To begin with, I'm a noob to JavaScript. How do I add functionality to the main loop, or something similar?

I'm trying to find a way to add functionality to Java Script's main event loop, from which the all its events are evaluated.

For example,

//every event based application has something similar at heart,
//usually, somewhere in the main function
 //process all events
 AbstractEvent **a = eventStack.begin()-1;
  application.processEvent(*a); //could be an "onclick","onmouseenter", exit, etc...
  //for Java Script, can I make stuff happen here

 //or here?