React Best Practices

Performance optimization guide for React and TanStack Start applications. Contains 53 rules across 8 categories, prioritized by impact to guide code generation and refactoring.

Adapted from Vercel Engineering's React Best Practices for the TanStack Start + tRPC + Vite stack.

For Thor-specific architectural patterns (data flow, tRPC procedures, forms, subscriptions), see docs/patterns/SKILL.md.

When to Apply

Reference these guidelines when:

• •Writing new React components or TanStack Start routes
• •Implementing data fetching (tRPC queries, mutations, subscriptions)
• •Reviewing code for performance issues
• •Refactoring existing React/TypeScript 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

2. Bundle Size Optimization (CRITICAL)

• •bundle-barrel-imports - Import directly, avoid barrel files
• •bundle-dynamic-imports - Use React.lazy() 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-cache-lru - Use LRU cache for cross-request caching in long-lived processes

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

• •client-swr-dedup - Use TanStack Query (tRPC) for automatic 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-manual-memo - No manual memoization (React Compiler handles it)

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-stable-keys - Use stable, unique keys for lists
• •rendering-use-id - Use useId for SSR-safe unique IDs
• •rendering-conditional-render - Use ternary, not && for conditionals
• •rendering-usetransition-loading - Prefer useTransition for loading state
• •rendering-compiler-purity - React Compiler purity rules (Rules of React enforcement)

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

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 - useEffectEvent for stable callback 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