ECMAScript Language Specification

This is NOT the official ECMAScript Language Specification

The official specification is the PDF Document located at http://www.ecma-international.org/publications/files/ecma-st/ECMA-262.pdf.

In order to help make the ECMAScript standard more accessible and to allow references to specific sections through the use of hyperlinks, this version was created by converting the original PDF version of the standard into HTML format through a combination of mechanical and manual means. It may contain errors and the official standard should be used for definitive purposes.

According to Ecma formal publications,

Ecma Standards and Technical Reports are made available to all interested persons or organizations, free of charge and copyright, in printed form and, as files in Acrobat ® PDF format.

This version was created by Bob Clary and includes errata. It is released under the same terms as the original ECMAScript Language Specification and is free of charge and copyright.

I would like to extend a special thank you and acknowledgement to "臣本不依" for his help in finding and fixing a number of errors which I introduced into this version.

Comments and errata should be addressed to ecma@bclary.com

An alternate version, created by Sean M. Burke, is also available. Sean's version breaks the document into sections rather than presenting it in one file.

Last Updated: January 12, 2004

Standard ECMA-262 3rd Edition -December 1999

Phone: +41 22 849.60.00 - Fax: +41 22 849.60.01 - URL: http://www.ecma.ch - Internet: helpdesk@ecma.ch

Brief History

This ECMA Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company's Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0.

The development of this Standard started in November 1996. The first edition of this ECMA Standard was adopted by the ECMA General Assembly of June 1997.

That ECMA Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The ECMA General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.

The current document defines the third edition of the Standard and includes powerful regular expressions, better string handling, new control statements, try/catch exception handling, tighter definition of errors, formatting for numeric output and minor changes in anticipation of forthcoming internationalisation facilities and future language growth.

Work on the language is not complete. The technical committee is working on significant enhancements, including mechanisms for scripts to be created and used across the Internet, and tighter coordination with other standards bodies such as groups within the World Wide Web Consortium and the Wireless Application Protocol Forum.

This Standard has been adopted as 3rd Edition of ECMA-262 by the ECMA General Assembly in December, 1999.

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 and semantics described in this specification.

A conforming implementation of this International standard shall interpret characters in conformance with the Unicode Standard, Version 2.1 or later, and ISO/IEC 10646-1 with either UCS-2 or UTF-16 as the adopted encoding form, implementation level 3. If the adopted ISO/IEC 10646-1 subset is not otherwise specified, it is presumed to be the BMP subset, collection 300. If the adopted encoding form is not otherwise specified, it presumed to be the UTF-16 encoding form.

A conforming implementation of ECMAScript is permitted to provide additional types, values, objects, properties, and functions beyond those described in this specification. In particular, a conforming implementation of ECMAScript is permitted to provide properties not described in this specification, and values for those properties, for objects that are described in this specification.

A conforming implementation of ECMAScript is permitted to support program and regular expression syntax not described in this specification. In particular, a conforming implementation of ECMAScript is permitted to support program syntax that makes use of the "future reserved words" listed in 7.5.3 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) plus its amendments and corrigenda.

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

Unicode Inc. (1998), Unicode Technical Report #8: The Unicode Standard , Version 2.1.

Unicode Inc. (1998), Unicode Technical Report #15: Unicode Normalization Forms.

ANSI/IEEE Std 754-1985: IEEE Standard for Binary Floating-Point Arithmetic. Institute of Electrical and Electronic 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. To accommodate non-professional programmers, some aspects of the language may be somewhat less strict.

ECMAScript was originally designed to be a Web scripting language, providing a mechanism to enliven Web pages in browsers and to perform server computation as part of a Web-based client-server architecture. ECMAScript can provide core scripting capabilities for a variety of host environments, and therefore the core scripting language is specified in this document apart from any particular host environment.

Some of the facilities of ECMAScript are similar to those used in other programming languages; in particular Java and Self, as described in:

  • Gosling, James, Bill Joy and Guy Steele. The Java™ Language Specification. Addison Wesley Publishing Co., 1996.
  • Ungar, David, and Smith, Randall B. Self: The Power of Simplicity. OOPSLA '87 Conference Proceedings, pp. 227-241, Orlando, FL, October 1987.
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 zero or more attributes that 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 that round out the definition of ECMAScript entities. These built-in objects include the Global object, the Object object, the Function object, the Array object, the String object, the Boolean object, the Number object, the Math object, the Date object, the RegExp object and the Error objects Error, EvalError, RangeError, ReferenceError, SyntaxError, TypeError and URIError.

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 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:

implicit prototype link explicit prototype link 15 -4

CF is a constructor (and also an object). Five objects have been created by using new expressions: cf1, cf2, cf3, cf4, and cf5. Each of these objects contains properties named q1 and q2. The dashed lines represent the implicit prototype relationship; so, for example, cf3's prototype is CFp. The constructor, CF, has two properties itself, named P1 and P2, which are not visible to CFp, cf1, cf2, cf3, cf4, or cf5. The property named CFP1 in CFp is shared by cf1, cf2, cf3, cf4, and cf5 (but not by CF), as are any properties found in CFp's implicit prototype chain that are not named q1, q2, or CFP1. Notice that there is no implicit prototype link between CF and CFp.

Unlike class-based object languages, properties can be added to objects dynamically by assigning values to them. That is, constructors are not required to name or assign values to all or any of the constructed object's properties. In the above diagram, one could add a new shared property for cf1, cf2, cf3, cf4, and cf5 by assigning a new value to the property in CFp.

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 an 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.

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 16-bit unsigned integer values.

NOTE
Although each value usually represents a single 16-bit unit of UTF-16 text, the language does not place any restrictions or requirements on the values except that they be 16-bit unsigned integers.

4.3.17 String Type

The type String is the set of all string values.

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 by calling the String constructor as a function (15.5.1).

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 represents 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 by calling the Number constructor as a function (15.7.1).

4.3.22 Infinity

The primitive value Infinity represents the positive infinite number value. This value is a member of the Number type.

4.3.23 NaN

The primitive value NaN represents the set of IEEE Standard "Not-a-Number" values. This value is a member of the Number type.

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

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

5.1.2 The Lexical and RegExp Grammars

A lexical grammar for ECMAScript is given in clause 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 InputElementDiv or InputElementRegExp, 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 (7.9). Simple white space and single-line comments are discarded and do not appear in the stream of input elements for the syntactic grammar. A MultiLineComment (that is, a comment of the form "/* --- */" regardless of whether it spans more than one line) is likewise simply discarded if it contains no line terminator; but if a MultiLineComment contains one or more line terminators, then it is replaced by a single line terminator, which becomes part of the stream of input elements for the syntactic grammar.

A RegExp grammar for ECMAScript is given in 15.10. This grammar also has as its terminal symbols the characters of the Unicode character set. It defines a set of productions, starting from the goal symbol Pattern, that describe how sequences of Unicode characters are translated into regular expression patterns.

Productions of the lexical and RegExp grammars are distinguished by having two colons "::" as separating punctuation. The lexical and RegExp grammars share some productions.

5.1.3 The Numeric String Grammar

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 9.3.1.

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

5.1.4 The Syntactic Grammar

The syntactic grammar for ECMAScript is given in clauses 11, 12, 13 and 14. This grammar has ECMAScript tokens defined by the lexical grammar as its terminal symbols (5.1.2). It defines a set of productions, starting from the goal symbol Program, that describe how sequences of tokens can form syntactically correct ECMAScript programs.

When a stream of 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 sections 11, 12, 13 and 14 is actually not a complete account of which token sequences are accepted as correct ECMAScript programs. Certain additional token sequences are also accepted, namely, those that would be described by the grammar if only semicolons were added to the sequence in certain places (such as before line terminator characters). Furthermore, certain token sequences that are described by the grammar are not considered acceptable if a terminator character appears in certain "awkward" places.

5.1.5 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. All nonterminal characters specified in this way are to be understood as the appropriate Unicode character from the ASCII range, as opposed to any similar-looking characters from other Unicode ranges.

Nonterminal symbols are shown in italic type. The definition of a nonterminal is introduced by the name of the nonterminal being defined followed by one or more colons. (The number of colons indicates to which grammar the production belongs.) One or more alternative right-hand sides for the nonterminal then follow on succeeding lines. For example, the syntactic definition:

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 Initialiseropt

is a convenient abbreviation for:

VariableDeclaration :
Identifier
Identifier Initialiser

and that:

IterationStatement :
for ( ExpressionNoInopt ; Expressionopt ; Expressionopt ) Statement

is a convenient abbreviation for:

IterationStatement :
for (; Expressionopt ; Expressionopt ) Statement
for ( ExpressionNoIn ; Expressionopt ; Expressionopt ) Statement

which in turn is an abbreviation for:

IterationStatement :
for (;; Expressionopt ) Statement
for (; Expression ; Expressionopt ) Statement
for ( ExpressionNoIn ;; Expressionopt ) Statement
for ( ExpressionNoIn ; Expression ; Expressionopt ) Statement

which in turn is an abbreviation for:

IterationStatement :
for (;;) Statement
for (;; Expression ) Statement
for (; Expression ;) Statement
for (; Expression ; Expression ) Statement
for ( ExpressionNoIn ;;) Statement
for ( ExpressionNoIn ;; Expression ) Statement
for ( ExpressionNoIn ; Expression ;) Statement
for ( ExpressionNoIn ; Expression ; Expression ) Statement

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

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

If the phrase "[lookahead ∉ set]" appears in the right-hand side of a production, it indicates that the production may not be used if the immediately following input terminal is a member of the given set. The set can be written as a list of terminals enclosed in curly braces. For convenience, the set can also be written as a nonterminal, in which case it represents the set of all terminals to which that nonterminal could expand. For example, given the definitions

DecimalDigit :: one of
0 1 2 3 4 5 6 7 8 9
DecimalDigits ::
DecimalDigit
DecimalDigits DecimalDigit

the definition

LookaheadExample ::
n [lookahead ∉ {1, 3, 5, 7, 9}] DecimalDigits
DecimalDigit [lookahead ∉ DecimalDigit ]

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

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

ReturnStatement :
return [no LineTerminator here] Expressionopt ;

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

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

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

NonZeroDigit :: one of
1 2 3 4 5 6 7 8 9

which is merely a convenient abbreviation for:

NonZeroDigit
1
2
3
4
5
6
7
8
9

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

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

Identifier ::
IdentifierName but not ReservedWord

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

Finally, a few nonterminal symbols are described by a descriptive phrase in 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 (or zero) such that abs(k)< abs(y) and x-k = q * y for some integer q.

The mathematical function floor(x) yields the largest integer (closest to positive infinity) that is not larger than x.

NOTE
floor(x) = x-(x modulo 1).

If an algorithm is defined to "throw an exception", execution of the algorithm is terminated and no result is returned. The calling algorithms are also terminated, until an algorithm step is reached that explicitly deals with the exception, using terminology such as "If an exception was thrown...". Once such an algorithm step has been encountered the exception is no longer considered to have occurred.

6. Source Text

ECMAScript source text is represented as a sequence of characters in the Unicode character encoding, version 2.1 or later, using the UTF-16 transformation format. The text is expected to have been normalised to Unicode Normalised Form C (canonical composition), as described in Unicode Technical Report #15. Conforming ECMAScript implementations are not required to perform any normalisation of text, or behave as though they were performing normalisation of text, themselves.

SourceCharacter ::
any Unicode character

ECMAScript source text can contain any of the Unicode characters. All Unicode white space characters are treated as white space, and all Unicode line/paragraph separators are treated as line separators. Non-Latin Unicode characters are allowed in identifiers, string literals, regular expression literals and comments.

Throughout the rest of this document, the phrase "code point" and the word "character" will be used to refer to a 16-bit unsigned value used to represent a single 16-bit unit of UTF-16 text. The phrase "Unicode character" will be used to refer to the abstract linguistic or typographical unit represented by a single Unicode scalar value (which may be longer than 16 bits and thus may be represented by more than one code point). This only refers to entities represented by single Unicode scalar values: the components of a combining character sequence are still individual "Unicode characters," even though a user might think of the whole sequence as a single character.

In string literals, regular expression literals and identifiers, any character (code point) may also be expressed as a Unicode escape sequence consisting of six characters, namely \u plus four hexadecimal digits. Within a comment, such an escape sequence is effectively ignored as part of the comment. Within a string literal or regular expression literal, the Unicode escape sequence contributes one character to the value of the literal. Within an identifier, the escape sequence contributes one character to the identifier.

NOTE 1
Although this document sometimes refers to a "transformation" between a "character" within a "string" and the 16-bit unsigned integer that is the UTF-16 encoding of that character, there is actually no transformation because a "character" within a "string" is actually represented using that 16-bit unsigned value.

NOTE 2
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.

There are two goal symbols for the lexical grammar. The InputElementDiv symbol is used in those syntactic grammar contexts where a division (/) or division-assignment (/=) operator is permitted. The InputElementRegExp symbol is used in other syntactic grammar contexts.

Note that contexts exist in the syntactic grammar where both a division and a RegularExpressionLiteral are permitted by the syntactic grammar; however, since the lexical grammar uses the InputElementDiv goal symbol in such cases, the opening slash is not recognised as starting a regular expression literal in such a context. As a workaround, one may enclose the regular expression literal in parentheses.

Syntax

InputElementDiv ::
WhiteSpace
LineTerminator
Comment
Token
DivPunctuator
InputElementRegExp ::
WhiteSpace
LineTerminator
Comment
Token
RegularExpressionLiteral
7.1 Unicode Format-Control Characters

The Unicode format-control characters (i. e., the characters in category "Cf" in the Unicode Character Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the formatting of a range of text in the absence of higher-level protocols for this (such as mark-up languages). It is useful to allow these in source text to facilitate editing and display.

The format control characters can occur anywhere in the source text of an ECMAScript program. These characters are removed from the source text before applying the lexical grammar. Since these characters are removed before processing string and regular expression literals, one must use a Unicode escape sequence (see 7.6) to include a Unicode format-control character inside a string or regular expression literal.

7.2 White Space

White space characters are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space 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:

Code Point Value Name Formal Name
\u0009 Tab <TAB>
\u000B Vertical Tab <VT>
\u000C Form Feed <FF>
\u0020 Space <SP>
\u00A0 No-break space <NBSP>
Other category "Zs" Any other Unicode "space separator" <USP>

Syntax

WhiteSpace ::
<TAB>
<VT>
<FF>
<SP>
<NBSP>
<USP>
7.3 Line Terminators

Like white space characters, line terminator characters are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space characters, line terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar. A line terminator cannot occur within any token, not even a string. Line terminators also affect the process of automatic semicolon insertion (7.9).

The following characters are considered to be line terminators:

Code Point Value Name Formal Name
\u000A Line Feed <LF>
\u000D Carriage Return <CR>
\u2028 Line separator <LS>
\u2029 Paragraph separator <PS>

Syntax

LineTerminator ::
<LF>
<CR>
<LS>
<PS>
7.4 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 (7.9).

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

Syntax

Comment ::
MultiLineComment
SingleLineComment
MultiLineComment ::
/* MultiLineCommentCharsopt */
MultiLineCommentChars ::
MultiLineNotAsteriskChar MultiLineCommentCharsopt
* PostAsteriskCommentCharsopt
PostAsteriskCommentChars ::
MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentCharsopt
* PostAsteriskCommentCharsopt
MultiLineNotAsteriskChar ::
SourceCharacter but not asterisk *
MultiLineNotForwardSlashOrAsteriskChar ::
SourceCharacter but not forward-slash / or asterisk *
SingleLineComment ::
// SingleLineCommentCharsopt
SingleLineCommentChars ::
SingleLineCommentChar SingleLineCommentCharsopt
SingleLineCommentChar ::
SourceCharacter but not LineTerminator
7.5 Tokens

Syntax

Token ::
ReservedWord
Identifier
Punctuator
NumericLiteral
StringLiteral
7.5.1 Reserved Words

Description

Reserved words cannot be used as identifiers.

Syntax

ReservedWord ::
Keyword
FutureReservedWord
NullLiteral
BooleanLiteral
7.5.2 Keywords

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

Syntax

Keyword :: one of
break else new var case finally return void catch for switch while continue function this with default if throw delete in try do instanceof typeof
7.5.3 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 enum int short boolean export interface static byte extends long super char final native synchronized class float package throws const goto private transient debugger implements protected volatile double import public
7.6 Identifiers

Description

Identifiers are interpreted according to the grammar given in Section 5.16 of the upcoming version 3.0 of the Unicode standard, with some small modifications. This grammar is based on both normative and informative character categories specified by the Unicode standard. The characters in the specified categories in version 2.1 of the Unicode standard must be treated as in those categories by all conforming ECMAScript implementations; however, conforming ECMAScript implementations may allow additional legal identifier characters based on the category assignment from later versions of Unicode.

This standard specifies one departure from the grammar given in the Unicode standard: The dollar sign ($) and the underscore (_) are permitted anywhere in an identifier. The dollar sign is intended for use only in mechanically generated code.

Unicode escape sequences are also permitted in identifiers, where they contribute a single character to the identifier, as computed by the CV of the UnicodeEscapeSequence (see 7.8.4). The \ preceding the UnicodeEscapeSequence does not contribute a character to the identifier. A UnicodeEscapeSequence cannot be used to put a character into an identifier that would otherwise be illegal. In other words, if a \ UnicodeEscapeSequence sequence were replaced by its UnicodeEscapeSequence's CV, the result must still be a valid Identifier that has the exact same sequence of characters as the original Identifier.

Two identifiers that are canonically equivalent according to the Unicode standard are not equal unless they are represented by the exact same sequence of code points (in other words, conforming ECMAScript implementations are only required to do bitwise comparison on identifiers). The intent is that the incoming source text has been converted to normalised form C before it reaches the compiler.

Syntax

Identifier ::
IdentifierName but not ReservedWord
IdentifierName ::
IdentifierStart
IdentifierName IdentifierPart
IdentifierStart ::
UnicodeLetter
$
_
\ UnicodeEscapeSequence
IdentifierPart ::
IdentifierStart
UnicodeCombiningMark
UnicodeDigit
UnicodeConnectorPunctuation
\ UnicodeEscapeSequence
UnicodeLetter
any character in the Unicode categories "Uppercase letter (Lu)", "Lowercase letter (Ll)", "Titlecase letter (Lt)", "Modifier letter (Lm)", "Other letter (Lo)", or "Letter number (Nl)".
UnicodeCombiningMark
any character in the Unicode categories "Non-spacing mark (Mn)" or "Combining spacing mark (Mc)"
UnicodeDigit
any character in the Unicode category "Decimal number (Nd)"
UnicodeConnectorPunctuation
any character in the Unicode category "Connector punctuation (Pc)"
UnicodeEscapeSequence
see 7.8.4.
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
7.7 Punctuators

Syntax

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

Syntax

Literal ::
NullLiteral
BooleanLiteral
NumericLiteral
StringLiteral
7.8.1 Null Literals

Syntax

NullLiteral ::
null

Semantics

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

7.8.2 Boolean Literals

Syntax

BooleanLiteral ::
true
false

Semantics

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

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

7.8.3 Numeric Literals

Syntax

NumericLiteral ::
DecimalLiteral
HexIntegerLiteral
DecimalLiteral ::
DecimalIntegerLiteral . DecimalDigitsopt ExponentPartopt
. DecimalDigits ExponentPartopt
DecimalIntegerLiteral ExponentPartopt
DecimalIntegerLiteral ::
0
NonZeroDigit DecimalDigitsopt
DecimalDigits ::
DecimalDigit
DecimalDigits DecimalDigit
DecimalDigit :: one of
0 1 2 3 4 5 6 7 8 9
NonZeroDigit :: one of
1 2 3 4 5 6 7 8 9
ExponentPart ::
ExponentIndicator SignedInteger
ExponentIndicator :: one of
e E
SignedInteger ::
DecimalDigits
+ DecimalDigits
- DecimalDigits
HexIntegerLiteral ::
0x HexDigit
0X HexDigit
HexIntegerLiteral HexDigit

The source character immediately following a NumericLiteral must not be an IdentifierStart or DecimalDigit.

NOTE
For example:

3in

is an error and not the two input elements 3 and in.

Semantics

A numeric literal stands for a value of the Number type. This value is determined in two steps: first, a mathematical value (MV) is derived from the literal; second, this mathematical value is rounded as described below.

  • The MV of NumericLiteral :: DecimalLiteral is the MV of DecimalLiteral.
  • The MV of NumericLiteral :: HexIntegerLiteral is the MV of HexIntegerLiteral.
  • The MV of DecimalLiteral :: DecimalIntegerLiteral . is the MV of DecimalIntegerLiteral.
  • The MV of DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits is the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits times 10 -n ), where n is the number of characters in DecimalDigits.
  • The MV of DecimalLiteral :: DecimalIntegerLiteral . ExponentPart is the MV of DecimalIntegerLiteral times 10 e ,where e is the MV of ExponentPart.
  • The MV of DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits ExponentPart is (the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits times 10-n )) times 10e, where n is the number of characters in DecimalDigits and e is the MV of ExponentPart.
  • The MV of DecimalLiteral ::. DecimalDigits is the MV of DecimalDigits times 10-n, where n is the number of characters in DecimalDigits.
  • The MV of DecimalLiteral :: . DecimalDigits ExponentPart is the MV of DecimalDigits times 10e-n,where n is the number of characters in DecimalDigits and e is the MV of ExponentPart.
  • The MV of DecimalLiteral :: DecimalIntegerLiteral is the MV of DecimalIntegerLiteral.
  • The MV of DecimalLiteral :: DecimalIntegerLiteral ExponentPart is the MV of DecimalIntegerLiteral times 10e, where e is the MV of ExponentPart.
  • The MV of DecimalIntegerLiteral :: 0 is 0.
  • The MV of DecimalIntegerLiteral :: NonZeroDigit DecimalDigits is (the MV of NonZeroDigit times 10n ) plus the MV of DecimalDigits, where n is the number of characters in DecimalDigits.
  • The MV of DecimalDigits :: DecimalDigit is the MV of DecimalDigit.
  • The MV of DecimalDigits :: DecimalDigits DecimalDigit is (the MV of DecimalDigits times 10) plus the MV of DecimalDigit.
  • The MV of ExponentPart :: ExponentIndicator SignedInteger is the MV of SignedInteger.
  • The MV of SignedInteger :: DecimalDigits is the MV of DecimalDigits.
  • The MV of SignedInteger :: + DecimalDigits is the MV of DecimalDigits.
  • The MV of SignedInteger :: - DecimalDigits is the negative of the MV of DecimalDigits.
  • The MV of DecimalDigit :: 0 or of HexDigit :: 0 is 0.
  • The MV of DecimalDigit :: 1 or of NonZeroDigit :: 1 or of HexDigit :: 1 is 1. The MV of DecimalDigit :: 2 or of NonZeroDigit :: 2 or of HexDigit :: 2 is 2.
  • The MV of DecimalDigit :: 3 or of NonZeroDigit :: 3 or of HexDigit :: 3 is 3.
  • The MV of DecimalDigit :: 4 or of NonZeroDigit :: 4 or of HexDigit :: 4 is 4.
  • The MV of DecimalDigit :: 5 or of NonZeroDigit :: 5 or of HexDigit :: 5 is 5. The MV of DecimalDigit :: 6 or of NonZeroDigit :: 6 or of HexDigit :: 6 is 6.
  • The MV of DecimalDigit :: 7 or of NonZeroDigit :: 7 or of HexDigit :: 7 is 7.
  • The MV of DecimalDigit :: 8 or of NonZeroDigit :: 8 or of HexDigit :: 8 is 8.
  • The MV of DecimalDigit :: 9 or of NonZeroDigit :: 9 or of HexDigit :: 9 is 9. The MV of HexDigit :: a or of HexDigit :: A is 10.
  • The MV of HexDigit :: b or of HexDigit :: B is 11.
  • The MV of HexDigit :: c or of HexDigit :: C is 12.
  • The MV of HexDigit :: d or of HexDigit :: D is 13.
  • The MV of HexDigit :: e or of HexDigit :: E is 14.
  • The MV of HexDigit :: f or of HexDigit :: F is 15.
  • The MV of HexIntegerLiteral :: 0x HexDigit is the MV of HexDigit.
  • The MV of HexIntegerLiteral :: 0X HexDigit is the MV of HexDigit.
  • The MV of HexIntegerLiteral :: HexIntegerLiteral HexDigit is (the MV of HexIntegerLiteral times 16) plus the MV of HexDigit.

Once the exact MV for a numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is +0; otherwise, the rounded value must be the number value for the MV (in the sense defined in 8.5), unless the literal is a DecimalLiteral and the literal has more than 20 significant digits, in which case the number value may be either the number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit or the number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit and then incrementing the literal at the 20th significant digit position. A digit is significant if it is not part of an ExponentPart and

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

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

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
0 [lookahead ∉ DecimalDigit]
HexEscapeSequence
UnicodeEscapeSequence
CharacterEscapeSequence ::
SingleEscapeCharacter
NonEscapeCharacter
SingleEscapeCharacter :: one of
' " \ b f n r t v
NonEscapeCharacter ::
SourceCharacter but not EscapeCharacter or LineTerminator
EscapeCharacter ::
SingleEscapeCharacter
DecimalDigit
x
u
HexEscapeSequence ::
x HexDigit HexDigit
UnicodeEscapeSequence ::
u HexDigit HexDigit HexDigit HexDigit

The definitions of the nonterminal HexDigit is given in section 7.8.3. 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 7.8.3.

  • The SV of StringLiteral :: "" is the empty character sequence.
  • The SV of StringLiteral :: '' is the empty character sequence.
  • The SV of StringLiteral :: " DoubleStringCharacters " is the SV of DoubleStringCharacters.
  • The SV of StringLiteral :: ' SingleStringCharacters ' is the SV of SingleStringCharacters.
  • The SV of DoubleStringCharacters :: DoubleStringCharacter is a sequence of one character, the CV of DoubleStringCharacter.
  • The SV of DoubleStringCharacters :: DoubleStringCharacter DoubleStringCharacters is a sequence of the CV of DoubleStringCharacter followed by all the characters in the SV of DoubleStringCharacters in order.
  • The SV of SingleStringCharacters :: SingleStringCharacter is a sequence of one character, the CV of SingleStringCharacter.
  • The SV of SingleStringCharacters :: SingleStringCharacter SingleStringCharacters is a sequence of the CV of SingleStringCharacter followed by all the characters in the SV of SingleStringCharacters in order.
  • The CV of DoubleStringCharacter :: SourceCharacter but not double-quote " or backslash \ or LineTerminator is the SourceCharacter character itself.
  • The CV of DoubleStringCharacter :: \ EscapeSequence is the CV of the EscapeSequence. The CV of SingleStringCharacter :: SourceCharacter but not single-quote ' or backslash \ or LineTerminator is the SourceCharacter character itself.
  • The CV of SingleStringCharacter :: \ EscapeSequence is the CV of the EscapeSequence.
  • The CV of EscapeSequence :: CharacterEscapeSequence is the CV of the CharacterEscapeSequence.
  • The CV of EscapeSequence :: 0 [lookahead ∉ DecimalDigit] is a <NUL> character (Unicode value 0000).
  • The CV of EscapeSequence :: HexEscapeSequence is the CV of the HexEscapeSequence.
  • The CV of EscapeSequence :: UnicodeEscapeSequence is the CV of the UnicodeEscapeSequence.
  • The CV of CharacterEscapeSequence :: SingleEscapeCharacter is the character whose code point value is determined by the SingleEscapeCharacter according to the following table:
Escape Sequence Code Point Value Name Symbol
\b \u0008 backspace <BS>
\t \u0009 horizontal tab <HT>
\n \u000A line feed (new line) <LF>
\v \u000B vertical tab <VT>
\f \u000C form feed <FF>
\r \u000D carriage return <CR>
\" \u0022 double quote "
\' \u0027 single quote '
\\ \u005C backslash \
  • The CV of CharacterEscapeSequence :: NonEscapeCharacter is the CV of the NonEscapeCharacter.
  • The CV of NonEscapeCharacter :: SourceCharacter but not EscapeCharacter or LineTerminator is the SourceCharacter character itself.
  • The CV of HexEscapeSequence :: x HexDigit HexDigit is the character whose code point value is (16 times the MV of the first HexDigit) plus the MV of the second HexDigit.
  • The CV of UnicodeEscapeSequence :: u HexDigit HexDigit HexDigit HexDigit is the character whose code point value is (4096 (that is, 163 ) times the MV of the first HexDigit) plus (256 (that is, 162 ) times the MV of the second HexDigit) plus (16 times the MV of the third HexDigit) plus the MV of the fourth HexDigit.

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.

7.8.5 Regular Expression Literals

A regular expression literal is an input element that is converted to a RegExp object (section 15.10) when it is scanned. The object is created before evaluation of the containing program or function begins. Evaluation of the literal produces a reference to that object; it does not create a new object. Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by new RegExp (section 15.10.4) or calling the RegExp constructor as a function (section 15.10.3).

The productions below describe the syntax for a regular expression literal and are used by the input element scanner to find the end of the regular expression literal. The strings of characters comprising the RegularExpressionBody and the RegularExpressionFlags are passed uninterpreted to the regular expression constructor, which interprets them according to its own, more stringent grammar. An implementation may extend the regular expression constructor's grammar, but it should not extend the RegularExpressionBody and RegularExpressionFlags productions or the productions used by these productions.

Syntax

RegularExpressionLiteral ::
/ RegularExpressionBody / RegularExpressionFlags
RegularExpressionBody ::
RegularExpressionFirstChar RegularExpressionChars
RegularExpressionChars ::
[empty]
RegularExpressionChars RegularExpressionChar
RegularExpressionFirstChar ::
NonTerminator but not * or \ or /
BackslashSequence
RegularExpressionChar ::
NonTerminator but not \ or /
BackslashSequence
BackslashSequence ::
\ NonTerminator
NonTerminator ::
SourceCharacter but not LineTerminator
RegularExpressionFlags ::
[empty]
RegularExpressionFlags IdentifierPart

NOTE
Regular expression literals may not be empty; instead of representing an empty regular expression literal, the characters // start a single-line comment. To specify an empty regular expression, use /(?:)/.

