What is the end of prototype chain in javascript -- null or Object.prototype?

What is the end of prototype chain in JavaScript ...?

Null. The only authority on the language is ECMA-262.

Are objects, which are not created from object literals, not linked to Object.prototype?

They may or many not be, e.g.

var x = Object.create(null)

has a [[Prototype]] of null, whereas:

var y = {};

has a [[Prototype]] of Object.prototype.

Say I have an object var x = { len: 4, breadth: 5}. Would JavaScript automatically create its prototype x.prototype?

No. Function objects have default prototype objects. Plain objects have a default [[Prototype]] (i.e. internal prototype property) that is Object.prototype (unless constructed as above).

And how long would the prototype chain be? Would x.prototype have only one prototype, Object.prototype, making a 3 point chain?

Two values: Object.prototype and null.

How does JavaScript internally create automatic prototypes?

However it likes, the language specification does not define implementation, only behaviour.

1. It is like, if New York has an envelope and inside, it says Colorado, and Colorado has an envelope, and inside, it says San Francisco, and San Francisco has an envelope, and inside, it says "none". So is San Francisco end of the chain, or is "none" the end of chain? It may depend on how you look at it. But one thing is for sure: it points up and up the chain, for inheritance purpose (prototypal inheritance), until it reaches null, which means can't go further up. And make sure you know that, to go up and up the chain, it is __proto__. It is not prototype.

2. Object.prototype, Animal.prototype are different from x.__proto__. The former are function objects (Object, Animal) having a prototype property pointing to a prototype object. And x.__proto__ is how the prototype chain is followed upward. To go up and up, it is x.__proto__.__proto__ and so on. See JavaScript's Pseudo Classical Inheritance diagram to understand it more.

3. Object.prototype refers to a prototype object. Quoted from MDN, null "represents the intentional absence of any object value. It is one of JavaScript's primitive values." So Object.prototype and null are not the same thing.

4. All JavaScript objects will have obj.__proto__ referring ultimately to what Object.prototype refers to. If it is not obj.__proto__, then it is obj.__proto__.__proto__. If not, just go up, and up, and it will reach the prototype object which Object.prototype refers to. And at this point, when you go up one level (by adding a .__proto__, then you get null. You can try it in Google Chrome's developer's tool:

   x = { a : 1 }
   > Object {a: 1}
   
   x.__proto__ === Object.prototype
   > true
   
   x.__proto__.__proto__
   > null
   
   Object.prototype.__proto__
   > null