• Now with Web 2.0 a lot of stress is being given to client side scripts especially JavaScript. A lot of the sites these days are using AJAX extensively, which in turn extensively use JavaScripts.
• Moreover a lot of JavaScript frameworks like jQuery,MooTools,script.aculo.us etc are being used. It becomes imperative that developers are on top of issues relating to JavaScripts.
• JavaScript is awesome because it is dynamic but at the same time it could get really complicated and debugging it would be equally complex. There are specific tools to handle JavaScript debugging for both IE and Firefox.

IE - JavaScript - Debug

IE8 which is in beta has the Developer Tools built into it. I has options to debug javascript code.

Firefox - JavaScript - Debug

Firefox has the Firebug add-on. I also has options to debug javascript code.