Semantics

A regular expression literal stands for a value of the Object type. This value is determined in two steps: first, the characters comprising the regular expression's RegularExpressionBody and RegularExpressionFlags production expansions are collected uninterpreted into two strings Pattern and Flags, respectively. Then the new RegExp constructor is called with two arguments Pattern and Flags and the result becomes the value of the RegularExpressionLiteral. If the call to new RegExp generates an error, an implementation may, at its discretion, either report the error immediately while scanning the program, or it may defer the error until the regular expression literal is evaluated in the course of program execution.

7.9 Automatic Semicolon Insertion

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

7.9.1 Rules of Automatic Semicolon Insertion
  • When, as the program is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:

    1. The offending token is separated from the previous token by at least one LineTerminator.

    2. The offending token is }.

  • When, as the program is parsed from left to right, the end of the input stream of tokens is encountered and the parser is unable to parse the input token stream as a single complete ECMAScript Program, then a semicolon is automatically inserted at the end of the input stream.
  • When, as the program is parsed from left to right, a token is encountered that is allowed by some production of the grammar, but the production is a restricted production and the token would be the first token for a terminal or nonterminal immediately following the annotation "[no LineTerminator here]" within the restricted production (and therefore such a token is called a restricted token), and the restricted token is separated from the previous token by at least one LineTerminator, then a semicolon is automatically inserted before the restricted token.

However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in the header of a for statement (section 12.6.3).

NOTE
These are the only restricted productions in the grammar:

PostfixExpression :
LeftHandSideExpression [no LineTerminator here] ++
LeftHandSideExpression [no LineTerminator here] --
ContinueStatement :
continue [no LineTerminator here] Identifieropt ;
BreakStatement :
break [no LineTerminator here] Identifieropt ;
ReturnStatement :
return [no LineTerminator here] Expressionopt ;
ThrowStatement :
throw [no LineTerminator here] Expression ;

The practical effect of these restricted productions is as follows:

  • When a ++ or -- token is encountered where the parser would treat it as a postfix operator, and at least one LineTerminator occurred between the preceding token and the ++ or -- token, then a semicolon is automatically inserted before the ++ or -- token.
  • When a continue, break, return, or throw token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the continue, break, return, or throw token.

The resulting practical advice to ECMAScript programmers is:

  • A postfix ++ or -- operator should appear on the same line as its operand.
  • An Expression in a return or throw statement should start on the same line as the return or throw token.
  • A label in a break or continue statement should be on the same line as the break or continue token.
7.9.2 Examples of Automatic Semicolon Insertion

The source

{ 1 2 } 3

is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source

{ 1
2 } 3

is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:

{ 1
;2 ;} 3;

which is a valid ECMAScript sentence.

The source

for (a; b
)

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the semicolon is needed for the header of a for statement. Automatic semicolon insertion never inserts one of the two semicolons in the header of a for statement.

The source

return
a + b

is transformed by automatic semicolon insertion into the following:

return;
a + b;

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

The source

a = b
++c

is transformed by automatic semicolon insertion into the following:

a = b;
++c;

NOTE
The token ++ is not treated as a postfix operator applying to the variable 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 as 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 has the value undefined.

8.2 The Null Type

The Null type has exactly one value, called null.

8.3 The Boolean Type

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

8.4 The String Type

The String type is the set of all finite ordered sequences of zero or more 16-bit unsigned integer values ("elements"). The String type is generally used to represent textual data in a running ECMAScript program, in which case each element in the string is treated as a code point value (see section 6). Each element is regarded as occupying a position within the sequence. These positions are indexed with nonnegative integers. The first element (if any) is at position 0, the next element (if any) at position 1, and so on. The length of a string is the number of elements (i. e., 16-bit values) within it. The empty string has length zero and therefore contains no elements.

When a string contains actual textual data, each element is considered to be a single UTF-16 unit. Whether or not this is the actual storage format of a String, the characters within a String are numbered as though they were represented using UTF-16. All operations on Strings (except as otherwise stated) treat them as sequences of undifferentiated 16-bit unsigned integers; they do not ensure the resulting string is in normalised form, nor do they ensure language-sensitive results.

NOTE
The rationale behind these decisions was to keep the implementation of Strings as simple and high-performing as possible. The intent is that textual data coming into the execution environment from outside (e. g., user input, text read from a file or received over the network, etc.) be converted to Unicode Normalised Form C before the running program sees it. Usually this would occur at the same time incoming text is converted from its original character encoding to Unicode (and would impose no additional overhead). Since it is recommended that ECMAScript source code be in Normalised Form C, string literals are guaranteed to be normalised (if source text is guaranteed to be normalised), as long as they do not contain any Unicode escape sequences.

8.5 The Number Type

The Number type has exactly 18437736874454810627 (that is, 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 denormalised, having the form

s * m * 2e

where s is +1 or -1, m is a positive integer less than 252, and e is -1074.

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

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

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

Some ECMAScript operators deal only with integers in the range -231 through 231-1, inclusive, or in the range 0 through 232-1, inclusive. These operators accept any value of the Number type but first convert each such value to one of 232 integer values. See the descriptions of the ToInt32 and ToUint32 operators in 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 host environment; therefore "ReadOnly" does not mean "constant and unchanging"!)
DontEnum The property is not to be enumerated by a for-in enumeration (section 12.6.4).
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 part of the language. They are defined by this specification purely for expository purposes. An implementation of ECMAScript must behave as if it produced and operated upon internal properties in the manner described here. For the purposes of this document, the names of internal properties are enclosed in double square brackets [[ ]]. When an algorithm uses an internal property of an object and the object does not implement the indicated internal property, a TypeError exception is thrown.

There are two types of access for normal (non-internal) 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 visible 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 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.
[[HasInstance]] (Value) Returns a boolean value indicating whether Value delegates behaviour to this object. Of the native ECMAScript objects, only Function objects implement [[HasInstance]].
[[Scope]] none A scope chain that defines the environment in which a Function object is executed.
[[Match]] (String, Index) Tests for a regular expression match and returns a MatchResult value (see section 15.10.2.1).

Every object (including host objects) must implement the [[Prototype]] and [[Class]] properties and the [[Get]], [[Put]], [[CanPut]], [[HasProperty]], [[Delete]], and [[DefaultValue]] methods. (Note, however, that the [[DefaultValue]] method may, for some objects, simply throw a TypeError exception.)

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. The value of a [[Class]] property is used internally to distinguish different kinds of built-in objects. Note that this specification does not provide any means for a program to access that value except through Object.prototype.toString (see 15.2.4.2).

For native objects the [[Get]], [[Put]], [[CanPut]], [[HasProperty]], [[Delete]] and [[DefaultValue]] methods behave as described in 8.6.2.1, 8.6.2.2, 8.6.2.3, 8.6.2.4, 8.6.2.5 and 8.6.2.6, respectively, except that Array objects have a slightly different implementation of the [[Put]] method (see 15.4.5.1). Host objects may implement these methods in any manner unless specified otherwise; for example, one possibility is that [[Get]] and [[Put]] for a particular host object indeed fetch and store property values but [[HasProperty]] always generates false.

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 (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. Call the [[CanPut]] method of [[Prototype]] of O with property name P.

6. Return Result(5).

8.6.2.4 [[HasProperty]](P)

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

1. If O has a property with name P, return true.

2. If the [[Prototype]] of O is null, return false.

3. Call the [[HasProperty]] method of [[Prototype]] with property name P.

4. 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. Throw a TypeError exception.

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. Throw a TypeError exception.

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 15.9), in which case it behaves as if the hint were String.

The above specification of [[DefaultValue]] for native objects can return only primitive values. If a host object implements its own [[DefaultValue]] method, it must ensure that its [[DefaultValue]] method can return only primitive values.

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 access the components of references:

  • GetBase(V). Returns the base object component of the reference V.
  • GetPropertyName(V). Returns the property name component of the reference V.

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

8.7.1 GetValue(V)

1. If Type(V) is not Reference, return V.

2. Call GetBase(V).

3. If Result(2) is null, throw a ReferenceError exception.

4. Call the [[Get]] method of Result(2), passing GetPropertyName( V) for the property name.

5. Return Result(4).

8.7.2 PutValue(V, W)

1. If Type(V) is not Reference, throw a ReferenceError exception.

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 (see 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, return and throw) that perform nonlocal transfers of control. Values of the Completion type are triples of the form (type, value, target), where type is one of normal, break, continue, return, or throw, value is any ECMAScript value or empty, and target is any ECMAScript identifier or empty.

The term "abrupt completion" refers to any completion with a type other than normal.

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 argument PreferredType. 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 (8.6.2.6).
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>
<NBSP>
<FF>
<VT>
<CR>
<LF>
<LS>
<PS>
<USP>
StrNumericLiteral :::
StrDecimalLiteral
HexIntegerLiteral
StrDecimalLiteral :::
StrUnsignedDecimalLiteral
+ StrUnsignedDecimalLiteral
- StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral :::
Infinity
DecimalDigits . DecimalDigitsopt ExponentPartopt
. DecimalDigits ExponentPartopt
DecimalDigits ExponentPartopt
DecimalDigits :::
DecimalDigit
DecimalDigits DecimalDigit
DecimalDigit ::: one of
0 1 2 3 4 5 6 7 8 9
ExponentPart :::
ExponentIndicator SignedInteger
ExponentIndicator ::: one of
e E
SignedInteger :::
DecimalDigits
+ DecimalDigits
- DecimalDigits
HexIntegerLiteral :::
0x 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 (see 7.8.3):

  • A StringNumericLiteral may be preceded and/ or followed by white space and/ or line terminators.
  • A StringNumericLiteral that is decimal may have any number of leading 0 digits.
  • A StringNumericLiteral that is decimal may be preceded by + or - to indicate its sign.
  • A StringNumericLiteral that is empty or contains only white space is converted to +0.

The conversion of a string to a number value is similar overall to the determination of the number value for a numeric literal (see 7.8.3), but some of the details are different, so the process for converting a string numeric literal to a value of Number type is given here in full. This value is determined in two steps: first, a mathematical value (MV) is derived from the string numeric literal; second, this mathematical value is rounded as described below.

  • The MV of StringNumericLiteral ::: [empty] is 0.
  • The MV of StringNumericLiteral ::: StrWhiteSpace is 0.
  • The MV of StringNumericLiteral ::: StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt is the MV of StrNumericLiteral, no matter whether white space is present or not.
  • The MV of StrNumericLiteral ::: StrDecimalLiteral is the MV of StrDecimalLiteral.
  • The MV of StrNumericLiteral ::: HexIntegerLiteral is the MV of HexIntegerLiteral.
  • The MV of StrDecimalLiteral ::: StrUnsignedDecimalLiteral is the MV of StrUnsignedDecimalLiteral.
  • The MV of StrDecimalLiteral::: + StrUnsignedDecimalLiteral is the MV of StrUnsignedDecimalLiteral.
  • The MV of StrDecimalLiteral::: - StrUnsignedDecimalLiteral is the negative of the MV of StrUnsignedDecimalLiteral. (Note that if the MV of StrUnsignedDecimalLiteral is 0, the negative of this MV is also 0. The rounding rule described below handles the conversion of this sign less mathematical zero to a floating-point +0 or -0 as appropriate.)
  • The MV of StrUnsignedDecimalLiteral::: Infinity is 1010000 (a value so large that it will round to +∞ ).
  • The MV of StrUnsignedDecimalLiteral::: DecimalDigits. is the MV of DecimalDigits. The MV of StrUnsignedDecimalLiteral::: DecimalDigits. DecimalDigits is the MV of the first DecimalDigits plus (the MV of the second DecimalDigits times 10-n ), where n is the number of characters in the second DecimalDigits.
  • The MV of StrUnsignedDecimalLiteral::: DecimalDigits. ExponentPart is the MV of DecimalDigits times 10e, where e is the MV of ExponentPart.
  • The MV of StrUnsignedDecimalLiteral::: DecimalDigits. DecimalDigits ExponentPart is (the MV of the first DecimalDigits plus (the MV of the second DecimalDigits times 10-n )) times 10e, where n is the number of characters in the second DecimalDigits and e is the MV of ExponentPart.
  • The MV of StrUnsignedDecimalLiteral:::. DecimalDigits is the MV of DecimalDigits times 10-n, where n is the number of characters in DecimalDigits.
  • The MV of StrUnsignedDecimalLiteral:::. DecimalDigits ExponentPart is the MV of DecimalDigits times 10e-n, where n is the number of characters in DecimalDigits and e is the MV of ExponentPart.
  • The MV of StrUnsignedDecimalLiteral::: DecimalDigits is the MV of DecimalDigits.
  • The MV of StrUnsignedDecimalLiteral::: DecimalDigits ExponentPart is the MV of DecimalDigits times 10e, where e is the MV of ExponentPart.
  • The MV of DecimalDigits ::: DecimalDigit is the MV of DecimalDigit. The MV of DecimalDigits ::: DecimalDigits DecimalDigit is (the MV of DecimalDigits times 10) plus the MV of DecimalDigit.
  • The MV of ExponentPart ::: ExponentIndicator SignedInteger is the MV of SignedInteger.
  • The MV of SignedInteger ::: DecimalDigits is the MV of DecimalDigits.
  • The MV of SignedInteger ::: + DecimalDigits is the MV of DecimalDigits.
  • The MV of SignedInteger ::: - DecimalDigits is the negative of the MV of DecimalDigits.
  • The MV of DecimalDigit ::: 0 or of HexDigit ::: 0 is 0.
  • The MV of DecimalDigit ::: 1 or of HexDigit ::: 1 is 1.
  • The MV of DecimalDigit ::: 2 or of HexDigit ::: 2 is 2.
  • The MV of DecimalDigit ::: 3 or of HexDigit ::: 3 is 3.
  • The MV of DecimalDigit ::: 4 or of HexDigit ::: 4 is 4.
  • The MV of DecimalDigit ::: 5 or of HexDigit ::: 5 is 5.
  • The MV of DecimalDigit ::: 6 or of HexDigit ::: 6 is 6.
  • The MV of DecimalDigit ::: 7 or of HexDigit ::: 7 is 7.
  • The MV of DecimalDigit ::: 8 or of HexDigit ::: 8 is 8.
  • The MV of DecimalDigit ::: 9 or of HexDigit ::: 9 is 9.
  • The MV of HexDigit ::: a or of HexDigit ::: A is 10.
  • The MV of HexDigit ::: b or of HexDigit ::: B is 11.
  • The MV of HexDigit ::: c or of HexDigit ::: C is 12.
  • The MV of HexDigit ::: d or of HexDigit ::: D is 13.
  • The MV of HexDigit ::: e or of HexDigit ::: E is 14.
  • The MV of HexDigit ::: f or of HexDigit ::: F is 15.
  • The MV of HexIntegerLiteral ::: 0x HexDigit is the MV of HexDigit.
  • The MV of HexIntegerLiteral ::: 0X HexDigit is the MV of HexDigit.
  • The MV of HexIntegerLiteral ::: HexIntegerLiteral HexDigit is (the MV of HexIntegerLiteral times 16) plus the MV of HexDigit.

Once the exact MV for a string numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is +0 unless the first non white space character in the string numeric literal is '-', in which case the rounded value is -0. Otherwise, the rounded value must be the number value for the MV (in the sense defined in 8.5), unless the literal includes a StrUnsignedDecimalLiteral and the literal has more than 20 significant digits, in which case the number value may be either the number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit or the number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit and then incrementing the literal at the 20th digit position. A digit is significant if it is not part of an ExponentPart and

  • it is not 0; or
  • there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.
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 ToUint16.

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

Otherwise, let n, k, and s be integers such that k >= 1, 10k-1 <= s <10k , the number value for s * 10n-k is m, and k is as small as possible. If there are multiple possibilities for s, choose the value of s for which s * 10n-k is closest in value to m. If there are two such possible values of s, choose the one that is even. Note that k is the number of digits in the decimal representation of s and that s is not divisible by 10.

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 Throw a TypeError exception.
Null Throw a TypeError exception.
Boolean Create a new Boolean object whose [[value]] property is set to the value of the boolean. See 15.6 for a description of Boolean objects.
Number Create a new Number object whose [[value]] property is set to the value of the number. See 15.7 for a description of Number objects.
String Create a new String object whose [[value]] property is set to the value of the string. See 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 two types of Function objects:

  • Program functions are defined in source text by a FunctionDeclaration or created dynamically either by using a FunctionExpression or by using the built-in Function object as a constructor.
  • Internal functions are built-in objects of the language, such as parseInt and Math.exp. An implementation may also provide implementation-dependent internal functions that are not described in this specification. These functions do not contain executable code defined by the ECMAScript grammar, so they are excluded from this discussion of execution contexts.
10.1.2 Types of Executable Code

There are three types of ECMAScript executable code:

  • Global code is source text that is treated as an ECMAScript Program. The global code of a particular Program does not include any source text that is parsed as part of a FunctionBody.
  • Eval code is the source text supplied to the built-in eval function. More precisely, if the parameter to the built-in eval function is a string, it is treated as an ECMAScript Program. The eval code for a particular invocation of eval is the global code portion of the string parameter.
  • Function code is source text that is parsed as part of a FunctionBody. The function code of a particular FunctionBody does not include any source text that is parsed as part of a nested FunctionBody. Function code also denotes the source text supplied when using the built-in Function object as a constructor. More precisely, the last parameter provided to the Function constructor is converted to a string and treated as the FunctionBody. If more than one parameter is provided to the Function constructor, all parameters except the last one are converted to strings and concatenated together, separated by commas. The resulting string is interpreted as the FormalParameterList for the FunctionBody defined by the last parameter. The function code for a particular instantiation of a Function does not include any source text that is parsed as part of a nested FunctionBody.
10.1.3 Variable Instantiation

Every execution context has associated with it a variable object. Variables and functions declared in the source text are added as properties of the variable object. For function 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. On entering an execution context, the properties are bound to the variable object in the following order:

  • For function code: for each formal parameter, as defined in the FormalParameterList, create a property of the variable object whose name is the Identifier and whose attributes are determined by the type of code. The values of the parameters are supplied by the caller as arguments to [[Call]]. If the caller supplies fewer parameter values than there are formal parameters, the extra formal parameters have value undefined. If two or more formal parameters share the same name, hence the same property, the corresponding property is given the value that was supplied for the last parameter with this name. If the value of this last parameter was not supplied by the caller, the value of the corresponding property is undefined.
  • For each FunctionDeclaration in the code, in source text order, create a property of the variable object whose name is the Identifier in the FunctionDeclaration, whose value is the result returned by creating a Function object as described in 13, and whose attributes are determined by the type of code. If the variable object already has a property with this name, replace its value and attributes. Semantically, this step must follow the creation of FormalParameterList properties.
  • For each VariableDeclaration or VariableDeclarationNoIn in the code, create a property of the variable object whose name is the Identifier in the VariableDeclaration or VariableDeclarationNoIn, whose value is undefined and whose attributes are determined by the type of code. If there is already a property of the variable object with the name of a declared variable, the value of the property and its attributes are not changed. Semantically, this step must follow the creation of the FormalParameterList and FunctionDeclaration properties. In particular, if a declared variable has the same name as a declared function or formal parameter, the variable declaration does not disturb the existing property.
10.1.4 Scope Chain and Identifier Resolution

Every execution context has associated with it a scope chain. A scope chain is a list of objects that are searched when evaluating an Identifier. When control enters an execution context, a scope chain is created and populated with an initial set of objects, depending on the type of code. During execution within an execution context, the scope chain of the execution context is affected only by with statements (see 12.10) and catch clauses (see 12.14).

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(1), passing the Identifier as the property name.

3. If Result(2) is true, return a value of type Reference whose base object is Result(1) 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 evaluating 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 (15.1), which is created before control enters any execution context. Initially the global object has the following properties:

  • Built-in objects such as Math, String, Date, parseInt, etc. These have attributes { DontEnum }.
  • Additional host defined properties. This may include a property whose value is the global object itself; for example, in the HTML document object model the window property of the global object is the global object itself.

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 function 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 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.

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 function code, an arguments object is created and initialised as follows:

  • The value of the internal [[Prototype]] property of the arguments object is the original Object prototype object, the one that is the initial value of Object.prototype (see 15.2.3.1).
  • A property is created with name callee and property attributes { DontEnum }. The initial value of this property is the Function object being executed. This allows anonymous functions to be recursive.
  • A property is created with name length and property attributes { DontEnum }. The initial value of this property is the number of actual parameter values supplied by the caller.
  • For each non-negative integer, arg, less than the value of the length property, a property is created with name ToString(arg) and property attributes { DontEnum }. The initial value of this property is the value of the corresponding actual parameter supplied by the caller. The first actual parameter value corresponds to arg = 0, the second to arg = 1, and so on. In the case when arg is less than the number of formal parameters for the Function object, this property shares its value with the corresponding property of the activation object. This means that changing this property changes the corresponding property of the activation object and vice versa.
10.2 Entering An Execution Context

Every function and constructor call enters a new execution context, even if a function is calling itself recursively. Every return exits an execution context. A thrown exception, if not caught, may also exit one or more execution contexts.

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
  • The scope chain is created and initialised to contain the global object and no others.
  • Variable instantiation is performed using the global object as the variable object and using property attributes { DontDelete }.
  • The this value is the global object.
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.

  • The scope chain is initialised to contain the same objects, in the same order, as the calling context's scope chain. This includes objects added to the calling context's scope chain by with statements and catch clauses.
  • Variable instantiation is performed using the calling context's variable object and using empty property attributes.
  • The this value is the same as the this value of the calling context.
10.2.3 Function Code
  • The scope chain is initialised to contain the activation object followed by the objects in the scope chain stored in the [[Scope]] property of the Function object.
  • Variable instantiation is performed using the activation object as the variable object and using property attributes { DontDelete }.
  • The caller provides the this value. If the this value provided by the caller is not an object (note that null is not an object), then the this value is the global object.
11 Expressions
11.1 Primary Expressions

Syntax

PrimaryExpression :
this
Identifier
Literal
ArrayLiteral
ObjectLiteral
(Expression )
11.1.1 The this Keyword

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

11.1.2 Identifier Reference

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

11.1.3 Literal Reference

A Literal is evaluated as described in 7.8.

11.1.4 Array Initialiser

An array initialiser is an expression describing the initialisation of an Array object, written in a form of a literal. It is a list of zero or more expressions, each of which represents an array element, enclosed in square brackets. The elements need not be literals; they are evaluated each time the array initialiser is evaluated.

Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is not preceded by an AssignmentExpression (i. e., a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array elements are not defined.

Syntax

ArrayLiteral :
[ Elisionopt ]
[ ElementList ]
[ ElementList , Elisionopt ]
ElementList :
Elisionopt AssignmentExpression
ElementList , Elisionopt AssignmentExpression
Elision :
,
Elision ,

Semantics

The production ArrayLiteral : [ Elisionopt ] is evaluated as follows:

1. Create a new array as if by the expression new Array().

2. Evaluate Elision; if not present, use the numeric value zero.

3. Call the [[Put]] method of Result(1) with arguments "length" and Result(2).

4. Return Result(1).

The production ArrayLiteral : [ ElementList ] is evaluated as follows:

1. Evaluate ElementList.

2. Return Result(1).

The production ArrayLiteral : [ ElementList , Elisionopt ] is evaluated as follows:

1. Evaluate ElementList.

2. Evaluate Elision; if not present, use the numeric value zero.

3. Call the [[Get]] method of Result(1) with argument "length".

4. Call the [[Put]] method of Result(1) with arguments "length" and (Result(2)+ Result(3)).

5. Return Result(1).

The production ElementList : Elisionopt AssignmentExpression is evaluated as follows:

1. Create a new array as if by the expression new Array().

2. Evaluate Elision; if not present, use the numeric value zero.

3. Evaluate AssignmentExpression.

4. Call GetValue(Result(3)).

5. Call the [[Put]] method of Result(1) with arguments Result(2) and Result(4).

6. Return Result(1)

The production ElementList : ElementList , Elisionopt AssignmentExpression is evaluated as follows:

1. Evaluate ElementList.

2. Evaluate Elision; if not present, use the numeric value zero.

3. Evaluate AssignmentExpression.

4. Call GetValue(Result(3)).

5. Call the [[Get]] method of Result(1) with argument "length".

6. Call the [[Put]] method of Result(1) with arguments (Result(2)+ Result(5)) and Result(4).

7. Return Result(1)

The production Elision : , is evaluated as follows:

1. Return the numeric value 1.

The production Elision : Elision , is evaluated as follows:

1. Evaluate Elision.

2. Return (Result(1)+ 1).

11.1.5 Object Initialiser

An object initialiser is an expression describing the initialisation of an Object, written in a form resembling a literal. It is a list of zero or more pairs of property names and associated values, enclosed in curly braces. The values need not be literals; they are evaluated each time the object initialiser is evaluated.

Syntax

ObjectLiteral :
{}
{ PropertyNameAndValueList }
PropertyNameAndValueList :
PropertyName : AssignmentExpression
PropertyNameAndValueList , PropertyName : AssignmentExpression
PropertyName :
Identifier
StringLiteral
NumericLiteral

Semantics

The production ObjectLiteral : {} is evaluated as follows:

1. Create a new object as if by the expression new Object().

2. Return Result(1).

The production ObjectLiteral : { PropertyNameAndValueList } is evaluated as follows:

1. Evaluate PropertyNameAndValueList.

2. Return Result(1);

The production PropertyNameAndValueList : PropertyName : AssignmentExpression is evaluated as follows:

1. Create a new object as if by the expression new Object().

2. Evaluate PropertyName.

3. Evaluate AssignmentExpression.

4. Call GetValue(Result(3)).

5. Call the [[Put]] method of Result(1) with arguments Result(2) and Result(4).

6. Return Result(1).

The production PropertyNameAndValueList : PropertyNameAndValueList , PropertyName : AssignmentExpression is evaluated as follows:

1. Evaluate PropertyNameAndValueList.

2. Evaluate PropertyName.

3. Evaluate AssignmentExpression.

4. Call GetValue(Result(3)).

5. Call the [[Put]] method of Result(1) with arguments Result(2) and Result(4).

6. Return Result(1).

The production PropertyName : Identifier is evaluated as follows:

1. Form a string literal containing the same sequence of characters as the Identifier.

2. Return Result(1).

The production PropertyName : StringLiteral is evaluated as follows:

1. Return the value of the StringLiteral.

The production PropertyName : NumericLiteral is evaluated as follows:

1. Form the value of the NumericLiteral.

2. Return ToString(Result(1)).

11.1.6 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
FunctionExpression
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, throw a TypeError exception.

4. If Result(2) does not implement the internal [[Construct]] method, throw a TypeError exception.

5. Call the [[Construct]] method on Result(2), providing no arguments (that is, an empty list of arguments).

6. 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 (11.2.4).

4. If Type(Result(2)) is not Object, throw a TypeError exception.

5. If Result(2) does not implement the internal [[Construct]] method, throw a TypeError exception.

6. Call the [[Construct]] method on Result(2), providing the list Result(3) as the argument values.

7. 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 (see 11.2.4).

3. Call GetValue(Result(1)).

4. If Type(Result(3)) is not Object, throw a TypeError exception.

5. If Result(3) does not implement the internal [[Call]] method, throw a TypeError exception.

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 (see 8.8).

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.

11.2.5 Function Expressions

The production MemberExpression : FunctionExpression is evaluated as follows:

1. Evaluate FunctionExpression.

2. Return Result(1).

11.3 Postfix Expressions

Syntax

PostfixExpression :
LeftHandSideExpression
LeftHandSideExpression [no LineTerminator here] ++
LeftHandSideExpression [no LineTerminator here] --
11.3.1 Postfix Increment Operator

The production PostfixExpression : LeftHandSideExpression [no LineTerminator here] ++ is evaluated as follows:

1. Evaluate LeftHandSideExpression.

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 (see 11.6.3).

5. Call PutValue(Result(1), Result(4)).

6. Return Result(3).

11.3.2 Postfix Decrement Operator

The production PostfixExpression : LeftHandSideExpression [no LineTerminator here] -- is evaluated as follows:

1. Evaluate LeftHandSideExpression.

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 (11.6.3).

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. If Type(Result(1)) is not Reference, return true.

3. Call GetBase(Result(1)).

4. Call GetPropertyName(Result(1)).

5. Call the [[Delete]] method on Result(3), providing Result(4) as the property name to delete.

6. Return Result(5).

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 not Reference, go to step 4.

3. If GetBase(Result(1)) is null, return "undefined".

4. Call GetValue(Result(1)).

5. Return a string determined by Type(Result(4)) 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 (see 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 (see 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 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:

  • If either operand is NaN, the result is NaN.
  • The sign of the result is positive if both operands have the same sign, negative if the operands have different signs.
  • Multiplication of an infinity by a zero results in NaN.
  • Multiplication of an infinity by an infinity results in an infinity. The sign is determined by the rule already stated above.
  • Multiplication of an infinity by a finite non-zero value results in a signed infinity. The sign is determined by the rule already stated above.
  • In the remaining cases, where neither an infinity or NaN is involved, the product is computed and rounded to the nearest representable value using IEEE 754 round-to-nearest mode. If the magnitude is too large to represent, the result is then an infinity of appropriate sign. If the magnitude is too small to represent, the result is then a zero of appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754.
11.5.2 Applying the / Operator

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

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

The % operator yields the remainder of its operands from an implied division; the left operand is the dividend and the right operand is the divisor.

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

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

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

  • If either operand is NaN, the result is NaN.
  • The sign of the result equals the sign of the dividend.
  • If the dividend is an infinity, or the divisor is a zero, or both, the result is NaN.
  • If the dividend is finite and the divisor is an infinity, the result equals the dividend.
  • If the dividend is a zero and the divisor is finite, the result is the same as the dividend.
  • In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the floating-point remainder r from a dividend n and a divisor d is defined by the mathematical relation r = n - (d *q) where q is an integer that is negative only if n/d is negative and positive only if n/d is positive, and whose magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient of n and d.
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 comparison algorithm for the relational operators, by 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:

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

The - operator performs subtraction when applied to two operands of numeric type, producing the difference of its operands; the left operand is the minuend and the right operand is the subtrahend. Given numeric operands a and b, it is always the case that a- b produces the same result as a+(- b).

