React Best Practices

Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 45 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.

When to Use

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-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

How to Use

Read individual rule files for detailed explanations and code examples:

rules/async-parallel.md
rules/bundle-barrel-imports.md
rules/_sections.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

When to Use

This skill is applicable to execute the workflow or actions described in the overview.

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.

React Best Practices

Version 0.1.0 Vercel Engineering January 2026

Note:

This document is mainly for agents and LLMs to follow when maintaining,

generating, or refactoring React and Next.js codebases at Vercel. Humans

may also find it useful, but guidance here is optimized for automation

and consistency by AI-assisted workflows.

---

Abstract

Comprehensive performance optimization guide for React and Next.js applications, designed for AI agents and LLMs. Contains 40+ rules across 8 categories, prioritized by impact from critical (eliminating waterfalls, reducing bundle size) to incremental (advanced patterns). Each rule includes detailed explanations, real-world examples comparing incorrect vs. correct implementations, and specific impact metrics to guide automated refactoring and code generation.

---

Table of Contents

1. Eliminating Waterfalls — CRITICAL

• 1.1 Defer Await Until Needed
• 1.2 Dependency-Based Parallelization
• 1.3 Prevent Waterfall Chains in API Routes
• 1.4 Promise.all() for Independent Operations
• 1.5 Strategic Suspense Boundaries

2. Bundle Size Optimization — CRITICAL

• 2.1 Avoid Barrel File Imports
• 2.2 Conditional Module Loading
• 2.3 Defer Non-Critical Third-Party Libraries
• 2.4 Dynamic Imports for Heavy Components
• 2.5 Preload Based on User Intent

3. Server-Side Performance — HIGH

• 3.1 Cross-Request LRU Caching
• 3.2 Minimize Serialization at RSC Boundaries
• 3.3 Parallel Data Fetching with Component Composition
• 3.4 Per-Request Deduplication with React.cache()
• 3.5 Use after() for Non-Blocking Operations

4. Client-Side Data Fetching — MEDIUM-HIGH

• 4.1 Deduplicate Global Event Listeners
• 4.2 Use SWR for Automatic Deduplication

5. Re-render Optimization — MEDIUM

• 5.1 Defer State Reads to Usage Point
• 5.2 Extract to Memoized Components
• 5.3 Narrow Effect Dependencies
• 5.4 Subscribe to Derived State
• 5.5 Use Functional setState Updates
• 5.6 Use Lazy State Initialization
• 5.7 Use Transitions for Non-Urgent Updates

6. Rendering Performance — MEDIUM

• 6.1 Animate SVG Wrapper Instead of SVG Element
• 6.2 CSS content-visibility for Long Lists
• 6.3 Hoist Static JSX Elements
• 6.4 Optimize SVG Precision
• 6.5 Prevent Hydration Mismatch Without Flickering
• 6.6 Use Activity Component for Show/Hide
• 6.7 Use Explicit Conditional Rendering

7. JavaScript Performance — LOW-MEDIUM

• 7.1 Batch DOM CSS Changes
• 7.2 Build Index Maps for Repeated Lookups
• 7.3 Cache Property Access in Loops
• 7.4 Cache Repeated Function Calls
• 7.5 Cache Storage API Calls
• 7.6 Combine Multiple Array Iterations
• 7.7 Early Length Check for Array Comparisons
• 7.8 Early Return from Functions
• 7.9 Hoist RegExp Creation
• 7.10 Use Loop for Min/Max Instead of Sort
• 7.11 Use Set/Map for O(1) Lookups
• 7.12 Use toSorted() Instead of sort() for Immutability

8. Advanced Patterns — LOW

• 8.1 Store Event Handlers in Refs
• 8.2 useLatest for Stable Callback Refs

---

1. Eliminating Waterfalls

Impact: CRITICAL

Waterfalls are the #1 performance killer. Each sequential await adds full network latency. Eliminating them yields the largest gains.

1.1 Defer Await Until Needed

Impact: HIGH (avoids blocking unused code paths)

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

async function handleRequest(userId: string, skipProcessing: boolean) {
  const userData = await fetchUserData(userId)
  
  if (skipProcessing) {
    // Returns immediately but still waited for userData
    return { skipped: true }
  }
  
  // Only this branch uses userData
  return processUserData(userData)
}

Correct: only blocks when needed

async function handleRequest(userId: string, skipProcessing: boolean) {
  if (skipProcessing) {
    // Returns immediately without waiting
    return { skipped: true }
  }
  
  // Fetch only when needed
  const userData = await fetchUserData(userId)
  return processUserData(userData)
}

Another example: early return optimization

// Incorrect: always fetches permissions
async function updateResource(resourceId: string, userId: string) {
  const permissions = await fetchPermissions(userId)
  const resource = await getResource(resourceId)
  
  if (!resource) {
    return { error: 'Not found' }
  }
  
  if (!permissions.canEdit) {
    return { error: 'Forbidden' }
  }
  
  return await updateResourceData(resource, permissions)
}

// Correct: fetches only when needed
async function updateResource(resourceId: string, userId: string) {
  const resource = await getResource(resourceId)
  
  if (!resource) {
    return { error: 'Not found' }
  }
  
  const permissions = await fetchPermissions(userId)
  
  if (!permissions.canEdit) {
    return { error: 'Forbidden' }
  }
  
  return await updateResourceData(resource, permissions)
}

This optimization is especially valuable when the skipped branch is frequently taken, or when the deferred operation is expensive.

1.2 Dependency-Based Parallelization

Impact: CRITICAL (2-10× improvement)

For operations with partial dependencies, use better-all to maximize parallelism. It automatically starts each task at the earliest possible moment.

Incorrect: profile waits for config unnecessarily

const [user, config] = await Promise.all([
  fetchUser(),
  fetchConfig()
])
const profile = await fetchProfile(user.id)

Correct: config and profile run in parallel

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)
  }
})

Reference: https://github.com/shuding/better-all

1.3 Prevent Waterfall Chains in API Routes

Impact: CRITICAL (2-10× improvement)

In API routes and Server Actions, start independent operations immediately, even if you don't await them yet.

Incorrect: config waits for auth, data waits for both

export async function GET(request: Request) {
  const session = await auth()
  const config = await fetchConfig()
  const data = await fetchData(session.user.id)
  return Response.json({ data, config })
}

Correct: auth and config start immediately

export async function GET(request: Request) {
  const sessionPromise = auth()
  const configPromise = fetchConfig()
  const session = await sessionPromise
  const [config, data] = await Promise.all([
    configPromise,
    fetchData(session.user.id)
  ])
  return Response.json({ data, config })
}

For operations with more complex dependency chains, use better-all to automatically maximize parallelism (see Dependency-Based Parallelization).

1.4 Promise.all() for Independent Operations

Impact: CRITICAL (2-10× improvement)

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

Incorrect: sequential execution, 3 round trips

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

Correct: parallel execution, 1 round trip

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

1.5 Strategic Suspense Boundaries

Impact: HIGH (faster initial paint)

Instead of awaiting data in async components before returning JSX, use Suspense boundaries to show the wrapper UI faster while data loads.

Incorrect: wrapper blocked by data fetching

async function Page() {
  const data = await fetchData() // Blocks entire page
  
  return (
    <div>
      <div>Sidebar</div>
      <div>Header</div>
      <div>
        <DataDisplay data={data} />
      </div>
      <div>Footer</div>
    </div>
  )
}

The entire layout waits for data even though only the middle section needs it.

Correct: wrapper shows immediately, data streams in

function Page() {
  return (
    <div>
      <div>Sidebar</div>
      <div>Header</div>
      <div>
        <Suspense fallback={<Skeleton />}>
          <DataDisplay />
        </Suspense>
      </div>
      <div>Footer</div>
    </div>
  )
}

async function DataDisplay() {
  const data = await fetchData() // Only blocks this component
  return <div>{data.content}</div>
}

Sidebar, Header, and Footer render immediately. Only DataDisplay waits for data.

