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description: React and Next.js performance optimization guidelines from Vercel Engineering. 45 rules across 8 categories, prioritized by impact.
---

# Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications. 45 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

| 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-` |

## 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

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