I want to debug my PhoneGap app in Xcode, but its Console can not show javascript errors.
For making javascript debugging work in Xcode I would take a look at the following.
http://phonegap.com/2011/05/18/debugging-phonegap-javascript/
http://www.daveoncode.com/2010/01/12/debugging-phonegap-applications-using-xcode-console/
As as far as additional troubleshooting goes...
To start with you could run the app in safari on you pc and utilize safari's debugger (or chrome as both are running similar rendering engines). This won't hit on the advanced logic errors and many of your api issues but it at the very least should help with troubleshooting many issues (basic javascript, HTML5 etc....).