I see that there's an onDisconnect() firebase method, however it looks like that method is intended to update other client devices that the current client device has gone offline. For me this isn't very useful. All of our API calls are coming from a central server which handles updates to our other client devices.
Basically, I only have 1 client device communicating with the firebase server... ever. So my question is, when that single device loses connection how will I know? This is probably the most frustrating part of all, Not only do I not know when I lose connection, but I'm still making calls as if each command is successful.
Offline Behavior
Every client sharing a Firebase maintains its own internal version of any active data. When data is updated or saved, it is written to this local version of the Firebase. The Firebase client then synchronizes that data with the Firebase servers and with other clients on a 'best-effort' basis.
As a result, all writes to Firebase will trigger local events immediately, before any data has even been written to the server. This means the app will remain responsive regardless of network latency or Internet connectivity.
Once connectivity is reestablished, we'll receive the appropriate set of events so that the client "catches up" with the current server state, without having to write any custom code.
Take a look at Detecting Connection State in the Firebase Guide. It describes the magic data location /.info/connected that you can monitor for changes in the client's authentication state.
Note: For version 3.0+ see onDisconnect