Firebase: change the location of the service worker

As Michael has called out, the method to use is useServiceWorker(<registration>).

https://firebase.google.com/docs/reference/js/firebase.messaging.Messaging#useServiceWorker

The Messaging class is what is returned when you callfirebase.message().

So the sample would be:

navigator.serviceWorker.register('./example/sw.js')
.then((registration) => {
  messaging.useServiceWorker(registration);

  // Request permission and get token.....
});

I just tried this on the demo app on Github without issue: https://github.com/firebase/quickstart-js/tree/master/messaging

You should be able to do firebase.messaging().useServiceWorker(registration) and pass in an existing service worker registration. Note that you should do this as early as possible, before any calls to e.g. getToken() or onMessage().