checking username available or not

[macintosh]

Often on websites, while registering, there's a button in the front of username box, which tells if the username is available or not. i think it is done by javascript and php. can someone help how i can create that javascript?

[ses5909]

Javascript isn't always used, but a server-side programming language like PHP or Java is used to query a database to check and see if a username exists.

Some newer sites may do this with a AJAX which of course means JS is involved.

Here is a link which may help:

http://www.mgroves.com/blog_archive.php?blogID=245