React Best Practices (TanStack Start Edition)

Comprehensive performance optimization guide for React and TanStack Start applications. Contains 45+ rules across 8 categories, prioritized by impact to guide automated refactoring and code generation. Customized for TanStack Start with server functions, Router SSR Query integration, and project-specific patterns.

When to Apply

Reference these guidelines when:

• Writing new React components or TanStack Start routes
• Implementing data fetching (server functions, Router SSR Query, TanStack Query)
• Reviewing code for performance issues (waterfalls, bundle size, re-renders)
• Refactoring existing React/TanStack code
• Optimizing bundle size, load times, or SSR performance
• Working with route loaders (createFileRoute, loader)
• Creating server functions (createServerFn)

Project-Specific Context

This project uses:

• TanStack Start: File-based routes with SSR/SSG support
• TanStack Router: Type-safe routing with loader patterns
• TanStack Query: Server state management with SSR hydration
• Router SSR Query Integration: dehydrate/hydrate for zero-refetch SSR
• Server Functions: Server-side logic with createServerFn
• Supabase: PostgreSQL with RLS for data layer
• Cloudflare Workers: Deployment target

Key Docs:

• 07-data-access-patterns.md – Router SSR Query patterns
• 01-architecture.md – Error handling, type safety
• 16-library-patterns.md – TanStack Query, Zustand, Zod

Rule Categories by Priority

Quick Reference

1. Eliminating Waterfalls (CRITICAL)

TanStack Start Specific:

• async-tanstack-loader-critical – Use ensureQueryData (await) vs prefetchQuery (fire-and-forget)
• async-tanstack-query-options – Create reusable queryOptions for zero-refetch SSR

General React:

• 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 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)

TanStack Start Specific:

• server-tanstack-router-dehydrate – Configure dehydrate/hydrate for zero-refetch SSR
• server-tanstack-parallel-fetching – Use Promise.all() in server functions

General React:

• 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 – Fire-and-forget for non-blocking side effects

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

• client-swr-dedup – Use TanStack Query 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