How can I detect whether a user has installed a specific iPhone app via JavaScript?

Answer 1

You very likely can't do this. Even in a native app, all you can call is UIApplication's canOpenURL: method, which just tells you if some app will open the URL, not which one. I have no idea if this function is exposed in JavaScript; very likely not (I wouldn't want malicious javascript probing my phone for which URLs it can open).