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.

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.

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.

Structure of the reference

In the JavaScript reference you can find the following chapters:

Standard built-in objects
This chapter documents all the JavaScript standard built-in objects, along with their methods and properties.
Statements and declarations
JavaScript applications consist of statements with an 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.
Expressions and operators
This chapter documents all the JavaScript language operators, expressions and keywords.
Functions
Chapter about JavaScript functions.
Classes
Chapter about JavaScript classes introduced in ECMAScript 2015.
Errors
Chapter about specific errors, exceptions and warnings thrown by JavaScript.
New in JavaScript
Chapter about JavaScript version history.

More reference pages

Global Objects

Functions and function scope

Statements

JavaScript applications consist of statements with an 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.

Statements and declarations by category

For an alphabetical listing see the sidebar on the left.

Control flow

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.
continue
Terminates execution of the statements in the current iteration of the current or labeled loop, and continues execution of the loop with the next iteration.
Empty
An empty statement is used to provide no statement, although the JavaScript syntax would expect one.
if...else
Executes a statement if a specified condition is true. If the condition is false, another statement can be executed.
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.

Declarations

var
Declares a variable, optionally initializing it to a value.
let
Declares a block scope local variable, optionally initializing it to a value.
const
Declares a read-only named constant.

Functions and classes

function
Declares a function with the specified parameters.
function*
Generators functions enable writing iterators more easily.
async function
Declares an async function with the specified parameters.
return
Specifies the value to be returned by a function.
class
Declares a class.

Iterations

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.
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.
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.

Others

debugger
Invokes any available debugging functionality. If no debugging functionality is available, this statement has no effect.
export
Used to export functions to make them available for imports in external modules, another scripts.
import
Used to import functions exported from an external module, another script.
label
Provides a statement with an identifier that you can refer to using a break or continue statement.
with
Extends the scope chain for a statement.

Specifications

Specification Status Comment
ECMAScript 1st Edition (ECMA-262)
The definition of 'Statements' in that specification.
Standard Initial definition
ECMAScript 3rd Edition (ECMA-262)
The definition of 'Statements' in that specification.
Standard  
ECMAScript 5.1 (ECMA-262)
The definition of 'Statements' in that specification.
Standard  
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'ECMAScript Language: Statements and Declarations' in that specification.
Standard New: function*, let, for...of, yield, class
ECMAScript Latest Draft (ECMA-262)
The definition of 'ECMAScript Language: Statements and Declarations' in that specification.
Living Standard  

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Legacy generator function No No2 — 58 No No No
async function55 Yes52 No4210.1
block Yes Yes Yes Yes Yes Yes
break Yes Yes Yes Yes Yes Yes
class42 Yes45 No ?10.1
const21 Yes361 211 Yes5.1
continue Yes Yes Yes Yes Yes Yes
debugger Yes Yes Yes Yes Yes Yes
do...while Yes Yes Yes6 Yes Yes
Empty statement (;) Yes Yes Yes Yes Yes Yes
export61153544 No4710.1
for Yes Yes Yes Yes Yes Yes
for each...in No No1.5 — 57 No No No
for...in Yes Yes Yes6 Yes Yes
for...of3812135 No258
function Yes Yes Yes Yes Yes Yes
function*391326 No2610
if...else Yes Yes Yes Yes Yes Yes
import61153544 No4710.1
label Yes Yes Yes Yes Yes Yes
let4112446 7 8111710
return Yes Yes Yes Yes Yes Yes
switch Yes Yes Yes Yes Yes Yes
throw Yes Yes Yes Yes Yes Yes
try...catch Yes Yes Yes6 Yes Yes
var Yes Yes Yes Yes Yes Yes
while Yes Yes Yes Yes Yes Yes
with Yes Yes Yes Yes Yes Yes
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Legacy generator function No No No4 — 58 No No No
async function Yes55 Yes52 No4210.1
block Yes Yes Yes Yes Yes Yes Yes
break Yes Yes Yes Yes Yes Yes Yes
class4242 Yes45 No ?10.1
const Yes Yes Yes361 211 Yes Yes
continue Yes Yes Yes Yes Yes Yes Yes
debugger Yes Yes Yes Yes Yes Yes Yes
do...while Yes Yes Yes Yes Yes Yes Yes
Empty statement (;) Yes Yes Yes Yes Yes Yes Yes
export No61 Yes544 No4710.1
for Yes Yes Yes Yes Yes Yes Yes
for each...in No No No4 — 57 No No No
for...in Yes Yes Yes Yes Yes Yes Yes
for...of5.1 Yes12 Yes No258
function Yes Yes Yes Yes Yes Yes Yes
function* Yes39 Yes26 No Yes10
if...else Yes Yes Yes Yes Yes Yes Yes
import No61 Yes544 No4710.1
label Yes Yes Yes Yes Yes Yes Yes
let414112446 7 8 ?1710
return Yes Yes Yes Yes Yes Yes Yes
switch Yes Yes Yes Yes Yes Yes Yes
throw Yes Yes Yes Yes Yes Yes Yes
try...catch Yes Yes Yes Yes Yes Yes Yes
var Yes Yes Yes Yes Yes Yes Yes
while Yes Yes Yes Yes Yes Yes Yes
with Yes Yes Yes Yes Yes Yes Yes

