The length property specifies the number of arguments expected by the function.

Property attributes of Function.length
Writable no
Enumerable no
Configurable yes


length is a property of a function object, and indicates how many arguments the function expects, i.e. the number of formal parameters. This number does not include the rest parameter. By contrast, arguments.length is local to a function and provides the number of arguments actually passed to the function.

Data property of the Function constructor

The Function constructor is itself a Function object. Its length data property has a value of 1. The property attributes are: Writable: false, Enumerable: false, Configurable: true.

Property of the Function prototype object

The length property of the Function prototype object has a value of 0.


console.log(Function.length); /* 1 */

console.log((function()        {}).length); /* 0 */
console.log((function(a)       {}).length); /* 1 */
console.log((function(a, b)    {}).length); /* 2 etc. */
console.log((function(...args) {}).length); /* 0, rest parameter is not counted */


Specification Status Comment
ECMAScript 1st Edition (ECMA-262) Standard Initial definition. Implemented in JavaScript 1.1.
ECMAScript 5.1 (ECMA-262)
The definition of 'Function.length' in that specification.
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Function.length' in that specification.
Standard The configurable attribute of this property is now true.
ECMAScript Latest Draft (ECMA-262)
The definition of 'Function.length' in that specification.

Browser compatibility

We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help!

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
Configurable: true ? 37 (37) ? ? ?
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)
Configurable: true ? ? 37.0 (37) ? ? ?

See also

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