JavaScript

This part of the JavaScript section on MDN serves as a repository of facts about the JavaScript language. Read more about this reference.

Global objects

This chapter documents all the JavaScript standard built-in objects, along with their methods and properties.

Value properties

These global properties return a simple value; they have no properties or methods.

• Infinity
• NaN
• undefined
• null literal

Function properties

These global functions—functions which are called globally rather than on an object—directly return their results to the caller.

• eval()
• uneval()
• isFinite()
• isNaN()
• parseFloat()
• parseInt()
• decodeURI()
• decodeURIComponent()
• encodeURI()
• encodeURIComponent()
• escape()
• unescape()

Fundamental objects

These are the fundamental, basic objects upon which all other objects are based. This includes objects that represent general objects, functions, and errors.

• Object
• Function
• Boolean
• Symbol
• Error
• EvalError
• InternalError
• RangeError
• ReferenceError
• SyntaxError
• TypeError
• URIError

Numbers and dates

These are the base objects representing numbers, dates, and mathematical calculations.

• Number
• Math
• Date

Text processing

These objects represent strings and support manipulating them.

• String
• RegExp

Indexed collections

These objects represent collections of data which are ordered by an index value. This includes (typed) arrays and array-like constructs.

• Array
• Int8Array
• Uint8Array
• Uint8ClampedArray
• Int16Array
• Uint16Array
• Int32Array
• Uint32Array
• Float32Array
• Float64Array

Keyed collections

These objects represent collections which use keys; these contain elements which are iterable in the order of insertion.

• Map
• Set
• WeakMap
• WeakSet

Structured data

These objects represent and interact with structured data buffers and data coded using JavaScript Object Notation (JSON).

• ArrayBuffer
• SharedArrayBuffer
• Atomics
• DataView
• JSON

Control abstraction objects

• Promise
• Generator
• GeneratorFunction
• AsyncFunction

Reflection

• Reflect
• Proxy

Internationalization

Additions to the ECMAScript core for language-sensitive functionalities.

• Intl
• Intl.Collator
• Intl.DateTimeFormat
• Intl.NumberFormat

WebAssembly

• WebAssembly
• WebAssembly.Module
• WebAssembly.Instance
• WebAssembly.Memory
• WebAssembly.Table
• WebAssembly.CompileError
• WebAssembly.LinkError
• WebAssembly.RuntimeError

Other

• arguments

Statements

This chapter documents all the JavaScript statements and declarations.

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*

Generator 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.

for await...of

Iterates over async iterable objects, 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, and other scripts.

import

Used to import functions exported from an external module, another script.

import.meta

A meta-property exposing context-specific metadata to a JavaScript module.

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.

Expressions and operators

This chapter documents all the JavaScript expressions and operators.

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 a special property of an 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.

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

Expression closures Obsolete since Gecko 60

The expression closure syntax is a shorthand for writing simple function.

Legacy generator function Obsolete since Gecko 58

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.

[for (x of y) x] Obsolete since Gecko 58

Array comprehensions.

(for (x of y) y) Obsolete since Gecko 58

Generator comprehensions.

Functions

This chapter documents how to work with JavaScript functions to develop your applications.

• arguments
• Arrow functions
• Default parameters
• Rest parameters

Additional reference pages

• Lexical grammar
• Data types and data structures
• Strict mode
• Deprecated features