React Best Practices

---
name: vercel-react-best-practices
description: 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.
license: MIT
metadata:
  author: vercel
  version: "1.0.0"
---

# Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 58 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-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-hoist-static-io` - Hoist static I/O (fonts, logos) to module level
- `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

## 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` ([source](https://raw.githubusercontent.com/vercel-labs/agent-skills/main/skills/react-best-practices/AGENTS.md))


---

## Companion Files

The following reference files are included for convenience:

### rules/async-parallel.md

---
title: Promise.all() for Independent Operations
impact: CRITICAL
impactDescription: 2-10× improvement
tags: async, parallelization, promises, waterfalls
---

## Promise.all() for Independent Operations

When async operations have no interdependencies, execute them concurrently using `Promise.all()`.

**Incorrect (sequential execution, 3 round trips):**

```typescript
const user = await fetchUser()
const posts = await fetchPosts()
const comments = await fetchComments()
```

**Correct (parallel execution, 1 round trip):**

```typescript
const [user, posts, comments] = await Promise.all([
  fetchUser(),
  fetchPosts(),
  fetchComments()
])
```

### rules/bundle-barrel-imports.md

---
title: Avoid Barrel File Imports
impact: CRITICAL
impactDescription: 200-800ms import cost, slow builds
tags: bundle, imports, tree-shaking, barrel-files, performance
---

## Avoid Barrel File Imports

Import directly from source files instead of barrel files to avoid loading thousands of unused modules. **Barrel files** are entry points that re-export multiple modules (e.g., `index.js` that does `export * from './module'`).

Popular icon and component libraries can have **up to 10,000 re-exports** in their entry file. For many React packages, **it takes 200-800ms just to import them**, affecting both development speed and production cold starts.

**Why tree-shaking doesn't help:** When a library is marked as external (not bundled), the bundler can't optimize it. If you bundle it to enable tree-shaking, builds become substantially slower analyzing the entire module graph.

**Incorrect (imports entire library):**

```tsx
import { Check, X, Menu } from 'lucide-react'
// Loads 1,583 modules, takes ~2.8s extra in dev
// Runtime cost: 200-800ms on every cold start

import { Button, TextField } from '@mui/material'
// Loads 2,225 modules, takes ~4.2s extra in dev
```

**Correct (imports only what you need):**

```tsx
import Check from 'lucide-react/dist/esm/icons/check'
import X from 'lucide-react/dist/esm/icons/x'
import Menu from 'lucide-react/dist/esm/icons/menu'
// Loads only 3 modules (~2KB vs ~1MB)

import Button from '@mui/material/Button'
import TextField from '@mui/material/TextField'
// Loads only what you use
```

**Alternative (Next.js 13.5+):**

```js
// next.config.js - use optimizePackageImports
module.exports = {
  experimental: {
    optimizePackageImports: ['lucide-react', '@mui/material']
  }
}

// Then you can keep the ergonomic barrel imports:
import { Check, X, Menu } from 'lucide-react'
// Automatically transformed to direct imports at build time
```

Direct imports provide 15-70% faster dev boot, 28% faster builds, 40% faster cold starts, and significantly faster HMR.

Libraries commonly affected: `lucide-react`, `@mui/material`, `@mui/icons-material`, `@tabler/icons-react`, `react-icons`, `@headlessui/react`, `@radix-ui/react-*`, `lodash`, `ramda`, `date-fns`, `rxjs`, `react-use`.

Reference: [How we optimized package imports in Next.js](https://vercel.com/blog/how-we-optimized-package-imports-in-next-js)