Post-Mortem: How Firebase RTDB Array Coercion Bricked My React Frontend

1. The Incident

I was building Friday OS—my self-hosted, local-first AI Personal OS. The stack uses Firebase Realtime Database (RTDB) for real-time state syncing. I hit "Delete" on one of the log entries in my dashboard. The backend returned a clean 200 OK and Firebase Console updated immediately, but my React frontend went completely white. The console output showed:

TypeError: items.map is not a function

2. Root Cause: The Non-Existent Array

Firebase RTDB is a JSON tree. Under the hood, it has no native array type. When you write an array: