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-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

How to Use

For detailed explanations and code examples, read the full compiled document: 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

Key Examples

Promise.all for Independent Operations (CRITICAL)

// ❌ Sequential: 3 round trips
const user = await fetchUser()
const posts = await fetchPosts()
const comments = await fetchComments()

// ✅ Parallel: 1 round trip
const [user, posts, comments] = await Promise.all([
  fetchUser(),
  fetchPosts(),
  fetchComments()
])

Avoid Barrel File Imports (CRITICAL)

// ❌ Imports entire library (200-800ms import cost)
import { Check, X, Menu } from 'lucide-react'

// ✅ Imports only what you need
import Check from 'lucide-react/dist/esm/icons/check'
import X from 'lucide-react/dist/esm/icons/x'

Dynamic Imports for Heavy Components (CRITICAL)

// ❌ Monaco bundles with main chunk ~300KB
import { MonacoEditor } from './monaco-editor'

// ✅ Monaco loads on demand
import dynamic from 'next/dynamic'
const MonacoEditor = dynamic(
  () => import('./monaco-editor').then(m => m.MonacoEditor),
  { ssr: false }
)

Use Functional setState (MEDIUM)

// ❌ Requires state as dependency, stale closure risk
const addItems = useCallback((newItems) => {
  setItems([...items, ...newItems])
}, [items])

// ✅ Stable callback, no stale closures
const addItems = useCallback((newItems) => {
  setItems(curr => [...curr, ...newItems])
}, [])

vercel-react-best-practices