Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 45 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-cache-react – Use React.cache() for per-request deduplication
• server-cache-lru – Use LRU cache for cross-request caching
• 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

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-dependencies – Use primitive dependencies in effects
• rerender-derived-state – Subscribe to derived booleans, not raw values
• rerender-functional-setstate – Use functional setState for stable callbacks
• rerender-lazy-state-init – Pass function to useState for expensive values
• rerender-transitions – Use startTransition for non-urgent updates

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-activity – Use Activity component for show/hide
• rendering-conditional-render – Use ternary, not && for conditionals

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-use-latest – useLatest for stable callback refs

How to Use

For detailed explanations and code examples, read the full compiled document: AGENTS.md

Each rule contains:

• Brief explanation of why it matters
• Incorrect code example with explanation
• Correct code example with explanation
• Additional context and references

Key Examples

Promise.all for Independent Operations (CRITICAL)

// ❌ Sequential: 3 round trips
const user = await fetchUser()
const posts = await fetchPosts()
const comments = await fetchComments()

// ✅ Parallel: 1 round trip
const [user, posts, comments] = await Promise.all([
  fetchUser(),
  fetchPosts(),
  fetchComments()
])

Avoid Barrel File Imports (CRITICAL)

// ❌ Imports entire library (200-800ms import cost)
import { Check, X, Menu } from 'lucide-react'

// ✅ Imports only what you need
import Check from 'lucide-react/dist/esm/icons/check'
import X from 'lucide-react/dist/esm/icons/x'

Dynamic Imports for Heavy Components (CRITICAL)

// ❌ Monaco bundles with main chunk ~300KB
import { MonacoEditor } from './monaco-editor'

// ✅ Monaco loads on demand
import dynamic from 'next/dynamic'
const MonacoEditor = dynamic(
  () => import('./monaco-editor').then(m => m.MonacoEditor),
  { ssr: false }
)

Use Functional setState (MEDIUM)

// ❌ Requires state as dependency, stale closure risk
const addItems = useCallback((newItems) => {
  setItems([...items, ...newItems])
}, [items])

// ✅ Stable callback, no stale closures
const addItems = useCallback((newItems) => {
  setItems(curr => [...curr, ...newItems])
}, [])

Constraints

필수 규칙 (MUST)

1. Waterfall 제거: Promise.all, Suspense 사용
2. 번들 최적화: barrel imports 금지, dynamic imports 사용
3. RSC 경계: 필요한 데이터만 직렬화

금지 사항 (MUST NOT)

1. Sequential await: 독립적인 fetch를 순차 실행하지 않음
2. Array mutations: sort() 대신 toSorted() 사용
3. Inline objects in React.cache: 캐시 미스 발생

References

• React Documentation
• Next.js Documentation
• SWR
• better-all
• Vercel Blog: Optimizing Package Imports
• Vercel Blog: Dashboard Performance

Metadata

버전

• 현재 버전: 1.0.0
• 최종 업데이트: 2026-01-22
• 호환 플랫폼: Claude, ChatGPT, Gemini
• 원본 출처: vercel/agent-skills

관련 스킬

• performance-optimization: 일반 성능 최적화
• state-management: 상태 관리

태그

#React #Next.js #performance #optimization #vercel #waterfalls #bundle-size #RSC #frontend