11.7 Bitwise Shift Operators

Syntax

ShiftExpression :
AdditiveExpression
ShiftExpression << AdditiveExpression
ShiftExpression >> AdditiveExpression
ShiftExpression >>> AdditiveExpression
11.7.1 The Left Shift Operator (<< )

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

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

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
RelationalExpression instanceof ShiftExpression
RelationalExpression in ShiftExpression
RelationalExpressionNoIn :
ShiftExpression
RelationalExpressionNoIn < ShiftExpression
RelationalExpressionNoIn > ShiftExpression
RelationalExpressionNoIn <= ShiftExpression
RelationalExpressionNoIn >= ShiftExpression
RelationalExpressionNoIn instanceof ShiftExpression

NOTE
The 'NoIn' variants are needed to avoid confusing the in operator in a relational expression with the in operator in a for statement.

Semantics

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

The RelationalExpressionNoIn productions are evaluated in the same manner as the RelationalExpression productions except that the contained RelationalExpressionNoIn is evaluated instead of the contained RelationalExpression.

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 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 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 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 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 code point value for the character at position k within Result(1).

20. Let n be the integer that is the code point value 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 code point value values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore strings that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both strings are already in normalised form.

11.8.6 The instanceof operator

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

1. Evaluate RelationalExpression.

2.Call GetValue(Result(1)).

3.Evaluate ShiftExpression.

4.Call GetValue(Result(3)).

5.If Result(4) is not an object, throw a TypeError exception.

6.If Result(4) does not have a [[HasInstance]] method, throw a TypeError exception.

7.Call the [[HasInstance]] method of Result(4) with parameter Result(2).

8.Return Result(7).

11.8.7 The in operator

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

1. Evaluate RelationalExpression.

2.Call GetValue(Result(1)).

3.Evaluate ShiftExpression.

4.Call GetValue(Result(3)).

5.If Result(4) is not an object, throw a TypeError exception.

6.Call ToString(Result(2)).

7.Call the [[HasProperty]] method of Result(4) with parameter Result(6).

8.Return Result(7).

11.9 Equality Operators

Syntax

EqualityExpression :
RelationalExpression
EqualityExpression == RelationalExpression
EqualityExpression != RelationalExpression
EqualityExpression === RelationalExpression
EqualityExpression !== RelationalExpression
EqualityExpressionNoIn :
RelationalExpressionNoIn
EqualityExpressionNoIn == RelationalExpressionNoIn
EqualityExpressionNoIn != RelationalExpressionNoIn
EqualityExpressionNoIn === RelationalExpressionNoIn
EqualityExpressionNoIn !== RelationalExpressionNoIn

Semantics

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

The EqualityExpressionNoIn productions are evaluated in the same manner as the EqualityExpression productions except that the contained EqualityExpressionNoIn and RelationalExpressionNoIn are evaluated instead of the contained EqualityExpression and RelationalExpression, respectively.

11.9.1 The Equals Operator (== )

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

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 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 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 or if they refer to objects joined to each other (see 13.1.2). 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:

String comparison can be forced by: ""+a ==""+ b.

Numeric comparison can be forced by: a -0 ==b -0.

Boolean comparison can be forced by: !a == !b.

The equality operators maintain the following invariants:

A != B is equivalent to !(A == B).

A == B is equivalent to B == A, except in the order of evaluation of A and B.

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 code point value 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. Therefore strings that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both strings are already in normalised form.

11.9.4 The Strict Equals Operator (=== )

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

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 below.)

6.Return Result(5).

11.9.5 The Strict Does-not-equal 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 below.)

6.If Result(5) is true, return false. Otherwise, return true.

11.9.6 The Strict 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), return false.

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 or if they refer to objects joined to each other (see 13.1.2). Otherwise, return false.

11.10 Binary Bitwise Operators

Syntax

BitwiseANDExpression :
EqualityExpression
BitwiseANDExpression & EqualityExpression
BitwiseANDExpressionNoIn :
EqualityExpressionNoIn
BitwiseANDExpressionNoIn & EqualityExpressionNoIn
BitwiseXORExpression :
BitwiseANDExpression
BitwiseXORExpression ^ BitwiseANDExpression
BitwiseXORExpressionNoIn :
BitwiseANDExpressionNoIn
BitwiseXORExpressionNoIn ^ BitwiseANDExpressionNoIn
BitwiseORExpression :
BitwiseXORExpression
BitwiseORExpression | BitwiseXORExpression
BitwiseORExpressionNoIn :
BitwiseXORExpressionNoIn
BitwiseORExpressionNoIn | BitwiseXORExpressionNoIn

Semantics

The production 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
LogicalANDExpressionNoIn :
BitwiseORExpressionNoIn
LogicalANDExpressionNoIn && BitwiseORExpressionNoIn
LogicalORExpression :
LogicalANDExpression
LogicalORExpression || LogicalANDExpression
LogicalORExpressionNoIn :
LogicalANDExpressionNoIn
LogicalORExpressionNoIn || LogicalANDExpressionNoIn

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).

The LogicalANDExpressionNoIn and LogicalORExpressionNoIn productions are evaluated in the same manner as the LogicalANDExpression and LogicalORExpression productions except that the contained LogicalANDExpressionNoIn, BitwiseORExpressionNoIn and LogicalORExpressionNoIn are evaluated instead of the contained LogicalANDExpression, BitwiseORExpression and LogicalORExpression, respectively.

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

11.12 Conditional Operator (?: )

Syntax

ConditionalExpression :
LogicalORExpression
LogicalORExpression ? AssignmentExpression : AssignmentExpression
ConditionalExpressionNoIn :
LogicalORExpressionNoIn
LogicalORExpressionNoIn ? AssignmentExpression : AssignmentExpressionNoIn

Semantics

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

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).

The ConditionalExpressionNoIn production is evaluated in the same manner as the ConditionalExpression production except that the contained LogicalORExpressionNoIn, AssignmentExpression and AssignmentExpressionNoIn are evaluated instead of the contained LogicalORExpression, first AssignmentExpression and second AssignmentExpression, respectively.

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

11.13 Assignment Operators

Syntax

AssignmentExpression :
ConditionalExpression
LeftHandSideExpression AssignmentOperator AssignmentExpression
AssignmentExpressionNoIn :
ConditionalExpressionNoIn
LeftHandSideExpression AssignmentOperator AssignmentExpressionNoIn
AssignmentOperator :oneof
= *= /= %= += -= <<= >>= >>>= &= ^= |=

Semantics

The AssignmentExpressionNoIn productions are evaluated in the same manner as the AssignmentExpression productions except that the contained ConditionalExpressionNoIn and AssignmentExpressionNoIn are evaluated instead of the contained ConditionalExpression and AssignmentExpression, respectively.

11.13.1 Simple Assignment (= )

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

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
ExpressionNoIn :
AssignmentExpressionNoIn
ExpressionNoIn , AssignmentExpressionNoIn

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).

The ExpressionNoIn production is evaluated in the same manner as the Expression production except that the contained ExpressionNoIn and AssignmentExpressionNoIn are evaluated instead of the contained Expression and AssignmentExpression, respectively.

12 Statements

Syntax

Statement :
Block
VariableStatement
EmptyStatement
ExpressionStatement
IfStatement
IterationStatement
ContinueStatement
BreakStatement
ReturnStatement
WithStatement
LabelledStatement
SwitchStatement
ThrowStatement
TryStatement

Semantics

A Statement can be part of a LabelledStatement, which itself can be part of a LabelledStatement, and so on. The labels introduced this way are collectively referred to as the "current label set" when describing the semantics of individual statements. A LabelledStatement has no semantic meaning other than the introduction of a label to a label set. The label set of an IterationStatement or a SwitchStatement initially contains the single element empty. The label set of any other statement is initially empty.

12.1 Block

Syntax

Block :
{ StatementListopt }
StatementList :
Statement
StatementList Statement

Semantics

The production Block : {} is evaluated as follows:

1.Return (normal, empty, empty).

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.If an exception was thrown, return (throw, V, empty) where V is the exception. (Execution now proceeds as if no exception were thrown.)

3.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 an exception was thrown, return (throw, V, empty) where V is the exception. (Execution now proceeds as if no exception were thrown.)

5.If Result(3). value is empty, let V = Result(1). value, otherwise let V = Result(3). value.

6.Return (Result(3). type, V, Result(3). target).

12.2 Variable statement

Syntax

VariableStatement :
var VariableDeclarationList ;
VariableDeclarationList :
VariableDeclaration
VariableDeclarationList , VariableDeclaration
VariableDeclarationListNoIn :
VariableDeclarationNoIn
VariableDeclarationListNoIn , VariableDeclarationNoIn
VariableDeclaration :
Identifier Initialiseropt
VariableDeclarationNoIn :
Identifier InitialiserNoInopt
Initialiser :
= AssignmentExpression
InitialiserNoIn :
= AssignmentExpressionNoIn

Description

If the variable statement occurs inside a FunctionDeclaration, the variables are defined with function-local scope in that function, as described in s10.1.3. Otherwise, they are defined with global scope (that is, they are created as members of the global object, as described in 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 undefined when created. A variable with an Initialiser is assigned the value of its AssignmentExpression when the VariableStatement is executed, not when the variable is created.

Semantics

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

1.Evaluate VariableDeclarationList.

2.Return (normal, empty, empty).

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 Initialiser is evaluated as follows:

1.Evaluate Identifier as described in 11.1.2.

2.Evaluate Initialiser.

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 Initialiser : = AssignmentExpression is evaluated as follows:

1.Evaluate AssignmentExpression.

2.Return Result(1).

The VariableDeclarationListNoIn, VariableDeclarationNoIn and InitialiserNoIn productions are evaluated in the same manner as the VariableDeclarationList, VariableDeclaration and Initialiser productions except that the contained VariableDeclarationListNoIn, VariableDeclarationNoIn, InitialiserNoIn and AssignmentExpressionNoIn are evaluated instead of the contained VariableDeclarationList, VariableDeclaration, Initialiser and AssignmentExpression, respectively.

12.3 Empty Statement

Syntax

EmptyStatement :
;

Semantics

The production EmptyStatement : ; is evaluated as follows:

1.Return (normal, empty, empty).

12.4 Expression Statement

Syntax

ExpressionStatement :
[lookahead ∉ {{, function}] Expression ;

Note that an ExpressionStatement cannot start with an opening curly brace because that might make it ambiguous with a Block. Also, an ExpressionStatement cannot start with the function keyword because that might make it ambiguous with a FunctionDeclaration.

Semantics

The production ExpressionStatement : [lookahead ∉ {{, function}] Expression; is evaluated as follows:

1.Evaluate Expression.

2.Call GetValue(Result(1)).

3.Return (normal, Result(2), empty).

12.5 The if Statement

Syntax

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

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

Semantics

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

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, empty, empty).

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 parenthesised control construct) and a body (which consists of a Statement).

Syntax

IterationStatement :
do Statement while ( Expression );
while ( Expression ) Statement
for ( ExpressionNoInopt; Expressionopt ; Expressionopt ) Statement
for (var VariableDeclarationListNoIn; Expressionopt ; Expressionopt ) Statement
for ( LeftHandSideExpression in Expression ) Statement
for (var VariableDeclarationNoIn in Expression ) Statement
12.6.1 The do-while Statement

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

1. Let V = empty.

2.Evaluate Statement.

3.If Result(2). value is not empty, let V = Result(2). value.

4.If Result(2). type is continue and Result(2). target is in the current label set, go to step 7.

5. If Result(2). type is break and Result(2). target is in the current label set, return (normal, V, empty).

6.If Result(2) is an abrupt completion, return Result(2).

7.Evaluate Expression.

8.Call GetValue(Result(7)).

9. Call ToBoolean(Result(8)).

10. If Result(9) is true, go to step 2.

11. Return (normal, V, empty);

12.6.2 The while statement

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

1. Let V = empty.

2.Evaluate Expression.

3.Call GetValue(Result(2)).

4. Call ToBoolean(Result(3)).

5.If Result(4) is false, return (normal, V, empty).

6.Evaluate Statement.

7.If Result(6). value is not empty, let V = Result( 6). value.

8.If Result(6). type is continue and Result(6). target is in the current label set, go to 2.

9. If Result(6). type is break and Result(6). target is in the current label set, return (normal, V, empty).

10. If Result(6) is an abrupt completion, return Result(6).

11. Go to step 2.

12.6.3 The for Statement

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

1. If ExpressionNoIn is not present, go to step 4.

2. Evaluate ExpressionNoIn.

3. Call GetValue(Result(2)). (This value is not used.)

4. Let V = empty.

5. If the first Expression is not present, go to step 10.

6. Evaluate the first 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). value is not empty, let V = Result(10). value

12. If Result(10). type is break and Result(10). target is in the current label set, go to step 19.

13. If Result(10). type is continue and Result(10). target is in the current label set, go to step 15.

14. If Result(10) is an abrupt completion, return Result(10).

15. If the second Expression is not present, go to step 5.

16. Evaluate the second Expression.

17. Call GetValue(Result(16). (This value is not used.)

18. Go to step 5.

19. Return (normal, V, empty).

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

1. Evaluate VariableDeclarationListNoIn.

2. Let V = empty.

3. If the first Expression is not present, go to step 8.

4. Evaluate the first Expression.

5. Call GetValue(Result(4)).

6. Call ToBoolean(Result(5)).

7. If Result(6) is false, go to step 17.

8. Evaluate Statement.

9. If Result(8). value is not empty, let V = Result(8). value.

10. If Result(8). type is break and Result(8). target is in the current label set, go to step 17.

11. If Result(8). type is continue and Result(8). target is in the current label set, go to step 13.

12. If Result(8) is an abrupt completion, return Result(8).

13. If the second Expression is not present, go to step 3.

14. Evaluate the second Expression.

15. Call GetValue(Result(14)). (This value is not used.)

16. Go to step 3.

17. Return (normal, V, empty).

12.6.4 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 V = empty.

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). value is not empty, let V = Result(8). value.

10. If Result(8). type is break and Result(8). target is in the current label set, go to step 14.

11. If Result(8). type is continue and Result(8). target is in the current label set, go to step 5.

12. If Result(8) is an abrupt completion, return Result(8).

13. Go to step 5.

14. Return (normal, V, empty).

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

1. Evaluate VariableDeclarationNoIn.

2. Evaluate Expression.

3. Call GetValue(Result(2)).

4. Call ToObject(Result(3)).

5. Let V = empty.

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). value is not empty, let V = Result(9). value.

11.If Result(9). type is break and Result(9). target is in the current label set, go to step 15.

12.If Result(9). type is continue and Result(9). target is in the current label set, go to step 6.

13. If Result(8) is an abrupt completion, return Result(8).

14. Go to step 6.

15. Return (normal, V, empty).

The mechanics of enumerating the properties (step 5 in the first algorithm, step 6 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 [no LineTerminator here] Identifieropt ;

Semantics

A program is considered syntactically incorrect if either of the following are true:

  • The program contains a continue statement without the optional Identifier, which is not nested, directly or indirectly (but not crossing function boundaries), within an IterationStatement.
  • The program contains a continue statement with the optional Identifier, where Identifier does not appear in the label set of an enclosing (but not crossing function boundaries) IterationStatement.

A ContinueStatement without an Identifier is evaluated as follows:

1. Return (continue, empty, empty).

A ContinueStatement with the optional Identifier is evaluated as follows:

1. Return (continue, empty, Identifier).

12.8 The break Statement

Syntax

BreakStatement :
break [no LineTerminator here] Identifieropt ;

Semantics

A program is considered syntactically incorrect if either of the following are true:

  • The program contains a break statement without the optional Identifier, which is not nested, directly or indirectly (but not crossing function boundaries), within an IterationStatement or a SwitchStatement.
  • The program contains a break statement with the optional Identifier, where Identifier does not appear in the label set of an enclosing (but not crossing function boundaries) Statement.

A BreakStatement without an Identifier is evaluated as follows:

1. Return (break, empty, empty).

A BreakStatement with an Identifier is evaluated as follows:

1. Return (break, empty, Identifier).

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 a FunctionBody. A return statement causes a function to cease execution and return a value to the caller. If Expression is omitted, the return value is undefined. Otherwise, the return value is the value of Expression.

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

1. If the Expression is not present, return (return, undefined, empty).

2. Evaluate Expression.

3. Call GetValue(Result(2)).

4. Return (return, Result(3), empty).

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. Let C be Result(5). If an exception was thrown in step 5, let C be (throw, V, empty), where V is the exception. (Execution now proceeds as if no exception were thrown.)

7. Remove Result(3) from the front of the scope chain.

8. Return C.

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

12.11 The switch Statement

Syntax

SwitchStatement :
switch ( Expression ) CaseBlock
CaseBlock :
{ CaseClausesopt }
{ CaseClausesopt DefaultClause CaseClausesopt }
CaseClauses :
CaseClause
CaseClauses CaseClause
CaseClause :
case Expression : StatementListopt
DefaultClause :
default : StatementListopt

Semantics

The production SwitchStatement : switch ( Expression ) CaseBlock is evaluated as follows:

1. Evaluate Expression.

2. Call GetValue(Result(1)).

3. Evaluate CaseBlock, passing it Result(2) as a parameter.

4. If Result(3). type is break and Result(3). target is in the current label set, return (normal, Result(3).value, empty).

5. Return Result(3).

The production CaseBlock : { CaseClausesopt } is given an input parameter, input, and is evaluated as follows:

1. Let V = empty.
2. Let A be the list of CaseClause items in source text order.
3. Let C be the next CaseClause in A. If there is no such CaseClause, then go to step 16.
4. Evaluate C.
5. If input is not equal to Result(4) as defined by the !== operator, then go to step 3.
6. If C does not have a StatementList, then go to step 10.
7. Evaluate C's StatementList and let R be the result.
8. If R is an abrupt completion, then return R.
9. Let V = R.value.
10. Let C be the next CaseClause in A. If there is no such CaseClause, then go to step 16.
11. If C does not have a StatementList, then go to step 10.
12. Evaluate C's StatementList and let R be the result.
13. If R.value is not empty, then let V = R.value.
14. If R is an abrupt completion, then return (R.type, V, R.target).
15. Go to step 10.
16. Return (normal, V, empty).

The production CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt } is given an input parameter, input, and is evaluated as follows:

1. Let V = empty.
2. Let A be the list of CaseClause items in the first CaseClauses, in source text order.
3. Let C be the next CaseClause in A. If there is no such CaseClause, then go to step 11.
4. Evaluate C.
5. If input is not equal to Result(4) as defined by the !== operator, then go to step 3.
6. If C does not have a StatementList, then go to step 20.
7. Evaluate C's StatementList and let R be the result.
8. If R is an abrupt completion, then return R.
9. Let V = R.value.
10. Go to step 20.
11. Let B be the list of CaseClause items in the second CaseClauses, in source text order.
12. Let C be the next CaseClause in B. If there is no such CaseClause, then go to step 26.
13. Evaluate C.
14. If input is not equal to Result(13) as defined by the !== operator, then go to step 12.
15. If C does not have a StatementList, then go to step 31.
16. Evaluate C's StatementList and let R be the result.
17. If R is an abrupt completion, then return R.
18. Let V = R.value.
19. Go to step 31.
20. Let C be the next CaseClause in A. If there is no such CaseClause, then go to step 26.
21. If C does not have a StatementList, then go to step 20.
22. Evaluate C's StatementList and let R be the result.
23. If R.value is not empty, then let V = R.value.
24. If R is an abrupt completion, then return (R.type, V, R.target).
25. Go to step 20.
26. If the DefaultClause does not have a StatementList, then go to step 30.
27. Evaluate the DefaultClause's StatementList and let R be the result.
28. If R.value is not empty, then let V = R.value.
29. If R is an abrupt completion, then return (R.type, V, R.target).
30. Let B be the list of CaseClause items in the second CaseClauses, in source text order.
31. Let C be the next CaseClause in B. If there is no such CaseClause, then go to step 37.
32. If C does not have a StatementList, then go to step 31.
33. Evaluate C's StatementList and let R be the result.
34. If R.value is not empty, then let V = R.value.
35. If R is an abrupt completion, then return (R.type, V, R.target).
36. Go to step 31.
37. Return (normal, V, empty).

The production CaseClause : case Expression : StatementListopt is evaluated as follows:

1. Evaluate Expression.

2. Call GetValue(Result(1)).

3. Return Result(2).

NOTE
Evaluating CaseClause does not execute the associated StatementList. It simply evaluates the Expression and returns the value, which the CaseBlock algorithm uses to determine which StatementList to start executing.

12.12 Labelled Statements

Syntax

LabelledStatement :
Identifier : Statement

Semantics

A Statement may be prefixed by a label. Labelled statements are only used in conjunction with labelled break and continue statements. ECMAScript has no goto statement.

An ECMAScript program is considered syntactically incorrect if it contains a LabelledStatement that is enclosed by a LabelledStatement with the same Identifier as label. This does not apply to labels appearing within the body of a FunctionDeclaration that is nested, directly or indirectly, within a labelled statement.

The production Identifier : Statement is evaluated by adding Identifier to the label set of Statement and then evaluating Statement. If theLabelledStatement itself has a non-empty label set, these labels are also added to the label set of Statement before evaluating it. If the result of evaluating Statement is (break, V, L) where L is equal to Identifier, the production results in (normal, V, empty).

Prior to the evaluation of a LabelledStatement, the contained Statement is regarded as possessing an empty label set, except if it is an IterationStatement or a SwitchStatement, in which case it is regarded as possessing a label set consisting of the single element, empty.

12.13 The throw statement

Syntax

ThrowStatement :
throw [no LineTerminator here] Expression ;

Semantics

The production ThrowStatement : throw [no LineTerminator here] Expression ; is evaluated as:

1. Evaluate Expression.

2. Call GetValue(Result(1)).

3. Return (throw, Result(2), empty).

12.14 The try statement

Syntax

TryStatement :
try Block Catch
try Block Finally
try Block Catch Finally
Catch :
catch (Identifier ) Block
Finally :
finally Block

Description

The try statement encloses a block of code in which an exceptional condition can occur, such as a runtime error or a throw statement. The catch clause provides the exception-handling code. When a catch clause catches an exception, its Identifier is bound to that exception.

Semantics

The production TryStatement : try Block Catch is evaluated as follows:

1. Evaluate Block.

2. If Result(1). type is not throw, return Result(1).

3. Evaluate Catch with parameter Result(1).

4. Return Result(3).

The production TryStatement : try Block Finally is evaluated as follows:

1. Evaluate Block.

2. Evaluate Finally.

3. If Result(2) .type is normal, return Result(1).

4. Return Result(2).

The production TryStatement : try Block Catch Finally is evaluated as follows:

1. Evaluate Block.

2. Let C = Result(1).

3. If Result(1). type is not throw, go to step 6.

4. Evaluate Catch with parameter Result(1).

5. Let C = Result(4).

6. Evaluate Finally.

7. If Result(6). type is normal, return C.

8. Return Result(6).

The production Catch : catch (Identifier ) Block is evaluated as follows:

1. Let C be the parameter that has been passed to this production.

2. Create a new object as if by the expression new Object().

3. Create a property in the object Result(2). The property's name is Identifier, valueisC. value, and attributes are { DontDelete }.

4. Add Result(2) to the front of the scope chain.

5. Evaluate Block.

6. Remove Result(2) from the front of the scope chain.

7. Return Result(5).

The production Finally : finally Block is evaluated as follows:

1. Evaluate Block.

2. Return Result(1).

13 Function Definition

Syntax

FunctionDeclaration :
function Identifier ( FormalParameterListopt ){ FunctionBody }
FunctionExpression :
function Identifieropt ( FormalParameterListopt ){ FunctionBody }
FormalParameterList :
Identifier
FormalParameterList , Identifier
FunctionBody :
SourceElements

Semantics

The production FunctionDeclaration : function Identifier ( FormalParameterListopt ){ FunctionBody } is processed for function declarations as follows:

1. Create a new Function object as specified in 13.2 with parameters specified by FormalParameterListopt, and body specified by FunctionBody. Pass in the scope chain of the running execution context as the Scope.

2. Create a property of the current variable object (as specified in 10.1.3) with name Identifier and value Result(1).

The production FunctionExpression : function ( FormalParameterListopt ){ FunctionBody } is evaluated as follows:

1. Create a new Function object as specified in 13.2 with parameters specified by FormalParameterListopt and body specified by FunctionBody. Pass in the scope chain of the running execution context as the Scope.

2. Return Result(1).

The production FunctionExpression : function Identifier ( FormalParameterListopt ){ FunctionBody } is evaluated as follows:

1. Create a new object as if by the expression new Object().

2. Add Result(1) to the front of the scope chain.

3. Create a new Function object as specified in 13.2 with parameters specified by FormalParameterListopt and body specified by FunctionBody. Pass in the scope chain of the running execution context as the Scope.

4. Create a property in the object Result(1). The property's name is Identifier, value is Result(3), and attributes are { DontDelete, ReadOnly }.

5. Remove Result(1) from the front of the scope chain.

6. Return Result(3).

NOTE
The Identifier in a FunctionExpression can be referenced from inside the FunctionExpression's FunctionBody to allow the function to call itself recursively. However, unlike in a FunctionDeclaration, the Identifier in a FunctionExpression cannot be referenced from and does not affect the scope enclosing the FunctionExpression.

The production FunctionBody : SourceElements is evaluated as follows:

1. Process SourceElements for function declarations.

2. Evaluate SourceElements.

3. Return Result(2).

13.1 Definitions

A couple of definitions are needed to describe the process of creating function objects:

13.1.1 Equated Grammar Productions

Two uses of the FunctionBody grammar production are defined to be equated when one of the following is true:

  • Both uses obtained their FunctionBody from the same location in the source text of the same ECMAScript program. This source text consists of global code and any contained function codes according to the definitions in 10.1.2.
  • Both uses obtained their FunctionBody from the same location in the source text of the same call to eval (15.1.2.1). This source text consists of eval code and any contained function codes according to the definitions in 10.1.2

NOTE
Two uses of FunctionBody obtained from a call to the Function constructor 15.3.1 and 15.3.2) are never equated. Also, two uses of FunctionBody obtained from two different calls to eval are never equated, even if those two calls to eval were passed the same argument.

13.1.2 Joined Objects

When two or more Function objects are joined, they have the following special behaviours:

  • Any time a non-internal property of an object O is created or set, the corresponding property is immediately also created or set with the same value and attributes in all objects joined with O.
  • Any time a non-internal property of an object O is deleted, the corresponding property is immediately also deleted in all objects joined with O.
  • If objects O and P are joined, they compare as == and === to each other.
  • Joining is transitive and symmetric, so that if objects O and P are joined and objects P and Q are joined, then objects O and Q are also automatically joined.

NOTE
Two or more objects joined to each other are effectively indistinguishable except that they may have different internal properties. The only such internal property that may differ in this specification is [[Scope]].

Joined objects are used as a tool for precise specification technique in this standard. They are not meant to be used as a guideline to how Function objects are implemented in practice. Rather, in practice an implementation may detect when the differences in the [[Scope]] properties of two or more joined Function objects are not externally observable and in those cases reuse the same Function object rather than making a set of joined Function objects. This is a legal optimisation because this standard only specifies observable behaviour of ECMAScript programs.

13.2 Creating Function Objects

Given an optional parameter list specified by FormalParameterList, a body specified by FunctionBody, and a scope chain specified by Scope, a Function object is constructed as follows:

1. If there already exists an object E that was created by an earlier call to this section's algorithm, and if that call to this section's algorithm was given a FunctionBody that is equated to the FunctionBody given now, then go to step 13. (If there is more than one object E satisfying these criteria, choose one at the implementation's discretion.)

2. Create a new native ECMAScript object and let F be that object.

3. Set the [[Class]] property of F to "Function".

4. Set the [[Prototype]] property of F to the original Function prototype object as specified in 15.3.3.1.

5. Set the [[Call]] property of F as described in 13.2.1.

6. Set the [[Construct]] property of F as described in 13.2.2.

7. Set the [[Scope]] property of F to a new scope chain (10.1.4) that contains the same objects as Scope.

8. Set the length property of F to the number of formal properties specified in FormalParameterList. If no parameters are specified, set the length property of F to 0. This property is given attributes as specified in 15.3.5.1.

9. Create a new object as would be constructed by the expression new Object().

10. Set the constructor property of Result(9) to F. This property is given attributes { DontEnum }.

11. Set the prototype property of F to Result(9). This property is given attributes as specified in 15.3.5.2.

12. Return F.

13. At the implementation's discretion, go to either step 2 or step 14.

14. Create a new native ECMAScript object joined to E and let F be that object. Copy all non-internal properties and their attributes from E to F so that all non-internal properties are identical in E and F.

15. Set the [[Class]] property of F to "Function".

16. Set the [[Prototype]] property of F to the original Function prototype object as specified in 15.3.3.1.

17. Set the [[Call]] property of F as described in 13.2.1.

18. Set the [[Construct]] property of F as described in 13.2.2.

19. Set the [[Scope]] property of F to a new scope chain (10.1.4) that contains the same objects as Scope.

20. Return F.

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

Step 1 allows an implementation to optimise the common case of a function A that has a nested function B where B is not dependent on A. In this case the implementation is allowed to reuse the same object for B instead of creating a new one every time A is called. Step 13 makes this optimisation optional; an implementation that chooses not to implement it will go to step 2.

For example, in the code

function A() {
   function B(x) {return x* x;} 
   return B; 
}

function C() { 
  return eval("(function(x) {return x* x;})"); 
}

var b1 = A(); 
var b2 = A(); 
function b3(x) {return x* x;} 
function b4(x) {return x* x;} 
var b5 = C(); 
var b6 = C();

an implementation is allowed, but not required, to join b1 and b2. In fact, it may make b1 and b2 the same object because there is no way to detect the difference between their [[Scope]] properties. On the other hand, an implementation must not join b3 and b4 because their source codes are not equated (13.1.1). Also, an implementation must not join b5 and b6 because they were produced by two different calls to eval and therefore their source codes are not equated.

In practice it's likely to be productive to join two Function objects only in the cases where an implementation can prove that the differences between their [[Scope]] properties are not observable, so one object can be reused. By following this policy, an implementation will only encounter the vacuous case of an object being joined with itself.

13.2.1 [[Call]]

When the [[Call]] property for a Function object F is called, the following steps are taken:

1. Establish a new execution context using F's FormalParameterList, the passed arguments list, and the this value as described in 10.2.3.

2. Evaluate F's FunctionBody.

3. Exit the execution context established in step 1, restoring the previous execution context.

4. If Result(2). type is throw then throw Result(2). value.

5. If Result(2). type is return then return Result(2). value.

6. (Result(2). type must be normal.) Return undefined.

