As far as I know there is no way to see if an email is a valid one. (otherwise spammers would be in heaven). But there is a technique used to make sure somebody is using a real email. I dont know what your application is but lets say somebody is signing up to be a member on your site, or any sort of application. You will probably need to give them information after they hit submit. (username, password, receipt info....) any of these things you could display on the "confirmation page" but why do that. If they are using a fake email for whatever reason, you'll never know. Just send them their username, password etc. to their email. This keeps them honest. They must check their email to do anything else, therefore it must be valid

By the way, a lot of these things have nothing to do with JavaScript