JavaScript: The Good Parts

Prototype

Every object is linked to a prototype object from which it can inherit properties. All objects created from object literals are linked to Object.prototype, an object that comes standard with JavaScript.

When you make a new object, you can select the object that should be its prototype. The mechanism that JavaScript provides to do this is messy and complex, but it can be significantly simplified. We will add a create method to the Object function. The beget method creates a new object that uses an old object as its prototype. There will be much more about functions in the next chapter.

if (typeof Object.create !== 'function') {
     Object.create = function (o) {
         var F = function () {};
         F.prototype = o;
         return new F();
     };
}
var another_stooge = Object.create(stooge);

The prototype link has no effect on updating. When we make changes to an object, the object's prototype is not touched:

another_stooge['first-name'] = 'Harry';
another_stooge['middle-name'] = 'Moses';
another_stooge.nickname = 'Moe';

The prototype link is used only in retrieval. If we try to retrieve a property value from an object, and if the object lacks the property name, then JavaScript attempts to retrieve the property value from the prototype object. And if that object is lacking the property, then it goes to its prototype, and so on until the process finally bottoms out with Object.prototype. If the desired property exists nowhere in the prototype chain, then the result is the undefined value. This is called ...