React Best Practices

Vercel React Best Practices

Comprehensive performance optimization guide for React applications, maintained by Vercel. Contains 70 rules across 8 categories, prioritized by impact. Sourced from vercel-labs/agent-skills.

ryOS notes

ryOS is a Vite + React SPA with a standalone Bun API (not Next.js App Router). When applying this skill:

• Prefer React.lazy + Suspense over next/dynamic (bundle-dynamic-imports)
• Most server-* rules (RSC, React.cache, after(), server actions) do not apply to the SPA; still apply waterfall/Promise.all ideas in api/ handlers (async-api-routes, async-parallel)
• The repo uses React Compiler–oriented eslint rules — avoid adding useMemo/useCallback by default unless an existing pattern requires it
• Full rule text: AGENTS.md and rules/*.md in this skill directory

When to Apply

Reference these guidelines when:

• Writing or reviewing React components in src/
• Implementing client data fetching or API route handlers
• Reviewing code for performance issues
• Refactoring for bundle size, re-renders, or load time

Rule Categories by Priority

Quick Reference

1. Eliminating Waterfalls (CRITICAL)

• async-cheap-condition-before-await - Check cheap sync conditions before awaiting flags or remote values
• 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-analyzable-paths - Prefer statically analyzable import and file-system paths to avoid broad bundles and traces
• 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-hoist-static-io - Hoist static I/O (fonts, logos) to module level
• server-no-shared-module-state - Avoid module-level mutable request state in RSC/SSR
• server-serialization - Minimize data passed to client components
• server-parallel-fetching - Restructure components to parallelize fetches
• server-parallel-nested-fetching - Chain nested fetches per item in Promise.all
• 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)

• 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-split-combined-hooks - Split hooks with independent dependencies
• rerender-move-effect-to-event - Put interaction logic in event handlers
• rerender-transitions - Use startTransition for non-urgent updates
• rerender-use-deferred-value - Defer expensive renders to keep input responsive
• rerender-use-ref-transient-values - Use refs for transient frequent values
• rerender-no-inline-components - Don't define components inside components