React Best Practices

Performance optimization guide for client-side React applications. Contains 47 rules across 8 categories, prioritized by impact. Adapted from Vercel Engineering's guidelines, filtered for client-only React + Vite (no Next.js / SSR).

When to Apply

Reference these guidelines when:

• Writing new React components or hooks
• Reviewing code for performance issues
• Refactoring existing React code
• Optimizing bundle size or render performance
• Writing or reviewing useEffect usage

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 promise chaining for partial dependencies
• async-api-routes — Start promises early, await late

2. Bundle Size Optimization (CRITICAL)

• bundle-barrel-imports — Import directly, avoid barrel files
• bundle-conditional — Load modules only when feature is activated
• bundle-defer-third-party — Load analytics/logging after mount
• bundle-preload — Preload on hover/focus for perceived speed

3. Client-Side Data Fetching (MEDIUM-HIGH)

• client-event-listeners — Deduplicate global event listeners
• client-passive-event-listeners — Use passive listeners for scroll/touch
• client-swr-dedup — Use SWR for automatic request deduplication
• client-localstorage-schema — Version and minimize localStorage data

4. Re-render Optimization (MEDIUM)

• rerender-derived-state-no-effect — Derive state during render, not effects
• rerender-defer-reads — Don't subscribe to state only used in callbacks
• rerender-simple-expression-in-memo — Avoid memo for simple primitives
• rerender-memo-with-default-value — Hoist default non-primitive props
• rerender-memo — Extract expensive work into memoized components
• rerender-dependencies — Use primitive dependencies in effects
• rerender-move-effect-to-event — Put interaction logic in event handlers
• 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
• rerender-use-ref-transient-values — Use refs for transient frequent values

5. useEffect Patterns (MEDIUM)

• useeffect-anti-patterns — Common mistakes: derived state in effects, effect chains, notifying parents
• useeffect-alternatives — Decision tree and alternatives: derived state, key prop, event handlers, useSyncExternalStore

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-conditional-render — Use ternary, not && for conditionals
• rendering-usetransition-loading — Prefer useTransition for loading state

7. JavaScript Performance (LOW-MEDIUM)

• js-batch-dom-css — Batch DOM reads/writes to avoid layout thrashing
• 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-init-once — Initialize app once per app load
• advanced-event-handler-refs — Store event handlers in refs
• advanced-use-latest — useEffectEvent for stable callback refs

How to Use

Read individual reference files for detailed explanations and code examples:

references/async-parallel.md
references/rerender-derived-state-no-effect.md
references/useeffect-anti-patterns.md

Each reference file contains:

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