apply第二个参数不是应该是个数组吗？对象也行吗？

Question

如图

Answer 1

Syntax
fun.apply(thisArg, [argsArray])

Parameters

• thisArg

The value of this provided for the call to fun. Note that this may not be the actual value seen by the method: if the method is a function in non-strict mode code, null and undefined will be replaced with the global object, and primitive values will be boxed.

• argsArray

An array-like object, specifying the arguments with which fun should be called, or null or undefined if no arguments should be provided to the function. Starting with ECMAScript 5 these arguments can be a generic array-like object instead of an array. See below for browser compatibility information.

从es5开始支持array-like的object啦，所有array-like就是有length属性，有index作为key的object啦。
你这个object正巧就有length属性，不过没有键值对，所以遍历出5个undefined

Answer 2

这个对象是类数组对象.

可能在apply在传递参数的时候会主动调用一次转换，将第二个参数转化为数组吧，这时候类数组对象就刚好转成数组了。

写个简单的例子：

var obj = {test: 1};

Array.apply(null, obj);

var obj2 = [].slice.call(obj);

Array.apply(null, obj2);

它们的结果是相同的，说明apply函数对第二个参数做过一次转换

Answer 3

折腾了一会儿，现在有个想法
首先，正如熊丸子（抱歉，不知道怎么做链接。。。）所说，{length: 5}是一个类数组对象，它类似于[undefined,undefined,undefined,undefined,undefined]
而Array方法就是Array对象的构造器，[].constructor == Array返回true
于是Array.apply(null, {length:5})就可以等效于Array.apply(null, [undefined, undefined, undefined, undefined, undefined])也即是Array(undefined, undefined, undefined, undefined, undefined)或者new Array(undefined, undefined, undefined, undefined, undefined)