JavaScript reference

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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) {

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
JavaScript 24 Firefox 24 Gecko 24
JavaScript 31 Firefox 31 Gecko 31

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



Additions to the ECMAScript core for language-sensitive functionalities.


Functions and function scope


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 functions enable writing iterators more easily.
A block statement is used to group zero or more statements. The block is delimited by a pair of curly brackets.
Terminates the current loop, switch, or label statement and transfers program control to the statement following the terminated statement.
Declares a read-only named constant.
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.
Invokes any available debugging functionality. If no debugging functionality is available, this statement has no effect.
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.
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.
Creates a loop that consists of three optional expressions, enclosed in parentheses and separated by semicolons, followed by a statement executed in the loop.
Iterates a specified variable over all values of object's properties. For each distinct property, a specified statement is executed.
Iterates over the enumerable properties of an object, in arbitrary order. For each distinct property, statements can be executed.
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.
Declares a function with the specified parameters.
Executes a statement if a specified condition is true. If the condition is false, another statement can be executed.
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.
Provides a statement with an identifier that you can refer to using a break or continue statement.
Declares a block scope local variable, optionally initializing it to a value.
Specifies the value to be returned by a function.
Evaluates an expression, matching the expression's value to a case clause, and executes statements associated with that case.
Throws a user-defined exception.
Marks a block of statements to try, and specifies a response, should an exception be thrown.
Declares a variable, optionally initializing it to a value.
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.
Extends the scope chain for a statement.
See New_in_JavaScript 1.7 & Iterators and generators

Expressions and operators

For an alphabetical listing see the sidebar on the left.

Primary expressions

Basic keywords and general expressions in JavaScript.

{{jsxref("Operators/this", "this")}}
The this keyword refers to the function's execution context.
{{jsxref("Operators/function", "function")}}
The function keyword defines a function expression.
{{jsxref("Global_Objects/Array", "[]")}}
Array literal syntax.
{{jsxref("Global_Objects/Object", "{}")}}
Object literal syntax.
{{jsxref("Global_Objects/RegExp", "/ab+c/i")}}
Regular expression literal syntax.
{{experimental_inline()}} {{jsxref("Operators/Array_comprehensions", "[for (x of y) x]")}}
Array comprehensions.
{{experimental_inline()}} {{jsxref("Operators/Generator_comprehensions", "(for (x of y) y)")}}
Generator comprehensions.
{{jsxref("Operators/Grouping", "( )")}}
Grouping operator.

Left-hand-side expressions

Left values are the destination of an assignment.

{{jsxref("Operators/Property_accessors", "Property accessors", "", 1)}}
Member operators provide access to a property or method of an object
( and object["property"]).
{{jsxref("Operators/new", "new")}}
The new operator creates an instance of a constructor.
{{experimental_inline()}} {{jsxref("Operators/super", "super")}}
The super keyword calls the parent constructor.
{{experimental_inline()}} {{jsxref("Operators/Spread_operator", "...obj")}}
The spread operator allows an expression to be expanded in places where multiple arguments (for function calls) or multiple elements (for array literals) are expected.

Increment and decrement

Postfix/prefix increment and postfix/prefix decrement operators.

{{jsxref("Operators/Arithmetic_Operators", "A++", "#Increment")}}
Postfix increment operator.
{{jsxref("Operators/Arithmetic_Operators", "A--", "#Decrement")}}
Postfix decrement operator.
{{jsxref("Operators/Arithmetic_Operators", "++A", "#Increment")}}
Prefix increment operator.
{{jsxref("Operators/Arithmetic_Operators", "--A", "#Decrement")}}
Prefix decrement operator.

Unary operators

A unary operation is operation with only one operand.

{{jsxref("Operators/delete", "delete")}}
The delete operator deletes a property from an object.
{{jsxref("Operators/void", "void")}}
The void operator discards an expression's return value.
{{jsxref("Operators/typeof", "typeof")}}
The typeof operator determines the type of a given object.
{{jsxref("Operators/Arithmetic_Operators", "+", "#Unary_plus")}}
The unary plus operator converts its operand to Number type.
{{jsxref("Operators/Arithmetic_Operators", "-", "#Unary_negation")}}
The unary negation operator converts its operand to Number type and then negates it.
{{jsxref("Operators/Bitwise_Operators", "~", "#Bitwise_NOT")}}
Bitwise NOT operator.
{{jsxref("Operators/Logical_Operators", "!", "#Logical_NOT")}}
Logical NOT operator.

Arithmetic operators

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

{{jsxref("Operators/Arithmetic_Operators", "+", "#Addition")}}
Addition operator.
{{jsxref("Operators/Arithmetic_Operators", "-", "#Subtraction")}}
Subtraction operator.
{{jsxref("Operators/Arithmetic_Operators", "/", "#Division")}}
Division operator.
{{jsxref("Operators/Arithmetic_Operators", "*", "#Multiplication")}}
Multiplication operator.
{{jsxref("Operators/Arithmetic_Operators", "%", "#Remainder")}}
Remainder operator.

