React Best Practices

Comprehensive performance optimization guide for React and Next.js applications, tuned by gracefullight (forked from Vercel Engineering). Contains 42 rules across 8 categories (some rules are enforced by Biome linter), prioritized by impact to guide automated refactoring and code generation.

Project-Specific Notes

This project uses Orval + React Query (@tanstack/react-query) for API layer:

• Orval generates type-safe React Query hooks from OpenAPI specs
• Always use Orval-generated hooks in Client Components instead of RSC data fetching
• React Query handles caching, deduplication, and devtools
• RSC Components should handle layout, SEO metadata, and pass static props only

React Compiler is enabled (reactCompiler: true in next.config.ts):

• React Compiler automatically handles memoization (useMemo, useCallback, React.memo)
• Only use manual memoization when profiling a bottleneck or for genuinely expensive computations

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-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

Note: Barrel file imports are enforced by Biome linter (noBarrelFile, noReExportAll)

3. Server-Side Performance (HIGH)

• server-after-nonblocking – Use after() for non-blocking operations

Note: RSC data fetching rules are excluded. This project uses Orval + React Query for API integration.

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

• client-react-compiler-optimal – React Compiler handles memoization, only use useMemo/useCallback when profiling bottlenecks
• client-orval-pattern – Always use Orval-generated hooks for API data fetching
• client-tanstack-query-dedup – Use @tanstack/query for automatic request deduplication
• client-event-listeners – Deduplicate global event listeners

IMPORTANT: Do NOT use RSC async data fetching in this project. Use Orval-generated React Query hooks instead.

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-dependencies – Use primitive dependencies in effects
• 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

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-activity – Use Activity component for show/hide

Note: Conditional rendering with && is enforced by Biome linter (noLeakedRender)

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-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

Note: RegExp hoisting is enforced by Biome linter (useTopLevelRegex)

8. Advanced Patterns (LOW)

• advanced-event-handler-refs – Store event handlers in refs
• advanced-use-latest – useLatest for stable callback refs

How to Use

Read individual rule files for detailed explanations and code examples:

rules/async-parallel.md
rules/bundle-dynamic-imports.md
rules/_sections.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 complete guide with all rules expanded: AGENTS.md