# React Best Practices

> React and Next.js performance optimization guidelines from Vercel Engineering. Use 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.

- Skill: `ulpi-io/react-best-practices` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add ulpi-io/react-best-practices`
- Raw SKILL.md: https://api.skillmd.com/api/skills/ulpi-io/react-best-practices/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Web & Frontend
- License: MIT
- Author: ulpi-io (https://skillmd.com/u/ulpi-io)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/ulpi-io/react-best-practices

---



# 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

| 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-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)

```typescript
// ❌ 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)

```tsx
// ❌ 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)

```tsx
// ❌ 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)

```tsx
// ❌ 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

### Required Rules (MUST)

1. **Eliminate waterfalls**: Use Promise.all, Suspense
2. **Bundle optimization**: Prohibit barrel imports, use dynamic imports
3. **RSC boundaries**: Serialize only the data you need

### Prohibited (MUST NOT)

1. **Sequential await**: Do not run independent fetches sequentially
2. **Array mutations**: Use toSorted() instead of sort()
3. **Inline objects in React.cache**: Causes cache misses

## References

- [React Documentation](https://react.dev)
- [Next.js Documentation](https://nextjs.org)
- [SWR](https://swr.vercel.app)
- [better-all](https://github.com/shuding/better-all)
- [Vercel Blog: Optimizing Package Imports](https://vercel.com/blog/how-we-optimized-package-imports-in-next-js)
- [Vercel Blog: Dashboard Performance](https://vercel.com/blog/how-we-made-the-vercel-dashboard-twice-as-fast)

## Metadata

### Version
- **Current version**: 1.0.0
- **Last updated**: 2026-01-22
- **Supported platforms**: Claude, ChatGPT, Gemini
- **Source**: vercel/agent-skills

### Related Skills
- [performance-optimization](../performance-optimization/SKILL.md): General performance optimization
- [state-management](../state-management/SKILL.md): State management

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