Board Thread:Support Requests - Getting Technical/@comment-3032314-20131126004329/@comment-11733175-20131126115407

is a deprecated way of running functions on certain events. It is part of wikibits.js (part of the standard mediawiki library as you guessed) as detailed here.

The current recommendation is to use jQuery to listen for events:

There are some issues with jQuery's implementation of event listening compared to native event listeners, but for most applications it works fine.