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-