-digit decimal number in the range 01 to 99. If nn≤m and the nnth capture is undefined, use the empty String instead. If nn>m, 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.
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
search
function is
intentionally generic; it does not require that its this
value be a String object. Therefore, it can be transferred to other
kinds of objects for use as a method.
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 S be the result of calling ToString, giving it the this value as its argument.
Let len be the number of characters in S.
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 S beginning with the character at position from.
The
length
property of
the slice
method
is 2.
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.
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,
"A<B>bold</B>and<CODE>coded</CODE>".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 S be the result of calling ToString, giving it the this value as its argument.
Let
A 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 = 232–1; else let lim = ToUint32(limit).
Let s be the number of characters in S.
Let p = 0.
If
separator is a RegExp object (its [[Class]] is "RegExp"
),
let R = separator; otherwise let R =
ToString(separator).
If lim = 0, return A.
If separator is undefined, then
Call
the [[DefineOwnProperty]] internal method of A with
arguments "
0
"
,
Property Descriptor {[[Value]]: S, [[Writable]]: true,
[[Enumerable]]: true, [[Configurable]]: true}, and
false.
Return A.
If s = 0, then
Call SplitMatch(S, 0, R) and let z be its MatchResult result.
If z is not failure, return A.
Call
the [[DefineOwnProperty]] internal method of A with
arguments "
0
"
,
Property Descriptor {[[Value]]: S, [[Writable]]: true,
[[Enumerable]]: true, [[Configurable]]: true}, and
false.
Return A.
Let q = p.
Repeat, while q ≠ s
Call SplitMatch(S, q, R) and let z be its MatchResult result.
If z is failure, then let q = q+1.
Else, z is not failure
z must be a State. Let e be z's endIndex and let cap be z's captures array.
If e = p, then let q = q+1.
Else, e ≠ p
Let T be a String value equal to the substring of S consisting of the characters at positions p (inclusive) through q (exclusive).
Call the [[DefineOwnProperty]] internal method of A with arguments ToString(lengthA), Property Descriptor {[[Value]]: T, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
Increment lengthA by 1.
If lengthA = lim, return A.
Let p = e.
Let i = 0.
Repeat, while i is not equal to the number of elements in cap.
Let i = i+1.
Call the [[DefineOwnProperty]] internal method of A with arguments ToString(lengthA), Property Descriptor {[[Value]]: cap[i], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
Increment lengthA by 1.
If lengthA = lim, return A.
Let q = p.
Let T be a String value equal to the substring of S consisting of the characters at positions p (inclusive) through s (exclusive).
Call the [[DefineOwnProperty]] internal method of A with arguments ToString(lengthA), Property Descriptor {[[Value]]: T, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
Return A.
The abstract operation SplitMatch takes three parameters, a String S, an integer q, and a String or RegExp R, and performs the following in order to return a MatchResult (see 15.10.2.1):
If
R is a RegExp object (its [[Class]] is "RegExp"
),
then
Call the [[Match]] internal method of R giving it the arguments S and q, and return the MatchResult result.
Type(R) must be String. Let r be the number of characters in R.
Let s be the number of characters in S.
If q+r > s then return the MatchResult failure.
If there exists an integer i between 0 (inclusive) and r (exclusive) such that the character at position q+i of S is different from the character at position i of R, then return failure.
The
length
property of
the split
method
is 2.
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.
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 S be the result of calling ToString, giving it the this value as its argument.
Let len be the number of characters in S.
Let intStart be ToInteger(start).
If end is undefined, let intEnd be len; else let intEnd be ToInteger(end).
Let finalStart be min(max(intStart, 0), len).
Let finalEnd be min(max(intEnd, 0), len).
Let from be min(finalStart, finalEnd).
Let to be max(finalStart, finalEnd).
Return a String whose length is to - from, containing characters from S, namely the characters with indices from through to −1, in ascending order.
The
length
property of
the substring
method is 2.
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.
The following steps are taken:
Call CheckObjectCoercible passing the this value as its argument.
Let S be the result of calling ToString, giving it the this value as its argument.
Let L be a String where each character of L is either the Unicode lowercase equivalent of the corresponding character of S or the actual corresponding character of S if no Unicode lowercase equivalent exists.
Return L.
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 S to L 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.
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.
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.
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.
The following steps are taken:
Call CheckObjectCoercible passing the this value as its argument.
Let S be the result of calling ToString, giving it the this value as its argument.
Let T be a String value that is a copy of S with both leading and trailing white space removed. The definition of white space is the union of WhiteSpace and LineTerminator.
Return T.
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.
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.
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 }.
String objects use a variation of the [[GetOwnProperty]] internal method used for other native ECMAScript objects (8.12.1). This special internal method is used to add access for named properties corresponding to individual characters of String objects.
Assume S is a String object and P is a String.
