React Native Best Practices

Overview

Performance optimization guide for React Native applications, covering JavaScript/React, Native (iOS/Android), and bundling optimizations. Based on Callstack's "Ultimate Guide to React Native Optimization".

When to Apply

Reference these guidelines when:
- Debugging slow/janky UI or animations
- Investigating memory leaks (JS or native)
- Optimizing app startup time (TTI)
- Reducing bundle or app size
- Writing native modules (Turbo Modules)
- Profiling React Native performance
- Reviewing React Native code for performance

Security Notes

• Treat shell commands in these references as local developer operations. Review them before running, prefer version-pinned tooling, and avoid piping remote scripts directly to a shell.
• Treat third-party libraries and plugins as dependencies that still require normal supply-chain controls: pin versions, verify provenance, and update through your standard review process.
• Treat remote chunk loading as first-party artifact delivery only. Prefer app-bundled chunks or signed CI release manifests; hosted chunks must come from trusted HTTPS origins you control and be pinned to the current app release.

Priority-Ordered Guidelines

Impact labels are triage hints: CRITICAL first, HIGH next, MEDIUM when evidence points there.

Quick Reference

Optimization Workflow

Follow this cycle for any performance issue: Measure → Optimize → Re-measure → Validate

1. Measure: Capture baseline metrics before changes. For runtime issues, prefer commit timeline, re-render counts, slow components, heaviest-commit breakdown, and startup/TTI when available. Component tree depth or count are optional context, not substitutes. Do not recommend memoization, atomic state, or compiler changes without a measured render or FPS problem.
2. Optimize: Apply the targeted fix from the relevant reference
3. Re-measure: Run the same measurement to get updated metrics
4. Validate: Confirm improvement (e.g., FPS 45→60, TTI 3.2s→1.8s, bundle 2.1MB→1.6MB)

If metrics did not improve, revert and try the next suggested fix.

Review Guardrails

• Check library versions before suggesting API-specific fixes. Example: FlashList v2 deprecates estimatedItemSize, so do not flag it as missing there.
• Do not suggest useMemo or useCallback dependency changes unless behavior is demonstrably incorrect or profiling shows wasted work tied to that value.
• Do not report stale closures speculatively. Show the stale read path, a repro, or profiler evidence before calling it out.
• When profiling a flow, measure the target interaction itself. Do not treat component tree depth or component count as the main performance evidence.

Critical: FPS & Re-renders

Profile first:

agent-device react-devtools status
agent-device react-devtools wait --connected
agent-device react-devtools profile start
agent-device react-devtools profile stop
agent-device react-devtools profile slow --limit 5
agent-device react-devtools profile rerenders --limit 5
agent-device react-devtools profile timeline --limit 20

Drive the target interaction with normal agent-device commands between profile start and profile stop.

Manual fallback when agent-device is unavailable: open React Native DevTools from Metro (j) or the Dev Menu, use the Profiler tab, and record the same interaction.

For release-build React component profiling, connect @callstack/inspector first so React DevTools can attach to the release app, then run the agent-device react-devtools flow above.

Common fixes:
- Replace ScrollView with FlatList/FlashList/Legend List for long lists
- After profiling shows cascading re-renders, use React Compiler for automatic memoization
- After profiling shows broad store/context updates, use atomic state (Jotai/Zustand) to reduce re-renders
- Use useDeferredValue for expensive computations

Critical: Bundle Size

Analyze bundle:

npx react-native bundle \
  --entry-file index.js \
  --bundle-output output.js \
  --platform ios \
  --sourcemap-output output.js.map \
  --dev false --minify true

npx source-map-explorer output.js --no-border-checks

Verify improvement after optimization:

# Record baseline size before changes
ls -lh output.js  # e.g., Before: 2.1 MB

# After applying fixes, re-bundle and compare
npx react-native bundle --entry-file index.js --bundle-output output.js \
  --platform ios --dev false --minify true
ls -lh output.js  # e.g., After: 1.6 MB  (24% reduction)

Common fixes:
- Avoid barrel imports (import directly from source)
- Remove unnecessary Intl polyfills only after checking Hermes API and method coverage
- Evaluate tree shaking (Expo SDK 52+ experimental unused import/export removal, or Re.Pack only if already configured)
- Enable R8 for Android native code shrinking

High: TTI Optimization

Measure TTI:
- Use react-native-performance for markers
- Only measure cold starts (exclude warm/hot/prewarm)

Common fixes:
- For React Native 0.78 and earlier, disable Android JS bundle compression to enable Hermes mmap
- Use native navigation (react-native-screens)
- Preload commonly-used expensive screens before navigating to them

High: Native Performance

Profile native:
- iOS: Xcode Instruments → Time Profiler
- Android: Android Studio → CPU Profiler

Common fixes:
- Use background threads for heavy native work
- Prefer async over sync Turbo Module methods
- Use C++ for cross-platform performance-critical code

References

Full documentation with code examples in references/:

JavaScript/React (js-*)

Native (native-*)

Bundling (bundle-*)

Problem → Skill Mapping

Attribution

Based on "The Ultimate Guide to React Native Optimization" by Callstack.