This is not the official ECMAScript Language Specification.
The official specification is the PDF document located at http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-262.pdf.
This HTML version was created from the original PDF version by a preposterous concatenation of hacks. It very likely contains errors and the official standard and errata are of course definitive. For copyright information, see ECMA's legal disclaimer in the document itself.
This version was created by Jason Orendorff and incorporates the official errata as of 31 July 2010.
Last updated: 5 November 2010.
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 published1 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.
This Ecma Standard has been adopted by the General Assembly of December 2009.
1Note: 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.
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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 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.
PDF page 12 / printed page 2 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 PDF page 13 / printed page 3 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.
PDF page 14 / printed page 4 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.
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.
PDF page 18 / printed page 8 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:
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:
,
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:
is a convenient abbreviation for:
PDF page 20 / printed page 10 and that:
for (
ExpressionNoInopt
;
Expressionopt ;
Expressionopt )
Statement
is a convenient abbreviation for:
for (
;
Expressionopt ;
Expressionopt )
Statement
for (
ExpressionNoIn ;
Expressionopt ;
Expressionopt )
Statement
which in turn is an abbreviation for:
for (
; ;
Expressionopt )
Statement
for (
;
Expression ;
Expressionopt )
Statement
for (
ExpressionNoIn ;
;
Expressionopt )
Statement
for (
ExpressionNoIn ;
Expression ;
Expressionopt )
Statement
which in turn is an abbreviation for:
for (
;
;
)
Statement
for (
;
;
Expression )
Statement
for (
;
Expression ;
)
Statement
for (
;
Expression ;
Expression )
Statement
for (
ExpressionNoIn ;
;
)
Statement
for (
ExpressionNoIn ;
;
Expression )
Statement
for (
ExpressionNoIn ;
Expression ;
)
Statement
for (
ExpressionNoIn ;
Expression ;
Expression )
Statement
so the nonterminal IterationStatement actually has eight alternative right-hand sides.
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
0
1
2
3
4
5
6
7
8
9
the definition
n
[lookahead ∉ {1,
3, 5,
7, 9}]
DecimalDigits
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.
PDF page 21 / printed page 11 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:
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:
1 2 3 4 5 6 7 8 9
which is merely a convenient abbreviation for:
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:
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:
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.
PDF page 22 / printed page 12 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:
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 PDF page 23 / printed page 13 composition), as described in Unicode Technical Report #15. Conforming ECMAScript implementations are not required to perform any normalisation of text, or behave as though they were performing normalisation of text, themselves. ECMAScript source text is assumed to be a sequence of 16-bit code units for the purposes of this specification. Such a source text may include sequences of 16-bit code units that are not valid UTF-16 character encodings. If an actual source text is encoded in a form other than 16-bit code units it must be processed as if it was first converted to UTF-16.
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);
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 |
|---|---|---|---|
\u200C |
Zero width non-joiner | <ZWNJ> | IdentifierPart |
\u200D |
Zero width joiner | <ZWJ> | IdentifierPart |
\uFEFF |
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.
PDF page 25 / printed page 15| Code Unit Value | Name | Formal Name |
|---|---|---|
| \u0009 | Tab | <TAB> |
| \u000B | Vertical Tab | <VT> |
| \u000C | Form Feed | <FF> |
| \u0020 | Space | <SP> |
| \u00A0 | No-break space | <NBSP> |
| \uFEFF | Byte Order Mark | <BOM> |
| Other category “Zs” | Any other Unicode “space separator” | <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.
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 |
|---|---|---|
\u000A |
Line Feed | <LF> |
\u000D |
Carriage Return | <CR> |
\u2028 |
Line separator | <LS> |
\u2029 |
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.
PDF page 26 / printed page 16Comments 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.
/*
MultiLineCommentCharsopt
*/
*
PostAsteriskCommentCharsopt
*
PostAsteriskCommentCharsopt
*
/
or
asterisk *
//
SingleLineCommentCharsopt
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.
$
_
\ UnicodeEscapeSequence
A reserved word is an IdentifierName that cannot be used as an Identifier.
The following tokens are ECMAScript keywords and may not be used as Identifiers in ECMAScript programs.
break do instanceof typeof
case else new var
catch finally return void
continue for switch while
debugger function this with
default if throw
delete in try
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.
class
enum
extends
super
const
export
import
The following tokens are also considered to be FutureReservedWords when they occur within strict mode code (see 10.1.1). The occurrence of any of these tokens within strict mode code in any context where the occurrence of a FutureReservedWord would produce an error must also produce an equivalent error:
implements
let
private
public
yield
interface
package
protected
static
{
}
(
)
[
]
.
;
,
<
>
<=
>=
==
!=
===
!==
+
-
*
%
++
--
<<
>>
>>>
&
|
^
!
~
&&
||
?
:
=
+=
-=
*=
%=
<<=
>>=
>>>=
&=
|=
^=
/ /=
null
The value of the null literal null is the sole value
of the Null type, namely null.
true
false
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.
. DecimalDigitsopt ExponentPartopt
. DecimalDigits ExponentPartopt
0
0 1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
e E
+ DecimalDigits
- DecimalDigits
0x HexDigit
0X HexDigit
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.
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.