When the [[GetOwnProperty]] internal method of S is called with property name P, the following steps are taken:
Let desc be the result of calling the default [[GetOwnProperty]] internal method (8.12.1) on S with argument P.
If desc is not undefined return desc.
If ToString(abs(ToInteger(P))) is not the same value as P, return undefined.
Let str be the String value of the [[PrimitiveValue]] internal property of S.
Let index be ToInteger(P).
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 }
When
Boolean
is called
as a function rather than as a constructor, it performs a type
conversion.
Returns a Boolean value (not a Boolean object) computed by ToBoolean(value).
When
Boolean
is called
as part of a new
expression it is a constructor: it initialises the newly created
object.
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.
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 1), the Boolean constructor has the
following property:
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 }.
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).
The
initial value of Boolean.prototype.constructor
is the built-in Boolean
constructor.
The following steps are taken:
Let B be the this value.
If Type(B) is Boolean, then let b be B.
Else
if Type(B) is Object and the value of the [[Class]] internal
property of B is "Boolean"
,
then let b be the value of the [[PrimitiveValue]] internal
property of B.
Else throw a TypeError exception.
If
b is true, then return "true"
;
else return "false"
.
The following steps are taken:
Let B be the this value.
If Type(B) is Boolean, then let b be B.
Else
if Type(B) is Object and the value of the [[Class]] internal
property of B is "Boolean"
,
then let b be the value of the [[PrimitiveValue]] internal
property of B.
Else throw a TypeError exception.
Return b.
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.
When
Number
is called
as a function rather than as a constructor, it performs a type
conversion.
Returns a Number value (not a Number object) computed by ToNumber(value) if value was supplied, else returns +0.
When
Number
is called
as part of a new
expression it is a constructor: it initialises the newly created
object.
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.
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 1), the Number constructor has the
following property:
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 }.
The
value of Number.MAX_VALUE
is the largest positive finite value of the Number type, which is
approximately 1.7976931348623157 × 10308.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
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 }.
The
value of Number.NaN
is NaN.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
The value of Number.NEGATIVE_INFINITY is −∞.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
The value of Number.POSITIVE_INFINITY is +∞.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
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.
The
initial value of Number.prototype.constructor
is the built-in Number
constructor.
The optional radix should be an integer value in the inclusive range 2 to 36. If radix not present or is undefined the Number 10 is used as the value of radix. 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 a
-z
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 generalization 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.
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.
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.
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
f be ToInteger(fractionDigits). (If fractionDigits
is undefined, this step produces the value 0
).
If f < 0 or f > 20, throw a RangeError exception.
Let x be this Number value.
If
x is NaN, return the String "NaN"
.
Let s be the empty String.
If x < 0, then
Let
s be "-
".
Let x = –x.
If x ≥ 1021, then
Let m = ToString(x).
Else, x < 1021
Let n be an integer for which the exact mathematical value of n ÷ 10f – x is as close to zero as possible. If there are two such n, pick the larger n.
If
n = 0, let m be the String "0"
.
Otherwise, let m be the String consisting of the digits of
the decimal representation of n (in order, with no leading
zeroes).
If f ≠ 0, then
Let k be the number of characters in m.
If k ≤ f, then
Let
z be the String consisting of f+1–k
occurrences of the character ‘0
’.
Let m be the concatenation of Strings z and m.
Let k = f + 1.
Let a be the first k–f characters of m, and let b be the remaining f characters of m.
Let
m be the concatenation of the three Strings a, "."
,
and b.
Return the concatenation of the Strings s and m.
The
length
property of
the toFixed
method
is 1.
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"
.
Return a String containing this Number value represented in decmal exponential notation with one digit before the significand's decimal point and fractionDigits digits after the significand's decimal point. If fractionDigits is undefined, include as many significand digits as necessary to uniquely specify the Number (just like in ToString except that in this case the Number is always output in exponential notation). Specifically, perform the following steps:
Let x be this Number value.
Let f be ToInteger(fractionDigits).
If
x is NaN, return the String "NaN"
.
Let s be the empty String.
If x < 0, then
Let
s be "-"
.
Let x = –x.
If x = +∞, then
Return
the concatenation of the Strings s and "Infinity"
.
If fractionDigits is not undefined and (f < 0 or f > 20), throw a RangeError exception.
If x = 0, then
Let f = 0.
Let
m be the String consisting of f+1 occurrences of the
character ‘0
’.
Let e = 0.
Else, x ≠ 0
If fractionDigits is not undefined, then
Let e and n be integers such that 10f ≤ n < 10f+1 and for which the exact mathematical value of n × 10e–f – x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10e–f is larger.
Else, fractionDigits is undefined
Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f+1, the number value for n × 10e–f is x, and f is as small as possible. Note that the decimal representation of n has f+1 digits, n is not divisible by 10, and the least significant digit of n is not necessarily uniquely determined by these criteria.
