Agent Skills: React 19 Best Practices

React 19 Best Practices

Comprehensive performance optimization guide for React 19/19.2 applications. Contains 41 rules across 8 categories, prioritized by impact from critical (concurrent rendering, server components) to incremental (component patterns).

When to Apply

• Writing new React components or refactoring existing ones
• Optimizing re-render performance or bundle size
• Using concurrent features (useTransition, useDeferredValue, Activity)
• Setting up Server Components or server/client boundaries
• Implementing form actions, optimistic updates, or data fetching
• Configuring React Compiler for automatic memoization
• Reviewing React code for common anti-patterns

Rule Categories

| Category | Impact | Rules | Key Topics | |----------|--------|-------|------------| | Concurrent Rendering | CRITICAL | 6 | useTransition, useDeferredValue, Activity, batching | | Server Components | CRITICAL | 6 | RSC boundaries, data fetching, streaming | | Actions & Forms | HIGH | 5 | Form actions, useActionState, useOptimistic | | Data Fetching | HIGH | 5 | use() hook, cache(), Suspense, error boundaries | | State Management | MEDIUM-HIGH | 5 | Derived values, context optimization, useReducer | | Memoization & Performance | MEDIUM | 5 | React Compiler, useMemo, useCallback, React.memo | | Effects & Events | MEDIUM | 5 | useEffectEvent, cleanup, external stores | | Component Patterns | LOW-MEDIUM | 4 | Composition, controlled vs uncontrolled, key reset |

Quick Reference

Critical patterns — get these right first:

• Fetch data in Server Components, not Client Components
• Push 'use client' boundaries as low as possible
• Use startTransition for expensive non-blocking updates
• Use <Activity> to preserve state across tab/page switches

Common mistakes — avoid these anti-patterns:

• Creating promises inside Client Components for use() (causes infinite loops)
• Memoizing everything (use React Compiler v1.0+ instead)
• Using effects for derived state or user event handling
• Placing 'use client' too high in the component tree

Table of Contents

1. Concurrent Rendering — CRITICAL
   • 1.1 Use Activity for Pre-Rendering and State Preservation — HIGH (eliminates navigation re-render cost, preserves user input state)
   • 1.2 Avoid Suspense Fallback Thrashing — HIGH (prevents 200-500ms layout shift flicker)
   • 1.3 Leverage Automatic Batching for Fewer Renders — HIGH (batches multiple setState calls into a single render in all contexts)
   • 1.4 Use useDeferredValue for Derived Expensive Values — CRITICAL (prevents jank in derived computations)
   • 1.5 Use useTransition for Non-Blocking Updates — CRITICAL (maintains <50ms input latency during heavy state updates)
   • 1.6 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 (significantly reduces client JS bundle, eliminates client-side data 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 (forms work without JS loaded, eliminates e.preventDefault() boilerplate)
   • 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 (0ms perceived latency for mutations, automatic rollback on server failure)
   • 3.5 Validate Forms on Server with Actions — MEDIUM (prevents client-only validation bypass, single source of truth for form errors)
4. Data Fetching — HIGH
   • 4.1 Fetch Data in Parallel with Promise.all — MEDIUM-HIGH (eliminates waterfalls, 2-5x faster)
   • 4.2 Use cache() for Request Deduplication — HIGH (eliminates duplicate fetches per server request)
   • 4.3 Use Error Boundaries with Suspense — MEDIUM (isolates failures to individual components, prevents full-page crashes)
   • 4.4 Use Suspense for Declarative Loading States — HIGH (eliminates loading state boilerplate, enables parallel data fetch coordination)
   • 4.5 Use the use() Hook for Promises in Render — HIGH (eliminates useEffect+useState fetch pattern, integrates with Suspense boundaries)
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 (eliminates impossible state combinations, enables unit-testable state logic)
6. Memoization & Performance — MEDIUM
   • 6.1 Avoid Premature Memoization — MEDIUM (removes 0.1-0.5ms per-render overhead from unnecessary memoization)
   • 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 expensive re-renders, 5-50ms savings per unchanged component)
   • 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 O(n) recalculations on re-renders with unchanged dependencies)
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 (prevents unnecessary effect re-runs from non-reactive value changes)
   • 7.5 Use useSyncExternalStore for External Subscriptions — MEDIUM (prevents tearing in concurrent rendering, ensures SSR-safe external state)
8. Component Patterns — LOW-MEDIUM
   • 8.1 Choose Controlled vs Uncontrolled Appropriately — LOW-MEDIUM (prevents form state sync bugs, enables real-time validation)
   • 8.2 Prefer Composition Over Props Explosion — LOW-MEDIUM (reduces prop drilling depth, enables independent component reuse)
   • 8.3 Use Key to Reset Component State — LOW-MEDIUM (forces full component remount, eliminates stale state after identity changes)
   • 8.4 Use Render Props for Inversion of Control — LOW-MEDIUM (enables parent-controlled rendering without child prop explosion)

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/blog/2025/10/07/react-compiler-1
5. https://react.dev/learn/you-might-not-need-an-effect
6. 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