Alternative: share promise across components

function Page() {
  // Start fetch immediately, but don't await
  const dataPromise = fetchData()
  
  return (
    <div>
      <div>Sidebar</div>
      <div>Header</div>
      <Suspense fallback={<Skeleton />}>
        <DataDisplay dataPromise={dataPromise} />
        <DataSummary dataPromise={dataPromise} />
      </Suspense>
      <div>Footer</div>
    </div>
  )
}

function DataDisplay({ dataPromise }: { dataPromise: Promise<Data> }) {
  const data = use(dataPromise) // Unwraps the promise
  return <div>{data.content}</div>
}

function DataSummary({ dataPromise }: { dataPromise: Promise<Data> }) {
  const data = use(dataPromise) // Reuses the same promise
  return <div>{data.summary}</div>
}

Both components share the same promise, so only one fetch occurs. Layout renders immediately while both components wait together.

When NOT to use this pattern:

• Critical data needed for layout decisions (affects positioning)

• SEO-critical content above the fold

• Small, fast queries where suspense overhead isn't worth it

• When you want to avoid layout shift (loading → content jump)

Trade-off: Faster initial paint vs potential layout shift. Choose based on your UX priorities.

---

2. Bundle Size Optimization

Impact: CRITICAL

Reducing initial bundle size improves Time to Interactive and Largest Contentful Paint.

2.1 Avoid Barrel File Imports

Impact: CRITICAL (200-800ms import cost, slow builds)

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

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

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+

// 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: https://vercel.com/blog/how-we-optimized-package-imports-in-next-js

2.2 Conditional Module Loading

Impact: HIGH (loads large data only when needed)

Load large data or modules only when a feature is activated.

Example: lazy-load animation frames

function AnimationPlayer({ enabled }: { enabled: boolean }) {
  const [frames, setFrames] = useState<Frame[] | null>(null)

  useEffect(() => {
    if (enabled && !frames && typeof window !== 'undefined') {
      import('./animation-frames.js')
        .then(mod => setFrames(mod.frames))
        .catch(() => setEnabled(false))
    }
  }, [enabled, frames])

  if (!frames) return <Skeleton />
  return <Canvas frames={frames} />
}

The typeof window !== 'undefined' check prevents bundling this module for SSR, optimizing server bundle size and build speed.

2.3 Defer Non-Critical Third-Party Libraries

Impact: MEDIUM (loads after hydration)

Analytics, logging, and error tracking don't block user interaction. Load them after hydration.

Incorrect: blocks initial bundle

import { Analytics } from '@vercel/analytics/react'

export default function RootLayout({ children }) {
  return (
    <html>
      <body>
        {children}
        <Analytics />
      </body>
    </html>
  )
}

Correct: loads after hydration

import dynamic from 'next/dynamic'

const Analytics = dynamic(
  () => import('@vercel/analytics/react').then(m => m.Analytics),
  { ssr: false }
)

export default function RootLayout({ children }) {
  return (
    <html>
      <body>
        {children}
        <Analytics />
      </body>
    </html>
  )
}

2.4 Dynamic Imports for Heavy Components

Impact: CRITICAL (directly affects TTI and LCP)

Use next/dynamic to lazy-load large components not needed on initial render.

Incorrect: Monaco bundles with main chunk ~300KB

import { MonacoEditor } from './monaco-editor'

function CodePanel({ code }: { code: string }) {
  return <MonacoEditor value={code} />
}

Correct: Monaco loads on demand

import dynamic from 'next/dynamic'

const MonacoEditor = dynamic(
  () => import('./monaco-editor').then(m => m.MonacoEditor),
  { ssr: false }
)

function CodePanel({ code }: { code: string }) {
  return <MonacoEditor value={code} />
}

2.5 Preload Based on User Intent

Impact: MEDIUM (reduces perceived latency)

Preload heavy bundles before they're needed to reduce perceived latency.

Example: preload on hover/focus

function EditorButton({ onClick }: { onClick: () => void }) {
  const preload = () => {
    if (typeof window !== 'undefined') {
      void import('./monaco-editor')
    }
  }

  return (
    <button
      onMouseEnter={preload}
      onFocus={preload}
      onClick={onClick}
    >
      Open Editor
    </button>
  )
}

Example: preload when feature flag is enabled

function FlagsProvider({ children, flags }: Props) {
  useEffect(() => {
    if (flags.editorEnabled && typeof window !== 'undefined') {
      void import('./monaco-editor').then(mod => mod.init())
    }
  }, [flags.editorEnabled])

  return <FlagsContext.Provider value={flags}>
    {children}
  </FlagsContext.Provider>
}

The typeof window !== 'undefined' check prevents bundling preloaded modules for SSR, optimizing server bundle size and build speed.

---

3. Server-Side Performance

Impact: HIGH

Optimizing server-side rendering and data fetching eliminates server-side waterfalls and reduces response times.

3.1 Cross-Request LRU Caching

Impact: HIGH (caches across requests)

React.cache() only works within one request. For data shared across sequential requests (user clicks button A then button B), use an LRU cache.

Implementation:

import { LRUCache } from 'lru-cache'

const cache = new LRUCache<string, any>({
  max: 1000,
  ttl: 5 * 60 * 1000  // 5 minutes
})

export async function getUser(id: string) {
  const cached = cache.get(id)
  if (cached) return cached

  const user = await db.user.findUnique({ where: { id } })
  cache.set(id, user)
  return user
}

// Request 1: DB query, result cached
// Request 2: cache hit, no DB query

Use when sequential user actions hit multiple endpoints needing the same data within seconds.

With Vercel's [Fluid Compute](https://vercel.com/docs/fluid-compute): LRU caching is especially effective because multiple concurrent requests can share the same function instance and cache. This means the cache persists across requests without needing external storage like Redis.

In traditional serverless: Each invocation runs in isolation, so consider Redis for cross-process caching.

Reference: https://github.com/isaacs/node-lru-cache

3.2 Minimize Serialization at RSC Boundaries

Impact: HIGH (reduces data transfer size)

The React Server/Client boundary serializes all object properties into strings and embeds them in the HTML response and subsequent RSC requests. This serialized data directly impacts page weight and load time, so size matters a lot. Only pass fields that the client actually uses.

Incorrect: serializes all 50 fields

async function Page() {
  const user = await fetchUser()  // 50 fields
  return <Profile user={user} />
}

'use client'
function Profile({ user }: { user: User }) {
  return <div>{user.name}</div>  // uses 1 field
}

Correct: serializes only 1 field

async function Page() {
  const user = await fetchUser()
  return <Profile name={user.name} />
}

'use client'
function Profile({ name }: { name: string }) {
  return <div>{name}</div>
}

3.3 Parallel Data Fetching with Component Composition

Impact: CRITICAL (eliminates server-side waterfalls)

React Server Components execute sequentially within a tree. Restructure with composition to parallelize data fetching.

Incorrect: Sidebar waits for Page's fetch to complete

export default async function Page() {
  const header = await fetchHeader()
  return (
    <div>
      <div>{header}</div>
      <Sidebar />
    </div>
  )
}

async function Sidebar() {
  const items = await fetchSidebarItems()
  return <nav>{items.map(renderItem)}</nav>
}

Correct: both fetch simultaneously

async function Header() {
  const data = await fetchHeader()
  return <div>{data}</div>
}

async function Sidebar() {
  const items = await fetchSidebarItems()
  return <nav>{items.map(renderItem)}</nav>
}

export default function Page() {
  return (
    <div>
      <Header />
      <Sidebar />
    </div>
  )
}

Alternative with children prop:

async function Layout({ children }: { children: ReactNode }) {
  const header = await fetchHeader()
  return (
    <div>
      <div>{header}</div>
      {children}
    </div>
  )
}

async function Sidebar() {
  const items = await fetchSidebarItems()
  return <nav>{items.map(renderItem)}</nav>
}

export default function Page() {
  return (
    <Layout>
      <Sidebar />
    </Layout>
  )
}

3.4 Per-Request Deduplication with React.cache()

Impact: MEDIUM (deduplicates within request)

Use React.cache() for server-side request deduplication. Authentication and database queries benefit most.

