In order to realize the full power of a class system, however, it's not enough to just define new methods on subclasses; sometimes, we need to combine a method of a parent class with additional logic on a subclass. In traditional object-oriented languages, this is often done by referencing a special super object; but in JavaScript, no such object exists, which means that we have to utilize JavaScript's apply or call methods instead.

For instance, Backbone.Model has a destroy method, but what if we want our Book class to also have its own destroy method? This method might take a number of pages and destroy them (reduce the total number of pages by that amount), but at the same time, we might want to keep the Backbone version around for its original purpose (which is to destroy the server-side version of the Book).

Luckily, because Backbone properly configured our Book class's prototype for us, calling the parent method from the subclass method is fairly straightforward...