为什么我需要从另一个来源指向Array构造函数，为什么我不能直接在数组对象上调用toString（）？

Question

var iframe = document.createElement('iframe');
document.body.appendChild(iframe);

var _Array = window.frames[window.frames.length-1].Array;

var arr = new _Array(1,2,3);

typeof arr; //'object'
arr instanceof Array; //false

I have 3 question about this code:

1. Object.prototype appears on the proto chain of both _Array and Array prototypes but Array and _Array are not in the same chain and because of that (arr instanceof Array) returns false?

2. I want to understand what's exactly going here, these are my thoughts: _Array points to the same constructor as where "windows.iframe[...].Array" points, then we create an instance of _Array, its proto points to Array.prototype, and Array.prototype.proto points to Object.prototype. So, both _Array and Array are constructors of array objects, which are inherited from Object.prototype. So, why do I need to call toString() directly from Object.prototype in order to get "[object Array]" on _Array and Array objects? When I call it like this: arr.toString() I get an empty string and when I call Object.prototype.toString.call(arr) I get "[object Array]". Why is that?

3. Can someone explain please why do I ever need to point to the Array constructor from another source? in this code _Array points to the same constructor as where "windows.iframe[...].Array" points to. Why is it useful?

Answer 1

An Array is just a data structure with behavior; there are many array implementations, and you can implement your own as well.

1. I want to understand what's exactly going here...

var iframe = document.createElement('iframe');
document.body.appendChild(iframe);

// Accessing a new type of Array
var _Array =
  window.frames[window.frames.length-1].Array;

// Instantiates new array of type "_Array" (declared in iframe)
var arr = new _Array(1,2,3); // [1,2,3]

// Shows the type of arr variable
typeof arr; //-> 'object'

// Asserting that the "arr" instance is not in an instance of type "Array" (native Javascript type), rather it is of the type declared in the iframe
arr instanceof Array; //-> false

2. (...) Why is it useful?

Please note that this code pretty useless on its own (it is an example snippet), its probable goal is to show

• basic type-related operations in Javascript
• possibility for iframes to hold data, such as new types ( Array )