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

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

As of ES2015, you can also use arrow functions.

Parameters

name Optional
The function name. Can be omitted, in which case the function is anonymous. The name is only local to the function body.
paramN Optional
The name of an argument to be passed to the function.
statements Optional
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 declaration (see function statement for details). The main difference between a function expression and a function declaration is the function name, which can be omitted in function expressions to create anonymous functions. A function expression can be used as an 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:

console.log(notHoisted) // undefined 
//  even though the variable name is hoisted, the definition isn't. so it's undefined.
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.

let math = {
  'factit': function factorial(n) {
    console.log(n)
    if (n <= 1) {
      return 1;
    }
    return n * factorial(n - 1);
  }
};

math.factit(3) //3;2;1;

The variable the function expression is assigned to will have a name property. The name doesn't change if it's assigned to a different variable. If function name is omitted, it will be the variable name (implicit name). If function name is present, 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"

console.log(foo === foo2); // true
console.log(typeof baz); // undefined
console.log(bar === baz); // false (errors because baz == undefined)

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
ECMAScript Latest Draft (ECMA-262)
The definition of 'Function definitions' in that specification.

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
functionChrome Full support 1Edge Full support 12Firefox Full support 1IE Full support 3Opera Full support 3Safari Full support 1WebView Android Full support 1Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 10.1Safari iOS Full support 1Samsung Internet Android Full support 1.0nodejs Full support Yes
Trailing comma in parametersChrome Full support 58Edge Full support 79Firefox Full support 52IE No support NoOpera Full support 45Safari No support NoWebView Android Full support 58Chrome Android Full support 58Firefox Android Full support 52Opera Android Full support 43Safari iOS No support NoSamsung Internet Android Full support 7.0nodejs Full support 8.0.0

Legend

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See also