function expression

The function keyword can be used to define a function inside an expression.

You can also define functions using the Function constructor and a function declaration.

Syntax

var myFunction = function [name]([param1[, param2[, ..., paramN]]]) {
   statements
};

As of ES2015, you can also use arrow functions.

Parameters

name
The function name. Can be omitted, in which case the function is anonymous. The name is only local to the function body.
param
The name of an argument to be passed to the function.
statements
The statements which comprise the body of the function.

Description

A function expression is very similar to and has almost the same syntax as a function statement (see function statement for details). The main difference between a function expression and a function statement is the function name, which can be omitted in function expressions to create anonymous functions. A function expression can be used as a IIFE (Immediately Invoked Function Expression) which runs as soon as it is defined. See also the chapter about functions for more information.

Function expression hoisting

Function expressions in JavaScript are not hoisted, unlike function declarations. You can't use function expressions before you define them:

notHoisted(); // TypeError: notHoisted is not a function

var notHoisted = function() {
   console.log('bar');
};

Named function expression

If you want to refer to the current function inside the function body, you need to create a named function expression. This name is then local only to the function body (scope). This also avoids using the non-standard arguments.callee property.

var math = {
  'factorial': function factorial(n) {
    if (n <= 1)
      return 1;
    return n * factorial(n - 1);
  }
};

The variable assigned to a function expression will have a name property. The name doesn't change if it's assigned to a different variable. If function name is ommited, it will be the variable name (implicit name). If function name presets, it will be the function name (explicit name). This also applies to arrow functions (arrows don't have a name so you can only give the variable an implicit name).

var foo = function() {}
foo.name // "foo"

var foo2 = foo
foo2.name // "foo"

var bar = function baz() {}
bar.name // "baz"

Examples

The following example defines an unnamed function and assigns it to x. The function returns the square of its argument:

var x = function(y) {
   return y * y;
};

More commonly it is used as a callback:

button.addEventListener('click', function(event) {
    console.log('button is clicked!')
})

Specifications

Specification Status Comment
ECMAScript Latest Draft (ECMA-262)
The definition of 'Function definitions' in that specification.
Living Standard  
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Function definitions' in that specification.
Standard  
ECMAScript 5.1 (ECMA-262)
The definition of 'Function definition' in that specification.
Standard  
ECMAScript 3rd Edition (ECMA-262)
The definition of 'Function definition' in that specification.
Standard Initial definition. Implemented in JavaScript 1.5.

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)
Trailing comma in parameters ? ? 52.0 (52.0) ? ? ?
Feature Android Chrome for Android Edge Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)
Trailing comma in parameters ? ? ? 52.0 (52.0) ? ? ? ?

See also