function

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

Syntax

var myFunction = 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.

Function expression hoisting

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

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

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

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

See also

• Functions and function scope
• Function
• function statement
• function* statement
• function* expression
• GeneratorFunction