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react-best-practices\ndescription: 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.\nmetadata:\n  author: vercel\n  version: \"1.0.0\"\n---\n\n# Vercel React Best Practices\n\nComprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 64 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.\n\n## When to Apply\n\nReference these guidelines when:\n- Writing new React components or Next.js pages\n- Implementing data fetching (client or server-side)\n- Reviewing code for performance issues\n- Refactoring existing React/Next.js code\n- Optimizing bundle size or load times\n\n## Rule Categories by Priority\n\n| Priority | Category | Impact | Prefix |\n|----------|----------|--------|--------|\n| 1 | Eliminating Waterfalls | CRITICAL | `async-` |\n| 2 | Bundle Size Optimization | CRITICAL | `bundle-` |\n| 3 | Server-Side Performance | HIGH | `server-` |\n| 4 | Client-Side Data Fetching | MEDIUM-HIGH | `client-` |\n| 5 | Re-render Optimization | MEDIUM | `rerender-` |\n| 6 | Rendering Performance | MEDIUM | `rendering-` |\n| 7 | JavaScript Performance | LOW-MEDIUM | `js-` |\n| 8 | Advanced Patterns | LOW | `advanced-` |\n\n## Quick Reference\n\n### 1. Eliminating Waterfalls (CRITICAL)\n\n- `async-defer-await` - Move await into branches where actually used\n- `async-parallel` - Use Promise.all() for independent operations\n- `async-dependencies` - Use better-all for partial dependencies\n- `async-api-routes` - Start promises early, await late in API routes\n- `async-suspense-boundaries` - Use Suspense to stream content\n\n### 2. Bundle Size Optimization (CRITICAL)\n\n- `bundle-barrel-imports` - Import directly, avoid barrel files\n- `bundle-dynamic-imports` - Use next/dynamic for heavy components\n- `bundle-defer-third-party` - Load analytics/logging after hydration\n- `bundle-conditional` - Load modules only when feature is activated\n- `bundle-preload` - Preload on hover/focus for perceived speed\n\n### 3. Server-Side Performance (HIGH)\n\n- `server-auth-actions` - Authenticate server actions like API routes\n- `server-cache-react` - Use React.cache() for per-request deduplication\n- `server-cache-lru` - Use LRU cache for cross-request caching\n- `server-dedup-props` - Avoid duplicate serialization in RSC props\n- `server-hoist-static-io` - Hoist static I/O (fonts, logos) to module level\n- `server-serialization` - Minimize data passed to client components\n- `server-parallel-fetching` - Restructure components to parallelize fetches\n- `server-after-nonblocking` - Use after() for non-blocking operations\n\n### 4. Client-Side Data Fetching (MEDIUM-HIGH)\n\n- `client-swr-dedup` - Use SWR for automatic request deduplication\n- `client-event-listeners` - Deduplicate global event listeners\n- `client-passive-event-listeners` - Use passive listeners for scroll\n- `client-localstorage-schema` - Version and minimize localStorage data\n\n### 5. Re-render Optimization (MEDIUM)\n\n- `rerender-defer-reads` - Don't subscribe to state only used in callbacks\n- `rerender-memo` - Extract expensive work into memoized components\n- `rerender-memo-with-default-value` - Hoist default non-primitive props\n- `rerender-dependencies` - Use primitive dependencies in effects\n- `rerender-derived-state` - Subscribe to derived booleans, not raw values\n- `rerender-derived-state-no-effect` - Derive state during render, not effects\n- `rerender-functional-setstate` - Use functional setState for stable callbacks\n- `rerender-lazy-state-init` - Pass function to useState for expensive values\n- `rerender-simple-expression-in-memo` - Avoid memo for simple primitives\n- `rerender-split-combined-hooks` - Split hooks with independent dependencies\n- `rerender-move-effect-to-event` - Put interaction logic in event handlers\n- `rerender-transitions` - Use startTransition for non-urgent updates\n- `rerender-use-deferred-value` - Defer expensive renders to keep input responsive\n- `rerender-use-ref-transient-values` - Use refs for transient frequent values\n- `rerender-no-inline-components` - Don't define components inside components\n\n### 6. Rendering Performance (MEDIUM)\n\n- `rendering-animate-svg-wrapper` - Animate div wrapper, not SVG element\n- `rendering-content-visibility` - Use content-visibility for long lists\n- `rendering-hoist-jsx` - Extract static JSX outside components\n- `rendering-svg-precision` - Reduce SVG coordinate precision\n- `rendering-hydration-no-flicker` - Use inline script for client-only data\n- `rendering-hydration-suppress-warning` - Suppress expected mismatches\n- `rendering-activity` - Use Activity component for show/hide\n- `rendering-conditional-render` - Use ternary, not && for conditionals\n- `rendering-usetransition-loading` - Prefer useTransition for loading state\n- `rendering-resource-hints` - Use React DOM resource hints for preloading\n- `rendering-script-defer-async` - Use defer or async on script tags\n\n### 7. JavaScript Performance (LOW-MEDIUM)\n\n- `js-batch-dom-css` - Group CSS changes via classes or cssText\n- `js-index-maps` - Build Map for repeated lookups\n- `js-cache-property-access` - Cache object properties in loops\n- `js-cache-function-results` - Cache function results in module-level Map\n- `js-cache-storage` - Cache localStorage/sessionStorage reads\n- `js-combine-iterations` - Combine multiple filter/map into one loop\n- `js-length-check-first` - Check array length before expensive comparison\n- `js-early-exit` - Return early from functions\n- `js-hoist-regexp` - Hoist RegExp creation outside loops\n- `js-min-max-loop` - Use loop for min/max instead of sort\n- `js-set-map-lookups` - Use Set/Map for O(1) lookups\n- `js-tosorted-immutable` - Use toSorted() for immutability\n- `js-flatmap-filter` - Use flatMap to map and filter in one pass\n\n### 8. Advanced Patterns (LOW)\n\n- `advanced-event-handler-refs` - Store event handlers in refs\n- `advanced-init-once` - Initialize app once per app load\n- `advanced-use-latest` - useLatest for stable callback refs\n\n## How to Use\n\nRead individual rule files for detailed explanations and code examples:\n\n```\nrules/async-parallel.md\nrules/bundle-barrel-imports.md\n```\n\nEach rule file contains:\n- Brief explanation of why it matters\n- Incorrect code example with explanation\n- Correct code example with explanation\n- Additional context and references\n\n## Full Compiled Document\n\nFor the complete guide with all rules expanded: `AGENTS.md`\n"},"changes":[],"summary":"First saved snapshot. No earlier version is available for comparison.","summary_kind":"deterministic","summary_metadata":{}}