Usage:

import { cache } from 'react'

export const getCurrentUser = cache(async () => {
  const session = await auth()
  if (!session?.user?.id) return null
  return await db.user.findUnique({
    where: { id: session.user.id }
  })
})

Within a single request, multiple calls to getCurrentUser() execute the query only once.

3.5 Use after() for Non-Blocking Operations

Impact: MEDIUM (faster response times)

Use Next.js's after() to schedule work that should execute after a response is sent. This prevents logging, analytics, and other side effects from blocking the response.

Incorrect: blocks response

import { logUserAction } from '@/app/utils'

export async function POST(request: Request) {
  // Perform mutation
  await updateDatabase(request)
  
  // Logging blocks the response
  const userAgent = request.headers.get('user-agent') || 'unknown'
  await logUserAction({ userAgent })
  
  return new Response(JSON.stringify({ status: 'success' }), {
    status: 200,
    headers: { 'Content-Type': 'application/json' }
  })
}

Correct: non-blocking

import { after } from 'next/server'
import { headers, cookies } from 'next/headers'
import { logUserAction } from '@/app/utils'

export async function POST(request: Request) {
  // Perform mutation
  await updateDatabase(request)
  
  // Log after response is sent
  after(async () => {
    const userAgent = (await headers()).get('user-agent') || 'unknown'
    const sessionCookie = (await cookies()).get('session-id')?.value || 'anonymous'
    
    logUserAction({ sessionCookie, userAgent })
  })
  
  return new Response(JSON.stringify({ status: 'success' }), {
    status: 200,
    headers: { 'Content-Type': 'application/json' }
  })
}

The response is sent immediately while logging happens in the background.

Common use cases:

• Analytics tracking

• Audit logging

• Sending notifications

• Cache invalidation

• Cleanup tasks

Important notes:

• after() runs even if the response fails or redirects

• Works in Server Actions, Route Handlers, and Server Components

Reference: https://nextjs.org/docs/app/api-reference/functions/after

---

4. Client-Side Data Fetching

Impact: MEDIUM-HIGH

Automatic deduplication and efficient data fetching patterns reduce redundant network requests.

4.1 Deduplicate Global Event Listeners

Impact: LOW (single listener for N components)

Use useSWRSubscription() to share global event listeners across component instances.

Incorrect: N instances = N listeners

function useKeyboardShortcut(key: string, callback: () => void) {
  useEffect(() => {
    const handler = (e: KeyboardEvent) => {
      if (e.metaKey && e.key === key) {
        callback()
      }
    }
    window.addEventListener('keydown', handler)
    return () => window.removeEventListener('keydown', handler)
  }, [key, callback])
}

When using the useKeyboardShortcut hook multiple times, each instance will register a new listener.

Correct: N instances = 1 listener

import useSWRSubscription from 'swr/subscription'

// Module-level Map to track callbacks per key
const keyCallbacks = new Map<string, Set<() => void>>()

function useKeyboardShortcut(key: string, callback: () => void) {
  // Register this callback in the Map
  useEffect(() => {
    if (!keyCallbacks.has(key)) {
      keyCallbacks.set(key, new Set())
    }
    keyCallbacks.get(key)!.add(callback)

    return () => {
      const set = keyCallbacks.get(key)
      if (set) {
        set.delete(callback)
        if (set.size === 0) {
          keyCallbacks.delete(key)
        }
      }
    }
  }, [key, callback])

  useSWRSubscription('global-keydown', () => {
    const handler = (e: KeyboardEvent) => {
      if (e.metaKey && keyCallbacks.has(e.key)) {
        keyCallbacks.get(e.key)!.forEach(cb => cb())
      }
    }
    window.addEventListener('keydown', handler)
    return () => window.removeEventListener('keydown', handler)
  })
}

function Profile() {
  // Multiple shortcuts will share the same listener
  useKeyboardShortcut('p', () => { /* ... */ }) 
  useKeyboardShortcut('k', () => { /* ... */ })
  // ...
}

4.2 Use SWR for Automatic Deduplication

Impact: MEDIUM-HIGH (automatic deduplication)

SWR enables request deduplication, caching, and revalidation across component instances.

Incorrect: no deduplication, each instance fetches

function UserList() {
  const [users, setUsers] = useState([])
  useEffect(() => {
    fetch('/api/users')
      .then(r => r.json())
      .then(setUsers)
  }, [])
}

Correct: multiple instances share one request

import useSWR from 'swr'

function UserList() {
  const { data: users } = useSWR('/api/users', fetcher)
}

For immutable data:

import { useImmutableSWR } from '@/lib/swr'

function StaticContent() {
  const { data } = useImmutableSWR('/api/config', fetcher)
}

For mutations:

import { useSWRMutation } from 'swr/mutation'

function UpdateButton() {
  const { trigger } = useSWRMutation('/api/user', updateUser)
  return <button onClick={() => trigger()}>Update</button>
}

Reference: https://swr.vercel.app

---

5. Re-render Optimization

Impact: MEDIUM

Reducing unnecessary re-renders minimizes wasted computation and improves UI responsiveness.

5.1 Defer State Reads to Usage Point

Impact: MEDIUM (avoids unnecessary subscriptions)

Don't subscribe to dynamic state (searchParams, localStorage) if you only read it inside callbacks.

Incorrect: subscribes to all searchParams changes

function ShareButton({ chatId }: { chatId: string }) {
  const searchParams = useSearchParams()

  const handleShare = () => {
    const ref = searchParams.get('ref')
    shareChat(chatId, { ref })
  }

  return <button onClick={handleShare}>Share</button>
}

Correct: reads on demand, no subscription

function ShareButton({ chatId }: { chatId: string }) {
  const handleShare = () => {
    const params = new URLSearchParams(window.location.search)
    const ref = params.get('ref')
    shareChat(chatId, { ref })
  }

  return <button onClick={handleShare}>Share</button>
}

5.2 Extract to Memoized Components

Impact: MEDIUM (enables early returns)

Extract expensive work into memoized components to enable early returns before computation.

Incorrect: computes avatar even when loading

function Profile({ user, loading }: Props) {
  const avatar = useMemo(() => {
    const id = computeAvatarId(user)
    return <Avatar id={id} />
  }, [user])

  if (loading) return <Skeleton />
  return <div>{avatar}</div>
}

Correct: skips computation when loading

const UserAvatar = memo(function UserAvatar({ user }: { user: User }) {
  const id = useMemo(() => computeAvatarId(user), [user])
  return <Avatar id={id} />
})

function Profile({ user, loading }: Props) {
  if (loading) return <Skeleton />
  return (
    <div>
      <UserAvatar user={user} />
    </div>
  )
}

Note: If your project has React Compiler enabled, manual memoization with memo() and useMemo() is not necessary. The compiler automatically optimizes re-renders.

5.3 Narrow Effect Dependencies

Impact: LOW (minimizes effect re-runs)

Specify primitive dependencies instead of objects to minimize effect re-runs.

Incorrect: re-runs on any user field change

useEffect(() => {
  console.log(user.id)
}, [user])

Correct: re-runs only when id changes

useEffect(() => {
  console.log(user.id)
}, [user.id])

For derived state, compute outside effect:

// Incorrect: runs on width=767, 766, 765...
useEffect(() => {
  if (width < 768) {
    enableMobileMode()
  }
}, [width])

// Correct: runs only on boolean transition
const isMobile = width < 768
useEffect(() => {
  if (isMobile) {
    enableMobileMode()
  }
}, [isMobile])

5.4 Subscribe to Derived State

Impact: MEDIUM (reduces re-render frequency)

Subscribe to derived boolean state instead of continuous values to reduce re-render frequency.

