Everything ends up being in a small testable chunk that you can stub out the interfaces for and test (see sinon.js, can't say enough good stuff about that). I've also got a small Backbone.sync implementation that just loads test fixtures and returns data from that, allowing me to have a db to test against that's fake and synchronous. With couchdb, all I have to do is take a DB dump of what I want to test, put it into a file in my test directory and I've got instant fixtures.