HabitGrid — Expo Habit Tracker

HabitGrid is the habit app I actually wanted: one screen, zero accounts, zero network. Every byte stays on the device. It doubles as my reference project for a modern Expo stack — typed routing with expo-router, a clean domain/UI split, and CI that runs the real logic tests without a device.

What it does

• ✅ One-tap tracking — mark a habit done today; tap again to undo.
• 🔥 Streaks — consecutive-day counting with a one-day grace rule, so a run survives until yesterday instead of dying at midnight.
• 🟦 14-day dot grid — GitHub-style history per habit.
• 📊 30-day progress bars — completion ratio at a glance.
• 📴 Fully offline — AsyncStorage persistence, no server, no tracking, dark UI.

The architecture that matters

lib/habits.ts      ← pure domain: streak math, grid helpers (no React imports)
lib/storage.ts     ← AsyncStorage adapter
app/index.tsx      ← expo-router screen: list, FAB, add modal
components/HabitCard.tsx

The domain layer is deliberately React-free: habit days are UTC YYYY-MM-DD strings, toggleToday is immutable, and streak edge cases (gaps, grace days, DST-proof day keys) run in plain Jest — 10 tests, 1 second, no emulator. That is the discipline I want in every mobile codebase: the interesting logic testable anywhere.

npm install --legacy-peer-deps
npx expo start          # scan the QR with Expo Go

MIT licensed.