Function: length

BaselineWidely available

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

Thelength data property of aFunction instance indicates the number of parameters expected by the function.

Try it

function func1() {}function func2(a, b) {}console.log(func1.length);// Expected output: 0console.log(func2.length);// Expected output: 2

Value

A number.

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

Description

AFunction object'slength property indicates how many arguments the function expects, i.e., the number of formal parameters:

Only parameters before the first one with adefault value are counted.
Adestructuring pattern counts as a single parameter.
Therest parameter is excluded.

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

TheFunction constructor is itself aFunction object. Itslength data property has a value of1.

Due to historical reasons,Function.prototype is a callable itself. Thelength property ofFunction.prototype has a value of0.

Examples

Using function length

console.log(Function.length); // 1console.log((() => {}).length); // 0console.log(((a) => {}).length); // 1console.log(((a, b) => {}).length); // 2 etc.console.log(((...args) => {}).length);// 0, rest parameter is not countedconsole.log(((a, b = 1, c) => {}).length);// 1, only parameters before the first one with// a default value are countedconsole.log((({ a, b }, [c, d]) => {}).length);// 2, destructuring patterns each count as// a single parameter