Function.length

length属性指明函数的形参个数。

| Function.length属性的属性特性 |

|:----|

| Writable | no |

| Enumerable | no |

| Configurable | yes |

描述

length 是函数对象的一个属性值，指该函数有多少个必须要传入的参数，即形参的个数。

Function 构造器的属性

Function 构造器本身也是个Function。他的length属性值为 1 。该属性 Writable:false, Enumerable:false, Configurable:true.

Function 原型对象的属性

Function 原型对象的 length 属性值为 0 。

示例

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

规范

Specification	Status	Comment
ECMAScript 1st Edition (ECMA-262)	Standard	Initial definition. Implemented in JavaScript 1.1.
ECMAScript 5.1 (ECMA-262)The definition of 'Function.length' in that specification.	Standard
ECMAScript 2015 (6th Edition, ECMA-262)The definition of 'Function.length' in that specification.	Standard	The configurable attribute of this property is now true.
ECMAScript Latest Draft (ECMA-262)The definition of 'Function.length' in that specification.	Draft

浏览器兼容性

Feature	Chrome	Edge	Firefox (Gecko)	Internet Explorer	Opera	Safari
Basic support	(Yes)	(Yes)	(Yes)	(Yes)	(Yes)	(Yes)
Configurable: true	?	?	37 (37)	?	?	?

Feature	Android	Chrome for Android	Edge	Firefox Mobile (Gecko)	IE Mobile	Opera Mobile	Safari Mobile
Basic support	(Yes)	(Yes)	(Yes)	(Yes)	(Yes)	(Yes)	(Yes)
Configurable: true	?	?	?	37.0 (37)	?	?	?