React

Vercel React Best Practices

Adapted from vercel-labs/agent-skills

Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. 57 rules across 8 categories, prioritized by impact.

When to Apply

• 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

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

Key pattern: If two fetches don't depend on each other, run them in parallel:

// BAD: Sequential
const user = await getUser(id)
const posts = await getPosts(id)

// GOOD: Parallel
const [user, posts] = await Promise.all([getUser(id), getPosts(id)])

2. Bundle Size Optimization (CRITICAL)

• bundle-barrel-imports: Import directly, avoid barrel files (import { X } from './X' not from './index')
• 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

Key pattern: Only send what the client needs:

// BAD: Passing entire object
<ClientComponent data={fullUserObject} />

// GOOD: Pick only needed fields
<ClientComponent name={user.name} avatar={user.avatar} />

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

Key pattern: Derive state during render, not in effects:

// BAD: Effect to derive state
const [items, setItems] = useState([])
const [filteredItems, setFilteredItems] = useState([])
useEffect(() => { setFilteredItems(items.filter(i => i.active)) }, [items])

// GOOD: Derive during render
const [items, setItems] = useState([])
const filteredItems = items.filter(i => i.active)

6. Rendering Performance (MEDIUM)

• rendering-content-visibility: Use content-visibility for long lists
• rendering-hoist-jsx: Extract static JSX outside components
• 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
• 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-combine-iterations: Combine multiple filter/map into one loop
• js-set-map-lookups: Use Set/Map for O(1) lookups
• js-early-exit: Return early from functions
• 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

Reference

For full rule explanations with detailed code examples, install the original skill:

npx skills add https://github.com/vercel-labs/agent-skills --skill vercel-react-best-practices

Individual rule files are available at rules/<rule-name>.md in the original repo.