13.2.2 [[Construct]]

When the [[Construct]] property for a Function object F is called, the following steps are taken:

1. Create a new native ECMAScript object.

2. Set the [[Class]] property of Result(1) to "Object".

3. Get the value of the prototype property of F.

4. If Result(3) is an object, set the [[Prototype]] property of Result(1) to Result(3).

5. If Result(3) is not an object, set the [[Prototype]] property of Result(1) to the original Object prototype object as described in 15.2.3.1.

6. Invoke the [[Call]] property of F, providing Result(1) as the this value and providing the argument list passed into [[Construct]] as the argument values.

7. If Type(Result(6)) is Object then return Result(6).

8. Return Result(1).

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. If Result(1) is an abrupt completion, return Result(1)

3. Evaluate SourceElement.

4. Return Result(3).

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 (see clause 13).

The production SourceElement : FunctionDeclaration is evaluated as follows:

1. Return (normal, empty, empty).

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.

Unless specified otherwise, the [[Class]] property of a built-in object is "Function" if that built-in object has a [[Call]] property, or "Object" if that built-in object does not have a [[Call]] property.

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.

Unless otherwise specified in the description of a particular function, if a function or constructor described in this section is given more arguments than the function is specified to allow, the behaviour of the function or constructor is undefined. In particular, an implementation is permitted (but not required) to throw a TypeError exception in this case.

NOTE
Implementations that add additional capabilities to the set of built-in functions are encouraged to do so by adding new functions rather than adding new parameters to existing functions.

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

Every built-in prototype object has the Object prototype object, which is the initial value of the expression Object.prototype (15.2.3.1), as the value of its internal [[Prototype]] property, except the Object prototype object itself.

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 largest number of named arguments shown in the section headings for the function description, including optional parameters.

NOTE
For example, the Function object that is the initial value of the slice property of the String prototype object is described under the section heading "String.prototype.slice(start , end)" which shows the two named arguments start and end; therefore the value of the length property of that Function object is 2.

In every case, the length property of a built-in Function object described in this 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 values of the [[Prototype]] and [[Class]] properties of the global object are implementation-dependent.

15.1.1 Value Properties of the Global Object
15.1.1.1 NaN

The initial value of NaN is NaN (8.5). This property has the attributes { DontEnum, DontDelete}.

15.1.1.2 Infinity

The initial value of Infinity is +∞ (8.5). This property has the attributes { DontEnum, DontDelete}.

15.1.1.3 undefined

The initial value of undefined is undefined (8.1). This property has the attributes { DontEnum, DontDelete}.

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 a Program. If the parse fails, throw a SyntaxError exception (but see also clause 16).

3. Evaluate the program from step 2.

4. If Result(3). type is normal and its completion value is a value V, then return the value V.

5. If Result(3). type is normal and its completion value is empty, then return the value undefined.

6. Result(3). type must be throw. Throw Result(3). value as an exception.

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, an EvalError exception may be thrown.

15.1.2.2 parseInt(string , radix)

The parseInt function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix. Leading whitespace in the string is ignored. If radix is undefined or 0, it is assumed to be 10 except when the number begins with the character pairs 0x or 0X, in which case a radix of 16 is assumed. Any radix-16 number may also optionally begin with the character pairs 0x or 0X.

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

1. Call ToString(string).

2. Let S be a newly created substring of Result(1) consisting of the first character that is not a StrWhiteSpaceChar and all characters following that character. (In other words, remove leading white space.)

3. Let sign be 1.

4. If S is not empty and the first character of S is a minus sign -, let sign be -1.

5. If S is not empty and the first character of S is a plus sign + or a minus sign -, then remove the first character from S.

6. Let R = ToInt32(radix).

7. If R =0, go to step 11.

8. If R <2 orR > 36, then return NaN.

9. If R = 16, go to step 13.

10. Go to step 14.

11. Let R = 10.

12. If the length of S is at least 1 and the first character of S is "0", then at the implementation's discretion either let R = 8 or leave R unchanged.

13. If the length of S is at least 2 and the first two characters of S are either "0x" or "0X", then remove the first two characters from S and let R = 16.

14. If S contains any character that is not a radix- R digit, then let Z be the substring of S consisting of all characters before the first such character; otherwise, let Z be S.

15. If Z is empty, return NaN.

16. Compute the mathematical integer value that is represented by Z in radix- R notation, using the letters A-Z and a-z for digits with values 10 through 35. (However, if R is 10 and Z contains more than 20 significant digits, every significant digit after the 20th may be replaced by a 0 digit, at the option of the implementation; and if R is not 2, 4, 8, 10, 16, or 32, then Result(16) may be an implementation-dependent approximation to the mathematical integer value that is represented by Z in radix- R notation.)

17. Compute the number value for Result(16).

18. Return sign * Result(17).

NOTE
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.

When radix is 0 or undefined and the string's number begins with a 0 digit not followed by an x or X, then the implementation may, at its discretion, interpret the number either as being octal or as being decimal. Implementations are encouraged to interpret numbers in this case as being decimal.

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 white space.)

3. If neither Result(2) nor any prefix of Result(2) satisfies the syntax of a StrDecimalLiteral (see 0), 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
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 isNaN(number)

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

15.1.2.5 isFinite(number)

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

15.1.3 URI Handling Function Properties

Uniform Resource Identifiers, or URIs, are strings that identify resources (e. g. web pages or files) and transport protocols by which to access them (e. g. HTTP or FTP) on the Internet. The ECMAScript language itself does not provide any support for using URIs except for functions that encode and decode URIs as described in 15.1.3.1, 15.1.3.2, 15.1.3.3 and 15.1.3.4.

NOTE
Many implementations of ECMAScript provide additional functions and methods that manipulate web pages; these functions are beyond the scope of this standard.

A URI is composed of a sequence of components separated by component separators. The general form is:

Scheme : First / Second ; Third ? Fourth

where the italicised names represent components and the ":", "/", ";" and"?" are reserved characters used as separators. The encodeURI and decodeURI functions are intended to work with complete URIs; they assume that any reserved characters in the URI are intended to have special meaning and so are not encoded. The encodeURIComponent and decodeURIComponent functions are intended to work with the individual component parts of a URI; they assume that any reserved characters represent text and so must be encoded so that they are not interpreted as reserved characters when the component is part of a complete URI.

The following lexical grammar specifies the form of encoded URIs. uri ::: uriCharactersopt

uriCharacters :::
uriCharacter uriCharactersopt
uriCharacter :::
uriReserved
uriUnescaped
uriEscaped
uriReserved ::: one of
; / ? : @ & = + $ ,
uriUnescaped :::
uriAlpha
DecimalDigit
uriMark
uriEscaped :::
% HexDigit HexDigit
uriAlpha ::: one of
a b c d e f g h i j k l m n o p q r s t u v w x y z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
uriMark ::: one of
- _ . ! ~ * ' ( )

When a character to be included in a URI is not listed above or is not intended to have the special meaning sometimes given to the reserved characters, that character must be encoded. The character is first transformed into a sequence of octets using the UTF-8 transformation, with surrogate pairs first transformed from their UCS-2 to UCS-4 encodings. (Note that for code points in the range [0,127] this results in a single octet with the same value.) The resulting sequence of octets is then transformed into a string with each octet represented by an escape sequence of the form "% xx".

The encoding and escaping process is described by the hidden function Encode taking two string arguments string and unescapedSet. This function is defined for expository purpose only.

1. Compute the number of characters in string.

2. Let R be the empty string.

3. Let k be 0.

4. If k equals Result(1), return R.

5. Let C be the character at position k within string.

6. If C is not in unescapedSet, go to step 9.

7. Let S be a string containing only the character C.

8. Go to step 24.

9. If the code point value of C is not less than 0xDC00 and not greater than 0xDFFF, throw a URIError exception.

10. If the code point value of C is less than 0xD800 or greater than 0xDBFF, let V be the code point value of C and go to step 16.

11. Increase k by 1.

12. If k equals Result(1), throw a URIError exception.

13. Get the code point value of the character at position k within string.

14. If Result(13) is less than 0xDC00 or greater than 0xDFFF, throw a URIError exception.

15. Let V be (((the code point value of C) - 0xD800) * 0x400 + (Result(13) - 0xDC00) + 0x10000).

16. Let Octets be the array of octets resulting by applying the UTF-8 transformation to V, and let L be the array size.

17. Let j be 0.

18. Get the value at position j within Octets.

19. Let S be a string containing three characters "% XY" where XY are two uppercase hexadecimal digits encoding the value of Result(18).

20. Let R be a new string value computed by concatenating the previous value of R and S.

21. Increase j by 1.

22. If j is equal to L, go to step 25.

23. Go to step 18.

24. Let R be a new string value computed by concatenating the previous value of R and S.

25. Increase k by 1.

26. Go to step 4.

The unescaping and decoding process is described by the hidden function Decode taking two string arguments string and reservedSet. This function is defined for expository purpose only.

1. Compute the number of characters in string.

2. Let R be the empty string.

3. Let k be 0.

4. If k equals Result(1), return R.

5. Let C be the character at position k within string.

6. If C is not '% ', go to step 40.

7. Let start be k.

8. If k + 2 is greater than or equal to Result(1), throw a URIError exception.

9. If the characters at position (k+ 1) and (k + 2) within string do not represent hexadecimal digits, throw a URIError exception.

10. Let B be the 8-bit value represented by the two hexadecimal digits at position (k +1) and(k +2).

11. Increment k by 2.

12. If the most significant bit in B is 0, let C be the character with code point value B and go to step 37.

13. Let n be the smallest non-negative number such that (B << n) & 0x80 is equal to 0.

14. If n equals 1 or n is greater than 4, throw a URIError exception.

15. Let Octets be an array of 8-bit integers of size n.

16. Put B into Octets at position 0.

17. If k +(3 *(n - 1)) is greater than or equal to Result(1), throw a URIError exception.

18. Let j be 1.

19. If j equals n, go to step 29.

20. Increment k by 1.

21. If the character at position k is not '% ', throw a URIError exception.

22. If the characters at position (k +1) and (k + 2) within string do not represent hexadecimal digits, throw a URIError exception.

23. Let B be the 8-bit value represented by the two hexadecimal digits at position (k +1) and(k +2).

24. If the two most significant bits in B are not 10, throw a URIError exception.

25. Increment k by 2.

26. Put B into Octets at position j.

27. Increment j by 1.

28. Go to step 19.

29. Let V be the value obtained by applying the UTF-8 transformation to Octets, that is, from an array of octets into a 32-bit value.

30. If V is less than 0x10000, go to step 36.

31. If V is greater than 0x10FFFF, throw a URIError exception.

32. Let L be (((V - 0x10000) & 0x3FF) + 0xDC00).

33. Let H be ((((V - 0x10000) >> 10) & 0x3FF) + 0xD800).

34. Let S be the string containing the two characters with code point values H and L.

35. Go to step 41.

36. Let C be the character with code point value V.

37. If C is not in reservedSet, go to step 40.

38. Let S be the substring of string from position start to position k included.

39. Go to step 41.

40. Let S be the string containing only the character C.

41. Let R be a new string value computed by concatenating the previous value of R and S.

42. Increase k by 1.

43. Go to step 4.

NOTE 1
The syntax of Uniform Resource Identifiers is given in RFC2396.

NOTE 2
A formal description and implementation of UTF-8 is given in the Unicode Standard, Version 2.0, Appendix A.

In UTF-8, characters are encoded using sequences of 1 to 6 octets. The only octet of a "sequence" of one has the higher-order bit set to 0, the remaining 7 bits being used to encode the character value. In a sequence of n octets, n> 1, the initial octet has the n higher-order bits set to 1, followed by a bit set to 0. The remaining bits of that octet contain bits from the value of the character to be encoded. The following octets all have the higher-order bit set to 1 and the following bit set to 0, leaving 6 bits in each to contain bits from the character to be encoded. The possible UTF-8 encodings of ECMAScript characters are:

Code Point Value Representation 1st Octet 2nd Octet 3rd Octet 4th Octet
0x0000 - 0x007F 00000000 0zzzzzzz 0zzzzzzz
0x0080 - 0x07FF 00000yyy yyzzzzzz 110yyyyy 10zzzzzz
0x0800 - 0xD7FF xxxxyyyy yyzzzzzz 1110xxxx 10yyyyyy 10zzzzzz
0xD800 - 0xDBFF
followed by
0xDC00 - 0xDFFF		 110110vv vvwwwwxx
followed by
110111yy yyzzzzzz		 11110uuu 10uuwwww 10xxyyyy 10zzzzzz
0xD800 -0xDBFF
not followed by
0xDC00 - 0xDFFF		 causes URIError
0xDC00 - 0xDFFF causes URIError
0xE000 - 0xFFFF xxxxyyyy yyzzzzzz 1110xxxx 10yyyyyy 10zzzzzz

Where

uuuuu = vvvv +1

to account for the addition of 0x10000 as in 3.7, Surrogates of the Unicode Standard version 2.0. The range of code point values 0xD800-0xDFFF is used to encode surrogate pairs; the above transformation combines a UCS-2 surrogate pair into a UCS-4 representation and encodes the resulting 21-bit value in UTF-8. Decoding reconstructs the surrogate pair.

15.1.3.1 decodeURI(encodedURI)

The decodeURI function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURI function is replaced with the character that it represents. Escape sequences that could not have been introduced by encodeURI are not replaced.

When the decodeURI function is called with one argument encodedURI, the following steps are taken:

1. Call ToString(encodedURI).

2. Let reservedURISet be a string containing one instance of each character valid in uriReserved plus "#".

3. Call Decode(Result(1), reservedURISet)

4. Return Result(3).

NOTE
The character "#" is not decoded from escape sequences even though it is not a reserved URI character.

15.1.3.2 decodeURIComponent(encodedURIComponent)

The decodeURIComponent function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURIComponent function is replaced with the character that it represents.

When the decodeURIComponent function is called with one argument encodedURIComponent, the following steps are taken:

1. Call ToString(encodedURIComponent).

2. Let reservedURIComponentSet be the empty string.

3. Call Decode(Result(1), reservedURIComponentSet)

4. Return Result(3).

15.1.3.3 encodeURI(uri)

The encodeURI function computes a new version of a URI in which each instance of certain characters is replaced by one, two or three escape sequences representing the UTF-8 encoding of the character.

When the encodeURI function is called with one argument uri, the following steps are taken:

1. Call ToString(uri).

2. Let unescapedURISet be a string containing one instance of each character valid in uriReserved and uriUnescaped plus "#".

3. Call Encode(Result(1), unescapedURISet)

4. Return Result(3).

NOTE
The character "#" is not encoded to an escape sequence even though it is not a reserved or unescaped URI character.

15.1.3.4 encodeURIComponent(uriComponent)

The encodeURIComponent function computes a new version of a URI in which each instance of certain characters is replaced by one, two or three escape sequences representing the UTF-8 encoding of the character.

When the encodeURIComponent function is called with one argument uriComponent, the following steps are taken:

1. Call ToString(uriComponent).

2. Let unescapedURIComponentSet be a string containing one instance of each character valid in uriUnescaped.

3. Call Encode(Result(1), unescapedURIComponentSet)

4. Return Result(3).

15.1.4 Constructor Properties of the Global Object
15.1.4.1 Object(...)

See 15.2.1 and 15.2.2.

15.1.4.2 Function(...)

See 15.3.1 and 15.3.2.

15.1.4.3 Array(...)

See 15.4.1 and 15.4.2.

15.1.4.4 String(...)

See 15.5.1 and 15.5.2.

15.1.4.5 Boolean(...)

See 15.6.1 and 15.6.2.

15.1.4.6 Number(...)

See 15.7.1 and 15.7.2.

15.1.4.7 Date(...)

See 15.9.2.

15.1.4.8 RegExp(...)

See 15.10.3 and 15.10.4.

15.1.4.9 Error(...)

See 15.11.1 and 15.11.2.

15.1.4.10 EvalError(...)

See 15.11.6.1.

15.1.4.11 RangeError(...)

See 15.11.6.2.

15.1.4.12 ReferenceError(...)

See 15.11.6.3.

15.1.4.13 SyntaxError(...)

See 15.11.6.4.

15.1.4.14 TypeError(...)

See 15.11.6.5.

15.1.4.15 URIError(...)

See 15.11.6.6.

15.1.5 Other Properties of the Global Object
15.1.5.1 Math

See 15.8.

15.2 Object Objects
15.2.1 The Object Constructor Called as a Function

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

15.2.1.1 Object([ value ] )

When the Object function is called with no arguments or with one argument value, the following steps are taken:

1. If value is null, undefined or not supplied, create and return a new Object object exactly if the object constructor had been called with the same arguments (15.2.2.1).

2. Return ToObject(value).

15.2.2 The Object Constructor

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

15.2.2.1 new Object([ value ] )

When the Object constructor is called with no arguments or with one argument value, the following steps are taken:

1. If value is not supplied, go to step 8.

2. If the type of value is not Object, go to step 5.

3. If the value is a native ECMAScript object, do not create a new object but simply return value.

4. 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.

5. If the type of value is String, return ToObject(value).

6. If the type of value is Boolean, return ToObject( value).

7. If the type of value is Number, return ToObject(value).

8. (The argument value was not supplied or its type was Null or Undefined.)
Create a new native ECMAScript object.
The [[Prototype]] property of the newly constructed object is set to the Object prototype object.
The [[Class]] property of the newly constructed object is set to "Object".
The newly constructed object has no [[Value]] property.
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 properties and the length property (whose value is 1), the Object constructor has the following properties:

15.2.3.1 Object.prototype

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

This property has 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 and the value of the internal [[Class]] property is "Object".

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.toLocaleString()

This function returns the result of calling toString(). (which means the result of calling the function that is found by looking up the toString property in the this object, which is not necessarily the implementation of toString in section 15.2.4.2)

NOTE 1
This function is provided to give all Objects a generic toLocaleString interface, even though not all may use it. Currently, Array, Number, and Date provide their own locale-sensitive toLocaleString methods.

NOTE 2
The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.2.4.4 Object.prototype.valueOf()

The valueOf method returns its this value. If the object is the result of calling the Object constructor with a host object (15.2.2.1), it is implementation-defined whether valueOf returns its this value or another value such as the host object originally passed to the constructor.

15.2.4.5 Object.prototype.hasOwnProperty(V)

When the hasOwnProperty method is called with argument V, the following steps are taken:

1. Let O be this object.

2. Call ToString(V).

3. If O doesn't have a property with the name given by Result(2), return false.

4. Return true.

NOTE
Unlike [[HasProperty]] (8.6.2.4), this method does not consider objects in the prototype chain.

15.2.4.6 Object.prototype.isPrototypeOf(V)

When the isPrototypeOf method is called with argument V, the following steps are taken:

1. Let O be this object.

2. If V is not an object, return false.

3. Let V be the value of the [[Prototype]] property of V.

4. if V is null, return false

5. If O and V refer to the same object or if they refer to objects joined to each other (13.1.2), return true.

6. Go to step 3.

15.2.4.7 Object.prototype.propertyIsEnumerable(V)

When the propertyIsEnumerable method is called with argument V, the following steps are taken:

1. Let O be this object.

2. Call ToString(V).

3. If O doesn't have a property with the name given by Result(2), return false.

4. If the property has the DontEnum attribute, return false.

5. Return true.

NOTE
This method does not consider objects in the prototype chain.

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 as 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. If P is not parsable as a FormalParameterListopt then throw a SyntaxError exception.

15. If body is not parsable as FunctionBody then throw a SyntaxError exception.

16. Create a new Function object as specified in 13.2 with parameters specified by parsing P as a FormalParameterListopt and body specified by parsing body as a FunctionBody. Pass in a scope chain consisting of the global object as the Scope parameter.

17. Return Result(16).

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

NOTE
It is permissible but not necessary to have one argument for each formal parameter to be specified. For example, all three of the following expressions produce the same result:

new Function("a", "b", "c", "return a+b+c")

new Function("a, b, c", "return a+b+c")

new Function("a,b", "c", "return a+b+c")

15.3.3 Properties of the Function Constructor

The value of the internal [[Prototype]] property of the Function constructor is the Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the Function constructor has the following properties:

15.3.3.1 Function.prototype

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

This property has the attributes { DontEnum, DontDelete, ReadOnly }.

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 (15.2.3.1).

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 white space, line terminators, and semicolons within the representation string is implementation-dependent.

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

15.3.4.3 Function.prototype.apply(thisArg, argArray)

The apply method takes two arguments, thisArg and argArray, and performs a function call using the [[Call]] property of the object. If the object does not have a [[Call]] property, a TypeError exception is thrown.

If thisArg is null or undefined, the called function is passed the global object as the this value. Otherwise, the called function is passed ToObject(thisArg) as the this value.

If argArray is null or undefined, the called function is passed no arguments. Otherwise, if argArray is neither an array nor an arguments object (see 10.1.8), a TypeError exception is thrown. If argArray is either an array or an arguments object, the function is passed the (ToUint32(argArray. length)) arguments argArray[ 0], argArray[ 1], ..., argArray[ ToUint32(argArray. length)- 1].

The length property of the apply method is 2.

15.3.4.4 Function.prototype.call(thisArg [ , arg1 [ , arg2, ... ] ] )

The call method takes one or more arguments, thisArg and (optionally) arg1, arg2 etc, and performs a function call using the [[Call]] property of the object. If the object does not have a [[Call]] property, a TypeError exception is thrown. The called function is passed arg1, arg2, etc. as the arguments.

If thisArg is null or undefined, the called function is passed the global object as the this value. Otherwise, the called function is passed ToObject(thisArg) as the this value.

The length property of the call method is 1.

15.3.5 Properties of Function Instances

In addition to the required internal properties, every function instance has a [[Call]] property, a [[Construct]] property and a [[Scope]] property (see 8.6.2 and 13.2). The value of the [[Class]] property is "Function".

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. This property has the attributes { DontDelete, ReadOnly, DontEnum }.

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. This property has the attribute { DontDelete }.

15.3.5.3 [[HasInstance]](V)

Assume F is a Function object.

When the [[HasInstance]] method of F is called with value V, the following steps are taken:

1. If V is not an object, return false.

2. Call the [[Get]] method of F with property name "prototype".

3. Let O be Result(2).

4. If O is not an object, throw a TypeError exception.

5. Let V be the value of the [[Prototype]] property of V.

6. If V is null, return false.

7. If O and V refer to the same object or if they refer to objects joined to each other (13.1.2), return true.

8. Go to step 5.

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 a nonnegative integer less than 232 . The value of the length property is numerically greater than the name of every property whose name is an array index; whenever a property of an Array object is created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever a property is added whose name is an array index, the length property is changed, if necessary, to be one more than the numeric value of that array index; and whenever the length property is changed, every property whose name is an array index whose value is not smaller than the new length is automatically deleted. This constraint applies only to 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([ item1[, item2 [,...]]])

When the Array function is called the following steps are taken:

1. Create and return a new Array object exactly as if the array constructor had been called with the same arguments (15.4.2).

15.4.2 The Array Constructor

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

15.4.2.1 new Array([ item0[, item1 [,...]]])

This description applies if and only if the Array constructor is given no arguments or at least two arguments.

The [[Prototype]] 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 (if supplied); the 1 property of the newly constructed object is set to item1 (if supplied); and, in general, for as many arguments as there are, the k property of the newly constructed object is set to argument k, where the first argument is considered to be argument number 0.

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 RangeError exception is thrown.

If the argument len is not a Number, then the length property of the newly constructed object is set to 1 and the 0 property of the newly constructed object is set to len.

15.4.3 Properties of the Array Constructor

The value of the internal [[Prototype]] property of the Array constructor is the Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the Array constructor has the following properties:

15.4.3.1 Array.prototype

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

This property has the attributes { DontEnum, DontDelete, ReadOnly }.

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).

The Array prototype object is itself an array; its [[Class]] is "Array", and it has a length property (whose initial value is +0) and the special internal [[Put]] method described in 15.2.3.1.

In following descriptions of functions that are properties of the Array prototype object, the phrase "this object" refers to the object that is the this value for the invocation of the function. It is permitted for the this to be an object for which the value of the internal [[Class]] property is not "Array".

NOTE
The Array prototype object does not have a valueOf property of its own; however, it inherits the valueOf property from the 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 result of calling this function is the same as if the built-in join method were invoked for this object with no argument.

The toString function is not generic; it throws a TypeError exception if its this value is not an Array object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.4.4.3 Array.prototype.toLocaleString()

The elements of the array are converted to strings using their toLocaleString methods, and these strings are then concatenated, separated by occurrences of a separator string that has been derived in an implementation-defined locale-specific way. The result of calling this function is intended to be analogous to the result of toString, except that the result of this function is intended to be locale-specific.

The result is calculated as follows:

1. Call the [[Get]] method of this object with argument "length".

2. Call ToUint32(Result(1)).

3. Let separator be the list-separator string appropriate for the host environment's current locale (this is derived in an implementation-defined way).

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 ToObject(Result(6)). toLocaleString().

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 ToObject(Result(12)). toLocaleString().

14. Let R be a string value produced by concatenating S and Result(13).

15. Increase k by 1.

16. Go to step 10.

The toLocaleString function is not generic; it throws a TypeError exception if its this value is not an Array object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

NOTE
The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.4.4.4 Array.prototype.concat([ item1[, item2 [,...]]])

When the concat method is called with zero or more arguments item1, item2, etc., it returns an array containing the array elements of the object followed by the array elements of each argument in order.

The following steps are taken:

1. Let A be a new array created as if by the expression new Array().

2. Let n be 0.

3. Let E be this object.

4. If E is not an Array object, go to step 16.

5. Let k be 0.

6. Call the [[Get]] method of E with argument "length".

7. If k equals Result(6) go to step 19.

8. Call ToString(k).

9. If E has a property named by Result(8), go to step 10, but if E has no property named by Result(8), go to step 13.

10. Call ToString(n).

11. Call the [[Get]] method of E with argument Result(8).

12. Call the [[Put]] method of A with arguments Result(10) and Result(11).

13. Increase n by 1.

14. Increase k by 1.

15. Go to step 7.

16. Call ToString(n).

17. Call the [[Put]] method of A with arguments Result(16) and E.

18. Increase n by 1.

19. Get the next argument in the argument list; if there are no more arguments, go to step 22.

20. Let E be Result(19).

21. Go to step 4.

22. Call the [[Put]] method of A with arguments "length" and n.

23. Return A.

The length property of the concat method is 1.

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

15.4.4.5 Array.prototype.join(separator)

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

The join method takes one argument, separator, and performs the following steps:

1. Call the [[Get]] method of this object with argument "length".

2. Call ToUint32(Result(1)).

3. If separator is undefined, 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.

The length property of the join method is 1.

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

15.4.4.6 Array.prototype.pop()

The last element of the array is removed from the array and returned.

1. Call the [[Get]] method of this object with argument "length".

2. Call ToUint32(Result(1)).

3. If Result(2) is not zero, go to step 6.

4. Call the [[Put]] method of this object with arguments "length" and Result(2).

5. Return undefined.

6. Call ToString(Result(2)- 1).

7. Call the [[Get]] method of this object with argument Result(6).

8. Call the [[Delete]] method of this object with argument Result(6).

9. Call the [[Put]] method of this object with arguments "length" and (Result(2)- 1).

10. Return Result(7).

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

15.4.4.7 Array.prototype.push([ item1[, item2 [,...]]])

The arguments are appended to the end of the array, in the order in which they appear. The new length of the array is returned as the result of the call.

When the push method is called with zero or more arguments item1, item2, etc., the following steps are taken:

1. Call the [[Get]] method of this object with argument "length".

2. Let n be the result of calling ToUint32(Result(1)).

3. Get the next argument in the argument list; if there are no more arguments, go to step 7.

4. Call the [[Put]] method of this object with arguments ToString(n) and Result(3).

5. Increase n by 1.

6. Go to step 3.

7. Call the [[Put]] method of this object with arguments "length" and n.

8. Return n.

The length property of the push method is 1.

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

15.4.4.8 Array.prototype.reverse()

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

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 does not have a property named by Result(8), go to step 19.

12. If this object does not have a property named by Result(7), go to step 16.

13. Call the [[Put]] method of this object with arguments Result(7) and Result(10).

14. Call the [[Put]] method of this object with arguments Result(8) and Result(9).

15. Go to step 25.

16. Call the [[Put]] method of this object with arguments Result(7) and Result(10).

17. Call the [[Delete]] method on this object, providing Result(8) as the name of the property to delete.

18. Go to step 25.

19. If this object does not have a property named by Result(7), go to step 23.

20. Call the [[Delete]] method on this object, providing Result(7) as the name of the property to delete..

21. Call the [[Put]] method of this object with arguments Result(8) and Result(9).

22. Go to step 25.

23. Call the [[Delete]] method on this object, providing Result(7) as the name of the property to delete.

24. Call the [[Delete]] method on this object, providing Result(8) as the name of the property to delete.

25. Increase k by 1.

26. Go to step 5.

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

15.4.4.9 Array.prototype.shift()

The first element of the array is removed from the array and returned.

1. Call the [[Get]] method of this object with argument "length".

2. Call ToUint32(Result(1)).

3. If Result(2) is not zero, go to step 6.

4. Call the [[Put]] method of this object with arguments "length" and Result(2).

5. Return undefined.

6. Call the [[Get]] method of this object with argument 0.

7. Let k be 1.

8. If k equals Result(2), go to step 18.

9. Call ToString(k).

10. Call ToString(k- 1).

11. If this object has a property named by Result(9), go to step 12; but if this object has no property named by Result(9), then go to step 15.

12. Call the [[Get]] method of this object with argument Result(9).

13. Call the [[Put]] method of this object with arguments Result(10) and Result(12).

14. Go to step 16.

15. Call the [[Delete]] method of this object with argument Result(10).

16. Increase k by 1.

17. Go to step 8.

18. Call the [[Delete]] method of this object with argument ToString(Result(2)- 1).

19. Call the [[Put]] method of this object with arguments "length" and (Result(2)- 1).

20. Return Result(6).

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

15.4.4.10 Array.prototype.slice(start, end)

The slice method takes two arguments, start and end, and returns an array containing the elements of the array from element start up to, but not including, element end (or through the end of the array if end is undefined). If start is negative, it is treated as (length+ start) where length is the length of the array. If end is negative, it is treated as (length+ end) where length is the length of the array. The following steps are taken:

1. Let A be a new array created as if by the expression new Array().

2. Call the [[Get]] method of this object with argument "length".

3. Call ToUint32(Result(2)).

4. Call ToInteger(start).

5. If Result(4) is negative, use max((Result(3)+ Result(4)), 0); else use min(Result(4), Result(3)).

6. Let k be Result(5).

7. If end is undefined, use Result(3); else use ToInteger(end).

8. If Result(7) is negative, use max((Result(3)+ Result(7)), 0); else use min(Result(7), Result(3)).

9. Let n be 0.

10. If k is greater than or equal to Result(8), go to step 19.

11. Call ToString(k).

12. If this object has a property named by Result(11), go to step 13; but if this object has no property named by Result(11), then go to step 16.