. is the MV of
DecimalIntegerLiteral.. DecimalDigits is
the MV of DecimalIntegerLiteral plus (the MV of
DecimalDigits times
10−n), where n is the number of
characters in DecimalDigits.. ExponentPart is
the MV of DecimalIntegerLiteral times
10e, where e is the MV of
ExponentPart.. 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..
DecimalDigits is the MV of
DecimalDigits times
10−n, where n is the number of
characters in DecimalDigits..
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.0 is 0.+
DecimalDigits is the MV of
DecimalDigits.-
DecimalDigits is the negative of the MV of
DecimalDigits.0 or of
HexDigit
:: 0 is 0.1 or of
NonZeroDigit
:: 1 or of
HexDigit
:: 1 is 1.2 or of
NonZeroDigit
:: 2 or of
HexDigit
:: 2 is 2.3 or of
NonZeroDigit
:: 3 or of
HexDigit
:: 3 is 3.4 or of
NonZeroDigit
:: 4 or of
HexDigit
:: 4 is 4.5 or of
NonZeroDigit
:: 5 or of
HexDigit
:: 5 is 5.6 or of
NonZeroDigit
:: 6 or of
HexDigit
:: 6 is 6.7 or of
NonZeroDigit
:: 7 or of
HexDigit
:: 7 is 7.8 or of
NonZeroDigit
:: 8 or of
HexDigit
:: 8 is 8.9 or of
NonZeroDigit
:: 9 or of
HexDigit
:: 9 is 9.a or of
HexDigit
:: A is 10.b or of
HexDigit
:: B is 11.c or of
HexDigit
:: C is 12.d or of
HexDigit
:: D is 13.e or of
HexDigit
:: E is 14.f or of
HexDigit
:: F is 15.0x
HexDigit is the MV of
HexDigit.0X
HexDigit is 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
0; orA 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.
" DoubleStringCharactersopt "
' SingleStringCharactersopt '
" or backslash \ or LineTerminator
\ EscapeSequence
' or backslash \ or LineTerminator
\ EscapeSequence
\ LineTerminatorSequence
0 [lookahead ∉ DecimalDigit]
' " \ b f n r t v
x
u
x HexDigit HexDigit
u HexDigit HexDigit HexDigit HexDigit
The definition of the nonterminal HexDigit is given in 7.8.3. SourceCharacter is defined in clause 6.
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.
"" is the empty
character sequence.'' is the empty
character sequence."
DoubleStringCharacters
" is the SV of
DoubleStringCharacters.'
SingleStringCharacters
' is the SV of
SingleStringCharacters.\
LineTerminatorSequence is the empty
character sequence."
or backslash
\ or
LineTerminator is the
SourceCharacter character itself.\
EscapeSequence is the CV of the
EscapeSequence.'
or backslash
\ or
LineTerminator is the
SourceCharacter character itself.\
EscapeSequence is the CV of the
EscapeSequence.0
[lookahead
∉ DecimalDigit] is a
<NUL> character (Unicode value 0000).| Escape Sequence | Code Unit Value | Name | Symbol |
|---|---|---|---|
\b |
\u0008 |
backspace | <BS> |
\t |
\u0009 |
horizontal tab | <HT> |
\n |
\u000A |
line feed (new line) | <LF> |
\v |
\u000B |
vertical tab | <VT> |
\f |
\u000C |
form feed | <FF> |
\r |
\u000D |
carriage return | <CR> |
\" |
\u0022 |
double quote | " |
\' |
\u0027 |
single quote | ' |
\\ |
\u005C |
backslash | \ |
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.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.
/ RegularExpressionBody / RegularExpressionFlags
* or \ or / or [
\ or / or [
\ RegularExpressionNonTerminator
[ RegularExpressionClassChars ]
] or \
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:
/(?:)/.
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:
}.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:
The practical effect of these restricted productions is as follows:
When a ++ or -- token is
encountered where the parser would treat it as a postfix operator, and at
least one LineTerminator occurred between the
preceding token and the ++ or
-- token, then a semicolon is automatically inserted
before the ++ or -- token.
When a continue, break,
return, or throw token is
encountered and a LineTerminator is encountered
before the next token, a semicolon is automatically inserted after the
continue, break,
return, or throw token.
The resulting practical advice to ECMAScript programmers is:
A postfix ++ or -- operator
should appear on the same line as its operand.
An Expression in a return
or throw statement should start on the same line as
the return or throw token.
An Identifier in a break or
continue statement should be on the same line as the
break or continue token.
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()PDF page 38 / printed page 28
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 PDF page 39 / printed page 29 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.)
PDF page 40 / printed page 30Some 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.
PDF page 41 / printed page 31| 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 PDF page 42 / printed page 32 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) → Undefined or Property Descriptor | Returns the Property Descriptor of the named own property of this object, or undefined if absent. |
| [[GetProperty]] | SpecOp (propertyName) → Undefined or Property Descriptor | Returns the fully populated Property Descriptor of the named property of this object, or undefined if absent. |
| [[Put]] | SpecOp (propertyName, any, Boolean) | Sets the specified named property to the value of the second parameter. The flag controls failure handling. |
| [[CanPut]] | SpecOp (propertyName) → Boolean | Returns a Boolean value indicating whether a [[Put]] operation with PropertyName can be performed. |
| [[HasProperty]] | SpecOp (propertyName) → Boolean | Returns a Boolean value indicating whether the object already has a property with the given name. |
| [[Delete]] | SpecOp (propertyName, Boolean) → Boolean | Removes the specified named own property from the object. The flag controls failure handling. |
| [[DefaultValue]] | SpecOp (Hint) → primitive | Hint is a String. Returns a default value for the object. |
| [[DefineOwnProperty]] | SpecOp (propertyName, PropertyDescriptor, Boolean) → Boolean | Creates or alters the named own property to have the state described by a Property Descriptor. The flag controls failure handling. |
Every object (including host objects) must implement all of the internal properties listed in Table 8. However, the [[DefaultValue]] internal method may, for some objects, simply throw a TypeError exception.
