ECMAScript Language Specification

Brief History

This ECMA Standard is based on several originating technologies, the most well-known being JavaScript (Netscape Communications) and Jscript (Microsoft Corporation). 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 has approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes from the first edition are editorial in nature.

The work on standardization of the language continues to support regular expressions, richer control statements and better string handling, in addition to the core language standardized in the first two editions of the ECMA Standard. These features and others, such as try/catch exception handling and better internationalization facilities, are being documented in anticipation of the third edition of the standard about the end of 1999 which will contain the second version of the language.

This Standard has been adopted as 2nd Edition of ECMA-262 by the ECMA General Assembly in August 1998.

Table of contents

1 -- Scope

This Standard defines the ECMAScript scripting language.

2 -- Conformance

A conforming implementation of ECMAScript must provide and support all the types, values, objects, properties, functions, and program syntax described in this specification.

A conforming implementation of this International standard shall interpret characters in conformance with the Unicode Standard, Version 2.0, and ISO/IEC 10646-1 with UCS-2 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.

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 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 section -- Future Reserved Word of this specification.

3 -- References

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), including amendments 1 through 9 and technical corrigendum 1.

ISO/IEC 646.IRV:1991 -- Information Processing -- ISO 7-bit Coded Character Set for Information Interchange.

Unicode Inc. (1996), The Unicode Standard™, Version 2.0. ISBN: 0-201-48345-9, Addison-Wesley Publishing Co., Menlo Park, California.

ANSI/IEEE Std 754-1985: IEEE Standard for Binary Floating-Point Arithmetic. Institute of Electrical and Electronics Engineers, New York (1985).

4 -- Overview

This section contains a non-normative overview of the ECMAScript language.

ECMAScript is an object-oriented programming language for performing computations and manipulating computational objects within a host environment. ECMAScript as defined here is not intended to be computationally self-sufficient; indeed, there are no provisions in this specification for input of external data or output of computed results. Instead, it is expected that the computational environment of an ECMAScript program will provide not only the objects and other facilities described in this specification but also certain environment-specific host objects, whose description and behaviour are beyond the scope of this specification except to indicate that they may provide certain properties that can be accessed and certain functions that can be called from an ECMAScript program.

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, and therefore there may be a number of informalities built into the language.

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 Javaä and Self, as described in:

4.1 -- Web Scripting

A web browser provides an ECMAScript host environment for client-side computation including, for instance, objects that represent windows, menus, pop-ups, dialog boxes, text areas, anchors, frames, history, cookies, and input/output. Further, the host environment provides a means to attach scripting code to events such as change of focus, page and image loading, unloading, error and abort, selection, form submission, and mouse actions. Scripting code appears within the HTML and the displayed page is a combination of user interface elements and fixed and computed text and images. The scripting code is reactive to user interaction and there is no need for a main program.

A web server provides a different host environment for server-side computation including objects representing requests, clients, and files; and mechanisms to lock and share data. By using browser-side and server side scripting together it is possible to distribute computation between the client and server while providing a customised user interface for a Web-based application.

Each Web browser and server that supports ECMAScript supplies its own host environment, completing the ECMAScript execution environment.

4.2 -- Language Overview

The following is an informal overview of ECMAScript--not all parts of the language are described. This overview is not part of the standard proper.

ECMAScript is object-based: basic language and host facilities are provided by objects, and an ECMAScript program is a cluster of communicating objects. An ECMAScript object is an unordered collection of properties each with 0 or more attributes which determine how each property can be used--for example, when the ReadOnly attribute for a property is set to true, any attempt by executed ECMAScript code to change the value of the property has no effect. Properties are containers that hold other objects, primitive values, or methods. 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 method is a function associated with an object via a property.

ECMAScript defines a collection of built-in objects which 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, and the Date object.

ECMAScript also defines a set of built-in operators that may not be, strictly speaking, functions or methods. ECMAScript operators include various unary operations, multiplicative operators, additive operators, bitwise shift operators, relational operators, equality operators, binary bitwise operators, binary logical operators, assignment operators, and the comma operator.

ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to serve as an easy-to-use scripting language. For example, a variable is not required to have its type declared nor are types associated with properties, and defined functions are not required to have their declarations appear textually before calls to them.

4.2.1 -- Objects

ECMAScript does not contain proper classes such as those in C++, Smalltalk, or Java, but rather, supports constructors which create objects by executing code that allocates storage for the objects and initialises all or part of them by assigning initial values to their properties. All functions including constructors are objects, but not all objects are constructors. Each constructor has a Prototype property that is used to implement prototype-based inheritance and shared properties. Objects are created by using constructors in new expressions; for example, new String("A String") creates a new string object. Invoking a constructor without using new has consequences that depend on the constructor. For example, String("A String") produces a primitive string, not an object.

ECMAScript supports prototype-based inheritance. Every constructor has an associated prototype, and every object created by that constructor has an implicit reference to the prototype (called the object's prototype) associated with its constructor. Furthermore, a prototype may have a non-null implicit reference to its prototype, and so on; this is called the prototype chain. When a reference is made to a property in an object, that reference is to the property of that name in the first object in the prototype chain that contains a property of that name. In other words, first the object mentioned directly is examined for such a property; if that object contains the named property, that is the property to which the reference refers; if that object does not contain the named property, the prototype for that object is examined next; and so on.

In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and inheritance is only of structure and behaviour. In ECMAScript, the state and methods are carried by objects, and structure, behaviour, and state are all inherited.

All objects that do not directly contain a particular property that their prototype contains share that property and its value. The following diagram 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, as are any properties found in CFp's implicit prototype chain which are not named q1, q2, or CFp1. Notice that there is no implicit prototype link between CFp and CF.

Unlike class-based object languages, properties can be added to objects dynamically by assigning values to them. That is, constructors are not required to name or assign values to all or any of the constructed object's properties. In the above diagram, one could add a new shared property for CF1, CF2, CF3, CF4, and CF5 by assigning a new value to the property in CFp.

4.3 -- Definitions

The following are informal definitions of key terms associated with ECMAScript.

4.3.1 -- Type

A type is a set of data values.

4.3.2 -- Primitive value

A primitive value is a member of one of the types Undefined, Null, Boolean, Number, or String. A primitive value is a datum that is represented directly at the lowest level of the language implementation.

4.3.3 -- Object

An object is a member of the type Object. It is an unordered collection of properties each of which contains a primitive value, object, or function. A function stored in a property of an object is called a method.

4.3.4 -- Constructor

A constructor is a function object that creates and initialises objects. Each constructor has an associated prototype object that is used to implement inheritance and shared properties.

4.3.5 -- Prototype

A prototype is an object used to implement structure, state, and behaviour inheritance in ECMAScript. When a constructor creates an object, that object implicitly references the constructor's associated prototype for the purpose of resolving property references. The constructor's associated prototype 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.

4.3.6 -- Native object

A native object is any object supplied by an ECMAScript implementation independent of the host environment. Standard native objects are defined in this specification. Some native objects are built-in; others may be constructed during the course of execution of an ECMAScript program.

4.3.7 -- Built-in object

A built-in object is any object supplied by an ECMAScript implementation, independent of the host environment, which is present at the start of the execution of an ECMAScript program. Standard built-in objects are defined in this specification, and the ECMAScript implementation may specify and define others. Every built-in object is a native object.

4.3.8 -- Host object

A host object is any object supplied by the host environment to complete the execution environment of ECMAScript. Any object that is not native is a host object.

4.3.9 -- Undefined value

The undefined value is a primitive value used when a variable has not been assigned a value.

4.3.10 -- Undefined type

The type Undefined has exactly one value, called undefined.

4.3.11 -- Null value

The null value is a primitive value that represents the null, empty, or non-existent reference.

4.3.12 -- Null type

The type Null has exactly one value, called null.

4.3.13 -- Boolean value

A boolean value is a member of the type Boolean and is one of two unique values, true and false.

4.3.14 -- Boolean type

The type Boolean represents a logical entity and consists of exactly two unique values. One is called true and the other is called false.

4.3.15 -- Boolean object

A boolean object is a member of the type Object and is an instance of the built-in Boolean object. That is, a boolean object is created by using the Boolean constructor in a new expression, supplying a boolean as an argument. The resulting object has an implicit (unnamed) property that is the boolean. A boolean object can be coerced to a boolean value. A boolean object can be used anywhere a boolean value is expected.

This is an example of one of the conveniences built into ECMAScript--in this case, the purpose is to accommodate programmers of varying backgrounds. Those familiar with imperative or procedural programming languages may find boolean, string and number values more natural, while those familiar with object-oriented languages may find boolean, string and number objects more intuitive.

4.3.16 -- String value

A string value is a member of the type String and is a finite ordered sequence of zero or more Unicode characters.

4.3.17 -- String type

The type String is the set of all finite ordered sequences of zero or more Unicode characters.

4.3.18 -- String object

A string object is a member of the type Object and is an instance of the built-in String object. That is, a string object is created by using the String constructor in a new expression, supplying a string as an argument. The resulting object has an implicit (unnamed) property that is the string. A string object can be coerced to a string value. A string object can be used anywhere a string value is expected.

4.3.19 -- Number value

A number value is a member of the type Number and is a direct representation of a number.

4.3.20 -- Number type

The type Number is a set of values representing numbers. In ECMAScript the set of values represent the double-precision 64-bit format IEEE 754 values including the special ``Not-a-Number'' (NaN) values, positive infinity, and negative infinity.

4.3.21 -- Number object

A number object is a member of the type Object and is an instance of the built-in Number object. That is, a number object is created by using the Number constructor in a new expression, supplying a number as an argument. The resulting object has an implicit (unnamed) property that is the number. A number object can be coerced to a number value. A number object can be used anywhere a number value is expected. Note that a number object can have shared properties by adding them to the Number prototype.

4.3.22 -- Infinity

The primitive value Infinity represents the positive infinite number value.

4.3.23 -- NaN

The primitive value NaN represents the set of IEEE Standard ``Not-a-Number'' values.

5 -- Notational Conventions

5.1 -- Syntactic and Lexical Grammars

This section describes the context-free grammars used in this specification to define the lexical and syntactic structure of an ECMAScript program.

5.1.1 -- Context-Free Grammars

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 one or more nonterminal and terminal symbols as its right-hand side. For each grammar, the terminal symbols are drawn from a specified alphabet.

Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a given context-free grammar specifies a language, namely, the (perhaps infinite) set of possible sequences of terminal symbols that can result from repeatedly replacing any nonterminal in the sequence with a right-hand side of a production for which the nonterminal is the left-hand side.

5.1.2 -- The lexical grammar

A lexical grammar for ECMAScript is given in Section 7. This grammar has as its terminal symbols the characters of the Unicode character set. It defines a set of productions, starting from the goal symbol InputElement, that describe how sequences of Unicode 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 (see section7.8). Simple white space and single-line comments are simply 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 multi-line comment 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.

Productions of the lexical grammar are distinguished by having two colons ``::'' as separating punctuation.

5.1.3 -- The numeric string grammar

A second 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 the characters of the Unicode character set. This grammar appears in section 9.3.1.

Productions of the numeric string grammar are distinguished by having three colons ``:::'' as punctuation.

5.1.4 -- The syntactic grammar

The syntactic grammar for ECMAScript is given in sections11, 12, 13, and 14. This grammar has ECMAScript tokens defined by the lexical grammar as its terminal symbols (see section 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 Unicode 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 syntax 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 sections11, 12, 13, and 14 is actually not a complete account of which token sequences are accepted as correct ECMAScript programs. Certain additional token sequences are also accepted, namely, those that would be described by the grammar if only semicolons were added to the sequence in certain places (such as before line terminator characters). Furthermore, certain token sequences that are described by the grammar are not considered acceptable if a terminator character appears in certain ``awkward'' places.

5.1.5 -- Grammar Notation

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.

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:

WithStatement :
with ( Expression ) Statement

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

ArgumentList :
AssignmentExpression
ArgumentList , AssignmentExpression

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

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

VariableDeclaration :
Identifier Initializeropt

is a convenient abbreviation for:

VariableDeclaration :
Identifier
Identifier Initializer

and that:

IterationStatement :
for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement
is a convenient abbreviation for:
IterationStatement :
for ( ; Expressionopt ; Expressionopt ) Statement
for ( Expression ; Expressionopt ; Expressionopt ) Statement
which in turn is an abbreviation for:
IterationStatement :
for ( ; ; Expressionopt ) Statement
for ( ; Expression ; Expressionopt ) Statement
for ( Expression ; ; Expressionopt ) Statement
for ( Expression ; Expression ; Expressionopt ) Statement
which in turn is an abbreviation for:
IterationStatement :
for ( ; ; ) Statement
for ( ; ; Expression ) Statement
for ( ; Expression ; ) Statement
for ( ; Expression ; Expression ) Statement
for ( Expression ; ; ) Statement
for ( Expression ; ; Expression ) Statement
for ( Expression ; Expression ; ) Statement
for ( Expression ; Expression ; Expression ) Statement

so the nonterminal IterationStatement actually has eight alternative right-hand sides.

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

ReturnStatement :
return [no LineTerminator here] Expressionopt ;

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

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

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

ZeroToThree :: one of
0 1 2 3

which is merely a convenient abbreviation for:

ZeroToThree ::
0
1
2
3

When an alternative in a production of the lexical grammar or the numeric string grammar appears to be a multicharacter token, it represents the sequence of characters that would make up such a token.

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

Identifier ::
IdentifierName but not ReservedWord

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

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

SourceCharacter ::
any Unicode character

5.2 -- Algorithm Conventions

The specification often uses a numbered list to specify steps in an algorithm. These algorithms are used to clarify semantics. In practice, there may be more efficient algorithms available to implement a given feature.

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''. Type(x) is used as shorthand for ``the type of x''.

Mathematical operations such as addition, subtraction, negation, multiplication, division, and the mathematical functions defined later in this section 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 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 ``generate a runtime error'', execution of the algorithm (and any calling algorithms) is terminated and no result is returned.

6 -- Source Text

ECMAScript source text is represented as a sequence of characters representable using the Unicode version 2.0 character encoding.

SourceCharacter ::
any Unicode character

Except within comments and string literals, every ECMAScript program shall consist of only characters from the first 128 Unicode characters (that is, the first half of row zero). Other Unicode characters may appear only within comments and string literals. In string literals, any Unicode character may also be expressed as a Unicode escape sequence consisting of six characters from the first 128 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, the Unicode escape sequence contributes one character to the string value of the literal.

Although the characters in an ECMAScript program are Unicode characters, they are treated as independent 16-bit values with none of the context-dependent interpretation specified in the Unicode standard. Such values are often called ``code points''. The Unicode standard refers to code points as ``coded character data elements''. Throughout this International standard the terms ``character'' and ``code point'' are understood to mean ``coded character data element''.

NOTE 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.

7 -- Lexical Conventions

The source text of an ECMAScript program is first converted into a sequence of input elements, which are either 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.

Syntax

InputElement ::
WhiteSpace
LineTerminator
Comment
Token

7.1 -- White Space

White space characters are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space may occur between any two tokens, and may occur within strings (where they are considered significant characters forming part of the literal string value), but cannot appear within any other kind of token.

The following characters are considered to be white space:
Unicode Value Name Formal Name
\u0009 Tab <TAB>
\u000B Vertical Tab <VT>
\u000C Form Feed <FF>
\u0020 Space <SP>

Syntax

WhiteSpace ::
<TAB>
<VT>
<FF>
<SP>

7.2 -- Line Terminators

Like whitespace characters, line terminator characters are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike whitespace 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. A line terminator cannot occur within any token, not even a string. Line terminators also affect the process of automatic semicolon insertion (see section 7.8).

The following characters are considered to be line terminators:
Unicode Value Name Formal Name
\u000A Line Feed <LF>
\u000D Carriage Return <CR>

Syntax

LineTerminator ::
<LF>
<CR>

7.3 -- Comments

Description

Comments can be either single or multi-line. Multi-line comments cannot nest.

Because a single-line comment can contain any character except a LineTerminator character, and because of the general rule that a token is always as long as possible, a single-line comment always consists of all characters from the // marker to the end of the line. However, the LineTerminator at the end of the line is not considered to be part of the single-line comment; it is recognised separately by the lexical grammar and becomes part of the stream of input elements for the syntactic grammar. This point is very important, because it implies that the presence or absence of single-line comments does not affect the process of automatic semicolon insertion (see section7.8.2).

Comments behave like white space and are discarded except that, if a MultiLineComment contains a line terminator character, then the entire comment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.

Syntax

Comment ::
MultiLineComment
SingleLineComment
MultiLineComment ::
/* MultiLineCommentCharsopt */
MultiLineCommentChars ::
MultiLineNotAsteriskChar MultiLineCommentCharsopt
* PostAsteriskCommentCharsopt
PostAsteriskCommentChars ::
MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentCharsopt
* PostAsteriskCommentCharsopt
MultiLineNotAsteriskChar ::
SourceCharacter but not asterisk *
MultiLineNotForwardSlashOrAsteriskChar ::
SourceCharacter but neither forward-slash / nor asterisk *
SingleLineComment ::
// SingleLineCommentCharsopt
SingleLineCommentChars ::
SingleLineCommentChar SingleLineCommentCharsopt
SingleLineCommentChar ::
SourceCharacter but not LineTerminator

7.4 -- Tokens

Syntax

Token ::
ReservedWord
Identifier
Punctuator
NumericLiteral
StringLiteral

7.4.1 -- Reserved Words

Description

Reserved words cannot be used as identifiers.

Syntax

ReservedWord ::
Keyword
FutureReservedWord
NullLiteral
BooleanLiteral

7.4.2 -- Keywords

The following tokens are ECMAScript keywords and may not be used as identifiers in ECMAScript programs.

Syntax

Keyword :: one of
break for new var
continue function return void
delete if this while
else in typeof with

7.4.3 -- Future Reserved Words

The following words are used as keywords in proposed extensions and are therefore reserved to allow for the possibility of future adoption of those extensions.

Syntax

FutureReservedWord :: one of
abstract do import short
boolean double instanceof static
byte enum int super
case export interface switch
catch extends long synchronized
char final native throw
class finally package throws
const float private transient
debugger goto protected try
default implements public volatile

7.5 -- Identifiers

Description

An identifier is a character sequence of unlimited length, where each character in the sequence must be a letter, a decimal digit, an underscore (_) character, or a dollar sign ($) character, and the first character may not be a decimal digit. ECMAScript identifiers are case sensitive: identifiers whose characters differ in any way, even if only in case, are considered to be distinct. The dollar sign ($) character is intended for use only in mechanically generated code.

Syntax

Identifier ::
IdentifierName but not ReservedWord
IdentifierName ::
IdentifierLetter
IdentifierName IdentifierLetter
IdentifierName DecimalDigit
IdentifierLetter :: one of
a b c d e f g h i j k l m n o p q r s t u v w x y z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
$ _                                                
DecimalDigit :: one of
0 1 2 3 4 5 6 7 8 9

7.6 -- Punctuators

Syntax

Punctuator :: one of
= > < == <= >=
!= , ! ~ ? :
. && || ++ -- +
- * / & | ^
% << >> >>> += -=
*= /= &= |= ^= %=
<<= >>= >>>= ( ) {
} [ ] ;    

7.7 -- Literals

Syntax

Literal ::
NullLiteral
BooleanLiteral
NumericLiteral
StringLiteral

7.7.1 -- Null Literals

Syntax

NullLiteral ::
null

Semantics

The value of the null literal null is the sole value of the Null type, namely null.

7.7.2 -- Boolean Literals

Syntax

BooleanLiteral ::
true
false

Semantics

The value of the Boolean literal true is a value of the Boolean type, namely true.

The value of the Boolean literal false is a value of the Boolean type, namely false.

7.7.3 -- Numeric Literals

Syntax

NumericLiteral ::
DecimalLiteral
HexIntegerLiteral
OctalIntegerLiteral
DecimalLiteral ::
DecimalIntegerLiteral . DecimalDigitsopt ExponentPartopt
. DecimalDigits ExponentPartopt
DecimalIntegerLiteral ExponentPartopt
DecimalIntegerLiteral ::
0
NonZeroDigit DecimalDigitsopt
DecimalDigits ::
DecimalDigit
DecimalDigits DecimalDigit
NonZeroDigit :: one of
1 2 3 4 5 6 7 8 9
ExponentPart ::
ExponentIndicator SignedInteger
ExponentIndicator :: one of
e E
SignedInteger ::
DecimalDigits
+ DecimalDigits
- DecimalDigits
HexIntegerLiteral ::
0x HexDigit
0X HexDigit
HexIntegerLiteral HexDigit
HexDigit :: one of
0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F
OctalIntegerLiteral ::
0 OctalDigit
OctalIntegerLiteral OctalDigit
OctalDigit :: one of
0 1 2 3 4 5 6 7

Semantics

A numeric literal stands for a value of the Number type. This value is determined in two steps: first, a mathematical value (MV) is derived from the literal; second, if this mathematical value is not representable using the number type, it is rounded to either the nearest representable value type above the mathematical value or the nearest representable value below the mathematical value.

The rounding mechanism is unspecified, but implementations are encouraged to use IEEE 754 round-to-nearest.

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 (in the sense defined in section8.5), 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

7.7.4 -- String Literals

A string literal is zero or more characters enclosed in single or double quotes. Each character may be represented by an escape sequence.

