Function: length
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. This number excludes the rest parameter and only includes parameters before the first one with a default value. 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
Specifications
Specification |
---|
ECMAScript Language Specification # sec-function-instances-length |
Browser compatibility
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