13. Call ToString(n).

14. Call the [[Get]] method of this object with argument Result(11).

15. Call the [[Put]] method of A with arguments Result(13) and Result(14).

16. Increase k by 1.

17. Increase n by 1.

18. Go to step 10.

19. Call the [[Put]] method of A with arguments "length" and n.

20. Return A.

The length property of the slice method is 2.

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

15.4.4.11 Array.prototype.sort(comparefn)

The elements of this array are sorted. The sort is not necessarily stable (that is, elements that compare equal do not necessarily remain in their original order). If comparefn is not undefined, it should be a function that accepts two arguments 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 not undefined and is not a consistent comparison function for the elements of this array (see below), the behaviour of sort is implementation-defined. Let len be ToUint32(this.length). If there exist integers i and j and an object P such that all of the conditions below are satisfied then the behaviour of sort is implementation-defined:

  • 0 <= i < len
  • 0 <= j < len
  • this does not have a property with name ToString(i)
  • P is obtained by following one or more [[Prototype]] properties starting at this
  • P has a property with name ToString(j)

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.

  • 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)] does not exist.
  • Then for all nonnegative integers j and k, each less than Result(2), if SortCompare(j, k)< 0 (see SortCompare below), then π(j)< π(k).

Here the notation old[j] is used to refer to the hypothetical result of calling the [[Get]] 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 comparefn is a consistent comparison function for a set of values S if all of the requirements below are met for all values a, b, and c (possibly the same value) in the set S: The notation a <CF b means comparefn(a, b)< 0; a =CF b means comparefn(a, b) = 0 (of either sign); and a >CF b means comparefn(a, b)> 0.

  • Calling comparefn(a, b) always returns the same value v when given a specific pair of values a and b as its two arguments. Furthermore, v has type Number, and v is not NaN. Note that this implies that exactly one of a <CF b, a =CF b, and a >CF b will be true for a given pair of a and b.
  • a =CF a (reflexivity)
  • If a =CF b, then b =CF a (symmetry)
  • If a =CF b and b =CF c, then a =CF c (transitivity of =CF)
  • If a <CF b and b <CF c, then a <CF c (transitivity of <CF)
  • If a >CF b and b >CF c, then a >CF c (transitivity of >CF)

NOTE
The above conditions are necessary and sufficient to ensure that comparefn divides the set S into equivalence classes and that these equivalence classes are totally ordered.

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

1. Call ToString(j).

2. Call ToString(k).

3. If this object does not have a property named by Result(1), and this object does not have a property named by Result(2), return +0.

4. If this object does not have a property named by Result(1), return 1.

5. If this object does not have a property named by Result(2), return -1.

6. Call the [[Get]] method of this object with argument Result(1).

7. Call the [[Get]] method of this object with argument Result(2).

8. Let x be Result(6).

9. Let y be Result(7).

10. If x and y are both undefined, return +0.

11. If x is undefined, return 1.

12. If y is undefined, return -1.

13. If the argument comparefn is undefined, go to step 16.

14. Call comparefn with arguments x and y.

15. Return Result(14).

16. Call ToString(x).

17. Call ToString(y).

18. If Result(16) < Result(17), return -1.

19. If Result(16) > Result(17), return 1.

20. Return +0.

NOTE 1
Because non-existent property values always compare greater than undefined property values, and undefined always compares greater than any other value, undefined property values always sort to the end of the result, followed by non-existent property values.

NOTE 2
The sort function is intentionally generic; it does not require that its this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method. Whether the sort function can be applied successfully to a host object is implementation-dependent.

15.4.4.12 Array.prototype.splice(start, deleteCount [, item1 [, item2[,...]]])

When the splice method is called with two or more arguments start, deleteCount and (optionally) item1, item2, etc., the deleteCount elements of the array starting at array index start are replaced by the arguments item1, item2, etc. The following steps are taken:

1. Let A be a new array created as if by the expression new Array().

2. Call the [[Get]] method of this object with argument "length".

3. Call ToUint32(Result(2)).

4. Call ToInteger(start).

5. If Result(4) is negative, use max((Result(3)+ Result(4)), 0); else use min(Result(4), Result(3)).

6. Compute min(max(ToInteger(deleteCount), 0), Result(3)- Result(5)).

7. Let k be 0.

8. If k equals Result(6), go to step 16.

9. Call ToString(Result(5)+ k).

10. If this object has a property named by Result(9), go to step 11; but if this object has no property named by Result(9), then go to step 14.

11. Call ToString(k).

12. Call the [[Get]] method of this object with argument Result(9).

13. Call the [[Put]] method of A with arguments Result(11) and Result(12).

14. Increment k by 1.

15. Go to step 8.

16. Call the [[Put]] method of A with arguments "length" and Result(6).

17. Compute the number of additional arguments item1, item2, etc.

18. If Result(17) is equal to Result(6), go to step 48.

19. If Result(17) is greater than Result(6), go to step 37.

20. Let k be Result(5).

21. If k is equal to (Result(3)- Result(6)), go to step 31.

22. Call ToString(k+ Result(6)).

23. Call ToString(k+ Result(17)).

24. If this object has a property named by Result(22), go to step 25; but if this object has no property named by Result(22), then go to step 28.

25. Call the [[Get]] method of this object with argument Result(22).

26. Call the [[Put]] method of this object with arguments Result(23) and Result(25).

27. Go to step 29.

28. Call the [[Delete]] method of this object with argument Result(23).

29. Increase k by 1.

30. Go to step 21.

31. Let k be Result(3).

32. If k is equal to (Result(3)- Result(6)+ Result(17)), go to step 48.

33. Call ToString(k- 1).

34. Call the [[Delete]] method of this object with argument Result(33).

35. Decrease k by 1.

36. Go to step 32.

37. Let k be (Result(3)- Result(6)).

38. If k is equal to Result(5), go to step 48.

39. Call ToString(k+ Result(6)- 1).

40. Call ToString(k+ Result(17)- 1)

41. If this object has a property named by Result(39), go to step 42; but if this object has no property named by Result(39), then go to step 45.

42. Call the [[Get]] method of this object with argument Result(39).

43. Call the [[Put]] method of this object with arguments Result(40) and Result(42).

44. Go to step 46.

45. Call the [[Delete]] method of this object with argument Result(40).

46. Decrease k by 1.

47. Go to step 38.

48. Let k be Result(5).

49. Get the next argument in the part of the argument list that starts with item1; if there are no more arguments, go to step 53.

50. Call the [[Put]] method of this object with arguments ToString( k) and Result(49).

51. Increase k by 1.

52. Go to step 49.

53. Call the [[Put]] method of this object with arguments "length" and (Result(3)- Result(6)+ Result(17)).

54. Return A.

The length property of the splice method is 2.

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

15.4.4.13 Array.prototype.unshift([ item1[, item2 [,...]]])

The arguments are prepended to the start of the array, such that their order within the array is the same as the order in which they appear in the argument list.

When the unshift method is called with zero or more arguments item1, item2, etc., the following steps are taken:

1. Call the [[Get]] method of this object with argument "length".

2. Call ToUint32(Result(1)).

3. Compute the number of arguments.

4. Let k be Result(2).

5. If k is zero, go to step 15.

6. Call ToString(k- 1).

7. Call ToString(k+ Result(3)- 1).

8. If this object has a property named by Result(6), go to step 9; but if this object has no property named by Result(6), then go to step 12.

9. Call the [[Get]] method of this object with argument Result(6).

10. Call the [[Put]] method of this object with arguments Result(7) and Result(9).

11. Go to step 13.

12. Call the [[Delete]] method of this object with argument Result(7).

13. Decrease k by 1.

14. Go to step 5.

15. Let k be 0.

16. Get the next argument in the part of the argument list that starts with item1; if there are no more arguments, go to step 21.

17. Call ToString(k).

18. Call the [[Put]] method of this object with arguments Result(17) and Result(16).

19. Increase k by 1.

20. Go to step 16.

21. Call the [[Put]] method of this object with arguments "length" and (Result(2)+ Result(3)).

22. Return (Result(2)+ Result(3)).

The length property of the unshift method is 1.

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

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 (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 ToNumber(V), throw a RangeError exception.

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). If value is not supplied, the empty string "" is returned.

15.5.2 The String Constructor

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

15.5.2.1 new String([ value ] )

The [[Prototype]] 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), or to the empty string if value is not supplied.

15.5.3 Properties of the String Constructor

The value of the internal [[Prototype]] property of the String constructor is the Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the String constructor has the following properties:

15.5.3.1 String.prototype

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

This property has 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 (9.7) and regarding the resulting 16-bit integer as the code point value of a character. If no arguments are supplied, the result is the empty string.

The length property of the fromCharCode function is 1.

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 throws a TypeError exception 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 throws a TypeError exception 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 code point value of the character at position pos in the string resulting from converting this object to a string. If there is no character at that position, the result is NaN.

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

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, whose value is the code point value of the character at position Result(2) in the string Result(1), 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.concat([ string1[, string2 [,...]]])

When the concat method is called with zero or more arguments string1, string2, etc., it returns a string consisting of the characters of this object (converted to a string) followed by the characters of each of string1, string2, etc. (where each argument is converted to a string). The result is a string value, not a String object. The following steps are taken:

1. Call ToString, giving it the this value as its argument.

2. Let R be Result(1).

3. Get the next argument in the argument list; if there are no more arguments, go to step 7.

4. Call ToString(Result(3)).

5. Let R be the string value consisting of the characters in the previous value of R followed by the characters Result(4).

6. Go to step 3.

7. Return R.

The length property of the concat method is 1.

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

15.5.4.7 String.prototype.indexOf(searchString, position)

If searchString appears as a substring of the result of converting this object to a string, at one or more positions that are greater than or equal to position, then the index of the smallest such position is returned; otherwise, -1 is returned. If position is undefined, 0 is assumed, so as to search all of the string.

The indexOf method takes two arguments, searchString and position, and performs the following steps:

1. Call ToString, giving it the this value as its argument.

2. Call ToString(searchString).

3. Call ToInteger(position). (If position is undefined, 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).

The length property of the indexOf method is 1.

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

15.5.4.8 String.prototype.lastIndexOf(searchString, position)

If searchString appears as a substring of the result of converting this object to a string at one or more positions that are smaller than or equal to position, then the index of the greatest such position is returned; otherwise, -1 is returned. If position is undefined, the length of the string value is assumed, so as to search all of the string.

The lastIndexOf method takes two arguments, searchString and position, and performs the following steps:

1. Call ToString, giving it the this value as its argument.

2. Call ToString(searchString).

3. Call ToNumber(position). (If position is undefined, 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 nonnegative 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).

The length property of the lastIndexOf method is 1.

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

15.5.4.9 String.prototype.localeCompare(that)

When the localeCompare method is called with one argument that, it returns a number other than NaN that represents the result of a locale-sensitive string comparison of this object (converted to a string) with that (converted to a string). The two strings are compared in an implementation-defined fashion. The result is intended to order strings in the sort order specified by the system default locale, and will be negative, zero, or positive, depending on whether this comes before that in the sort order, the strings are equal, or this comes after that in the sort order, respectively.

The localeCompare method, if considered as a function of two arguments this and that, is a consistent comparison function (as defined in 15.4.4.11) on the set of all strings. Furthermore, localeCompare returns 0 or -0 when comparing two strings that are considered canonically equivalent by the Unicode standard.

The actual return values are left implementation-defined to permit implementers to encode additional information in the result value, but the function is required to define a total ordering on all strings and to return 0 when comparing two strings that are considered canonically equivalent by the Unicode standard.

NOTE 1
The localeCompare method itself is not directly suitable as an argument to Array.prototype.sort because the latter requires a function of two arguments.

NOTE 2
This function is intended to rely on whatever language-sensitive comparison functionality is available to the ECMAScript environment from the host environment, and to compare according to the rules of the host environment's current locale. It is strongly recommended that this function treat strings that are canonically equivalent according to the Unicode standard as identical (in other words, compare the strings as if they had both been converted to Normalised Form C or D first). It is also recommended that this function not honour Unicode compatibility equivalences or decompositions.

If no language-sensitive comparison at all is available from the host environment, this function may perform a bitwise comparison.

NOTE 3
The localeCompare function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

NOTE 4
The second parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.5.4.10 String.prototype.match(regexp)

If regexp is not an object whose [[Class]] property is "RegExp", it is replaced with the result of the expression new RegExp( regexp). Let string denote the result of converting the this value to a string. Then do one of the following:

  • If regexp. global is false: Return the result obtained by invoking RegExp.prototype.exec (see 15.10.6.2) on regexp with string as parameter.
  • If regexp. global is true: Set the regexp.lastIndex property to 0 and invoke RegExp.prototype.exec repeatedly until there is no match. If there is a match with an empty string (in other words, if the value of regexp. lastIndex is left unchanged), increment regexp. lastIndex by 1. Let n be the number of matches. If n=0, then the value returned is null; otherwise, the value returned is an array with the length property set to n and properties 0 through n–1 corresponding to the first elements of the results of all matching invocations of RegExp.prototype.exec.

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

15.5.4.11 String.prototype.replace(searchValue, replaceValue)

Let string denote the result of converting the this value to a string.

If searchValue is a regular expression (an object whose [[Class]] property is "RegExp"), do the following: If searchValue. global is false, then search string for the first match of the regular expression searchValue. If searchValue.global is true, then search string for all matches of the regular expression searchValue. Do the search in the same manner as in String.prototype.match, including the update of searchValue. lastIndex. Let m be the number of left capturing parentheses in searchValue (NCapturingParens as specified in 15.10.2.1).

If searchValue is not a regular expression, let searchString be ToString(searchValue) and search string for the first occurrence of searchString. Let m be 0.

If replaceValue is a function, then for each matched substring, call the function with the following m + 3 arguments. Argument 1 is the substring that matched. If searchValue is a regular expression, the next m arguments are all of the captures in the MatchResult (see 15.10.2.1). Argument m + 2 is the offset within string where the match occurred, and argument m + 3 is string. The result is a string value derived from the original input by replacing each matched substring with the corresponding return value of the function call, converted to a string if need be.

Otherwise, let newstring denote the result of converting replaceValue to a string. The result is a string value derived from the original input string by replacing each matched substring with a string derived from newstring by replacing characters in newstring by replacement text as specified in the following table. These $ replacements are done left-to-right, and, once such a replacement is performed, the new replacement text is not subject to further replacements. For example, "$1,$2".replace(/(\$(\d))/g, "$$1-$1$2") returns "$1-$11,$1-$22". A $ in newstring that does not match any of the forms below is left as is.

Characters Replacement text
$$ $
$& The matched substring.
$` The portion of string that precedes the matched substring.
$' The portion of string that follows the matched substring.
$n The nth capture, where n is a single digit 1-9 and $n is not followed by a decimal digit. If n <= m and the nth capture is undefined, use the empty string instead. If n> m, the result is implementation-defined.
$nn The nnth capture, where nn is a two-digit decimal number 01-99. If nn<= m and the nn th capture is undefined, use the empty string instead. If nn> m, the result is implementation-defined.

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

15.5.4.12 String.prototype.search(regexp)

If regexp is not an object whose [[Class]] property is "RegExp", it is replaced with the result of the expression new RegExp( regexp). Let string denote the result of converting the this value to a string. The value string is searched from its beginning for an occurrence of the regular expression pattern regexp. The result is a number indicating the offset within the string where the pattern matched, or -1 if there was no match.

NOTE 1
This method ignores the lastIndex and global properties of regexp. The lastIndex property of regexp is left unchanged.

NOTE 2
The search function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.13 String.prototype.slice(start, end)

The slice method takes two arguments, start and end, and returns a substring of the result of converting this object to a string, starting from character position start and running to, but not including, character position end (or through the end of the string if end is undefined). If start is negative, it is treated as (sourceLength+ start) where sourceLength is the length of the string. If end is negative, it is treated as (sourceLength+ end) where sourceLength is the length of the string. The result is a string value, not a String object. The following steps are taken:

1. Call ToString, giving it the this value as its argument.

2. Compute the number of characters in Result(1).

3. Call ToInteger(start).

4. If end is undefined, use Result(2); else use ToInteger(end).

5. If Result(3) is negative, use max(Result(2)+ Result(3), 0); else use min(Result(3), Result(2)).

6. If Result(4) is negative, use max(Result(2)+ Result(4), 0); else use min(Result(4), Result(2)).

7. Compute max(Result(6)- Result(5), 0).

8. Return a string containing Result(7) consecutive characters from Result(1) beginning with the character at position Result(5).

The length property of the slice method is 2.

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

15.5.4.14 String.prototype.split(separator, limit)

Returns an Array object into which substrings of the result of converting this object to a string have been stored. The substrings are determined by searching from left to right for occurrences of separator; these occurrences are not part of any substring in the returned array, but serve to divide up the string value. The value of separator may be a string of any length or it may be a RegExp object (i. e., an object whose [[Class]] property is "RegExp"; see 15.10).

The value of separator may be an empty string, an empty regular expression, or a regular expression that can match an empty string. In this case, separator does not match the empty substring at the beginning or end of the input string, nor does it match the empty substring at the end of the previous separator match. (For example, if separator is the empty string, the string is split up into individual characters; the length of the result array equals the length of the string, and each substring contains one character.) If separator is a regular expression, only the first match at a given position of the this string is considered, even if backtracking could yield a non-empty-substring match at that position. (For example, "ab".split(/a*?/) evaluates to the array ["a","b"], while "ab".split(/a*/) evaluates to the array["","b"].)

If the this object is (or converts to) the empty string, the result depends on whether separator can match the empty string. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty string.

If separator is a regular expression that contains capturing parentheses, then each time separator is matched the results (including any undefined results) of the capturing parentheses are spliced into the output array. (For example, "A<B>bold</B>and<CODE>coded</CODE>".split(/<(\/)?([^<>]+)>/) evaluates to the array ["A", undefined, "B", "bold", "/", "B", "and", undefined, "CODE", "coded", "/", "CODE", ""].)

If separator is undefined, then the result array contains just one string, which is the this value (converted to a string). If limit is not undefined, then the output array is truncated so that it contains no more than limit elements.

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

1. Let S = ToString(this).

2. Let A be a new array created as if by the expression new Array().

3. If limit is undefined, let lim =232 -1; else let lim = ToUint32(limit).

4. Let s be the number of characters in S.

5. Let p =0.

6. If separator is a RegExp object (its [[Class]] is "RegExp"), let R = separator; otherwise let R = ToString(separator).

7. If lim =0, return A.

8. If separator is undefined, go to step 33.

9. If s =0, go to step 31.

10. Let q = p.

11. If q = s, go to step 28.

12. Call SplitMatch(R, S, q) and let z be its MatchResult result.

13. If z is failure, go to step 26.

14. z must be a State. Let e be z's endIndex and let cap be z's captures array.

15. If e = p, go to step 26.

16. Let T be a string value equal to the substring of S consisting of the characters at positions p (inclusive) through q (exclusive).

17. Call the [[Put]] method of A with arguments A.length and T.

18. If A.length = lim, return A.

19. Let p = e.

20. Let i =0.

21. If i is equal to the number of elements in cap, go to step 10.

22. Let i = i+ 1.

23. Call the [[Put]] method of A with arguments A. length and cap[ i].

24. If A. length = lim, return A.

25. Go to step 21.

26. Let q = q+ 1.

27. Go to step 11.

28. Let T be a string value equal to the substring of S consisting of the characters at positions p (inclusive) through s (exclusive).

29. Call the [[Put]] method of A with arguments A. length and T.

30. Return A.

31. Call SplitMatch(R, S, 0) and let z be its MatchResult result.

32. If z is not failure, return A.

33. Call the [[Put]] method of A with arguments "0" and S.

34. Return A.

The internal helper function SplitMatch takes three parameters, a string S, an integer q, and a string or RegExp R, and performs the following in order to return a MatchResult (see 15.10.2.1):

1. If R is a RegExp object (its [[Class]] is "RegExp"), go to step 8.

2. R must be a string. Let r be the number of characters in R.

3. Let s be the number of characters in S.

4. If q+ r > s then return the MatchResult failure.

5. If there exists an integer i between 0 (inclusive) and r (exclusive) such that the character at position q+ i of S is different from the character at position i of R, then return failure.

6. Let cap be an empty array of captures (see 15.10.2.1).

7. Return the State (q+ r, cap). (see 15.10.2.1)

8. Call the [[Match]] method of R giving it the arguments S and q, and return the MatchResult result.

The length property of the split method is 2.

NOTE 1
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.

NOTE 2
The split method ignores the value of separator. global for separators that are RegExp objects.

15.5.4.15 String.prototype.substring(start, end)

The substring method takes two arguments, start and end, and returns a substring of the result of converting this object to a string, starting from character position start and running to, but not including, character position end of the string (or through the end of the string is end is undefined). The result is a string value, not a String object.

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

If start is larger than end, they are swapped. The following steps are taken:

1. Call ToString, giving it the this value as its argument.

2. Compute the number of characters in Result(1).

3. Call ToInteger(start).

4. If end is undefined, use Result(2); else use ToInteger(end).

5. Compute min(max(Result(3), 0), Result(2)).

6. Compute min(max(Result(4), 0), Result(2)).

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.

The length property of the substring method is 2.

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

15.5.4.16 String.prototype.toLowerCase()

If this object is not already a string, it is converted to a string. The characters in that string are converted one by one to lower case. The result is a string value, not a String object.

The characters are converted one by one. The result of each conversion is the original character, unless that character has a Unicode lowercase equivalent, in which case the lowercase equivalent is used instead.

NOTE 1
The result should be derived according to the case mappings in the Unicode character database (this explicitly includes not only the UnicodeData. txt file, but also the SpecialCasings. txt file that accompanies it in Unicode 2.1.8 and later).

NOTE 2
The toLowerCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.17 String.prototype.toLocaleLowerCase()

This function works exactly the same as toLowerCase except that its result is intended to yield the correct result for the host environment's current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.

NOTE 1
The toLocaleLowerCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

NOTE 2
The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.5.4.18 String.prototype.toUpperCase()

This function behaves in exactly the same way as String.prototype.toLowerCase, except that characters are mapped to their uppercase equivalents as specified in the Unicode Character Database.

NOTE 1
Because both toUpperCase and toLowerCase have context-sensitive behaviour, the functions are not symmetrical. In other words, s.toUpperCase(). toLowerCase() is not necessarily equal to s.toLowerCase().

NOTE 2
The toUpperCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.19 String.prototype.toLocaleUpperCase()

This function works exactly the same as toUpperCase except that its result is intended to yield the correct result for the host environment's current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.

NOTE 1
The toLocaleUpperCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

NOTE 2
The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.5.5 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).

15.6.2 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).

15.6.3 Properties of the Boolean Constructor

The value of the internal [[Prototype]] property of the Boolean constructor is the Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the Boolean constructor has the following property:

15.6.3.1 Boolean.prototype

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

This property has the attributes { 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; a TypeError exception is thrown if the this value is not 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 throws a TypeError exception 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 throws a TypeError exception 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) if value was supplied, else returns +0.

15.7.2 The Number Constructor

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

15.7.2.1 new Number([ value ] )

The [[Prototype]] 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) if value was supplied, else 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 (15.3.4).

Besides the internal properties and the length property (whose value is 1), the Number constructor has the following property:

15.7.3.1 Number.prototype

The initial value of Number.prototype is the Number prototype object (15.7.4).

This property has the attributes { 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 1.7976931348623157 * 10308 .

This property has the attributes { DontEnum, DontDelete, ReadOnly }.

15.7.3.3 Number.MIN_VALUE

The value of Number.MIN_VALUE is the smallest positive value of the number type, which is approximately 5 * 10-324 .

This property has the attributes { DontEnum, DontDelete, ReadOnly }.

15.7.3.4 Number.NaN

The value of Number.NaN is NaN.

This property has the attributes { DontEnum, DontDelete, ReadOnly }.

15.7.3.5 Number.NEGATIVE_INFINITY

The value of Number.NEGATIVE_INFINITY is -∞.

This property has the attributes { DontEnum, DontDelete, ReadOnly }.

15.7.3.6 Number.POSITIVE_INFINITY

The value of Number.POSITIVE_INFINITY is +∞.

This property has 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; a TypeError exception is thrown if the this value is not 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 radix is the number 10 or undefined, then this number value is given as an argument to the ToString operator; the resulting string value is returned.

If radix 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 throws a TypeError exception 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.toLocaleString()

Produces a string value that represents the value of the Number formatted according to the conventions of the host environment's current locale. This function is implementation-dependent, and it is permissible, but not encouraged, for it to return the same thing as toString.

NOTE
The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.7.4.4 Number.prototype.valueOf()

Returns this number value.

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

15.7.4.5 Number.prototype.toFixed(fractionDigits)

Return a string containing the number represented in fixed-point notation with fractionDigits digits after the decimal point. If fractionDigits is undefined, 0 is assumed. Specifically, perform the following steps:

1. Let f be ToInteger(fractionDigits). (If fractionDigits is undefined, this step produces the value 0).

2. If f <0 or f > 20, throw a RangeError exception.

3. Let x be this number value.

4. If x is NaN, return the string "NaN".

5. Let s be the empty string.

6. If x >= 0, go to step 9.

7. Let s be "-".

8. Let x =- x.

9. If x >= 1021, let m = ToString(x) and go to step 20.

10. Let n be an integer for which the exact mathematical value of n ÷ 10f - x is as close to zero as possible. If there are two such n, pick the larger n.

11. If n =0, let m be the string "0". Otherwise, let m be the string consisting of the digits of the decimal representation of n (in order, with no leading zeroes).

12. If f =0, go to step 20.

13. Let k be the number of characters in m.

14. If k > f, go to step 18.

15. Let z be the string consisting of f+ 1- k occurrences of the character '0'.

16. Let m be the concatenation of strings z and m.

17. Let k = f +1.

18. Let a be the first k- f characters of m, and let b be the remaining f characters of m.

19. Let m be the concatenation of the three strings a, ".", and b.

20. Return the concatenation of the strings s and m.

The length property of the toFixed method is 1.

If the toFixed method is called with more than one argument, then the behaviour is undefined (see clause 15).

An implementation is permitted to extend the behaviour of toFixed for values of fractionDigits less than 0 or greater than 20. In this case toFixed would not necessarily throw RangeError for such values.

NOTE
The output of toFixed may be more precise than toString for some values because toString only prints enough significant digits to distinguish the number from adjacent number values. For example, (1000000000000000128).toString() returns "1000000000000000100", while (1000000000000000128).toFixed(0) returns "1000000000000000128".

15.7.4.6 Number.prototype.toExponential(fractionDigits)

Return a string containing the number represented in exponential notation with one digit before the significand's decimal point and fractionDigits digits after the significand's decimal point. If fractionDigits is undefined, include as many significand digits as necessary to uniquely specify the number (just like in ToString except that in this case the number is always output in exponential notation). Specifically, perform the following steps:

1. Let x be this number value.

2. Let f be ToInteger(fractionDigits).

3. If x is NaN, return the string "NaN".

4. Let s be the empty string.

5. If x >= 0, go to step 8.

6. Let s be "-".

7. Let x =- x.

8. If x = +∞, let m = "Infinity" and go to step 30.

9. If fractionDigits is undefined, go to step 14.

10. If f <0 or f > 20, throw a RangeError exception.

11. If x =0, go to step 16.

12. Let e and n be integers such that 10f <= n <10f+1 and for which the exact mathematical value of n * 10e- f - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n * 10e- f is larger.

13. Go to step 20.

14. If x != 0, go to step 19.

15. Let f =0.

16. Let m be the string consisting of f+ 1 occurrences of the character '0'.

17. Let e =0.

18. Go to step 21.

19. Let e, n, and f be integers such that f >= 0, 10f <= n <10f+ 1 , the number value for n * 10e- f is x, and f is as small as possible. Note that the decimal representation of n has f+ 1 digits, n is not divisible by 10, and the least significant digit of n is not necessarily uniquely determined by these criteria.

20. Let m be the string consisting of the digits of the decimal representation of n (in order, with no leading zeroes).

21. If f =0, go to step 24.

22. Let a be the first character of m, and let b be the remaining f characters of m.

23. Let m be the concatenation of the three strings a, ".", and b.

24. If e =0, let c = "+" and d = "0" and go to step 29.

25. If e >0, let c = "+" and go to step 28.

26. Let c = "-".

27. Let e =- e.

28. Let d be the string consisting of the digits of the decimal representation of e (in order, with no leading zeroes).

29. Let m be the concatenation of the four strings m, "e", c, and d.

30. Return the concatenation of the strings s and m. The length property of the toExponential method is 1.

If the toExponential method is called with more than one argument, then the behaviour is undefined (see clause 15).

An implementation is permitted to extend the behaviour of toExponential for values of fractionDigits less than 0 or greater than 20. In this case toExponential would not necessarily throw RangeError for such values.

NOTE
For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 19 be used as a guideline:

Let e, n, and f be integers such that f >= 0, 10f <= n <10f+1 , the number value for n * 10e-f is x, and f is as small as possible. If there are multiple possibilities for n, choose the value of n for which n * 10e-f is closest in value to x. If there are two such possible values of n, choose the one that is even.

15.7.4.7 Number.prototype.toPrecision(precision)

Return a string containing the number represented either in exponential notation with one digit before the significand's decimal point and precision- 1 digits after the significand's decimal point or in fixed notation with precision significant digits. If precision is undefined, call ToString (9.8.1) instead. Specifically, perform the following steps:

1. Let x be this number value.

2. If precision is undefined, return ToString(x).

3. Let p be ToInteger(precision).

4. If x is NaN, return the string "NaN".

5. Let s be the empty string.

6. If x >= 0, go to step 9.

7. Let s be "-".

8. Let x = -x.

9. If x = +∞, let m = "Infinity" and go to step 30.

10. If p <1 or p > 21, throw a RangeError exception.

11. If x != 0, go to step 15.

12. Let m be the string consisting of p occurrences of the character '0'.

13. Let e =0.

14. Go to step 18.

15. Let e and n be integers such that 10p- 1 <= n <10p and for which the exact mathematical value of n * 10e- p+ 1 - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n * 10e- p+ 1 is larger.

