Vercel React Best Practices

---
name: vercel-react-best-practices
description: React and Next.js performance optimization guidelines from Vercel Engineering. Use when writing, reviewing, or refactoring React/Next.js code. Triggers on tasks involving React components, Next.js pages, data fetching, bundle optimization, or performance improvements.
allowed-tools: Bash,Read,Write,Edit,Grep,Glob,Task
---

# Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications from Vercel Engineering. Contains 45 rules across 8 categories, prioritized by impact.

## 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-cache-react` - Use React.cache() for per-request deduplication
- `server-cache-lru` - Use LRU cache for cross-request caching
- `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

### 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-dependencies` - Use primitive dependencies in effects
- `rerender-derived-state` - Subscribe to derived booleans, not raw values
- `rerender-functional-setstate` - Use functional setState for stable callbacks
- `rerender-lazy-state-init` - Pass function to useState for expensive values
- `rerender-transitions` - Use startTransition for non-urgent updates

### 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-activity` - Use Activity component for show/hide
- `rendering-conditional-render` - Use ternary, not && for conditionals

### 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-use-latest` - useLatest for stable callback refs

## Key Performance Rules

### 1.1 Defer Await Until Needed

Move `await` operations into the branches where they're actually used to avoid blocking code paths that don't need them.

**Incorrect: blocks both branches**
```typescript
async function handleRequest(userId: string, skipProcessing: boolean) {
  const userData = await fetchUserData(userId)

  if (skipProcessing) {
    // Returns immediately but still waited for userData
    return { skipped: true }
  }

  return processUserData(userData)
}
```

**Correct: only blocks when needed**
```typescript
async function handleRequest(userId: string, skipProcessing: boolean) {
  if (skipProcessing) {
    // Returns immediately without waiting
    return { skipped: true }
  }

  const userData = await fetchUserData(userId)
  return processUserData(userData)
}
```

### 1.2 Dependency-Based Parallelization

For operations with partial dependencies, use `better-all` to maximize parallelism.

**Incorrect: profile waits for config unnecessarily**
```typescript
const [user, config] = await Promise.all([
  fetchUser(),
  fetchConfig()
])
const profile = await fetchProfile(user.id)
```

**Correct: config and profile run in parallel**
```typescript
import { all } from 'better-all'

const { user, config, profile } = await all({
  async user() { return fetchUser() },
  async config() { return fetchConfig() },
  async profile() {
    return fetchProfile((await this.$.user).id)
  }
})
```

### 2.1 Avoid Barrel File Imports

Import directly from modules instead of barrel files to enable better tree-shaking.

**Incorrect: barrel file imports**
```typescript
import { Button, Input, Card, Modal } from '@/components'
```

**Correct: direct imports**
```typescript
import Button from '@/components/Button'
import Input from '@/components/Input'
import Card from '@/components/Card'
import Modal from '@/components/Modal'
```

### 2.4 Dynamic Imports for Heavy Components

Use `next/dynamic` for heavy components that aren't needed on initial render.

```typescript
import dynamic from 'next/dynamic'

const HeavyChart = dynamic(() => import('@/components/HeavyChart'), {
  loading: () => <p>Loading...</p>,
  ssr: false  // Disable if no SSR needed
})
```

### 3.6 Per-Request Deduplication with React.cache()

Use `React.cache()` to deduplicate calls within the same request.

```typescript
import { cache } from 'react'

const getUser = cache(async (id: string) => {
  return db.user.findUnique({ where: { id } })
})

// Both calls return the same promise within the same request
const user1 = await getUser('123')
const user2 = await getUser('123')  // No additional database query
```

### 4.3 Use SWR for Automatic Deduplication

Use SWR for automatic request deduplication and caching.

```typescript
import useSWR from 'swr'

function useUser(id: string) {
  return useSWR(`/api/user/${id}`, fetcher)
}
```

### 5.2 Extract to Memoized Components

Extract expensive work into memoized components to prevent unnecessary re-renders.

```typescript
// Before: Expensive calculation on every render
function Dashboard({ data }) {
  const summary = computeSummary(data)  // Runs every render

  return <div>{summary}</div>
}

// After: Memoized component
const Summary = memo(({ data }) => {
  return <div>{computeSummary(data)}</div>
})

function Dashboard({ data }) {
  return <Summary data={data} />
}
```

### 5.7 Use Transitions for Non-Urgent Updates

Use `startTransition` for non-urgent updates to keep the UI responsive.

```typescript
import { useState, startTransition } from 'react'

function SearchResults({ query }) {
  const [results, setResults] = useState([])

  function handleChange(e) {
    startTransition(() => {
      setResults(search(e.target.value))
    })
  }

  return <input onChange={handleChange} />
}
```

---

**Version**: 1.0.0
**Author**: Vercel Engineering
**License**: MIT
**Reference**: https://github.com/vercel-labs/agent-skills/tree/main/skills/react-best-practices