WHERE USED

JavaScript is a programming language that allows you to implement complex things on web pages — every time a web page does more than just sit there and display static information for you to look at — displaying timely content updates, or interactive maps, or animated 2D/3D graphics, or scrolling video jukeboxes, etc. — you can bet that JavaScript is probably involved. It is the third layer of the layer cake of standard web technologies, two of which (HTML and CSS) we have covered in much more detail in other parts of the Learning Area.
JavaScript is very easy to implement. All you need to do is put your code in the HTML document and tell the browser that it is JavaScript. It works on web users’ computers — even when they are offline!J avaScript allows you to create highly responsive interfaces that improve the user experience and provide dynamic functionality, without having to wait for the server to react and show another page.

ADVANTAGES

Javascript is executed on the client side
This means that the code is executed on the user's processor instead of the web server thus saving bandwidth and strain on the web server.
Javascript is a relatively easy language
The Javascript language is relatively easy to learn and comprises of syntax that is close to English. It uses the DOM model that provides plenty of prewritten functionality to the various objects on pages making it a breeze to develop a script to solve a custom purpose.
Javascript is relatively fast to the end user
As the code is executed on the user's computer, results and processing is completed almost instantly depending on the task (tasks in javascript on web pages are usually simple so as to prevent being a memory hog) as it does not need to be processed in the site's web server and sent back to the user consuming local as well as server bandwidth.
Extended functionality to web pages
Third party add-ons like Greasemonkey enable Javascript developers to write snippets of Javascript which can execute on desired web pages to extend its functionality. If you use a website and require a certain feature to be included, you can write it yourself and use an add-on like Greasemonkey to implement it on the web page.

Summary

JavaScript is a wonderful technology to use on the web. It is not that hard to learn and it is very versatile. It plays nicely with other web technologies — such as HTML and CSS — and can even interact with plugins such as Flash. JavaScript allows us to build highly responsive user interfaces, prevent frustrating page reloads and even fix support issues for CSS. Using the right browser add-ons (such as Google Gears or Yahoo Browser Plus) you can even use JavaScript to make online systems available offline and sync automatically once the computer goes online.
JavaScript is also not restricted to browsers. The speed and small memory footprint of JavaScript in comparison to other languages brings up more and more uses for it — from automating repetitive tasks in programs like Illustrator, up to using it as a server-side language with a standalone parser. The future is wide open; no matter what you do as a web developer in the nearer future, I am quite sure you will have to work with JavaScript sooner or later.