Can It Be Done Like This (ajax)?

I was playing a little bit with a lot of ajax frameworks. I have even
wrote one small by myself. But one thing still drives me crazy: Whenever
I'm doing an ajax call I need to specify handler to handle data coming
from a server script. That means that almost for every action I need two
functions.

Since I'm still new to all of this I'd like to ask how can i achieve
something like this:

function function_name(el) {
var i_need_this = get_what_i_need(el.id);
return _i_need_this;
}

where get_what_i_need function is returning some information using ajax
from a server.