Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 57 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

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-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

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

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

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-init-once - Initialize app once per app load
• •advanced-use-latest - useLatest for stable callback refs

How to Use This Skill

⚠️ IMPORTANT: DO NOT load AGENTS.md (2935 lines) - it consumes too much context!

Instead, use this efficient loading strategy:

1. Start with Quick Reference (~200 lines)

For overview and quick lookup:

QUICK-REFERENCE.md

2. Load Only Relevant Category (~200-600 lines)

Based on the issue type, load the specific category file:

3. Load Individual Rules (Optional)

For specific guidance, read individual rule files:

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

Workflow for Agents

1. Identify issue category (e.g., "component re-renders too often")
2. Load QUICK-REFERENCE.md to confirm category (→ Re-render Optimization)
3. Load categories/05-rerender.md (~527 lines vs 2935 for full AGENTS.md)
4. Apply relevant rules
5. (Optional) Load specific rule file if more detail needed

Context savings: ~90% reduction (300-600 lines vs 2935 lines)

File Structure

vercel-react-best-practices/
├── SKILL.md                 # This file (index + usage guide)
├── QUICK-REFERENCE.md       # Condensed overview (~200 lines)
├── AGENTS.md                # Full document (⚠️ 2935 lines - avoid loading!)
├── categories/              # Category-specific files
│   ├── 01-waterfalls.md     # Eliminating Waterfalls (CRITICAL)
│   ├── 02-bundle.md         # Bundle Size Optimization (CRITICAL)
│   ├── 03-server.md         # Server-Side Performance (HIGH)
│   ├── 04-client.md         # Client-Side Data Fetching (MEDIUM-HIGH)
│   ├── 05-rerender.md       # Re-render Optimization (MEDIUM)
│   ├── 06-rendering.md      # Rendering Performance (MEDIUM)
│   ├── 07-javascript.md     # JavaScript Performance (LOW-MEDIUM)
│   └── 08-advanced.md       # Advanced Patterns (LOW)
└── rules/                   # Individual rule files (granular detail)

Priority Guide

When optimizing code, follow this order:

1. •CRITICAL (categories 1-2): Waterfalls, bundle size → 2-10× improvement
2. •HIGH (category 3): Server-side performance → reduces load time, security
3. •MEDIUM-HIGH (category 4): Client-side fetching → better UX
4. •MEDIUM (categories 5-6): Re-renders, rendering → improves responsiveness
5. •LOW-MEDIUM (category 7): JavaScript optimizations → cumulative gains
6. •LOW (category 8): Advanced patterns → specific edge cases