Agent Skills: Vercel React Best Practices

Vercel React Best Practices

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

Security Notice (Critical)

IMPORTANT: Code security must be maintained during performance optimization.

• Never expose sensitive data when optimizing data serialization
• Sanitize user-supplied code before rendering or evaluation
• Validate all dynamic output to prevent injection attacks
• Never use HTML comments (<!-- -->) to store sensitive information
• Avoid direct evaluation or insertion of code samples without validation

When to Apply

Reference these guidelines when:

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

Understanding the Rules

Each rule includes:

• Name: Identifier for the optimization pattern
• Description: Concise explanation of what the rule addresses
• Code Examples: Showing incorrect (before) and correct (after) implementations
• Impact Level: Priority rating to guide optimization efforts

Rule Enforcement

When reviewing code:

• Critical rules: Must be addressed immediately (waterfalls, bundle bloat)
• High rules: Should be fixed in current PR if feasible
• Medium rules: Note for future optimization or current work if easy
• Low rules: Consider for dedicated performance optimization efforts

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-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-auth-actions - Authenticate server actions like API routes
• 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
• 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
• 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

Additional Best Practices

Accessibility (a11y)

Ensure performance optimizations don't compromise accessibility:

• Lazy loading must not break keyboard navigation
• Dynamic imports should maintain focus management
• Animations should respect prefers-reduced-motion
• Loading states should be announced to screen readers
• Use semantic HTML even when optimizing for performance

Testing and Error Boundaries

Integrate performance with reliability:

• Test code coverage after refactoring for performance
• Implement error boundaries around Suspense boundaries
• Test lazy-loaded components in isolation
• Validate that optimizations don't break functionality
• Monitor error rates after performance changes

Next.js Rendering Modes

Choose appropriate rendering strategy based on content requirements:

• SSG (Static): For content that rarely changes
• ISR (Incremental Static): For content with predictable update patterns
• SSR (Server-Side): For personalized or real-time content
• CSR (Client-Side): For highly interactive, user-specific data
• Document rendering mode choices clearly in code comments, but never include secrets (API keys, tokens), credentials, or internal-only endpoints in comments

State Management and Context (Essential)

• Split contexts by update frequency
• Use context selectors to minimize re-renders
• Keep context providers close to consumers
• Avoid prop drilling by using composition patterns

Advanced State Patterns

• Consider external stores (Zustand, Jotai) for global state

Monitoring and Profiling

Measure performance impact:

• Use React DevTools Profiler for component performance
• Monitor Core Web Vitals (LCP, FID, CLS)
• Track bundle size changes in CI/CD
• Set performance budgets for routes
• Profile before and after optimizations
• Use Lighthouse CI for automated performance checks