Continuous testing has changed my life and Guard is top dog when it comes to continuous testing for Rails development. It's simple, reliable and extremely easy to extend. Autotest and Watchr don't hold a candle to Guard.
Some people whine and complain that JSLint is evil and there may be some legitimacy to that claim, but the simple fact is that JSLint has saved my ass more than once. Before JSLint, a single missing semicolon would take down an entire application for IE6/7 users, but ever since we've introduced JSLint into our development flow, we've had zero issues.
Integrating JSLint into your continuous testing flow is now possible with the new guard-jslint-on-rails plugin. Catch those pesky lint issues before you deploy to production. Catch them even before your continuous integration process runs. Heck, now you'll catch them before you commit them to your version control repository!
First: install the gem...
$ gem install guard-jslint-on-rails
Next: configure guard...
$ guard init jslint-on-rails