Syntax

StringLiteral ::
" DoubleStringCharactersopt "
' SingleStringCharactersopt '
DoubleStringCharacters ::
DoubleStringCharacter DoubleStringCharactersopt
SingleStringCharacters ::
SingleStringCharacter SingleStringCharactersopt
DoubleStringCharacter ::
SourceCharacter but not double-quote " or backslash \ or LineTerminator
EscapeSequence
SingleStringCharacter ::
SourceCharacter but not single-quote ' or backslash \ or LineTerminator
EscapeSequence
EscapeSequence ::
CharacterEscapeSequence
OctalEscapeSequence
HexEscapeSequence
UnicodeEscapeSequence
CharacterEscapeSequence ::
\ SingleEscapeCharacter
\ NonEscapeCharacter
SingleEscapeCharacter :: one of
' " \ b f n r t
NonEscapeCharacter ::
SourceCharacter but not EscapeCharacter or LineTerminator
EscapeCharacter ::
SingleEscapeCharacter
OctalDigit
x
u
HexEscapeSequence ::
\x HexDigit HexDigit
OctalEscapeSequence ::
\ OctalDigit
\ OctalDigit OctalDigit
\ ZeroToThree OctalDigit OctalDigit
ZeroToThree :: one of
0 1 2 3
UnicodeEscapeSequence ::
\u HexDigit HexDigit HexDigit HexDigit

The definitions of the nonterminals HexDigit and OctalDigit are given in section 7.7.3. SourceCharacter is described in sections 2 and 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 section 7.7.3.

NOTE A LineTerminator character cannot appear in a string literal, even if preceded by a backslash \. The correct way to cause a line terminator character to be part of the string value of a string literal is to use an escape sequence such as \n or \u000A.

7.8 -- Automatic semicolon insertion

Certain ECMAScript statements (empty statement, variable statement, expression statement, continue statement, break statement, and return statement) must each be terminated with a semicolon. Such a semicolon may always appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source text in certain situations. These situations are described by saying that semicolons are automatically inserted into the source code token stream in those situations.

7.8.1 -- Rules of automatic semicolon insertion

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.

The practical effect of these restricted productions is as follows:

  1. When the token ++ or -- 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.
  2. When the token return is encountered and a LineTerminator is encountered before the next token is encountered, a semicolon is automatically inserted after the token return.

The resulting practical advice to ECMAScript programmers is:

  1. A postfix ++ or -- operator should appear on the same line as its operand.
  2. An Expression in a return statement should start on the same line as the return token.

7.8.2 -- Examples of Automatic Semicolon Insertion

The source 

	{ 1 2 } 3
is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source 
	{ 1
	2 } 3
is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following: 
	{ 1
	;2 ;} 3;
which is a valid ECMAScript sentence.

The source 

	for (a; b
	)
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the place where a semicolon is needed is within the header of a for statement. Automatic semicolon insertion never occurs within the header of a for statement.

The source 

	return
	a + b
is transformed by automatic semicolon insertion into the following: 
	return;
	a + b;

NOTE The expression a + b is not treated as a value to be returned by the return statement, because a LineTerminator separates it from the token return.

The source 

	a = b
	++c
is transformed by automatic semicolon insertion into the following: 
	a = b;
	++c;
NOTE The token ++ is not treated as a postfix operator applying to the variable b, because a LineTerminator occurs between b and ++.

The source 

	if (a > b)
	else c = d
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the else token, even though no production of the grammar applies at that point, because an automatically inserted semicolon would then be parsed as an empty statement.

The source 

	a = b + c
	(d + e).print()
is not transformed by automatic semicolon insertion, because the parenthesised expression that begins the second line can be interpreted as an argument list for a function call: 
	a = b + c(d + e).print()
In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on automatic semicolon insertion.

8 -- Types

A value is an entity that takes on one of nine types. There are nine types ( Undefined, Null, Boolean, String, Number, Object, Reference, List, and Completion ). Values of type Reference, List, and Completion are used only as intermediate results of expression evaluation and cannot be stored to properties of objects.

8.1 -- The Undefined type

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

8.2 -- The Null type

The Null type has exactly one value, called null.

8.3 -- The Boolean type

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

8.4 -- The String type

The String type is the set of all finite ordered sequences of zero or more Unicode characters (more properly referred to as code points; see section 6). Each character is regarded as occupying a position within the sequence. These positions are identified by nonnegative integers. The leftmost character (if any) is at position 0, the next character (if any) at position 1, and so on. The length of a string is the number of distinct positions within it. The empty string has length zero and therefore contains no characters.

8.5 -- The Number type

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, assuming that the globally defined variable NaN has not been altered by program execution.) In some implementations, external code might be able to detect a difference between various Non-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, assuming that the globally defined variable Infinity has not been altered by program execution.)

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 there is a corresponding negative number 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 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 denormalized, having the form
s * m * 2e
where s is +1 or -1, m is a positive integer less than 252, and e is -1074.

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

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

In this specification, the phrase ``the number value for x'' where x represents an exact nonzero real mathematical quantity (which might even be an irrational number such as π) means a number value chosen in the following manner. Consider the set of all finite values of the Number type, with -0 removed and with two additional values added to it that are not representable in the Number type, namely 21024 (which is +1 * 253 * 2971) and -21024 (which is -1 * 253 * 2971). Choose the member of this set that is closest in value to x. If two values of the set are equally close, then the one with an even significand is chosen; for this purpose, the two extra values 21024 and -21024 are considered to have even significands. Finally, if 21024 was chosen, replace it with +∞; if -21024 was chosen, replace it with -∞; if +0 was chosen, replace it with -0 if and only if x is less than zero; any other chosen value is used unchanged. The result is the number value for x. (This procedure corresponds exactly to the behaviour of the IEEE 754 ``round to nearest'' mode.)

Some ECMAScript operators deal only with integers in the range -231 through 231-1, inclusive, or in the range 0 through 232-1, inclusive. These operators accept any value of the Number type but first convert each such value to one of 232 integer values. See the descriptions of the ToInt32 and ToUint32 operators in sections 9.5 and 9.6, respectively.

8.6 -- The Object type

An Object is an unordered collection of properties. Each property consists of a name, a value and a set of attributes.

8.6.1 -- Property attributes

A property can have zero or more attributes from the following set:
Attribute Description
ReadOnly

The property is a read-only property. Attempts by ECMAScript code to write to the property will be ignored. (Note, however, that in some cases the value of a property with the ReadOnly attribute may change over time because of actions taken by the underlying implementation; therefore ``ReadOnly'' does not mean ``constant and unchanging''!).

DontEnum

The property is not to be enumerated by a for-in enumeration (section 12.6.3).

DontDelete

Attempts to delete the property will be ignored. See the description of the delete operator in section 11.4.1.

Internal

Internal properties have no name and are not directly accessible via the property accessor operators. How these properties are accessed is implementation specific. How and when some of these properties are used is specified by the language specification.

8.6.2 -- Internal Properties and Methods

Internal properties and methods are not exposed in the language. For the purposes of this document, their names 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 runtime error is generated.

There are two types of access for exposed properties: get and put, corresponding to retrieval and assignment, respectively.

Native ECMAScript objects have an internal property called [[Prototype]]. The value of this property is either null or an object and is used for implementing inheritance. Properties of the [[Prototype]] object are exposed as properties of the child object for the purposes of get access, but not for put access.

The following table summarises the internal properties used by this specification. The description indicates their behaviour for native ECMAScript objects. Host objects may implement these internal methods with any implementation-dependent behaviour, or it may be that a host object implements only some internal methods and not others.

Property Parameters Description
[[Prototype]] none

The prototype of this object.

[[Class]] none

A string value indicating the kind of this object.

[[Value]] none

Internal state information associated with this object.

[[Get]] (PropertyName)

Returns the value of the property.

[[Put]] (PropertyName, Value)

Sets the specified property to Value.

[[CanPut]] (PropertyName)

Returns a boolean value indicating whether a [[Put]] operation with the specified PropertyName will succeed.

[HasProperty]] (PropertyName)

Returns a boolean value indicating whether the object already has a member with the given name.

[[Delete]] (PropertyName)

Removes the specified property from the object.

[[DefaultValue]] (Hint)

Returns a default value for the object, which should be a primitive value (not an object or reference).

[[Construct]] a list of argument values provided by the caller

Constructs an object. Invoked via the new operator. Objects that implement this internal method are called constructors.

[[Call]] a list of argument values provided by the caller

Executes code associated with the object. Invoked via a function call expression. Objects that implement this internal method are called functions.

Every object must implement the [[Class]] property and the [[Get]], [[Put]], [[HasProperty]], [[Delete]], and [[DefaultValue]] methods, even host objects. (Note, however, that the [[DefaultValue]] method may, for some objects, simply generate a runtime error.)

The value of the [[Prototype]] property must be either an object or null, and every [[Prototype]] chain must have finite length (that is, starting from any object, recursively accessing the [[Prototype]] property must eventually lead to a null value). Whether or not a native object can have a host object as its [[Prototype]] depends on the implementation.

The value of the [[Class]] property is defined by this specification for every kind of built-in object. The value of the [[Class]] property of a host object may be any value, even a value used by a built-in object for its [[Class]] property. Note that this specification does not provide any means for a program to access the value of a [[Class]] property; that value is used internally to distinguish different kinds of built-in objects.

Every native object implements the [[Get]], [[Put]], [[CanPut]], [[HasProperty]], and [[Delete]] methods in the manner described in sections 8.6.2.1, 8.6.2.2, 8.6.2.3, 8.6.3.4, and 8.6.2.5, respectively, except that Array objects have a slightly different implementation of the [[Put]] method (section 15.4.5.1). Host objects may implement these methods in any manner; 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.

In the following algorithm descriptions, assume O is a native ECMAScript object and P is a string.

8.6.2.1 -- [[Get]](P)

When the [[Get]] method of O is called with property name P, the following steps are taken:

  1. If O doesn't have a property with name P, go to step 4.
  2. Get the value of the property.
  3. Return Result(2).
  4. If the [[Prototype]] of O is null, return undefined.
  5. Call the [[Get]] method of [[Prototype]] with property name P.
  6. Return Result(5).
8.6.2.2 -- [[Put]](P, V)

When the [[Put]] method of O is called with property P and value V, the following steps are taken:

  1. Call the [[CanPut]] method of O with name P.
  2. If Result(1) is false, return.
  3. If O doesn't have a property with name P, go to step 6.
  4. Set the value of the property to V. The attributes of the property are not changed.
  5. Return.
  6. Create a property with name P, set its value to V and give it empty attributes.
  7. Return.

Note, however, that if O is an Array object, it has a more elaborate [[Put]] method (section 15.4.5.1).

8.6.2.3 -- [[CanPut]](P)

The [[CanPut]] method is used only by the [[Put]] method.

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

  1. If O doesn't have a property with name P, go to step 4.
  2. If the property has the ReadOnly attribute, return false.
  3. Return true.
  4. If the [[Prototype]] of O is null, return true.
  5. If the [[Prototype]] of O is a host object that does not implement the [[CanPut]] method, return false.
  6. Call the [[CanPut]] method of [[Prototype]] of O with property name P.
  7. Return Result(6).
8.6.2.4 -- [[HasProperty]](P)
  1. When the [[HasProperty]] method of O is called with property name P, the following steps are taken:
  2. If O has a property with name P, return true.
  3. If the [[Prototype]] of O is null, return false.
  4. Call the [[HasProperty]] method of [[Prototype]] with property name P.
  5. Return Result(3).
8.6.2.5 -- [[Delete]](P)

When the [[Delete]] method of O is called with property name P, the following steps are taken:

  1. If O doesn't have a property with name P, return true.
  2. If the property has the DontDelete attribute, return false.
  3. Remove the property with name P from O.
  4. Return true.
8.6.2.6 -- [[DefaultValue]](hint)

When the [[DefaultValue]] method of O is called with hint String, the following steps are taken:

  1. Call the [[Get]] method of object O with argument "toString".
  2. If Result(1) is not an object, go to step 5.
  3. Call the [[Call]] method of Result(1), with O as the this value and an empty argument list.
  4. If Result(3) is a primitive value, return Result(3).
  5. Call the [[Get]] method of object O with argument "valueOf".
  6. If Result(5) is not an object, go to step 9.
  7. Call the [[Call]] method of Result(5), with O as the this value and an empty argument list.
  8. If Result(7) is a primitive value, return Result(7).
  9. Generate a runtime error.

When the [[DefaultValue]] method of O is called with hint Number, the following steps are taken:

  1. Call the [[Get]] method of object O with argument "valueOf".
  2. If Result(1) is not an object, go to step 5.
  3. Call the [[Call]] method of Result(1), with O as the this value and an empty argument list.
  4. If Result(3) is a primitive value, return Result(3).
  5. Call the [[Get]] method of object O with argument "toString".
  6. If Result(5) is not an object, go to step 9.
  7. Call the [[Call]] method of Result(5), with O as the this value and an empty argument list.
  8. If Result(7) is a primitive value, return Result(7).
  9. Generate a runtime error.

When the [[DefaultValue]] method of O is called with no hint, then it behaves as if the hint were Number, unless O is a Date object (see section  ), in which case it behaves as if the hint were String.

8.7 -- The Reference Type

The internal Reference type is not a language data type. It is defined by this specification purely for expository purposes. An implementation of ECMAScript must behave as if it produced and operated upon references in the manner described here. However, a value of type Reference is used only as an intermediate result of expression evaluation and cannot be stored as the value of a variable or property.

The Reference type is used to explain the behaviour of such operators as delete, typeof, and the assignment operators. For example, the left-hand operand of an assignment is expected to produce a reference. The behaviour of assignment could, instead, be explained entirely in terms of a case analysis on the syntactic form of the left-hand operand of an assignment operator, but for one difficulty: function calls are permitted to return references. This possibility is admitted purely for the sake of host objects. No built-in ECMAScript function defined by this specification returns a reference and there is no provision for a user-defined function to return a reference. (Another reason not to use a syntactic case analysis is that it would be lengthy and awkward, affecting many parts of the specification.)

Another use of the Reference type is to explain the determination of the this value for a function call.

A Reference is a reference to a property of an object. A Reference consists of two components, the base object and the property name.

The following abstract operations are used in this specification to describe the behaviour of references:

8.7.1 -- GetBase(V)

  1. If Type(V) is Reference, return the base object component of V.
  2. Generate a runtime error.

8.7.2 -- GetPropertyName(V)

  1. If Type(V) is Reference, return the property name component of V.
  2. Generate a runtime error.

8.7.3 -- GetValue(V)

  1. If Type(V) is not Reference, return V.
  2. Call GetBase(V).
  3. If Result(2) is null, generate a runtime error.
  4. Call the [[Get]] method of Result(2), passing GetPropertyName(V) for the property name.
  5. Return Result(4).

8.7.4 -- PutValue(V, W)

  1. If Type(V) is not Reference, generate a runtime error.
  2. Call GetBase(V).
  3. If Result(2) is null, go to step 6.
  4. Call the [[Put]] method of Result(2), passing GetPropertyName(V) for the property name and W for the value.
  5. Return.
  6. Call the [[Put]] method for the global object, passing GetPropertyName(V) for the property name and W for the value.
  7. Return.

8.8 -- The List type

The internal List type is not a language data type. It is defined by this specification purely for expository purposes. An implementation of ECMAScript must behave as if it produced and operated upon List values in the manner described here. However, a value of the List type is used only as an intermediate result of expression evaluation and cannot be stored as the value of a variable or property.

The List type is used to explain the evaluation of argument lists (section 11.2.4) in new expressions and in function calls. Values of the List type are simply ordered sequences of values. These sequences may be of any length.

8.9 -- The Completion Type

The internal Completion type is not a language data type. It is defined by this specification purely for expository purposes. An implementation of ECMAScript must behave as if it produced and operated upon Completion values in the manner described here. However, a value of the Completion type is used only as an intermediate result of statement evaluation and cannot be stored as the value of a variable or property.

The Completion type is used to explain the behaviour of statements (break, continue, and return) that perform nonlocal transfers of control. Values of the Completion type have one of the following forms:

Any completion of one of the four forms that carries a value V is called a value completion. Any completion of one of the first two forms is called a normal completion; any other completion is called an abrupt completion. Any completion of a form that mentions break is called a break completion. Any completion of a form that mentions continue is called a continue completion. Any completion of a form that mentions return is called a return completion.

9 -- Type Conversion

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 operators. These operators are not a part of the language; they are defined here to aid the specification of the semantics of the language. The conversion operators are polymorphic; that is, they can accept a value of any standard type, but not of type Reference, List, or Completion (the internal types).

9.1 -- ToPrimitive

The operator ToPrimitive takes a Value argument and an optional PreferredType argument. The operator ToPrimitive converts its value 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 the following table:

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 internal [[DefaultValue]] method of the object, passing the optional hint PreferredType. The behaviour of the [[DefaultValue]] method is defined by this specification for all native ECMAScript objects (see section 8.6.2.6). If the return value is of type Object or Reference, a runtime error is generated.

9.2 -- ToBoolean

The operator ToBoolean converts its argument to a value of type Boolean according to the following table:

Input Type Result
Undefined false
Null false
Boolean

The result equals the input argument (no conversion).

Number

The result is false if the argument is +0, -0, or NaN; otherwise the result is true.

String

The result is false if the argument is the empty string (its length is zero); otherwise the result is true.

Object true

9.3 -- ToNumber

The operator ToNumber converts its argument to a value of type Number according to the following table:

Input 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:

  1. Call ToPrimitive(input argument, hint Number).
  2. Call ToNumber(Result(1)).
  3. Return Result(2).

9.3.1 -- ToNumber Applied to the String Type

ToNumber applied to strings applies the following grammar to the input string. If the grammar cannot interpret the string as an expansion of StringNumericLiteral, then the result of ToNumber is NaN.

StringNumericLiteral :::
StrWhiteSpaceopt
StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt
StrWhiteSpace :::
StrWhiteSpaceChar StrWhiteSpaceopt
StrWhiteSpaceChar :::
<TAB>
<SP>
<FF>
<VT>
<CR>
<LF>
StrNumericLiteral :::
StrDecimalLiteral
+ StrDecimalLiteral
- StrDecimalLiteral
HexIntegerLiteral
StrDecimalLiteral :::
Infinity
DecimalDigits . DecimalDigitsopt ExponentPartopt
. DecimalDigits ExponentPartopt
DecimalDigits ExponentPartopt
DecimalDigits :::
DecimalDigit
DecimalDigits DecimalDigit
DecimalDigit ::: one of
0 1 2 3 4 5 6 7 8 9
ExponentPart :::
ExponentIndicator SignedInteger
ExponentIndicator ::: one of
e E
SignedInteger :::
DecimalDigits
+ DecimalDigits
- DecimalDigits
HexIntegerLiteral :::
0x HexDigit
0X HexDigit
HexIntegerLiteral HexDigit
HexDigit ::: one of
0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

Some differences should be noted between the syntax of a StringNumericLiteral and a NumericLiteral (section 7.7.3):

The conversion of a string to a number value is similar overall to the determination of the number value for a numeric literal (section 7.7.3), 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, ideally using IEEE 754 round-to-nearest mode, to a representable value of the number type.

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-whitespace 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 section 8.5), unless the literal includes a StrDecimalLiteral 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 (either it is not 0 or (there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right)).

9.4 -- ToInteger

The operator ToInteger converts its argument to an integral numeric value. This operator functions as follows:

  1. Call ToNumber on the input argument.
  2. If Result(1) is NaN, return +0.
  3. If Result(1) is +0, -0, +∞ , or -∞ , return Result(1).
  4. Compute sign(Result(1)) * floor(abs(Result(1))).
  5. Return Result(4).

9.5 -- ToInt32: (signed 32 bit integer)

The operator ToInt32 converts its argument to one of 232 integer values in the range -231 through 231-1 , inclusive. This operator functions as follows:

  1. Call ToNumber on the input argument.
  2. If Result(1) is NaN, +0, -0, +∞, or -∞ , return +0.
  3. Compute sign(Result(1)) * floor(abs(Result(1))).
  4. Compute Result(3) modulo 232; that is, a finite integer value k of Number type with positive sign and less than 232 in magnitude such the mathematical difference of Result(3) and k is mathematically an integer multiple of 232.
  5. If Result(4) is greater than or equal to 231, return Result(4) - 232, otherwise return Result(4).
NOTE Given the above definition of ToInt32:

The ToInt32 operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.

ToInt32(ToUint32(x)) is equal to ToInt32(x) for all values of x. (It is to preserve this latter property that +∞ and -∞ are mapped to +0.)

ToInt32 maps -0 to +0.

9.6 -- ToUint32: (unsigned 32 bit integer)

The operator ToUint32 converts its argument to one of 232 integer values in the range 0 through 232-1, inclusive. This operator functions as follows:

  1. Call ToNumber on the input argument.
  2. If Result(1) is NaN, +0, -0, +∞ , or -∞ , return +0.
  3. Compute sign(Result(1)) * floor(abs(Result(1))).
  4. Compute Result(3) modulo 232; that is, a finite integer value k of Number type with positive sign and less than 232 in magnitude such the mathematical difference of Result(3) and k is mathematically an integer multiple of 232.
  5. Return Result(4).
NOTE Given the above definition of ToUInt32::

Step 5 is the only difference between ToUint32 and ToInt32.

The ToUint32 operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.

ToUint32(ToInt32(x)) is equal to ToUint32(x) for all values of x. (It is to preserve this latter property that +∞ and -∞ are mapped to +0.)

ToUint32 maps -0 to +0.

9.7 -- ToUint16: (unsigned 16 bit integer)

The operator ToUint16 converts its argument to one of 216 integer values in the range 0 through 216-1, inclusive. This operator functions as follows:

  1. Call ToNumber on the input argument.
  2. If Result(1) is NaN, +0, -0, +∞ , or -∞ , return +0.
  3. Compute sign(Result(1)) * floor(abs(Result(1))).
  4. Compute Result(3) modulo 216; that is, a finite integer value k of Number type with positive sign and less than 216 in magnitude such the mathematical difference of Result(3) and k is mathematically an integer multiple of 216.
  5. Return Result(4).
NOTE Given the above definition of ToUInt16::

The substitution of 216 for 232 in step 4 is the only difference between ToUint32 and ToUnit16.

ToUint16 maps -0 to +0.

