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Skill: react-best-practices

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React and Next.js performance optimization guidelines from Vercel Engineering. This skill should be used when writing, reviewing, or refactoring React/Next.js code to ensure optimal performance patterns. Triggers on tasks involving React components, Next.js pages, data fetching, bundle optimization, or performance improvements.

Licence: MIT · Upstream: https://github.com/vercel-labs/agent-skills · Redistributed unmodified below.

Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 70 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 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

Rule Categories by Priority

Quick Reference

1. Eliminating Waterfalls (CRITICAL)

• async-cheap-condition-before-await - Check cheap sync conditions before awaiting flags or remote values
• 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-analyzable-paths - Prefer statically analyzable import and file-system paths to avoid broad bundles and traces
• 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-hoist-static-io - Hoist static I/O (fonts, logos) to module level
• server-no-shared-module-state - Avoid module-level mutable request state in RSC/SSR
• server-serialization - Minimize data passed to client components
• server-parallel-fetching - Restructure components to parallelize fetches
• server-parallel-nested-fetching - Chain nested fetches per item in Promise.all
• 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-split-combined-hooks - Split hooks with independent dependencies
• rerender-move-effect-to-event - Put interaction logic in event handlers
• rerender-transitions - Use startTransition for non-urgent updates
• rerender-use-deferred-value - Defer expensive renders to keep input responsive
• rerender-use-ref-transient-values - Use refs for transient frequent values
• rerender-no-inline-components - Don't define components inside components

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
• rendering-resource-hints - Use React DOM resource hints for preloading
• rendering-script-defer-async - Use defer or async on script tags

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
• js-flatmap-filter - Use flatMap to map and filter in one pass
• js-request-idle-callback - Defer non-critical work to browser idle time

8. Advanced Patterns (LOW)

• advanced-effect-event-deps - Don't put useEffectEvent results in effect deps
• 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

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