It's the most popular language for providing this kind of functionality - it's used in sites such as Google and Yahoo! - and it can also make web pages quicker, because it runs inside the web browser and doesn't always need to work with a server on the internet to do its thing.

For example, for http://www.unitedstatesartists.org/ we used it to produce a “rollover” map of the United States to display information about a number of artistic projects. When you move your mouse over a project's image on the map, a box displaying summary information about the project pops up. We felt that in this case JavaScript provided an easy and intuitive way to convey this information to the user.

However, we use JavaScript appropriately: We design our web pages to still be functional even if JavaScript is not available in a browser. In our rollover example, if a user doesn't have JavaScript enabled, the web page won't “break”, and the links on the map will still function - it's just that if a user's browser supports it, the pop-ups provide a nice design effect.

There exist a number of popular libraries of JavaScript code that we love to use, e.g. jQuery and Prototype, because they provide great ready-made features, their code is extensively-tested because of its widespread adoption, and they have built-in compatibility with the most common web browsers. Also, JavaScript is the primary language used to provide "Ajax" functionality, where we can update only parts of a web page from the web server, without having to have the server resend the whole page.

So JavaScript has a lot to offer, from providing simple visual enhancements to pages to improving web site performance under stress. Whatever website we're building, we aim to use JavaScript to our advantage to improve the web experience!