Relational operators

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

{{jsxref("Operators/in", "in")}}
The in operator determines whether an object has a given property.
{{jsxref("Operators/instanceof", "instanceof")}}
The instanceof operator determines whether an object is an instance of another object.
{{jsxref("Operators/Comparison_Operators", "<", "#Less_than_operator")}}
Less than operator.
{{jsxref("Operators/Comparison_Operators", ">", "#Greater_than_operator")}}
Greater than operator.
{{jsxref("Operators/Comparison_Operators", "<=", "#Less_than_or_equal_operator")}}
Less than or equal operator.
{{jsxref("Operators/Comparison_Operators", ">=", "#Greater_than_or_equal_operator")}}
Greater than or equal operator.

Equality operators

The result of evaluating an equality operator is always of type Boolean based on whether the comparison is true.

{{jsxref("Operators/Comparison_Operators", "==", "#Equality")}}
Equality operator.
{{jsxref("Operators/Comparison_Operators", "!=", "#Inequality")}}
Inequality operator.
{{jsxref("Operators/Comparison_Operators", "===", "#Identity")}}
Identity operator.
{{jsxref("Operators/Comparison_Operators", "!==", "#Nonidentity")}}
Nonidentity operator.

Bitwise shift operators

Operations to shift all bits of the operand.

{{jsxref("Operators/Bitwise_Operators", "<<", "#Left_shift")}}
Bitwise left shift operator.
{{jsxref("Operators/Bitwise_Operators", ">>", "#Right_shift")}}
Bitwise right shift operator.
{{jsxref("Operators/Bitwise_Operators", ">>>", "#Unsigned_right_shift")}}
Bitwise unsigned right shift operator.

Binary bitwise operators

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

{{jsxref("Operators/Bitwise_Operators", "&", "#Bitwise_AND")}}
Bitwise AND.
{{jsxref("Operators/Bitwise_Operators", "|", "#Bitwise_OR")}}
Bitwise OR.
{{jsxref("Operators/Bitwise_Operators", "^", "#Bitwise_XOR")}}
Bitwise XOR.

Binary logical operators

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

{{jsxref("Operators/Logical_Operators", "&&", "#Logical_AND")}}
Logical AND.
{{jsxref("Operators/Logical_Operators", "||", "#Logical_OR")}}
Logical OR.

Conditional (ternary) operator

{{jsxref("Operators/Conditional_Operator", "(condition ? ifTrue : ifFalse)")}}

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

Assignment operators

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

{{jsxref("Operators/Assignment_Operators", "=", "#Assignment")}}
Assignment operator.
{{jsxref("Operators/Assignment_Operators", "*=", "#Multiplication_assignment")}}
Multiplication assignment.
{{jsxref("Operators/Assignment_Operators", "/=", "#Division_assignment")}}
Division assignment.
{{jsxref("Operators/Assignment_Operators", "%=", "#Remainder_assignment")}}
Remainder assignment.
{{jsxref("Operators/Assignment_Operators", "+=", "#Addition_assignment")}}
Addition assignment.
{{jsxref("Operators/Assignment_Operators", "-=", "#Subtraction_assignment")}}
Subtraction assignment
{{jsxref("Operators/Assignment_Operators", "<<=", "#Left_shift_assignment")}}
Left shift assignment.
{{jsxref("Operators/Assignment_Operators", ">>=", "#Right_shift_assignment")}}
Right shift assignment.
{{jsxref("Operators/Assignment_Operators", ">>>=", "#Unsigned_right_shift_assignment")}}
Unsigned right shift assignment.
{{jsxref("Operators/Assignment_Operators", "&=", "#Bitwise_AND_assignment")}}
Bitwise AND assignment.
{{jsxref("Operators/Assignment_Operators", "^=", "#Bitwise_XOR_assignment")}}
Bitwise XOR assignment.
{{jsxref("Operators/Assignment_Operators", "|=", "#Bitwise_OR_assignment")}}
Bitwise OR assignment.
{{experimental_inline()}} {{jsxref("Operators/Destructuring_assignment", "[a, b] = [1, 2]")}}
{{experimental_inline()}} {{jsxref("Operators/Destructuring_assignment", "{a, b} = {a:1, b:2}")}}

Destructuring assignment allows you to assign the properties of an array or object to variables using syntax that looks similar to array or object literals.

Comma operator

{{jsxref("Operators/Comma_Operator", ",")}}

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

Lexical Grammar

Appendix - Deprecated Features

Document Tags and Contributors

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