Backbone and its ilk are full MVC themselves. Typically, you'd have models on the backend which are synced to the frontend via XHR/websockets/etc, so that both

Backbone and its ilk are full MVC themselves. Typically, you'd have models on the backend which are synced to the frontend via XHR/websockets/etc, so that both sides have a representation. Backbone also has controllers (in the Cocoa sense rather than the Rails sense), in the form of collections. Unlike Rails, though, most of the functionality is going to be in your views and your models.