React Best Practices

Performance in React isn't one thing — it's a stack of decisions made at every layer, from how you fetch data to how you structure a conditional. These guidelines cover 8 categories derived from Vercel Engineering's production patterns. Apply the HIGH priority categories first; revisit MEDIUM/LOW during review or refactoring phases.

Workflow

1. Identify the component type — Server Component (default in Next.js App Router) or Client Component ("use client"). This determines which categories apply.
2. Check for waterfalls — Sequential awaits where parallel fetching is possible are the most common performance killer.
3. Check bundle contributions — Barrel imports and unguarded dynamic dependencies inflate initial load.
4. Audit server-side patterns — Caching, serialization, and auth in server actions.
5. Review client-side state — Re-renders, SWR usage, event listener patterns.
6. Apply low-priority optimizations — JS micro-optimizations and advanced patterns during dedicated perf reviews.

For the full rule catalog, read the references/ file for the relevant category before implementing.

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Categories

1. Eliminating Waterfalls — CRITICAL

Sequential async operations block the render pipeline. Every unnecessary sequential await adds to Time to First Byte and visible loading time.

Key rules:

• Defer await: Move await into the branch where the value is first used, not at the top of a component.
• Parallel fetching: Use Promise.all() or Promise.allSettled() for independent operations.
• Suspense boundaries: Wrap independent data-fetching components in <Suspense> to stream UI progressively.

// Instead of sequential:
const user = await getUser();
const posts = await getPosts();

// Use parallel:
const [user, posts] = await Promise.all([getUser(), getPosts()]);

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2. Bundle Size Optimization — CRITICAL

Every kilobyte added to the initial bundle delays interactivity. Barrel files and eager imports are the primary culprits in Next.js apps.

Key rules:

• Direct imports: Import specific files, not barrel index files (import Button from '@ui/Button' not import { Button } from '@ui').
• Dynamic imports: Use next/dynamic with ssr: false for components not needed on initial render (charts, modals, editors).
• Conditional loading: Load heavy modules (e.g., PDF renderers, video players) only when the feature activates.

const PDFViewer = dynamic(() => import('@/components/PDFViewer'), { ssr: false });

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3. Server-Side Performance — HIGH

Server Components run on the server per request. Inefficient I/O, redundant serialization, and missing caches multiply costs across concurrent users.

Key rules:

• React.cache() for per-request deduplication — wrap data fetching functions called in multiple components within a single render.
• LRU cache for cross-request caching of stable data (e.g., config lookups, CMS content with short TTLs).
• Hoist static I/O — fonts, default images, and config loaded at module level, not inside render functions.
• Minimize RSC props — data passed to Client Components must be serialized; pass only what the client actually renders.

Read references/server-performance.md for all 8 rules with examples (server-auth-actions, server-cache-react, server-cache-lru, server-dedup-props, server-hoist-static-io, server-serialization, server-parallel-fetching, server-after-nonblocking).

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4. Client-Side Data Fetching — MEDIUM-HIGH

Client-side fetching patterns affect perceived performance and network efficiency. Uncoordinated requests duplicate work; passive event listeners block scrolling.

Key rules:

• SWR deduplication: Use useSWR for client data fetching — it automatically deduplicates concurrent requests for the same key.
• Passive event listeners: Add { passive: true } to scroll and touch handlers to avoid blocking the main thread.

Read references/client-fetching.md for all 4 rules (client-swr-dedup, client-event-listeners, client-passive-event-listeners, client-localstorage-schema).

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5. Re-render Optimization — MEDIUM

Unnecessary re-renders are the most common runtime performance issue. React re-renders when state changes — the goal is making sure only the components that need to update actually do.

Key rules:

• useMemo with default value: Provide stable defaults to avoid undefined → value transitions that cause downstream re-renders.
• Derived state without effect: Compute derived values during render, not in useEffect with setState.
• Functional setState: Use the callback form setState(prev => ...) when new state depends on previous.
• Lazy state init: Pass a function to useState for expensive initial values: useState(() => computeExpensive()).

