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.
