Qreact-best-practices

Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 62 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-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-hoist-static-io - Hoist static I/O (fonts, logos) to module level
• 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
• rerender-no-inline-components - Don't define components inside components

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
• rendering-resource-hints - Use React DOM resource hints for preloading
• rendering-script-defer-async - Use defer or async on script tags

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
• js-flatmap-filter - Use flatMap to map and filter in one pass

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

Code Patterns

Essential React optimization patterns (3 levels):

Basic: Memo + useMemo — Cache expensive computations

const Avatar = memo(({ user }) => {
  const id = useMemo(() => computeAvatarId(user), [user])
  return <Avatar id={id} />
})

Error Handling: Suspense + ErrorBoundary — Resilient async rendering

<ErrorBoundary fallback={<ErrorUI />}>
  <Suspense fallback={<Skeleton />}>
    <AsyncData />
  </Suspense>
</ErrorBoundary>

Advanced: useTransition — Non-blocking updates

const handler = () => startTransition(() => setScrollY(window.scrollY))

Comment Template

JSDoc for React components: @param, @returns, @since (internal). Example:

/**
 * Paginated user list with lazy rendering.
 * @param {User[]} users
 * @returns {ReactNode} List with controls
 */

Lint Rules

ESLint: plugin:react/recommended, plugin:react-hooks/recommended Key rules: react-hooks/rules-of-hooks (error), exhaustive-deps (warn), jsx-no-target-blank (error) TypeScript: tsc --noEmit before commit

Security Checklist

1. XSS — Never use dangerouslySetInnerHTML; sanitize with DOMPurify
2. innerHTML — Grep; use React declarative rendering instead
3. Audit — Run npm audit monthly, enable dependabot
4. Sensitive Data — No passwords/tokens/PII in state; use secure httpOnly cookies
5. URL Validation — Validate user URLs before href assignment

Anti-patterns Summary

Reference rules/ for full details. These 5 pairs highlight common mistakes:

Wrong	Correct	Rule
if (loading && <Component />)	{loading ? <Skeleton /> : <Component />}	rendering-conditional-render
const Avatar = () => { ... } inside component	Extract to module scope	rerender-no-inline-components
setState(old => old + 1) in effect loop	Use functional setState in callbacks	rerender-functional-setstate
await data; return jsx; blocking entire page	Use Suspense boundaries per section	async-suspense-boundaries
import { a, b, c } from './utils' barrel	import { a } from './utils/a'	bundle-barrel-imports

How to Use

Read individual rule files for detailed explanations and code examples:

rules/async-parallel.md
rules/bundle-barrel-imports.md
rules/rerender-memo.md
rules/async-suspense-boundaries.md
rules/rerender-transitions.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

Reference: see references/react-patterns.md for detailed patterns.

For the complete guide with all rules expanded: references/react-patterns.md