All objects have an internal property called [[Prototype]]. The value of this property is either null or an object and is used for implementing inheritance. Whether or not a native object can have a host object as its [[Prototype]] depends on the implementation. Every [[Prototype]] chain must have finite length (that is, starting from any object, recursively accessing the [[Prototype]] internal property must eventually lead to a null value). Named data properties of the [[Prototype]] object are inherited (are visible as properties of the child object) for the purposes of get access, but not for put access. Named accessor properties are inherited for both get access and put access.
Every ECMAScript object has a Boolean-valued [[Extensible]] internal property that controls whether or not named properties may be added to the object. If the value of the [[Extensible]] internal property is false then additional named properties may not be added to the object. In addition, if [[Extensible]] is false the value of the [[Class]] and [[Prototype]] internal properties of the object may not be modified. Once the value of an [[Extensible]] internal property has been set to false it may not be subsequently changed to true.
NOTE This specification defines no ECMAScript language operators or built-in functions that permit a program to modify an object’s [[Class]] or [[Prototype]] internal properties or to change the value of [[Extensible]] from false to true. PDF page 43 / printed page 33 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.
PDF page 44 / printed page 34| Internal Property | Value Type Domain | Description |
|---|---|---|
| [[PrimitiveValue]] | primitive | Internal state information associated with this object. Of the standard built-in ECMAScript objects, only Boolean, Date, Number, and String objects implement [[PrimitiveValue]]. |
| [[Construct]] | SpecOp(a List of any) → Object | Creates an object. Invoked via the new operator.
The arguments to the SpecOp are the arguments passed to the
new operator. Objects that implement this
internal method are called constructors. |
| [[Call]] | SpecOp(any, a List of any) → any or Reference | Executes code associated with the object. Invoked via a function call expression. The arguments to the SpecOp are 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]] | Lexical Environment | 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 [[FormalParameters]]. |
| [[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 PDF page 45 / printed
page 35 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:
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:
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.
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:
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:
When the abstract operation IsDataDescriptor is called with property descriptor Desc, the following steps are taken:
When the abstract operation IsGenericDescriptor is called with property descriptor Desc, the following steps are taken:
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).
new Object() where
Object is the standard built-in constructor
with that name.When the abstract operation ToPropertyDescriptor is called with object Desc, the following steps are taken:
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:
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:
When the [[Get]] internal method of O is called with property name P, the following steps are taken:
When the [[CanPut]] internal method of O is called with property name P, the following steps are taken:
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:
When the [[HasProperty]] internal method of O is called with property name P, the following steps are taken:
When the [[Delete]] internal method of O is called with property name P and the Boolean flag Throw, the following steps are taken:
When the [[DefaultValue]] internal method of O is called with hint String, the following steps are taken:
When the [[DefaultValue]] internal method of O is called with hint Number, the following steps are taken:
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:
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:
PDF page 54 / printed page 44| 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.
+ StrUnsignedDecimalLiteral
- StrUnsignedDecimalLiteral
Infinity
. DecimalDigitsopt ExponentPartopt
. DecimalDigits ExponentPartopt
0 1 2 3 4 5 6 7 8 9
e E
+ DecimalDigits
- DecimalDigits
0x HexDigit
0X HexDigit
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):
0 digits.+ or - to
indicate its sign.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.
+
StrUnsignedDecimalLiteral is the MV of
StrUnsignedDecimalLiteral.-
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.)Infinity is
1010000 (a value so large that it will round to
+∞).. is the MV of
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.. ExponentPart is
the MV of DecimalDigits times
10e, where e is the MV of
ExponentPart.. 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..