Read references/rerender-optimization.md for all 12 rules (rerender-defer-reads, rerender-memo, rerender-memo-with-default-value, rerender-dependencies, rerender-derived-state, rerender-derived-state-no-effect, rerender-functional-setstate, rerender-lazy-state-init, rerender-simple-expression-in-memo, rerender-move-effect-to-event, rerender-transitions, rerender-use-ref-transient-values).

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6. Rendering Performance — MEDIUM

Browser rendering is the last mile. DOM mutations, layout thrashing, and unoptimized SVGs cause jank even when React itself is efficient.

Key rules:

• content-visibility: auto: Apply to off-screen content sections to skip layout and paint for invisible areas.
• Hoist JSX constants: Move static JSX outside component functions to avoid object recreation on every render.
• Suppress hydration warnings carefully: Only use suppressHydrationWarning for genuinely client-only values (e.g., timestamps, random IDs) — not to hide real mismatches.

Read references/rendering-performance.md for all 9 rules (rendering-animate-svg-wrapper, rendering-content-visibility, rendering-hoist-jsx, rendering-svg-precision, rendering-hydration-no-flicker, rendering-hydration-suppress-warning, rendering-activity, rendering-conditional-render, rendering-usetransition-loading).

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7. JavaScript Performance — LOW-MEDIUM

These micro-optimizations compound at scale. Prioritize during dedicated performance audits, not during initial development.

Key rules:

• Index maps over repeated array finds: Pre-compute a Map or record from arrays used in loops or repeated lookups.
• Early exit: Add guard clauses to exit expensive functions as early as possible.
• Set/Map lookups: Use Set for membership checks and Map for key-value lookups — both are O(1) vs array's O(n).

Read references/js-performance.md for all 12 rules (js-batch-dom-css, js-index-maps, js-cache-property-access, js-cache-function-results, js-cache-storage, js-combine-iterations, js-length-check-first, js-early-exit, js-hoist-regexp, js-min-max-loop, js-set-map-lookups, js-tosorted-immutable).

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8. Advanced Patterns — LOW

These patterns solve specific edge cases. Apply only when the simpler alternatives have been exhausted.

• Event handler refs: Store event handlers in refs when they need to be stable across renders without useCallback dependencies.
• Init once: Use module-level initialization for app-wide singletons (analytics, feature flags) to prevent re-initialization on HMR.
• useLatest: Keep a ref that always points to the current callback — useful for event listeners that outlive their render cycle.

Read references/advanced-patterns.md for implementation details on all 3 patterns (advanced-event-handler-refs, advanced-init-once, advanced-use-latest).

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Output Format

When auditing existing code, report issues as:

[CATEGORY] [PRIORITY] rule-name
File: path/to/file.tsx:line
Issue: What the current code does wrong
Fix: What to change and why it matters

When writing new code, apply the applicable rules silently and note any trade-offs made.

Quality Checklist

• No sequential awaits where parallel fetching is possible
• No barrel file imports from large libraries
• Server Components don't pass unnecessary data to Client Components
• React.cache() used for functions called in multiple components per request
• No derived state computed in useEffect + setState
• Heavy client-only components use next/dynamic
• Static JSX objects hoisted out of render functions

Common Antipatterns

• Putting "use client" at the top of a file "just to be safe" — pushes serialization costs to the client unnecessarily
• useEffect(() => { setDerived(compute(value)) }, [value]) — compute during render instead
• import { everything } from 'library' — import specific files
• const x = { key: 'value' } inside render — hoist to module level if static

References

• references/server-performance.md — 8 server-side rules
• references/client-fetching.md — 4 client-side data fetching rules
• references/rerender-optimization.md — 12 re-render optimization rules
• references/rendering-performance.md — 9 browser rendering rules
• references/js-performance.md — 12 JavaScript micro-optimization rules
• references/advanced-patterns.md — 3 advanced React patterns

See Also

• vercel-composition-patterns — Para patterns de composición React avanzados (compound components, CVA, React 19 APIs)