Javascript is a programming language that is used to make web pages interactive.

First u need to create a javascript file and place them in separate 
files (using a .js extension helps identify them as Javascript).
You then just link the Javascript to your HTML by inserting a <script> tag. 
The same Javascript can then be added to several pages just by adding the
appropriate tag into each of the pages to set up the link.It runs on your 
visitor s computer and so does not require constant downloads from your web site.