Incorrect: re-renders on every pixel change

function Sidebar() {
  const width = useWindowWidth()  // updates continuously
  const isMobile = width < 768
  return <nav className={isMobile ? 'mobile' : 'desktop'}>
}

Correct: re-renders only when boolean changes

function Sidebar() {
  const isMobile = useMediaQuery('(max-width: 767px)')
  return <nav className={isMobile ? 'mobile' : 'desktop'}>
}

5.5 Use Functional setState Updates

Impact: MEDIUM (prevents stale closures and unnecessary callback recreations)

When updating state based on the current state value, use the functional update form of setState instead of directly referencing the state variable. This prevents stale closures, eliminates unnecessary dependencies, and creates stable callback references.

Incorrect: requires state as dependency

function TodoList() {
  const [items, setItems] = useState(initialItems)
  
  // Callback must depend on items, recreated on every items change
  const addItems = useCallback((newItems: Item[]) => {
    setItems([...items, ...newItems])
  }, [items])  // ❌ items dependency causes recreations
  
  // Risk of stale closure if dependency is forgotten
  const removeItem = useCallback((id: string) => {
    setItems(items.filter(item => item.id !== id))
  }, [])  // ❌ Missing items dependency - will use stale items!
  
  return <ItemsEditor items={items} onAdd={addItems} onRemove={removeItem} />
}

The first callback is recreated every time items changes, which can cause child components to re-render unnecessarily. The second callback has a stale closure bug—it will always reference the initial items value.

Correct: stable callbacks, no stale closures

function TodoList() {
  const [items, setItems] = useState(initialItems)
  
  // Stable callback, never recreated
  const addItems = useCallback((newItems: Item[]) => {
    setItems(curr => [...curr, ...newItems])
  }, [])  // ✅ No dependencies needed
  
  // Always uses latest state, no stale closure risk
  const removeItem = useCallback((id: string) => {
    setItems(curr => curr.filter(item => item.id !== id))
  }, [])  // ✅ Safe and stable
  
  return <ItemsEditor items={items} onAdd={addItems} onRemove={removeItem} />
}

Benefits:

1. Stable callback references - Callbacks don't need to be recreated when state changes

2. No stale closures - Always operates on the latest state value

3. Fewer dependencies - Simplifies dependency arrays and reduces memory leaks

4. Prevents bugs - Eliminates the most common source of React closure bugs

When to use functional updates:

• Any setState that depends on the current state value

• Inside useCallback/useMemo when state is needed

• Event handlers that reference state

• Async operations that update state

When direct updates are fine:

• Setting state to a static value: setCount(0)

• Setting state from props/arguments only: setName(newName)

• State doesn't depend on previous value

Note: If your project has React Compiler enabled, the compiler can automatically optimize some cases, but functional updates are still recommended for correctness and to prevent stale closure bugs.

5.6 Use Lazy State Initialization

Impact: MEDIUM (wasted computation on every render)

Pass a function to useState for expensive initial values. Without the function form, the initializer runs on every render even though the value is only used once.

Incorrect: runs on every render

function FilteredList({ items }: { items: Item[] }) {
  // buildSearchIndex() runs on EVERY render, even after initialization
  const [searchIndex, setSearchIndex] = useState(buildSearchIndex(items))
  const [query, setQuery] = useState('')
  
  // When query changes, buildSearchIndex runs again unnecessarily
  return <SearchResults index={searchIndex} query={query} />
}

function UserProfile() {
  // JSON.parse runs on every render
  const [settings, setSettings] = useState(
    JSON.parse(localStorage.getItem('settings') || '{}')
  )
  
  return <SettingsForm settings={settings} onChange={setSettings} />
}

Correct: runs only once

function FilteredList({ items }: { items: Item[] }) {
  // buildSearchIndex() runs ONLY on initial render
  const [searchIndex, setSearchIndex] = useState(() => buildSearchIndex(items))
  const [query, setQuery] = useState('')
  
  return <SearchResults index={searchIndex} query={query} />
}

function UserProfile() {
  // JSON.parse runs only on initial render
  const [settings, setSettings] = useState(() => {
    const stored = localStorage.getItem('settings')
    return stored ? JSON.parse(stored) : {}
  })
  
  return <SettingsForm settings={settings} onChange={setSettings} />
}

Use lazy initialization when computing initial values from localStorage/sessionStorage, building data structures (indexes, maps), reading from the DOM, or performing heavy transformations.

For simple primitives (useState(0)), direct references (useState(props.value)), or cheap literals (useState({})), the function form is unnecessary.

5.7 Use Transitions for Non-Urgent Updates

Impact: MEDIUM (maintains UI responsiveness)

Mark frequent, non-urgent state updates as transitions to maintain UI responsiveness.

Incorrect: blocks UI on every scroll

function ScrollTracker() {
  const [scrollY, setScrollY] = useState(0)
  useEffect(() => {
    const handler = () => setScrollY(window.scrollY)
    window.addEventListener('scroll', handler, { passive: true })
    return () => window.removeEventListener('scroll', handler)
  }, [])
}

Correct: non-blocking updates

import { startTransition } from 'react'

function ScrollTracker() {
  const [scrollY, setScrollY] = useState(0)
  useEffect(() => {
    const handler = () => {
      startTransition(() => setScrollY(window.scrollY))
    }
    window.addEventListener('scroll', handler, { passive: true })
    return () => window.removeEventListener('scroll', handler)
  }, [])
}

---

6. Rendering Performance

Impact: MEDIUM

Optimizing the rendering process reduces the work the browser needs to do.

6.1 Animate SVG Wrapper Instead of SVG Element

Impact: LOW (enables hardware acceleration)

Many browsers don't have hardware acceleration for CSS3 animations on SVG elements. Wrap SVG in a <div> and animate the wrapper instead.

Incorrect: animating SVG directly - no hardware acceleration

function LoadingSpinner() {
  return (
    <svg 
      className="animate-spin"
      width="24" 
      height="24" 
      viewBox="0 0 24 24"
    >
      <circle cx="12" cy="12" r="10" stroke="currentColor" />
    </svg>
  )
}

Correct: animating wrapper div - hardware accelerated

function LoadingSpinner() {
  return (
    <div className="animate-spin">
      <svg 
        width="24" 
        height="24" 
        viewBox="0 0 24 24"
      >
        <circle cx="12" cy="12" r="10" stroke="currentColor" />
      </svg>
    </div>
  )
}

This applies to all CSS transforms and transitions (transform, opacity, translate, scale, rotate). The wrapper div allows browsers to use GPU acceleration for smoother animations.

6.2 CSS content-visibility for Long Lists

Impact: HIGH (faster initial render)

Apply content-visibility: auto to defer off-screen rendering.

CSS:

.message-item {
  content-visibility: auto;
  contain-intrinsic-size: 0 80px;
}

Example:

function MessageList({ messages }: { messages: Message[] }) {
  return (
    <div className="overflow-y-auto h-screen">
      {messages.map(msg => (
        <div key={msg.id} className="message-item">
          <Avatar user={msg.author} />
          <div>{msg.content}</div>
        </div>
      ))}
    </div>
  )
}

For 1000 messages, browser skips layout/paint for ~990 off-screen items (10× faster initial render).

6.3 Hoist Static JSX Elements

Impact: LOW (avoids re-creation)

Extract static JSX outside components to avoid re-creation.