DecimalDigits is the MV of
DecimalDigits times
10−n, where n is the number of
characters in DecimalDigits.+
DecimalDigits is the MV of
DecimalDigits.-
DecimalDigits is the negative of the MV of
DecimalDigits.0 or of
HexDigit
::: 0 is 0.1 or of
HexDigit
::: 1 is 1.2 or of
HexDigit
::: 2 is 2.3 or of
HexDigit
::: 3 is 3.4 or of
HexDigit
::: 4 is 4.5 or of
HexDigit
::: 5 is 5.6 or of
HexDigit
::: 6 is 6.7 or of
HexDigit
::: 7 is 7.8 or of
HexDigit
::: 8 is 8.9 or of
HexDigit
::: 9 is 9.a or of
HexDigit
::: A is 10.b or of
HexDigit
::: B is 11.c or of
HexDigit
::: C is 12.d or of
HexDigit
::: D is 13.e or of
HexDigit
::: E is 14.f or of
HexDigit
::: F is 15.0x
HexDigit is the MV of
HexDigit.0X
HexDigit is 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
0; orThe abstract operation ToInteger converts its argument to an integral numeric value. This abstract operation functions as follows:
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:
PDF page 57 / printed page 47NOTE Given the above definition of ToInt32:
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:
NOTE Given the above definition of ToUInt32:
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:
NOTE Given the above definition of ToUint16:
The abstract operation ToString converts its argument to a value of type String according to Table 13:
PDF page 58 / printed page 48| Argument Type | Result |
|---|---|
| Undefined | "undefined" |
| Null | "null" |
| Boolean | If the argument is true, then the result is "true". If the argument is false, then the result is "false". |
| Number | See 9.8.1. |
| String | Return the input argument (no conversion) |
| Object | Apply the following steps:
|
The abstract operation ToString converts a Number m to String format as follows:
0’..’, followed by the remaining
k−n digits of the decimal representation of
s.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.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)..’, 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:
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:
PDF page 59 / printed page 49 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:
PDF page 60 / printed page 50| Argument Type | Result |
|---|---|
| Undefined | Return false. |
| Null | Return false. |
| Boolean | Return false. |
| Number | Return false. |
| String | Return false. |
| Object | If the argument object has a [[Call]] internal method, then return true, otherwise return false. |
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:
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 PDF page 62 / printed page 52 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 |
|---|---|
| CreateImmutableBinding(N) | Create a new but uninitialized immutable binding in an environment record. The String value N is the text of the bound name. |
| InitializeImmutableBinding(N, V) | 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. |
PDF page 63 / printed page 53 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:
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.
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.
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.
The concrete Environment Record method DeleteBinding for declarative environment records can only delete bindings that have been explicitly designated as being subject to deletion.
Declarative Environment Records always return undefined as their ImplicitThisValue.
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.
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.
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:
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.
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.
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:
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.
Object Environment Records return undefined as their ImplicitThisValue unless their provideThis flag is true.
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:
When the abstract operation NewDeclarativeEnvironment is called with either a Lexical Environment or null as argument E the following steps are performed:
When the abstract operation NewObjectEnvironmentis called with an Object O and a Lexical Environment E (or null) as arguments, the following steps are performed:
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 PDF page 67 / printed page 57 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 this keyword
within ECMAScript code associated with this execution context. |
The LexicalEnvironment and VariableEnvironment components of an execution context are always Lexical Environments. When an execution context is created its LexicalEnvironment and VariableEnvironment components initially have the same value. The value of the VariableEnvironment component never changes while the value of the LexicalEnvironment component may change during execution of code within an execution context.
In most situations only the running execution context (the top of the execution context stack) is directly manipulated by algorithms within this specification. Hence when the terms “LexicalEnvironment”, “VariableEnvironment” and “ThisBinding” are used without qualification they are in reference to those components of the running execution context.
An execution context is purely a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to access an execution context.
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:
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.
PDF page 68 / printed page 58The following steps are performed when control enters the execution context for global code:
The following steps are performed to initialize a global execution context for ECMAScript code C:
The following steps are performed when control enters the execution context for 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:
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:
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:
new Object() where
Object is the standard built-in constructor
with that name.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:
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:
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:
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:
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:
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:
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.
this
( 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.
[ Elisionopt ]
[ ElementList ]
[ ElementList , Elisionopt ]
, Elisionopt AssignmentExpression
,
,
The production ArrayLiteral
: [
Elisionopt ]
is evaluated as follows:
new Array() where
Array is the standard built-in constructor with
that name.The production ArrayLiteral
: [
ElementList ] is
evaluated as follows:
The production ArrayLiteral
: [
ElementList ,
Elisionopt ]
is evaluated as follows:
The production ElementList : Elisionopt AssignmentExpression is evaluated as follows:
new Array() where
Array is the standard built-in constructor with
that name.The production ElementList
: ElementList
, Elisionopt
AssignmentExpression is evaluated as
follows:
The production Elision
: , is evaluated as
follows:
The production Elision
: Elision
, is evaluated as follows:
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.
{ }
{ PropertyNameAndValueList }
{ PropertyNameAndValueList , }
, PropertyAssignment
: AssignmentExpression
get PropertyName ( ) { FunctionBody }
set PropertyName ( PropertySetParameterList ) { FunctionBody }
The production ObjectLiteral
: {
} is evaluated as follows:
new Object() where
Object is the standard built-in constructor
with that name.The productions ObjectLiteral
: {
PropertyNameAndValueList
} and
ObjectLiteral
: {
PropertyNameAndValueList ,
} are evaluated as follows:
The production PropertyNameAndValueList : PropertyAssignment is evaluated as follows:
new Object() where
Object is the standard built-in constructor
with that name.The production
PropertyNameAndValueList
: PropertyNameAndValueList
, PropertyAssignment is
evaluated as follows:
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:
PDF page 76 / printed page 66 The
production PropertyAssignment
: get
PropertyName (
) {
FunctionBody } is
evaluated as follows:
The production PropertyAssignment
: set
PropertyName (
PropertySetParameterList )
{ FunctionBody
} is evaluated as follows:
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:
The production PropertyName : StringLiteral is evaluated as follows:
The production PropertyName : NumericLiteral is evaluated as follows:
The production PrimaryExpression
: (
Expression ) is evaluated
as follows:
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.
