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.
The length
data property of a Function
instance indicates the number of parameters expected by the function.
Try it
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
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 Language Specification # sec-function-instances-length |
Browser compatibility
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