9.8 -- ToString

The operator ToString converts its argument to a value of type String according to the following table:

Input 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 note below.
String

Return the input argument (no conversion)

Object

Apply the following steps:

  1. Call ToPrimitive(input argument, hint String).
  2. Call ToString(Result(1)).
  3. Return Result(2).

9.8.1 -- ToString Applied to the Number Type

The operator ToString converts a number m to string format as follows:

  1. If m is NaN, return the string "NaN".
  2. If m is +0 or -0, return the string "0".
  3. If m is less than zero, return the string concatenation of the string "-" and ToString(-m).
  4. If m is infinity, return the string "Infinity".
  5. Otherwise, let n, k, and s be integers such that k = 1, 10k-1 = s < 10k, the number value for s * 10n - k is m, and k is as small as possible. Note that k is the number of digits in the decimal representation of s, that s is not divisible by 10, and that the least significant digit of s is not necessarily uniquely determined by these criteria.
  6. If k = n = 21, return the string consisting of the k digits of the decimal representation of s (in order, with no leading zeroes), followed by n - k occurrences of the character `0'
  7. If 0 < n = 21, return the string consisting of the most significant n digits of the decimal representation of s, followed by a decimal point `.', followed by the remaining k - n digits of the decimal representation of s.
  8. If -6 < n = 0, return the string consisting of the character `0', followed by a decimal point `.', followed by -n occurrences of the character `0', followed by the k digits of the decimal representation of s.
  9. Otherwise, if k = 1, return the string consisting of the single digit of s, followed by lowercase character `e', followed by a plus sign `+' or minus sign `-' according to whether n-1 is positive or negative, followed by the decimal representation of the integer abs(n-1) (with no leading zeros).
  10. Return the string consisting of the most significant digit of the decimal representation of s, followed by a decimal point `.', followed by the remaining k-1 digits of the decimal representation of s, followed by the lowercase character `e', followed by a plus sign `+' or minus sign `-' according to whether n-1 is positive or negative, followed by the decimal representation of the integer abs(n-1) (with no leading zeros).
NOTE The following observations may be useful as guidelines for implementations, but are not part of the normative requirements of this standard.

If x is any number value other than -0, then ToNumber(ToString(x)) is exactly the same number value as x.

The least significant digit of s is not always uniquely determined by the requirements listed in step 5.

For implementations which 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 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. Note that k is the number of digits in the decimal representation of s, that s is not divisible by 10. If there are two such possible values of s, choose the one that is even.

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

Gay, David M. Correctly Rounded Binary-Decimal and Decimal-Binary Conversions. Numerical Analysis -- Manuscript 90-10. AT&T Bell Laboratories (Murray Hill, New Jersey). November 30, 1990. Available as http://cm.bell-labs.com/cm/cs/doc/90/4-10.ps.gz. Associated code available as http://cm.bell-labs.com/netlib/fp/dtoa.c.gz and as http://cm.bell-labs.com/netlib/fp/g_fmt.c.gz and may also be found at the various netlib mirror sites.

9.9 -- ToObject

The operator ToObject converts its argument to a value of type Object according to the following table:

Input Type Result
Undefined Generate a runtime error.
Null Generate a runtime error.
Boolean

Create a new boolean object whose default value is the value of the boolean. See section 15.6 for a description of boolean objects.

Number

Create a new number object whose default value is the value of the number. See section 15.7 for a description of number objects.

String

Create a new string object whose default value is the value of the string. See section 15.5 for a description of string objects.

Object

The result is the input argument (no conversion).

10 -- Execution Contexts

When control is transferred to ECMAScript executable code, control is entering an execution context. Active execution contexts logically form a stack. The top execution context on this logical stack is the running execution context.

10.1 -- Definitions

10.1.1 -- Function Objects

There are four types of function objects:

10.1.2 -- Types of Executable Code

There are five types of executable ECMAScript source text:

10.1.3 -- Variable instantiation

Every execution context has associated with it a variable object. Variables declared in the source text are added as properties of the variable object. For global and eval code, functions defined in the source text are added as properties of the variable object. Function declarations in other types of code are not allowed by the grammar. For function, anonymous, and implementation-supplied code, parameters are added as properties of the variable object.

Which object is used as the variable object and what attributes are used for the properties depends on the type of code, but the remainder of the behaviour is generic:

10.1.4 -- Scope Chain and Identifier Resolution

Every execution context has associated with it a scope chain. This is logically a list of objects that are searched when binding an Identifier. When control enters an execution context, the scope chain is created and is populated with an initial set of objects, depending on the type of code. When control leaves the execution context, the scope chain is destroyed.

During execution, the scope chain of the execution context is affected only by WithStatement. When execution enters a with block, the object specified in the with statement is added to the front of the scope chain. When execution leaves a with block, whether normally or via a break or continue statement, the object is removed from the scope chain. The object being removed will always be the first object in the scope chain.

During execution, the syntactic production PrimaryExpression : Identifier is evaluated using the following algorithm:

  1. Get the next object in the scope chain. If there isn't one, go to step 5.
  2. Call the [[HasProperty]] method of Result(l), passing the Identifier as the property.
  3. If Result(2) is true, return a value of type Reference whose base object is Result(l) and whose property name is the Identifier.
  4. Go to step 1.
  5. Return a value of type Reference whose base object is null and whose property name is the Identifier.

The result of binding an identifier is always a value of type Reference with its member name component equal to the identifier string.

10.1.5 -- Global Object

There is a unique global object, which is created before control enters any execution context. Initially the global object has the following properties:

As control enters execution contexts, and as ECMAScript code is executed, additional properties may be added to the global object and the initial properties may be changed.

10.1.6 -- Activation object

When control enters an execution context for declared function code, anonymous code or implementation-supplied code, an object called the activation object is created and associated with the execution context. The activation object is initialised with a property with name arguments and property attributes { DontDelete }. The initial value of this property is the arguments object described below.

The activation object is then used as the variable object for the purposes of variable instantiation.

When a value is to be returned from the call to a function, its activation object is no longer needed and may be permanently decommissioned.

The activation object is purely a specification mechanism. It is impossible for an ECMAScript program to access the activation object. It can access members of the activation object, but not the activation object itself. When the call operation is applied to a Reference value whose base object is an activation object, null is used as the this value of the call.

10.1.7 -- This

There is a this value associated with every active execution context. The this value depends on the caller and the type of code being executed and is determined when control enters the execution context. The this value associated with an execution context is immutable.

10.1.8 -- Arguments Object

When control enters an execution context for declared function code, anonymous code, or implementation-supplied code, an arguments object is created and initialised as follows:

10.2 -- Entering An Execution Context

When control enters an execution context, the scope chain is created and initialised, variable instantiation is performed, and the this value is determined.

The initialisation of the scope chain, variable instantiation, and the determination of the this value depend on the type of code being entered.

10.2.1 -- Global Code

10.2.2 -- Eval Code

When control enters an execution context for eval code, the previous active execution context, referred to as the calling context, is used to determine the scope chain, the variable object, and the this value. If there is no calling context, then initialising the scope chain, variable instantiation, and determination of the this value are performed just as for global code.

10.2.3 -- Function and Anonymous Code

10.2.4 -- Implementation-supplied Code

11 -- Expressions

11.1 -- Primary Expressions

Syntax

PrimaryExpression :
this
Identifier
Literal
( Expression )

11.1.1 -- The this keyword

The this keyword evaluates to the this value of the execution context.

11.1.2 -- Identifier reference

An Identifier is evaluated using the scoping rules stated in section 10.1.4. The result of an Identifier is always a value of type Reference.

11.1.3 -- Literal reference

A Literal is evaluated as described in section 7.7.

11.1.4 -- The Grouping Operator

The production PrimaryExpression : ( Expression ) is evaluated as follows:

  1. Evaluate Expression. This may be of type Reference.
  2. Return Result(1).
NOTE This algorithm does not apply GetValue to Result(1). The principal motivation for this is so that operators such as delete and typeof may be applied to parenthesised expressions.

11.2 -- Left-Hand-Side Expressions

Syntax

MemberExpression :
PrimaryExpression
MemberExpression [ Expression ]
MemberExpression . Identifier
new MemberExpression Arguments
NewExpression :
MemberExpression
new NewExpression
CallExpression :
MemberExpression Arguments
CallExpression Arguments
CallExpression [ Expression ]
CallExpression . Identifier
Arguments :
()
( ArgumentList )
ArgumentList :
AssignmentExpression
ArgumentList , AssignmentExpression
LeftHandSideExpression :
NewExpression
CallExpression

11.2.1 -- Property Accessors

Properties are accessed by name, using either the dot notation:

MemberExpression . Identifier
CallExpression . Identifier
or the bracket notation:
MemberExpression [ Expression ]
CallExpression [ Expression ]

The dot notation is explained by the following syntactic conversion:

MemberExpression . Identifier
is identical in its behaviour to
MemberExpression [ <identifier-string> ]
and similarly
CallExpression . Identifier
is identical in its behaviour to
CallExpression [ <identifier-string> ]
where <identifier-string> is a string literal containing the same sequence of characters as the Identifier.

The production MemberExpression : MemberExpression [ Expression ] is evaluated as follows:

  1. Evaluate MemberExpression.
  2. Call GetValue(Result(1)).
  3. Evaluate Expression.
  4. Call GetValue(Result(3)).
  5. Call ToObject(Result(2)).
  6. Call ToString(Result(4)).
  7. Return a value of type Reference whose base object is Result(5) and whose property name is Result(6).

The production CallExpression : CallExpression [ Expression ] is evaluated in exactly the same manner, except that the contained CallExpression is evaluated in step 1.

11.2.2 -- The new operator

The production NewExpression : new NewExpression is evaluated as follows:

  1. Evaluate NewExpression.
  2. Call GetValue(Result(1)).
  3. If Type(Result(2)) is not Object, generate a runtime error.
  4. If Result(2) does not implement the internal [[Construct]] method, generate a runtime error.
  5. Call the [[Construct]] method on Result(2), providing no arguments (that is, an empty list of arguments).
  6. If Type(Result(5)) is not Object, generate a runtime error.
  7. Return Result(5).

The production MemberExpression : new MemberExpression Arguments is evaluated as follows:

  1. Evaluate MemberExpression.
  2. Call GetValue(Result(1)).
  3. Evaluate Arguments, producing an internal list of argument values (section 11.2.4).
  4. If Type(Result(2)) is not Object, generate a runtime error.
  5. If Result(2) does not implement the internal [[Construct]] method, generate a runtime error.
  6. Call the [[Construct]] method on Result(2), providing the list Result(3) as the argument values.
  7. If Type(Result(6)) is not Object, generate a runtime error.
  8. Return Result(6).

11.2.3 -- Function Calls

The production CallExpression : MemberExpression Arguments is evaluated as follows:

  1. Evaluate MemberExpression.
  2. Evaluate Arguments, producing an internal list of argument values (section 11.2.4).
  3. Call GetValue(Result(1)).
  4. If Type(Result(3)) is not Object, generate a runtime error.
  5. If Result(3) does not implement the internal [[Call]] method, generate a runtime error.
  6. If Type(Result(1)) is Reference, Result(6) is GetBase(Result(1)). Otherwise, Result(6) is null.
  7. If Result(6) is an activation object, Result(7) is null. Otherwise, Result(7) is the same as Result(6).
  8. Call the [[Call]] method on Result(3), providing Result(7) as the this value and providing the list Result(2) as the argument values.
  9. Return Result(8).

The production CallExpression : CallExpression Arguments is evaluated in exactly the same manner, except that the contained CallExpression is evaluated in step 1.

NOTE Result(8) will never be of type Reference if Result(3) is a native ECMAScript object. Whether calling a host object can return a value of type Reference is implementation-dependent.

11.2.4 -- Argument Lists

The evaluation of an argument list produces an internal list of values (section 8).

The production Arguments : ( ) is evaluated as follows:

  1. Return an empty internal list of values.

The production Arguments : ( ArgumentList ) is evaluated as follows:

  1. Evaluate ArgumentList.
  2. Return Result(1).

The production ArgumentList : AssignmentExpression is evaluated as follows:

  1. Evaluate AssignmentExpression.
  2. Call GetValue(Result(1)).
  3. Return an internal list whose sole item is Result(2).

The production ArgumentList : ArgumentList , AssignmentExpression is evaluated as follows:

  1. Evaluate ArgumentList.
  2. Evaluate AssignmentExpression.
  3. Call GetValue(Result(2)).
  4. Return an internal list whose length is one greater than the length of Result(1) and whose items are the items of Result(1), in order, followed at the end by Result(3), which is the last item of the new list.

11.3 -- Postfix expressions

Syntax

PostfixExpression :
LeftHandSideExpression
LeftHandSideExpression [no LineTerminator here] ++
LeftHandSideExpression [no LineTerminator here] --

11.3.1 -- Postfix increment operator

The production MemberExpression : MemberExpression++ is evaluated as follows:

  1. Evaluate MemberExpression.
  2. Call GetValue(Result(1)).
  3. Call ToNumber(Result(2)).
  4. Add the value 1 to Result(3), using the same rules as for the + operator (section 11.6.3, Applying the additive operators (+, -) to numbers).
  5. Call PutValue(Result(1), Result(4)).
  6. Return Result(3).

11.3.2 -- Postfix decrement operator

The production MemberExpression : MemberExpression -- is evaluated as follows:

  1. Evaluate MemberExpression.
  2. Call GetValue(Result(1)).
  3. Call ToNumber(Result(2)).
  4. Subtract the value 1 from Result(3), using the same rules as for the - operator (section 11.6.3, Applying the additive operators (+, -) to numbers).
  5. Call PutValue(Result(1), Result(4)).
  6. Return Result(3).

11.4 -- Unary operators

Syntax

UnaryExpression :
PostfixExpression
delete UnaryExpression
void UnaryExpression
typeof UnaryExpression
++ UnaryExpression
-- UnaryExpression
+ UnaryExpression
- UnaryExpression
~ UnaryExpression
! UnaryExpression

11.4.1 -- The delete operator

The production UnaryExpression : delete UnaryExpression is evaluated as follows:

  1. Evaluate UnaryExpression.
  2. Call GetBase(Result(1)).
  3. Call GetPropertyName(Result(1)).
  4. If Type(Result(2)) is not Object, return true.
  5. If Result(2) does not implement the internal [[Delete]] method, go to step 8.
  6. Call the [[Delete]] method on Result(2), providing Result(3) as the property name to delete.
  7. Return Result(6).
  8. Call the [[HasProperty]] method on Result(2), providing Result(3) as the property name to check for.
  9. If Result(8) is true, return false.
  10. Return true.

11.4.2 -- The void operator

The production UnaryExpression : void UnaryExpression is evaluated as follows:

  1. Evaluate UnaryExpression.
  2. Call GetValue(Result(1)).
  3. Return undefined.

11.4.3 -- The typeof operator

The production UnaryExpression : typeof UnaryExpression is evaluated as follows:

  1. Evaluate UnaryExpression.
  2. If Type(Result(1)) is Reference and GetBase(Result(1)) is null, return "undefined".
  3. Call GetValue(Result(1)).
  4. Return a string determined by Type(Result(3)) according to the following table:
    Type Result
    Undefined "undefined"
    Null "object"
    Boolean "boolean"
    Number "number"
    String "string"
    Object (native and doesn't implement [[Call]]) "object"
    Object (native and implements [[Call]]) "function"
    Object (host) Implementation-dependent

11.4.4 -- Prefix increment operator

The production UnaryExpression : ++ UnaryExpression is evaluated as follows:

  1. Evaluate UnaryExpression.
  2. Call GetValue(Result(1)).
  3. Call ToNumber(Result(2)).
  4. Add the value 1 to Result(3), using the same rules as for the + operator (section 11.6.3).
  5. Call PutValue(Result(1), Result(4)).
  6. Return Result(4).

11.4.5 -- Prefix decrement operator

The production UnaryExpression : -- UnaryExpression is evaluated as follows:

  1. Evaluate UnaryExpression.
  2. Call GetValue(Result(1)).
  3. Call ToNumber(Result(2)).
  4. Subtract the value 1 from Result(3), using the same rules as for the - operator (section 11.6.3).
  5. Call PutValue(Result(1), Result(4)).
  6. Return Result(4).

11.4.6 -- Unary + operator

The unary + operator converts its operand to Number type.

The production UnaryExpression : + UnaryExpression is evaluated as follows:

  1. Evaluate UnaryExpression.
  2. Call GetValue(Result(1)).
  3. Call ToNumber(Result(2)).
  4. Return Result(3).

11.4.7 -- Unary - operator

The unary - operator converts its operand to Number type and then negates it. Note that negating +0 produces -0, and negating -0 produces +0.

The production UnaryExpression : - UnaryExpression is evaluated as follows:

  1. Evaluate UnaryExpression.
  2. Call GetValue(Result(1)).
  3. Call ToNumber(Result(2)).
  4. If Result(3) is NaN, return NaN.
  5. Negate Result(3); that is, compute a number with the same magnitude but opposite sign.
  6. Return Result(5).

11.4.8 -- The bitwise NOT operator ( ~ )

The production UnaryExpression : ~ UnaryExpression is evaluated as follows:

  1. Evaluate UnaryExpression.
  2. Call GetValue(Result(1)).
  3. Call ToInt32(Result(2)).
  4. Apply bitwise complement to Result(3). The result is a signed 32-bit integer.
  5. Return Result(4).

11.4.9 -- Logical NOT operator ( !)

The production UnaryExpression : ! UnaryExpression is evaluated as follows:

  1. Evaluate UnaryExpression.
  2. Call GetValue(Result(1)).
  3. Call ToBoolean(Result(2)).
  4. If Result(3) is true, return false.
  5. Return true.

11.5 -- Multiplicative operators

Syntax

MultiplicativeExpression :
UnaryExpression
MultiplicativeExpression * UnaryExpression
MultiplicativeExpression / UnaryExpression
MultiplicativeExpression % UnaryExpression

Semantics

The production MultiplicativeExpression : MultiplicativeExpression @ UnaryExpression, where @ stands for one of the operators in the above definitions, is evaluated as follows:

  1. Evaluate MultiplicativeExpression.
  2. Call GetValue(Result(1)).
  3. Evaluate UnaryExpression.
  4. Call GetValue(Result(3)).
  5. Call ToNumber(Result(2)).
  6. Call ToNumber(Result(4)).
  7. Apply the specified operation (*, /, or %) to Result(5) and Result(6). See the notes below (11.5.1, 11.5.2, 11.5.3).
  8. Return Result(7).

11.5.1 -- Applying the * operator

The * operator performs multiplication, producing the product of its operands. Multiplication is commutative. Multiplication is not always associative in ECMAScript, because of finite precision.

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

11.5.2 -- Applying the / operator

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

11.5.3 -- Applying the % operator

The binary % operator is said to yield the remainder of its operands from an implied division; the left operand is the dividend and the right operand is the divisor. In C and C++, the remainder operator accepts only integral operands, but 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 a ECMAScript floating-point remainder operation is determined by the rules of IEEE arithmetic:

11.6 -- Additive operators

Syntax

AdditiveExpression :
MultiplicativeExpression
AdditiveExpression + MultiplicativeExpression
AdditiveExpression - MultiplicativeExpression

11.6.1 -- The addition operator ( + )

The addition operator either performs string concatenation or numeric addition.

The production AdditiveExpression : AdditiveExpression + MultiplicativeExpression is evaluated as follows:

  1. Evaluate AdditiveExpression.
  2. Call GetValue(Result(1)).
  3. Evaluate MultiplicativeExpression.
  4. Call GetValue(Result(3)).
  5. Call ToPrimitive(Result(2)).
  6. Call ToPrimitive(Result(4)).
  7. If Type(Result(5)) is String or Type(Result(6)) is String, go to step 12. (Note that this step differs from step 3 in the algorithm for comparison for the relational operators in using or instead of and.)
  8. Call ToNumber(Result(5)).
  9. Call ToNumber(Result(6)).
  10. Apply the addition operation to Result(8) and Result(9). See the note below (11.6.3).
  11. Return Result(10).
  12. Call ToString(Result(5)).
  13. Call ToString(Result(6)).
  14. Concatenate Result(12) followed by Result(13).
  15. Return Result(14).

NOTE 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.

11.6.2 -- The subtraction operator ( - )

The production AdditiveExpression : AdditiveExpression - MultiplicativeExpression is evaluated as follows:

  1. Evaluate AdditiveExpression.
  2. Call GetValue(Result(1)).
  3. Evaluate MultiplicativeExpression.
  4. Call GetValue(Result(3)).
  5. Call ToNumber(Result(2)).
  6. Call ToNumber(Result(4)).
  7. Apply the subtraction operation to Result(5) and Result(6). See the note below (11.6.3).
  8. Return Result(7).

11.6.3 -- Applying the additive operators (+, -) to numbers

The + operator performs addition when applied to two operands of numeric type, producing the sum of the operands. The - operator performs subtraction, producing the difference of two numeric operands.

Addition is a commutative operation, but not always associative.

The result of an addition is determined using the rules of IEEE 754 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).

11.7 -- Bitwise shift operators

Syntax

ShiftExpression :
AdditiveExpression
ShiftExpression << AdditiveExpression
ShiftExpression >> AdditiveExpression
ShiftExpression >>> AdditiveExpression

Semantics

The result of evaluating ShiftExpression is always truncated to 32 bits. If the result of evaluating ShiftExpression produces a fractional component, the fractional component is discarded. The result of evaluating an AdditiveExpresion that is the right-hand operand of a shift operator is always truncated to five bits.

11.7.1 -- The left shift operator ( << )

Performs a bitwise left shift operation on the left operand by the amount specified by the right operand.

The production ShiftExpression : ShiftExpression << AdditiveExpression is evaluated as follows:

  1. Evaluate ShiftExpression.
  2. Call GetValue(Result(1)).
  3. Evaluate AdditiveExpression.
  4. Call GetValue(Result(3)).
  5. Call ToInt32(Result(2)).
  6. Call ToUint32(Result(4)).
  7. Mask out all but the least significant 5 bits of Result(6), that is, compute Result(6) & 0x1F.
  8. Left shift Result(5) by Result(7) bits. The result is a signed 32 bit integer.
  9. Return Result(8).

