Debugging JavaScript

Anyone who uses JavaScript has probably wished from time to time
 that they could step through a script. Debugging comes in handy when
 you need to track down an error or figure out why 
 a client-side script is behaving oddly.

Unfortunately, there aren't many tools for the job. I know of
 two, Microsoft Script Debugger and Venkman JavaScript Debugger. 
 A lot of developers, myself included, used what we called the poor man's debugger. 
 Behind that grand name was a series of alert calls, 
 which let us trace the program's execution and print out its state along the way.
 It looks like those days may be over.

If you use Mozilla, you can install optional components
 along with the browser, including the 
 Venkman JavaScript Debugger. I didn't find it especially intuitive
 until I came across this detailed guide, Learning the JavaScript debugger Venkman
(via czilla.cz)