[ Expression ]
. IdentifierName
new MemberExpression Arguments
new NewExpression
[ Expression ]
. IdentifierName
( )
( ArgumentList )
, AssignmentExpression
Properties are accessed by name, using either the dot notation:
.
IdentifierName
. IdentifierName
or the bracket notation:
[
Expression ]
[
Expression ]
The dot notation is explained by the following syntactic conversion:
.
IdentifierName
is identical in its behaviour to
[
<identifier-name-string> ]
and similarly
.
IdentifierName
is identical in its behaviour to
[
<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:
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:
The production MemberExpression
: new
MemberExpression
Arguments is evaluated as follows:
The production CallExpression : MemberExpression Arguments is evaluated as follows:
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:
The production Arguments
: (
ArgumentList ) is
evaluated as follows:
The production ArgumentList : AssignmentExpression is evaluated as follows:
The production ArgumentList
: ArgumentList
, AssignmentExpression is
evaluated as follows:
The production MemberExpression : FunctionExpression is evaluated as follows:
The production PostfixExpression
: LeftHandSideExpression
[no
LineTerminator here]
++ is evaluated as follows:
+ operator (see
11.6.3).The production PostfixExpression
: LeftHandSideExpression
[no
LineTerminator here]
-- is evaluated as follows:
- operator
(11.6.3).delete UnaryExpression
void UnaryExpression
typeof UnaryExpression
++ UnaryExpression
-- UnaryExpression
+ UnaryExpression
- UnaryExpression
~ UnaryExpression
! UnaryExpression
delete OperatorThe production UnaryExpression
: delete
UnaryExpression is evaluated as follows:
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.
void OperatorThe production UnaryExpression
: void
UnaryExpression is evaluated as follows:
NOTE GetValue must be called even though its value is not used because it may have observable side-effects.
typeof OperatorThe production UnaryExpression
: typeof
UnaryExpression is evaluated as follows:
| Type of val | Result |
|---|---|
| Undefined | "undefined" |
| Null | "object" |
| Boolean | "boolean" |
| Number | "number" |
| String | "string" |
| Object (native and does not implement [[Call]]) | "object" |
| Object (native or host and does implement [[Call]]) | "function" |
| Object (host and does not implement [[Call]]) | Implementation-defined except may not be "undefined", "boolean", "number", or "string". |
The production UnaryExpression
: ++
UnaryExpression is evaluated as follows:
+ operator (see
11.6.3).The production UnaryExpression
: --
UnaryExpression is evaluated as follows:
- operator
(see 11.6.3).+ OperatorThe unary + operator converts its operand to Number type.
The production UnaryExpression
: +
UnaryExpression is evaluated as follows:
- OperatorThe 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:
~ )The production UnaryExpression
: ~
UnaryExpression is evaluated as follows:
! )The production UnaryExpression
: !
UnaryExpression is evaluated as follows:
* UnaryExpression
/ UnaryExpression
% UnaryExpression
The production
MultiplicativeExpression
: MultiplicativeExpression
@
UnaryExpression, where @ stands for one of the operators in the above
definitions, is evaluated as follows:
* OperatorThe * 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:
/ OperatorThe / 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:
% OperatorThe % 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:
+ MultiplicativeExpression
- MultiplicativeExpression
+ )The addition operator either performs string concatenation or numeric addition.
The production AdditiveExpression
: AdditiveExpression
+
MultiplicativeExpression is evaluated as
follows:
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.
PDF page 85 / printed page 75 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:
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:
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).
<< AdditiveExpression
>> AdditiveExpression
>>> AdditiveExpression
<< )Performs a bitwise left shift operation on the left operand by the amount specified by the right operand.
PDF page 86 / printed page 76 The
production ShiftExpression
: ShiftExpression
<<
AdditiveExpression is evaluated as follows:
>> )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:
>>> )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:
< ShiftExpression
> ShiftExpression
<= ShiftExpression
>= ShiftExpression
instanceof ShiftExpression
in ShiftExpression
< ShiftExpression
> ShiftExpression
<= ShiftExpression
>= ShiftExpression
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.
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:
> )The production RelationalExpression
: RelationalExpression
> ShiftExpression is
evaluated as follows:
<= )The production RelationalExpression
: RelationalExpression
<= ShiftExpression is
evaluated as follows:
>= )The production RelationalExpression
: RelationalExpression
>= ShiftExpression is
evaluated as follows:
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:
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.
instanceof operatorThe production
RelationalExpression:
RelationalExpression instanceof
ShiftExpression is evaluated as follows:
in operatorThe production
RelationalExpression
: RelationalExpression
in ShiftExpression is
evaluated as follows:
== RelationalExpression
!= RelationalExpression
=== RelationalExpression
!== RelationalExpression
== RelationalExpressionNoIn
!= RelationalExpressionNoIn
=== RelationalExpressionNoIn
!== RelationalExpressionNoIn
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:
!= )The production EqualityExpression
: EqualityExpression
!= RelationalExpression
is evaluated as follows:
The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed as follows:
NOTE 1 Given the above definition of equality:
"" + a == "" +
b.+a ==
+b.!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:
!== )The production EqualityExpression
: EqualityExpression
!== RelationalExpression
is evaluated as follows:
The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:
NOTE This algorithm differs from the SameValue Algorithm (9.12) in its treatment of signed zeroes and NaNs.
& EqualityExpression
& EqualityExpressionNoIn
^ BitwiseANDExpression
^ BitwiseANDExpressionNoIn
| BitwiseXORExpression
| BitwiseXORExpressionNoIn
The production A :
A @ B,
where @ is one of the bitwise operators
in the productions above, is evaluated as follows:
&& BitwiseORExpression
&& BitwiseORExpressionNoIn
|| LogicalANDExpression
|| LogicalANDExpressionNoIn
The production
LogicalANDExpression
: LogicalANDExpression
&&
BitwiseORExpression is evaluated as follows:
The production
LogicalORExpression
: LogicalORExpression
|| LogicalANDExpression
is evaluated as follows:
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.
? : )? AssignmentExpression : AssignmentExpression
? AssignmentExpression : AssignmentExpressionNoIn
The production
ConditionalExpression
: LogicalORExpression
? AssignmentExpression
: AssignmentExpression is
evaluated as follows:
PDF page 94 / printed page 84 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.
= *= /= %= += -= <<= >>= >>>= &= ^= |=
The AssignmentExpressionNoIn productions are evaluated in the same manner as the AssignmentExpression productions except that the contained ConditionalExpressionNoIn and AssignmentExpressionNoIn are evaluated instead of the contained ConditionalExpression and AssignmentExpression, respectively.
= )The production
AssignmentExpression
: LeftHandSideExpression
= AssignmentExpression is
evaluated as follows:
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.
op= )The production
AssignmentExpression
: LeftHandSideExpression
@ =
AssignmentExpression, where @
represents one of the operators indicated above, is evaluated as follows:
NOTE See NOTE 11.13.1.
, ), AssignmentExpression
, AssignmentExpressionNoIn
The production Expression
: Expression
, AssignmentExpression is
evaluated as follows:
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.
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.
{ StatementListopt }
The production Block
: {
} is evaluated as follows:
The production Block
: {
StatementList } is
evaluated as follows:
The production StatementList : Statement is evaluated as follows:
The production StatementList : StatementList Statement is evaluated as follows:
var VariableDeclarationList ;
, VariableDeclaration
, VariableDeclarationNoIn
= AssignmentExpression
= 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.
The production VariableStatement
: var
VariableDeclarationList ;
is evaluated as follows:
The production VariableDeclarationList :VariableDeclaration is evaluated as follows:
The production
VariableDeclarationList
: VariableDeclarationList
, VariableDeclaration is
evaluated as follows:
The production VariableDeclaration : Identifier is evaluated as follows:
The production VariableDeclaration : Identifier Initialiser is evaluated as follows:
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:
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".
;
The production EmptyStatement
: ; is evaluated as
follows:
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.
The production
ExpressionStatement
:
[lookahead ∉
{{, function}]
Expression ; is evaluated
as follows:
if Statementif ( Expression ) Statement else Statement
if ( Expression ) Statement
Each else for which the choice of associated
if is ambiguous shall be associated with the nearest
possible if that would otherwise have no corresponding
else.
The production IfStatement
: if (
Expression )
Statement else
Statement is evaluated as follows:
The production IfStatement
: if (
Expression )
Statement is evaluated as follows:
do Statement while ( Expression ) ;
while ( Expression ) Statement
for (ExpressionNoInopt; Expressionopt ; Expressionopt ) Statement
for ( var VariableDeclarationListNoIn ; Expressionopt ; Expressionopt ) Statement
for ( LeftHandSideExpression in Expression ) Statement
for ( var VariableDeclarationNoIn in Expression ) Statement
do–while StatementThe production do
Statement while
( Expression
) ; is evaluated as
follows:
while StatementThe production IterationStatement
: while
( Expression
) Statement is evaluated
as follows:
for StatementThe production IterationStatement
: for (
ExpressionNoInopt ;
Expressionopt ;
Expressionopt)
Statement is evaluated as follows:
The production IterationStatement
: for (
var VariableDeclarationListNoIn
; Expressionopt
; Expressionopt
) Statement is evaluated
as follows:
for–in StatementThe production IterationStatement
: for (
LeftHandSideExpression in
Expression )
Statement is evaluated as follows:
The production IterationStatement
: for (
var VariableDeclarationNoIn
in Expression
) Statement is evaluated
as follows:
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.
continue StatementA program is considered syntactically incorrect if either of the following is true:
continue statement without
the optional Identifier, which is not nested,
directly or indirectly (but not crossing function boundaries), within an
IterationStatement.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:
A ContinueStatement with the optional Identifier is evaluated as follows:
break StatementA program is considered syntactically incorrect if either of the following is true:
break statement without the
optional Identifier, which is not nested,
directly or indirectly (but not crossing function boundaries), within an
IterationStatement or a
SwitchStatement.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:
A BreakStatement with an Identifier is evaluated as follows:
return StatementAn 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:
with Statementwith ( 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.
The production WithStatement
: with (
Expression )
Statement is evaluated as follows:
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.
switch Statementswitch ( Expression ) CaseBlock
{ CaseClausesopt }
{ CaseClausesopt DefaultClause CaseClausesopt }
case Expression : StatementListopt
default : StatementListopt
The production SwitchStatement
: switch
( Expression
) CaseBlock is evaluated
as follows:
The production CaseBlock
: {
CaseClausesopt
} is given an input parameter, input,
and is evaluated as follows:
The production CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt } is given an input parameter, input, and is evaluated as follows:
The production CaseClause
: case
Expression :
StatementListopt is evaluated as
follows:
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.
