Javascript is a powerful tool in the web programmers toolbox however, it's also one of our greatest headaches. Dealing with browser inconsistencies is always a source of great pain. You test on multiple platforms, find everyone you know with a mac running safari and think you have your code locked down however it rarely always works out this way. Being able to detect javascript errors in the wild can be a great resource for you to really see how your code is performing on a day to day basis.
The key to it is that it uses a global function that IE and Mozilla both have to create a global-level error handler for the page - "onerror". He shows you how, with a few simple lines, you can create the function (with an alert inside) and, together with a little bit of Ajax and PHP magic, write it out to an error logging script for review...
