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.
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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
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)