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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
.
The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
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.
paramN
- 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:
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.
var 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 | Status | Comment |
---|---|---|
ECMAScript Latest Draft (ECMA-262) The definition of 'Function definitions' in that specification. |
Draft | |
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
Desktop | Mobile | Server | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Basic support | Chrome Full support Yes | Edge Full support Yes | Firefox Full support 1 | IE Full support Yes | Opera Full support Yes | Safari Full support Yes | WebView Android Full support Yes | Chrome Android Full support Yes | Edge Mobile Full support Yes | Firefox Android Full support 4 | Opera Android Full support Yes | Safari iOS Full support Yes | Samsung Internet Android Full support Yes | nodejs Full support Yes |
Trailing comma in parameters | Chrome Full support 58 | Edge No support No | Firefox Full support 52 | IE No support No | Opera Full support 45 | Safari No support No | WebView Android Full support 58 | Chrome Android Full support 58 | Edge Mobile No support No | Firefox Android Full support 52 | Opera Android Full support 45 | Safari iOS No support No | Samsung Internet Android Full support 7.0 | nodejs Full support 8.0.0 |
Legend
- Full support
- Full support
- No support
- No support