Visible Button

I have a form with three <textarea> and one <input> element. When either of
these are changed I want an "Update" button to be visible at the bottom of
the form. I've tried this:

On every element I've added this trigger:
onChange='jsVwBtn()'

Function like this:
function jsVwBtn()
{document.form1.btnupdate.type = "button";}

Default the btnupdate type is set to "hidden".
Some of you might start to laugh now, but as you continue do that please
give some "state of the art" way to do this