[waltsu]

I hope that you realize that when you've divided your listeners to Product and Cart objects, you've created a "lightweight" Model representation. Next thing you probably need is to fetch those Products and Carts from the server, and you add fetch()-function to them. Soon you'll realize that you've rewritten for example Backbone's Model class. With the exception that your solution has probably more bugs.

[Arnor]

That's a good point. However, it's till tempered by the fact that I haven't added ~40kb to the app to get it.