Javascript is client side, for the most part you can assume y our users have it... but that's not 100%, I know AOL for example used to have javascript issues, and I know IE 6 still have javascript issues, most things will work there, but they don't strictly implement the standard.
Some people will disable it as some sites use javascript to do nefarious things, like pop open a gazillion windows, or make it difficult to exit their page.
But for the most part, most people have IE or Opera or a Mozilla variant, and they all support most of the javascript standard.
forum216
should help you
-Rob