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---
title: "Function: length"
slug: Web/JavaScript/Reference/Global_Objects/Function/length
page-type: javascript-instance-data-property
browser-compat: javascript.builtins.Function.length
---
{{JSRef}}
The **`length`** data property of a {{jsxref("Function")}} instance indicates the number of parameters expected by the function.
{{EmbedInteractiveExample("pages/js/function-length.html")}}
## 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_assignment) 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
```
## Specifications
{{Specifications}}
## Browser compatibility
{{Compat}}
## See also
- {{jsxref("Function")}}