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.

js>>
Event.observeOnce = function(element, event, observer){
var handler = function(){
Event.stopObserving(element, event, handler);
return observer.apply(null, $A(arguments));
};
Event.observe(element, event, handler);
};
<<--

The usage is same to Event.observe.

js>>
Event.observeOnce(document, 'dom:loaded', function(){
/* some code goes here */
});
<<--

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