11.7.2 -- The signed right shift operator ( >> )

Performs a sign-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

The production ShiftExpression : ShiftExpression >> AdditiveExpression is evaluated as follows:

  1. Evaluate ShiftExpression.
  2. Call GetValue(Result(1)).
  3. Evaluate AdditiveExpression.
  4. Call GetValue(Result(3)).
  5. Call ToInt32(Result(2)).
  6. Call ToUint32(Result(4)).
  7. Mask out all but the least significant 5 bits of Result(6), that is, compute Result(6) & 0x1F.
  8. Perform sign-extending right shift of Result(5) by Result(7) bits. The most significant bit is propagated. The result is a signed 32 bit integer.
  9. Return Result(8).

11.7.3 -- The unsigned right shift operator ( >>> )

Performs a zero-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

The production ShiftExpression : ShiftExpression >>> AdditiveExpression is evaluated as follows:

  1. Evaluate ShiftExpression.
  2. Call GetValue(Result(1)).
  3. Evaluate AdditiveExpression.
  4. Call GetValue(Result(3)).
  5. Call ToUint32(Result(2)).
  6. Call ToUint32(Result(4)).
  7. Mask out all but the least significant 5 bits of Result(6), that is, compute Result(6) & 0x1F.
  8. Perform zero-filling right shift of Result(5) by Result(7) bits. Vacated bits are filled with zero. The result is an unsigned 32 bit integer.
  9. Return Result(8).

11.8 -- Relational operators

Syntax

RelationalExpression :
ShiftExpression
RelationalExpression < ShiftExpression
RelationalExpression > ShiftExpression
RelationalExpression <= ShiftExpression
RelationalExpression >= ShiftExpression

Semantics

The result of evaluating RelationalExpression is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.

11.8.1 -- The less-than operator ( < )

The production RelationalExpression : RelationalExpression < ShiftExpression is evaluated as follows:

  1. Evaluate RelationalExpression.
  2. Call GetValue(Result(1)).
  3. Evaluate ShiftExpression.
  4. Call GetValue(Result(3)).
  5. Perform the comparison Result(2) < Result(4). (See section 11.8.5).
  6. If Result(5) is undefined, return false. Otherwise, return Result(5).

11.8.2 -- The greater-than operator ( > )

The production RelationalExpression : RelationalExpression > ShiftExpression is evaluated as follows:

  1. Evaluate RelationalExpression.
  2. Call GetValue(Result(1)).
  3. Evaluate ShiftExpression.
  4. Call GetValue(Result(3)).
  5. Perform the comparison Result(4) < Result(2). (See section 11.8.5).
  6. If Result(5) is undefined, return false. Otherwise, return Result(5).

11.8.3 -- The less-than-or-equal operator ( <= )

The production RelationalExpression : RelationalExpression <= ShiftExpression is evaluated as follows:

  1. Evaluate RelationalExpression.
  2. Call GetValue(Result(1)).
  3. Evaluate ShiftExpression.
  4. Call GetValue(Result(3)).
  5. Perform the comparison Result(4) < Result(2). (See section 11.8.5).
  6. If Result(5) is true or undefined, return false. Otherwise, return true.

11.8.4 -- The greater-than-or-equal operator ( >= )

The production RelationalExpression : RelationalExpression >= ShiftExpression is evaluated as follows:

  1. Evaluate RelationalExpression.
  2. Call GetValue(Result(1)).
  3. Evaluate ShiftExpression.
  4. Call GetValue(Result(3)).
  5. Perform the comparison Result(2) < Result(4). (See section 11.8.5).
  6. If Result(5) is true or undefined, return false. Otherwise, return true.

11.8.5 -- The abstract relational comparison algorithm

The comparison x < y, where x and y are values, produces true, false, or undefined (which indicates that at least one operand is NaN). Such a comparison is performed as follows:

  1. Call ToPrimitive(x, hint Number).
  2. Call ToPrimitive(y, hint Number).
  3. If Type(Result(1)) is String and Type(Result(2)) is String, go to step 16. (Note that this step differs from step 7 in the algorithm for the addition operator + in using and instead of or.)
  4. Call ToNumber(Result(1)).
  5. Call ToNumber(Result(2)).
  6. If Result(4) is NaN, return undefined.
  7. If Result(5) is NaN, return undefined.
  8. If Result(4) and Result(5) are the same number value, return false.
  9. If Result(4) is +0 and Result(5) is -0, return false.
  10. If Result(4) is -0 and Result(5) is +0, return false.
  11. If Result(4) is +∞, return false.
  12. If Result(5) is +∞, return true.
  13. If Result(5) is -∞, return false.
  14. If Result(4) is -∞, return true.
  15. If the mathematical value of Result(4) is less than the mathematical value of Result(5) -- note that these mathematical values are both finite and not both zero -- return true. Otherwise, return false.
  16. If Result(2) is a prefix of Result(1), return false. (A string value p is a prefix of string value q if q can be the result of concatenating p and some other string r. Note that any string is a prefix of itself, because r may be the empty string.)
  17. If Result(1) is a prefix of Result(2), return true.
  18. Let k be the smallest nonnegative integer such that the character at position k within Result(1) is different from the character at position k within Result(2). (There must be such a k, for neither string is a prefix of the other.)
  19. Let m be the integer that is the Unicode encoding for the character at position k within Result(1).
  20. Let n be the integer that is the Unicode encoding for the character at position k within Result(2).
  21. If m < n, return true. Otherwise, return false.

NOTE The comparison of strings uses a simple lexicographic ordering on sequences of Unicode code point 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 2.0 specification.

11.9 -- Equality operators

Syntax

EqualityExpression :
RelationalExpression
EqualityExpression == RelationalExpression
EqualityExpression != RelationalExpression

Semantics

The result of evaluating EqualityExpression is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.

11.9.1 -- The equals operator ( == )

The production EqualityExpression : EqualityExpression == RelationalExpression is evaluated as follows:

  1. Evaluate EqualityExpression.
  2. Call GetValue(Result(1)).
  3. Evaluate RelationalExpression.
  4. Call GetValue(Result(3)).
  5. Perform the comparison Result(4) == Result(2). (See section 11.9.3).
  6. Return Result(5).

11.9.2 -- The does-not-equals operator ( != )

The production EqualityExpression : EqualityExpression != RelationalExpression is evaluated as follows:

  1. Evaluate EqualityExpression.
  2. Call GetValue(Result(1)).
  3. Evaluate RelationalExpression.
  4. Call GetValue(Result(3)).
  5. Perform the comparison Result(4) == Result(2). (See section 11.9.3).
  6. If Result(5) is true, return false. Otherwise, return true.

11.9.3 -- The abstract equality comparison algorithm

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

  1. If Type(x) is different from Type(y), go to step 14.
  2. If Type(x) is Undefined, return true.
  3. If Type(x) is Null, return true.
  4. If Type(x) is not Number, go to step 11.
  5. If x is NaN, return false.
  6. If y is NaN, return false.
  7. If x is the same number value as y, return true.
  8. If x is +0 and y is -0, return true.
  9. If x is -0 and y is +0, return true.
  10. Return false.
  11. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions). Otherwise, return false..
  12. If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false.
  13. Return true if x and y refer to the same object. Otherwise, return false.
  14. If x is null and y is undefined, return true.
  15. If x is undefined and y is null, return true.
  16. If Type(x) is Number and Type(y) is String,
    return the result of the comparison x == ToNumber(y).
  17. If Type(x) is String and Type(y) is Number,
    return the result of the comparison ToNumber(x) == y.
  18. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
  19. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
  20. If Type(x) is either String or Number and Type(y) is Object,
    return the result of the comparison x == ToPrimitive(y).
  21. If Type(x) is Object and Type(y) is either String or Number,
    return the result of the comparison ToPrimitive(x) == y.
  22. Return false.

NOTE Given the above definition of equality::

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.

Comparison of strings uses a simple equality test on sequences of Unicode code point 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 2.0 specification.

11.10 -- Binary bitwise operators

Syntax

BitwiseANDExpression :
EqualityExpression
BitwiseANDExpression & EqualityExpression
BitwiseXORExpression :
BitwiseANDExpression
BitwiseXORExpression ^ BitwiseANDExpression
BitwiseORExpression :
BitwiseXORExpression
BitwiseORExpression | BitwiseXORExpression

Semantics

The production A : A @ B, where @ is one of the bitwise operators in the productions above, is evaluated as follows:

  1. Evaluate A.
  2. Call GetValue(Result(1)).
  3. Evaluate B.
  4. Call GetValue(Result(3)).
  5. Call ToInt32(Result(2)).
  6. Call ToInt32(Result(4)).
  7. Apply the bitwise operator @ to Result(5) and Result(6). The result is a signed 32 bit integer.
  8. Return Result(7).

11.11 -- Binary logical operators

Syntax

LogicalANDExpression :
BitwiseORExpression
LogicalANDExpression && BitwiseORExpression
LogicalORExpression :
LogicalANDExpression
LogicalORExpression || LogicalANDExpression

Semantics

The production LogicalANDExpression : LogicalANDExpression && BitwiseORExpression is evaluated as follows:

  1. Evaluate LogicalANDExpression.
  2. Call GetValue(Result(1)).
  3. Call ToBoolean(Result(2)).
  4. If Result(3) is false, return Result(2).
  5. Evaluate BitwiseORExpression.
  6. Call GetValue(Result(5)).
  7. Return Result(6).

The production LogicalORExpression : LogicalORExpression || LogicalANDExpression is evaluated as follows:

  1. Evaluate LogicalORExpression.
  2. Call GetValue(Result(1)).
  3. Call ToBoolean(Result(2)).
  4. If Result(3) is true, return Result(2).
  5. Evaluate LogicalANDExpression.
  6. Call GetValue(Result(5)).
  7. Return Result(6).

NOTE The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.

11.12 -- Conditional operator ( ?: )

Syntax

ConditionalExpression :
LogicalORExpression
LogicalORExpression ? AssignmentExpression : AssignmentExpression

Semantics

The production ConditionalExpression : LogicalORExpression ? AssignmentExpression : AssignmentExpression is evaluated as follows:

  1. Evaluate LogicalORExpression.
  2. Call GetValue(Result(1)).
  3. Call ToBoolean(Result(2)).
  4. If Result(3) is false, go to step 8.
  5. Evaluate the first AssignmentExpression.
  6. Call GetValue(Result(5)).
  7. Return Result(6).
  8. Evaluate the second AssignmentExpression.
  9. Call GetValue(Result(8)).
  10. Return Result(9).

NOTE The grammar for a ConditionalExpression in ECMAScript is a little bit different from that in C and Java, which each allow the second subexpression to be an Expression but restrict the third expression to be a ConditionalExpression. The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the centre expression.

11.13 -- Assignment operators

Syntax

AssignmentExpression :
ConditionalExpression
LeftHandSideExpression AssignmentOperator AssignmentExpression
AssignmentOperator :: one of
= *= /= %= += -= <<= >>= >>>= &= ^= |=

11.13.1 -- Simple Assignment ( = )

The production AssignmentExpression : LeftHandSideExpression = AssignmentExpression is evaluated as follows:

  1. Evaluate LeftHandSideExpression.
  2. Evaluate AssignmentExpression.
  3. Call GetValue(Result(2)).
  4. Call PutValue(Result(1), Result(3)).
  5. Return Result(3).

11.13.2 -- Compound assignment ( op= )

The production AssignmentExpression : LeftHandSideExpression @= AssignmentExpression, where @ represents one of the operators indicated above, is evaluated as follows:

  1. Evaluate LeftHandSideExpression.
  2. Call GetValue(Result(1)).
  3. Evaluate AssignmentExpression.
  4. Call GetValue(Result(3)).
  5. Apply operator @ to Result(2) and Result(4).
  6. Call PutValue(Result(1), Result(5)).
  7. Return Result(5).

11.14 -- Comma operator ( , )

Syntax

Expression :
AssignmentExpression
Expression , AssignmentExpression

Semantics

The production Expression : Expression , AssignmentExpression is evaluated as follows:

  1. Evaluate Expression.
  2. Call GetValue(Result(1)).
  3. Evaluate AssignmentExpression.
  4. Call GetValue(Result(3)).
  5. Return Result(4).

12 -- Statements

Syntax

Statement :
Block
VariableStatement
EmptyStatement
ExpressionStatement
IfStatement
IterationStatement
ContinueStatement
BreakStatement
ReturnStatement
WithStatement

12.1 -- Block

Syntax

Block :
{ StatementListopt }
StatementList :
Statement
StatementList Statement

Semantics

The production Block : { } is evaluated as follows:

  1. Return ``normal completion''.

The production Block : { StatementList }is evaluated as follows:

  1. Evaluate StatementList.
  2. Return Result(1).

The production StatementList : Statement is evaluated as follows:

  1. Evaluate Statement.
  2. Return Result(1).

The production StatementList : StatementList Statement is evaluated as follows:

  1. Evaluate StatementList.
  2. If Result(1) is an abrupt completion, return Result(1).
  3. Evaluate Statement.
  4. If Result(3) is a value completion, return Result(3).
  5. If Result(1) is not a value completion, return Result(3).
  6. Let V be the value carried by Result(1).
  7. If Result(3) is ``abrupt completion because of break'',
    return ``abrupt completion after value V because of break''.
  8. If Result(3) is ``abrupt completion because of continue'',
    return ``abrupt completion after value V because of continue''.
  9. Return ``normal completion after value V''.

12.2 -- Variable statement

Syntax

VariableStatement :
var VariableDeclarationList ;
VariableDeclarationList :
VariableDeclaration
VariableDeclarationList , VariableDeclaration
VariableDeclaration :
Identifier Initializeropt
Initializer :
= AssignmentExpression

Description

If the variable statement occurs inside a FunctionDeclaration, the variables are defined with function-local scope in that function, as described in section 10.1.3. Otherwise, they are defined with global scope (that is, they are created as members of the global object, as described in section 10.1.3) using property attributes { DontDelete }. Variables are created when the execution scope is entered. A Block does not define a new execution scope. Only Program and FunctionDeclaration produce a new scope. Variables are initialised to the undefined value when created. A variable with an Initializer is assigned the value of its AssignmentExpression when the VariableStatement is executed, not when the variable is created.

Semantics

The production VariableStatement : var VariableDeclarationList ; is evaluated as follows:

  1. Evaluate VariableDeclarationList.
  2. Return ``normal completion''.

The production VariableDeclarationList :VariableDeclaration is evaluated as follows:

  1. Evaluate VariableDeclaration.

The production VariableDeclarationList : VariableDeclarationList , VariableDeclaration is evaluated as follows:

  1. Evaluate VariableDeclarationList.
  2. Evaluate VariableDeclaration.

The production VariableDeclaration : Identifier is evaluated as follows:

  1. Return a string value containing the same sequence of characters as in the Identifier.

The production VariableDeclaration : Identifier Initializer is evaluated as follows:

  1. Evaluate Identifier as described in section 11.1.2.
  2. Evaluate Initializer.
  3. Call GetValue(Result(2)).
  4. Call PutValue(Result(1), Result(3)).
  5. Return a string value containing the same sequence of characters as in the Identifier.

The production Initializer : = AssignmentExpression is evaluated as follows:

  1. Evaluate AssignmentExpression.
  2. Return Result(1).

12.3 -- Empty statement

Syntax

EmptyStatement :
;

Semantics

The production EmptyStatement : ; is evaluated as follows:

  1. Return ``normal completion''.

12.4 -- Expression statement

Syntax

ExpressionStatement :
Expression ;

Semantics

The production ExpressionStatement : Expression ; is evaluated as follows:

  1. Evaluate Expression.
  2. Call GetValue(Result(1)).
  3. Return ``normal completion after value V'', where the value V is Result(2).

12.5 -- The if statement

Syntax

IfStatement :
if ( Expression ) Statement else Statement
if ( Expression ) Statement

Each else for which the choice of associated if is ambiguous shall be associated with the nearest possible if that would otherwise have no corresponding else.

Semantics

The production IfStatement : if ( Expression ) Statement else Statement is evaluated as follows:

  1. Evaluate Expression.
  2. Call GetValue(Result(1)).
  3. Call ToBoolean(Result(2)).
  4. If Result(3) is false, go to step 7.
  5. Evaluate the first Statement.
  6. Return Result(5).
  7. Evaluate the second Statement.
  8. Return Result(7).

The production IfStatement : if ( Expression ) Statement is evaluated as follows:

  1. Evaluate Expression.
  2. Call GetValue(Result(1)).
  3. Call ToBoolean(Result(2)).
  4. If Result(3) is false, return ``normal completion''.
  5. Evaluate Statement.
  6. Return Result(5).

12.6 -- Iteration statements

An iteration statement consists of a header (which consists of a keyword and a parenthesized control construct) and a body (which consists of a Statement).

Syntax

IterationStatement :
while ( Expression ) Statement
for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement
for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement
for ( LeftHandSideExpression in Expression ) Statement
for ( var Identifier Initializeropt in Expression ) Statement

12.6.1 -- The while statement

The production IterationStatement : while ( Expression ) Statement is evaluated as follows:

  1. Let C be ``normal completion''.
  2. Evaluate Expression.
  3. Call GetValue(Result(1)).
  4. Call ToBoolean(Result(2)).
  5. If Result(3) is false, go to step 12.
  6. Evaluate Statement.
  7. If Result(6) is a value completion, change C to be ``normal completion after value V'' where V is the value carried by Result(6).
  8. If Result(6) is a break completion, go to step 12.
  9. If Result(6) is a continue completion, go to step 2.
  10. If Result(6) is a return completion, return Result(6).
  11. Go to step 2.
  12. Return C.

12.6.2 -- The for statement

The production IterationStatement : for (Expressionopt ; Expressionopt ; Expressionopt) Statement is evaluated as follows:

  1. If the first Expression is not present, go to step 4.
  2. Evaluate the first Expression.
  3. Call GetValue(Result(2)). (This value is not used.)
  4. Let C be ``normal completion''.
  5. If the second Expression is not present, go to step 10.
  6. Evaluate the second Expression.
  7. Call GetValue(Result(6)).
  8. Call ToBoolean(Result(7)).
  9. If Result(8) is false, go to step 19.
  10. Evaluate Statement.
  11. If Result(10) is a value completion, change C to be ``normal completion after value V'' where V is the value carried by Result(10).
  12. If Result(10) is a break completion, go to step 19.
  13. If Result(10) is a continue completion, go to step 15.
  14. If Result(10) is a return completion, return Result(10).
  15. If the third Expression is not present, go to step 5.
  16. Evaluate the third Expression.
  17. Call GetValue(Result(16). (This value is not used.)
  18. Go to step 5.
  19. Return C.

The production IterationStatement : for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement is evaluated as follows:

  1. Evaluate VariableDeclarationList.
  2. Let C be ``normal completion''.
  3. If the second Expression is not present, go to step 8.
  4. Evaluate the second Expression.
  5. Call GetValue(Result(4)).
  6. Call ToBoolean(Result(5)).
  7. If Result(6) is false, go to step 15.
  8. Evaluate Statement.
  9. If Result(8) is a value completion, change C to be ``normal completion after value V'' where V is the value carried by Result(8).
  10. If Result(8) is a break completion, go to step 17.
  11. If Result(8) is a continue completion, go to step 13.
  12. If Result(8) is a return completion, return Result(8).
  13. If the third Expression is not present, go to step 3.
  14. Evaluate the third Expression.
  15. Call GetValue(Result(14)). (This value is not used.)
  16. Go to step 3.
  17. Return C.

12.6.3 -- The for..in statement

The production IterationStatement : for ( LeftHandSideExpression in Expression ) Statement is evaluated as follows:

  1. Evaluate the Expression.
  2. Call GetValue(Result(1)).
  3. Call ToObject(Result(2)).
  4. Let C be ``normal completion''.
  5. Get the name of the next property of Result(3) that doesn't have the DontEnum attribute. If there is no such property, go to step 14.
  6. Evaluate the LeftHandSideExpression ( it may be evaluated repeatedly).
  7. Call PutValue(Result(6), Result(5)).
  8. Evaluate Statement.
  9. If Result(8) is a value completion, change C to be ``normal completion after value V'' where V is the value carried by Result(8).
  10. If Result(8) is a break completion, go to step 14.
  11. If Result(8) is a continue completion, go to step 5.
  12. If Result(8) is a return completion, return Result(8).
  13. Go to step 5.
  14. Return C.

The production IterationStatement : for ( var VariableDeclaration in Expression ) Statement is evaluated as follows:

  1. Evaluate VariableDeclaration.
  2. Evaluate Expression.
  3. Call GetValue(Result(2)).
  4. Call ToObject(Result(3)).
  5. Let C be ``normal completion''.
  6. Get the name of the next property of Result(4) that doesn't have the DontEnum attribute. If there is no such property, go to step 15.
  7. Evaluate Result(1) as if it were an Identifier; see 11.1.2 (yes, it may be evaluated repeatedly).
  8. Call PutValue(Result(7), Result(6)).
  9. Evaluate Statement.
  10. If Result(9) is a value completion, change C to be ``normal completion after value V'' where V is the value carried by Result(9).
  11. If Result(9) a break completion, go to step 15.
  12. If Result(9) a continue completion, go to step 6.
  13. If Result(9) a return completion, return Result(9).
  14. Go to step 6.
  15. Return C.

The mechanics of enumerating the properties (step 5 in the first algorithm, step 10 in the second) is implementation-dependent. The order of enumeration is defined by the object. 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.

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.

12.7 -- The continue statement

Syntax

ContinueStatement :
continue ;

Semantics

An ECMAScript program is considered syntactically incorrect if it contains a continue statement that is not within a while or for statement. The continue statement is evaluated as:

  1. Return ``abrupt completion because of continue''.

12.8 -- The break statement

Syntax

BreakStatement :
break ;

Semantics

An ECMAScript program is considered syntactically incorrect if it contains a break statement that is not within a while or for statement. The break statement is evaluated as:

  1. Return ``abrupt completion because of break''.

12.9 -- The return statement

Syntax

ReturnStatement :
return [no LineTerminator here] Expressionopt ;

Semantics

An ECMAScript program is considered syntactically incorrect if it contains a return statement that is not within the Block of a FunctionDeclaration. It causes a function to cease execution and return a value to the caller. If Expression is omitted, the return value is the undefined value. Otherwise, the return value is the value of Expression.

