React Best Practices

Core Rules

1. Rendering Optimization

• React Server Components (RSC): Favor RSC for data fetching by default. Use "use client" only for interactivity or browser-only APIs.
• Hydration: Minimize the waterfall by using Suspense and streaming. Never perform state updates in useEffect that could be handled during rendering.
• Memoization: Avoid useMemo and useCallback for trivial operations. Use them only when referential equality is required to prevent expensive re-renders in deep trees.

2. State Management (The Antigravity Way)

• Server State: MANDATORY use of TanStack Query. Disable refetchOnWindowFocus for data that rarely changes.
• URL as State: Use search parameters (URL) for UI state like filters, tabs, and pagination to ensure shareability.
• Local UI State: Use useOptimistic for instant feedback on mutations.

3. Styling & Layout

• Tailwind CSS: Use the clsx and tailwind-merge pattern for dynamic class strings.
• Images: Use next/image for automatic optimization. Always provide width, height, and alt.
• Layouts: Use the Next.js layout.tsx pattern for persistent navigation and persistent state.

4. Advanced Performance

• Code Splitting: Use next/dynamic or React.lazy for components that are not visible on the initial fold.
• Bundle Analysis: Audit the bundle periodically. Avoid heavy libraries (e.g., Moment.js, Lodash) where lightweight alternatives or native JS exist.
• Strict Mode: Always develop with React.StrictMode enabled to catch side-effect bugs early.

Interaction Protocol

• Input: React codebase fragments or performance profiling reports.
• Output: Refactored code with specific performance and best-practice improvements.

Tag: Start your response with [REACT-BEST-PRACTICES].

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-barrel-imports - Import directly, avoid barrel files
• 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

3. Server-Side Performance (HIGH)

• server-cache-react - Use React.cache() for per-request deduplication
• server-cache-lru - Use LRU cache for cross-request caching
• server-serialization - Minimize data passed to client components
• server-parallel-fetching - Restructure components to parallelize fetches
• server-after-nonblocking - Use after() for non-blocking operations

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

• client-swr-dedup - Use SWR for automatic request deduplication
• client-event-listeners - Deduplicate global event listeners

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
• rendering-conditional-render - Use ternary, not && for conditionals

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-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-barrel-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 the complete guide with all rules expanded: AGENTS.md