Incorrect: recreates element every render

function LoadingSkeleton() {
  return <div className="animate-pulse h-20 bg-gray-200" />
}

function Container() {
  return (
    <div>
      {loading && <LoadingSkeleton />}
    </div>
  )
}

Correct: reuses same element

const loadingSkeleton = (
  <div className="animate-pulse h-20 bg-gray-200" />
)

function Container() {
  return (
    <div>
      {loading && loadingSkeleton}
    </div>
  )
}

This is especially helpful for large and static SVG nodes, which can be expensive to recreate on every render.

Note: If your project has React Compiler enabled, the compiler automatically hoists static JSX elements and optimizes component re-renders, making manual hoisting unnecessary.

6.4 Optimize SVG Precision

Impact: LOW (reduces file size)

Reduce SVG coordinate precision to decrease file size. The optimal precision depends on the viewBox size, but in general reducing precision should be considered.

Incorrect: excessive precision

<path d="M 10.293847 20.847362 L 30.938472 40.192837" />

Correct: 1 decimal place

<path d="M 10.3 20.8 L 30.9 40.2" />

Automate with SVGO:

npx svgo --precision=1 --multipass icon.svg

6.5 Prevent Hydration Mismatch Without Flickering

Impact: MEDIUM (avoids visual flicker and hydration errors)

When rendering content that depends on client-side storage (localStorage, cookies), avoid both SSR breakage and post-hydration flickering by injecting a synchronous script that updates the DOM before React hydrates.

Incorrect: breaks SSR

function ThemeWrapper({ children }: { children: ReactNode }) {
  // localStorage is not available on server - throws error
  const theme = localStorage.getItem('theme') || 'light'
  
  return (
    <div className={theme}>
      {children}
    </div>
  )
}

Server-side rendering will fail because localStorage is undefined.

Incorrect: visual flickering

function ThemeWrapper({ children }: { children: ReactNode }) {
  const [theme, setTheme] = useState('light')
  
  useEffect(() => {
    // Runs after hydration - causes visible flash
    const stored = localStorage.getItem('theme')
    if (stored) {
      setTheme(stored)
    }
  }, [])
  
  return (
    <div className={theme}>
      {children}
    </div>
  )
}

Component first renders with default value (light), then updates after hydration, causing a visible flash of incorrect content.

Correct: no flicker, no hydration mismatch

function ThemeWrapper({ children }: { children: ReactNode }) {
  return (
    <>
      <div id="theme-wrapper">
        {children}
      </div>
      <script
        dangerouslySetInnerHTML={{
          __html: `
            (function() {
              try {
                var theme = localStorage.getItem('theme') || 'light';
                var el = document.getElementById('theme-wrapper');
                if (el) el.className = theme;
              } catch (e) {}
            })();
          `,
        }}
      />
    </>
  )
}

The inline script executes synchronously before showing the element, ensuring the DOM already has the correct value. No flickering, no hydration mismatch.

This pattern is especially useful for theme toggles, user preferences, authentication states, and any client-only data that should render immediately without flashing default values.

6.6 Use Activity Component for Show/Hide

Impact: MEDIUM (preserves state/DOM)

Use React's <Activity> to preserve state/DOM for expensive components that frequently toggle visibility.

Usage:

import { Activity } from 'react'

function Dropdown({ isOpen }: Props) {
  return (
    <Activity mode={isOpen ? 'visible' : 'hidden'}>
      <ExpensiveMenu />
    </Activity>
  )
}

Avoids expensive re-renders and state loss.

6.7 Use Explicit Conditional Rendering

Impact: LOW (prevents rendering 0 or NaN)

Use explicit ternary operators (? :) instead of && for conditional rendering when the condition can be 0, NaN, or other falsy values that render.

Incorrect: renders "0" when count is 0

function Badge({ count }: { count: number }) {
  return (
    <div>
      {count && <span className="badge">{count}</span>}
    </div>
  )
}

// When count = 0, renders: <div>0</div>
// When count = 5, renders: <div><span class="badge">5</span></div>

Correct: renders nothing when count is 0

function Badge({ count }: { count: number }) {
  return (
    <div>
      {count > 0 ? <span className="badge">{count}</span> : null}
    </div>
  )
}

// When count = 0, renders: <div></div>
// When count = 5, renders: <div><span class="badge">5</span></div>

---

7. JavaScript Performance

Impact: LOW-MEDIUM

Micro-optimizations for hot paths can add up to meaningful improvements.

7.1 Batch DOM CSS Changes

Impact: MEDIUM (reduces reflows/repaints)

Avoid changing styles one property at a time. Group multiple CSS changes together via classes or cssText to minimize browser reflows.

Incorrect: multiple reflows

function updateElementStyles(element: HTMLElement) {
  // Each line triggers a reflow
  element.style.width = '100px'
  element.style.height = '200px'
  element.style.backgroundColor = 'blue'
  element.style.border = '1px solid black'
}

Correct: add class - single reflow

// CSS file
.highlighted-box {
  width: 100px;
  height: 200px;
  background-color: blue;
  border: 1px solid black;
}

// JavaScript
function updateElementStyles(element: HTMLElement) {
  element.classList.add('highlighted-box')
}

Correct: change cssText - single reflow

function updateElementStyles(element: HTMLElement) {
  element.style.cssText = `
    width: 100px;
    height: 200px;
    background-color: blue;
    border: 1px solid black;
  `
}

React example:

// Incorrect: changing styles one by one
function Box({ isHighlighted }: { isHighlighted: boolean }) {
  const ref = useRef<HTMLDivElement>(null)
  
  useEffect(() => {
    if (ref.current && isHighlighted) {
      ref.current.style.width = '100px'
      ref.current.style.height = '200px'
      ref.current.style.backgroundColor = 'blue'
    }
  }, [isHighlighted])
  
  return <div ref={ref}>Content</div>
}

// Correct: toggle class
function Box({ isHighlighted }: { isHighlighted: boolean }) {
  return (
    <div className={isHighlighted ? 'highlighted-box' : ''}>
      Content
    </div>
  )
}

Prefer CSS classes over inline styles when possible. Classes are cached by the browser and provide better separation of concerns.

7.2 Build Index Maps for Repeated Lookups

Impact: LOW-MEDIUM (1M ops to 2K ops)

Multiple .find() calls by the same key should use a Map.

Incorrect (O(n) per lookup):

function processOrders(orders: Order[], users: User[]) {
  return orders.map(order => ({
    ...order,
    user: users.find(u => u.id === order.userId)
  }))
}

Correct (O(1) per lookup):

function processOrders(orders: Order[], users: User[]) {
  const userById = new Map(users.map(u => [u.id, u]))

  return orders.map(order => ({
    ...order,
    user: userById.get(order.userId)
  }))
}

Build map once (O(n)), then all lookups are O(1).

For 1000 orders × 1000 users: 1M ops → 2K ops.

7.3 Cache Property Access in Loops

Impact: LOW-MEDIUM (reduces lookups)

Cache object property lookups in hot paths.

Incorrect: 3 lookups × N iterations

for (let i = 0; i < arr.length; i++) {
  process(obj.config.settings.value)
}

Correct: 1 lookup total

const value = obj.config.settings.value
const len = arr.length
for (let i = 0; i < len; i++) {
  process(value)
}

7.4 Cache Repeated Function Calls

Impact: MEDIUM (avoid redundant computation)

Use a module-level Map to cache function results when the same function is called repeatedly with the same inputs during render.

