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-tanstack-query - Preferred: Use TanStack Query (useQuery) for data fetching with built-in error handling
• •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

TanStack Query for Error Handling

Use useQuery from TanStack Query instead of manual try...catch for cleaner error handling:

❌ Avoid: Manual try...catch

function UserProfile({ userId }) {
  const [data, setData] = useState(null);
  const [error, setError] = useState(null);
  const [isLoading, setIsLoading] = useState(true);

  useEffect(() => {
    const fetchData = async () => {
      try {
        const response = await fetch(`/api/users/${userId}`);
        if (!response.ok) throw new Error("Failed to fetch");
        setData(await response.json());
      } catch (err) {
        setError(err);
      } finally {
        setIsLoading(false);
      }
    };
    fetchData();
  }, [userId]);

  // Manual loading/error/data handling...
}

✅ Preferred: TanStack Query useQuery

import { useQuery } from "@tanstack/react-query";

function UserProfile({ userId }) {
  const { data, error, isLoading, refetch } = useQuery({
    queryKey: ["user", userId],
    queryFn: () => fetch(`/api/users/${userId}`).then((res) => res.json()),
    staleTime: 5 * 60 * 1000, // 5 minutes
    retry: 3,
  });

  if (isLoading) return <Skeleton />;
  if (error) return <ErrorDisplay error={error} onRetry={refetch} />;
  return <UserCard user={data} />;
}

Benefits of TanStack Query:

• •Automatic error/loading states
• •Built-in retry logic
• •Request deduplication
• •Background refetching
• •Cache invalidation
• •Optimistic updates
• •No manual try...catch needed

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

React 19 Compiler Optimizations

Important: When using React 19 with the React Compiler (React Forget), many manual optimization patterns are no longer needed. The compiler automatically handles these optimizations at build time.

Deprecated Patterns in React 19

Code Examples

Before (React 18):

const MemoizedComponent = memo(function MyComponent({ data }) {
  const processed = useMemo(() => expensiveCalc(data), [data]);
  const handleClick = useCallback(() => doSomething(data), [data]);

  return <button onClick={handleClick}>{processed}</button>;
});

const ForwardedInput = forwardRef((props, ref) => <input ref={ref} {...props} />);

// Context Provider
<ThemeContext.Provider value={theme}>{children}</ThemeContext.Provider>;

After (React 19):

function MyComponent({ data }) {
  const processed = expensiveCalc(data); // Auto-memoized
  const handleClick = () => doSomething(data); // Auto-memoized

  return <button onClick={handleClick}>{processed}</button>;
}

function Input({ ref, ...props }) {
  // ref as regular prop
  return <input ref={ref} {...props} />;
}

// Context as Provider directly
<ThemeContext value={theme}>{children}</ThemeContext>;

When These Rules Still Apply

The re-render optimization rules in section 5 are still useful for:

• •Understanding performance concepts
• •Projects not yet using React 19
• •Edge cases where compiler optimization may not apply
• •Server Components (which don't use hooks)

How to Use

Read individual rule files for detailed explanations and code examples:

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

Each rule file contains:

• •Brief explanation of why it matters
• •Incorrect code example with explanation
• •Correct code example with explanation
• •Additional context and references

Full Compiled Document

For the complete guide with all rules expanded: AGENTS.md