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Building an Offline-First App With React Native and Firebase

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When the power goes out on a schedule, a generator runs every day — and it comes with a maintenance schedule: oil, filters and spark plugs, all by engine hours. Most portable generators have no hour meter, so I built Generator Tracker: a one-tap Start/Stop logbook with fuel tracking, maintenance reminders and monthly charts. The code is open source (MIT): https://github.com/yuri-val/GeneratorTracker

This post isn't about the app itself, but about the part that turned out to be the most interesting engineering problem: sync for an app people use precisely when there's no connection.

Stack

  • Expo SDK 57, React Native 0.86, React 19.2, TypeScript.
  • AsyncStorage as the local store; Firebase Auth + Firestore for optional sync.
  • iOS: native SwiftUI forms via @expo/ui and system tabs with Liquid Glass. Android: Material 3. Web stays on React Native Paper.

Principle 1: the phone is the source of truth

During a blackout mobile internet...

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