Let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
If f ≠ 0, then
Let a be the first character of m, and let b be the remaining f characters of m.
Let
m be the concatenation of the three Strings a, "."
,
and b.
If e = 0, then
Let
c = "+".
Let
d = "0".
Else
If
e > 0, then let c = "+".
Else, e ≤ 0
Let
c = "-"
.
Let e = –e.
Let d be the String consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
Let
m be the concatenation of the four Strings m, "e"
,
c, and d.
Return the concatenation of the Strings s and m.
The
length
property of
the toExponential
method is 1.
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 e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f+1, the number value for n × 10e–f is x, and f is as small as possible. If there are multiple possibilities for n, choose the value of n for which n × 10e–f is closest in value to x. If there are two such possible values of n, choose the one that is even.
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 x be this Number value.
If precision is undefined, return ToString(x).
Let p be ToInteger(precision).
If
x is NaN, return the String "NaN"
.
Let s be the empty String.
If x < 0, then
Let
s be "-"
.
Let x = –x.
If x = +∞, then
Return
the concatenation of the Strings s and "Infinity"
.
If p < 1 or p > 21, throw a RangeError exception.
If x = 0, then
Let
m be the String consisting of p occurrences of the
character ‘0
’.
Let e = 0.
Else x ≠ 0,
Let e and n be integers such that 10p–1 ≤ n < 10p and for which the exact mathematical value of n × 10e–p+1 – x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10e–p+1 is larger.
Let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
If e < –6 or e ≥ p, then
Let a be the first character of m, and let b be the remaining p–1 characters of m.
Let
m be the concatenation of the three Strings a, "."
,
and b.
If e = 0, then
Let
c = "+"
and d = "0"
.
Else e ≠ 0,
If e > 0, then
Let
c = "+"
.
Else e < 0,
Let
c = "-"
.
Let e = –e.
Let d be the String consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
Let
m be the concatenation of the five Strings s, m,
"e"
,
c, and d.
If e = p–1, then return the concatenation of the Strings s and m.
If e ≥ 0, then
Let
m be the concatenation of the first e+1 characters
of m, the character ‘.
’,
and the remaining p– (e+1) characters of m.
Else e < 0,
Let
m be the concatenation of the String "0."
,
–(e+1) occurrences of the character ‘0
’,
and the String m.
Return the concatenation of the Strings s and m.
The
length
property of
the toPrecision
method is 1.
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.
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.
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 x” has a technical meaning defined in 8.5.
The Number value for e, the base of the natural logarithms, which is approximately 2.7182818284590452354.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
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 }.
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 }.
The Number value for the base-2 logarithm of e, 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
.
The Number value for the base-10 logarithm of e, 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
.
The Number value for π, 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 }.
The Number value for the square root of ½, 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
.
The Number value for the square root of 2, which is approximately 1.4142135623730951.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
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
,
and sqrt
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
(http://www.netlib.org/fdlibm).
Returns the absolute value of x; the result has the same magnitude as x but has positive sign.
If x is NaN, the result is NaN.
If x is −0, the result is +0.
If x is −∞, the result is +∞.
Returns an implementation-dependent approximation to the arc cosine of x. The result is expressed in radians and ranges from +0 to +π.
If x is NaN, the result is NaN.
If x is greater than 1, the result is NaN.
If x is less than −1, the result is NaN.
If x is exactly 1, the result is +0.
Returns an implementation-dependent approximation to the arc sine of x. The result is expressed in radians and ranges from −π/2 to +π/2.
If x is NaN, the result is NaN.
If x is greater than 1, the result is NaN.
If x is less than –1, the result is NaN.
If x is +0, the result is +0.
If x is −0, the result is −0.
Returns an implementation-dependent approximation to the arc tangent of x. The result is expressed in radians and ranges from −π/2 to +π/2.
If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is −0, the result is −0.
If x is +∞, the result is an implementation-dependent approximation to +π/2.
If x is −∞, the result is an implementation-dependent approximation to −π/2.
Returns an implementation-dependent approximation to the arc tangent of the quotient y/x of the arguments y and x, where the signs of y and x are used to determine the quadrant of the result. Note that it is intentional and traditional for the two-argument arc tangent function that the argument named y be first and the argument named x be second. The result is expressed in radians and ranges from −π to +π.
If either x or y is NaN, the result is NaN.
If y>0 and x is +0, the result is an implementation-dependent approximation to +π/2.
If y>0 and x is −0, the result is an implementation-dependent approximation to +π/2.
If y is +0 and x>0, the result is +0.
If y is +0 and x is +0, the result is +0.
If y is +0 and x is −0, the result is an implementation-dependent approximation to +π.
If y is +0 and x<0, the result is an implementation-dependent approximation to +π.
If y is −0 and x>0, the result is −0.
If y is −0 and x is +0, the result is −0.