: Statement
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.
throw StatementThe production ThrowStatement
: throw
[no
LineTerminator here]
Expression ; is evaluated
as:
try Statementtry Block Catch
try Block Finally
try Block Catch Finally
catch ( Identifier ) Block
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.
The production TryStatement
: try
Block Catch is evaluated
as follows:
The production TryStatement
: try
Block Finally is
evaluated as follows:
The production TryStatement
: try
Block Catch
Finally is evaluated as follows:
The production Catch
: catch
( Identifier
) Block is evaluated as
follows:
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:
It is a SyntaxError if a TryStatement with a Catch occurs within strict code and the Identifier of the Catch production is either "eval" or "arguments".
debugger statement;
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:
function Identifier ( FormalParameterListopt ) { FunctionBody }
function Identifieropt ( FormalParameterListopt ) { FunctionBody }
, Identifier
The production
FunctionDeclaration
: function
Identifier (
FormalParameterListopt
) {
FunctionBody } is
instantiated as follows during Declaration Binding
instantiation (10.5):
The production FunctionExpression
: function
(
FormalParameterListopt
) {
FunctionBody } is
evaluated as follows:
The production FunctionExpression
: function
Identifier (
FormalParameterListopt
) {
FunctionBody } is
evaluated as follows:
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:
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.
PDF page 109 / printed page 99 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:
new Object()
where Object is the
standard built-in constructor with that name.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:
When the [[Construct]] internal method for a Function object F is called with a possibly empty list of arguments, the following steps are taken:
The production Program : SourceElementsopt is evaluated as follows:
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:
The production SourceElement : Statement is evaluated as follows:
The production SourceElement : FunctionDeclaration is evaluated as follows:
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
PDF page 112 / printed page 102
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.
PDF page 113 / printed page 103 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:
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, the number may also optionally begin with the character
pairs 0x or 0X.
When the parseInt function is called, the following steps are taken:
-, let sign be
−1.+ or a minus sign
-, then remove the first character from
S.0x” or
“0X”, then remove the first two
characters from S and let R = 16.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.)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:
PDF page 115 / printed page 105 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.
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.
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:
:
First /
Second ;
Third ?
Fourth
where the italicised names represent components and
“:”,
“/”,
“;” and
“?” are reserved characters used as
separators. The
encodeURI and
decodeURI functions
are intended to work with complete URIs; they assume that any reserved
characters in the URI are intended to have special meaning and so are not
encoded. The
encodeURIComponent and
decodeURIComponent
functions are intended to work with the individual component parts of a URI;
they assume that any reserved characters represent text and so must be encoded
so that they are not interpreted as reserved characters when the component is
part of a complete URI.
The following lexical grammar specifies the form of encoded URIs.
; / ? : @ & = + $ ,
% HexDigit HexDigit
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
- _ . ! ~ * ' ( )
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.
PDF page 117 / printed page 107 The unescaping and decoding process is described by the abstract operation Decode taking two String arguments string and reservedSet.
%’, then
%’
%’, throw a
URIError exception.NOTE The syntax of Uniform Resource Identifiers is given in 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 |
|---|---|---|---|---|---|
| 0x0000 - 0x007F | 00000000 0zzzzzzz |
0zzzzzzz |
|||
| 0x0080 - 0x07FF | 00000yyy yyzzzzzz | 110yyyyy | 10zzzzzz | ||
| 0x0800 - 0xD7FF | xxxxyyyy yyzzzzzz | 1110xxxx | 10yyyyyy | 10zzzzzz | |
| 0xD800 - 0xDBFF followed by 0xDC00 - 0xDFFF | 110110vv vvwwwwxx followed by 110111yy yyzzzzzz | 11110uuu | 10uuwwww | 10xxyyyy | 10zzzzzz |
| 0xD800 - 0xDBFF not followed by 0xDC00 - 0xDFFF | causes URIError | ||||
| 0xDC00 - 0xDFFF | causes URIError | ||||
| 0xE000 - 0xFFFF | xxxxyyyy yyzzzzzz | 1110xxxx | 10yyyyyy | 10zzzzzz |
Where
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:
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:
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:
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:
See 15.2.1 and 15.2.2.
See 15.3.1 and 15.3.2.
See 15.4.1 and 15.4.2.
See 15.5.1 and 15.5.2.
See 15.6.1 and 15.6.2.
See 15.7.1 and 15.7.2.
See 15.10.3 and 15.10.4.
See 15.11.1 and 15.11.2.
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:
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:
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).
PDF page 122 / printed page 112 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:
When the getOwnPropertyDescriptor function is called, the following steps are taken:
When the getOwnPropertyNames function is called, the following steps are taken:
new Array () where
Array is the standard built-in constructor with
that name.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:
new Object() where
Object is the standard built-in constructor
with that name.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:
The defineProperties function is used to add own properties and/or update the attributes of existing own properties of an object. When the defineProperties function is called, the following steps are taken:
If 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:
When the freeze function is called, the following steps are taken:
When the preventExtensions function is called, the following steps are taken:
When the isSealed function is called with argument O, the following steps are taken:
When the isFrozen function is called with argument O, the following steps are taken:
When the isExtensible function is called with argument O, the following steps are taken:
When the keys function is called with argument O, the following steps are taken:
new Array(n) where
Array is the standard built-in constructor with
that name.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:
When the toLocaleString method is called, the following steps are taken:
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:
When the hasOwnProperty method is called with argument V, the following steps are taken:
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:
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:
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:
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:
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:
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:
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:
length property
of Target minus the length of A.length own property of
F to either 0 or L, whichever is larger.length own property of
F to 0.length own
property of F to the values specified in
15.3.5.1.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:
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:
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:
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:
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:
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:
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:
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:
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:
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:
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:
new Array() where
Array is the standard built-in constructor with
that name.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:
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.
