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    Reference

    About this Reference

    The JavaScript Reference serves as a repository of facts about the JavaScript language. The entire language is described here in detail. As you write JavaScript code, you'll refer to these pages often (thus the title "JavaScript Reference"). If you're learning JavaScript, or need help understanding some of its capabilities or features, check out the JavaScript Guide

    What you should already know

    The JavaScript language is intended to be used within some larger environment, be it a browser, server-side scripts, or similar. For the most part, this reference attempts to be environment-agnostic and does not target a web browser environment. For demonstration purposes, this reference uses a function, println, which is not part of JavaScript and can be mapped to environment-specific functionality to display given values. For example, in a web browser println might have been defined as follows:

    function println(string) {
      console.log(string);
    }

    Mapping to equivalent functionality in other environments is left as an exercise for the reader.

    Formatting conventions

    This reference includes descriptive syntax sections to demonstrate appropriate or common usage of the subject of documentation. Within these sections, all text literals to be reproduced verbatim are non-italicized, with the exception of ellipses. Words in italics represent user-defined names or statements. Any portions enclosed in square brackets ([ and ]) are optional. A comma-delimited sequence that includes an ellipsis (...) indicates that the sequence is a list and all items in the sequence except the first are optional (e.g. only param1 is required in "param1, param2, ..., paramN").

    JavaScript history

    Recent versions of Mozilla-based browsers support newer versions of JavaScript. The following table lists the JavaScript version supported by different Mozilla-based browser versions.

    Browsers that do not support at least JavaScript 1.5 are very rare today, since JavaScript 1.5 was introduced back in 1999. If you're interested in historic information, please refer to the Wikipedia article on ECMAScript.

    JavaScript/Browser support history

    JavaScript (SpiderMonkey) version Mozilla release Gecko version
    JavaScript 1.5 Navigator 6.0, Mozilla Application Suite, Firefox 1.0 Gecko 0.6x-1.7
    JavaScript 1.6 Firefox 1.5 Gecko 1.8
    JavaScript 1.7 Firefox 2 Gecko 1.8.1
    JavaScript 1.8 Firefox 3 Gecko 1.9
    JavaScript 1.8.5 Firefox 4 Gecko 2.0
    JavaScript 1.8.6 Firefox 17 Gecko 17

    Where to find JavaScript information

    JavaScript documentation of core language features (pure ECMAScript, for the most part) includes the following:

    If you are new to JavaScript, start with the Guide. Once you have a firm grasp of the fundamentals, you can use the Reference to get more details on individual objects and language constructs.

    Global Objects

    Value properties

    Global properties returning a simple value.

    Function properties

    Global functions returning the result of a specific routine.

    Fundamental objects

    General language objects, functions and errors.

    Numbers and dates

    Objects dealing with numbers, dates and mathematical calculations.

    Text processing

    Objects for manipulating texts.

    Indexed collections

    Collections ordered by an index. Array-type objects.

    Keyed collections

    Collections of objects as keys. Elements iterable in insertion order.

    Structured data

    Data buffers and JavaScript Object Notation.

    Control abstraction objects

    Reflection

    Internationalization

    Additions to the ECMAScript core for language-sensitive functionalities.

    Other

    Functions and function scope

    Statements

    JavaScript statements consist of keywords used with the appropriate syntax. A single statement may span multiple lines. Multiple statements may occur on a single line if each statement is separated by a semicolon. This isn't a keyword, but a group of keywords.

