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 "Gizmo":
function Hoozit(id) {
this.id = id;
}
Hoozit.prototype = new Gizmo();
Hoozit.prototype.test = function (id) {
return this.id === id;
};
Why does he write Hoozit.prototype = new Gizmo() instead of Hoozit.prototype = Gizmo.prototype?
Is there any difference between these two?
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.
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;
return new NewObj();
};
// Paraphrasing of Nicholas Zakas's Prototype Inheritance helper
function inheritPrototype(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.
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.
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
}
