Fix javascript problems on single page

I need someone who's knowledgeable in javascript/html to fix broken functionalities on a simple invoice generating page. There are bunch of little nagging issues like the followings: 1. upon change of a country's currency sign (usd, eur, yen etc..) currency formats and item prices don't update properly or not at all. 2. then upon change of item quantity or price, it doesn't update total(subtotal, item) prices at all. 3. invoice file number wouldn't continue from left off last time when a random number is typed in manually. - Basically fix all visible calculation related and country currency issues.