Object Constructor and prototyping
In the world of OOP, the previous ways of defining an object is too limiting in many situations. We need a way to create an object "type" that can be used multiple times without having to redefine the object every time to meet each particular instance's needs. The standard way to achieve this is to use the Object Constructor function.
Lets use a real world item "cat" as an example. A property of a cat may be its color or name. A method may be to "meeyow". The important thing to realize, however is that every cat will have a different name or even meeyow noise. To create an object type that accommodates this need for flexibility, we'll use an object constructor:
Here the function "cat()" is an object constructor, and its properties and methods are declared inside it by prefixing them with the keyword "this." Objects defined using an object constructor are then instantiated using the new keyword. Notice how we're able to easily define multiple instances of cat, each with its own name- that's the flexibility object constructor brings to custom objects. Constructors create the blueprints for objects, not the object itself.
Adding methods to our object using prototype
Lets extend our original cat() object above with an additional method to change the cat's name, using prototype:
IE5 doesn't support the shift() and unshift() methods of Array that NS4+ does, so lets prototype them in!
The possibilities are endless.