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

Detailed Rules

Eliminating Waterfalls (CRITICAL)

async-defer-await

Move await into branches where the value is actually needed.

// ❌ BAD: Awaiting early blocks everything
async function getData() {
  const data = await fetchData();
  if (condition) {
    return data;
  }
  return null;
}

// ✅ GOOD: Defer await until needed
async function getData() {
  const dataPromise = fetchData();
  if (condition) {
    return await dataPromise;
  }
  return null;
}

async-parallel

Use Promise.all() for independent operations.

// ❌ BAD: Sequential fetches
const user = await fetchUser(id);
const posts = await fetchPosts(id);
const comments = await fetchComments(id);

// ✅ GOOD: Parallel fetches
const [user, posts, comments] = await Promise.all([
  fetchUser(id),
  fetchPosts(id),
  fetchComments(id),
]);

async-suspense-boundaries

Use Suspense boundaries to stream content progressively.

// ✅ GOOD: Suspense enables streaming
export default function Page() {
  return (
    <div>
      <Header />
      <Suspense fallback={<Loading />}>
        <SlowComponent />
      </Suspense>
    </div>
  );
}

Bundle Size Optimization (CRITICAL)

bundle-barrel-imports

Import directly from source files, not barrel files (index.ts).

// ❌ BAD: Barrel import pulls in entire library
import { Button } from '@/components';

// ✅ GOOD: Direct import
import { Button } from '@/components/Button';

bundle-dynamic-imports

Use next/dynamic for heavy components.

// ❌ BAD: Always loads heavy component
import HeavyChart from './HeavyChart';

// ✅ GOOD: Dynamic import
import dynamic from 'next/dynamic';
const HeavyChart = dynamic(() => import('./HeavyChart'), {
  loading: () => <ChartSkeleton />,
});

bundle-defer-third-party

Load analytics and logging after hydration.

// ✅ GOOD: Defer analytics loading
useEffect(() => {
  import('./analytics').then((module) => module.init());
}, []);

Server-Side Performance (HIGH)

server-cache-react

Use React.cache() for per-request deduplication.

// ✅ GOOD: Cached per-request
import { cache } from 'react';

const getUser = cache(async (id: string) => {
  return await db.user.findUnique({ where: { id } });
});

server-parallel-fetching

Restructure components to parallelize server fetches.

// ❌ BAD: Parent awaits before child renders
async function Page() {
  const user = await getUser();
  return <Profile user={user} />;
}

// ✅ GOOD: Start fetch in parent, await in child
async function Page() {
  const userPromise = getUser();
  return <Profile userPromise={userPromise} />;
}

async function Profile({ userPromise }) {
  const user = await userPromise;
  return <div>{user.name}</div>;
}

Re-render Optimization (MEDIUM)

rerender-memo

Extract expensive calculations into memoized components.

// ✅ GOOD: Expensive component memoized
const ExpensiveList = memo(function ExpensiveList({ items }) {
  return items.map((item) => <ExpensiveItem key={item.id} item={item} />);
});

rerender-functional-setstate

Use functional setState for stable callbacks.

// ❌ BAD: Callback changes on every render
const increment = () => setCount(count + 1);

// ✅ GOOD: Stable callback
const increment = useCallback(() => setCount((c) => c + 1), []);

rerender-transitions

Use startTransition for non-urgent updates.

// ✅ GOOD: Non-urgent update doesn't block input
const [isPending, startTransition] = useTransition();

function handleChange(value) {
  setInputValue(value); // Urgent
  startTransition(() => {
    setSearchResults(filterData(value)); // Non-urgent
  });
}

Credits & Attribution

This skill is adapted from Vercel Engineering.

Original repository: https://github.com/vercel-labs/agent-skills

Copyright (c) Vercel, Inc. - MIT License
Adapted by webconsulting.at for this skill collection