Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 57 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

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)

• 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

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