The idea of a HACKING file is 'stolen' from the Subversion project, as it's a particularly nice thing to have around. It will be the mother of many more specific topics.
There is another Javascript XMPP/BOSH library out there called JSJaC, which as of late has been evolving into a much nicer piece of code. At the time, it simply did not meet my needs because:
Again, some of these issues have been resolved, but Xmpp4Js is mature enough that there is no reason to switch back now.
Xmpp4Js is built in such a way that there is room to experiment with that technology. It should be as simple as implementing another Xmpp4Js.Transport. Currently there is no major advantage to switching from BOSH, but scalability concerns in the future may necessitate exploring other avenues including Bayeaux.
xmpp4js is an xmpp connection library for Javascript. It is written with a heavy emphasis on resuability and extensibility, and is naturally heavilly tested.