React Best Practices

Vercel React Best Practices

Measurement-led React and Next.js performance skill for routing slow-page work into the right packet before refactoring. The canonical focus is waterfalls, bundle weight, RSC or client-boundary mistakes, hydration/script cost, and rerender churn; the deep AGENTS.md remains available, but day-to-day use should start from the lighter reference bundle and route-outs.

When to use this skill

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-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

Instructions

Step 1: Classify the primary bottleneck before suggesting fixes

Start with references/perf-triage-modes.md and put the task into one dominant packet:

• waterfalls / async sequencing
• bundle weight / lazy loading
• RSC / server-client boundary mistakes
• rerender churn / memoization / context spread
• hydration or third-party script cost
• measurement-first unknowns

Step 2: Use the smallest support packet that answers the question

• references/perf-triage-modes.md — choose the main React / Next.js perf mode and first fixes
• references/measurement-and-tooling-checklist.md — decide what to measure with React Profiler, bundle analyzers, Web Vitals, or PR budget tooling
• references/boundaries-and-route-outs.md — keep the boundary with state-management, web-accessibility, responsive-design, and performance-optimization explicit
• AGENTS.md — deep rule catalog when a code generation or large refactor pass needs the full 45-rule reference

Step 3: Prefer measurement-led fixes over blanket cargo culting

Use the measurement checklist before recommending broad memoization or cache-heavy rewrites. React Compiler, bundle analyzers, and web vitals instrumentation have changed which old React performance heuristics are still worth applying blindly.

Step 4: Prioritize high-impact fixes first

1. Remove waterfalls and late-started async work.
2. Cut route JS and heavy client boundaries.
3. Fix hydration or script-placement mistakes.
4. Address rerender churn once the larger loading and boundary costs are understood.

Step 5: Route adjacent work out instead of absorbing everything

• App-wide state model choice → ../state-management/SKILL.md
• Accessibility-specific failures → ../web-accessibility/SKILL.md
• Layout adaptation / overflow / reflow work → ../responsive-design/SKILL.md
• Non-React or cross-stack bottlenecks → ../performance-optimization/SKILL.md

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])
}, [])

Best practices

1. Start with the lightest support packet that matches the problem; use AGENTS.md only when the smaller references are not enough.