1. Prior to Firefox 13, const is implemented, but re-assignment is not failing.

2. Prior to Firefox 46, a TypeError was thrown on redeclaration instead of a SyntaxError.

3. From version 15: this feature is behind the Experimental JavaScript Features preference.

4. From version 54: this feature is behind the dom.moduleScripts.enabled preference. To change preferences in Firefox, visit about:config.

5. Prior to Firefox 51, using the for...of loop construct with the const keyword threw a SyntaxError ("missing = in const declaration").

6. Prior to Firefox 44, let is only available to code blocks in HTML wrapped in a <script type="application/javascript;version=1.7"> block (or higher version) and has different semantics (e.g. no temporal dead zone).

7. Prior to Firefox 46, a TypeError is thrown on redeclaration instead of a SyntaxError.

8. Firefox 54 adds support of let in workers.

See also

Operators and other keywords

This chapter documents all the JavaScript language operators, expressions and keywords.

Expressions and operators by category

For an alphabetical listing see the sidebar on the left.

Primary expressions

Basic keywords and general expressions in JavaScript.

this
The this keyword refers to the function's execution context.
function
The function keyword defines a function expression.
class
The class keyword defines a class expression.
function*
The function* keyword defines a generator function expression.
yield
Pause and resume a generator function.
yield*
Delegate to another generator function or iterable object.
async function*
The async function defines an async function expression.
await
Pause and resume an async function and wait for the promise's resolution/rejection.
[]
Array initializer/literal syntax.
{}
Object initializer/literal syntax.
/ab+c/i
Regular expression literal syntax.
( )
Grouping operator.

Left-hand-side expressions

Left values are the destination of an assignment.

Property accessors
Member operators provide access to a property or method of an object
(object.property and object["property"]).
new
The new operator creates an instance of a constructor.
new.target
In constructors, new.target refers to the constructor that was invoked by new.
super
The super keyword calls the parent constructor.
...obj
Spread syntax 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.

A++
Postfix increment operator.
A--
Postfix decrement operator.
++A
Prefix increment operator.
--A
Prefix decrement operator.

Unary operators

A unary operation is operation with only one operand.

delete
The delete operator deletes a property from an object.
void
The void operator discards an expression's return value.
typeof
The typeof operator determines the type of a given object.
+
The unary plus operator converts its operand to Number type.
-
The unary negation operator converts its operand to Number type and then negates it.
~
Bitwise NOT operator.
!
Logical NOT operator.

Arithmetic operators

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

+
Addition operator.
-
Subtraction operator.
/
Division operator.
*
Multiplication operator.
%
Remainder operator.
**
Exponentiation operator.

Relational operators

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

in
The in operator determines whether an object has a given property.
instanceof
The instanceof operator determines whether an object is an instance of another object.
<
Less than operator.
>
Greater than operator.
<=
Less than or equal operator.
>=
Greater than or equal operator.

Note: => is not an operator, but the notation for Arrow functions.

Equality operators

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

==
Equality operator.
!=
Inequality operator.
===
Identity operator.
!==
Nonidentity operator.

Bitwise shift operators

Operations to shift all bits of the operand.

<<
Bitwise left shift operator.
>>
Bitwise right shift operator.
>>>
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.

&
Bitwise AND.
|
Bitwise OR.
^
Bitwise XOR.

Binary logical operators

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

&&
Logical AND.
||
Logical OR.

Conditional (ternary) 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.

