When to use javascript?

Hi all,
I am a newbie to javascript .I know only basics from w3schools.But don't know how to apply in live projects....How can i know when to use javascripts in HTML?
pls clear my confusion??

it is based on your software requirement, for example it you want to validate a user input before sending request to user javascript will be there for u, and also for any functionality u think it should run and compiled at client side ( your browser) and save time to send request and processing load at web servers

I think this may answer your query en.wikipedia.org/wiki/JavaScript#Use_in_web_pages

Java script is an easy way to make web sites interactive and not just text.

Generally many things should be faster with Javascript, as it can do stuff without contacting the server, but you shouldn't rely on for anything that you want to make sure is done, as it can be turned off, and also don't want to rely on it to care about your security, like validating user input.