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name: vercel-react-best-practices description: React performance optimization guidelines from Vercel Engineering. This skill should be used when writing, reviewing, or refactoring React code to ensure optimal performance patterns. Triggers on tasks involving React components, data fetching, bundle optimization, or performance improvements. license: MIT metadata: author: vercel version: "1.0.0"

Vercel React Best Practices

Comprehensive performance optimization guide for React applications, maintained by Vercel. Contains 57 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.

When to Apply

Reference these guidelines when:

• Writing new React components
• Implementing data fetching (client or server-side)
• Reviewing code for performance issues
• Refactoring existing React code
• Optimizing bundle size or load times

Rule Categories by Priority

Priority	Category	Impact	Prefix
1	Eliminating Waterfalls	CRITICAL	async-
2	Bundle Size Optimization	CRITICAL	bundle-
3	Server-Side Performance	HIGH	server-
4	Client-Side Data Fetching	MEDIUM-HIGH	client-
5	Re-render Optimization	MEDIUM	rerender-
6	Rendering Performance	MEDIUM	rendering-
7	JavaScript Performance	LOW-MEDIUM	js-
8	Advanced Patterns	LOW	advanced-

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-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. 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-dedup-props - Avoid duplicate serialization in RSC props
• server-serialization - Minimize data passed to client components
• server-parallel-fetching - Restructure components to parallelize fetches

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

• client-swr-dedup - Use SWR for automatic request 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

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-conditional-render - Use ternary, not && for conditionals
• rendering-usetransition-loading - Prefer useTransition for loading state

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 - useLatest for stable callback refs

TanStack Start Addendum (HIGH)

• start-isomorphic-loader - Use server functions for server-only operations in loaders
• start-execution-boundaries - Use createServerOnlyFn/createClientOnlyFn appropriately
• start-server-functions - Organize server functions with static imports and validation
• start-server-routes - Use createFileRoute server handlers with proper Response objects
• start-middleware - Implement middleware with proper chaining and execution order
• start-env-security - Separate server and client environment variables correctly
• start-prerendering - Configure static prerendering for appropriate routes
• start-build-config - Configure Vite plugins in correct order with proper TypeScript settings
• start-error-boundaries - Implement route-level error boundaries with ErrorComponent

Next.js Addendum (HIGH)

• nextjs-api-routes-waterfalls - Prevent waterfall chains in API routes and Server Actions
• nextjs-optimize-package-imports - Use optimizePackageImports for barrel imports
• nextjs-dynamic-imports - Use next/dynamic for heavy components
• nextjs-defer-third-party - Defer third-party analytics with next/dynamic
• nextjs-server-actions-auth - Authenticate Server Actions like API routes
• nextjs-after-nonblocking - Use after() for non-blocking operations
• nextjs-fetch-memoization - Understand Next.js fetch memoization vs React.cache
• nextjs-use-search-params - Avoid unnecessary subscriptions with useSearchParams

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