there are a bajillion libraries out there for unit testing javascript (something test::unit-ish would be qunit). rails + backbone + JST style templating actually makes TDDing your js relatively painless

We're looking into integration-level testing for our Javascript since it'd give us full coverage of the bugs we see.

I forgot to mention we also use Socket.io for real time push updates to user Backbone models. A good number of bugs don't crop up until another user modifies data and those changes get pushed out to another person.

Testing with front end javascript is more difficult due to different interpreters, much more variation in environment, etc. That doesn't mean you can't have tests for your core business logic at least in something like Vows or Mocha.

You don't. You use QUnit and move on with your life.