The production ReturnStatement :: return [no LineTerminator here] Expressionopt ; is evaluated as:

  1. If the Expression is not present, return ``abrupt completion because of return undefined''.
  2. Evaluate Expression.
  3. Call GetValue(Result(2)).
  4. Return ``abrupt completion because of return V'', where the value V is Result(3).

12.10 -- The with statement

Syntax

WithStatement :
with ( Expression ) Statement

Description

The with statement adds a computed object to the front of the scope chain of the current execution context, then executes a statement with this augmented scope chain, then restores the scope chain.

Semantics

The production WithStatement : with ( Expression ) Statement is evaluated as follows:

  1. Evaluate Expression.
  2. Call GetValue(Result(1)).
  3. Call ToObject(Result(2)).
  4. Add Result(3) to the front of the scope chain.
  5. Evaluate Statement using the augmented scope chain from step 4.
  6. Remove Result(3) from the front of the scope chain.
  7. Return Result(5).

NOTE No matter how control leaves the embedded Statement, whether normally or by some form of abrupt completion, the scope chain is always restored to its former state.

13 -- Function Definition

Syntax

FunctionDeclaration :
function Identifier ( FormalParameterListopt ) Block
FormalParameterList :
Identifier
FormalParameterList , Identifier

Semantics

Defines a property of the global object whose name is the Identifier following the function keyword and whose value is a function object with the given parameter list and statements. If the function definition is supplied text to the eval function and the calling context has an activation object, then the declared function is added to the activation object instead of to the global object. See section 10.1.3.

The production FunctionDeclaration : function Identifier ( ) Block is processed for function declarations as follows:

  1. Create a new Function object (15.3.2.1) with no parameters, the Block as the body, and Identifier as its name.
  2. Put this new Function object as the new value of the property named Identifier in the global object or the activation object, as appropriate (see above).

The production FunctionDeclaration : function Identifier ( FormalParameterList ) Block is processed for function declarations as follows:

  1. Create a new Function object (15.3.2.1) with the parameters specified by the FormalParameterList, the Block as the body, and Identifier as its name.
  2. Put this new Function object as the new value of the property named Identifier in the global object or the activation object, as appropriate (see above).

14 -- Program

Syntax

Program :
SourceElements
SourceElements :
SourceElement
SourceElements SourceElement
SourceElement :
Statement
FunctionDeclaration

Semantics

The production Program : SourceElements is evaluated as follows:

  1. Process SourceElements for function declarations.
  2. Evaluate SourceElements.
  3. Return Result(2).

The production SourceElements : SourceElement is processed for function declarations as follows:

  1. Process SourceElement for function declarations.

The production SourceElements : SourceElement is evaluated as follows:

  1. Evaluate SourceElement.
  2. Return Result(1).

The production SourceElements : SourceElements SourceElement is processed for function declarations as follows:

  1. Process SourceElements for function declarations.
  2. Process SourceElement for function declarations.

The production SourceElements : SourceElements SourceElement is evaluated as follows:

  1. Evaluate SourceElements.
  2. Evaluate SourceElement.
  3. If Result(2) is a value completion, return Result(2).
  4. Return Result(1).

The production SourceElement : Statement is processed for function declarations by taking no action.

The production SourceElement : Statement is evaluated as follows:

  1. Evaluate Statement.
  2. Return Result(1).

The production SourceElement : FunctionDeclaration is processed for function declarations as follows:

  1. Process FunctionDeclaration for function declarations.

The production SourceElement : FunctionDeclaration is evaluated as follows:

  1. Return ``normal completion''.

15 -- Native ECMAScript objects

There are certain built-in objects available whenever an ECMAScript program begins execution. One, the global object, is in the scope chain of the executing program. Others are accessible as initial properties of the global object.

Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they are functions intended for use with the new operator. For each built-in function, this specification describes the arguments required by that function and properties of the function object. For each built-in constructor, this specification furthermore describes properties of the prototype object of that constructor and properties of specific object instances returned by a new expression that invokes that constructor.

Unless otherwise specified in the description of a particular function, if a function or constructor described in this section 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.

Every built-in function and every built-in constructor has the Function prototype object, which is the value of the expression Function.prototype (15.3.2.1), as the value of its internal [[Prototype]] property, except the Function prototype object itself.

Every built-in prototype object has the Object prototype object, which is the value of the expression Object.prototype (15.2.3.1), as the value of its internal [[Prototype]] property, except the Object prototype object itself. Every native prototype object associated with a program-created function also has the Object prototype object as the value of its internal [[Prototype]] property.

None of the built-in functions described in this section shall implement the internal [[Construct]] method unless otherwise specified in the description of a particular function. None of the built-in functions described in this section shall initially have a prototype property unless otherwise specified in the description of a particular function. Every built-in function object described in this section -- 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 number of named arguments shown in the section heading for the function description; for example, the function object that is the initial value of the indexOf property of the String prototype object is described under the section heading ``indexOf(searchString, position)'' which shows the two named arguments searchString and position; therefore the value of the length property of that function object is 2. Sometimes the same function object is described under more than one heading to emphasise its different behaviours when given different numbers of actual arguments; in such a case, unless otherwise specified, the length value is the largest number of arguments shown in any applicable section heading. For example, the function object that is the initial value of the Object property of the global object is described under four separate headings: as a function of one argument (section 15.2.1.1), as a function of zero arguments (section 15.2.1.2), as a constructor of one argument (15.2.2.1), and as a constructor of zero arguments (15.2.2.2). The largest number of arguments described is 1, so the value of the length property of that function object is 1.

In every case, a length property of a built-in function object described in this section has the attributes { ReadOnly, DontDelete, DontEnum } (and no others). Every other property described in this section has the attribute { DontEnum } (and no others) unless otherwise specified.

15.1 -- The Global Object

The global object does not have a [[Construct]] property; it is not possible to use the global object as a constructor with the new operator.

The global object does not have a [[Call]] property; it is not possible to invoke the global object as a function.

The value of the [[Prototype]] property of the global object is implementation-dependent.

15.1.1 -- Value properties of the Global Object

15.1.1.1 -- NaN

The initial value of NaN is NaN.

15.1.1.2 -- Infinity

The initial value of Infinity is +∞.

15.1.2 -- Function properties of the Global Object

15.1.2.1 -- eval(x)

When the eval function is called with one argument x, the following steps are taken:

  1. If x is not a string value, return x.
  2. Parse x as an ECMAScript Program. If the parse fails, generate a runtime error.
  3. Evaluate the program from step 2.
  4. If Result(3) is ``normal completion after value V'', return the value V.
  5. Return undefined.

If value of the eval property is used in any way other than a direct call (that is, other than by the explicit use of its name as an Identifier which is the MemberExpression in a CallExpression), or if the eval property is assigned to, a runtime error may be generated.

15.1.2.2 -- parseInt(string, radix)

The parseInt function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix.

When the parseInt function is called, the following steps are taken:

  1. Call ToString(string).
  2. Compute a substring of Result(1) consisting of the leftmost character that is not a StrWhiteSpaceChar and all characters to the right of that character. (In other words, remove leading whitespace.)
  3. Let sign be 1.
  4. If Result(2) is not empty and the first character of Result(2) is a minus sign -, let sign be -1.
  5. If Result(2) is not empty and the first character of Result(2) is a plus sign + or a minus sign -, then Result(5) is the substring of Result(2) produced by removing the first character; otherwise, Result(5) is Result(2).
  6. If the radix argument is not supplied, go to step 12.
  7. Call ToInt32(radix).
  8. If Result(7) is zero, go to step 12; otherwise, if Result(7) < 2 or Result(7) > 36, return NaN.
  9. Let R be Result(7).
  10. If R = 16 and the length of Result(5) is at least 2 and the first two characters of Result(5) are either ``0x'' or ``0X'', let S be the substring of Result(5) consisting of all but the first two characters; otherwise, let S be Result(5).
  11. Go to step 22.
  12. If Result(5) is empty or the first character of Result(5) is not 0, go to step 20.
  13. If the length of Result(5) is at least 2 and the second character of Result(5) is x or X, go to step 17.
  14. Let R be 8.
  15. Let S be Result(5).
  16. Go to step 22.
  17. Let R be 16.
  18. Let S be the substring of Result(5) consisting of all but the first two characters.
  19. Go to step 22.
  20. Let R be 10.
  21. Let S be Result(5).
  22. If S contains any character that is not a radix-R digit, then let Z be the substring of S consisting of all characters to the left of the leftmost such character; otherwise, let Z be S.
  23. If Z is empty, return NaN.
  24. Compute the mathematical integer value that is represented by Z in radix-R notation. (But if R is 10 and Z contains more than 20 significant digits, every 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 Result(24) may be an implementation-dependent approximation to the mathematical integer value that is represented by Z in radix-R notation.)
  25. Compute the number value for Result(24).
  26. Return sign * Result(25).

Note that parseInt may interpret only a leading portion of the string as an integer value; it ignores any characters that cannot be interpreted as part of the notation of an integer, and no indication is given that any such characters were ignored.

15.1.2.3 -- parseFloat(string)

The parseFloat function produces a number value dictated by interpretation of the contents of the string argument as a decimal literal.

When the parseFloat function is called, the following steps are taken:

  1. Call ToString(string).
  2. Compute a substring of Result(1) consisting of the leftmost character that is not a StrWhiteSpaceChar and all characters to the right of that character.(In other words, remove leading whitespace.)
  3. If neither Result(2) nor any prefix of Result(2) satisfies the syntax of a StrDecimalLiteral (see 9.3.1), return NaN.
  4. Compute the longest prefix of Result(2), which might be Result(2) itself, which satisfies the syntax of a StrDecimalLiteral.
  5. Return the number value for the MV of Result(4).

Note that parseFloat may interpret only a leading portion of the string as a number value; it ignores any characters that cannot be interpreted as part of the notation of an decimal literal, and no indication is given that any such characters were ignored.

15.1.2.4 -- escape(string)

The escape function computes a new version of a string value in which certain characters have been replaced by a hexadecimal escape sequence.

For those characters being replaced whose Unicode encoding is 0xFF or less, a two-digit escape sequence of the form %xx is used. For those characters being replaced whose Unicode encoding is greater than 0xFF, a four-digit escape sequence of the form %uxxxx is used.

When the escape function is called with one argument string, the following steps are taken:

  1. Call ToString(string).
  2. Compute the number of characters in Result(1).
  3. Let R be the empty string.
  4. Let k be 0.
  5. If k equals Result(2), return R.
  6. Get the character at position k within Result(1).
  7. If Result(6) is one of the 69 nonblank ISO/IEC 646 IRV characters ABCDEFGHIJKLMNOPQRSTUVWXYZ abcdefghijklmnopqrstuvwxyz 0123456789 @*_+-./ , go to step 14.
  8. Compute the 16-bit unsigned integer that is the Unicode character encoding of Result(6).
  9. If Result(8), is less than 256, go to step 12.
  10. Let S be a string containing six characters ``%uwxyz'' where wxyz are four hexadecimal digits encoding the value of Result(8).
  11. Go to step 15.
  12. Let S be a string containing three characters ``%xy'' where xy are two hexadecimal digits encoding the value of Result(8).
  13. Go to step 15.
  14. Let S be a string containing the single character Result(6).
  15. Let R be a new string value computed by concatenating the previous value of R and S.
  16. Increase k by 1.
  17. Go to step 5.
NOTE The encoding is partly based on the encoding described in RFC1738, but the entire encoding specified in this standard is described above without regard to the contents of RFC1738.
15.1.2.5 -- unescape(string)

The unescape function computes a new version of a string value in which each escape sequences of the sort that might be introduced by the escape function is replaced with the character that it represents.

When the unescape function is called with one argument string, the following steps are taken:

  1. Call ToString(string).
  2. Compute the number of characters in Result(1).
  3. Let R be the empty string.
  4. Let k be 0.
  5. If k equals Result(2), return R.
  6. Let c be the character at position k within Result(1).
  7. If c is not %, go to step 18.
  8. If k is greater than Result(2)-6, go to step 14.
  9. If the character at position k+1 within result(1) is not u, go to step 14.
  10. If the four characters at positions k+2, k+3, k+4, and k+5 within Result(1) are not all hexadecimal digits, go to step 14.
  11. Let c be the character whose Unicode encoding is the integer represented by the four hexadecimal digits at positions k+2, k+3, k+4, and k+5 within Result(1).
  12. Increase k by 5.
  13. Go to step 18.
  14. If k is greater than Result(2)-3, go to step 18.
  15. If the two characters at positions k+1 and k+2 within Result(1) are not both hexadecimal digits, go to step 18.
  16. Let c be the character whose Unicode encoding is the integer represented by two zeroes plus the two hexadecimal digits at positions k+1 and k+2 within Result(1).
  17. Increase k by 2.
  18. Let R be a new string value computed by concatenating the previous value of R and c.
  19. Increase k by 1.
  20. Go to step 5.

15.1.2.6 -- isNaN(number)

Applies ToNumber to its argument, then returns true if the result is NaN, and otherwise returns false.

15.1.2.7 -- isFinite(number)

Applies ToNumber to its argument, then returns false if the result is NaN, +∞, or -∞ , and otherwise returns true.

15.1.3 -- Constructor Properties of the Global Object

15.1.3.1 -- Object( . . . )

See sections 15.2.1 and 15.2.2.

15.1.3.2 -- Function( . . . )

See sections 15.3.1 and 15.3.2.

15.1.3.3 -- Array( . . . )

See sections 15.4.1 and 15.4.2.

15.1.3.4 -- String( . . . )

See sections 15.5.1 and 15.5.2.

15.1.3.5 -- Boolean( . . . )

See sections 15.6.1 and 15.6.2.

15.1.3.6 -- Number( . . . )

See sections 15.7.1 and 15.7.2.

15.1.3.7 -- Date( . . . )

See section 15.9.2 .

15.1.4 -- Other Properties of the Global Object

15.1.4.1 -- Math

See section 15.8.

15.2 -- Object Objects

15.2.1 -- The Object Constructor Called as a Function

When Object is called as a function rather than as a constructor, it performs a type conversion.

15.2.1.1 -- Object(value)

When the Object function is called with one argument value, the following steps are taken:

  1. If the value is null or undefined, create and return a new object with no properties (other than internal properties) exactly as if the object constructor had been called on that same value (15.2.2.1).
  2. Return ToObject(value).

15.2.1.2 -- Object()

When the Object function is called with no arguments, the following step is taken:

  1. Create and return a new object with no properties (other than internal properties) exactly if the object constructor had been called with no argument (15.2.2.2).

15.2.2 -- The Object Constructor

When Object is called as part of a new expression, it is a constructor that may create an object.

15.2.2.1 -- new Object(value)

When the Object constructor is called with one argument value, the following steps are taken:

  1. If the type of the value is not Object, go to step 4.
  2. If the value is a native ECMAScript object, do not create a new object but simply return value.
  3. If the value is a host object, then actions are taken and a result is returned in an implementation-dependent manner that may depend on the host object.
  4. If the type of the value is String, return ToObject(value).
  5. If the type of the value is Boolean, return ToObject(value).
  6. If the type of the value is Number, return ToObject(value).
  7. (The type of the value must be Null or Undefined.) Create a new native ECMAScript object. Return the newly created native object.

15.2.2.2 -- new Object()

When the Object constructor is called with no argument, the following step is taken:

  1. Create a new native ECMAScript object. Return the newly created native object.

15.2.3 -- Properties of the Object Constructor

The value of the internal [[Prototype]] property of the Object constructor is the Function prototype object.

Besides the internal [[Call]] and [[Construct]] properties and the length property, the Object constructor has the following properties:

15.2.3.1 -- Object.prototype

The initial value of Object.prototype is the built-in Object prototype object (15.2.4).

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.2.4 -- Properties of the Object Prototype Object

The value of the internal [[Prototype]] property of the Object prototype object is null.

15.2.4.1 -- Object.prototype.constructor

The initial value of Object.prototype.constructor is the built-in Object constructor.

15.2.4.2 -- Object.prototype.toString()

When the toString method is called, the following steps are taken:

  1. Get the [[Class]] property of this object.
  2. Compute a string value by concatenating the three strings "[object ", Result(1), and "]".
  3. Return Result(2).
15.2.4.3 -- Object.prototype.valueOf()

As a rule, the valueOf method for an object simply returns the object; but if the object is a ``wrapper'' for a host object, as may perhaps be created by the Object constructor (see section 15.2.2.1), then the contained host object should be returned.

15.2.5 -- Properties of Object Instances

Object instances have no special properties beyond those inherited from the Object prototype object.

15.3 -- Function Objects

15.3.1 -- The Function Constructor Called as a Function

When Function is called as a function rather than as a constructor, it creates and initialises a new function object. Thus the function call Function(...) is equivalent to the object creation expression new Function(...) with the same arguments.

15.3.1.1 -- Function(p1, p2, . . . , pn, body)

When the Function function is called with some arguments p1, p2, . . . , pn, body (where n might be 0, that is, there are no ``p'' arguments, and where body might also not be provided), the following steps are taken:

  1. Create and return a new Function object exactly if the function constructor had been called with the same arguments (15.3.2.1).

15.3.2 -- The Function Constructor

When Function is called as part of a new expression, it is a constructor: it initialises the newly created object.

15.3.2.1 -- new Function(p1, p2, . . . , pn, body)

The last argument specifies the body (executable code) of a function; any preceding arguments specify formal parameters.

When the Function constructor is called with some arguments p1, p2, . . . , pn, body (where n might be 0, that is, there are no ``p'' arguments, and where body might also not be provided), the following steps are taken:

  1. Let P be the empty string.
  2. If no arguments were given, let body be the empty string and go to step 13.
  3. If one argument was given, let body be that argument and go to step 13.
  4. Let Result(4) be the first argument.
  5. Let P be ToString(Result(4)).
  6. Let k be 2.
  7. If k equals the number of arguments, let body be the k'th argument and go to step 13.
  8. Let Result(8) be the k'th argument.
  9. Call ToString(Result(8)).
  10. Let P be the result of concatenating the previous value of P, the string "," (a comma), and Result(9).
  11. Increase k by 1.
  12. Go to step 7.
  13. Call ToString(body).
  14. Let F be the newly constructed Function object.
  15. The [[Class]] property of F is set to "Function".
  16. The [[Prototype]] property of F is set to the original Function prototype object, the one that is the initial value of Function.prototype (15.3.3.1).
  17. Set the [[Call]] property of F to a method such that, when it is invoked, the executable function will be invoked whose formal parameters are specified by P and whose body is specified by Result(13). The string value P must be parsable as a FormalParameterListopt; the string value result(13) must be parsable as a StatementListopt. (Note that both P and Result(13) may contain whitespace, line terminators, and comments.) However, if either P or Result(13) is syntactically incorrect, or otherwise cannot be interpreted as part of a correct ECMAScript function definition, then the [[Call]] property of F is not set and a runtime error is generated..
  18. Set the [[Construct]] property of F to a method that, when it is invoked, If the result of invoking the [[Call]] method is an object, that object becomes the result of the invocation of the [[Construct]] method; otherwise the new object becomes the result of the invocation of the [[Construct]] method.
  19. If the toString method of F is later invoked, it will use ``anonymous'' as the name of the function in rendering the function as a string.
  20. Compute, as an integer number value of positive sign, the number of formal parameters that resulted from the parse of P as a FormalParameterListopt.
  21. The length property of F is set to Result(20). This property is given attributes { DontDelete, DontEnum, ReadOnly }.
  22. Create a new object as if by the expression new Object().
  23. The prototype property of F is set to Result(22). This property is given attributes { DontEnum }.
  24. The constructor property of Result(22) is set to F. This property is given attributes { DontEnum }.
  25. Return F.

Note that 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")

A prototype property is automatically created for every function, against the possibility that the function will be used as a constructor.

15.3.3 -- Properties of the Function Constructor

15.3.3.1 -- Function.prototype

The initial value of Function.prototype is the built-in Function prototype object (15.3.4).

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.3.3.2 -- Function.length

The length property is 1. (Of course, the Function constructor accepts more than one argument, because it accepts a variable number of arguments.)

15.3.4 -- Properties of the Function Prototype Object

The Function prototype object is itself a Function object (its [[Class]] is "Function") that, when invoked, accepts any arguments and returns undefined.

The value of the internal [[Prototype]] property of the Function prototype object is the Object prototype object (15.3.2.1).

It is a function with an ``empty body''; if it is invoked, it merely returns undefined.

The Function prototype object does not have a valueOf property of its own; however, it inherits the valueOf property from the Object prototype object.

15.3.4.1 -- Function.prototype.constructor

The initial value of Function.prototype.constructor is the built-in Function constructor.

15.3.4.2 -- Function.prototype.toString()

An implementation-dependent representation of the function is returned. This representation has the syntax of a FunctionDeclaration. Note in particular that the use and placement of whitespace, line terminators, and semicolons within the representation string is implementation-dependent.

The toString function is not generic; it generates a runtime error if its this value is not a Function object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.3.5 -- Properties of Function Instances

Every function instance has a [[Call]] property and a [[Construct]] property.

15.3.5.1 -- length

The value of the length property is usually 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.

15.3.5.2 -- prototype

The value of the prototype property is used to initialise the internal [[Prototype]] property of a newly created object before the Function object is invoked as a constructor for that newly created object.

15.4 -- Array Objects

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. Every Array object has a length property whose value is always an integer with positive sign and less than 232. It is always the case that 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 properties of the Array object itself and is unaffected by length or array index properties that may be inherited from its prototype.

15.4.1 -- The Array Constructor Called as a Function

When Array is called as a function rather than as a constructor, it creates and initialises a new array object. Thus the function call Array (...) is equivalent to the object creation expression new Array (...) with the same arguments.

15.4.1.1 -- Array(item0, item1, . . .)

An array is created and returned as if by the expression new Array(item0, item1, . . .).

15.4.1.2 -- Array(len)

An array is created and returned as if by the expression new Array(len).

15.4.1.3 -- Array()

An array is created and returned as if by the expression new Array().

