JavaScript reference

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.