react-native-best-practices

Files (44)

• POWER.md
• SKILL.md
• openai.yaml · agents/
• bundle-analyze-app.md · references/
• bundle-analyze-js.md · references/
• bundle-barrel-exports.md · references/
• bundle-code-splitting.md · references/
• bundle-hermes-mmap.md · references/
• bundle-library-size.md · references/
• bundle-native-assets.md · references/
• bundle-r8-android.md · references/
• bundle-tree-shaking.md · references/
• bundle-treemap-source-map-explorer.png · references/images/
• controlled-textinput-pingpong.png · references/images/
• devtools-flamegraph.png · references/images/
• emerge-xray-ios.png · references/images/
• expo-atlas-treemap.png · references/images/
• flashlight-flatlist-vs-flashlist.png · references/images/
• fps-drop-graph.png · references/images/
• memory-heap-snapshot.png · references/images/
• tti-warm-start-diagram.png · references/images/
• view-hierarchy-flattening.png · references/images/
• xcode-instruments-templates.png · references/images/
• xcode-thread-view.png · references/images/
• js-animations-reanimated.md · references/
• js-atomic-state.md · references/
• js-bottomsheet.md · references/
• js-concurrent-react.md · references/
• js-lists-flatlist-flashlist.md · references/
• js-measure-fps.md · references/
• js-memory-leaks.md · references/
• js-profile-react.md · references/
• js-react-compiler.md · references/
• js-uncontrolled-components.md · references/
• native-android-16kb-alignment.md · references/
• native-measure-tti.md · references/
• native-memory-leaks.md · references/
• native-memory-patterns.md · references/
• native-platform-setup.md · references/
• native-profiling.md · references/
• native-sdks-over-polyfills.md · references/
• native-threading-model.md · references/
• native-turbo-modules.md · references/
• native-view-flattening.md · references/

SKILL.md (first lines)

# 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:**
```bash
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`](https://github.com/callstackincubator/inspector#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:**
```bash
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:**
```bash
# 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:**

From ilteoood/harness, MIT.

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