I want to debug my PhoneGap app in Xcode, but its Console can not show javascript errors.
Note that with 0.9.2 (released today), console.log has been standardized across the platforms for logging (with debug.log deprecated).
There is a function that is available on the desktop WebView that is not exposed in the iOS UIWebView that will catch all errors (I'm trying to hack that functionality into a plugin, which uses private APIs, but the plugin would only be for development), but for now do what Kris suggested above and put try catch blocks on code and use console.log
To quickly catch possible syntax errors, when developing I have the page loaded in desktop Safari and quickly refresh it with the webkit error console viewable.