--- name: vercel-react-best-practices description: React and Next.js performance optimization guidelines from Vercel Engineering. This skill should be used 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. license: MIT metadata: author: vercel version: "1.0.0" ---

# Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 70 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-cheap-condition-before-await` - Check cheap sync conditions before awaiting flags or remote values - `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-analyzable-paths` - Prefer statically analyzable import and file-system paths to avoid broad bundles and traces - `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-hoist-static-io` - Hoist static I/O (fonts, logos) to module level - `server-no-shared-module-state` - Avoid module-level mutable request state in RSC/SSR - `server-serialization` - Minimize data passed to client components - `server-parallel-fetching` - Restructure components to parallelize fetches - `server-parallel-nested-fetching` - Chain nested fetches per item in Promise.all - `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 - `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-split-combined-hooks` - Split hooks with independent dependencies - `rerender-move-effect-to-event` - Put interaction logic in event handlers - `rerender-transitions` - Use startTransition for non-urgent updates - `rerender-use-deferred-value` - Defer expensive renders to keep input responsive - `rerender-use-ref-transient-values` - Use refs for transient frequent values - `rerender-no-inline-components` - Don't define components inside components

### 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-hydration-suppress-warning` - Suppress expected mismatches - `rendering-activity` - Use Activity component for show/hide - `rendering-conditional-render` - Use ternary, not && for conditionals - `rendering-usetransition-loading` - Prefer useTransition for loading state - `rendering-resource-hints` - Use React DOM resource hints for preloading - `rendering-script-defer-async` - Use defer or async on script tags

### 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 - `js-flatmap-filter` - Use flatMap to map and filter in one pass - `js-request-idle-callback` - Defer non-critical work to browser idle time

### 8. Advanced Patterns (LOW)

- `advanced-effect-event-deps` - Don't put `useEffectEvent` results in effect deps - `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

## How to Use

Read individual rule files for detailed explanations and code examples:

``` rules/async-parallel.md rules/bundle-barrel-imports.md ```

Each rule file contains: - Brief explanation of why it matters - Incorrect code example with explanation - Correct code example with explanation - Additional context and references

## Full Compiled Document

For the complete guide with all rules expanded: `AGENTS.md`