One very handy feature of Visual Studio 2008 is debugging in Javascript. I beleive that working with javascript in VS2008 has became simply brilliant!
Having said that I was building a silverlight application and I tried to do a breakpoint in my Javascript code and guess what…. It did not work !
Ian Griffths (my Silverlight teacher 
) pointed out to me that this problem was caused by having Silverlight debugging selected in Visual Studio… So in order to debug Javascript you must go to the website Property Pages -> Select “Start Options” and uncheck the Silverlight checkbox in the Debuggers sectionas shown in the image below
If this still is not working for you check your Internet Explorer settings . Basically you MUST have the “Disable Script Debugging” in IE unchecked. To find out if this is set correctly go to the “Tools” menu in IE -> “Internet Options” -> ”Advanced” and you should see it now. This must be unchecked.
