Why would you use JavaScript I?
Originally JavaScript was designed to do what you think Java should do -- elegant dynamic animation on the client side
Then along came Java and JavaScript was eclipsed as it was just Netscape and not so well designed
Netscape tried to make it a server side language (LiveWire) instead of Perl but Java was much better here as well
JavaScript lived on for simple arithmetic on client side to produce somewhat better frames (as clicking on a link can call a JavaScript function instead of just loading a URL) and to quickly check forms client side without going back to server which is slow
A serious weakness was that dynamic aspects of JavaScript could only be used at “load” time and other changes of JavaScript required a “reload” which is real ugly and slow