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    length

    概述

    length 属性指明函数的形参个数。

    length 属性的属性特性:
    writable false
    enumerable false
    configurable true

    描述

    length 是函数对象的一个属性值,指明该函数期望多少个参数,意即形参的个数。数量不包括剩余参数。相比之下,  arguments.length 是函数被调用时实际传参的个数。

    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 。

    例子

    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 */
     

    规范

    Specification Status Comment
    ECMAScript 1st Edition. Standard Initial definition. Implemented in JavaScript 1.1.
    length property of the Function constructor:
    ECMAScript 5.1 (ECMA-262)
    Function.length

    length property of the Function prototype object:
    ECMAScript 5.1 (ECMA-262)
    Function.length

    length property of Function instances:
    ECMAScript 5.1 (ECMA-262)
    Function.length
    Standard  
    length property of the Function constructor:
    ECMAScript 6 (ECMA-262)
    Function.length

    length property of the Function prototype object:
    ECMAScript 6 (ECMA-262)
    Function.length

    length property of Function instances:
    ECMAScript 6 (ECMA-262)
    Function.length
    Draft The configurable attribute of this property is now true.

    浏览器兼容性

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

    参考

     

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