Incorrect: redundant computation

function ProjectList({ projects }: { projects: Project[] }) {
  return (
    <div>
      {projects.map(project => {
        // slugify() called 100+ times for same project names
        const slug = slugify(project.name)
        
        return <ProjectCard key={project.id} slug={slug} />
      })}
    </div>
  )
}

Correct: cached results

// Module-level cache
const slugifyCache = new Map<string, string>()

function cachedSlugify(text: string): string {
  if (slugifyCache.has(text)) {
    return slugifyCache.get(text)!
  }
  const result = slugify(text)
  slugifyCache.set(text, result)
  return result
}

function ProjectList({ projects }: { projects: Project[] }) {
  return (
    <div>
      {projects.map(project => {
        // Computed only once per unique project name
        const slug = cachedSlugify(project.name)
        
        return <ProjectCard key={project.id} slug={slug} />
      })}
    </div>
  )
}

Simpler pattern for single-value functions:

let isLoggedInCache: boolean | null = null

function isLoggedIn(): boolean {
  if (isLoggedInCache !== null) {
    return isLoggedInCache
  }
  
  isLoggedInCache = document.cookie.includes('auth=')
  return isLoggedInCache
}

// Clear cache when auth changes
function onAuthChange() {
  isLoggedInCache = null
}

Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components.

Reference: https://vercel.com/blog/how-we-made-the-vercel-dashboard-twice-as-fast

7.5 Cache Storage API Calls

Impact: LOW-MEDIUM (reduces expensive I/O)

localStorage, sessionStorage, and document.cookie are synchronous and expensive. Cache reads in memory.

Incorrect: reads storage on every call

function getTheme() {
  return localStorage.getItem('theme') ?? 'light'
}
// Called 10 times = 10 storage reads

Correct: Map cache

const storageCache = new Map<string, string | null>()

function getLocalStorage(key: string) {
  if (!storageCache.has(key)) {
    storageCache.set(key, localStorage.getItem(key))
  }
  return storageCache.get(key)
}

function setLocalStorage(key: string, value: string) {
  localStorage.setItem(key, value)
  storageCache.set(key, value)  // keep cache in sync
}

Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components.

Cookie caching:

let cookieCache: Record<string, string> | null = null

function getCookie(name: string) {
  if (!cookieCache) {
    cookieCache = Object.fromEntries(
      document.cookie.split('; ').map(c => c.split('='))
    )
  }
  return cookieCache[name]
}

Important: invalidate on external changes

window.addEventListener('storage', (e) => {
  if (e.key) storageCache.delete(e.key)
})

document.addEventListener('visibilitychange', () => {
  if (document.visibilityState === 'visible') {
    storageCache.clear()
  }
})

If storage can change externally (another tab, server-set cookies), invalidate cache:

7.6 Combine Multiple Array Iterations

Impact: LOW-MEDIUM (reduces iterations)

Multiple .filter() or .map() calls iterate the array multiple times. Combine into one loop.

Incorrect: 3 iterations

const admins = users.filter(u => u.isAdmin)
const testers = users.filter(u => u.isTester)
const inactive = users.filter(u => !u.isActive)

Correct: 1 iteration

const admins: User[] = []
const testers: User[] = []
const inactive: User[] = []

for (const user of users) {
  if (user.isAdmin) admins.push(user)
  if (user.isTester) testers.push(user)
  if (!user.isActive) inactive.push(user)
}

7.7 Early Length Check for Array Comparisons

Impact: MEDIUM-HIGH (avoids expensive operations when lengths differ)

When comparing arrays with expensive operations (sorting, deep equality, serialization), check lengths first. If lengths differ, the arrays cannot be equal.

In real-world applications, this optimization is especially valuable when the comparison runs in hot paths (event handlers, render loops).

Incorrect: always runs expensive comparison

function hasChanges(current: string[], original: string[]) {
  // Always sorts and joins, even when lengths differ
  return current.sort().join() !== original.sort().join()
}

Two O(n log n) sorts run even when current.length is 5 and original.length is 100. There is also overhead of joining the arrays and comparing the strings.

Correct (O(1) length check first):

function hasChanges(current: string[], original: string[]) {
  // Early return if lengths differ
  if (current.length !== original.length) {
    return true
  }
  // Only sort/join when lengths match
  const currentSorted = current.toSorted()
  const originalSorted = original.toSorted()
  for (let i = 0; i < currentSorted.length; i++) {
    if (currentSorted[i] !== originalSorted[i]) {
      return true
    }
  }
  return false
}

This new approach is more efficient because:

• It avoids the overhead of sorting and joining the arrays when lengths differ

• It avoids consuming memory for the joined strings (especially important for large arrays)

• It avoids mutating the original arrays

• It returns early when a difference is found

7.8 Early Return from Functions

Impact: LOW-MEDIUM (avoids unnecessary computation)

Return early when result is determined to skip unnecessary processing.

Incorrect: processes all items even after finding answer

function validateUsers(users: User[]) {
  let hasError = false
  let errorMessage = ''
  
  for (const user of users) {
    if (!user.email) {
      hasError = true
      errorMessage = 'Email required'
    }
    if (!user.name) {
      hasError = true
      errorMessage = 'Name required'
    }
    // Continues checking all users even after error found
  }
  
  return hasError ? { valid: false, error: errorMessage } : { valid: true }
}

Correct: returns immediately on first error

function validateUsers(users: User[]) {
  for (const user of users) {
    if (!user.email) {
      return { valid: false, error: 'Email required' }
    }
    if (!user.name) {
      return { valid: false, error: 'Name required' }
    }
  }

  return { valid: true }
}

7.9 Hoist RegExp Creation

Impact: LOW-MEDIUM (avoids recreation)

Don't create RegExp inside render. Hoist to module scope or memoize with useMemo().

Incorrect: new RegExp every render

function Highlighter({ text, query }: Props) {
  const regex = new RegExp(`(${query})`, 'gi')
  const parts = text.split(regex)
  return <>{parts.map((part, i) => ...)}</>
}

Correct: memoize or hoist

const EMAIL_REGEX = /^[^\s@]+@[^\s@]+\.[^\s@]+$/

function Highlighter({ text, query }: Props) {
  const regex = useMemo(
    () => new RegExp(`(${escapeRegex(query)})`, 'gi'),
    [query]
  )
  const parts = text.split(regex)
  return <>{parts.map((part, i) => ...)}</>
}

Warning: global regex has mutable state

const regex = /foo/g
regex.test('foo')  // true, lastIndex = 3
regex.test('foo')  // false, lastIndex = 0

Global regex (/g) has mutable lastIndex state:

7.10 Use Loop for Min/Max Instead of Sort

Impact: LOW (O(n) instead of O(n log n))

Finding the smallest or largest element only requires a single pass through the array. Sorting is wasteful and slower.

Incorrect (O(n log n) - sort to find latest):

interface Project {
  id: string
  name: string
  updatedAt: number
}

function getLatestProject(projects: Project[]) {
  const sorted = [...projects].sort((a, b) => b.updatedAt - a.updatedAt)
  return sorted[0]
}

Sorts the entire array just to find the maximum value.

Incorrect (O(n log n) - sort for oldest and newest):

function getOldestAndNewest(projects: Project[]) {
  const sorted = [...projects].sort((a, b) => a.updatedAt - b.updatedAt)
  return { oldest: sorted[0], newest: sorted[sorted.length - 1] }
}

Still sorts unnecessarily when only min/max are needed.

Correct (O(n) - single loop):

function getLatestProject(projects: Project[]) {
  if (projects.length === 0) return null
  
  let latest = projects[0]
  
  for (let i = 1; i < projects.length; i++) {
    if (projects[i].updatedAt > latest.updatedAt) {
      latest = projects[i]
    }
  }
  
  return latest
}

function getOldestAndNewest(projects: Project[]) {
  if (projects.length === 0) return { oldest: null, newest: null }
  
  let oldest = projects[0]
  let newest = projects[0]
  
  for (let i = 1; i < projects.length; i++) {
    if (projects[i].updatedAt < oldest.updatedAt) oldest = projects[i]
    if (projects[i].updatedAt > newest.updatedAt) newest = projects[i]
  }
  
  return { oldest, newest }
}

Single pass through the array, no copying, no sorting.

