react-best-practices

name	react-best-practices
description	React and Next.js performance optimization guidelines from Vercel Engineering. This skill should be used when writing, reviewing, or refactoring React/Next.js code to ensure optimal performance patterns. Triggers on tasks involving React components, Next.js pages, data fetching, bundle optimization, or performance improvements.
license	MIT
metadata	{"author":"vercel","version":"1.0.0"}

Vercel React Best Practices

57 performance rules across 8 categories, prioritized by impact.

React Compiler (React 19+): If the React Compiler is enabled in your project, it automatically memoizes components, hooks return values, and expensive computations. Manual useMemo, useCallback, and React.memo are no longer needed in most cases — the compiler handles them. Rules marked ⚙️ below are automatically optimized by the compiler. Keep these rules as fallback guidance for projects without the compiler enabled.

useActionState (React 19+): For form submissions with server actions, prefer useActionState over manual useState + onSubmit handlers. It integrates with React's transition system and works with progressive enhancement.

Rule Categories by Priority

Priority	Category	Impact	Prefix
1	Eliminating Waterfalls	Core	async-
2	Bundle Size Optimization	Core	bundle-
3	Server-Side Performance	Recommended	server-
4	Client-Side Data Fetching	Recommended	client-
5	Re-render Optimization	Situational	rerender-
6	Rendering Performance	Situational	rendering-
7	JavaScript Performance	Supplemental	js-
8	Advanced Patterns	Supplemental	advanced-

Quick Reference

1. Eliminating Waterfalls (Core)

• async-defer-await - Move await into branches where actually used
• async-parallel - Use Promise.all() for independent operations
• async-dependencies - Use better-all for partial dependencies
• async-api-routes - Start promises early, await late in API routes
• async-suspense-boundaries - Use Suspense to stream content

2. Bundle Size Optimization (Core)

• bundle-barrel-imports - Import directly, avoid barrel files
• bundle-dynamic-imports - Use next/dynamic for heavy components
• bundle-defer-third-party - Load analytics/logging after hydration
• bundle-conditional - Load modules only when feature is activated
• bundle-preload - Preload on hover/focus for perceived speed

3. Server-Side Performance (Recommended)

• server-auth-actions - Authenticate server actions like API routes
• server-cache-react - Use React.cache() for per-request deduplication
• server-cache-lru - Use LRU cache for cross-request caching
• server-dedup-props - Avoid duplicate serialization in RSC props
• server-serialization - Minimize data passed to client components
• server-parallel-fetching - Restructure components to parallelize fetches
• server-after-nonblocking - Use after() for non-blocking operations

4. Client-Side Data Fetching (Recommended)

• client-swr-dedup - Use SWR for automatic request deduplication
• client-event-listeners - Deduplicate global event listeners
• client-passive-event-listeners - Use passive listeners for scroll
• client-localstorage-schema - Version and minimize localStorage data

5. Re-render Optimization (Situational)

• rerender-defer-reads - Don't subscribe to state only used in callbacks
• rerender-memo - Extract expensive work into memoized components ⚙️
• rerender-memo-with-default-value - Hoist default non-primitive props ⚙️
• rerender-dependencies - Use primitive dependencies in effects
• rerender-derived-state - Subscribe to derived booleans, not raw values
• rerender-derived-state-no-effect - Derive state during render, not effects
• rerender-functional-setstate - Use functional setState for stable callbacks ⚙️
• rerender-lazy-state-init - Pass function to useState for expensive values
• rerender-simple-expression-in-memo - Avoid memo for simple primitives ⚙️
• rerender-move-effect-to-event - Put interaction logic in event handlers
• rerender-transitions - Use startTransition for non-urgent updates
• rerender-use-ref-transient-values - Use refs for transient frequent values

⚙️ = Automatically handled by React Compiler when enabled. No manual useMemo/useCallback/React.memo needed.

6. Rendering Performance (Situational)

• rendering-animate-svg-wrapper - Animate div wrapper, not SVG element
• rendering-content-visibility - Use content-visibility for long lists
• rendering-hoist-jsx - Extract static JSX outside components
• rendering-svg-precision - Reduce SVG coordinate precision
• rendering-hydration-no-flicker - Use inline script for client-only data
• rendering-hydration-suppress-warning - Suppress expected mismatches
• rendering-activity - Use Activity component for show/hide
• rendering-conditional-render - Use ternary, not && for conditionals
• rendering-usetransition-loading - Prefer useTransition for loading state

7. JavaScript Performance (Supplemental)

• js-batch-dom-css - Group CSS changes via classes or cssText
• js-index-maps - Build Map for repeated lookups
• js-cache-property-access - Cache object properties in loops
• js-cache-function-results - Cache function results in module-level Map
• js-cache-storage - Cache localStorage/sessionStorage reads
• js-combine-iterations - Combine multiple filter/map into one loop
• js-length-check-first - Check array length before expensive comparison
• js-early-exit - Return early from functions
• js-hoist-regexp - Hoist RegExp creation outside loops
• js-min-max-loop - Use loop for min/max instead of sort
• js-set-map-lookups - Use Set/Map for O(1) lookups
• js-tosorted-immutable - Use toSorted() for immutability

8. Advanced Patterns (Supplemental)

• advanced-event-handler-refs - Store event handlers in refs
• advanced-init-once - Initialize app once per app load
• advanced-use-latest - useLatest for stable callback refs

References

• Individual rule files: rules/{prefix}-{name}.md
• Full compiled reference: AGENTS.md (80 KB — contains all 57 rules with examples; consult individual rule files for targeted lookup)