JavaScript: What are .extend and .prototype used for?

Question

Javascript's inheritance is prototype based, so you extend the prototypes of objects such as Date, Math, and even your own custom ones.

Date.prototype.lol = function() {
 alert('hi');
};

( new Date ).lol() // alert message

In the snippet above, I define a method for all Date objects ( already existing ones and all new ones ).

extend is usually a high level function that copies the prototype of a new subclass that you want to extend from the base class.

So you can do something like:

extend( Fighter, Human )

And the Fighter constructor/object will inherit the prototype of Human, so if you define methods such as live and die on Human then Fighter will also inherit those.

Updated Clarification:

"high level function" meaning .extend isn't built-in but often provided by a library such as jQuery or Prototype.

.extend() is added by many third-party libraries to make it easy to create objects from other objects. See http://api.jquery.com/jQuery.extend/ or http://www.prototypejs.org/api/object/extend for some examples.

.prototype refers to the "template" (if you want to call it that) of an object, so by adding methods to an object's prototype (you see this a lot in libraries to add to String, Date, Math, or even Function) those methods are added to every new instance of that object.

The extend method for example in jQuery or PrototypeJS, copies all properties from the source to the destination object.

Now about the prototype property, it is a member of function objects, it is part of the language core.

Any function can be used as a constructor, to create new object instances. All functions have this prototype property.

When you use the new operator with on a function object, a new object will be created, and it will inherit from its constructor prototype.

For example:

function Foo () {
}
Foo.prototype.bar = true;

var foo = new Foo();

foo.bar; // true
foo instanceof Foo; // true
Foo.prototype.isPrototypeOf(foo); // true

Javascript inheritance seems to be like an open debate everywhere. It can be called "The curious case of Javascript language".

The idea is that there is a base class and then you extend the base class to get an inheritance-like feature (not completely, but still).

The whole idea is to get what prototype really means. I did not get it until I saw John Resig's code (close to what jQuery.extend does) wrote a code chunk that does it and he claims that base2 and prototype libraries were the source of inspiration.

Here is the code.

    /* Simple JavaScript Inheritance
     * By John Resig http://ejohn.org/
     * MIT Licensed.
     */  
     // Inspired by base2 and Prototype
    (function(){
  var initializing = false, fnTest = /xyz/.test(function(){xyz;}) ? /\b_super\b/ : /.*/;

  // The base Class implementation (does nothing)
  this.Class = function(){};

  // Create a new Class that inherits from this class
  Class.extend = function(prop) {
    var _super = this.prototype;

    // Instantiate a base class (but only create the instance,
    // don't run the init constructor)
    initializing = true;
    var prototype = new this();
    initializing = false;

    // Copy the properties over onto the new prototype
    for (var name in prop) {
      // Check if we're overwriting an existing function
      prototype[name] = typeof prop[name] == "function" &&
        typeof _super[name] == "function" && fnTest.test(prop[name]) ?
        (function(name, fn){
          return function() {
            var tmp = this._super;

            // Add a new ._super() method that is the same method
            // but on the super-class
            this._super = _super[name];

            // The method only need to be bound temporarily, so we
            // remove it when we're done executing
            var ret = fn.apply(this, arguments);        
            this._super = tmp;

            return ret;
          };
        })(name, prop[name]) :
        prop[name];
    }

    // The dummy class constructor
    function Class() {
      // All construction is actually done in the init method
      if ( !initializing && this.init )
        this.init.apply(this, arguments);
    }

    // Populate our constructed prototype object
    Class.prototype = prototype;

    // Enforce the constructor to be what we expect
    Class.prototype.constructor = Class;

    // And make this class extendable
    Class.extend = arguments.callee;

    return Class;
  };
})();

There are three parts which are doing the job. First, you loop through the properties and add them to the instance. After that, you create a constructor for later to be added to the object.Now, the key lines are:

// Populate our constructed prototype object
Class.prototype = prototype;

// Enforce the constructor to be what we expect
Class.prototype.constructor = Class;

You first point the Class.prototype to the desired prototype. Now, the whole object has changed meaning that you need to force the layout back to its own one.

And the usage example:

var Car = Class.Extend({
  setColor: function(clr){
    color = clr;
  }
});

var volvo = Car.Extend({
   getColor: function () {
      return color;
   }
});

Read more about it here at Javascript Inheritance by John Resig 's post.

Some extend functions in third party libraries are more complex than others. Knockout.js for instance contains a minimally simple one that doesn't have some of the checks that jQuery's does:

function extend(target, source) {
    if (source) {
        for(var prop in source) {
            if(source.hasOwnProperty(prop)) {
                target[prop] = source[prop];
            }
        }
    }
    return target;
}