mediago-dev Vercel React Best Practices
Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 67 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.
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
| Priority | Category | Impact | Prefix |
|---|---|---|---|
| 1 | Eliminating Waterfalls | CRITICAL | async- |
| 2 | Bundle Size Optimization | CRITICAL | bundle- |
| 3 | Server-Side Performance | HIGH | server- |
| 4 | Client-Side Data Fetching | MEDIUM-HIGH | client- |
| 5 | Re-render Optimization | MEDIUM | rerender- |
| 6 | Rendering Performance | MEDIUM | rendering- |
| 7 | JavaScript Performance | LOW-MEDIUM | js- |
| 8 | Advanced Patterns | LOW | advanced- |
Quick Reference
1. Eliminating Waterfalls (CRITICAL)
async-cheap-condition-before-await- Check cheap sync conditions before awaiting flags or remote valuesasync-defer-await- Move await into branches where actually usedasync-parallel- Use Promise.all() for independent operationsasync-dependencies- Use better-all for partial dependenciesasync-api-routes- Start promises early, await late in API routesasync-suspense-boundaries- Use Suspense to stream content
2. Bundle Size Optimization (CRITICAL)
bundle-barrel-imports- Import directly, avoid barrel filesbundle-dynamic-imports- Use next/dynamic for heavy componentsbundle-defer-third-party- Load analytics/logging after hydrationbundle-conditional- Load modules only when feature is activatedbundle-preload- Preload on hover/focus for perceived speed
3. Server-Side Performance (HIGH)
server-auth-actions- Authenticate server actions like API routesserver-cache-react- Use React.cache() for per-request deduplicationserver-cache-lru- Use LRU cache for cross-request cachingserver-dedup-props- Avoid duplicate serialization in RSC propsserver-hoist-static-io- Hoist static I/O (fonts, logos) to module levelserver-serialization- Minimize data passed to client componentsserver-parallel-fetching- Restructure components to parallelize fetchesserver-parallel-nested-fetching- Chain nested fetches per item in Promise.allserver-after-nonblocking- Use after() for non-blocking operations
4. Client-Side Data Fetching (MEDIUM-HIGH)
client-swr-dedup- Use SWR for automatic request deduplicationclient-event-listeners- Deduplicate global event listenersclient-passive-event-listeners- Use passive listeners for scrollclient-localstorage-schema- Version and minimize localStorage data
5. Re-render Optimization (MEDIUM)
rerender-defer-reads- Don't subscribe to state only used in callbacksrerender-memo- Extract expensive work into memoized componentsrerender-memo-with-default-value- Hoist default non-primitive propsrerender-dependencies- Use primitive dependencies in effectsrerender-derived-state- Subscribe to derived booleans, not raw valuesrerender-derived-state-no-effect- Derive state during render, not effectsrerender-functional-setstate- Use functional setState for stable callbacksrerender-lazy-state-init- Pass function to useState for expensive valuesrerender-simple-expression-in-memo- Avoid memo for simple primitivesrerender-split-combined-hooks- Split hooks with independent dependenciesrerender-move-effect-to-event- Put interaction logic in event handlersrerender-transitions- Use startTransition for non-urgent updatesrerender-use-deferred-value- Defer expensive renders to keep input responsivererender-use-ref-transient-values- Use refs for transient frequent valuesrerender-no-inline-components- Don't define components inside components
6. Rendering Performance (MEDIUM)
rendering-animate-svg-wrapper- Animate div wrapper, not SVG elementrendering-content-visibility- Use content-visibility for long listsrendering-hoist-jsx- Extract static JSX outside componentsrendering-svg-precision- Reduce SVG coordinate precisionrendering-hydration-no-flicker- Use inline script for client-only datarendering-hydration-suppress-warning- Suppress expected mismatchesrendering-activity- Use Activity component for show/hiderendering-conditional-render- Use ternary, not && for conditionalsrendering-usetransition-loading- Prefer useTransition for loading staterendering-resource-hints- Use React DOM resource hints for preloadingrendering-script-defer-async- Use defer or async on script tags
7. JavaScript Performance (LOW-MEDIUM)
js-batch-dom-css- Group CSS changes via classes or cssTextjs-index-maps- Build Map for repeated lookupsjs-cache-property-access- Cache object properties in loopsjs-cache-function-results- Cache function results in module-level Mapjs-cache-storage- Cache localStorage/sessionStorage readsjs-combine-iterations- Combine multiple filter/map into one loopjs-length-check-first- Check array length before expensive comparisonjs-early-exit- Return early from functionsjs-hoist-regexp- Hoist RegExp creation outside loopsjs-min-max-loop- Use loop for min/max instead of sortjs-set-map-lookups- Use Set/Map for O(1) lookupsjs-tosorted-immutable- Use toSorted() for immutabilityjs-flatmap-filter- Use flatMap to map and filter in one passjs-request-idle-callback- Defer non-critical work to browser idle time
8. Advanced Patterns (LOW)
advanced-event-handler-refs- Store event handlers in refsadvanced-init-once- Initialize app once per app loadadvanced-use-latest- useLatest for stable callback refs
How to Use
Read individual rule files for detailed explanations and code examples:
rules/async-parallel.md
rules/bundle-barrel-imports.md
Each rule file contains:
- Brief explanation of why it matters
- Incorrect code example with explanation
- Correct code example with explanation
- Additional context and references
Full Compiled Document
For the complete guide with all rules expanded: AGENTS.md