If y is −0 and x is −0, the result is an implementation-dependent approximation to −π.
If y is −0 and x<0, the result is an implementation-dependent approximation to −π.
If y<0 and x is +0, the result is an implementation-dependent approximation to −π/2.
If y<0 and x is −0, the result is an implementation-dependent approximation to −π/2.
If y>0 and y is finite and x is +∞, the result is +0.
If y>0 and y is finite and x is −∞, the result if an implementation-dependent approximation to +π.
If y<0 and y is finite and x is +∞, the result is −0.
If y<0 and y is finite and x is −∞, the result is an implementation-dependent approximation to −π.
If y is +∞ and x is finite, the result is an implementation-dependent approximation to +π/2.
If y is −∞ and x is finite, the result is an implementation-dependent approximation to −π/2.
If y is +∞ and x is +∞, the result is an implementation-dependent approximation to +π/4.
If y is +∞ and x is −∞, the result is an implementation-dependent approximation to +3π/4.
If y is −∞ and x is +∞, the result is an implementation-dependent approximation to −π/4.
If y is −∞ and x is −∞, the result is an implementation-dependent approximation to −3π/4.
Returns the smallest (closest to −∞) Number value that is not less than x and is equal to a mathematical integer. If x is already an integer, the result is x.
If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is −0, the result is −0.
If x is +∞, the result is +∞.
If x is −∞, the result is −∞.
If x is less than 0 but greater than -1, the result is −0.
The
value of Math.ceil(x)
is the same as the value of -Math.floor(-x)
.
Returns an implementation-dependent approximation to the cosine of x. The argument is expressed in radians.
If x is NaN, the result is NaN.
If x is +0, the result is 1.
If x is −0, the result is 1.
If x is +∞, the result is NaN.
If x is −∞, the result is NaN.
Returns an implementation-dependent approximation to the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms).
If x is NaN, the result is NaN.
If x is +0, the result is 1.
If x is −0, the result is 1.
If x is +∞, the result is +∞.
If x is −∞, the result is +0.
Returns the greatest (closest to +∞) Number value that is not greater than x and is equal to a mathematical integer. If x is already an integer, the result is x.
If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is −0, the result is −0.
If x is +∞, the result is +∞.
If x is −∞, the result is −∞.
If x is greater than 0 but less than 1, the result is +0.
NOTE The
value of Math.floor(x)
is the same as the value of -Math.ceil(-x)
.
Returns an implementation-dependent approximation to the natural logarithm of x.
If x is NaN, the result is NaN.
If x is less than 0, the result is NaN.
If x is +0 or −0, the result is −∞.
If x is 1, the result is +0.
If x is +∞, the result is +∞.
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
2.
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
2.
Returns an implementation-dependent approximation to the result of raising x to the power y.
If y is NaN, the result is NaN.
If y is +0, the result is 1, even if x is NaN.
If y is −0, the result is 1, even if x is NaN.
If x is NaN and y is nonzero, the result is NaN.
If abs(x)>1 and y is +∞, the result is +∞.
If abs(x)>1 and y is −∞, the result is +0.
If abs(x)==1 and y is +∞, the result is NaN.
If abs(x)==1 and y is −∞, the result is NaN.
If abs(x)<1 and y is +∞, the result is +0.
If abs(x)<1 and y is −∞, the result is +∞.
If x is +∞ and y>0, the result is +∞.
If x is +∞ and y<0, the result is +0.
If x is −∞ and y>0 and y is an odd integer, the result is −∞.
If x is −∞ and y>0 and y is not an odd integer, the result is +∞.
If x is −∞ and y<0 and y is an odd integer, the result is −0.
If x is −∞ and y<0 and y is not an odd integer, the result is +0.
If x is +0 and y>0, the result is +0.
If x is +0 and y<0, the result is +∞.
If x is −0 and y>0 and y is an odd integer, the result is −0.
If x is −0 and y>0 and y is not an odd integer, the result is +0.
If x is −0 and y<0 and y is an odd integer, the result is −∞.
If x is −0 and y<0 and y is not an odd integer, the result is +∞.
If x<0 and x is finite and y is finite and y is not an integer, the result is NaN.
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.
Returns the Number value that is closest to x and is equal to a mathematical integer. If two integer Number values are equally close to x, then the result is the Number value that is closer to +∞. If x is already an integer, the result is x.
If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is −0, the result is −0.
If x is +∞, the result is +∞.
If x is −∞, the result is −∞.
If x is greater than 0 but less than 0.5, the result is +0.
If x is less than 0 but greater than or equal to -0.5, the result is −0.
NOTE 1 Math.round(3.5)
returns 4, but
Math.round(–3.5)
returns –3.
NOTE 2 The value of Math.round(x)
is the same as the value of Math.floor(x+0.5)
,
except when x
is
−0
or is less than 0
but greater than or equal to -0.5;
for these cases Math.round(x)
returns −0,
but Math.floor(x+0.5)
returns +0.
