# Vercel React Best Practices

> 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.

Source: https://skillsagentes.com/skills/vercel-labs/agent-skills/vercel-react-best-practices
Repository: https://github.com/vercel-labs/agent-skills
Author: vercel-labs
License: MIT
Updated: hace 4 meses
Context cost: 82 tok installed, 1.8k tok once triggered, 57.6k tok with every bundled file
Bundle: 76 files, 225 KB
Permissions requested: none declared

## Install

```bash
npx -y skills add vercel-labs/agent-skills --skill react-best-practices --agent claude-code
```

## What it does

- Aplica 70 reglas de rendimiento de React/Next.js de Vercel Engineering, agrupadas en 8 categorías priorizadas por impacto
- Guía el refactor y la generación de código hacia patrones como eliminar waterfalls, reducir el bundle y optimizar re-renders
- Referencia archivos de reglas individuales en rules/ con explicaciones y ejemplos de código correcto e incorrecto

## Use it when

- Al escribir nuevos componentes de React o páginas de Next.js
- Al implementar data fetching en cliente o servidor
- Al revisar o refactorizar código React/Next.js por problemas de rendimiento
- Al optimizar el tamaño del bundle o los tiempos de carga

## What triggers it

- "Revisa este componente de Next.js en busca de problemas de rendimiento"
- "Refactoriza este data fetching para evitar waterfalls"
- "¿Cómo reduzco el tamaño del bundle en esta página?"
- "Optimiza los re-renders de este componente React"

## Files

- AGENTS.md — 106 KB
- README.md — 3 KB
- SKILL.md — 7 KB
- metadata.json — 921 B
- rules/_sections.md — 2 KB
- rules/_template.md — 631 B
- rules/advanced-effect-event-deps.md — 2 KB
- rules/advanced-event-handler-refs.md — 1 KB
- rules/advanced-init-once.md — 958 B
- rules/advanced-use-latest.md — 1 KB
- rules/async-api-routes.md — 1 KB
- rules/async-cheap-condition-before-await.md — 1 KB
- rules/async-defer-await.md — 2 KB
- rules/async-dependencies.md — 1 KB
- rules/async-parallel.md — 654 B
- rules/async-suspense-boundaries.md — 2 KB
- rules/bundle-analyzable-paths.md — 2 KB
- rules/bundle-barrel-imports.md — 3 KB
- rules/bundle-conditional.md — 949 B
- rules/bundle-defer-third-party.md — 920 B
- rules/bundle-dynamic-imports.md — 791 B
- rules/bundle-preload.md — 1 KB
- rules/client-event-listeners.md — 2 KB
- rules/client-localstorage-schema.md — 2 KB
- rules/client-passive-event-listeners.md — 2 KB
- rules/client-swr-dedup.md — 1 KB
- rules/js-batch-dom-css.md — 3 KB
- rules/js-cache-function-results.md — 2 KB
- rules/js-cache-property-access.md — 532 B
- rules/js-cache-storage.md — 2 KB
- rules/js-combine-iterations.md — 753 B
- rules/js-early-exit.md — 1 KB
- rules/js-flatmap-filter.md — 1 KB
- rules/js-hoist-regexp.md — 1 KB
- rules/js-index-maps.md — 837 B
- rules/js-length-check-first.md — 2 KB
- rules/js-min-max-loop.md — 2 KB
- rules/js-request-idle-callback.md — 3 KB
- rules/js-set-map-lookups.md — 532 B
- rules/js-tosorted-immutable.md — 2 KB
- rules/rendering-activity.md — 564 B
- rules/rendering-animate-svg-wrapper.md — 1 KB
- rules/rendering-conditional-render.md — 980 B
- rules/rendering-content-visibility.md — 815 B
- rules/rendering-hoist-jsx.md — 1 KB
- rules/rendering-hydration-no-flicker.md — 2 KB
- rules/rendering-hydration-suppress-warning.md — 872 B
- rules/rendering-resource-hints.md — 2 KB
- rules/rendering-script-defer-async.md — 2 KB
- rules/rendering-svg-precision.md — 588 B
- rules/rendering-usetransition-loading.md — 2 KB
- rules/rerender-defer-reads.md — 973 B
- rules/rerender-dependencies.md — 824 B
- rules/rerender-derived-state-no-effect.md — 1 KB
- rules/rerender-derived-state.md — 728 B
- rules/rerender-functional-setstate.md — 3 KB
- rules/rerender-lazy-state-init.md — 2 KB
- rules/rerender-memo-with-default-value.md — 1 KB
- rules/rerender-memo.md — 1 KB
- rules/rerender-move-effect-to-event.md — 1 KB
- rules/rerender-no-inline-components.md — 2 KB
- rules/rerender-simple-expression-in-memo.md — 1018 B
- rules/rerender-split-combined-hooks.md — 2 KB
- rules/rerender-transitions.md — 1 KB
- rules/rerender-use-deferred-value.md — 2 KB
- rules/rerender-use-ref-transient-values.md — 2 KB
- rules/server-after-nonblocking.md — 2 KB
- rules/server-auth-actions.md — 3 KB
- rules/server-cache-lru.md — 1 KB
- rules/server-cache-react.md — 2 KB
- rules/server-dedup-props.md — 2 KB
- rules/server-hoist-static-io.md — 4 KB
- rules/server-no-shared-module-state.md — 2 KB
- rules/server-parallel-fetching.md — 2 KB
- rules/server-parallel-nested-fetching.md — 992 B
- rules/server-serialization.md — 996 B

## SKILL.md

Reproduced verbatim from vercel-labs/agent-skills under MIT. This section is the upstream document and is in English.

# 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

| 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-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`

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Skills Agentes — https://skillsagentes.com/skills/vercel-labs/agent-skills/vercel-react-best-practices