15.4.2 -- The Array Constructor

When Array is called as part of a new expression, it is a constructor: it initialises the newly created object.

15.4.2.1 -- new Array(item0, item1, . . .)

This description applies if and only if the Array constructor is given two or more arguments.

The [[Prototype]] 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]] property of the newly constructed object is set to "Array".

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; the 1 property of the newly constructed object is set to item1; 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.

15.4.2.2 -- new Array(len)

The [[Prototype]] 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]] property of the newly constructed object is set to "Array".

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 runtime error is generated.

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.

15.4.2.3 -- new Array()

The [[Prototype]] 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]] property of the newly constructed object is set to "Array".

The length property of the newly constructed object is set to +0.

15.4.3 -- Properties of the Array Constructor

The value of the internal [[Prototype]] property of the Array constructor is the Function prototype object.

Besides the internal [[Call]] and [[Construct]] properties, the Array constructor has the following properties:

15.4.3.1 -- Array.prototype

The initial value of Array.prototype is the built-in Array prototype object (15.4.4).

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.4.3.2 -- Array.length

The length property is 1. (Of course, the Array constructor accepts more than one argument, because it accepts a variable number of arguments.)

15.4.4 -- Properties of the Array Prototype Object

The value of the internal [[Prototype]] property of the Array prototype object is the Object prototype object (15.2.3.1).

Note that the Array prototype object is itself an array; it has a length property (whose initial value is +0) and the special internal [[Put]] method described in section 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 this to refer to an object for which the value of the internal [[Class]] property is not "Array".

The Array prototype object does not have a valueOf property of its own; however, it inherits the valueOf property from the Object prototype object.

15.4.4.1 -- Array.prototype.constructor

The initial value of Array.prototype.constructor is the built-in Array constructor.

15.4.4.2 -- Array.prototype.toString()

The elements of this object are converted to strings, and these strings are then concatenated, separated by comma characters. The result is the same as if the built-in join method were invoked for this object with no argument.

15.4.4.3 -- Array.prototype.join(separator)

The elements of the array are converted to strings, and these strings are then concatenated, separated by occurrences of the separator. If no separator is provided, a single comma is used as the separator.

When the join method is called with one argument separator, the following steps are taken:

  1. Call the [[Get]] method of this object with argument "length".
  2. Call ToUint32(Result(1)).
  3. If separator is not supplied, let separator be the single-character string ",".
  4. Call ToString(separator).
  5. If Result(2) is zero, return the empty string.
  6. Call the [[Get]] method of this object with argument "0".
  7. If Result(6) is undefined or null, use the empty string; otherwise, call ToString(Result(6)).
  8. Let R be Result(7).
  9. Let k be 1.
  10. If k equals Result(2), return R.
  11. Let S be a string value produced by concatenating R and Result(4).
  12. Call the [[Get]] method of this object with argument ToString(k).
  13. If Result(12) is undefined or null, use the empty string; otherwise, call ToString(Result(12)).
  14. Let R be a string value produced by concatenating S and Result(13).
  15. Increase k by 1.
  16. Go to step 10.

Note that the join function is intentionally generic; it does not require that its this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method. Whether the join function can be applied successfully to a host object is implementation-dependent.

15.4.4.4 -- Array.prototype.reverse()

The elements of the array are rearranged so as to reverse their order. The object is returned as the result of the call.

  1. Call the [[Get]] method of this object with argument "length".
  2. Call ToUint32(Result(1)).
  3. Compute floor(Result(2)/2).
  4. Let k be 0.
  5. If k equals Result(3), return this object.
  6. Compute Result(2)-k-1.
  7. Call ToString(k).
  8. Call ToString(Result(6)).
  9. Call the [[Get]] method of this object with argument Result(7).
  10. Call the [[Get]] method of this object with argument Result(8).
  11. If this object has a property named by Result(8), go to step 12; but if this object has no property named by Result(8), then go to either step 12 or step 14, depending on the implementation.
  12. Call the [[Put]] method of this object with arguments Result(7) and Result(10).
  13. Go to step 15.
  14. Call the [[Delete]] method on this object, providing Result(7) as the name of the property to delete.
  15. If this object has a property named by Result(7), go to step 16; but if this object has no property named by Result(7), then go to either step 16 or step 18, depending on the implementation.
  16. Call the [[Put]] method of this object with arguments Result(8) and Result(9).
  17. Go to step 19.
  18. Call the [[Delete]] method on this object, providing Result(8) as the name of the property to delete.
  19. Increase k by 1.
  20. Go to step 5.

Note that the reverse function is intentionally generic; it does not require that its this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method. Whether the reverse function can be applied successfully to a host object is implementation-dependent.

15.4.4.5 -- Array.prototype.sort(comparefn)

The elements of this array are sorted. The sort is not necessarily stable. If comparefn is supplied, 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.

If comparefn is supplied and is not a consistent comparison function for the elements of this array (see below), the result is implementation-defined. Otherwise the following steps are taken:

  1. Call the [[Get]] method of this object with argument "length".
  2. Call ToUint32(Result(1)).
  3. Perform an implementation-dependent sequence of calls to the [[Get]] , [[Put]], and [[Delete]] methods of this object and to SortCompare (described below), where the first argument for each call to [[Get]], [[Put]], or [[Delete]] is a nonnegative integer less than Result(2) and where the arguments for calls to SortCompare are results of previous calls to the [[Get]] method.
  4. Return this object.

The returned object must have the following two properties.

  1. There must be some mathematical permutation π of the nonnegative integers less than Result(2), such that for every nonnegative integer j less than Result(2), if property old[j] existed, then new[π (j)] is exactly the same value as old[j],. but if property old[j] did not exist, then new[π (j)] either does not exist or exists with value undefined.
  2. Then for all nonnegative integers j and k, each less than Result(2), if old[j] compares less than old[k] (see SortCompare below), then π (j) < π(k).

Here the notation old[j] is used to refer to the hypothetical result of calling the [[Get]] method of this object with argument j before this function is executed, and the notation new[j] to refer to the hypothetical result of calling the [[Get]] method of this object with argument j after this function has been executed.

A function is a consistent comparison function for a set of values if

When the SortCompare operator is called with two arguments x and y, the following steps are taken:

  1. If x and y are both undefined, return +0.
  2. If x is undefined, return 1.
  3. If y is undefined, return -1.
  4. If the argument comparefn was not provided in the call to sort, go to step 7.
  5. Call comparefn with arguments x and y.
  6. Return Result(5).
  7. Call ToString(x).
  8. Call ToString(y).
  9. If Result(7) < Result(8), return -1.
  10. If Result(7) > Result(8), return 1.
  11. Return +0.

Note that, because undefined always compares greater than any other value, undefined and non-existent property values always sort to the end of the result. It is implementation-dependent whether or not such properties will exist or not at the end of the array when the sort is concluded.

Note that the sort function is intentionally generic; it does not require that its this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method. Whether the sort function can be applied successfully to a host object is implementation-dependent .

15.4.5 -- Properties of Array Instances

Array instances inherit properties from the Array prototype object and also have the following properties.

15.4.5.1 -- [[Put]](P, V)

Array objects use a variation of the [[Put]] method used for other native ECMAScript objects (section 8.6.2.2).

Assume A is an Array object and P is a string.

When the [[Put]] method of A is called with property P and value V, the following steps are taken:

  1. Call the [[CanPut]] method of A with name P.
  2. If Result(1) is false, return.
  3. If A doesn't have a property with name P, go to step 7.
  4. If P is "length", go to step 12.
  5. Set the value of property P of A to V.
  6. Go to step 8.
  7. Create a property with name P, set its value to V and give it empty attributes.
  8. If P is not an array index, return.
  9. If ToUint32(P) is less than the value of the length property of A, then return.
  10. Change (or set) the value of the length property of A to ToUint32(P)+1.
  11. Return.
  12. Compute ToUint32(V).
  13. If Result(12) is not equal to ToInteger(V), generate a runtime error.
  14. For every integer k that is less than the value of the length property of A but not less than Result(12), if A itself has a property (not an inherited property) named ToString(k), then delete that property.
  15. Set the value of property P of A to Result(12).
  16. Return.
15.4.5.2 -- length

The length property of this Array object is always numerically greater than the name of every property whose name is an array index.

The length property has the attributes { DontEnum, DontDelete }.

15.5 -- String Objects

15.5.1 -- The String Constructor Called as a Function

When String is called as a function rather than as a constructor, it performs a type conversion.

15.5.1.1 -- String(value)

Returns a string value (not a string object) computed by ToString(value).

15.5.1.2 -- String()

Returns the empty string "".

15.5.2 -- The String Constructor

When String is called as part of a new expression, it is a constructor: it initialises the newly created object.

15.5.2.1 -- new String(value)

The [[Prototype]] property of the newly constructed object is set to the original String prototype object, the one that is the initial value of String.prototype (15.5.3.1).

The [[Class]] property of the newly constructed object is set to "String".

The [[Value]] property of the newly constructed object is set to ToString(value).

15.5.2.2 -- new String()

The [[Prototype]] property of the newly constructed object is set to the original String prototype object, the one that is the initial value of String.prototype (15.5.3.1).

The [[Class]] property of the newly constructed object is set to "String".

The [[Value]] property of the newly constructed object is set to the empty string.

15.5.3 -- Properties of the String Constructor

The value of the internal [[Prototype]] property of the String constructor is the Function prototype object.

Besides the internal [[Call]] and [[Construct]] properties and the length property, the String constructor has the following properties:

15.5.3.1 -- String.prototype

The initial value of String.prototype is the built-in String prototype object (15.5.4).

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.5.3.2 -- String.fromCharCode(char0, char1, . . .)

Returns a string value containing as many characters as the number of arguments. Each argument specifies one character of the resulting string, with the first argument specifying the first character, and so on, from left to right. An argument is converted to a character by applying the operation ToUint16 (section 9.7) and regarding the resulting 16-bit integer as the Unicode code point encoding of a character. If no arguments are supplied, the result is the empty string.

15.5.4 -- Properties of the String Prototype Object

The String prototype object is itself a string object (its [[Class]] is "String") whose value is an empty string.

The value of the internal [[Prototype]] property of the String prototype object is the Object prototype object (15.2.3.1).

15.5.4.1 -- String.prototype.constructor

The initial value of String.prototype.constructor is the built-in String constructor.

15.5.4.2 -- String.prototype.toString()

Returns this string value. (Note that, for a string object, the toString method happens to return the same thing as the valueOf method.)

The toString function is not generic; it generates a runtime error if its this value is not a string object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.5.4.3 -- String.prototype.valueOf()

Returns this string value.

The valueOf function is not generic; it generates a runtime error if its this value is not a string object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.5.4.4 -- String.prototype.charAt(pos)

Returns a string containing the character at position pos in the string resulting from converting this object to a string. If there is no character at that position, the result is the empty string. The result is a string value, not a string object.

If pos is a value of Number type that is an integer, then the result of x.charAt(pos) is equal to the result of x.substring(pos, pos+1).

When the charAt method is called with one argument pos, the following steps are taken:

  1. Call ToString, giving it the this value as its argument.
  2. Call ToInteger(pos).
  3. Compute the number of characters in Result(1).
  4. If Result(2) is less than 0 or is not less than Result(3), return the empty string.
  5. Return a string of length 1, containing one character from Result(1), namely the character at position Result(2), where the first (leftmost) character in Result(1) is considered to be at position 0, the next one at position 1, and so on.
NOTE The charAt function is intentionally generic; it does not require that its this value be a string object. Therefore, it can be transferred to other kinds of objects for use as a method.
15.5.4.5 -- String.prototype.charCodeAt(pos)

Returns a number (a nonnegative integer less than 216) representing the Unicode code point encoding 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:

  1. Call ToString, giving it the this value as its argument.
  2. Call ToInteger(pos).
  3. Compute the number of characters in Result(1).
  4. If Result(2) is less than 0 or is not less than Result(3), return NaN.
  5. Return a value of Number type, of positive sign, whose magnitude is the Unicode encoding of one character from Result(1), namely the character at position Result(2), where the first (leftmost) character in Result(1) is considered to be at position 0, the next one at position 1, and so on.
NOTE The charCodeAt function is intentionally generic; it does not require that its this value be a string object. Therefore, it can be transferred to other kinds of objects for use as a method.
15.5.4.6 -- String.prototype.indexOf(searchString, position)

If the given searchString appears as a substring of the result of converting this object to a string, at one or more positions that are at or to the right of the specified position, then the index of the leftmost such position is returned; otherwise, -1 is returned. If position is undefined or not supplied, 0 is assumed, so as to search all of the string.

When the indexOf method is called with two arguments searchString and position, the following steps are taken:

  1. Call ToString, giving it the this value as its argument.
  2. Call ToString(searchString).
  3. Call ToInteger(position). (If position is undefined or not supplied, this step produces the value 0).
  4. Compute the number of characters in Result(1).
  5. Compute min(max(Result(3), 0), Result(4)).
  6. Compute the number of characters in the string that is Result(2).
  7. Compute the smallest possible integer k not smaller than Result(5) such that k+Result(6) is not greater than Result(4), and for all nonnegative integers j less than Result(6), the character at position k+j of Result(1) is the same as the character at position j of Result(2); but if there is no such integer k, then compute the value -1.
  8. Return Result(7).
NOTE The indexOf function is intentionally generic; it does not require that its this value be a string object. Therefore, it can be transferred to other kinds of objects for use as a method.
15.5.4.7 -- String.prototype.lastIndexOf(searchString, position)

If the given searchString appears as a substring of the result of converting this object to a string, at one or more positions that are at or to the left of the specified position, then the index of the rightmost such position is returned; otherwise, -1 is returned. If position is undefined or not supplied, the length of the string value is assumed, so as to search all of the string.

When the lastIndexOf method is called with two arguments searchString and position, the following steps are taken:

  1. Call ToString, giving it the this value as its argument.
  2. Call ToString(searchString).
  3. Call ToNumber(position). (If position is undefined or not supplied, this step produces the value NaN).
  4. If Result(3) is NaN, use +∞; otherwise, call ToInteger(Result(3)).
  5. Compute the number of characters in Result(1).
  6. Compute min(max(Result(4), 0), Result(5)).
  7. Compute the number of characters in the string that is Result(2).
  8. Compute the largest possible integer k not larger than Result(6) such that k+Result(7) is not greater than Result(5), and for all nonnegative integers j less than Result(7), the character at position k+j of Result(1) is the same as the character at position j of Result(2); but if there is no such integer k, then compute the value -1.
  9. Return Result(8).
NOTE The lastIndexOf function is intentionally generic; it does not require that its this value be a string object. Therefore, it can be transferred to other kinds of objects for use as a method.
15.5.4.8 -- String.prototype.split(separator)

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 the given separator; these occurrences are not part of any substring in the returned array, but serve to divide up the string value. The separator may be a string of any length.

As a special case, if the 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 the separator is not supplied, then the result array contains just one string, which is the string.

When the split method is called with one argument separator, the following steps are taken:

  1. Call ToString, giving it the this value as its argument.
  2. Create a new Array object of length 0 and call it A.
  3. If separator is not supplied, call the [[Put]] method of A with 0 and Result(1) as arguments, and then return A.
  4. Call ToString(separator).
  5. Compute the number of characters in Result(1).
  6. Compute the number of characters in the string that is Result(4).
  7. Let p be 0.
  8. If Result(6) is zero (the separator string is empty), go to step 17.
  9. Compute the smallest possible integer k not smaller than p such that k+Result(6) is not greater than Result(5), and for all nonnegative integers j less than Result(6), the character at position k+j of Result(1) is the same as the character at position j of Result(2); but if there is no such integer k, then go to step 14.
  10. Compute a string value equal to the substring of Result(1), consisting of the characters at positions p through k-1, inclusive.
  11. Call the [[Put]] method of A with A.length and Result(10) as arguments.
  12. Let p be k+Result(6).
  13. Go to step 9.
  14. Compute a string value equal to the substring of Result(1), consisting of the characters from position p to the end of Result(1).
  15. Call the [[Put]] method of A with A.length and Result(14) as arguments.
  16. Return A.
  17. If p equals Result(5), return A.
  18. Compute a string value equal to the substring of Result(1), consisting of the single character at position p.
  19. Call the [[Put]] method of A with A.length and Result(18) as arguments.
  20. Increase p by 1.
  21. Go to step 17.
NOTE The split function is intentionally generic; it does not require that its this value be a string object. Therefore, it can be transferred to other kinds of objects for use as a method.
15.5.4.9 -- String.prototype.substring(start)

Returns a substring of the result of converting this object to a string, starting from character position start and running to the end of the string. The result is a string value, not a string object.

If the argument is NaN or negative, it is replaced with zero; if the argument is larger than the length of the string, it is replaced with the length of the string.

When the substring method is called with one argument start, the following steps are taken:

  1. Call ToString, giving it the this value as its argument.
  2. Call ToInteger(start).
  3. Compute the number of characters in Result(1).
  4. Compute min(max(Result(2), 0), Result(3)).
  5. Return a string whose length is the difference between Result(3) and Result(4), containing characters from Result(1), namely the characters with indices Result(4) through Result(3)-1, in ascending order.

15.5.4.10 -- String.prototype.substring(start, end)

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. 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.

When the substring method is called with two arguments start and end, the following steps are taken:

  1. Call ToString, giving it the this value as its argument.
  2. Call ToInteger(start).
  3. Call ToInteger (end).
  4. Compute the number of characters in Result(1).
  5. Compute min(max(Result(2), 0), Result(4)).
  6. Compute min(max(Result(3), 0), Result(4)).
  7. Compute min(Result(5), Result(6)).
  8. Compute max(Result(5), Result(6)).
  9. Return a string whose length is the difference between Result(8) and Result(7), containing characters from Result(1), namely the characters with indices Result(7) through Result(8)-1, in ascending order.

NOTE The substring function is intentionally generic; it does not require that its this value be a string object. Therefore, it can be transferred to other kinds of objects for use as a method.
15.5.4.11 -- String.prototype.toLowerCase

Returns a string equal in length to the length of the result of converting this object to a string. The result is a string value, not a string object.

Every character of the result is equal to the corresponding character of the string, unless that character has a Unicode 2.0 lowercase equivalent, in which case the lowercase equivalent is used instead. (The canonical Unicode 2.0 case mapping shall be used, which does not depend on implementation or locale.)

Note that the toLowerCase function is intentionally generic; it does not require that its this value be a string object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.12 -- String.prototype.toUpperCase

Returns a string equal in length to the length of the result of converting this object to a string. The result is a string value, not a string object.

Every character of the result is equal to the corresponding character of the string, unless that character has a Unicode 2.0 uppercase equivalent, in which case the uppercase equivalent is used instead. (The canonical Unicode 2.0 case mapping shall be used, which does not depend on implementation or locale.)

Note that the toUpperCase function is intentionally generic; it does not require that its this value be a string object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.5 -- Properties of String Instances

String instances inherit properties from the String prototype object and also have a [[Value]] property and a length property.

The [[Value]] property is the string value represented by this string object.

15.5.5.1 -- length

The number of characters in the String value represented by this string object.

Once a string object is created, this property is unchanging. It has the attributes { DontEnum, DontDelete, ReadOnly }.

15.6 -- Boolean Objects

15.6.1 -- The Boolean Constructor Called as a Function

When Boolean is called as a function rather than as a constructor, it performs a type conversion.

15.6.1.1 -- Boolean(value)

Returns a Boolean value (not a boolean object) computed by ToBoolean(value).

15.6.1.2 -- Boolean()

Returns false.

15.6.2 -- The Boolean Constructor

When Boolean is called as part of a new expression, it is a constructor: it initialises the newly created object.

15.6.2.1 -- new Boolean(value)

The [[Prototype]] 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]] property of the newly constructed boolean object is set to "Boolean".

The [[Value]] property of the newly constructed boolean object is set to ToBoolean(value).

15.6.2.2 -- new Boolean()

The [[Prototype]] 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]] property of the newly constructed boolean object is set to "Boolean".

The [[Value]] property of the newly constructed boolean object is set to false.

15.6.3 -- Properties of the Boolean Constructor

The value of the internal [[Prototype]] property of the Boolean constructor is the Function prototype object.

Besides the internal [[Call]] and [[Construct]] properties and the length property, the Boolean constructor has the following property:

15.6.3.1 -- Boolean.prototype

The initial value of Boolean.prototype is the built-in Boolean prototype object (15.6.4).

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.6.4 -- Properties of the Boolean Prototype Object

The Boolean prototype object is itself a boolean object (its [[Class]] is "Boolean") whose value is false.

The value of the internal [[Prototype]] property of the Boolean prototype object is the Object prototype object (15.2.3.1).

In following descriptions of functions that are properties of the Boolean prototype object, the phrase ``this boolean object'' refers to the object that is the this value for the invocation of the function; it is a runtime error if this does not refer to an object for which the value of the internal [[Class]] property is "Boolean". Also, the phrase ``this boolean value'' refers to the boolean value represented by this boolean object, that is, the value of the internal [[Value]] property of this boolean object.

15.6.4.1 -- Boolean.prototype.constructor

The initial value of Boolean.prototype.constructor is the built-in Boolean constructor.

15.6.4.2 -- Boolean.prototype.toString()

If this boolean value is true, then the string "true" is returned. Otherwise, this boolean value must be false, and the string "false" is returned.

The toString function is not generic; it generates a runtime error if its this value is not a boolean object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.6.4.3 -- Boolean.prototype.valueOf()

Returns this boolean value.

The valueOf function is not generic; it generates a runtime error if its this value is not a boolean object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.6.5 -- Properties of Boolean Instances

Boolean instances have no special properties beyond those inherited from the Boolean prototype object.

15.7 -- Number Objects

15.7.1 -- The Number Constructor Called as a Function

When Number is called as a function rather than as a constructor, it performs a type conversion.

15.7.1.1 -- Number(value)

Returns a number value (not a Number object) computed by ToNumber(value).

15.7.1.2 -- Number()

Returns +0.

15.7.2 -- The Number Constructor

When Number is called as part of a new expression, it is a constructor: it initialises the newly created object.

