React Best Practices

React Best Practices

Performance optimization guide for React and Next.js applications. 57 rules across 8 categories, prioritized by impact. Originally adopted from Vercel Engineering (MIT).

Core rules are compiled into GLOBAL.md. This skill contains additional detail.

When to Apply

Reference these guidelines when:

• Writing new React components or Next.js pages
• Implementing data fetching (client or server-side)
• Reviewing code for performance issues
• Refactoring existing React/Next.js code
• Optimizing bundle size or load times

Rule Categories by Priority

Quick Reference

1. Eliminating Waterfalls (CRITICAL)

• 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 (CRITICAL)

• 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 (HIGH)

• 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 (MEDIUM-HIGH)

• 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 (MEDIUM)

These rules answer a measured re-render cost — don't add memo/useMemo/useCallback defensively; /change-review treats profiling-free memoization as churn, not optimization.

• 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

6. Rendering Performance (MEDIUM)

• 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 (LOW-MEDIUM)

• 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 (LOW)

• 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

Additional References

• /avoid-effects — When useEffect is and isn’t appropriate (it usually isn't); full React doc in that skill’s references/react-dev-full-article.md. Read before adding any useEffect.