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name: react-native-best-practices description: Provides React Native performance optimization guidelines for FPS, TTI, bundle size, memory leaks, re-renders, and animations. Applies to tasks involving Hermes optimization, JS thread blocking, bridge overhead, FlashList, native modules, or debugging jank and frame drops. license: MIT

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

| Priority | Category | Impact | Prefix | |----------|----------|--------|--------| | 1 | FPS & Re-renders | CRITICAL | js-* | | 2 | Bundle Size | CRITICAL | bundle-* | | 3 | TTI Optimization | HIGH | native-*, bundle-* | | 4 | Native Performance | HIGH | native-* | | 5 | Memory Management | MEDIUM-HIGH | js-*, native-* | | 6 | Animations | MEDIUM | js-* |

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-*)

| File | Impact | Description | |------|--------|-------------| | js-lists-flatlist-flashlist.md | CRITICAL | Replace ScrollView with virtualized lists | | js-profile-react.md | MEDIUM | agent-device react-devtools profiling | | js-measure-fps.md | HIGH | FPS monitoring and measurement | | js-memory-leaks.md | MEDIUM | JS memory leak hunting | | js-atomic-state.md | HIGH | Jotai/Zustand patterns | | js-concurrent-react.md | HIGH | useDeferredValue, useTransition | | js-react-compiler.md | HIGH | Automatic memoization | | js-animations-reanimated.md | MEDIUM | Reanimated worklets | | js-bottomsheet.md | HIGH | Bottom sheet optimization | | js-uncontrolled-components.md | HIGH | TextInput optimization |

Native (native-*)

| File | Impact | Description | |------|--------|-------------| | native-turbo-modules.md | HIGH | Building fast native modules | | native-sdks-over-polyfills.md | HIGH | Native vs JS libraries | | native-measure-tti.md | HIGH | TTI measurement setup | | native-threading-model.md | HIGH | Turbo Module threads | | native-profiling.md | MEDIUM | Xcode/Android Studio profiling | | native-platform-setup.md | MEDIUM | iOS/Android tooling guide | | native-view-flattening.md | MEDIUM | View hierarchy debugging | | native-memory-patterns.md | MEDIUM | C++/Swift/Kotlin memory | | native-memory-leaks.md | MEDIUM | Native memory leak hunting | | native-android-16kb-alignment.md | CRITICAL | Third-party library alignment for Google Play |

Bundling (bundle-*)

| File | Impact | Description | |------|--------|-------------| | bundle-barrel-exports.md | CRITICAL | Avoid barrel imports | | bundle-analyze-js.md | CRITICAL | JS bundle visualization | | bundle-tree-shaking.md | HIGH | Dead code elimination | | bundle-analyze-app.md | HIGH | App size analysis | | bundle-r8-android.md | HIGH | Android code shrinking | | bundle-hermes-mmap.md | HIGH | Disable bundle compression | | bundle-native-assets.md | HIGH | Asset catalog setup | | bundle-library-size.md | MEDIUM | Evaluate dependencies | | bundle-code-splitting.md | MEDIUM | Remote chunk loading safeguards |

Problem → Skill Mapping

| Problem | Start With | |---------|------------| | App feels slow/janky | js-measure-fps.md → js-profile-react.md | | Too many re-renders | js-profile-react.md → js-react-compiler.md | | Slow startup (TTI) | native-measure-tti.md → bundle-analyze-js.md | | Large app size | bundle-analyze-app.md → bundle-r8-android.md | | Memory growing | js-memory-leaks.md or native-memory-leaks.md | | Animation drops frames | js-animations-reanimated.md | | Bottom sheet jank/re-renders | js-bottomsheet.md → js-animations-reanimated.md | | List scroll jank | js-lists-flatlist-flashlist.md | | TextInput lag | js-uncontrolled-components.md | | Native module slow | native-turbo-modules.md → native-threading-model.md | | Native library alignment issue | native-android-16kb-alignment.md |

Attribution

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