Prototypal Inheritance In Javascript

I've been watching Douglas Crockford's talks at YUI Theater, and I have a question about JavaScript inheritance... Douglas gives this example to show that 'Hoozit' inherits from 'G

Solution 1:

The reason is that using Hoozit.prototype = Gizmo.prototype would mean that modifying Hoozit's prototype object would also modify objects of type Gizmo, which is not expected behavior.

Hoozit.prototype = new Gizmo() inherits from Gizmo, and then leaves Gizmo alone.

Solution 2:

The other answers address this, but if you DO want to inherit the prototype, you can use some parasitic magic:

Object.prototype.inherit = function(p) {
    NewObj = function(){};
    NewObj.prototype = p;
    returnnewNewObj(); 
};

// Paraphrasing of Nicholas Zakas's Prototype Inheritance helperfunctioninheritPrototype(subType, superType) {
    var prototype = Object.inherit(superType.prototype);
    prototype.constructor = subType;
    subType.prototype = prototype;
};

Now you can replace the:

Hoozit.prototype = new Gizmo();

with

inheritPrototype(Hoozit, Gizmo);

It might not be worth the trouble unless you have a real big Gizmo constructor (the only win in my suggestion is that you don't have to call Gizmo's constructor to hook up the prototype). I have examples of many of these types of patterns in TDD JavaScript Examples.

Solution 3:

If he writes Hoozit.prototype = Gizmo.prototype any modfication he makes later to the prototype of Hoozit will be reflected in the prototype of Gizmo.

Solution 4:

In addition to Triptych's answer: Hoozit instances will also inherit all instance properties of Gizmo, not only the ones defined in the prototype; eg:

function Gizmo() {
    this.foo = 'bar'; // foo is visible in every Hoozit instance
}