/vercel-react-best-practices

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.

Vercel React Best Practices

When to Load

• **Trigger**: React or Next.js development, component writing, data fetching patterns, bundle optimization
• **Skip**: Project does not use React or Next.js

Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 47 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

Full Compiled Document

For the complete guide with all rules expanded and detailed code examples, see [AGENTS.md](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