Traditionally testing on any project is left till last, but even then testing of client-side code is usually overlooked entirely.
With the rise-and-rise of jQuery the world at large is finally recognising that JavaScript is not a second-class citizen.
Web applications the world-round are leveraging JavaScript - and in particular jQuery - to build large scale applications that serve millions. Of course to ensure a high-level of quality and robustness it's necessary to test all this code.
Enter stage left - QUnit.