=
Assignment operator.
*=
Multiplication assignment.
/=
Division assignment.
%=
Remainder assignment.
+=
Addition assignment.
-=
Subtraction assignment
<<=
Left shift assignment.
>>=
Right shift assignment.
>>>=
Unsigned right shift assignment.
&=
Bitwise AND assignment.
^=
Bitwise XOR assignment.
|=
Bitwise OR assignment.
[a, b] = [1, 2]
{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

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

Non-standard features

Legacy generator function
The function keyword can be used to define a legacy generator function inside an expression. To make the function a legacy generator, the function body should contains at least one yield expression.
Expression closures
The expression closure syntax is a shorthand for writing simple function.
[for (x of y) x]
Array comprehensions.
(for (x of y) y)
Generator comprehensions.

Specifications

Specification Status Comment
ECMAScript 1st Edition (ECMA-262)
The definition of 'Expressions' in that specification.
Standard Initial definition
ECMAScript 5.1 (ECMA-262)
The definition of 'Expressions' in that specification.
Standard  
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'ECMAScript Language: Expressions' in that specification.
Standard New: Spread operator, destructuring assignment, super keyword.
ECMAScript Latest Draft (ECMA-262)
The definition of 'ECMAScript Language: Expressions' in that specification.
Living Standard  

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Array comprehensions No No30 No No No
async function expression55 ?52 No4210.1
await55 Yes52 ?4210.1
class42 Yes45 No Yes Yes
Comma operator Yes Yes Yes3 Yes Yes
Conditional operator (c ? t : f) Yes Yes Yes Yes Yes Yes
delete Yes Yes Yes Yes Yes Yes
Destructuring assignment4914411 No Yes8
Expression closures No No Yes No No No
function Yes Yes Yes Yes Yes Yes
function* Yes Yes26 No Yes10
Generator comprehensions No No30 No No No
Grouping operator () Yes Yes Yes Yes Yes Yes
in Yes Yes Yes Yes Yes Yes
instanceof Yes Yes Yes Yes Yes Yes
Legacy generator function No No2 — 58 No No No
new Yes Yes Yes Yes Yes Yes
new.target46 Yes41 No Yes Yes
Object initializer1 Yes11 Yes1
Pipeline operator (|>) No No582 No No No
Property accessors Yes Yes Yes Yes Yes Yes
super42 Yes45 No Yes Yes
this Yes Yes Yes Yes Yes Yes
typeof Yes Yes Yes Yes Yes Yes
void Yes Yes Yes Yes Yes Yes
yield39 Yes263 No Yes10
yield* Yes Yes273 No Yes10
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Array comprehensions No No No30 No No No
async function expression5555 ?52 No4210.1
await5555 Yes52 ?4210.1
class4242 Yes45 No Yes Yes
Comma operator Yes Yes Yes Yes Yes Yes Yes
Conditional operator (c ? t : f) Yes Yes Yes Yes Yes Yes Yes
delete Yes Yes Yes Yes Yes Yes Yes
Destructuring assignment494914411 No Yes8
Expression closures No No No Yes No No No
function Yes Yes Yes Yes Yes Yes Yes
function* Yes Yes Yes26 No Yes10
Generator comprehensions No No No30 No No No
Grouping operator () Yes Yes Yes Yes Yes Yes Yes
in Yes Yes Yes Yes Yes Yes Yes
instanceof Yes Yes Yes Yes Yes Yes Yes
Legacy generator function No No No4 — 58 No No No
new Yes Yes Yes Yes Yes Yes Yes
new.target4646 Yes41 No Yes Yes
Object initializer11 Yes41 Yes1
Pipeline operator (|>) No No No582 No No No
Property accessors Yes Yes Yes Yes Yes Yes Yes
super4242 Yes45 No Yes Yes
this Yes Yes Yes Yes Yes Yes Yes
typeof Yes Yes Yes Yes Yes Yes Yes
void Yes Yes Yes Yes Yes Yes Yes
yield3939 Yes263 No Yes10
yield* Yes Yes Yes273 No Yes10

1. Firefox provided a non-standard destructuring implementation from Firefox 2 to 40.

2. From version 58: this feature is behind the --enable-pipeline-operator compile flag.

3. Starting with Firefox 33, the parsing of the yield expression has been updated to conform with the ES2015 specification.

See also

Comments

E4X (extension)

Global statements:

Global functions:

Global constructors:

Appendix A - Reserved Words

Appendix B - Deprecated Features

Document Tags and Contributors

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