16. Let m be the string consisting of the digits of the decimal representation of n (in order, with no leading zeroes).

17. If e <-6 or e >= p, go to step 22.

18. If e = p- 1, go to step 30.

19. If e >= 0, let m be the concatenation of the first e+ 1 characters of m, the character '. ', and the remaining p-(e+ 1) characters of m and go to step 30.

20. Let m be the concatenation of the string "0.",-( e+ 1) occurrences of the character '0', and the string m.

21. Go to step 30.

22. Let a be the first character of m, and let b be the remaining p- 1 characters of m.

23. Let m be the concatenation of the three strings a, ".", and b.

24. If e =0, let c = "+" and d = "0" and go to step 29.

25. If e >0, let c = "+" and go to step 28.

26. Let c = "-".

27. Let e =- e.

28. Let d be the string consisting of the digits of the decimal representation of e (in order, with no leading zeroes).

29. Let m be the concatenation of the four strings m, "e", c, and d.

30. Return the concatenation of the strings s and m.

The length property of the toPrecision method is 1.

If the toPrecision method is called with more than one argument, then the behaviour is undefined (see clause 15).

An implementation is permitted to extend the behaviour of toPrecision for values of precision less than 1 or greater than 21. In this case toPrecision would not necessarily throw RangeError for such values.

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 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 value of the internal [[Class]] property of the Math object is "Math".

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 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 has 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 has 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 has 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.

This property has the attributes { DontEnum, DontDelete, ReadOnly }.

NOTE
The value of Math.LOG2E is approximately the reciprocal of the value of Math.LN2.

15.8.1.5 LOG10E

The number value for the base-10 logarithm of e, the base of the natural logarithms; this value is approximately 0.4342944819032518.

This property has the attributes { DontEnum, DontDelete, ReadOnly }.

NOTE

The value of Math.LOG10E is approximately the reciprocal of the value of Math.LN10.

15.8.1.6 PI

The number value for , the ratio of the circumference of a circle to its diameter, which is approximately 3.1415926535897932.

This property has the attributes { DontEnum, DontDelete, ReadOnly }.

15.8.1.7 SQRT1_2

The number value for the square root of 1/2, which is approximately 0.7071067811865476.

This property has the attributes { DontEnum, DontDelete, ReadOnly }.

NOTE
The value of Math.SQRT1_ 2 is approximately the reciprocal of the value of Math.SQRT2.

15.8.1.8 SQRT2

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

This property has 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).

In the function descriptions below, the symbols NaN, -0, +0, -∞ and +∞ refer to the number values described in 8.5.

NOTE
The behaviour of the functions acos, asin, atan, atan2, cos, exp, log, pow, sin, and sqrt is not precisely specified here except to require specific results for certain argument values that represent boundary cases of interest. For other argument values, these functions are intended to compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of approximation algorithms. The general intent is that an implementer should be able to use the same mathematical library for ECMAScript on a given hardware platform that is available to C programmers on that platform.

Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard) that implementations use the approximation algorithms for IEEE 754 arithmetic contained in fdlibm, the freely distributable mathematical library from Sun Microsystems (fdlibm-comment@sunpro.eng.sun.com). This specification also requires specific results for certain argument values that represent boundary cases of interest

15.8.2.1 abs(x)

Returns the absolute value of x; the result has the same magnitude as x but has positive sign.

  • If x is NaN, the result is NaN.
  • If x is -0, the result is +0.
  • If x is -∞, the result is +∞.
15.8.2.2 acos(x)

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

  • If x is NaN, the result is NaN.
  • If x is greater than 1, the result is NaN.
  • If x is less than -1, the result is NaN.
  • If x is exactly 1, the result is +0.
15.8.2.3 asin(x)

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

  • If x is NaN, the result is NaN.
  • If x is greater than 1, the result is NaN.
  • If x is less than -1, the result is NaN.
  • If x is +0, the result is +0.
  • If x is -0, the result is -0.
15.8.2.4 atan(x)

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

  • If x is NaN, the result is NaN.
  • If x is +0, the result is +0.
  • If x is -0, the result is -0.
  • If x is +∞ , the result is an implementation-dependent approximation to +π/2.
  • If x is -∞, the result is an implementation-dependent approximation to -π/2.
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 y and x are used to determine the quadrant of the result. Note that it is intentional and traditional for the two-argument arc tangent function that the argument named y be first and the argument named x be second. The result is expressed in radians and ranges from -π to +π.

  • If either x or y is NaN, the result is NaN.
  • If y> 0 and x is +0, the result is an implementation-dependent approximation to +π/2.
  • If y> 0 and x is -0, the result is an implementation-dependent approximation to +π/2.
  • If y is +0 and x> 0, the result is +0.
  • If y is +0 and x is +0, the result is +0.
  • If y is +0 and x is -0, the result is an implementation-dependent approximation to +π.
  • If y is +0 and x< 0, the result is an implementation-dependent approximation to +π.
  • If y is -0 and x > 0, the result is -0.
  • If y is -0 and x is +0, the result is -0.
  • If y is -0 and x is -0, the result is an implementation-dependent approximation to -π.
  • If y is -0 and x< 0, the result is an implementation-dependent approximation to -π.
  • If y< 0 and x is +0, the result is an implementation-dependent approximation to -π/2.
  • If y< 0 and x is -0, the result is an implementation-dependent approximation to -π/2.
  • If y> 0 and y is finite and x is +∞, the result is +0.
  • If y> 0 and y is finite and x is -∞, the result if an implementation-dependent approximation to +π.
  • If y< 0 and y is finite and x is +∞, the result is -0.
  • If y< 0 and y is finite and x is -∞, the result is an implementation-dependent approximation to -π.
  • If y is +∞ and x is finite, the result is an implementation-dependent approximation to +π/2.
  • If y is -∞ and x is finite, the result is an implementation-dependent approximation to -π/2.
  • If y is +∞ and x is +∞, the result is an implementation-dependent approximation to +π/4.
  • If y is +∞ and x is -∞, the result is an implementation-dependent approximation to +3π/4.
  • If y is -∞ and x is +∞, the result is an implementation-dependent approximation to -π/4.
  • If y is -∞ and x is -∞, the result is an implementation-dependent approximation to -3π/4.
15.8.2.6 ceil(x)

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

  • If x is NaN, the result is NaN.
  • If x is +0, the result is +0.
  • If x is -0, the result is -0.
  • If x is +∞, the result is +∞.
  • If x is -∞, the result is -∞.
  • If x is less than 0 but greater than -1, the result is -0.

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

15.8.2.7 cos(x)

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

  • If x is NaN, the result is NaN.
  • If x is +0, the result is 1.
  • If x is -0, the result is 1.
  • If x is +∞, the result is NaN.
  • If x is -∞, the result is NaN.
15.8.2.8 exp(x)

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

  • If x is NaN, the result is NaN.
  • If x is +0, the result is 1.
  • If x is -0, the result is 1.
  • If x is +∞ , the result is +∞.
  • If x is -∞, the result is +0.
15.8.2.9 floor(x)

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

  • If x is NaN, the result is NaN.
  • If x is +0, the result is +0.
  • If x is -0, the result is -0.
  • If x is +∞, the result is +∞.
  • If x is -∞, the result is -∞.
  • If x is greater than 0 but less than 1, the result is +0.

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

15.8.2.10 log(x)

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

  • If x is NaN, the result is NaN.
  • If x is less than 0, the result is NaN.
  • If x is +0 or -0, the result is -∞.
  • If x is 1, the result is +0.
  • If x is +∞, the result is +∞.
15.8.2.11 max([ value1 [, value2 [,...]]])

Given zero or more arguments, calls ToNumber on each of the arguments and returns the largest of the resulting values.

  • If no arguments are given, the result is -∞.
  • If any value is NaN, the result is NaN.
  • The comparison of values to determine the largest value is done as in 11.8.5 except that +0 is considered to be larger than -0.

The length property of the max method is 2.

15.8.2.12 min([ value1 [, value2 [,...]]])

Given zero or more arguments, calls ToNumber on each of the arguments and returns the smallest of the resulting values.

  • If no arguments are given, the result is +∞.
  • If any value is NaN, the result is NaN.
  • The comparison of values to determine the smallest value is done as in 11.8.5 except that +0 is considered to be larger than -0.

The length property of the min method is 2.

15.8.2.13 pow(x, y)

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

  • If y is NaN, the result is NaN.
  • If y is +0, the result is 1, even if x is NaN.
  • If y is -0, the result is 1, even if x is NaN.
  • If x is NaN and y is nonzero, the result is NaN.
  • If abs(x)> 1 and y is +∞, the result is +∞ .
  • If abs(x)> 1 and y is -∞, the result is +0.
  • If abs(x)== 1 and y is +∞ , the result is NaN.
  • If abs(x)== 1 and y is -∞, the result is NaN.
  • If abs(x)< 1 and y is +∞ , the result is +0.
  • If abs(x)< 1 and y is -∞, the result is +∞.
  • If x is +∞ and y> 0, the result is +∞ .
  • If x is +∞ and y< 0, the result is +0.
  • If x is -∞ and y> 0 and y is an odd integer, the result is -∞.
  • If x is -∞ and y> 0 and y is not an odd integer, the result is +∞ .
  • If x is -∞ and y< 0 and y is an odd integer, the result is -0.
  • If x is -∞ and y< 0 and y is not an odd integer, the result is +0.
  • If x is +0 and y> 0, the result is +0.
  • If x is +0 and y< 0, the result is +∞ .
  • If x is -0 and y > 0 and y is an odd integer, the result is -0.
  • If x is -0 and y> 0 and y is not an odd integer, the result is +0.
  • If x is -0 and y < 0 and y is an odd integer, the result is -∞.
  • If x is -0 and y< 0 and y is not an odd integer, the result is +∞ .
  • If x< 0 and x is finite and y is finite and y is not an integer, the result is NaN.
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 x and is equal to a mathematical integer. If two integer number values are equally close to x, then the result is the number value that is closer to +∞ .If x is already an integer, the result is x.

  • If x is NaN, the result is NaN.
  • If x is +0, the result is +0.
  • If x is -0, the result is -0.
  • If x is +∞ , the result is +∞ .
  • If x is -∞, the result is -∞.
  • If x is greater than 0 but less than 0.5, the result is +0.
  • If x is less than 0 but greater than or equal to -0.5, the result is -0.

NOTE 1
Math.round(3.5) returns 4, but Math.round(-3.5) returns -3.

NOTE 2
The value of Math.round(x) is the same as the value of Math.floor(x+ 0.5), except when x is -0 or is less than 0 but greater than or equal to -0.5; for these cases Math.round(x) returns -0, but Math.floor(x+ 0.5) returns +0.

15.8.2.16 sin(x)

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

  • If x is NaN, the result is NaN.
  • If x is +0, the result is +0.
  • If x is -0, the result is -0.
  • If x is +∞ or -∞, the result is NaN.
15.8.2.17 sqrt(x)

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

  • If x is NaN, the result is NaN.
  • If x less than 0, the result is NaN.
  • If x is +0, the result is +0.
  • If x is -0, the result is -0.
  • If x is +∞ , the result is +∞ .
15.8.2.18 tan(x)

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

  • If x is NaN, the result is NaN.
  • If x is +0, the result is +0.
  • If x is -0, the result is -0.
  • If x is +∞ or ∞, the result is NaN.
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, 129 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)  = 0 if   0               <= DayWithinYear(t)< 31 
                 =  1 if  31               <= DayWithinYear(t) < 59+InLeapYear(t) 
                 =  2 if  59+InLeapYear(t) <= DayWithinYear(t) < 90+InLeapYear(t) 
                 =  3 if  90+InLeapYear(t) <= DayWithinYear(t) < 120+InLeapYear(t) 
                 =  4 if 120+InLeapYear(t) <= DayWithinYear(t) < 151+InLeapYear(t) 
                 =  5 if 151+InLeapYear(t) <= DayWithinYear(t) < 181+InLeapYear(t) 
                 =  6 if 181+InLeapYear(t) <= DayWithinYear(t) < 212+InLeapYear(t) 
                 =  7 if 212+InLeapYear(t) <= DayWithinYear(t) < 243+InLeapYear(t) 
                 =  8 if 243+InLeapYear(t) <= DayWithinYear(t) < 273+InLeapYear(t) 
                 =  9 if 273+InLeapYear(t) <= DayWithinYear(t) < 304+InLeapYear(t) 
                 = 10 if 304+InLeapYear(t) <= DayWithinYear(t) < 334+InLeapYear(t) 
                 = 11 if 334+InLeapYear(t) <= DayWithinYear(t) < 365+InLeapYear(t)

where

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

A month value of 0 specifies January; 1 specifies February; 2 specifies March; 3 specifies April; 4 specifies May; 5 specifies June; 6 specifies July; 7 specifies August; 8 specifies September; 9 specifies October; 10 specifies November; and 11 specifies December. Note that MonthFromTime(0) = 0, corresponding to Thursday, 01 January, 1970.

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 0 specifies Sunday; 1 specifies Monday; 2 specifies Tuesday; 3 specifies Wednesday; 4 specifies Thursday; 5 specifies Friday; and 6 specifies Saturday. Note that WeekDay(0) = 4, corresponding to Thursday, 01 January, 1970.

15.9.1.8 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.

15.9.1.9 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 host environment provides functionality for determining daylight saving time, the implementation of ECMAScript is free to map the year in question to an equivalent year (same leap-year- ness and same starting week day for the year) for which the host environment provides daylight saving time information. The only restriction is that all equivalent years should produce the same result.

15.9.1.9b 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 HoursPerDay = 24

MinutesPerHour = 60

SecondsPerMinute = 60

msPerSecond = 1000

msPerMinute = msPerSecond * SecondsPerMinute = 60000

msPerHour = msPerMinute * MinutesPerHour = 3600000

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)) > 8.64 x 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
The function call Date(...) is not equivalent to the object creation expression new Date(...) with the same arguments.

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

All of the arguments are optional; any arguments supplied are accepted but are completely ignored. A string is created and returned as if by the expression (new Date()).toString().

15.9.3 The Date Constructor

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

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

When Date is called with two to seven arguments, it computes the date from year, month, and (optionally) date, hours, minutes, seconds and ms.

The [[Prototype]] property of the newly constructed object is set to the original Date prototype object, the one that is the initial value of Date.prototype (15.9.4.1).

The [[Class]] 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 date is supplied use ToNumber(date); else use 1.

4. If hours is supplied use ToNumber(hours); else use 0.

5. If minutes is supplied use ToNumber(minutes); else use 0.

6. If seconds is supplied use ToNumber(seconds); else use 0.

7. If ms is supplied use ToNumber(ms); else use 0.

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(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 (15.9.4.1).

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 (15.9.4.2); let V be the time value for this date.

6. Go to step 4.

15.9.3.3 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 (15.9.4.1).

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 (15.3.4).

Besides the internal 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 (15.9.5).

This property has 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.toUTCString())

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 toUTCString method.

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

When the UTC function is called with fewer than two arguments, the behaviour is implementation-dependent. When the UTC function is called with two to seven arguments, it computes the date from year, month and (optionally) date, hours, minutes, seconds and ms. The following steps are taken:

1. Call ToNumber(year).

2. Call ToNumber(month).

3. If date is supplied use ToNumber(date); else use 1.

4. If hours is supplied use ToNumber(hours); else use 0.

5. If minutes is supplied use ToNumber(minutes); else use 0.

6. If seconds is supplied use ToNumber(seconds); else use 0.

7. If ms is supplied use ToNumber(ms); else use 0.

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))).

The length property of the UTC function is 7.

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

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. None of these functions are generic; a TypeError exception is thrown if the this value is not an object for which the value of the 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 the current time zone in a convenient, human-readable form.

NOTE
It is intended that for any Date value d, the result of Date.prototype.parse(d.toString()) (section 15.9.4.2) is equal to d.

15.9.5.3 Date.prototype.toDateString()

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

15.9.5.4 Date.prototype.toTimeString()

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

15.9.5.5 Date.prototype.toLocaleString()

This function returns a string value. The contents of the string are implementation-dependent, but are intended to represent the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.

NOTE
The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.9.5.6 Date.prototype.toLocaleDateString()

This function returns a string value. The contents of the string are implementation-dependent, but are intended to represent the "date" portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.

NOTE
The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.9.5.7 Date.prototype.toLocaleTimeString()

This function returns a string value. The contents of the string are implementation-dependent, but are intended to represent the "time" portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.

NOTE
The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.9.5.8 Date.prototype.valueOf()

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

15.9.5.9 Date.prototype.getTime()

1. If the this value is not an object whose [[Class]] property is "Date", throw a TypeError exception.

2. Return this time value.

15.9.5.10 Date.prototype.getFullYear()

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return YearFromTime(LocalTime(t)).

15.9.5.11 Date.prototype.getUTCFullYear()

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return YearFromTime(t).

15.9.5.12 Date.prototype.getMonth()

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return MonthFromTime(LocalTime(t)).

15.9.5.13 Date.prototype.getUTCMonth()

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return MonthFromTime(t).

15.9.5.14 Date.prototype.getDate()

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return DateFromTime(LocalTime(t)).

15.9.5.15 Date.prototype.getUTCDate()

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return DateFromTime(t).

15.9.5.16 Date.prototype.getDay()

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return WeekDay(LocalTime(t)).

15.9.5.17 Date.prototype.getUTCDay()

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return WeekDay(t).

15.9.5.18 Date.prototype.getHours()

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return HourFromTime(LocalTime(t)).

15.9.5.19 Date.prototype.getUTCHours()

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return HourFromTime(t).

15.9.5.20 Date.prototype.getMinutes()

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return MinFromTime(LocalTime(t)).

15.9.5.21 Date.prototype.getUTCMinutes()

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return MinFromTime(t).

15.9.5.22 Date.prototype.getSeconds()

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return SecFromTime(LocalTime(t)).

15.9.5.23 Date.prototype.getUTCSeconds()

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return SecFromTime(t).

15.9.5.24 Date.prototype.getMilliseconds()

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return msFromTime(LocalTime(t)).

15.9.5.25 Date.prototype.getUTCMilliseconds()

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return msFromTime(t).

15.9.5.26 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.

15.9.5.27 Date.prototype.setTime(time)

1. If the this value is not a Date object, throw a TypeError exception.

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.

15.9.5.28 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.

15.9.5.29 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.

15.9.5.30 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.

The length property of the setSeconds method is 2.

15.9.5.31 Date.prototype.setUTCSeconds(sec [, ms ] )

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

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.

The length property of the setUTCSeconds method is 2.

15.9.5.33 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.

The length property of the setMinutes method is 3.

15.9.5.34 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.

The length property of the setUTCMinutes method is 3.

15.9.5.35 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.

The length property of the setHours method is 4.

15.9.5.36 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.

The length property of the setUTCHours method is 4.

15.9.5.36 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.

15.9.5.37 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.

15.9.5.38 Date.prototype.setMonth(month [, 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(month).

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.

The length property of the setMonth method is 2.

15.9.5.39 Date.prototype.setUTCMonth(month [, date ] )

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

1. Let t be this time value.

2. Call ToNumber(month).

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.

The length property of the setUTCMonth method is 2.

15.9.5.40 Date.prototype.setFullYear(year [, month [, date ] ] )

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

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

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 month is not specified, compute MonthFromTime(t); otherwise, call ToNumber(month).

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.

The length property of the setFullYear method is 3.

15.9.5.41 Date.prototype.setUTCFullYear(year [, month [, date ] ] )

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

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

1. Let t be this time value; but if this time value is NaN, let t be +0.

2. Call ToNumber(year).

3. If month is not specified, compute MonthFromTime( t); otherwise, call ToNumber(month).

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.

The length property of the setUTCFullYear method is 3.

15.9.5.42 Date.prototype.toUTCString()

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

15.9.6 Properties of Date Instances

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

15.10 RegExp (Regular Expression) Objects

A RegExp object contains a regular expression and the associated flags.

NOTE
The form and functionality of regular expressions is modelled after the regular expression facility in the Perl 5 programming language.

15.10.1 Patterns

The RegExp constructor applies the following grammar to the input pattern string. An error occurs if the grammar cannot interpret the string as an expansion of Pattern.

Syntax

Pattern ::
Disjunction
Disjunction ::
Alternative
Alternative | Disjunction
Alternative ::
[empty]
Alternative Term
Term ::
Assertion
Atom
Atom Quantifier
Assertion ::
^
$
\b
\B
Quantifier ::
QuantifierPrefix
QuantifierPrefix ?
QuantifierPrefix ::
*
+
?
{ DecimalDigits }
{ DecimalDigits ,}
{ DecimalDigits , DecimalDigits }
Atom ::
PatternCharacter
.
\ AtomEscape
CharacterClass
( Disjunction )
(?: Disjunction )
(?= Disjunction )
(?! Disjunction )
PatternCharacter ::
SourceCharacter but not any of: ^ $ \ . * + ? ( ) [ ] { } |
AtomEscape ::
DecimalEscape
CharacterEscape
CharacterClassEscape
CharacterEscape ::
ControlEscape
c ControlLetter
HexEscapeSequence
UnicodeEscapeSequence
IdentityEscape
ControlEscape :: one of
f n r t v
ControlLetter :: 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
IdentityEscape ::
SourceCharacter but not IdentifierPart
DecimalEscape ::
DecimalIntegerLiteral [lookahead ∉ DecimalDigit]
CharacterClassEscape :: one of
d D s S w W
CharacterClass ::
[ [lookahead ∉ {^}] ClassRanges ]
[^ ClassRanges ]
ClassRanges ::
[empty]
NonemptyClassRanges
NonemptyClassRanges ::
ClassAtom
ClassAtom NonemptyClassRangesNoDash
ClassAtom - ClassAtom ClassRanges
NonemptyClassRangesNoDash ::
ClassAtom
ClassAtomNoDash NonemptyClassRangesNoDash
ClassAtomNoDash - ClassAtom ClassRanges
ClassAtom ::
-ClassAtomNoDash
ClassAtomNoDash ::
SourceCharacter but not one of \ ] -
\ ClassEscape
ClassEscape ::
DecimalEscape
b
CharacterEscape
CharacterClassEscape
15.10.2 Pattern Semantics

A regular expression pattern is converted into an internal function using the process described below. An implementation is encouraged to use more efficient algorithms than the ones listed below, as long as the results are the same.

15.10.2.1 Notation

The descriptions below use the following variables:

  • Input is the string being matched by the regular expression pattern. The notation input[ n] means the nth character of input, where n can range between 0 (inclusive) and InputLength (exclusive).
  • InputLength is the number of characters in the Input string.
  • NCapturingParens is the total number of left capturing parentheses (i. e. the total number of times the Atom :: ( Disjunction ) production is expanded) in the pattern. A left capturing parenthesis is any ( pattern character that is matched by the ( terminal of the Atom :: ( Disjunction ) production.
  • IgnoreCase is the setting of the RegExp object's ignoreCase property.
  • Multiline is the setting of the RegExp object's multiline property.

Furthermore, the descriptions below use the following internal data structures:

  • A CharSet is a mathematical set of characters.
  • A State is an ordered pair (endIndex, captures) where endIndex is an integer and captures is an internal array of NCapturingParens values. States are used to represent partial match states in the regular expression matching algorithms. The endIndex is one plus the index of the last input character matched so far by the pattern, while captures holds the results of capturing parentheses. The nth element of captures is either a string that represents the value obtained by the nth set of capturing parentheses or undefined if the nth set of capturing parentheses hasn't been reached yet. Due to backtracking, many states may be in use at any time during the matching process.
  • A MatchResult is either a State or the special token failure that indicates that the match failed. A
  • Continuation function is an internal closure (i. e. an internal function with some arguments already bound to values) that takes one State argument and returns a MatchResult result. If an internal closure references variables bound in the function that creates the closure, the closure uses the values that these variables had at the time the closure was created. The continuation attempts to match the remaining portion (specified by the closure's already-bound arguments) of the pattern against the input string, starting at the intermediate state given by its State argument. If the match succeeds, the continuation returns the final State that it reached; if the match fails, the continuation returns failure.
  • A Matcher function is an internal closure that takes two arguments --a State and a Continuation --and returns a MatchResult result. The matcher attempts to match a middle subpattern (specified by the closure's already-bound arguments) of the pattern against the input string, starting at the intermediate state given by its State argument. The Continuation argument should be a closure that matches the rest of the pattern. After matching the subpattern of a pattern to obtain a new State, the matcher then calls Continuation on that state to test if the rest of the pattern can match as well. If it can, the matcher returns the state returned by the continuation; if not, the matcher may try different choices at its choice points, repeatedly calling Continuation until it either succeeds or all possibilities have been exhausted.
  • An AssertionTester function is an internal closure that takes a State argument and returns a boolean result. The assertion tester tests a specific condition (specified by the closure's already-bound arguments) against the current place in the input string and returns true if the condition matched or false if not.
  • An EscapeValue is either a character or an integer. An EscapeValue is used to denote the interpretation of a DecimalEscape escape sequence: a character ch means that the escape sequence is interpreted as the character ch, while an integer n means that the escape sequence is interpreted as a backreference to the nth set of capturing parentheses.
15.10.2.2 Pattern

The production Pattern :: Disjunction evaluates as follows:

1. Evaluate Disjunction to obtain a Matcher m.

2. Return an internal closure that takes two arguments, a string str and an integer index, and performs the following:

1. Let Input be the given string str. This variable will be used throughout the functions in 15.10.2.

2. Let InputLength be the length of Input. This variable will be used throughout the functions in 15.10.2.

3. Let c be a Continuation that always returns its State argument as a successful MatchResult.

4. Let cap be an internal array of NCapturingParens undefined values, indexed 1 through NCapturingParens.

5. Let x be the State (index, cap).

6. Call m(x, c) and return its result.

Informative comments: A Pattern evaluates ("compiles") to an internal function value. RegExp.prototype.exec can then apply this function to a string and an offset within the string to determine whether the pattern would match starting at exactly that offset within the string, and, if it does match, what the values of the capturing parentheses would be. The algorithms in 15.10.2 are designed so that compiling a pattern may throw a SyntaxError exception; on the other hand, once the pattern is successfully compiled, applying its result function to find a match in a string cannot throw an exception (except for any host-defined exceptions that can occur anywhere such as out-of-memory).

15.10.2.3 Disjunction

The production Disjunction :: Alternative evaluates by evaluating Alternative to obtain a Matcher and returning that Matcher.

The production Disjunction :: Alternative | Disjunction evaluates as follows:

1. Evaluate Alternative to obtain a Matcher m1.

2. Evaluate Disjunction to obtain a Matcher m2.

3. Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following:

1. Call m1(x, c) and let r be its result.

2. If r isn't failure, return r.

3. Call m2(x, c) and return its result.

Informative comments: The | regular expression operator separates two alternatives. The pattern first tries to match the left Alternative (followed by the sequel of the regular expression); if it fails, it tries to match the right Disjunction (followed by the sequel of the regular expression). If the left Alternative, the right Disjunction, and the sequel all have choice points, all choices in the sequel are tried before moving on to the next choice in the left Alternative. If choices in the left Alternative are exhausted, the right Disjunction is tried instead of the left Alternative. Any capturing parentheses inside a portion of the pattern skipped by | produce undefined values instead of strings. Thus, for example,

/a|ab/.exec("abc")

returns the result "a" and not "ab". Moreover,

/((a)|(ab))((c)|(bc))/.exec("abc")

returns the array

["abc", "a", "a", undefined, "bc", undefined, "bc"]

and not

["abc", "ab", undefined, "ab", "c", "c", undefined]

15.10.2.4 Alternative

The production Alternative :: [empty] evaluates by returning a Matcher that takes two arguments, a State x and a Continuation c, and returns the result of calling c( x).

The production Alternative :: Alternative Term evaluates as follows:

1. Evaluate Alternative to obtain a Matcher m1.

2. Evaluate Term to obtain a Matcher m2.

3. Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following:

1. Create a Continuation d that takes a State argument y and returns the result of calling m2(y, c).

2. Call m1(x, d) and return its result.

Informative comments: Consecutive Terms try to simultaneously match consecutive portions of the input string. If the left Alternative, the right Term, and the sequel of the regular expression all have choice points, all choices in the sequel are tried before moving on to the next choice in the right Term, and all choices in the right Term are tried before moving on to the next choice in the left Alternative.

15.10.2.5 Term

The production Term :: Assertion evaluates by returning an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following:

1. Evaluate Assertion to obtain an AssertionTester t.

2. Call t(x) and let r be the resulting boolean value.

3. If r is false, return failure.

4. Call c(x) and return its result.

The production Term :: Atom evaluates by evaluating Atom to obtain a Matcher and returning that Matcher.

The production Term :: Atom Quantifier evaluates as follows:

1. Evaluate Atom to obtain a Matcher m.

2. Evaluate Quantifier to obtain the three results: an integer min, an integer (or ∞) max, and boolean greedy.

3. If max is finite and less than min, then throw a SyntaxError exception.

4. Let parenIndex be the number of left capturing parentheses in the entire regular expression that occur to the left of this production expansion's Term. This is the total number of times the Atom :: ( Disjunction ) production is expanded prior to this production's Term plus the total number of Atom :: ( Disjunction ) productions enclosing this Term.

5. Let parenCount be the number of left capturing parentheses in the expansion of this production's Atom. This is the total number of Atom :: ( Disjunction ) productions enclosed by this production's Atom.

6. Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following:

1. Call RepeatMatcher(m, min, max, greedy, x, c, parenIndex, parenCount) and return its result.

The internal helper function RepeatMatcher takes eight parameters, a Matcher m, an integer min, an integer (or ∞) max, a boolean greedy,a Statex, a Continuation c, an integer parenIndex, andan integer parenCount, and performs the following:

1. If max is zero, then call c(x) and return its result.

2. Create an internal Continuation closure d that takes one State argument y and performs the following:

1. If min is zero and y's endIndex is equal to x's endIndex, then return failure.

2. If min is zero then let min2 be zero; otherwise let min2 be min- 1.

3. If max is, ∞ then let max2 be ∞; otherwise let max2 be max- 1.

4. Call RepeatMatcher(m, min2, max2, greedy, y, c, parenIndex, parenCount) and return its result.

3. Let cap be a fresh copy of x's captures internal array.

4. For every integer k that satisfies parenIndex < k and k <= parenIndex+ parenCount, set cap[k] to undefined.

5. Let e be x's endIndex.

6. Let xr be the State (e, cap).

7. If min is not zero, then call m(xr, d) and return its result.

8. If greedy is true, then go to step 12.

9. Call c(x) and let z be its result.

10. If z is not failure, return z.

11. Call m(xr, d) and return its result.

12. Call m(xr, d) and let z be its result.

13. If z is not failure, return z.

14. Call c(x) and return its result.

Informative comments: An Atom followed by a Quantifier is repeated the number of times specified by the Quantifier. A quantifier can be non-greedy, in which case the Atom pattern is repeated as few times as possible while still matching the sequel, or it can be greedy, in which case the Atom pattern is repeated as many times as possible while still matching the sequel. The Atom pattern is repeated rather than the input string that it matches, so different repetitions of the Atom can match different input substrings.

If the Atom and the sequel of the regular expression all have choice points, the Atom is first matched as many (or as few, if non-greedy) times as possible. All choices in the sequel are tried before moving on to the next choice in the last repetition of Atom. All choices in the last (n th ) repetition of Atom are tried before moving on to the next choice in the next-to-last (n- 1) st repetition of Atom; at which point it may turn out that more or fewer repetitions of Atom are now possible; these are exhausted (again, starting with either as few or as many as possible) before moving on to the next choice in the (n-1) st repetition of Atom and so on.

Compare

/a[a-z]{2,4}/.exec("abcdefghi")

which returns "abcde" with

/a[a-z]{2,4}?/.exec("abcdefghi")

which returns "abc".

Consider also

/(aa|aabaac|ba|b|c)*/.exec("aabaac")

which, by the choice point ordering above, returns the array

["aaba", "ba"]

and not any of:

["aabaac", "aabaac"] ["aabaac", "c"]

The above ordering of choice points can be used to write a regular expression that calculates the greatest common divisor of two numbers (represented in unary notation). The following example calculates the gcd of 10 and 15:

"aaaaaaaaaa, aaaaaaaaaaaaaaa".replace(/^(a+)\1*,\1+$/,"$1")

which returns the gcd in unary notation "aaaaa".

Step 4 of the RepeatMatcher clears Atom's captures each time Atom is repeated. We can see its behaviour in the regular expression

/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")

which returns the array

["zaacbbbcac", "z", "ac", "a", undefined, "c"]

and not

["zaacbbbcac", "z", "ac", "a", "bbb", "c"]

because each iteration of the outermost * clears all captured strings contained in the quantified Atom, which in this case includes capture strings numbered 2, 3, and 4.

Step 1 of theRepeatMatcher's closure d states that, once the minimum number of repetitions has been satisfied, any more expansions of Atom that match the empty string are not considered for further repetitions. This prevents the regular expression engine from falling into an infinite loop on patterns such as:

/(a*)*/.exec("b")

or the slightly more complicated:

/(a*)b\1+/.exec("baaaac")

which returns the array

["b", ""]

15.10.2.6 Assertion

The production Assertion :: ^ evaluates by returning an internal AssertionTester closure that takes a State argument x and performs the following:

1. Let e be x's endIndex.

2. If e is zero, return true.

3. If Multiline is false, return false.

4. If the character Input[ e- 1] is one of the line terminator characters <LF>, <CR>, <LS>, or <PS>, return true.

5. Return false.

The production Assertion :: $ evaluates by returning an internal AssertionTester closure that takes a State argument x and performs the following:

1. Let e be x's endIndex.

2. If e is equal to InputLength, return true.

3. If multiline is false, return false.

4. If the character Input[ e] is one of the line terminator characters <LF>, <CR>, <LS>, or <PS>, return true.

5. Return false.

The production Assertion :: \b evaluates by returning an internal AssertionTester closure that takes a State argument x and performs the following:

1. Let e be x's endIndex.

2. Call IsWordChar(e-1) and let a be the boolean result.

3. Call IsWordChar(e) and let b be the boolean result.

4. If a is true and b is false, return true.

5. If a is false and b is true, return true.

6. Return false.

The production Assertion :: \B evaluates by returning an internal AssertionTester closure that takes a State argument x and performs the following:

1. Let e be x's endIndex.

2. Call IsWordChar(e- 1) and let a be the boolean result.

3. Call IsWordChar(e) and let b be the boolean result.

4. If a is true and b is false, return false.

5. If a is false and b is true, return false.

6. Return true.

The internal helper function IsWordChar takes an integer parameter e and performs the following:

1. If e == -1 or e == InputLength, return false.

2. Let c be the character Input[ e].

3. If c is one of the sixty-three characters in the table below, return true.

a b c d e f g h i j k l m n o p q r s t u v w x y z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
0 1 2 3 4 5 6 7 8 9 _

4. Return false.

15.10.2.7 Quantifier

The production Quantifier :: QuantifierPrefix evaluates as follows:

1. Evaluate QuantifierPrefix to obtain the two results: an integer min and an integer (or ∞) max.

2. Return the three results min , max, and true.

The production Quantifier :: QuantifierPrefix ? evaluates as follows:

1. Evaluate QuantifierPrefix to obtain the two results: an integer min and an integer (or ∞ ) max.

2. Return the three results min , max, and false.

The production QuantifierPrefix :: * evaluates by returning the two results 0 and ∞.

The production QuantifierPrefix :: + evaluates by returning the two results 1 and ∞.

The production QuantifierPrefix :: ? evaluates by returning the two results 0 and 1.

The production QuantifierPrefix :: { DecimalDigits } evaluates as follows:

1. Let i be the MV of DecimalDigits (see 7.8.3).

2. Return the two results i and i.

The production QuantifierPrefix :: { DecimalDigits ,} evaluates as follows:

1. Let i be the MV of DecimalDigits.

2. Return the two results i and ∞.

The production QuantifierPrefix :: { DecimalDigits , DecimalDigits } evaluates as follows:

1. Let i be the MV of the first DecimalDigits.

2. Let j be the MV of the second DecimalDigits.

3. Return the two results i and j.

15.10.2.8 Atom

The production Atom :: PatternCharacter evaluates as follows:

1. Let ch be the character represented by PatternCharacter.

2. Let A be a one-element CharSet containing the character ch.

3. Call CharacterSetMatcher(A, false) and return its Matcher result.

The production Atom :: . evaluates as follows:

1. Let A be the set of all characters except the four line terminator characters <LF>, <CR>, <LS>, or <PS>.

2. Call CharacterSetMatcher(A, false) and return its Matcher result.

The production Atom :: \ AtomEscape evaluates by evaluating AtomEscape to obtain a Matcher and returning that Matcher.

The production Atom :: CharacterClass evaluates as follows:

1. Evaluate CharacterClass to obtain a CharSet A and a boolean invert.

2. Call CharacterSetMatcher(A, invert) and return its Matcher result.

The production Atom :: ( Disjunction ) evaluates as follows:

1. Evaluate Disjunction to obtain a Matcher m.

2. Let parenIndex be the number of left capturing parentheses in the entire regular expression that occur to the left of this production expansion's initial left parenthesis. This is the total number of times the Atom :: ( Disjunction ) production is expanded prior to this production's Atom plus the total number of Atom :: ( Disjunction ) productions enclosing this Atom.

3. Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following:

1. Create an internal Continuation closure d that takes one State argument y and performs the following:

1. Let cap be a fresh copy of y's captures internal array.

2. Let xe be x's endIndex.

3. Let ye be y's endIndex.

4. Let s be a fresh string whose characters are the characters of Input at positions xe (inclusive) through ye (exclusive).

5. Set cap[ parenIndex+ 1] to s.

6. Let z be the State (ye, cap).

7. Call c(z) and return its result.

2. Call m(x, d) and return its result. The production Atom :: (?: Disjunction ) evaluates by evaluating Disjunction to obtain a Matcher and returning that Matcher.

The production Atom :: (?= Disjunction ) evaluates as follows:

1. Evaluate Disjunction to obtain a Matcher m.

2. Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following:

1. Let d be a Continuation that always returns its State argument as a successful MatchResult.

2. Call m(x, d) and let r be its result.

3. If r is failure, return failure.

4. Let y be r's State.

5. Let cap be y's captures internal array.

6. Let xe be x's endIndex.

7. Let z be the State (xe, cap).

8. Call c(z) and return its result.

The production Atom :: (?! Disjunction ) evaluates as follows:

1. Evaluate Disjunction to obtain a Matcher m.

2. Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following:

1. Let d be a Continuation that always returns its State argument as a successful MatchResult.

2. Call m(x, d) and let r be its result.

3. If r isn't failure, return failure.

4. Call c(x) and return its result.

The internal helper function CharacterSetMatcher takes two arguments, a CharSet A and a boolean flag invert, and performs the following:

1. Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following:

1. Let e be x's endIndex.

2. If e == InputLength, return failure.

3. Let c be the character Input[ e].

4. Let cc be the result of Canonicalize(c).

5. If invert is true, go to step 8.

6. If there does not exist a member a of set A such that Canonicalize(a)== cc, then return failure.

7. Go to step 9.

8. If there exists a member a of set A such that Canonicalize(a)== cc, then return failure.

9. Let cap be x's captures internal array.

10. Let y be the State (e+ 1, cap).

11. Call c(y) and return its result.

The internal helper function Canonicalize takes a character parameter ch and performs the following:

1. If IgnoreCase is false, return ch.

2. Let u be ch converted to upper case as if by calling String.prototype.toUpperCase on the one-character string ch.

3. If u does not consist of a single character, return ch.

4. Let cu be u's character.

5. If ch's code point value is greater than or equal to decimal 128 and cu's code point value is less than decimal 128, then return ch.

6. Return cu.

Informative comments: Parentheses of the form ( Disjunction ) serve both to group the components of the Disjunction pattern together and to save the result of the match. The result can be used either in a backreference (\ followed by a nonzero decimal number), referenced in a replace string, or returned as part of an array from the regular expression matching function. To inhibit the capturing behaviour of parentheses, use the form (?: Disjunction ) instead.

The form (?= Disjunction ) specifies a zero-width positive lookahead. In order for it to succeed, the pattern inside Disjunction must match at the current position, but the current position is not advanced before matching the sequel. If Disjunction can match at the current position in several ways, only the first one is tried. Unlike other regular expression operators, there is no backtracking into a (?= form (this unusual behaviour is inherited from Perl). This only matters when the Disjunction contains capturing parentheses and the sequel of the pattern contains backreferences to those captures.

For example,

/(?=(a+))/.exec("baaabac")

matches the empty string immediately after the first b and therefore returns the array:

["", "aaa"]

To illustrate the lack of backtracking into the lookahead, consider:

/(?=(a+))a*b\1/.exec("baaabac")

This expression returns

["aba", "a"]

and not:

["aaaba", "a"]

The form (?! Disjunction ) specifies a zero-width negative lookahead. In order for it to succeed, the pattern inside Disjunction must fail to match at the current position. The current position is not advanced before matching the sequel. Disjunction can contain capturing parentheses, but backreferences to them only make sense from within Disjunction itself. Backreferences to these capturing parentheses from elsewhere in the pattern always return undefined because the negative lookahead must fail for the pattern to succeed. For example,

/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")

looks for an a not immediately followed by some positive number n of a's, a b, another n a's (specified by the first \2) and a c. The second \2 is outside the negative lookahead, so it matches against undefined and therefore always succeeds. The whole expression returns the array:

["baaabaac", "ba", undefined, "abaac"]

In case-insignificant matches all characters are implicitly converted to upper case immediately before they are compared. However, if converting a character to upper case would expand that character into more than one character (such as converting "ß" ((u00DF) into" SS"), then the character is left as-is instead. The character is also left as-is if it is not an ASCII character but converting it to upper case would make it into an ASCII character. This prevents Unicode characters such as \u0131 and \u017F from matching regular expressions such as /[a-z]/i, which are only intended to match ASCII letters. Furthermore, if these conversions were allowed, then /[^\W]/i would match each of a, b,..., h, but not i or s.

15.10.2.9 AtomEscape

The production AtomEscape :: DecimalEscape evaluates as follows:

1. Evaluate DecimalEscape to obtain an EscapeValue E.

2. If E is not a character then go to step 6.

3. Let ch be E's character.

4. Let A be a one-element CharSet containing the character ch.

5. Call CharacterSetMatcher(A, false) and return its Matcher result.

6. E must be an integer. Let n be that integer.

7. If n= 0 or n> NCapturingParens then throw a SyntaxError exception.

8. Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following:

1. Let cap be x's captures internal array.

2. Let s be cap[ n].

3. If s is undefined, then call c(x) and return its result.

4. Let e be x's endIndex.

5. Let len be s's length.

6. Let f be e+ len.

7. If f> InputLength, return failure.

8. If there exists an integer i between 0 (inclusive) and len (exclusive) such that Canonicalize(s[ i]) is not the same character as Canonicalize(Input [e+ i]), then return failure.

9. Let y be the State (f, cap).

10. Call c(y) and return its result.

The production AtomEscape :: CharacterEscape evaluates as follows:

1. Evaluate CharacterEscape to obtain a character ch.

2. Let A be a one-element CharSet containing the character ch.

3. Call CharacterSetMatcher(A, false) and return its Matcher result.

The production AtomEscape :: CharacterClassEscape evaluates as follows:

1. Evaluate CharacterClassEscape to obtain a CharSet A.

2. Call CharacterSetMatcher(A, false) and return its Matcher result.

Informative comments: An escape sequence of the form \ followed by a nonzero decimal number n matches the result of the nth set of capturing parentheses (see 15.10.2.11). It is an error if the regular expression has fewer than n capturing parentheses. If the regular expression has n or more capturing parentheses but the nth one is undefined because it hasn't captured anything, then the backreference always succeeds.

15.10.2.10 CharacterEscape

The production CharacterEscape :: ControlEscape evaluates by returning the character according to the table below:

ControlEscape Unicode Value Name Symbol
t \u0009 horizontal tab <HT>
n \u000A line feed (new line) <LF>
v \u000B vertical tab <VT>
f \u000C form feed <FF>
r \u000D carriage return <CR>

The production CharacterEscape :: c ControlLetter evaluates as follows:

1. Let ch be the character represented by ControlLetter.

2. Let i be ch's code point value.

3. Let j be the remainder of dividing i by 32.

4. Return the Unicode character numbered j.

The production CharacterEscape :: HexEscapeSequence evaluates by evaluating the CV of the HexEscapeSequence (see 7.8.4) and returning its character result.

The production CharacterEscape :: UnicodeEscapeSequence evaluates by evaluating the CV of the UnicodeEscapeSequence (see 7.8.4) and returning its character result.

The production CharacterEscape :: IdentityEscape evaluates by returning the character represented by IdentityEscape.

15.10.2.11 DecimalEscape

The production DecimalEscape :: DecimalIntegerLiteral [lookahead DecimalDigit] evaluates as follows.

1. Let i be the MV of DecimalIntegerLiteral.

2. If i is zero, return the EscapeValue consisting of a <NUL> character (Unicode value 0000).

3. Return the EscapeValue consisting of the integer i.

The definition of "the MV of DecimalIntegerLiteral" is in 7.8.3.

Informative comments: If \ is followed by a decimal number n whose first digit is not 0, then the escape sequence is considered to be a backreference. It is an error if n is greater than the total number of left capturing parentheses in the entire regular expression. \0 represents the NUL character and cannot be followed by a decimal digit.

15.10.2.12 CharacterClassEscape

The production CharacterClassEscape :: d evaluates by returning the ten-element set of characters containing the characters 0 through 9 inclusive.

The production CharacterClassEscape :: D evaluates by returning the set of all characters not included in the set returned by CharacterClassEscape :: d.

The production CharacterClassEscape :: s evaluates by returning the set of characters containing the characters that are on the right-hand side of the WhiteSpace (7.2) or LineTerminator (7.3) productions.

The production CharacterClassEscape :: S evaluates by returning the set of all characters not included in the set returned by CharacterClassEscape :: s.

The production CharacterClassEscape :: w evaluates by returning the set of characters containing the sixty-three characters:

a b c d e f g h i j k l m n o p q r s t u v w x y z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
0 1 2 3 4 5 6 7 8 9 _

The production CharacterClassEscape :: W evaluates by returning the set of all characters not included in the set returned by CharacterClassEscape :: w.

15.10.2.13 CharacterClass

The production CharacterClass :: [ [lookahead {^}] ClassRanges ] evaluates by evaluating ClassRanges to obtain a CharSet and returning that CharSet and the boolean false.

The production CharacterClass :: [^ ClassRanges ] evaluates by evaluating ClassRanges to obtain a CharSet and returning that CharSet and the boolean true.

15.10.2.14 ClassRanges

The production ClassRanges :: [empty] evaluates by returning the empty CharSet.

The production ClassRanges :: NonemptyClassRanges evaluates by evaluating NonemptyClassRanges to obtain a CharSet and returning that CharSet.

15.10.2.15 NonemptyClassRanges

The production NonemptyClassRanges :: ClassAtom evaluates by evaluating ClassAtom to obtain a CharSet and returning that CharSet.

The production NonemptyClassRanges :: ClassAtom NonemptyClassRangesNoDash evaluates as follows:

1. Evaluate ClassAtom to obtain a CharSet A.

2. Evaluate NonemptyClassRangesNoDash to obtain a CharSet B.

3. Return the union of CharSets A and B.

The production NonemptyClassRanges :: ClassAtom - ClassAtom ClassRanges evaluates as follows:

1. Evaluate the first ClassAtom to obtain a CharSet A.

2. Evaluate the second ClassAtom to obtain a CharSet B.

3. Evaluate ClassRanges to obtain a CharSet C.

4. Call CharacterRange(A, B) and let D be the resulting CharSet.

5. Return the union of CharSets D and C.

The internal helper function CharacterRange takes two CharSet parameters A and B and performs the following:

1. If A does not contain exactly one character or B does not contain exactly one character then throw a SyntaxError exception.

2. Let a be the one character in CharSet A.

3. Let b be the one character in CharSet B.

4. Let i be the code point value of character a.

5. Let j be the code point value of character b.

6. If i > j then throw a SyntaxError exception.

7. Return the set containing all characters numbered i through j, inclusive.

15.10.2.16 NonemptyClassRangesNoDash

The production NonemptyClassRangesNoDash :: ClassAtom evaluates by evaluating ClassAtom to obtain a CharSet and returning that CharSet.

The production NonemptyClassRangesNoDash :: ClassAtomNoDash NonemptyClassRangesNoDash evaluates as follows:

1. Evaluate ClassAtomNoDash to obtain a CharSet A.

2. Evaluate NonemptyClassRangesNoDash to obtain a CharSet B.

3. Return the union of CharSets A and B.

The production NonemptyClassRangesNoDash :: ClassAtomNoDash - ClassAtom ClassRanges evaluates as follows:

1. Evaluate ClassAtomNoDash to obtain a CharSet A.

2. Evaluate ClassAtom to obtain a CharSet B.

3. Evaluate ClassRanges to obtain a CharSet C.

4. Call CharacterRange(A, B) and let D be the resulting CharSet.

5. Return the union of CharSets D and C.

Informative comments: ClassRanges can expand into single ClassAtoms and/ or ranges of two ClassAtoms separated by dashes. In the latter case the ClassRanges includes all characters between the first ClassAtom and the second ClassAtom, inclusive; an error occurs if either ClassAtom does not represent a single character (for example, if one is \w) or if thefirstClassAtom's code point value is greater than the second ClassAtom's code point value.

Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. Thus, for example, the pattern /[E-F]/ i matches only the letters E, F, e, and f, while the pattern /[E-f]/ i matches all upper and lower-case ASCII letters as well as the symbols [, \, ], ^, _, and `.

A - character can be treated literally or it can denote a range. It is treated literally if it is the first or last character of ClassRanges, the beginning or end limit of a range specification, or immediately follows a range specification.

15.10.2.17 ClassAtom

The production ClassAtom :: - evaluates by returning the CharSet containing the one character -.

The production ClassAtom :: ClassAtomNoDash evaluates by evaluating ClassAtomNoDash to obtain a CharSet and returning that CharSet.

15.10.2.18 ClassAtomNoDash

The production ClassAtomNoDash :: SourceCharacter but not one of \ ] - evaluates by returning a one-element CharSet containing the character represented by SourceCharacter.

The production ClassAtomNoDash :: \ ClassEscape evaluates by evaluating ClassEscape to obtain a CharSet and returning that CharSet.

15.10.2.19 ClassEscape

The production ClassEscape :: DecimalEscape evaluates as follows:

1. Evaluate DecimalEscape to obtain an EscapeValue E.

2. If E is not a character then throw a SyntaxError exception.

3. Let ch be E's character.

4. Return the one-element CharSet containing the character ch.

The production ClassEscape :: b evaluates by returning the CharSet containing the one character <BS> (Unicode value 0008).

The production ClassEscape :: CharacterEscape evaluates by evaluating CharacterEscape to obtain a character and returning a one-element CharSet containing that character.

The production ClassEscape :: CharacterClassEscape evaluates by evaluating CharacterClassEscape to obtain a CharSet and returning that CharSet.

Informative comments: A ClassAtom can use any of the escape sequences that are allowed in the rest of the regular expression except for \b, \B, and backreferences. Inside a CharacterClass, \b means the backspace character, while \B and backreferences raise errors. Using a backreference inside a ClassAtom causes an error.

15.10.3 The RegExp Constructor Called as a Function
15.10.3.1 RegExp(pattern, flags)

If pattern is an object R whose [[Class]] property is "RegExp" and flags is undefined, then return R unchanged. Otherwise call the RegExp constructor (15.10.4.1), passing it the pattern and flags arguments and return the object constructed by that constructor.

15.10.4 The RegExp Constructor

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

15.10.4.1 new RegExp(pattern, flags)

If pattern is an object R whose [[Class]] property is "RegExp" and flags is undefined, then let P be the pattern used to construct R and let F be the flags used to construct R. If pattern is an object R whose [[Class]] property is "RegExp" and flags is not undefined, then throw a TypeError exception. Otherwise, let P be the empty string if pattern is undefined and ToString(pattern) otherwise, and let F be the empty string if flags is undefined and ToString(flags) otherwise.

The global property of the newly constructed object is set to a Boolean value that is true if F contains the character "g" and false otherwise.

The ignoreCase property of the newly constructed object is set to a Boolean value that is true if F contains the character "i" and false otherwise.

The multiline property of the newly constructed object is set to a Boolean value that is true if F contains the character "m" and false otherwise.

If F contains any character other than "g", "i", or" m", or if it contains the same one more than once, then throw a SyntaxError exception.

If P's characters do not have the form Pattern, then throw a SyntaxError exception. Otherwise let the newly constructed object have a [[Match]] property obtained by evaluating ("compiling") Pattern. Note that evaluating Pattern may throw a SyntaxError exception. (Note: if pattern is a StringLiteral, the usual escape sequence substitutions are performed before the string is processed by RegExp. If pattern must contain an escape sequence to be recognised by RegExp, the"\" character must be escaped within the StringLiteral to prevent its being removed when the contents of the StringLiteral are formed.)

The source property of the newly constructed object is set to an implementation-defined string value in the form of a Pattern based on P.

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

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

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

15.10.5 Properties of the RegExp Constructor

The value of the internal [[Prototype]] property of the RegExp constructor is the Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 2), the RegExp constructor has the following properties:

15.10.5.1 RegExp.prototype

The initial value of RegExp.prototype is the RegExp prototype object (15.10.6).

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

15.10.6 Properties of the RegExp Prototype Object

The value of the internal [[Prototype]] property of the RegExp prototype object is the Object prototype. The value of the internal [[Class]] property of the RegExp prototype object is "Object".

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

In the following descriptions of functions that are properties of the RegExp prototype object, the phrase "this RegExp object" refers to the object that is the this value for the invocation of the function; a TypeError exception is thrown if the this value is not an object for which the value of the internal [[Class]] property is "RegExp".

15.10.6.1 RegExp.prototype.constructor

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

15.10.6.2 RegExp.prototype.exec(string)

Performs a regular expression match of string against the regular expression and returns an Array object containing the results of the match, or null if the string did not match

The string ToString(string) is searched for an occurrence of the regular expression pattern as follows:

1. Let S be the value of ToString(string).

2. Let length be the length of S.

3. Let lastIndex be the value of the lastIndex property.

4. Let i be the value of ToInteger(lastIndex).

5. If the global property is false, let i =0.

6. If i <0 or i > length then set lastIndex to 0 and return null.

7. Call [[Match]], giving it the arguments S and i. If [[Match]] returned failure, go to step 8; otherwise let r be its State result and go to step 10.

8. Let i = i+ 1.

9. Go to step 6.

10. Let e be r's endIndex value.

11. If the global property is true, setlastIndex to e.

12. Let n be the length of r's captures array. (This is the same value as 15.10.2.1's NCapturingParens.)

13. Return a new array with the following properties:

  • The index property is set to the position of the matched substring within the complete string S.
  • The input property is set to S.
  • The length property is set to n +1.
  • The 0 property is set to the matched substring (i. e. the portion of S between offset i inclusive and offset e exclusive).
  • For each integer i such that i >0 and i <= n, set the property named ToString(i) to the ith element of r's captures array.
15.10.6.3 RegExp.prototype.test(string)

Equivalent to the expression RegExp.prototype.exec(string) != null.

15.10.6.4 RegExp.prototype.toString()

Let src be a string in the form of a Pattern representing the current regular expression. src may or may not be identical to the source property or to the source code supplied to the RegExp constructor; however, if src were supplied to the RegExp constructor along with the current regular expression's flags, the resulting regular expression must behave identically to the current regular expression.

toString returns a string value formed by concatenating the strings "/", src, and "/"; plus "g" if the global property is true, "i" if the ignoreCase property is true, and" m" if the multiline property is true.

NOTE
An implementation may choose to take advantage of src being allowed to be different from the source passed to the RegExp constructor to escape special characters in src. For example, in the regular expression obtained from new RegExp("/"), src could be, among other possibilities, "/" or "\/". The latter would permit the entire result ("/\//") of the toString call to have the form RegularExpressionLiteral.

15.10.7 Properties of RegExp Instances

RegExp instances inherit properties from their [[Prototype]] object as specified above and also have the following properties.

15.10.7.1 source

The value of the source property is string in the form of a Pattern representing the current regular expression. This property shall have the attributes { DontDelete, ReadOnly, DontEnum }.

15.10.7.2 global

The value of the global property is a Boolean value indicating whether the flags contained the character "g". This property shall have the attributes { DontDelete, ReadOnly, DontEnum }.

15.10.7.3 ignoreCase

The value of the ignoreCase property is a Boolean value indicating whether the flags contained the character "i". This property shall have the attributes { DontDelete, ReadOnly, DontEnum }.

15.10.7.4 multiline

The value of the multiline property is a Boolean value indicating whether the flags contained the character "m". This property shall have the attributes { DontDelete, ReadOnly, DontEnum }.

15.10.7.5 lastIndex

The value of the lastIndex property is an integer that specifies the string position at which to start the next match. This property shall have the attributes { DontDelete, DontEnum }.

15.11 Error Objects

Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also serve as base objects for user-defined exception classes.

15.11.1 The Error Constructor Called as a Function

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

15.11.1.1 Error (message)

The [[Prototype]] property of the newly constructed object is set to the original Error prototype object, the one that is the initial value of Error.prototype (15.11.3.1).

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

If the argument message is not undefined, the message property of the newly constructed object is set to ToString(message).

15.11.2 The Error Constructor

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

15.11.2.1 new Error (message)

The [[Prototype]] property of the newly constructed object is set to the original Error prototype object, the one that is the initial value of Error.prototype (15.11.3.1).

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

If the argument message is not undefined, the message property of the newly constructed object is set to ToString(message).

15.11.3 Properties of the Error Constructor

The value of the internal [[Prototype]] property of the Error constructor is the Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the Error constructor has the following property:

15.11.3.1 Error.prototype

The initial value of Error.prototype is the Error prototype object (15.11.4).

This property has the attributes { DontEnum, DontDelete, ReadOnly }.

15.11.4 Properties of the Error Prototype Object

The Error prototype object is itself an Error object (its [[Class]] is "Error").

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

15.11.4.1 Error.prototype.constructor

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

15.11.4.2 Error.prototype.name

The initial value of Error.prototype.name is "Error".

15.11.4.3 Error.prototype.message

The initial value of Error.prototype.message is an implementation-defined string.

15.11.4.4 Error.prototype.toString ()

Returns an implementation defined string.

15.11.5 Properties of Error Instances

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

15.11.6 Native Error Types Used in This Standard

One of the NativeError objects below is thrown when a runtime error is detected. All of these objects share the same structure, as described in 15.11.7.

15.11.6.1 EvalError

Indicates that the global function eval was used in a way that is incompatible with its definition. See 15.1.2.1.

15.11.6.2 RangeError

Indicates a numeric value has exceeded the allowable range. See 15.4.2.2, 15.4.5.1, 15.7.4.5, 15.7.4.6, and 15.7.4.7.

15.11.6.3 ReferenceError

Indicate that an invalid reference value has been detected. See 8.7.1, and 8.7.2.

15.11.6.4 SyntaxError

Indicates that a parsing error has occurred. See 15.1.2.1, 15.3.2.1, 15.10.2.5, 15.10.2.9, 15.10.2.15, 15.10.2.19, and 15.10.4.1.

15.11.6.5 TypeError

Indicates the actual type of an operand is different than the expected type. See 8.6.2, 8.6.2.6, 9.9, 11.2.2, 11.2.3, 11.8.6, 11.8.7, 15.3.4.2, 15.3.4.3, 15.3.4.4, 15.3.5.3, 15.4.4.2, 15.4.4.3, 15.5.4.2, 15.5.4.3, 15.6.4, 15.6.4.2, 15.6.4.3, 15.7.4, 15.7.4.2, 15.7.4.4, 15.9.5, 15.9.5.9, 15.9.5.27, 15.10.4.1, and 15.10.6.

15.11.6.6 URIError

Indicates that one of the global URI handling functions was used in a way that is incompatible with its definition. See 15.1.3.