Returns an implementation-dependent approximation to the sine of x. The argument is expressed in radians.
If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is −0, the result is −0.
If x is +∞ or −∞, the result is NaN.
Returns an implementation-dependent approximation to the square root of x.
If x is NaN, the result is NaN.
If x is less than 0, the result is NaN.
If x is +0, the result is +0.
If x is −0, the result is −0.
If x is +∞, the result is +∞.
Returns an implementation-dependent approximation to the tangent of x. The argument is expressed in radians.
If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is −0, the result is −0.
If x is +∞ or −∞, the result is NaN.
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.
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 –9,007,199,254,740,991 to 9,007,199,254,740,991; 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.
A given time value t belongs to day number
where the number of milliseconds per day is
msPerDay = 86400000
The remainder is called the time within the day:
TimeWithinDay(t) = t modulo msPerDay
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 4) and ((not divisible by 100) or (divisible by 400)). The number of days in year number y is therefore defined by
DaysInYear(y)
= 365 if (y modulo 4) ≠
0
= 366 if (y modulo 4) = 0 and (y modulo 100) ≠
0
= 365 if (y modulo 100) = 0 and (y modulo 400) ≠
0
= 366 if (y modulo 400) = 0
All non-leap years have 365 days with the usual number of days per month and leap years have an extra day in February. The day number of the first day of year y is given by:
DayFromYear(y) = 365 × (y−1970) + floor((y−1969)/4) − floor((y−1901)/100) + floor((y−1601)/400)
The time value of the start of a year is:
TimeFromYear(y) = msPerDay × DayFromYear(y)
A time value determines a year by:
YearFromTime(t) = the largest integer y (closest to positive infinity) such that TimeFromYear(y) ≤ t
The leap-year function is 1 for a time within a leap year and otherwise is zero:
InLeapYear(t) =
0 if DaysInYear(YearFromTime(t)) = 365
= 1 if
DaysInYear(YearFromTime(t)) = 366
Months are identified by an integer in the range 0 to 11, inclusive. The mapping MonthFromTime(t) from a time value t to a month number is defined by:
MonthFromTime(t) =
0 if 0 ≤ DayWithinYear(t)
< 31
= 1 if 31 ≤
DayWithinYear (t) < 59+InLeapYear(t)
=
2 if 59+InLeapYear(t) ≤
DayWithinYear (t) < 90+InLeapYear(t)
=
3 if 90+InLeapYear(t) ≤
DayWithinYear (t) < 120+InLeapYear(t)
=
4 if 120+InLeapYear(t) ≤
DayWithinYear (t) < 151+InLeapYear(t)
=
5 if 151+InLeapYear(t) ≤
DayWithinYear (t) < 181+InLeapYear(t)
=
6 if 181+InLeapYear(t) ≤
DayWithinYear (t) < 212+InLeapYear(t)
=
7 if 212+InLeapYear(t) ≤
DayWithinYear (t) < 243+InLeapYear(t)
=
8 if 243+InLeapYear(t) ≤
DayWithinYear (t) < 273+InLeapYear(t)
=
9 if 273+InLeapYear(t) ≤
DayWithinYear (t) < 304+InLeapYear(t)
=
10 if 304+InLeapYear(t) ≤
DayWithinYear (t) < 334+InLeapYear(t)
=
11 if 334+InLeapYear(t) ≤
DayWithinYear (t) < 365+InLeapYear(t)
where
DayWithinYear(t) = Day(t)−DayFromYear(YearFromTime(t))
A month value of 0 specifies January; 1 specifies February; 2 specifies March; 3 specifies April; 4 specifies May; 5 specifies June; 6 specifies July; 7 specifies August; 8 specifies September; 9 specifies October; 10 specifies November; and 11 specifies December. Note that MonthFromTime(0) = 0, corresponding to Thursday, 01 January, 1970.
A date number is identified by an integer in the range 1 through 31, inclusive. The mapping DateFromTime(t) from a time value t to a month number is defined by:
DateFromTime(t) =
DayWithinYear(t)+1 if MonthFromTime(t)=0
=
DayWithinYear(t)−30 if
MonthFromTime(t)=1
= DayWithinYear(t)−58−InLeapYear(t) if
MonthFromTime(t)=2
= DayWithinYear(t)−89−InLeapYear(t) if
MonthFromTime(t)=3
=
DayWithinYear(t)−119−InLeapYear(t) if
MonthFromTime(t)=4
=
DayWithinYear(t)−150−InLeapYear(t) if
MonthFromTime(t)=5
=
DayWithinYear(t)−180−InLeapYear(t) if
MonthFromTime(t)=6
=
DayWithinYear(t)−211−InLeapYear(t) if
MonthFromTime(t)=7
=
DayWithinYear(t)−242−InLeapYear(t) if
MonthFromTime(t)=8
=
DayWithinYear(t)−272−InLeapYear(t) if
MonthFromTime(t)=9
=
DayWithinYear(t)−303−InLeapYear(t) if
MonthFromTime(t)=10
=
DayWithinYear(t)−333−InLeapYear(t) if
MonthFromTime(t)=11
The weekday for a particular time value t is defined as
WeekDay(t) = (Day(t) + 4) modulo 7
A weekday value of 0 specifies Sunday; 1 specifies Monday; 2 specifies Tuesday; 3 specifies Wednesday; 4 specifies Thursday; 5 specifies Friday; and 6 specifies Saturday. Note that WeekDay(0) = 4, corresponding to Thursday, 01 January, 1970.
