Expressions and operators
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.
[]
 Array initializer/literal syntax.
{}
 Object initializer/literal syntax.
/ab+c/i
 Regular expression literal syntax.
( )
 Grouping operator.
Lefthandside 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
andobject["property"]
). new
 The
new
operator creates an instance of a constructor.  new.target
 In constructors,
new.target
refers to the constructor that was invoked bynew
. super
 The
super
keyword calls the parent constructor. ...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.
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.
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.
Binary logical operators
Logical operators are typically used with boolean (logical) values, and when they are, they return a boolean value.
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.
Nonstandard 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 oneyield
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 (ECMA262) The definition of 'Expressions' in that specification. 
Standard  Initial definition 
ECMAScript 5.1 (ECMA262) The definition of 'Expressions' in that specification. 
Standard  
ECMAScript 2015 (6th Edition, ECMA262) The definition of 'ECMAScript Language: Expressions' in that specification. 
Standard  New: Spread operator, destructuring assignment, super keyword. 
ECMAScript 2017 Draft (ECMA262) The definition of 'ECMAScript Language: Expressions' in that specification. 
Draft 
See also
License
© 2016 Mozilla Contributors
Licensed under the Creative Commons AttributionShareAlike License v2.5 or later.
https://developer.mozilla.org/enus/docs/web/javascript/reference/operators