Function: length

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

The length data property of a Function instance indicates the number of parameters expected by the function.

Try it

Value

A number.

Property attributes of `Function: length`
Writable	no
Enumerable	no
Configurable	yes

Description

A Function object's length property indicates how many arguments the function expects, i.e. the number of formal parameters:

Only parameters before the first one with a default value are counted.
A destructuring pattern counts as a single parameter.
The rest parameter is excluded.

By contrast, arguments.length is local to a function and provides the number of arguments actually passed to the function.

The Function constructor is itself a Function object. Its length data property has a value of 1.

Due to historical reasons, Function.prototype is a callable itself. The length property of Function.prototype has a value of 0.

Examples

Using function length

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

console.log((() => {}).length); // 0
console.log(((a) => {}).length); // 1
console.log(((a, b) => {}).length); // 2 etc.

console.log(((...args) => {}).length);
// 0, rest parameter is not counted

console.log(((a, b = 1, c) => {}).length);
// 1, only parameters before the first one with
// a default value are counted

console.log((({ a, b }, [c, d]) => {}).length);
// 2, destructuring patterns each count as
// a single parameter

Specifications

Specification
ECMAScript Language Specification # sec-function-instances-length

Browser compatibility

	Desktop					Mobile						Server
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet	WebView Android	Deno	Node.js
`length`	1	12	1	3	1	18	4	10.1	1	1.0	4.4	1.0	0.10.0
`configurable_true`	43	12	37	30	10	43	37	30	10	4.0	43	1.0	4.0.0