Object class comes twice in prototype chain of DOMWindow?

Question

Why do we have 2 class Object and again Object in prototype chain of window?

window --> DOMWindow --->Object --->Object ---> null

Can anyone please give me some point about this design?

Follwing is the output on chrome.

Answer 1

First: The DOMWindow as shown in the console is a result of the smart Dev Tools: The constructors name is shown in this case. When you explicitly use window.__proto__.toString() , [object Object] would be shown three times.

Note about the design

To answer your question about the design, I cite the ES5 specification (emphasis is mine):

All objects have an internal property called [[Prototype]] . The value of this property is either null or an object and is used for implementing inheritance. Whether or not a native object can have a host object as its [[Prototype]] depends on the implementation. Every [[Prototype]] chain must have finite length (that is, starting from any object, recursively accessing the [[Prototype]] internal property must eventually lead to a null value).

So, it's not weird that you see null in the end.

More details

Some (technical) notes beforehand:

• Object.getPrototypeOf( obj ) returns the [[Prototype]] property of an object obj .
• The [[Class]] property of an object represents its internal class. Its name can be extracted using Object.prototype.toString.call( obj ) .

Table:

   toString() result:   [[Class]]   #  Additional notes
1. [object DOMWindow]    global     # The global object
2. [object Object]       Object     # [[Prototype]] of the Global object
3. [object Object]       Object     # [[Prototype]] of 2 (dummy?)
4. [object Object]       Object     # [[Prototype]] of 3 === Object.prototype
5. [object Null]         Null       # Object.prototype.__proto__ === null

1. The global object .
   Examples: window in browser-JavaScript, global in Node.js .
2. According to section 15.1 , the [[Prototype]] and [[Class]] properties of global are implementation-dependent. In Chrome, the implementation of DOMWindow looks like the one as described in this IDL .
3. In Chrome, this constructor has literally no name. In Firefox, this is the Global Scope Polluter class.
   This seems to be a dummy object, hence the lack of properties.
4. The previous object is a true instance of Object .
   This explains the logged __defineGetter , etc. properties in the console.
5. The [[Prototype]] property of Object.prototype is null , see section 15.2.4 .

Additional references

• Code to get the [[Class]] properties:

   var w = window; while (1) { console.log(Object.prototype.toString.call(w)); if (w == null) break; w = Object.getPrototypeOf(w); } 

• 15.1 - The Global Object
• 15.2.4 - Properties of the Object Prototype Object
• WebCore 's DOMWindow.idl ^{What is an Interface definition language ?} for DOMWindow (AFAIK, it's not V8s IDL, because V8's constructors are not postfixed with Constructor ).