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The function
keyword can be used to define a function inside an expression.
Syntax
function [name]([param1[, param2[, ..., paramN]]]) { statements }
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.
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; };
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); } };
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 2017 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
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |