React Native

React Native Development Rules

Component Performance

• FlatList for any list over 10 items — ScrollView with map loads everything in memory, FlatList virtualizes
• keyExtractor must return stable unique strings — using index causes bugs on reorder and deletion
• React.memo prevents re-renders when props unchanged — wrap pure display components
• useCallback for functions passed to child components — new function reference triggers child re-render
• Avoid inline styles in render — creates new object every render, extract to StyleSheet.create

State Management

• useState is fine for component-local state — don't add Redux/Zustand for a toggle
• Lift state to lowest common ancestor only — higher causes unnecessary re-renders
• useMemo for expensive computations — but don't overuse, caching has overhead
• Context re-renders all consumers on any change — split contexts by update frequency
• Avoid storing derived data in state — compute during render from source state

Navigation

• React Navigation is the standard — Expo Router for file-based routing in Expo projects
• Stack screens stay mounted by default — clean up subscriptions and timers in useEffect cleanup
• Pass serializable params only — functions and complex objects break deep linking and state persistence
• useFocusEffect for screen-specific side effects — runs on focus, not just mount
• navigation.reset for auth flows — clears back stack, prevents returning to login after sign-in

Styling

• StyleSheet.create outside component body — creates styles once, not every render
• Flexbox defaults differ from web — flexDirection: 'column', no display: flex needed
• Dimensions in density-independent pixels — don't use pixel values from design tools directly
• Platform.select for platform-specific styles — cleaner than conditionals in style objects
• No CSS inheritance — text styles don't cascade, each Text needs explicit styling

Native Modules

• Expo modules cover most needs — avoid ejecting for common features like camera, location, notifications
• expo-dev-client enables native modules without full eject — best of both worlds
• React Native New Architecture (Fabric, TurboModules) is opt-in — check library compatibility before enabling
• Native crashes don't show in JS debugger — check Xcode/Android Studio logs

Performance Debugging

• Hermes engine should be enabled — significantly faster startup and lower memory
• InteractionManager.runAfterInteractions defers heavy work — keeps animations smooth
• useNativeDriver: true for animations — runs on UI thread, not JS thread
• console.log in production kills performance — remove or use __DEV__ guard
• Flipper for debugging — network, layout, performance profiling

Images

• Use resizeMode appropriately — cover crops, contain letterboxes, stretch distorts
• Prefetch images for smooth UX: Image.prefetch(url) before displaying
• Local images need explicit dimensions — remote images can use aspect ratio if one dimension set
• SVGs via react-native-svg — better scaling than PNGs for icons
• Cache remote images with react-native-fast-image — default Image has no persistent cache

Common Mistakes

• async in useEffect directly — must define async function inside, then call it
• Missing key warnings in lists — always use unique, stable keys
• Assuming web React patterns work — no DOM, no CSS, different event system
• Forgetting cleanup in useEffect — subscriptions, timers, listeners leak without cleanup return
• Testing only on one platform — iOS and Android differ in behavior, test both regularly

Platform Differences

• Android needs explicit overflow: 'hidden' for border radius clipping — iOS clips by default
• Shadows: iOS uses shadow* props, Android uses elevation
• StatusBar behavior differs — test visibility and color on both platforms
• Back button is Android-only — handle with BackHandler or navigation listeners
• Push notifications setup differs significantly — platform-specific configuration required

Build & Release

• npx react-native clean for unexplained build failures — clears caches and derived data
• iOS: cd ios && pod install after adding native dependencies — often forgotten step
• Android: cd android && ./gradlew clean for stubborn build issues
• EAS Build (Expo) simplifies CI/CD — handles signing, versioning, submission
• Test release builds locally before submitting — development and production behavior differ