Debugging of Javascript

1) In IE –
Simply use external .js file

<script language="javascript" src="JavaScripts/WSCallViaHTC.js" type="text/javascript"></script>

2) In Mozilla –

a) FireBug enables you to step through client script and examine HTML DOM elements.
  It also provides a script console, a command line, and other tools. 

b) The JavaScript Debugger (also known as "Venkman") provides a JavaScript
  debugging environment that includes a source-code browser and other features. 

c) Web Developer Extension enables you to inspect the DOM and CSS styles. 

Suggested Link:

Debugging Javascripts --
http://ajax.asp.net/docs/overview/92684ea0-7bb4-4a34-9203-3aa6394ce375.aspx