15.7.2.1 -- new Number(value)

The [[Prototype]] 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]] property of the newly constructed object is set to "Number".

The [[Value]] property of the newly constructed object is set to ToNumber(value).

15.7.2.2 -- new Number()

The [[Prototype]] 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]] property of the newly constructed object is set to "Number".

The [[Value]] property of the newly constructed object is set to +0.

15.7.3 -- Properties of the Number Constructor

The value of the internal [[Prototype]] property of the Number constructor is the Function prototype object.

Besides the internal [[Call]] and [[Construct]] properties and the length property, the Number constructor has the following property:

15.7.3.1 -- Number.prototype

The initial value of Number.prototype is the built-in Number prototype object.

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.7.3.2 -- Number.MAX_VALUE

The value of Number.MAX_VALUE is the largest positive finite value of the number type, which is approximately 1.7976931348623157e308.

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.7.3.3 -- Number.MIN_VALUE

The value of Number.MIN_VALUE is the smallest positive nonzero value of the number type, which is approximately 5e-324.

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.7.3.4 -- Number.NaN

The value of Number.NaN is NaN.

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.7.3.5 -- Number.NEGATIVE_INFINITY

The value of Number.NEGATIVE_INFINITY is -∞ .

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.7.3.6 -- Number.POSITIVE_INFINITY

The value of Number.POSITIVE_INFINITY is +∞.

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.7.4 -- Properties of the Number Prototype Object

The Number prototype object is itself a Number object (its [[Class]] is "Number") whose value is +0.

The value of the internal [[Prototype]] property of the Number prototype object is the Object prototype object (15.2.3.1).

In following descriptions of functions that are properties of the Number prototype object, the phrase ``this Number object'' refers to the object that is the this value for the invocation of the function; it is a runtime error if this does not refer to an object for which the value of the internal [[Class]] property is "Number". Also, the phrase ``this number value'' refers to the number value represented by this Number object, that is, the value of the internal [[Value]] property of this Number object.

15.7.4.1 -- Number.prototype.constructor

The initial value of Number.prototype.constructor is the built-in Number constructor.

15.7.4.2 -- Number.prototype.toString(radix)

If the radix is the number 10 or not supplied, then this number value is given as an argument to the ToString operator; the resulting string value is returned.

If the radix is supplied and is an integer from 2 to 36, but not 10, the result is a string, the choice of which is implementation-dependent.

The toString function is not generic; it generates a runtime error if its this value is not a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.7.4.3 -- Number.prototype.valueOf()

Returns this number value.

The valueOf function is not generic; it generates a runtime error if its this value is not a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.7.5 -- Properties of Number Instances

Number instances have no special properties beyond those inherited from the Number prototype object.

15.8 -- The Math Object

The Math object is merely a single object that has some named properties, some of which are functions.

The value of the internal [[Prototype]] property of the Math object is the Object prototype object (15.2.3.1).

The Math object does not have a [[Construct]] 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]] 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 section 8.5.

15.8.1 -- Value Properties of the Math Object

15.8.1.1 -- E

The number value for e, the base of the natural logarithms, which is approximately 2.7182818284590452354.

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.8.1.2 -- LN10

The number value for the natural logarithm of 10, which is approximately 2.302585092994046.

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.8.1.3 -- LN2

The number value for the natural logarithm of 2, which is approximately 0.6931471805599453.

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.8.1.4 -- LOG2E

The number value for the base-2 logarithm of e, the base of the natural logarithms; this value is approximately 1.4426950408889634. (Note that the value of Math.LOG2E is approximately the reciprocal of the value of Math.LN2.)

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.8.1.5 -- LOG10E

The number value for the base-10 logarithm of e, the base of the natural logarithms; this value is approximately 0.4342944819032518. (Note that the value of Math.LOG10E is approximately the reciprocal of the value of Math.LN10.)

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.8.1.6 -- PI

The number value for π, the ratio of the circumference of a circle to its diameter, which is approximately 3.14159265358979323846.

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.8.1.7 -- SQRT1_2

The number value for the square root of 1/2, which is approximately 0.7071067811865476. (Note that the value of Math.SQRT1_2 is approximately the reciprocal of the value of Math.SQRT2.)

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.8.1.8 -- SQRT2

The number value for the square root of 2, which is approximately 1.4142135623730951.

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.8.2 -- Function Properties of the Math Object

Every function listed in this section applies the ToNumber 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).

The behaviour of the functions acos, asin, atan, atan2, cos, exp, log, pow, sin, and sqrt is not precisely specified here except to require specific results for certain argument values that represent boundary cases of interest.. For other argument values, these functions are intended to compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of approximation algorithms. The general intent is that an implementor 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 (fdlibm-comment@sunpro.eng.1.com).

15.8.2.1 -- abs(x)

Returns the absolute value of its argument; in general, the result has the same magnitude as the argument but has positive sign.

15.8.2.2 -- acos(x)

Returns an implementation-dependent approximation to the arc cosine of the argument. The result is expressed in radians and ranges from +0 to +π.

15.8.2.3 -- asin(x)

Returns an implementation-dependent approximation to the arc sine of the argument. The result is expressed in radians and ranges from -π/2 to +π/2.

15.8.2.4 -- atan(x)

Returns an implementation-dependent approximation to the arc tangent of the argument. The result is expressed in radians and ranges from -π/2 to +π/2.

15.8.2.5 -- atan2(y, x)

Returns an implementation-dependent approximation to the arc tangent of the quotient y/x of the arguments y and x, where the signs of the arguments 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 .

15.8.2.6 -- ceil(x)

Returns the smallest (closest to -∞) number value that is not less than the argument and is equal to a mathematical integer. If the argument is already an integer, the result is the argument itself.

The value of Math.ceil(x) is the same as the value of -Math.floor(-x).

15.8.2.7 -- cos(x)

Returns an implementation-dependent approximation to the cosine of the argument. The argument is expressed in radians.

15.8.2.8 -- exp(x)

Returns an implementation-dependent approximation to the exponential function of the argument (e raised to the power of the argument, where e is the base of the natural logarithms).

15.8.2.9 -- floor(x)

Returns the greatest (closest to +∞) number value that is not greater than the argument and is equal to a mathematical integer. If the argument is already an integer, the result is the argument itself.

NOTE The value of Math.floor(x) is the same as the value of -Math.ceil(-x).
15.8.2.10 -- log(x)

Returns an implementation-dependent approximation to natural logarithm of the argument.

15.8.2.11 -- max(x, y)

Returns the larger of the two arguments.

15.8.2.12 -- min(x, y)

Returns the smaller of the two arguments.

15.8.2.13 -- pow(x, y)

Returns an implementation-dependent approximation to the result of raising x to the power y.

15.8.2.14 -- random()

Returns a number value with positive sign, greater than or equal to 0 but less than 1, chosen randomly or pseudo randomly with approximately uniform distribution over that range, using an implementation-dependent algorithm or strategy. This function takes no arguments.

15.8.2.15 -- round(x)

Returns the number value that is closest to the argument and is equal to a mathematical integer. If two integer number values are equally close to the argument, then the result is the number value that is closer to +∞. If the argument is already an integer, the result is the argument itself.

NOTE Math.round(3.5) returns 4, but Math.round(-3.5) returns -3.

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.

15.8.2.16 -- sin(x)

Returns an implementation-dependent approximation to the sine of the argument. The argument is expressed in radians.

15.8.2.17 -- sqrt(x)

Returns an implementation-dependent approximation to the square root of the argument.

15.8.2.18 -- tan(x)

Returns an implementation-dependent approximation to the tangent of the argument. The argument is expressed in radians.

15.9 -- Date Objects

15.9.1 -- Overview of Date Objects and Definitions of Internal Operators

A Date object contains a number indicating a particular instant in time to within a millisecond. The number may also be NaN, indicating that the Date object does not represent a specific instant of time.

The following sections define a number of functions for operating on time values. Note that, in every case, if any argument to such a function is NaN, the result will be NaN.

15.9.1.1 -- Time Range

Time is measured in ECMAScript in milliseconds since 01 January, 1970 UTC. 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.

15.9.1.2 -- Day Number and Time within Day

A given time value t belongs to day number

Day(t) = floor(t / msPerDay)
where the number of milliseconds per day is
msPerDay = 86400000
The remainder is called the time within the day:
TimeWithinDay(t) = t modulo msPerDay

15.9.1.3 -- Year Number

ECMAScript uses an extrapolated Gregorian system to map a day number to a year number and to determine the month and date within that year. In this system, leap years are precisely those which are (divisible by 4) and ((not divisible by 100) or (divisible by 400)). The number of days in year number y is therefore defined by

DaysInYear(y)
= 365 if (y modulo 4) ≠ 0
= 366 if (y modulo 4) = 0 and (y modulo 100) ≠ 0
= 365 if (y modulo 100) = 0 and (y modulo 400) ≠ 0
= 366 if (y modulo 400) = 0

All non-leap years have 365 days with the usual number of days per month and leap years have an extra day in February. The day number of the first day of year y is given by:

DayFromYear(y)
= 365 * (y-1970) + floor((y-1969)/4) -floor((y-1901)/100) + floor((y-1601)/400)

The time value of the start of a year is:

TimeFromYear(y)
= msPerDay * DayFromYear(y)

A time value determines a year by:

YearFromTime(t)
= the largest integer y (closest to positive infinity) such that TimeFromYear(y) ≤ t

The leap-year function is 1 for a time within a leap year and otherwise is zero:

InLeapYear(t)
= 0 if DaysInYear(YearFromTime(t)) = 365
= 1 if DaysInYear(YearFromTime(t)) = 366
15.9.1.4 -- Month Number

Months are identified by an integer in the range 0 to 11, inclusive. The mapping MonthFromTime(t) from a time value t to a month number is defined by:
MonthFromTime(t) = 0if 0 ≤ DayWithinYear(t) < 31
= 1if 31 ≤ DayWithinYear(t) < 59+InLeapYear(t)
= 2if 59+InLeapYear(t) ≤ DayWithinYear (t) < 90+InLeapYear(t)
= 3if 90+InLeapYear(t) ≤ DayWithinYear (t) < 120+InLeapYear(t)
= 4if 120+InLeapYear(t) ≤ DayWithinYear (t) < 151+InLeapYear(t)
= 5if 151+InLeapYear(t) ≤ DayWithinYear (t) < 181+InLeapYear(t)
= 6if 181+InLeapYear(t) ≤ DayWithinYear (t) < 212+InLeapYear(t)
= 7if 212+InLeapYear(t) ≤ DayWithinYear (t) < 243+InLeapYear(t)
= 8if 243+InLeapYear(t) ≤ DayWithinYear (t) < 273+InLeapYear(t)
= 9if 273+InLeapYear(t) ≤ DayWithinYear (t) < 304+InLeapYear(t)
= 10if 304+InLeapYear(t) ≤ DayWithinYear (t) < 334+InLeapYear(t)
= 11if 334+InLeapYear(t) ≤ DayWithinYear (t) < 365+InLeapYear(t)
where

DayWithinYear(t)
= Day(t)-DayFromYear(YearFromTime(t))

A month value of

Note that MonthFromTime(0) = 0, corresponding to Thursday, 01 January, 1970.

15.9.1.5 -- Date Number

A date number is identified by an integer in the range 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

15.9.1.6 -- Week Day

The weekday for a particular time value t is defined as

WeekDay(t)
= (Day(t) + 4) modulo 7

A weekday value of

Note that WeekDay(0) = 4, corresponding to Thursday, 01 January, 1970.

15.9.1.7 -- Local Time Zone Adjustment

An implementation of ECMAScript is expected to determine the local time zone adjustment. The local time zone adjustment is a value LocalTZA measured in milliseconds which when added to UTC represents the local standard time. Daylight saving time is not reflected by LocalTZA. The value LocalTZA does not vary with time but depends only on the geographic location.

15.9.1.8 -- Daylight Saving Time Adjustment

An implementation of ECMAScript is expected to determine the daylight saving time algorithm. The algorithm to determine the daylight saving time adjustment DaylightSavingTA(t), measured in milliseconds, must depend only on four things:

(1) the time since the beginning of the year
t - TimeFromYear(YearFromTime(t))
(2) whether t is in a leap year
InLeapYear(t)
(3) the week day of the beginning of the year
WeekDay(TimeFromYear(YearFromTime(t))
and
(4) the geographic location.

The implementation of ECMAScript should not try to determine whether the exact time was subject to daylight saving time, but just whether daylight saving time would have been in effect if the current daylight saving time algorithm had been used at the time. This avoids complications such as taking into account the years that the locale observed daylight saving time year round.

If the underlying operating system 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 operating system provides daylight saving time information. The only restriction is that all equivalent years should produce the same result.

15.9.1.9 -- Local Time

Conversion from UTC to local time is defined by

LocalTime(t)
= t + LocalTZA + DaylightSavingTA(t)

Conversion from local time to UTC is defined by

UTC(t)
= t - LocalTZA - DaylightSavingTA (t - LocalTZA)

Note that UTC(LocalTime(t)) is not necessarily always equal to t.

15.9.1.10 -- Hours, Minutes, Second, and Milliseconds

The following functions are useful in decomposing time values:

HourFromTime(t)
= floor(t / msPerHour) modulo HoursPerDay
MinFromTime(t)
= floor(t / msPerMinute) modulo MinutesPerHour
SecFromTime(t)
= floor(t / msPerSecond) modulo SecondsPerMinute
msFromTime(t)
= t modulo msPerSecond
where
15.9.1.11 -- MakeTime(hour, min, sec, ms)

The operator MakeTime calculates a number of milliseconds from its four arguments, which must be ECMAScript number values. This operator functions as follows:

  1. If hour is not finite or min is not finite or sec is not finite or ms is not finite, return NaN.
  2. Call ToInteger(hour).
  3. Call ToInteger(min).
  4. Call ToInteger(sec).
  5. Call ToInteger(ms).
  6. Compute Result(2) * msPerHour + Result(3) * msPerMinute + Result(4) * msPerSecond + Result(5), performing the arithmetic according to IEEE 754 rules (that is, as if using the ECMAScript operators * and +).
  7. Return Result(6).
15.9.1.12 -- MakeDay(year, month, date)

The operator MakeDay calculates a number of days from its three arguments, which must be ECMAScript number values. This operator functions as follows:

  1. If year is not finite or month is not finite or date is not finite, return NaN.
  2. Call ToInteger(year).
  3. Call ToInteger(month).
  4. Call ToInteger(date).
  5. Compute Result(2) + floor(Result(3)/12).
  6. Compute Result(3) modulo 12.
  7. Find a value t such that YearFromTime(t)==Result(5) and MonthFromTime(t)==Result(6) and DateFromTime(t)==1; but if this is not possible (because some argument is out of range), return NaN.
  8. Compute Day(Result(7)) + Result(4) -1.
  9. Return Result(8).
15.9.1.13 -- MakeDate(day, time)

The operator MakeDate calculates a number of milliseconds from its two arguments, which must be ECMAScript number values. This operator functions as follows:

  1. If day is not finite or time is not finite, return NaN.
  2. Compute day * msPerDay + time.
  3. Return Result(2).

15.9.1.14 -- TimeClip(time)

The operator TimeClip calculates a number of milliseconds from its argument, which must be an ECMAScript number value. This operator functions as follows:

  1. If time is not finite, return NaN.
  2. If abs(Result(1)) > 8.64e15 (that is, 8.64 * 1015), return NaN.
  3. Return an implementation-dependent choice of either ToInteger(Result(2)) or ToInteger(Result(2)) + (+0). (Adding a positive zero converts -0 to +0.)
NOTE The point of step 3 is that an implementation is permitted a choice of internal representations of time values, for example as a 64-bit signed integer or as a 64-bit floating-point value. Depending on the implementation, this internal representation may or may not distinguish -0 and +0.

15.9.2 -- The Date Constructor Called As a Function

When Date is called as a function rather than as a constructor, it returns a string representing the current time (UTC). Note that the function call Date (...) is not equivalent to the object creation expression new Date(...) with the same arguments.

15.9.2.1 -- Date(year, month, date, hours, minutes, seconds, ms)

The arguments are accepted but are completely ignored. A string is created and returned as if by the expression (new Date ()).toString().

15.9.2.2 -- Date(year, month, date, hours, minutes, seconds)

The arguments are accepted but are completely ignored. A string is created and returned as if by the expression (new Date ()).toString().

15.9.2.3 -- Date(year, month, date, hours, minutes)

The arguments are accepted but are completely ignored. A string is created and returned as if by the expression (new Date ()).toString().

15.9.2.4 -- Date(year, month, date, hours)

The arguments are accepted but are completely ignored. A string is created and returned as if by the expression (new Date ()).toString().

15.9.2.5 -- Date(year, month, day)

The arguments are accepted but are completely ignored. A string is created and returned as if by the expression (new Date ()).toString().

15.9.2.6 -- Date(year, month)

The arguments are accepted but are completely ignored. A string is created and returned as if by the expression (new Date ()).toString().

15.9.2.7 -- Date(value)

The argument is accepted but is completely ignored. A string is created and returned as if by the expression (new Date ()).toString().

15.9.2.8 -- Date()

A string is created and returned as if by the expression (new Date ()).toString().

15.9.3 -- The Date Constructor

When Date is called as part of a new expression, it is a constructor: it initialises the newly created object.

15.9.3.1 -- new Date(year, month, date, hours, minutes, seconds, ms)

The [[Prototype]] property of the newly constructed object is set to the original Date prototype object, the one that is the initial value of Date.prototype.

The [[Class]] property of the newly constructed object is set to "Date".

The [[Value]] property of the newly constructed object is set as follows:

  1. Call ToNumber(year).
  2. Call ToNumber(month).
  3. Call ToNumber(date).
  4. Call ToNumber(hours).
  5. Call ToNumber(minutes).
  6. Call ToNumber(seconds).
  7. Call ToNumber(ms).
  8. If Result(1) is not NaN and 0 ≤ ToInteger(Result(1)) ≤ 99, Result(8) is 1900+ToInteger(Result(1)); otherwise, Result(8) is Result(1).
  9. Compute MakeDay(Result(8), Result(2), Result(3)).
  10. Compute MakeTime(Result(4), Result(5), Result(6), Result(7)).
  11. Compute MakeDate(Result(9), Result(10)).
  12. Set the [[Value]] property of the newly constructed object to TimeClip(UTC(Result(11))).
15.9.3.2 -- new Date(year, month, date, hours, minutes, seconds)

The [[Prototype]] property of the newly constructed object is set to the original Date prototype object, the one that is the initial value of Date.prototype .

The [[Class]] property of the newly constructed object is set to "Date".

The [[Value]] property of the newly constructed object is set as follows:

  1. Call ToNumber(year).
  2. Call ToNumber(month).
  3. Call ToNumber(date).
  4. Call ToNumber(hours).
  5. Call ToNumber(minutes).
  6. Call ToNumber(seconds).
  7. If Result(1) is not NaN and 0 ≤ ToInteger(Result(1)) ≤ 99, Result(7) is 1900+ToInteger(Result(1)); otherwise, Result(7) is Result(1).
  8. Compute MakeDay(Result(7), Result(2), Result(3)).
  9. Compute MakeTime(Result(4), Result(5), Result(6), 0).
  10. Compute MakeDate(Result(8), Result(9)).
  11. Set the [[Value]] property of the newly constructed object to TimeClip(UTC(Result(10))).

15.9.3.3 -- new Date(year, month, date, hours, minutes)

The [[Prototype]] property of the newly constructed object is set to the original Date prototype object, the one that is the initial value of Date.prototype .

The [[Class]] property of the newly constructed object is set to "Date".

The [[Value]] property of the newly constructed object is set as follows:

  1. Call ToNumber(year).
  2. Call ToNumber(month).
  3. Call ToNumber(date).
  4. Call ToNumber(hours).
  5. Call ToNumber(minutes).
  6. If Result(1) is not NaN and 0 ≤ ToInteger(Result(1)) ≤ 99, Result(6) is 1900+ToInteger(Result(1)); otherwise, Result(6) is Result(1).
  7. Compute MakeDay(Result(6), Result(2), Result(3)).
  8. Compute MakeTime(Result(4), Result(5), 0, 0).
  9. Compute MakeDate(Result(7), Result(8)).
  10. Set the [[Value]] property of the newly constructed object to TimeClip(UTC(Result(9))).
15.9.3.4 -- new Date(year, month, date, hours)

The [[Prototype]] property of the newly constructed object is set to the original Date prototype object, the one that is the initial value of Date.prototype .

The [[Class]] property of the newly constructed object is set to "Date".

The [[Value]] property of the newly constructed object is set as follows:

  1. Call ToNumber(year).
  2. Call ToNumber(month).
  3. Call ToNumber(date).
  4. Call ToNumber(hours).
  5. If Result(1) is not NaN and 0 ≤ ToInteger(Result(1)) ≤ 99, Result(5) is 1900+ToInteger(Result(1)); otherwise, Result(5) is Result(1).
  6. Compute MakeDay(Result(5), Result(2), Result(3)).
  7. Compute MakeTime(Result(4), 0, 0, 0).
  8. Compute MakeDate(Result(6), Result(7)).
  9. Set the [[Value]] property of the newly constructed object to TimeClip(UTC(Result(8))).
15.9.3.5 -- new Date(year, month, date)

The [[Prototype]] property of the newly constructed object is set to the original Date prototype object, the one that is the initial value of Date.prototype .

The [[Class]] property of the newly constructed object is set to "Date".

The [[Value]] property of the newly constructed object is set as follows:

  1. Call ToNumber(year).
  2. Call ToNumber(month).
  3. Call ToNumber(date).
  4. If Result(1) is not NaN and 0 ≤ ToInteger(Result(1)) ≤ 99, Result(4) is 1900+ToInteger(Result(1)); otherwise, Result(4) is Result(1).
  5. Compute MakeDay(Result(4), Result(2), Result(3)).
  6. Compute MakeDate(Result(5), 0).
  7. Set the [[Value]] property of the newly constructed object to TimeClip(UTC(Result(6))).
15.9.3.6 -- new Date(year, month)

The [[Prototype]] property of the newly constructed object is set to the original Date prototype object, the one that is the initial value of Date.prototype .

