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.
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The length data property of a Function instance indicates the number of parameters expected by the function.
Try it
function func1() {}
function func2(a, b) {}
console.log(func1.length);
// Expected output: 0
console.log(func2.length);
// Expected output: 2
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
js
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® 2025 Language Specification # sec-function-instances-length |