Alternative: Math.min/Math.max for small arrays

const numbers = [5, 2, 8, 1, 9]
const min = Math.min(...numbers)
const max = Math.max(...numbers)

This works for small arrays but can be slower for very large arrays due to spread operator limitations. Use the loop approach for reliability.

7.11 Use Set/Map for O(1) Lookups

Impact: LOW-MEDIUM (O(n) to O(1))

Convert arrays to Set/Map for repeated membership checks.

Incorrect (O(n) per check):

const allowedIds = ['a', 'b', 'c', ...]
items.filter(item => allowedIds.includes(item.id))

Correct (O(1) per check):

const allowedIds = new Set(['a', 'b', 'c', ...])
items.filter(item => allowedIds.has(item.id))

7.12 Use toSorted() Instead of sort() for Immutability

Impact: MEDIUM-HIGH (prevents mutation bugs in React state)

.sort() mutates the array in place, which can cause bugs with React state and props. Use .toSorted() to create a new sorted array without mutation.

Incorrect: mutates original array

function UserList({ users }: { users: User[] }) {
  // Mutates the users prop array!
  const sorted = useMemo(
    () => users.sort((a, b) => a.name.localeCompare(b.name)),
    [users]
  )
  return <div>{sorted.map(renderUser)}</div>
}

Correct: creates new array

function UserList({ users }: { users: User[] }) {
  // Creates new sorted array, original unchanged
  const sorted = useMemo(
    () => users.toSorted((a, b) => a.name.localeCompare(b.name)),
    [users]
  )
  return <div>{sorted.map(renderUser)}</div>
}

Why this matters in React:

1. Props/state mutations break React's immutability model - React expects props and state to be treated as read-only

2. Causes stale closure bugs - Mutating arrays inside closures (callbacks, effects) can lead to unexpected behavior

Browser support: fallback for older browsers

// Fallback for older browsers
const sorted = [...items].sort((a, b) => a.value - b.value)

.toSorted() is available in all modern browsers (Chrome 110+, Safari 16+, Firefox 115+, Node.js 20+). For older environments, use spread operator:

Other immutable array methods:

• .toSorted() - immutable sort

• .toReversed() - immutable reverse

• .toSpliced() - immutable splice

• .with() - immutable element replacement

---

8. Advanced Patterns

Impact: LOW

Advanced patterns for specific cases that require careful implementation.

8.1 Store Event Handlers in Refs

Impact: LOW (stable subscriptions)

Store callbacks in refs when used in effects that shouldn't re-subscribe on callback changes.

Incorrect: re-subscribes on every render

function useWindowEvent(event: string, handler: () => void) {
  useEffect(() => {
    window.addEventListener(event, handler)
    return () => window.removeEventListener(event, handler)
  }, [event, handler])
}

Correct: stable subscription

import { useEffectEvent } from 'react'

function useWindowEvent(event: string, handler: () => void) {
  const onEvent = useEffectEvent(handler)

  useEffect(() => {
    window.addEventListener(event, onEvent)
    return () => window.removeEventListener(event, onEvent)
  }, [event])
}

Alternative: use `useEffectEvent` if you're on latest React:

useEffectEvent provides a cleaner API for the same pattern: it creates a stable function reference that always calls the latest version of the handler.

8.2 useLatest for Stable Callback Refs

Impact: LOW (prevents effect re-runs)

Access latest values in callbacks without adding them to dependency arrays. Prevents effect re-runs while avoiding stale closures.

Implementation:

function useLatest<T>(value: T) {
  const ref = useRef(value)
  useEffect(() => {
    ref.current = value
  }, [value])
  return ref
}

Incorrect: effect re-runs on every callback change

function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {
  const [query, setQuery] = useState('')

  useEffect(() => {
    const timeout = setTimeout(() => onSearch(query), 300)
    return () => clearTimeout(timeout)
  }, [query, onSearch])
}

Correct: stable effect, fresh callback

function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {
  const [query, setQuery] = useState('')
  const onSearchRef = useLatest(onSearch)

  useEffect(() => {
    const timeout = setTimeout(() => onSearchRef.current(query), 300)
    return () => clearTimeout(timeout)
  }, [query])
}

---

References

1. https://react.dev 2. https://nextjs.org 3. https://swr.vercel.app 4. https://github.com/shuding/better-all 5. https://github.com/isaacs/node-lru-cache 6. https://vercel.com/blog/how-we-optimized-package-imports-in-next-js 7. https://vercel.com/blog/how-we-made-the-vercel-dashboard-twice-as-fast

React Best Practices

A structured repository for creating and maintaining React Best Practices optimized for agents and LLMs.

Structure

• rules/ - Individual rule files (one per rule)
• _sections.md - Section metadata (titles, impacts, descriptions)
• _template.md - Template for creating new rules
• area-description.md - Individual rule files
• src/ - Build scripts and utilities
• metadata.json - Document metadata (version, organization, abstract)
• __AGENTS.md__ - Compiled output (generated)
• __test-cases.json__ - Test cases for LLM evaluation (generated)

Getting Started

1. Install dependencies:

   pnpm install

2. Build AGENTS.md from rules:

   pnpm build

3. Validate rule files:

   pnpm validate

4. Extract test cases:

   pnpm extract-tests

Creating a New Rule

1. Copy rules/_template.md to rules/area-description.md 2. Choose the appropriate area prefix:

• async- for Eliminating Waterfalls (Section 1)
• bundle- for Bundle Size Optimization (Section 2)
• server- for Server-Side Performance (Section 3)
• client- for Client-Side Data Fetching (Section 4)
• rerender- for Re-render Optimization (Section 5)
• rendering- for Rendering Performance (Section 6)
• js- for JavaScript Performance (Section 7)
• advanced- for Advanced Patterns (Section 8)

3. Fill in the frontmatter and content 4. Ensure you have clear examples with explanations 5. Run pnpm build to regenerate AGENTS.md and test-cases.json

Rule File Structure

Each rule file should follow this structure:

---
title: Rule Title Here
impact: MEDIUM
impactDescription: Optional description
tags: tag1, tag2, tag3
---

## Rule Title Here

Brief explanation of the rule and why it matters.

**Incorrect (description of what's wrong):**

// Bad code example

**Correct (description of what's right):**

// Good code example

Optional explanatory text after examples.

Reference: [Link](https://example.com)

## File Naming Convention

- Files starting with `_` are special (excluded from build)
- Rule files: `area-description.md` (e.g., `async-parallel.md`)
- Section is automatically inferred from filename prefix
- Rules are sorted alphabetically by title within each section
- IDs (e.g., 1.1, 1.2) are auto-generated during build

## Impact Levels

- `CRITICAL` - Highest priority, major performance gains
- `HIGH` - Significant performance improvements
- `MEDIUM-HIGH` - Moderate-high gains
- `MEDIUM` - Moderate performance improvements
- `LOW-MEDIUM` - Low-medium gains
- `LOW` - Incremental improvements

## Scripts

- `pnpm build` - Compile rules into AGENTS.md
- `pnpm validate` - Validate all rule files
- `pnpm extract-tests` - Extract test cases for LLM evaluation
- `pnpm dev` - Build and validate

## Contributing

When adding or modifying rules:

1. Use the correct filename prefix for your section
2. Follow the `_template.md` structure
3. Include clear bad/good examples with explanations
4. Add appropriate tags
5. Run `pnpm build` to regenerate AGENTS.md and test-cases.json
6. Rules are automatically sorted by title - no need to manage numbers!

## Acknowledgments

Originally created by [@shuding](https://x.com/shuding) at [Vercel](https://vercel.com).

Sections

This file defines all sections, their ordering, impact levels, and descriptions. The section ID (in parentheses) is the filename prefix used to group rules.

---

1. Eliminating Waterfalls (async)

Impact: CRITICAL Description: Waterfalls are the #1 performance killer. Each sequential await adds full network latency. Eliminating them yields the largest gains.