15.11.7 NativeError Object Structure

When an ECMAScript implementation detects a runtime error, it throws an instance of one of the NativeError objects defined in 15.11.6. Each of these objects has the structure described below, differing only in the name used as the constructor name instead of NativeError, in thename property of the prototype object, and in the implementation-defined message property of the prototype object.

For each error object, references to NativeError in the definition should be replaced with the appropriate error object name from 15.11.6.

15.11.7.1 NativeError Constructors Called as Functions

When a NativeError constructor is called as a function rather than as a constructor, it creates and initialises a new object. A call of the object as a function is equivalent to calling it as a constructor with the same arguments.

15.11.7.2 NativeError (message)

The [[Prototype]] property of the newly constructed object is set to the prototype object for this error constructor. The [[Class]] property of the newly constructed object is set to "Error".

If the argument message is not undefined, the message property of the newly constructed object is set to ToString( message).

15.11.7.3 The NativeError Constructors

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

15.11.7.4 New NativeError (message)

The [[Prototype]] property of the newly constructed object is set to the prototype object for this NativeError constructor. The [[Class]] property of the newly constructed object is set to "Error".

If the argument message is not undefined, the message property of the newly constructed object is set to ToString( message).

15.11.7.5 Properties of the NativeError Constructors

The value of the internal [[Prototype]] property of a NativeError constructor is the Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), each NativeError constructor has the following property:

15.11.7.6 NativeError. prototype

The initial value of NativeError. prototype is a NativeError prototype object (15.11.7.7). Each NativeError constructor has a separate prototype object.

This property has the attributes { DontEnum, DontDelete, ReadOnly }.

15.11.7.7 Properties of the NativeError Prototype Objects

Each NativeError prototype object is an Error object (its [[Class]] is "Error").

The value of the internal [[Prototype]] property of each NativeError prototype object is the Error prototype object (15.11.4).

15.11.7.8 NativeError. prototype.constructor

The initial value of the constructor property of the prototype for a given NativeError constructor is the NativeError constructor function itself (15.11.7).

15.11.7.9 NativeError. prototype.name

The initial value of the name property of the prototype for a given NativeError constructor is the name of the constructor (the name used instead of NativeError).

15.11.7.10 NativeError. prototype.message

The initial value of the message property of the prototype for a given NativeError constructor is an implementation-defined string.

NOTE
The prototypes for the NativeError constructors do not themselves provide a toString function, but instances of errors will inherit it from the Error prototype object.

15.11.7.11 Properties of NativeError Instances

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

16 Errors

An implementation should report runtime errors at the time the relevant language construct is evaluated. An implementation may report syntax errors in the program at the time the program is read in, or it may, at its option, defer reporting syntax errors until the relevant statement is reached. An implementation may report syntax errors in eval code at the time eval is called, or it may, at its option, defer reporting syntax errors until the relevant statement is reached.

An implementation may treat any instance of the following kinds of runtime errors as a syntax error and therefore report it early:

  • Improper uses of return, break, and continue.
  • Using the eval property other than via a direct call.
  • Errors in regular expression literals.
  • Attempts to call PutValue on a value that is not a reference (for example, executing the assignment statement 3= 4).

An implementation shall not report other kinds of runtime errors early even if the compiler can prove that a construct cannot execute without error under any circumstances. An implementation may issue an early warning in such a case, but it should not report the error until the relevant construct is actually executed.

An implementation shall report all errors as specified, except for the following:

  • An implementation may extend program and regular expression syntax. To permit this, all operations (such as calling eval, using a regular expression literal, or using the Function or RegExp constructor) that are allowed to throw SyntaxError are permitted to exhibit implementation-defined behaviour instead of throwing SyntaxError when they encounter an implementation-defined extension to the program or regular expression syntax.
  • An implementation may provide additional types, values, objects, properties, and functions beyond those described in this specification. This may cause constructs (such as looking up a variable in the global scope) to have implementation-defined behaviour instead of throwing an error (such as ReferenceError).
  • An implementation is not required to detect EvalError. If it chooses not to detect EvalError, the implementation must allow eval to be used indirectly and/ or allow assignments to eval.
  • An implementation may define behaviour other than throwing RangeError for toFixed, toExponential, and toPrecision when the fractionDigits or precision argument is outside the specified range.
Annex A
(informative)
Grammar Summary
A.1 Lexical Grammar
SourceCharacter::See clause 6
any Unicode character
InputElementDiv ::See clause 7
WhiteSpace
LineTerminator
Comment
Token DivPunctuator
InputElementRegExp ::See clause 7
WhiteSpace
LineTerminator
Comment
Token
RegularExpressionLiteral
WhiteSpace ::See 7.2
<TAB>
<VT>
<FF>
<SP>
<NBSP>
<USP>
LineTerminator ::See 7.3
<LF>
<CR>
<LS>
<PS>
Comment ::See 7.4
MultiLineComment
SingleLineComment
MultiLineComment ::See 7.4
/* MultiLineCommentCharsopt */
MultiLineCommentChars ::See 7.4
MultiLineNotAsteriskChar MultiLineCommentCharsopt
* PostAsteriskCommentCharsopt
PostAsteriskCommentChars :: See 7.4
MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentCharsopt
* PostAsteriskCommentCharsopt
MultiLineNotAsteriskChar ::See 7.4
SourceCharacter but not asterisk *
MultiLineNotForwardSlashOrAsteriskChar ::See 7.4
SourceCharacter but not forward-slash / or asterisk *
SingleLineComment ::See 7.4
// SingleLineCommentCharsopt
SingleLineCommentChars ::See 7.4
SingleLineCommentChar SingleLineCommentCharsopt
SingleLineCommentChar ::See 7.4
SourceCharacter but not LineTerminator
Token ::See 7.5
ReservedWord
Identifier
Punctuator
NumericLiteral
StringLiteral
ReservedWord ::See 7.5.1
Keyword
FutureReservedWord
NullLiteral
BooleanLiteral
Keyword :: one ofSee 7.5.2
break else new var case finally return void catch for switch while continue function this with default if throw delete in try do instanceof typeof
FutureReservedWord :: one ofSee 7.5.3
abstract enum int short boolean export interface static byte extends long super char final native synchronized class float package throws const goto private transient debugger implements protected volatile double import public
Identifier :: See 7.6
IdentifierName but not ReservedWord
IdentifierName ::See 7.6
IdentifierStart
IdentifierName IdentifierPart
IdentifierStart ::See 7.6
UnicodeLetter
$
_

\ UnicodeEscapeSequence
IdentifierPart ::See 7.6
IdentifierStart
UnicodeCombiningMark
UnicodeDigit
UnicodeConnectorPunctuation
\ UnicodeEscapeSequence
UnicodeLetter See 7.6
any character in the Unicode categories "Uppercase letter (Lu)", "Lowercase letter (Ll)", "Titlecase letter (Lt)", "Modifier letter (Lm)", "Other letter (Lo)", or "Letter number (Nl)".
UnicodeCombiningMarkSee 7.6
any character in the Unicode categories "Non-spacing mark (Mn)" or "Combining spacing mark (Mc)"
UnicodeDigitSee 7.6
any character in the Unicode category "Decimal number (Nd)"
UnicodeConnectorPunctuation See 7.6
any character in the Unicode category "Connector punctuation (Pc)"
HexDigit :: one ofSee 7.6
0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F
Punctuator :: one ofSee 7.7
{ } ( ) [ ]
. ; , < > <=
>= == != === !==
+ - * % ++ --
<< >> >>> & | ^
! ~ && || ? :
= += -= *= %= <<=
>>= >>>= &= |= ^=
DivPunctuator :: one of See 7.7
/
/=
Literal :: See 7.8
NullLiteral
BooleanLiteral
NumericLiteral
StringLiteral
NullLiteral ::See 7.8.1
null
BooleanLiteral ::See 7.8.2
true
false
NumericLiteral ::See 7.8.3
DecimalLiteral
HexIntegerLiteral
DecimalLiteral ::See 7.8.3
DecimalIntegerLiteral . DecimalDigitsopt ExponentPartopt
. DecimalDigits ExponentPartopt
DecimalIntegerLiteral ExponentPartopt
DecimalIntegerLiteral ::See 7.8.3
0
NonZeroDigit DecimalDigitsopt
DecimalDigits ::See 7.8.3
DecimalDigit
DecimalDigits DecimalDigit
DecimalDigit :: one ofSee 7.8.3
0 1 2 3 4 5 6 7 8 9
ExponentIndicator :: one ofSee 7.8.3
e E
SignedInteger ::See 7.8.3
DecimalDigits
+ DecimalDigits
- DecimalDigits
HexIntegerLiteral :: See 7.8.3
0x HexDigit
0X HexDigit
HexIntegerLiteral HexDigit
StringLiteral ::See 7.8.4
" DoubleStringCharactersopt "
' SingleStringCharactersopt '
DoubleStringCharacters ::See 7.8.4
DoubleStringCharacter DoubleStringCharactersopt
SingleStringCharacters ::See 7.8.4
SingleStringCharacter SingleStringCharactersopt
DoubleStringCharacter ::See 7.8.4
SourceCharacter but not double-quote " or backslash \ or LineTerminator
\ EscapeSequence
SingleStringCharacter ::See 7.8.4
SourceCharacter but not single-quote ' or backslash \ or LineTerminator
\ EscapeSequence
EscapeSequence ::See 7.8.4
CharacterEscapeSequence
0 [lookahead ∉ DecimalDigit]
HexEscapeSequence
UnicodeEscapeSequence
CharacterEscapeSequence ::See 7.8.4
SingleEscapeCharacter
NonEscapeCharacter
SingleEscapeCharacter :: one ofSee 7.8.4
' " \ b f n r t v
EscapeCharacter ::See 7.8.4
SingleEscapeCharacter
DecimalDigit
x
u
HexEscapeSequence ::See 7.8.4
x HexDigit HexDigit
UnicodeEscapeSequence ::See 7.8.4
u HexDigit HexDigit HexDigit HexDigit
RegularExpressionLiteral :: See 7.8.5
/ RegularExpressionBody / RegularExpressionFlags
RegularExpressionBody ::See 7.8.5
RegularExpressionFirstChar RegularExpressionChars
RegularExpressionChars ::See 7.8.5
[empty]
RegularExpressionChars RegularExpressionChar
RegularExpressionFirstChar ::See 7.8.5
NonTerminator but not * or \ or /
BackslashSequence
RegularExpressionChar ::See 7.8.5
NonTerminator but not \ or /
BackslashSequence
BackslashSequence ::See 7.8.5
\ NonTerminator
NonTerminator ::See 7.8.5
SourceCharacter but not LineTerminator
RegularExpressionFlags ::See 7.8.5
[empty]
RegularExpressionFlags IdentifierPart
A.2 Number Conversions
StringNumericLiteral ::: See 9.3.1
StrWhiteSpaceopt
StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt
StrWhiteSpace :::See 9.3.1
StrWhiteSpaceChar StrWhiteSpaceopt
StrWhiteSpaceChar :::See 9.3.1
<TAB>
<SP>
<NBSP>
<FF>
<VT>
<CR>
<LF>
<LS>
<PS>
<USP>
StrNumericLiteral ::: See 9.3.1
StrDecimalLiteral
HexIntegerLiteral
StrDecimalLiteral :::See 9.3.1
StrUnsignedDecimalLiteral
+ StrUnsignedDecimalLiteral
- StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral :::See 9.3.1
Infinity
DecimalDigits . DecimalDigitsopt ExponentPartopt
. DecimalDigits ExponentPartopt
DecimalDigits ExponentPartopt
DecimalDigits :::See 9.3.1
DecimalDigit
DecimalDigits DecimalDigit
DecimalDigit ::: one ofSee 9.3.1
0 1 2 3 4 5 6 7 8 9
ExponentPart :::See 9.3.1
ExponentIndicator SignedInteger
ExponentIndicator ::: one ofSee 9.3.1
e E
SignedInteger :::See 9.3.1
DecimalDigits
+ DecimalDigits
- DecimalDigits
HexIntegerLiteral :::See 9.3.1
0x HexDigit
0X HexDigit
HexIntegerLiteral HexDigit
HexDigit ::: one ofSee 9.3.1
0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F
A.3 Expressions
PrimaryExpression : See 11.1
this
Identifier
Literal
ArrayLiteral
ObjectLiteral
( Expression )
ArrayLiteral : See 11.1.4
[ Elisionopt ]
[ ElementList ]
[ ElementList , Elisionopt ]
ElementList :See 11.1.4
Elisionopt AssignmentExpression
ElementList , Elisionopt AssignmentExpression
Elision :See 11.1.4
,
Elision ,
ObjectLiteral :See 11.1.5
{}
{ PropertyNameAndValueList }
PropertyNameAndValueList :See 11.1.5
PropertyName : AssignmentExpression
PropertyNameAndValueList , PropertyName : AssignmentExpression
PropertyName :See 11.1.5
Identifier
StringLiteral
NumericLiteral
MemberExpression :See 11.2
PrimaryExpression
FunctionExpression
MemberExpression [ Expression ]
MemberExpression . Identifier
new MemberExpression Arguments
NewExpression :See 11.2
MemberExpression
new NewExpression
CallExpression :See 11.2
MemberExpression Arguments
CallExpression Arguments
CallExpression [ Expression ]
CallExpression . Identifier
Arguments :See 11.2
()
( ArgumentList )
ArgumentList : See 11.2
AssignmentExpression
ArgumentList , AssignmentExpression
LeftHandSideExpression :See 11.2
NewExpression
CallExpression
PostfixExpression :See 11.3
LeftHandSideExpression
LeftHandSideExpression [no LineTerminator here] ++
LeftHandSideExpression [no LineTerminator here] --
UnaryExpression :See 11.4
PostfixExpression
delete UnaryExpression
void UnaryExpression
typeof UnaryExpression
++ UnaryExpression
-- UnaryExpression
+ UnaryExpression
- UnaryExpression
~ UnaryExpression
! UnaryExpression
MultiplicativeExpression :See 11.5
UnaryExpression
MultiplicativeExpression * UnaryExpression
MultiplicativeExpression / UnaryExpression
MultiplicativeExpression % UnaryExpression
AdditiveExpression :See 11.6
MultiplicativeExpression
AdditiveExpression + MultiplicativeExpression
AdditiveExpression - MultiplicativeExpression
ShiftExpression :See 11.7
AdditiveExpression
ShiftExpression << AdditiveExpression
ShiftExpression >> AdditiveExpression
ShiftExpression >>> AdditiveExpression
RelationalExpression :See 11.8
ShiftExpression
RelationalExpression < ShiftExpression
RelationalExpression > ShiftExpression
RelationalExpression <= ShiftExpression
RelationalExpression >= ShiftExpression
RelationalExpression instanceof ShiftExpression
RelationalExpression in ShiftExpression
RelationalExpressionNoIn : See 11.8
ShiftExpression
RelationalExpressionNoIn < ShiftExpression
RelationalExpressionNoIn > ShiftExpression
RelationalExpressionNoIn <= ShiftExpression
RelationalExpressionNoIn >= ShiftExpression
RelationalExpressionNoIn instanceof ShiftExpression
EqualityExpression :See 11.9
RelationalExpression
EqualityExpression == RelationalExpression
EqualityExpression != RelationalExpression
EqualityExpression === RelationalExpression
EqualityExpression !== RelationalExpression
EqualityExpressionNoIn :See 11.9
RelationalExpressionNoIn
EqualityExpressionNoIn == RelationalExpressionNoIn
EqualityExpressionNoIn != RelationalExpressionNoIn
EqualityExpressionNoIn === RelationalExpressionNoIn
EqualityExpressionNoIn !== RelationalExpressionNoIn
BitwiseANDExpression :See 11.10
EqualityExpression
BitwiseANDExpression & EqualityExpression
BitwiseANDExpressionNoIn :See 11.10
EqualityExpressionNoIn
BitwiseANDExpressionNoIn & EqualityExpressionNoIn
BitwiseXORExpression :See 11.10
BitwiseANDExpression
BitwiseXORExpression ^ BitwiseANDExpression
BitwiseXORExpressionNoIn :See 11.10
BitwiseANDExpressionNoIn
BitwiseXORExpressionNoIn ^ BitwiseANDExpressionNoIn
BitwiseORExpression :See 11.10
BitwiseXORExpression
BitwiseORExpression | BitwiseXORExpression
BitwiseORExpressionNoIn :See 11.10
BitwiseXORExpressionNoIn
BitwiseORExpressionNoIn | BitwiseXORExpressionNoIn
LogicalANDExpression :See 11.11
BitwiseORExpression
LogicalANDExpression && BitwiseORExpression
LogicalANDExpressionNoIn : See 11.11
BitwiseORExpressionNoIn
LogicalANDExpressionNoIn && BitwiseORExpressionNoIn
LogicalORExpression :See 11.11
LogicalANDExpression
LogicalORExpression || LogicalANDExpression
LogicalORExpressionNoIn :See 11.11
LogicalANDExpressionNoIn
LogicalORExpressionNoIn || LogicalANDExpressionNoIn
ConditionalExpression :See 11.12
LogicalORExpression
LogicalORExpression ? AssignmentExpression : AssignmentExpression
ConditionalExpressionNoIn :See 11.12
LogicalORExpressionNoIn
LogicalORExpressionNoIn ? AssignmentExpressionNoIn : AssignmentExpressionNoIn
AssignmentExpression :See 11.13
ConditionalExpression
LeftHandSideExpression AssignmentOperator AssignmentExpression
AssignmentExpressionNoIn :See 11.13
ConditionalExpressionNoIn
LeftHandSideExpression AssignmentOperator AssignmentExpressionNoIn
AssignmentOperator :one ofSee 11.13
= *= /= %= += -= <<= >>= >>>= &= ^= |=
Expression :See 11.14
AssignmentExpression
Expression , AssignmentExpression
ExpressionNoIn :See 11.14
AssignmentExpressionNoIn
ExpressionNoIn , AssignmentExpressionNoIn
A.4 Statements
Statement :See clause 12
Block
VariableStatement
EmptyStatement
ExpressionStatement
IfStatement
IterationStatement
ContinueStatement
BreakStatement
ReturnStatement
WithStatement
LabelledStatement
SwitchStatement
ThrowStatement
TryStatement
Block :See 12.1
{ StatementListopt }
StatementList :See 12.1
Statement
StatementList Statement
VariableStatement :See 12.2
var VariableDeclarationList ;
VariableDeclarationList :See 12.2
VariableDeclaration
VariableDeclarationList , VariableDeclaration
VariableDeclarationListNoIn :See 12.2
VariableDeclarationNoIn
VariableDeclarationListNoIn , VariableDeclarationNoIn
VariableDeclaration :See 12.2
Identifier Initialiseropt
VariableDeclarationNoIn :See 12.2
Identifier InitialiserNoInopt
Initialiser :See 12.2
= AssignmentExpression
InitialiserNoIn :See 12.2
= AssignmentExpressionNoIn
EmptyStatement :See 12.3
;
ExpressionStatement :See 12.4
[lookahead ∉ {{, function}] Expression ;
IfStatement :See 12.5
if ( Expression ) Statement else Statement
if ( Expression ) Statement
IterationStatement :See 12.6
do Statement while ( Expression );
while ( Expression ) Statement
for (ExpressionNoInopt; Expressionopt ; Expressionopt ) Statement
for (var VariableDeclarationListNoIn; Expressionopt ; Expressionopt ) Statement
for ( LeftHandSideExpression in Expression ) Statement
for (var VariableDeclarationNoIn in Expression ) Statement
ContinueStatement :See 12.7
continue [no LineTerminator here] Identifieropt ;
BreakStatement :See 12.8
break [no LineTerminator here] Identifieropt ;
ReturnStatement :See 12.9
return [no LineTerminator here] Expressionopt ;
WithStatement :See 12.10
with ( Expression ) Statement
SwitchStatement :See 12.11
switch ( Expression ) CaseBlock
CaseBlock :See 12.11
{ CaseClausesopt }
{ CaseClausesopt DefaultClause CaseClausesopt }
CaseClauses :See 12.11
CaseClause
CaseClauses CaseClause
CaseClause :See 12.11
case Expression : StatementListopt
DefaultClause :See 12.11
default : StatementListopt
LabelledStatement :See 12.12
Identifier : Statement
ThrowStatement : See 12.13
throw [no LineTerminator here] Expression ;
TryStatement :See 12.14
try Block Catch
try Block Finally
try Block Catch Finally
Catch :See 12.14
catch (Identifier ) Block
Finally :See 12.14
finally Block
A.5 Functions and Programs
FunctionDeclaration :See clause 13
function Identifier ( FormalParameterListopt ){ FunctionBody }
FunctionExpression :See clause 13
function Identifieropt ( FormalParameterListopt ){ FunctionBody }
FormalParameterList :See clause 13
Identifier
FormalParameterList , Identifier
FunctionBody :See clause 13
SourceElements
Program :See clause 14
SourceElements
SourceElements :See clause 14
SourceElement
SourceElements SourceElement
SourceElement :See clause 14
Statement FunctionDeclaration
A.6 Universal Resource Identifier Character Classes
uri ::: See 15.1.3
uriCharactersopt
uriCharacters :::See 15.1.3
uriCharacter uriCharactersopt
uriCharacter :::See 15.1.3
uriReserved
uriUnescaped
uriEscaped
uriReserved ::: one ofSee 15.1.3
; / ? : @ & = + $ ,
uriUnescaped :::See 15.1.3
uriAlpha
DecimalDigit
uriMark
uriEscaped :::See 15.1.3
% HexDigit HexDigit
uriAlpha ::: one ofSee 15.1.3
a b c d e f g h i j k l m n o p q r s t u v w x y z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
uriMark ::: one ofSee 15.1.3
- _ . ! ~ * ' ( )
A.7 Regular Expressions
Pattern :: See 15.10.1
Disjunction
Disjunction ::See 15.10.1
Alternative
Alternative | Disjunction
Alternative ::See 15.10.1
[empty]
Alternative Term
Term ::See 15.10.1
Assertion
Atom
Atom Quantifier
Assertion ::See 15.10.1
^
$
\ b
\ B
Quantifier ::See 15.10.1
QuantifierPrefix
QuantifierPrefix ?
QuantifierPrefix ::See 15.10.1
*
+
?
{ DecimalDigits }
{ DecimalDigits ,}
{ DecimalDigits , DecimalDigits }
Atom ::See 15.10.1
PatternCharacter
.

\ AtomEscape
CharacterClass
( Disjunction )
(?: Disjunction )
(?= Disjunction )
(?! Disjunction )

PatternCharacter ::SourceCharacter but not any of:See 15.10.1
^ $ \ . * + ? ( ) [ ] { } |
AtomEscape ::
See 15.10.1 DecimalEscape CharacterEscape CharacterClassEscape
CharacterEscape ::See 15.10.1
ControlEscape
c ControlLetter
HexEscapeSequence
UnicodeEscapeSequence
IdentityEscape
ControlEscape :: one ofSee 15.10.1
f n r t v
ControlLetter :: one ofSee 15.10.1
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
IdentityEscape ::See 15.10.1
SourceCharacter but not IdentifierPart
DecimalEscape ::See 15.10.1
DecimalIntegerLiteral [lookahead ∉ DecimalDigit]
CharacterClass ::See 15.10.1
[ [lookahead ∉ {^}] ClassRanges ]
[^ ClassRanges ]
ClassRanges ::See 15.10.1
[empty]
NonemptyClassRanges
NonemptyClassRanges ::See 15.10.1
ClassAtom
ClassAtom NonemptyClassRangesNoDash
ClassAtom - ClassAtom ClassRanges
NonemptyClassRangesNoDash ::See 15.10.1
ClassAtom>
ClassAtomNoDash NonemptyClassRangesNoDash
ClassAtomNoDash - ClassAtom ClassRanges
ClassAtom ::See 15.10.1
-
ClassAtomNoDash
ClassAtomNoDash ::See 15.10.1
SourceCharacter but not one of \ ] -
\ ClassEscape
ClassEscape ::See 15.10.1
DecimalEscape
b
CharacterEscape
CharacterClassEscape
Annex B
(informative)
Compatibility
B.1 Additional Syntax

Past editions of ECMAScript have included additional syntax and semantics for specifying octal literals and octal escape sequences. These have been removed from this edition of ECMAScript. This non-normative annex presents uniform syntax and semantics for octal literals and octal escape sequences for compatibility with some older ECMAScript programs.

B.1.1 Numeric Literals

The syntax and semantics of 7.8.3 can be extended as follows:

Syntax

NumericLiteral ::
DecimalLiteral
HexIntegerLiteral
OctalIntegerLiteral
OctalDigit :: one of
0 1 2 3 4 5 6 7
OctalIntegerLiteral ::
0 OctalDigit
OctalIntegerLiteral OctalDigit

Semantics

  • The MV of NumericLiteral :: OctalIntegerLiteral is the MV of OctalIntegerLiteral.
  • The MV of OctalDigit :: 0 is 0.
  • The MV of OctalDigit :: 1 is 1.
  • The MV of OctalDigit :: 2 is 2.
  • The MV of OctalDigit :: 3 is 3.
  • The MV of OctalDigit :: 4 is 4.
  • The MV of OctalDigit :: 5 is 5.
  • The MV of OctalDigit :: 6 is 6.
  • The MV of OctalDigit :: 7 is 7.
  • The MV of OctalIntegerLiteral :: 0 OctalDigit is the MV of OctalDigit.
  • The MV of OctalIntegerLiteral :: OctalIntegerLiteral OctalDigit is (the MV of OctalIntegerLiteral times 8) plus the MV of OctalDigit.
B.1.2 String Literals

The syntax and semantics of 7.8.4 can be extended as follows:

Syntax

EscapeSequence ::
CharacterEscapeSequence
OctalEscapeSequence
HexEscapeSequence
UnicodeEscapeSequence
OctalEscapeSequence ::
OctalDigit [lookahead ∉ DecimalDigit]
ZeroToThree OctalDigit [lookahead ∉ DecimalDigit]
FourToSeven OctalDigit
ZeroToThree OctalDigit OctalDigit
ZeroToThree :: one of
0 1 2 3
FourToSeven :: one of
4 5 6 7

Semantics

  • The CV of EscapeSequence :: OctalEscapeSequence is the CV of the OctalEscapeSequence.
  • The CV of OctalEscapeSequence :: OctalDigit [lookahead DecimalDigit] is the character whose code point value is the MV of the OctalDigit.
  • The CV of OctalEscapeSequence :: ZeroToThree OctalDigit [lookahead DecimalDigit] is the character whose code point value is (8 times the MV of the ZeroToThree) plus the MV of the OctalDigit.
  • The CV of OctalEscapeSequence :: FourToSeven OctalDigit is the character whose code point value is (8 times the MV of the FourToSeven) plus the MV of the OctalDigit.
  • The CV of OctalEscapeSequence :: ZeroToThree OctalDigit OctalDigit is the character whose code point value is (64 (that is, 82 ) times the MV of the ZeroToThree) plus (8 times the MV of the first OctalDigit) plus theMV of the second OctalDigit.
  • The MV of ZeroToThree :: 0 is 0.
  • The MV of ZeroToThree :: 1 is 1.
  • The MV of ZeroToThree :: 2 is 2.
  • The MV of ZeroToThree :: 3 is 3.
  • The MV of FourToSeven :: 4 is 4.
  • The MV of FourToSeven :: 5 is 5.
  • The MV of FourToSeven :: 6 is 6.
  • The MV of FourToSeven :: 7 is 7.
B.2 Additional Properties

Some implementations of ECMAScript have included additional properties for some of the standard native objects. This non-normative annex suggests uniform semantics for such properties without making the properties or their semantics part of this standard.

B.2.1 escape (string)

The escape function is a property of the global object. It computes a new version of a string value in which certain characters have been replaced by a hexadecimal escape sequence.

For those characters being replaced whose code point value is 0xFF or less, a two-digit escape sequence of the form %xx is used. For those characters being replaced whose code point value 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 (represented as a 16-bit unsigned integer) at position k within Result(1).

7. If Result(6) is one of the 69 nonblank characters "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@*_+-./ " then go to step 13.

8. If Result(6), is less than 256, go to step 11.

9. Let S be a string containing six characters "%uwxyz" where wxyz are four hexadecimal digits encoding the value of Result(6).

10. Go to step 14.

11. Let S be a string containing three characters "%xy" where xy are two hexadecimal digits encoding the value of Result(6).

12. Go to step 14.

13. Let S be a string containing the single character Result(6).

14. Let R be a new string value computed by concatenating the previous value of R and S.

15. Increase k by 1. 16. 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.

B.2.2 unescape (string)

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

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

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 code point value 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 code point value 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.

B.2.3 String.prototype.substr (start, length)

The substr method takes two arguments, start and length, and returns a substring of the result of converting this object to a string, starting from character position start and running for length characters (or through the end of the string is length is undefined). If start is negative, it is treated as (sourceLength+ start) where sourceLength is the length of the string. The result is a string value, not a String object. The following steps are taken:

1. Call ToString, giving it the this value as its argument.

2. Call ToInteger(start).

3. If length is undefined, use +∞ ; otherwise call ToInteger(length).

4. Compute the number of characters in Result(1).

5. If Result(2) is positive or zero, use Result(2); else use max(Result(4)+ Result(2), 0).

6. Compute min(max(Result(3), 0), Result(4)- Result(5)).

7. If Result(6) <= 0, return the empty string "".

8. Return a string containing Result(6) consecutive characters from Result(1) beginning with the character at position Result(5).

The length property of the substr method is 2.

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

B.2.4 Date.prototype.getYear()

NOTE
The getFullYear method is preferred for nearly all purposes, because it avoids the "year 2000 problem."

When the getYear method is called with no arguments the following steps are taken:

1. Let t be this time value.

2. If t is NaN, return NaN.

3. Return YearFromTime(LocalTime(t)) 1900.

B.2.5 Date.prototype.setYear(year)

NOTE
The setFullYear method is preferred for nearly all purposes, because it avoids the "year 2000 problem."

When the setYear method is called with one argument year the following steps are taken:

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.

B.2.6 Date.prototype.toGMTString()

NOTE
The property toUTCString is preferred. The toGMTString property is provided principally for compatibility with old code. It is recommended that the toUTCString property be used in new ECMAScript code.

The Function object that is the initial value of Date.prototype.toGMTString is the same Function object that is the initial value of Date.prototype.toUTCString.

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