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.
An implementation of ECMAScript is expected to determine the daylight saving time algorithm. The algorithm to determine the daylight saving time adjustment DaylightSavingTA(t), measured in milliseconds, must depend only on four things:
(1) the time since the beginning of the year
t – TimeFromYear(YearFromTime(t))
(2) whether t is in a leap year
InLeapYear(t)
(3) the week day of the beginning of the year
WeekDay(TimeFromYear(YearFromTime(t))
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.
Conversion from UTC to local time is defined by
LocalTime(t) = t + LocalTZA + DaylightSavingTA(t)
Conversion from local time to UTC is defined by
UTC(t) = t – LocalTZA – DaylightSavingTA(t – LocalTZA)
Note that UTC(LocalTime(t)) is not necessarily always equal to t.
The following functions are useful in decomposing time values:
HourFromTime(t) = floor(t / msPerHour) modulo HoursPerDay
MinFromTime(t) = floor(t / msPerMinute) modulo MinutesPerHour
SecFromTime(t) = floor(t / msPerSecond) modulo SecondsPerMinute
msFromTime(t) = t modulo msPerSecond
where
HoursPerDay = 24
MinutesPerHour = 60
SecondsPerMinute = 60
msPerSecond = 1000
msPerMinute = 60000 = msPerSecond × SecondsPerMinute
msPerHour = 3600000 = msPerMinute × MinutesPerHour
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 h be ToInteger(hour).
Let m be ToInteger(min).
Let s be ToInteger(sec).
Let milli be ToInteger(ms).
Let
t be h *
msPerHour +
m
*
msPerMinute +
s *
msPerSecond +
milli, performing the arithmetic according to IEEE 754 rules
(that is, as if using the ECMAScript operators *
and +
).
Return t.
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 y be ToInteger(year).
Let m be ToInteger(month).
Let dt be ToInteger(date).
Let ym be y + floor(m /12).
Let mn be m modulo 12.
Find
a value t such that YearFromTime(t) ==
ym and MonthFromTime(t) ==
mn) and DateFromTime(t) ==
1; but if this is not possible (because some argument is out of
range), return NaN.
Return Day(t) + dt − 1.
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.
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 x 1015, 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.
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
Z
Where the fields are as follows:
YYYY | is the decimal digits of the year 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. |
T | “T ” appears literally in the string, to indicate the beginning of the time element. |
HH | is the number of complete hours that have passed since midnight as two decimal digits. |
: | “: ” (colon) appears literally twice in the string. |
mm | is the number of complete minutes since the start of the hour as two decimal digits. |
ss | is the number of complete seconds since the start of the minute as two decimal digits. |
. | “. ” (dot) appears literally in the string. |
sss | is the number of complete milliseconds since the start of the second as three decimal digits. |
Z | is the time zone offset specified as “Z ” (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 “T
”
and 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 mm
or
ss
fields are absent “00
” is used as the value
and the value of an absent sss
file is “000
”. The
value of an absent time zone offset is “Z
”.
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.
ECMAScript requires the ability to specify 6 digit years (extended years); approximately 285,616 years, either forward or backward, from 01 January, 1970 UTC. To represent years before 0 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 2 extra year digits and is always prefixed with a + or – sign. The year 0 is considered positive and hence prefixed with a + sign.
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.
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
.
When
Date
is called as
part of a new
expression, it is a constructor: it initialises the newly created
object.
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 y be ToNumber(year).
Let m be ToNumber(month).
If date is supplied then let dt be ToNumber(date); else let dt be 1.
If hours is supplied then let h be ToNumber(hours); else let h be 0.
If minutes is supplied then let min be ToNumber(minutes); else let min be 0.
If seconds is supplied then let s be ToNumber(seconds); else let s be 0.
If ms is supplied then let milli be ToNumber(ms); else let milli be 0.
If y is not NaN and 0 ≤ ToInteger(y) ≤ 99, then let yr be 1900+ToInteger(y); otherwise, let yr be y.
Let finalDate be MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli)).
Set the [[PrimitiveValue]] internal property of the newly constructed object to TimeClip(UTC(finalDate)).
