A recent link from the Mir.aculo.us website talks about unit testing in Javascript. This is an important part of an Ajax toolset.

Jon Tirsen come up with a system that will look quite familiar to JUnit users:

Event.simulateEvent('tobeedited','mouseout');
assertEqual("transparent", Element.getStyle('tobeedited','background-color'));

You can see more here.

DWR has had it's own unit test framework in Javascript for a while. If you download the latest war file you can take a look for yourself. There is an easy link off the index page.

The DWR version contains a front-end which shows you the tests and the familiar green bar as the tests are running. It would be interesting to try using Jon's framework with DWR's UI.