Function: length

## Value

A number.

{{js_property_attributes(0, 0, 1)}}

## Description

A {{jsxref("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](/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters) are counted.
- A [destructuring pattern](/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring) counts as a single parameter.
- The [rest parameter](/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters) is excluded.

By contrast, {{jsxref("Functions/arguments/length", "arguments.length")}} is local to a function and provides the number of arguments actually passed to the function.

The {{jsxref("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