Advantages include Cross-browser support, validating data on the client, and being able to create more sophisticated user interfaces.

JavaScript effects are also much faster to download than some other front-end technologies like Flash and Java applets. In fact, unless you're writing a massive JavaScript application, it's quite likely that no significant extra download time will be added to a page by using JavaScript on it. Nor do users need to download a plugin before they can view your JavaScript, as they would with Flash for example, they simply need a browser that supports it - and, of course, most modern browsers do. Other advantages include the fact that you don't need any extra tools to write JavaScript, any plain text or HTML editor will do, so there's no expensive development software to buy. It's also an easy language to learn, and there's a thriving and supportive online community of JavaScript developers and information resources. The disadvantages of JavaScript, as with most web development, are almost entirely related to browser compatibility. While the advances in browser programmability we've seen over recent years are, generally speaking, a good thing, if you don't implement them with care you can create a lot of inconsistencies and broken pages quite unintentionally using JavaScript. Code that works just great on IE4 might not work at all on Netscape 4, what works in NN6 doesn't always work in NN 4, and so on. In essence, there are two main problems with JavaScript and browsers: · The different JavaScript versions in different browsers. · Browser programmability: the HTML elements and features of the browser that can be accessed through any scripting language. (IE4 , for example, makes most of the page and HTML accessible to scripts, but Navigator 4 limits what can be accessed and manipulated.) it allow Developer to add Dynamic content like image swapping , Rollover, which is not available in HTML or CSS CSS basically used for ----> styling ur pages JavaScript used to allow ----> script access to objects embedded in other application.