React Best Practices

Comprehensive performance optimization guide for React applications, adapted from Vercel Engineering. Contains 30 rules across 7 categories, prioritized by impact to guide automated refactoring and code generation.

Note: This version has been customized for pure React 19.2:
- Server-side data fetching and SWR rules removed (project uses react-query)
- Next.js-specific rules removed (next/dynamic, SSR hydration, etc.)
- React Compiler is enabled - Memoization rules (memo, useMemo, useCallback) removed as the compiler handles this automatically

When to Apply

Reference these guidelines when:

• Writing new React components
• Reviewing code for performance issues
• Refactoring existing React 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-suspense-boundaries - Use Suspense to stream content

2. Bundle Size Optimization (CRITICAL)

• bundle-barrel-imports - Import directly, avoid barrel files
• bundle-conditional - Load modules only when feature is activated
• bundle-preload - Preload on hover/focus for perceived speed

3. Client-Side Patterns (MEDIUM-HIGH)

• client-localstorage-schema - Type-safe localStorage access
• client-passive-event-listeners - Use passive event listeners for scroll/touch

4. Re-render Optimization (MEDIUM)

• rerender-defer-reads - Don't subscribe to state only used in callbacks
• 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

5. Rendering Performance (MEDIUM)

• rendering-animate-svg-wrapper - Animate div wrapper, not SVG element
• rendering-content-visibility - Use content-visibility for long lists
• rendering-svg-precision - Reduce SVG coordinate precision
• rendering-activity - Use Activity component for show/hide
• rendering-conditional-render - Use ternary, not && for conditionals

6. 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

7. Advanced Patterns (LOW)

• advanced-event-handler-refs - Store event handlers in refs

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