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:
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.
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.
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:
new Array() where
Array is the standard built-in constructor with
that name.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:
The behaviour of sort is also implementation defined if obj is sparse and any of the following conditions are true:
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.
PDF page 139 / printed page 129 The returned object must have the following two properties.
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.
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:
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:
new Array() where
Array is the standard built-in constructor with
that name.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:
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:
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 (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:
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:
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:
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:
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.
PDF page 146 / printed page 136 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:
new Array(len) where
Array is the standard built-in constructor with
that name and len is the value of len.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:
new Array() where
Array is the standard built-in constructor with
that name.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:
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:
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:
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.)
PDF page 152 / printed page 142 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,
.
When the charAt method is called with one argument pos, the following steps are taken:
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:
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:
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:
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:
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:
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. Furthermore, localeCompare returns 0 or −0 when comparing two Strings that are considered canonically equivalent by the Unicode standard.
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:
new RegExp(regexp)
where RegExp is the standard built-in
constructor with that name.new Array() where
Array is the standard built-in constructor
with that name.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:
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 PDF page 156 / printed page 146 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. |
$n |
The nth capture, where n is a single digit in the
range 1 to 9 and $n is not followed
by a decimal digit. If n≤m and the
nth capture is undefined, use the
empty String instead. If n>m, the result is
implementation-defined. |
$nn |
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:
new RegExp(regexp) where
RegExp is the standard built-in constructor
with that name.lastIndex and
global properties of regexp are
ignored when performing the search. The
lastIndex property of regexp is
left unchanged.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:
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:
new Array() where
Array is the standard built-in constructor with
that name..length = lim, 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):
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:
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:
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:
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:
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:
The following steps are taken:
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 properties:
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:
0’.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, PDF page 166
/ printed page 156
(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:
0’.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).
PDF page 167 / printed page 157 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:
i. 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:
0’..’, and the remaining
p−(e+1) characters of m.0’,
and the String 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.
PDF page 169 / printed page 159 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, sqrt, and tan is not precisely specified here except to require specific results for certain argument values that represent boundary cases of interest. For other argument values, these functions are intended to compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of approximation algorithms. The general intent is that an implementer should be able to use the same mathematical library for ECMAScript on a given hardware platform that is available to C programmers on that platform. Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard) that implementations use the approximation algorithms for IEEE 754 arithmetic contained in fdlibm, the freely distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).
Returns the absolute value of x; the result has the same magnitude as x but has positive sign.
Returns an implementation-dependent approximation to the arc cosine of x. The result is expressed in radians and ranges from +0 to +π.
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.
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.
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.
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.
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.
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).
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.
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.
Given zero or more arguments, calls ToNumber on each of the arguments and returns the largest of the resulting values.
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.
The length property of the
min method is 2.
Returns an implementation-dependent approximation to the result of raising x to the power y.
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.
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.
Returns an implementation-dependent approximation to the square root of x.
Returns an implementation-dependent approximation to the tangent of x. The argument is expressed in radians.
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
The remainder is called the time within the day:
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:
The time value of the start of a year is:
A time value determines a year by:
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
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
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
(2) whether t is in a leap year
(3) the week day of the beginning of the year
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
Conversion from local time to UTC is defined by
Note that UTC(LocalTime(t)) is not necessarily always equal to t.
The following functions are useful in decomposing time values:
PDF page 177 / printed page 167where
The operator MakeTime calculates a number of milliseconds from its four arguments, which must be ECMAScript Number values. This operator functions as follows:
* msPerHour
+ m *
msPerMinute + s
* msPerSecond +
milli, performing the arithmetic according to IEEE 754 rules
(that is, as if using the ECMAScript operators *
and +).The operator MakeDay calculates a number of days from its three arguments, which must be ECMAScript Number values. This operator functions as follows:
The operator MakeDate calculates a number of milliseconds from its two arguments, which must be ECMAScript Number values. This operator functions as follows:
The operator TimeClip calculates a number of milliseconds from its argument, which must be an ECMAScript Number value. This operator functions as follows:
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.sssZ
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 from 00 to 24. |
: |
“:” (colon) appears literally
twice in the string. |
mm |
is the number of complete minutes since the start of the hour as two decimal digits from 00 to 59. |
ss |
is the number of complete seconds since the start of the minute as two decimal digits from 00 to 59. |
. |
“.” (dot) appears literally in the
string. |
sss |
is the number of complete milliseconds since the start of the second as three decimal digits. |
| Both the "." and the milliseconds field may be omitted. | |
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 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:
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).
PDF page 180 / printed page 170 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:
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())
PDF page 181 / printed page 171 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:
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.
Returns the difference between local time and UTC time in minutes.