MDN may have intermittent access issues April 18 13:00 - April 19 01:00 UTC. See whistlepig.mozilla.org for all notifications.

mozilla
Your Search Results

    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.
    [for (x of y) x]
    Array comprehensions.
    (for (x of y) y)
    Generator comprehensions.
    ( )
    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.
    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.

    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

    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.

    Specifications

    Specification Status Comment
    ECMAScript 1st Edition. Standard Initial definition.
    ECMAScript 5.1 (ECMA-262)
    The definition of 'Expressions' in that specification.
    Standard  
    ECMAScript 6 (ECMA-262)
    The definition of 'ECMAScript Language: Expressions' in that specification.
    Release Candidate New: Spread operator, destructuring assignment, super keyword, Array comprehensions, Generator comprehensions

    See also

    Document Tags and Contributors

    Last updated by: fscholz,
    Hide Sidebar