Backbone provides a very powerful class system, but sometimes, even this class system isn't enough. For instance, let's say you have several classes that all contain several identical methods. The popular Don't Repeat Yourself (DRY) programming principle suggests that you should eliminate the duplicate versions of the methods and instead, define them all together in just one place in your code.

Normally, you could do so by refactoring these methods into a common parent class of these classes. But what if these classes can't share a common parent class? For instance, what if one of these classes is a Model class and the other a Collection class?

In this case, you need to rely on something called a mixin. A mixin is just an object that holds one or more methods (or even primitive properties) that you want to share between several classes. For example, if we wanted to share a couple of logging-related methods between several classes, we could create a mixin of these methods, as follows...