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Function: length

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

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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. This number excludes the rest parameter and only includes parameters before the first one with a default value. 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

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