React Native Best Practices Agent Skill

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

• 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.
• Optimize: Apply the targeted fix from the relevant reference
• Re-measure: Run the same measurement to get updated metrics
• 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/][references]:

JavaScript/React (js-*)

| File | Impact | Description |

|------|--------|-------------|

| [js-lists-flatlist-flashlist.md][js-lists-flatlist-flashlist] | CRITICAL | Replace ScrollView with virtualized lists |

| [js-profile-react.md][js-profile-react] | MEDIUM | agent-device react-devtools profiling |

| [js-measure-fps.md][js-measure-fps] | HIGH | FPS monitoring and measurement |

| [js-memory-leaks.md][js-memory-leaks] | MEDIUM | JS memory leak hunting |

| [js-atomic-state.md][js-atomic-state] | HIGH | Jotai/Zustand patterns |

| [js-concurrent-react.md][js-concurrent-react] | HIGH | useDeferredValue, useTransition |

| [js-react-compiler.md][js-react-compiler] | HIGH | Automatic memoization |

| [js-animations-reanimated.md][js-animations-reanimated] | MEDIUM | Reanimated worklets |

| [js-bottomsheet.md][js-bottomsheet] | HIGH | Bottom sheet optimization |

| [js-uncontrolled-components.md][js-uncontrolled-components] | HIGH | TextInput optimization |

Native (native-*)

| File | Impact | Description |

|------|--------|-------------|

| [native-turbo-modules.md][native-turbo-modules] | HIGH | Building fast native modules |

| [native-sdks-over-polyfills.md][native-sdks-over-polyfills] | HIGH | Native vs JS libraries |

| [native-measure-tti.md][native-measure-tti] | HIGH | TTI measurement setup |

| [native-threading-model.md][native-threading-model] | HIGH | Turbo Module threads |

| [native-profiling.md][native-profiling] | MEDIUM | Xcode/Android Studio profiling |

| [native-platform-setup.md][native-platform-setup] | MEDIUM | iOS/Android tooling guide |

| [native-view-flattening.md][native-view-flattening] | MEDIUM | View hierarchy debugging |

| [native-memory-patterns.md][native-memory-patterns] | MEDIUM | C++/Swift/Kotlin memory |

| [native-memory-leaks.md][native-memory-leaks] | MEDIUM | Native memory leak hunting |

| [native-android-16kb-alignment.md][native-android-16kb-alignment] | CRITICAL | Third-party library alignment for Google Play |

Bundling (bundle-*)

| File | Impact | Description |

|------|--------|-------------|

| [bundle-barrel-exports.md][bundle-barrel-exports] | CRITICAL | Avoid barrel imports |

| [bundle-analyze-js.md][bundle-analyze-js] | CRITICAL | JS bundle visualization |

| [bundle-tree-shaking.md][bundle-tree-shaking] | HIGH | Dead code elimination |

| [bundle-analyze-app.md][bundle-analyze-app] | HIGH | App size analysis |

| [bundle-r8-android.md][bundle-r8-android] | HIGH | Android code shrinking |

| [bundle-hermes-mmap.md][bundle-hermes-mmap] | HIGH | Disable bundle compression |

| [bundle-native-assets.md][bundle-native-assets] | HIGH | Asset catalog setup |

| [bundle-library-size.md][bundle-library-size] | MEDIUM | Evaluate dependencies |

| [bundle-code-splitting.md][bundle-code-splitting] | MEDIUM | Remote chunk loading safeguards |

Problem → Skill Mapping

| Problem | Start With |

|---------|------------|

| App feels slow/janky | [js-measure-fps.md][js-measure-fps] → [js-profile-react.md][js-profile-react] |

| Too many re-renders | [js-profile-react.md][js-profile-react] → [js-react-compiler.md][js-react-compiler] |

| Slow startup (TTI) | [native-measure-tti.md][native-measure-tti] → [bundle-analyze-js.md][bundle-analyze-js] |

| Large app size | [bundle-analyze-app.md][bundle-analyze-app] → [bundle-r8-android.md][bundle-r8-android] |

| Memory growing | [js-memory-leaks.md][js-memory-leaks] or [native-memory-leaks.md][native-memory-leaks] |

| Animation drops frames | [js-animations-reanimated.md][js-animations-reanimated] |

| Bottom sheet jank/re-renders | [js-bottomsheet.md][js-bottomsheet] → [js-animations-reanimated.md][js-animations-reanimated] |

| List scroll jank | [js-lists-flatlist-flashlist.md][js-lists-flatlist-flashlist] |

| TextInput lag | [js-uncontrolled-components.md][js-uncontrolled-components] |

| Native module slow | [native-turbo-modules.md][native-turbo-modules] → [native-threading-model.md][native-threading-model] |

| Native library alignment issue | [native-android-16kb-alignment.md][native-android-16kb-alignment] |

[references]: references/

[js-lists-flatlist-flashlist]: references/js-lists-flatlist-flashlist.md

[js-profile-react]: references/js-profile-react.md

[js-measure-fps]: references/js-measure-fps.md

[js-memory-leaks]: references/js-memory-leaks.md

[js-atomic-state]: references/js-atomic-state.md

[js-concurrent-react]: references/js-concurrent-react.md

[js-react-compiler]: references/js-react-compiler.md

[js-animations-reanimated]: references/js-animations-reanimated.md

[js-bottomsheet]: references/js-bottomsheet.md

[js-uncontrolled-components]: references/js-uncontrolled-components.md

[native-turbo-modules]: references/native-turbo-modules.md

[native-sdks-over-polyfills]: references/native-sdks-over-polyfills.md

[native-measure-tti]: references/native-measure-tti.md

[native-threading-model]: references/native-threading-model.md

[native-profiling]: references/native-profiling.md

[native-platform-setup]: references/native-platform-setup.md

[native-view-flattening]: references/native-view-flattening.md

[native-memory-patterns]: references/native-memory-patterns.md

[native-memory-leaks]: references/native-memory-leaks.md

[native-android-16kb-alignment]: references/native-android-16kb-alignment.md

[bundle-barrel-exports]: references/bundle-barrel-exports.md

[bundle-analyze-js]: references/bundle-analyze-js.md

[bundle-tree-shaking]: references/bundle-tree-shaking.md

[bundle-analyze-app]: references/bundle-analyze-app.md

[bundle-r8-android]: references/bundle-r8-android.md

[bundle-hermes-mmap]: references/bundle-hermes-mmap.md

[bundle-native-assets]: references/bundle-native-assets.md

[bundle-library-size]: references/bundle-library-size.md

[bundle-code-splitting]: references/bundle-code-splitting.md

Attribution

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