How to do Crash Reporting for Web Apps

When I develop iPhone / iPad apps I always use a crash reporting system. My current favorite is Crashlytics - which works great.

For Android apps I've been using the rather more basic ACRA crash reporter.

Now that I'm building a webapp to be run on a client's own server, it seems a good idea to build in some kind of crash reporting system, probably linked off the window.onerror event.

Of course, I could build my own system. But does anyone know a good drop-in solution that sends and collates reports?

Incidentally, I'd prefer a completely client-side solution (i.e. just JavaScript) rather than anything that requires server support.

What is app crash report?

Firebase Crashlytics, a real time crash reporting tool, helps you prioritize and fix your most pervasive crashes based on the impact on real users. Crashlytics also easily integrates into your Android, iOS, macOS, tvOS, and watchOS apps.

Both Exceptional and Airbrake offer javascript implementations