The [[Class]] property of the newly constructed object is set to "Date".

The [[Value]] property of the newly constructed object is set as follows:

  1. Call ToNumber(year).
  2. Call ToNumber(month).
  3. If Result(1) is not NaN and 0 ≤ ToInteger(Result(1)) ≤ 99, Result(3) is 1900+ToInteger(Result(1)); otherwise, Result(3) is Result(1).
  4. Compute MakeDay(Result(3), Result(2), 1).
  5. Compute MakeDate(Result(4), 0).
  6. Set the [[Value]] property of the newly constructed object to TimeClip(UTC(Result(5))).
15.9.3.7 -- new Date(value)

The [[Prototype]] property of the newly constructed object is set to the original Date prototype object, the one that is the initial value of Date.prototype .

The [[Class]] property of the newly constructed object is set to "Date".

The [[Value]] property of the newly constructed object is set as follows:

  1. Call ToPrimitive(value).
  2. If Type(Result(1)) is String, then go to step 5.
  3. Let V be ToNumber(Result(1)).
  4. Set the [[Value]] property of the newly constructed object to TimeClip(V) and return.
  5. Parse Result(1) as a date, in exactly the same manner as for the parse method; let V be the time value for this date.
  6. Go to step 4.
15.9.3.8 -- new Date()

The [[Prototype]] property of the newly constructed object is set to the original Date prototype object, the one that is the initial value of Date.prototype .

The [[Class]] property of the newly constructed object is set to "Date".

The [[Value]] property of the newly constructed object is set to the current time (UTC).

15.9.4 -- Properties of the Date Constructor

The value of the internal [[Prototype]] property of the Date constructor is the Function prototype object.

Besides the internal [[Call]] and [[Construct]] properties and the length property (whose value is 7), the Date constructor has the following properties:

15.9.4.1 -- Date.prototype

The initial value of Date.prototype is the built-in Date prototype object.

This property shall have the attributes { DontEnum, DontDelete, ReadOnly }.

15.9.4.2 -- Date.parse(string)

The parse function applies the ToString operator to its argument and interprets the resulting string as a date; it returns a number, the UTC time value corresponding to the date. 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.

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.toGMTString())
However, the expression 
	Date.parse(x.toLocaleString())
is not required to produce the same number value as the preceding three expressions and, in general, the value produced by Date.parse is implementation-dependent when given any string value that could not be produced in that implementation by the toString or toGMTString method.

15.9.4.3 -- Date.UTC(year, month, date, hours, minutes, seconds, ms)

When the UTC function is called with seven arguments, the following steps are taken:

  1. Call ToNumber(year).
  2. Call ToNumber(month).
  3. Call ToNumber(date).
  4. Call ToNumber(hours).
  5. Call ToNumber(minutes).
  6. Call ToNumber(seconds).
  7. Call ToNumber(ms).
  8. If Result(1) is not NaN and 0 ≤ ToInteger(Result(1)) ≤ 99, Result(8) is 1900+ToInteger(Result(1)); otherwise, Result(8) is Result(1).
  9. Compute MakeDay(Result(8), Result(2), Result(3)).
  10. Compute MakeTime(Result(4), Result(5), Result(6), Result(7)).
  11. Return TimeClip(MakeDate(Result(9), Result(10))).
NOTE The UTC function differs from the Date constructor in two ways: it returns a time value as a number, rather than creating a Date object, and it interprets the arguments in UTC rather than as local time.
15.9.4.4 -- Date.UTC(year, month, date, hours, minutes, seconds)

When the UTC function is called with six arguments, the following steps are taken:

  1. Call ToNumber(year).
  2. Call ToNumber(month).
  3. Call ToNumber(date).
  4. Call ToNumber(hours).
  5. Call ToNumber(minutes).
  6. Call ToNumber(seconds).
  7. If Result(1) is not NaN and 0 ≤ ToInteger(Result(1)) ≤ 99, Result(7) is 1900+ToInteger(Result(1)); otherwise, Result(7) is Result(1).
  8. Compute MakeDay(Result(7), Result(2), Result(3)).
  9. Compute MakeTime(Result(4), Result(5), Result(6), 0).
  10. Return TimeClip(MakeDate(Result(8), Result(9))).

NOTE The UTC function differs from the Date constructor in two ways: it returns a time value as a number, rather than creating a Date object, and it interprets the arguments in UTC rather than as local time.
15.9.4.5 -- Date.UTC(year, month, date, hours, minutes)

When the UTC function is called with five arguments, the following steps are taken:

  1. Call ToNumber(year).
  2. Call ToNumber(month).
  3. Call ToNumber(date).
  4. Call ToNumber(hours).
  5. Call ToNumber(minutes).
  6. If Result(1) is not NaN and 0 ≤ ToInteger(Result(1)) ≤ 99, Result(6) is 1900+ToInteger(Result(1)); otherwise, Result(6) is Result(1).
  7. Compute MakeDay(Result(6), Result(2), Result(3)).
  8. Compute MakeTime(Result(4), Result(5), 0, 0).
  9. Return TimeClip(MakeDate(Result(7), Result(8))).

NOTE The UTC function differs from the Date constructor in two ways: it returns a time value as a number, rather than creating a Date object, and it interprets the arguments in UTC rather than as local time.
15.9.4.6 -- Date.UTC(year, month, date, hours)

When the UTC function is called with four arguments, the following steps are taken:

  1. Call ToNumber(year).
  2. Call ToNumber(month).
  3. Call ToNumber(date).
  4. Call ToNumber(hours).
  5. If Result(1) is not NaN and 0 ≤ ToInteger(Result(1)) ≤ 99, Result(5) is 1900+ToInteger(Result(1)); otherwise, Result(5) is Result(1).
  6. Compute MakeDay(Result(5), Result(2), Result(3)).
  7. Compute MakeTime(Result(4), 0, 0, 0).
  8. Return TimeClip(MakeDate(Result(6), Result(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.
15.9.4.7 -- Date.UTC(year, month, date)

When the UTC function is called with three arguments, the following steps are taken:

  1. Call ToNumber(year).
  2. Call ToNumber(month).
  3. Call ToNumber(date).
  4. If Result(1) is not NaN and 0 ≤ ToInteger(Result(1)) ≤ 99, Result(4) is 1900+ToInteger(Result(1)); otherwise, Result(4) is Result(1).
  5. Compute MakeDay(Result(4), Result(2), Result(3)).
  6. Return TimeClip(MakeDate(Result(5), 0)).

NOTE The UTC function differs from the Date constructor in two ways: it returns a time value as a number, rather than creating a Date object, and it interprets the arguments in UTC rather than as local time.
15.9.4.8 -- Date.UTC(year, month)

The behaviour of the UTC function with two arguments is implementation-dependent.

15.9.4.9 -- Date.UTC(year)

The behaviour of the UTC function with one argument is implementation-dependent.

15.9.4.10 -- Date.UTC()

The behaviour of the UTC function with no arguments is implementation-dependent.

15.9.5 -- Properties of the Date Prototype Object

The Date prototype object is itself a Date object (its [[Class]] is "Date") whose value is NaN.

The value of the internal [[Prototype]] property of the Date prototype object is the Object prototype object (15.2.3.1).

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; it is a runtime error if this does not refer to an object for which the value of the internal [[Class]] 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 internal [[Value]] property of this Date object.

15.9.5.1 -- Date.prototype.constructor

The initial value of Date.prototype.constructor is the built-in Date constructor.

15.9.5.2 -- Date.prototype.toString()

This function returns a string value. The contents of the string are implementation-dependent, but are intended to represent the Date in a convenient, human-readable form in the current time zone.

The toString function is not generic; it generates a runtime error if its this value is not a Date object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.9.5.3 -- Date.prototype.valueOf()

The valueOf function returns a number, which is this time value.

The valueOf function is not generic; it generates a runtime error if its this value is not a Date object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.9.5.4 -- Date.prototype.getTime()
  1. If the this value is not an object whose [[Class]] property is "Date", generate a runtime error.
  2. Return this time value.
15.9.5.5 -- Date.prototype.getYear()
NOTE This function is not part of this specification. The function getFullYear is much to be preferred for nearly all purposes, because it avoids the ``year 2000 problem.'' If implemented, getYear may follow the following rules:
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return YearFromTime(LocalTime(t)) -1900.
15.9.5.6 -- Date.prototype.getFullYear()
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return YearFromTime(LocalTime(t)).
15.9.5.7 -- Date.prototype.getUTCFullYear()
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return YearFromTime(t).
15.9.5.8 -- Date.prototype.getMonth()
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return MonthFromTime(LocalTime(t)).
15.9.5.9 -- Date.prototype.getUTCMonth()
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return MonthFromTime(t).
15.9.5.10 -- Date.prototype.getDate()
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return DateFromTime(LocalTime(t)).
15.9.5.11 -- Date.prototype.getUTCDate()
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return DateFromTime(t).
15.9.5.12 -- Date.prototype.getDay()
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return WeekDay(LocalTime(t)).
15.9.5.13 -- Date.prototype.getUTCDay()
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return WeekDay(t).
15.9.5.14 -- Date.prototype.getHours()
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return HourFromTime(LocalTime(t)).
15.9.5.15 -- Date.prototype.getUTCHours()
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return HourFromTime(t).
15.9.5.16 -- Date.prototype.getMinutes()
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return MinFromTime(LocalTime(t)).
15.9.5.17 -- Date.prototype.getUTCMinutes()
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return MinFromTime(t).
15.9.5.18 -- Date.prototype.getSeconds()
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return SecFromTime(LocalTime(t)).
15.9.5.19 -- Date.prototype.getUTCSeconds()
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return SecFromTime(t).
15.9.5.20 -- Date.prototype.getMilliseconds()
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return msFromTime(LocalTime(t)).
15.9.5.21 -- Date.prototype.getUTCMilliseconds()
  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return msFromTime(t).
15.9.5.22 -- Date.prototype.getTimezoneOffset()

Returns the difference between local time and UTC time in minutes.

  1. Let t be this time value.
  2. If t is NaN, return NaN.
  3. Return (t -LocalTime(t)) / msPerMinute.
15.9.5.23 -- Date.prototype.setTime(time)
  1. If the this value is not a Date object, generate a runtime error.
  2. Call ToNumber(time).
  3. Call TimeClip(Result(1)).
  4. Set the [[Value]] property of the this value to Result(2).
  5. Return the value of the [[Value]] property of the this value.
15.9.5.24 -- Date.prototype.setMilliseconds(ms)
  1. Let t be the result of LocalTime(this time value).
  2. Call ToNumber(ms).
  3. Compute MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t), Result(2)).
  4. Compute UTC(MakeDate(Day(t), Result(3))).
  5. Set the [[Value]] property of the this value to TimeClip(Result(4)).
  6. Return the value of the [[Value]] property of the this value.
15.9.5.25 -- Date.prototype.setUTCMilliseconds(ms)
  1. Let t be this time value.
  2. Call ToNumber(ms).
  3. Compute MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t), Result(2)).
  4. Compute MakeDate(Day(t), Result(3)).
  5. Set the [[Value]] property of the this value to TimeClip(Result(4)).
  6. Return the value of the [[Value]] property of the this value.
15.9.5.26 -- Date.prototype.setSeconds(sec [, ms ] )

If ms is not specified, this behaves as if ms were specified with the value getMilliseconds( ).

  1. Let t be the result of LocalTime(this time value).
  2. Call ToNumber(sec).
  3. If ms is not specified, compute msFromTime(t); otherwise, call ToNumber(ms).
  4. Compute MakeTime(HourFromTime(t), MinFromTime(t), Result(2), Result(3)).
  5. Compute UTC(MakeDate(Day(t), Result(4))).
  6. Set the [[Value]] property of the this value to TimeClip(Result(5)).
  7. Return the value of the [[Value]] property of the this value.
15.9.5.27 -- Date.prototype.setUTCSeconds(sec [, ms ] )

If ms is not specified, this behaves as if ms were specified with the value getUTCMilliseconds( ).

  1. Let t be this time value.
  2. Call ToNumber(sec).
  3. If ms is not specified, compute msFromTime(t); otherwise, call ToNumber(ms).
  4. Compute MakeTime(HourFromTime(t), MinFromTime(t), Result(2), Result(3)).
  5. Compute MakeDate(Day(t), Result(4)).
  6. Set the [[Value]] property of the this value to TimeClip(Result(5)).
  7. Return the value of the [[Value]] property of the this value.
15.9.5.28 -- Date.prototype.setMinutes(min [, sec [, ms ]] )

If sec is not specified, this behaves as if sec were specified with the value getSeconds ( ).

If ms is not specified, this behaves as if ms were specified with the value getMilliseconds( ).

  1. Let t be the result of LocalTime(this time value).
  2. Call ToNumber(min).
  3. If sec is not specified, compute SecFromTime(t); otherwise, call ToNumber(sec).
  4. If ms is not specified, compute msFromTime(t); otherwise, call ToNumber(ms).
  5. Compute MakeTime(HourFromTime(t), Result(2), Result(3), Result(4)).
  6. Compute UTC(MakeDate(Day(t), Result(5))).
  7. Set the [[Value]] property of the this value to TimeClip(Result(6)).
  8. Return the value of the [[Value]] property of the this value.
15.9.5.29 -- Date.prototype.setUTCMinutes(min [, sec [, ms ]] )

If sec is not specified, this behaves as if sec were specified with the value getUTCSeconds ( ).

If ms is not specified, this behaves as if ms were specified with the value getUTCMilliseconds( ).

  1. Let t be this time value.
  2. Call ToNumber(min).
  3. If sec is not specified, compute SecFromTime(t); otherwise, call ToNumber(sec).
  4. If ms is not specified, compute msFromTime(t); otherwise, call ToNumber(ms).
  5. Compute MakeTime(HourFromTime(t), Result(2), Result(3), Result(4)).
  6. Compute MakeDate(Day(t), Result(5)).
  7. Set the [[Value]] property of the this value to TimeClip(Result(6)).
  8. Return the value of the [[Value]] property of the this value.
15.9.5.30 -- Date.prototype.setHours(hour [, min [, sec [, ms ]]] )

If min is not specified, this behaves as if min were specified with the value getMinutes( ).

If sec is not specified, this behaves as if sec were specified with the value getSeconds ( ).

If ms is not specified, this behaves as if ms were specified with the value getMilliseconds( ).

  1. Let t be the result of LocalTime(this time value).
  2. Call ToNumber(hour).
  3. If min is not specified, compute MinFromTime(t); otherwise, call ToNumber(min).
  4. If sec is not specified, compute SecFromTime(t); otherwise, call ToNumber(sec).
  5. If ms is not specified, compute msFromTime(t); otherwise, call ToNumber(ms).
  6. Compute MakeTime(Result(2), Result(3), Result(4), Result(5)).
  7. Compute UTC(MakeDate(Day(t), Result(6))).
  8. Set the [[Value]] property of the this value to TimeClip(Result(7)).
  9. Return the value of the [[Value]] property of the this value.
15.9.5.31 -- Date.prototype.setUTCHours(hour [, min [, sec [, ms ]]] )

If min is not specified, this behaves as if min were specified with the value getUTCMinutes( ).

If sec is not specified, this behaves as if sec were specified with the value getUTCSeconds ( ).

If ms is not specified, this behaves as if ms were specified with the value getUTCMilliseconds( ).

  1. Let t be this time value.
  2. Call ToNumber(hour).
  3. If min is not specified, compute MinFromTime(t); otherwise, call ToNumber(min).
  4. If sec is not specified, compute SecFromTime(t); otherwise, call ToNumber(sec).
  5. If ms is not specified, compute msFromTime(t); otherwise, call ToNumber(ms).
  6. Compute MakeTime(Result(2), Result(3), Result(4), Result(5)).
  7. Compute MakeDate(Day(t), Result(6)).
  8. Set the [[Value]] property of the this value to TimeClip(Result(7)).
  9. Return the value of the [[Value]] property of the this value.
15.9.5.32 -- Date.prototype.setDate(date)
  1. Let t be the result of LocalTime(this time value).
  2. Call ToNumber(date).
  3. Compute MakeDay(YearFromTime(t), MonthFromTime(t), Result(2)).
  4. Compute UTC(MakeDate(Result(3), TimeWithinDay(t))).
  5. Set the [[Value]] property of the this value to TimeClip(Result(4)).
  6. Return the value of the [[Value]] property of the this value.
15.9.5.33 -- Date.prototype.setUTCDate(date)
  1. Let t be this time value.
  2. Call ToNumber(date).
  3. Compute MakeDay(YearFromTime(t), MonthFromTime(t), Result(2)).
  4. Compute MakeDate(Result(3), TimeWithinDay(t)).
  5. Set the [[Value]] property of the this value to TimeClip(Result(4)).
  6. Return the value of the [[Value]] property of the this value.
15.9.5.34 -- Date.prototype.setMonth(mon [, date ] )

If date is not specified, this behaves as if date were specified with the value getDate( ).

  1. Let t be the result of LocalTime(this time value).
  2. Call ToNumber(mon).
  3. If date is not specified, compute DateFromTime(t); otherwise, call ToNumber(date).
  4. Compute MakeDay(YearFromTime(t), Result(2), Result(3)).
  5. Compute UTC(MakeDate(Result(4), TimeWithinDay(t))).
  6. Set the [[Value]] property of the this value to TimeClip(Result(5)).
  7. Return the value of the [[Value]] property of the this value.
15.9.5.35 -- Date.prototype.setUTCMonth(mon [, date ] )

If date is not specified, this behaves as if date were specified with the value getUTCDate( ).

  1. Let t be this time value.
  2. Call ToNumber(mon).
  3. If date is not specified, compute DateFromTime(t); otherwise, call ToNumber(date).
  4. Compute MakeDay(YearFromTime(t), Result(2), Result(3)).
  5. Compute MakeDate(Result(4), TimeWithinDay(t)).
  6. Set the [[Value]] property of the this value to TimeClip(Result(5)).
  7. Return the value of the [[Value]] property of the this value.
15.9.5.36 -- Date.prototype.setFullYear(year [, mon [, date ]] )

If mon is not specified, this behaves as if mon were specified with the value getMonth( ).

If date is not specified, this behaves as if date were specified with the value getDate( ).

  1. Let t be the result of LocalTime(this time value); but if this time value is NaN, let t be +0.
  2. Call ToNumber(year).
  3. If mon is not specified, compute MonthFromTime(t); otherwise, call ToNumber(mon).
  4. If date is not specified, compute DateFromTime(t); otherwise, call ToNumber(date).
  5. Compute MakeDay(Result(2), Result(3), Result(4)).
  6. Compute UTC(MakeDate(Result(5), TimeWithinDay(t))).
  7. Set the [[Value]] property of the this value to TimeClip(Result(6)).
  8. Return the value of the [[Value]] property of the this value.
15.9.5.37 -- Date.prototype.setUTCFullYear(year [, mon [, date ]] )

If mon is not specified, this behaves as if mon were specified with the value getUTCMonth( ).

If date is not specified, this behaves as if date were specified with the value getUTCDate( ).

  1. Let t be this time value; but if this time value is NaN, let t be +0.
  2. Call ToNumber(year).
  3. If mon is not specified, compute MonthFromTime(t); otherwise, call ToNumber(mon).
  4. If date is not specified, compute DateFromTime(t); otherwise, call ToNumber(date).
  5. Compute MakeDay(Result(2), Result(3), Result(4)).
  6. Compute MakeDate(Result(5), TimeWithinDay(t)).
  7. Set the [[Value]] property of the this value to TimeClip(Result(6)).
  8. Return the value of the [[Value]] property of the this value.
15.9.5.38 -- Date.prototype.setYear(year)
NOTE This function is not part of this specification. The function setFullYear is much to be preferred for nearly all purposes, because it avoids the ``year 2000 problem.'' If implemented, setYear may follow the following rules:
  1. Let t be the result of LocalTime(this time value); but if this time value is NaN, let t be +0.
  2. Call ToNumber(year).
  3. If Result(2) is NaN, set the [[Value]] property of the this value to NaN and return NaN.
  4. If Result(2) is not NaN and 0 = ToInteger(Result(2)) = 99 then Result(4) is ToInteger(Result(2)) + 1900. Otherwise, Result(4) is Result(2).
  5. Compute MakeDay(Result(4), MonthFromTime(t), DateFromTime(t)).
  6. Compute UTC(MakeDate(Result(5), TimeWithinDay(t))).
  7. Set the [[Value]] property of the this value to TimeClip(Result(6)).
  8. Return the value of the [[Value]] property of the this value.
15.9.5.39 -- Date.prototype.toLocaleString()

This function returns a string value. The contents of the string are implementation-dependent, but are intended to represent the Date in a convenient, human-readable form appropriate to the geographic or cultural locale.

15.9.5.40 -- Date.prototype.toUTCString()

This function returns a string value. The contents of the string are implementation-dependent, but are intended to represent the Date in a convenient, human-readable form in UTC.

15.9.5.41 -- Date.prototype.toGMTString()

The function object that is the initial value of Date.prototype.toGMTString is the same function object that is the initial value of Date.prototype.toUTCString. The toGMTString property is provided principally for compatibility with old code. It is recommended that the toUTCString property be used in new ECMAScript code.

15.9.6 -- Properties of Date Instances

Date instances have no special properties beyond those inherited from the Date prototype object.

16 -- Errors

This specification specifies the last possible moment an error occurs. A given implementation may generate errors sooner (e.g., at compile-time). Doing so may cause differences in behaviour among implementations. Notably, if runtime errors become catchable in future versions, a given error would not be catchable if an implementation generates the error at compile-time rather than runtime.

An ECMAScript compiler should detect errors at compile time in all code presented to it, even code that detailed analysis might prove to be ``dead'' (never executed). A programmer should not rely on the trick of placing code within an if (false) statement, for example, to try to suppress compile-time error detection.

In general, if a compiler can prove that a construct cannot execute without error under any circumstances, then it may issue a compile-time error even though the construct might never be executed at all.

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Index