    Generators
    Generators functions enable writing iterators more easily.
    block
    A block statement is used to group zero or more statements. The block is delimited by a pair of curly brackets.
    break
    Terminates the current loop, switch, or label statement and transfers program control to the statement following the terminated statement.
    const
    Declares a read-only named constant.
    continue
    Terminates execution of the statements in the current iteration of the current or labelled loop, and continues execution of the loop with the next iteration.
    debugger
    Invokes any available debugging functionality. If no debugging functionality is available, this statement has no effect.
    do...while
    Creates a loop that executes a specified statement until the test condition evaluates to false. The condition is evaluated after executing the statement, resulting in the specified statement executing at least once.
    export
    Allows a signed script to provide properties, functions, and objects to other signed or unsigned scripts. This feature is not in ECMA-262, Edition 3.
    for
    Creates a loop that consists of three optional expressions, enclosed in parentheses and separated by semicolons, followed by a statement executed in the loop.
    for each...in
    Iterates a specified variable over all values of object's properties. For each distinct property, a specified statement is executed.
    for...in
    Iterates over the enumerable properties of an object, in arbitrary order. For each distinct property, statements can be executed.
    for...of
    Iterates over iterable objects (including arrays, array-like objects, iterators and generators), invoking a custom iteration hook with statements to be executed for the value of each distinct property.
    function
    Declares a function with the specified parameters.
    if...else
    Executes a statement if a specified condition is true. If the condition is false, another statement can be executed.
    import
    Allows a script to import properties, functions, and objects from a signed script that has exported the information. This feature is not in ECMA 262, Edition 3.
    label
    Provides a statement with an identifier that you can refer to using a break or continue statement.
    let
    Declares a block scope local variable, optionally initializing it to a value.
    return
    Specifies the value to be returned by a function.
    switch
    Evaluates an expression, matching the expression's value to a case clause, and executes statements associated with that case.
    throw
    Throws a user-defined exception.
    try...catch
    Marks a block of statements to try, and specifies a response, should an exception be thrown.
    var
    Declares a variable, optionally initializing it to a value.
    while
    Creates a loop that executes a specified statement as long as the test condition evaluates to true. The condition is evaluated before executing the statement.
    with
    Extends the scope chain for a statement.
    yield
    See New_in_JavaScript 1.7 & Iterators and generators

    Operators and other keywords

    There are several special operators that do not fit into any other category:

    Arithmetic Operators

    (+, -, *, /, %, ++, --, unary -, unary +)

    Arithmetic operators take numerical values (either literals or variables) as their operands and return a single numerical value.

    Assignment Operators

    (=, *=, /=, %=, +=, -=, <<=, >>=, >>>=, &=, ^=, |=)

    An assignment operator assigns a value to its left operand based on the value of its right operand.

    Bitwise Operators

    (&, |, ^, ~, <<, >>, >>>)

    Bitwise operators treat their operands as a set of 32 bits (zeros and ones) and return standard JavaScript numerical values.

    Comparison Operators

    (==, !=, ===, !==, >, >=, <, <=)

    A comparison operator compares its operands and returns a logical value based on whether the comparison is true.

    Logical Operators

    (&&, ||, !)

    Logical operators are typically used with boolean (logical) values, and when they are, they return a boolean value.

    String Operators

    (+ and +=)

    The string operators concatenate two string values together, returning another string that is the union of the two strings.

    Member Operators

    (object.property and object["property"])

    Member operators provide access to a property or method of an object.

    Special Operators
    Conditional Operator

    (condition ? ifTrue : ifFalse)

    The conditional operator returns one of two values based on the logical value of the condition.

    Comma Operator

    (,)

    The comma operator allows multiple expressions to be evaluated in a single statement and returns the result of the last expression.

    delete Operator

    (delete)

    The delete operator deletes a property from an object.

    in Operator

    (in)

    The in operator determines whether an object has a given property.

    instanceof Operator

    (instanceof)

    The instanceof operator determines whether an object is an instance of another object.

    new Operator

    (new)

    The new operator creates an instance of a constructor.

    typeof Operator

    (typeof)

    The typeof operator determines the type of a given object.

    void Operator

    (void)

    The void operator discards an expression's return value.

    yield Operator

    (yield)

    The yield operator determines what is returned in a generator by that generator's iterator.

    Operator Precedence
    Operator precedence defines the order in which operators are evaluated.
    Keywords
    get Modifier

    (get)

    The get modifier defines a property in an object literal to be a getter.

    set Modifier

    (set)

    The set modifier defines a property in an object literal to be a setter.

    function Keyword

    (function)

    The function keyword defines a function expression.

    this Keyword

    (this)

    The this keyword refers to the function's execution context.

    Comments

    E4X (extension)

    Global statements:

    Global functions:

    Global constructors:

    Appendix A - Reserved Words

    Appendix B - Deprecated Features

    Document Tags and Contributors

    Contributors to this page: Sheppy
    Last updated by: Sheppy,