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 v be ToPrimitive(value).
If Type(v) is String, then
Parse
v as a date, in exactly the same manner as for the parse
method (15.9.4.2); let V be the time value for this date.
Else, let V be ToNumber(v).
Set the [[PrimitiveValue]] internal property of the newly constructed object to TimeClip(V) and return.
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.
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 7
),
the Date constructor has the following properties:
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 }.
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. Unrecognizable Strings or dates containing illegal
element values in the format String shall cause Date.parse
to return NaN.
If x is any Date object whose milliseconds amount is zero within a particular implementation of ECMAScript, then all of the following expressions should produce the same numeric value in that implementation, if all the properties referenced have their initial values:
x
.valueOf()
Date.parse(
x
.toString())
Date.parse(
x
.toUTCString())
Date.parse(
x
.toISOString())
However, the expression
Date.parse(
x.toLocaleString())
is
not required to produce the same Number value as the preceding three
expressions and, in general, the value produced by Date.parse
is implementation-dependent when given any String value that does
not conform to the Date Time String Format (15.9.1.15) and that
could not be produced in that implementation by the toString
or toUTCString
method.
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 y be ToNumber(year).
Let m be ToNumber(month).
If date is supplied then let dt be ToNumber(date); else let dt be 1.
If hours is supplied then let h be ToNumber(hours); else let h be 0.
If minutes is supplied then let min be ToNumber(minutes); else let min be 0.
If seconds is supplied then let s be ToNumber(seconds); else let s be 0.
If ms is supplied then let milli be ToNumber(ms); else let milli be 0.
If y is not NaN and 0 ≤ ToInteger(y) ≤ 99, then let yr be 1900+ToInteger(y); otherwise, let yr be y.
Return TimeClip(MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli))).
The
length
property of
the UTC
function
is 7.
NOTE The
UTC
function differs from
the Date
constructor in
two ways: it returns a time value as a Number, rather than creating
a Date object, and it interprets the arguments in UTC rather than as
local time.
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
.
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.
The
initial value of Date.prototype.constructor
is the built-in Date
constructor.
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 d
whose milliseconds amount is zero, the result of
Date.parse(
d
.toString())
is equal to d
.valueOf()
.
See 15.9.4.2.
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.
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.
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.
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.
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.
The
valueOf
function
returns a Number, which is this time value.
Return this time value.
Let t be this time value.
If t is NaN, return NaN.
Return YearFromTime(LocalTime(t)).
Let t be this time value.
If t is NaN, return NaN.
Return YearFromTime(t).
Let t be this time value.
If t is NaN, return NaN.
Return MonthFromTime(LocalTime(t)).
Let t be this time value.
If t is NaN, return NaN.
Return MonthFromTime(t).
Let t be this time value.
If t is NaN, return NaN.
Return DateFromTime(LocalTime(t)).
Let t be this time value.
If t is NaN, return NaN.
Return DateFromTime(t).
Let t be this time value.
If t is NaN, return NaN.
Let t be this time value.
If t is NaN, return NaN.
Return WeekDay(t).
Let t be this time value.
If t is NaN, return NaN.
Return HourFromTime(LocalTime(t)).
Let t be this time value.
If t is NaN, return NaN.
Return HourFromTime(t).
Let t be this time value.
If t is NaN, return NaN.
Return MinFromTime(LocalTime(t)).
Let t be this time value.
If t is NaN, return NaN.
Return MinFromTime(t).
Let t be this time value.
If t is NaN, return NaN.
Return SecFromTime(LocalTime(t)).
Let t be this time value.
If t is NaN, return NaN.
Return SecFromTime(t).
Let t be this time value.
If t is NaN, return NaN.
Return msFromTime(LocalTime(t)).
Let t be this time value.
If t is NaN, return NaN.
Return msFromTime(t).
Returns the difference between local time and UTC time in minutes.
Let t be this time value.
If t is NaN, return NaN.
Return (t − LocalTime(t)) / msPerMinute.
Set the [[PrimitiveValue]] internal property of this Date object to v.
Return v.
Let t be the result of LocalTime(this time value).
Let time be MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t), ToNumber(ms)).
Set the [[PrimitiveValue]] internal property of this Date object to u.
Return u.
Let t be this time value.
Let time be MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t), ToNumber(ms)).
Set the [[PrimitiveValue]] internal property of this Date object to v.
Return v.
If ms
is not specified, this behaves as if ms
were specified with the value getMilliseconds()
.
Let t be the result of LocalTime(this time value).
Let s be ToNumber(sec).
If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), MinFromTime(t), s, milli)).
Set the [[PrimitiveValue]] internal property of this Date object to u.
Return u.
The
length
property of
the setSeconds
method is 2.
If ms
is not specified, this behaves as if ms
were specified with the value getUTCMilliseconds()
.
Let t be this time value.
