In prototype.js, Event.observe method is very useful. But it is a bit inconvenient if you want to handle events only once such as an initialization. I have often encountered such situation, so I wrote a small utility function being as follows.

   1  Event.observeOnce = function(element, event, observer){
   2    var handler = function(){
   3      Event.stopObserving(element, event, handler);
   4      return observer.apply(null, $A(arguments));
   5    };
   6    Event.observe(element, event, handler);
   7  };

The usage is same to Event.observe.

   1  Event.observeOnce(document, 'dom:loaded', function(){
   2    /* some code goes here */
   3  });

It may be useful if you copy this into public/javascripts/application.js for Rails applications.