Javascript To Detect Textbox

I am wondering if any of you Javascript experts can enlighten me on
which event to call. The use situation is that the tetxbox field is
populated from the DB and the add/update and delete will work only when
a textbox for reason for change is entered. This mean if a user simply
type in the same value as before, the ASP.NET textbox will be marked
with changed. But it should not limit that a user can not change the
whole value in the textbox at all.

I have looked at keyup, mousedown, etc. but it will calling the
function after every key is entered and that is what I don't want to
happen.