Let s be ToNumber(sec).
If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), MinFromTime(t), s, milli)).
Let v be TimeClip(date).
Set the [[PrimitiveValue]] internal property of this Date object to v.
Return v.
The
length
property of
the setUTCSeconds
method is 2.
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 t be the result of LocalTime(this time value).
Let m be ToNumber(min).
If sec is not specified, then let s be SecFromTime(t); otherwise, let s be ToNumber(sec).
If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), m, s, milli)).
Set the [[PrimitiveValue]] internal property of this Date object to u.
Return u.
The
length
property of
the setMinutes
method is 3.
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 t be this time value.
Let m be ToNumber(min).
If sec is not specified, then let s be SecFromTime(t); otherwise, let s be ToNumber(sec).
If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), m, s, milli)).
Let v be TimeClip(date).
Set the [[PrimitiveValue]] internal property of this Date object to v.
Return v.
The
length
property of
the setUTCMinutes
method is 3.
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 t be the result of LocalTime(this time value).
Let h be ToNumber(hour).
If min is not specified, then let m be MinFromTime(t); otherwise, let m be ToNumber(min).
If If sec is not specified, then let s be SecFromTime(t); otherwise, let s be ToNumber(sec).
If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
Set the [[PrimitiveValue]] internal property of this Date object to u.
Return u.
The
length
property of
the setHours
method is 4.
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 t be this time value.
Let h be ToNumber(hour).
If min is not specified, then let m be MinFromTime(t); otherwise, let m be ToNumber(min).
If sec is not specified, then let s be SecFromTime(t); otherwise, let s be ToNumber(sec).
If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
Let v be TimeClip(newDate).
Set the [[PrimitiveValue]] internal property of this Date object to v.
Return v.
The
length
property of
the setUTCHours
method is 4.
Let t be the result of LocalTime(this time value).
Let dt be ToNumber(date).
Let newDate be MakeDate(MakeDay(YearFromTime(t), MonthFromTime(t), dt), TimeWithinDay(t)).
Set the [[PrimitiveValue]] internal property of this Date object to u.
Return u.
Let t be this time value.
Let dt be ToNumber(date).
Let newDate be MakeDate(MakeDay(YearFromTime(t), MonthFromTime(t), dt), TimeWithinDay(t)).
Let v be TimeClip(newDate).
Set the [[PrimitiveValue]] internal property of this Date object to v.
Return v.
If
date is not
specified, this behaves as if date
were specified with the value getDate()
.
Let t be the result of LocalTime(this time value).
Let m be ToNumber(month).
If date is not specified, then let dt be DateFromTime(t); otherwise, let dt be ToNumber(date).
Let newDate be MakeDate(MakeDay(YearFromTime(t), m, dt), TimeWithinDay(t)).
Set the [[PrimitiveValue]] internal property of this Date object to u.
Return u.
The
length
property of
the setMonth
method is 2.
If
date is not
specified, this behaves as if date
were specified with the value getUTCDate()
.
Let t be this time value.
Let m be ToNumber(month).
If date is not specified, then let dt be DateFromTime(t); otherwise, let dt be ToNumber(date).
Let newDate be MakeDate(MakeDay(YearFromTime(t), m, dt), TimeWithinDay(t)).
Let v be TimeClip(newDate).
Set the [[PrimitiveValue]] internal property of this Date object to v.
Return v.
The
length
property of
the setUTCMonth
method is 2.
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 t be the result of LocalTime(this time value); but if this time value is NaN, let t be +0.
Let y be ToNumber(year).
If month is not specified, then let m be MonthFromTime(t); otherwise, let m be ToNumber(month).
If date is not specified, then let dt be DateFromTime(t); otherwise, let dt be ToNumber(date).
Let newDate be MakeDate(MakeDay(y, m, dt), TimeWithinDay(t)).
Set the [[PrimitiveValue]] internal property of this Date object to u.
Return u.
The
length
property of
the setFullYear
method is 3.
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 t be this time value; but if this time value is NaN, let t be +0.
Let y be ToNumber(year).
If month is not specified, then let m be MonthFromTime(t); otherwise, let m be ToNumber(month).
If date is not specified, then let dt be DateFromTime(t); otherwise, let dt be ToNumber(date).
Let newDate be MakeDate(MakeDay(y, m, dt), TimeWithinDay(t)).
Let v be TimeClip(newDate).
Set the [[PrimitiveValue]] internal property of this Date object to v.
Return v.
The
length
property of
the setUTCFullYear
method is 3.
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 “T
”
used to separate the date and time elements.
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.
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 O be the result of calling ToObject, giving it the this value as its argument.
Let tv be ToPrimitive(O, 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 O with argument "toISOString
".
If IsCallable(toISO) is false, throw a TypeError exception.
Return the result of calling the [[Call]] internal method of toISO with O 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.
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.
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.
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 ::