Relation between [[Prototype]] and prototype in JavaScript

Question

From http://www.jibbering.com/faq/faq_notes/closures.html :

Note: ECMAScript defines an internal [[prototype]] property of the internal Object type. This

Answer 1

To answer your question directly: logically it is an object's private copy of the prototype property of its constructor. Using metalanguage this is how objects are created:

// not real JS

var Ctr = function(...){...};
Ctr.prototype = {...}; // some object with methods and properties

// the object creation sequence: var x = new Ctr(a, b, c);
var x = {};
x["[[prototype]]"] = Ctr.prototype;
var result = Ctr.call(x, a, b, c);
if(typeof result == "object"){ x = result; }
// our x is fully constructed and initialized at this point

At this point we can modify the prototype, and the change will be reflected by all objects of the class, because they refer to the prototype by reference:

Ctr.prototype.log = function(){ console.log("...logging..."); };

x.log();  // ...logging..

But if we change the prototype on the constructor, already created objects will continue referring to the old object:

Ctr.prototype = {life: 42};
// let's assume that the old prototype didn't define "life"

console.log(x.life);  // undefined
x.log();              // ...logging...

In the full accordance with the standard [[prototype]] is not available, but Mozilla extends the standard with __proto__ property (read-only), which is exposing the normally hidden [[prototype]]:

• the Mozilla's documentation
• overview of Mozilla's extensions: __count__, __proto__, __parent__

Again, __proto__ can be legalized in the next ES3.1 standard.