Agent Skills: React 19 Best Practices

React 19 Best Practices

Comprehensive performance optimization guide for React 19 applications. Contains 40 rules across 8 categories, prioritized by impact from critical (concurrent rendering, server components) to incremental (component patterns).

Table of Contents

1. Concurrent Rendering — CRITICAL
   • 1.1 Avoid Suspense Fallback Thrashing — HIGH (prevents flickering, smoother UX)
   • 1.2 Leverage Automatic Batching for Fewer Renders — HIGH (32% fewer renders in heavy updates)
   • 1.3 Use useDeferredValue for Derived Expensive Values — CRITICAL (prevents jank in derived computations)
   • 1.4 Use useTransition for Non-Blocking Updates — CRITICAL (keeps UI responsive during heavy updates)
   • 1.5 Write Concurrent-Safe Components — MEDIUM-HIGH (prevents bugs in concurrent rendering)
2. Server Components — CRITICAL
   • 2.1 Avoid Client-Only Libraries in Server Components — MEDIUM-HIGH (prevents build errors, correct component placement)
   • 2.2 Enable Streaming with Nested Suspense — MEDIUM-HIGH (progressive loading, faster TTFB)
   • 2.3 Fetch Data in Server Components — CRITICAL (38% less client JS, no client waterfalls)
   • 2.4 Minimize Server/Client Boundary Crossings — CRITICAL (reduces serialization overhead, smaller bundles)
   • 2.5 Pass Only Serializable Props to Client Components — HIGH (prevents runtime errors, ensures correct hydration)
   • 2.6 Use Composition to Mix Server and Client Components — HIGH (maintains server rendering for static content)
3. Actions & Forms — HIGH
   • 3.1 Use Form Actions Instead of onSubmit — HIGH (progressive enhancement, simpler code)
   • 3.2 Use useActionState for Form State Management — HIGH (declarative form handling, automatic pending states)
   • 3.3 Use useFormStatus for Submit Button State — MEDIUM-HIGH (proper loading indicators, prevents double submission)
   • 3.4 Use useOptimistic for Instant UI Feedback — HIGH (instant perceived response, auto-rollback on failure)
   • 3.5 Validate Forms on Server with Actions — MEDIUM (secure validation, consistent error handling)
4. Data Fetching — HIGH
   • 4.1 Fetch Data in Parallel with Promise.all — MEDIUM-HIGH (eliminates waterfalls, 2-5× faster)
   • 4.2 Use cache() for Request Deduplication — HIGH (eliminates duplicate fetches per render)
   • 4.3 Use Error Boundaries with Suspense — MEDIUM (graceful error recovery, isolated failures)
   • 4.4 Use Suspense for Declarative Loading States — HIGH (cleaner code, coordinated loading UI)
   • 4.5 Use the use() Hook for Promises in Render — HIGH (cleaner async component code, Suspense integration)
5. State Management — MEDIUM-HIGH
   • 5.1 Calculate Derived Values During Render — MEDIUM (eliminates sync bugs, simpler code)
   • 5.2 Split Context to Prevent Unnecessary Re-renders — MEDIUM (reduces re-renders from context changes)
   • 5.3 Use Functional State Updates for Derived Values — MEDIUM-HIGH (prevents stale closures, stable callbacks)
   • 5.4 Use Lazy Initialization for Expensive Initial State — MEDIUM-HIGH (prevents expensive computation on every render)
   • 5.5 Use useReducer for Complex State Logic — MEDIUM (clearer state transitions, easier testing)
6. Memoization & Performance — MEDIUM
   • 6.1 Avoid Premature Memoization — MEDIUM (memoization has overhead, measure first)
   • 6.2 Leverage React Compiler for Automatic Memoization — MEDIUM (automatic optimization, less manual code)
   • 6.3 Use React.memo for Expensive Pure Components — MEDIUM (skips re-render when props unchanged)
   • 6.4 Use useCallback for Stable Function References — MEDIUM (prevents child re-renders from reference changes)
   • 6.5 Use useMemo for Expensive Calculations — MEDIUM (skips expensive recalculation on re-renders)
7. Effects & Events — MEDIUM
   • 7.1 Always Clean Up Effect Side Effects — MEDIUM (prevents memory leaks, stale callbacks)
   • 7.2 Avoid Effects for Derived State and User Events — MEDIUM (eliminates sync bugs, simpler code)
   • 7.3 Avoid Object and Array Dependencies in Effects — MEDIUM (prevents infinite loops, unnecessary re-runs)
   • 7.4 Use useEffectEvent for Non-Reactive Logic — MEDIUM (separates reactive from non-reactive code)
   • 7.5 Use useSyncExternalStore for External Subscriptions — MEDIUM (correct subscription handling, SSR compatible)
8. Component Patterns — LOW-MEDIUM
   • 8.1 Choose Controlled vs Uncontrolled Appropriately — LOW-MEDIUM (correct data flow, proper form handling)
   • 8.2 Prefer Composition Over Props Explosion — LOW-MEDIUM (more flexible, reusable components)
   • 8.3 Use Key to Reset Component State — LOW-MEDIUM (correct state isolation, proper resets)
   • 8.4 Use Render Props for Inversion of Control — LOW-MEDIUM (flexible rendering, shared logic)

References

1. https://react.dev
2. https://react.dev/blog/2024/12/05/react-19
3. https://react.dev/blog/2025/10/01/react-19-2
4. https://react.dev/learn/you-might-not-need-an-effect
5. https://github.com/facebook/react

Related Skills

• For Next.js 16 App Router, see nextjs-16-app-router skill
• For client-side form handling, see react-hook-form skill
• For data caching with TanStack Query, see tanstack-query skill