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

Implemented in: JavaScript 1.5
ECMA Version: ECMA-262, Edition 3


function [name]([param1[, param2[, ..., paramN]]]) {


The function name. Can be omitted, in which case the function is anonymous. The name is only local to the function body.
The name of an argument to be passed to the function. A function can have up to 255 arguments.
The statements which comprise the body of the function.


A function expression is very similar to and has almost the same syntax as a function declaration (see function for details). See Functions for information on the differences between function statements and function expressions.


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);

See also

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 Last updated by: Sheppy,