Function.length
The length property indicates the number of parameters expected by the function.
Property attributes of Function.length |
|
|---|---|
| Writable | no |
| Enumerable | no |
| Configurable | yes |
Description
length is a property of a function object, and 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.
Data property of the Function constructor
The Function constructor is itself a Function object. Its length data property has a value of 1. The property attributes are: Writable: false, Enumerable: false, Configurable: true.
Property of the Function prototype object
The length property of the Function prototype object has a value of 0.
Examples
Using function length
console.log(Function.length); /* 1 */
console.log((function() {}).length); /* 0 */
console.log((function(a) {}).length); /* 1 */
console.log((function(a, b) {}).length); /* 2 etc. */
console.log((function(...args) {}).length);
// 0, rest parameter is not counted
console.log((function(a, b = 1, c) {}).length);
// 1, only parameters before the first one with
// a default value is counted
Specifications
| Specification |
|---|
| ECMAScript Language Specification # sec-function-instances-length |
Browser compatibility
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