2. Bundle Size Optimization (bundle)

Impact: CRITICAL Description: Reducing initial bundle size improves Time to Interactive and Largest Contentful Paint.

3. Server-Side Performance (server)

Impact: HIGH Description: Optimizing server-side rendering and data fetching eliminates server-side waterfalls and reduces response times.

4. Client-Side Data Fetching (client)

Impact: MEDIUM-HIGH Description: Automatic deduplication and efficient data fetching patterns reduce redundant network requests.

5. Re-render Optimization (rerender)

Impact: MEDIUM Description: Reducing unnecessary re-renders minimizes wasted computation and improves UI responsiveness.

6. Rendering Performance (rendering)

Impact: MEDIUM Description: Optimizing the rendering process reduces the work the browser needs to do.

7. JavaScript Performance (js)

Impact: LOW-MEDIUM Description: Micro-optimizations for hot paths can add up to meaningful improvements.

8. Advanced Patterns (advanced)

Impact: LOW Description: Advanced patterns for specific cases that require careful implementation.

Rule Title Here

Impact: MEDIUM (optional impact description)

Brief explanation of the rule and why it matters. This should be clear and concise, explaining the performance implications.

Incorrect (description of what's wrong):

// Bad code example here
const bad = example()

Correct (description of what's right):

// Good code example here
const good = example()

Reference: Link to documentation or resource

Store Event Handlers in Refs

Store callbacks in refs when used in effects that shouldn't re-subscribe on callback changes.

Incorrect (re-subscribes on every render):

function useWindowEvent(event: string, handler: () => void) {
  useEffect(() => {
    window.addEventListener(event, handler)
    return () => window.removeEventListener(event, handler)
  }, [event, handler])
}

Correct (stable subscription):

function useWindowEvent(event: string, handler: () => void) {
  const handlerRef = useRef(handler)
  useEffect(() => {
    handlerRef.current = handler
  }, [handler])

  useEffect(() => {
    const listener = () => handlerRef.current()
    window.addEventListener(event, listener)
    return () => window.removeEventListener(event, listener)
  }, [event])
}

Alternative: use `useEffectEvent` if you're on latest React:

import { useEffectEvent } from 'react'

function useWindowEvent(event: string, handler: () => void) {
  const onEvent = useEffectEvent(handler)

  useEffect(() => {
    window.addEventListener(event, onEvent)
    return () => window.removeEventListener(event, onEvent)
  }, [event])
}

useEffectEvent provides a cleaner API for the same pattern: it creates a stable function reference that always calls the latest version of the handler.

useLatest for Stable Callback Refs

Access latest values in callbacks without adding them to dependency arrays. Prevents effect re-runs while avoiding stale closures.

Implementation:

function useLatest<T>(value: T) {
  const ref = useRef(value)
  useEffect(() => {
    ref.current = value
  }, [value])
  return ref
}

Incorrect (effect re-runs on every callback change):

function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {
  const [query, setQuery] = useState('')

  useEffect(() => {
    const timeout = setTimeout(() => onSearch(query), 300)
    return () => clearTimeout(timeout)
  }, [query, onSearch])
}

Correct (stable effect, fresh callback):

function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {
  const [query, setQuery] = useState('')
  const onSearchRef = useLatest(onSearch)

  useEffect(() => {
    const timeout = setTimeout(() => onSearchRef.current(query), 300)
    return () => clearTimeout(timeout)
  }, [query])
}

Prevent Waterfall Chains in API Routes

In API routes and Server Actions, start independent operations immediately, even if you don't await them yet.

Incorrect (config waits for auth, data waits for both):

export async function GET(request: Request) {
  const session = await auth()
  const config = await fetchConfig()
  const data = await fetchData(session.user.id)
  return Response.json({ data, config })
}

Correct (auth and config start immediately):

export async function GET(request: Request) {
  const sessionPromise = auth()
  const configPromise = fetchConfig()
  const session = await sessionPromise
  const [config, data] = await Promise.all([
    configPromise,
    fetchData(session.user.id)
  ])
  return Response.json({ data, config })
}

For operations with more complex dependency chains, use better-all to automatically maximize parallelism (see Dependency-Based Parallelization).

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):

async function handleRequest(userId: string, skipProcessing: boolean) {
  const userData = await fetchUserData(userId)
  
  if (skipProcessing) {
    // Returns immediately but still waited for userData
    return { skipped: true }
  }
  
  // Only this branch uses userData
  return processUserData(userData)
}

Correct (only blocks when needed):

async function handleRequest(userId: string, skipProcessing: boolean) {
  if (skipProcessing) {
    // Returns immediately without waiting
    return { skipped: true }
  }
  
  // Fetch only when needed
  const userData = await fetchUserData(userId)
  return processUserData(userData)
}

Another example (early return optimization):

// Incorrect: always fetches permissions
async function updateResource(resourceId: string, userId: string) {
  const permissions = await fetchPermissions(userId)
  const resource = await getResource(resourceId)
  
  if (!resource) {
    return { error: 'Not found' }
  }
  
  if (!permissions.canEdit) {
    return { error: 'Forbidden' }
  }
  
  return await updateResourceData(resource, permissions)
}

// Correct: fetches only when needed
async function updateResource(resourceId: string, userId: string) {
  const resource = await getResource(resourceId)
  
  if (!resource) {
    return { error: 'Not found' }
  }
  
  const permissions = await fetchPermissions(userId)
  
  if (!permissions.canEdit) {
    return { error: 'Forbidden' }
  }
  
  return await updateResourceData(resource, permissions)
}

This optimization is especially valuable when the skipped branch is frequently taken, or when the deferred operation is expensive.

Dependency-Based Parallelization

For operations with partial dependencies, use better-all to maximize parallelism. It automatically starts each task at the earliest possible moment.

Incorrect (profile waits for config unnecessarily):

const [user, config] = await Promise.all([
  fetchUser(),
  fetchConfig()
])
const profile = await fetchProfile(user.id)

Correct (config and profile run in parallel):

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)
  }
})

Reference: https://github.com/shuding/better-all

Promise.all() for Independent Operations

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

Incorrect (sequential execution, 3 round trips):

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

Correct (parallel execution, 1 round trip):

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

Strategic Suspense Boundaries

Instead of awaiting data in async components before returning JSX, use Suspense boundaries to show the wrapper UI faster while data loads.

Incorrect (wrapper blocked by data fetching):

async function Page() {
  const data = await fetchData() // Blocks entire page
  
  return (
    <div>
      <div>Sidebar</div>
      <div>Header</div>
      <div>
        <DataDisplay data={data} />
      </div>
      <div>Footer</div>
    </div>
  )
}

The entire layout waits for data even though only the middle section needs it.

Correct (wrapper shows immediately, data streams in):

function Page() {
  return (
    <div>
      <div>Sidebar</div>
      <div>Header</div>
      <div>
        <Suspense fallback={<Skeleton />}>
          <DataDisplay />
        </Suspense>
      </div>
      <div>Footer</div>
    </div>
  )
}

async function DataDisplay() {
  const data = await fetchData() // Only blocks this component
  return <div>{data.content}</div>
}

Sidebar, Header, and Footer render immediately. Only DataDisplay waits for data.

Alternative (share promise across components):

function Page() {
  // Start fetch immediately, but don't await
  const dataPromise = fetchData()
  
  return (
    <div>
      <div>Sidebar</div>
      <div>Header</div>
      <Suspense fallback={<Skeleton />}>
        <DataDisplay dataPromise={dataPromise} />
        <DataSummary dataPromise={dataPromise} />
      </Suspense>
      <div>Footer</div>
    </div>
  )
}

function DataDisplay({ dataPromise }: { dataPromise: Promise<Data> }) {
  const data = use(dataPromise) // Unwraps the promise
  return <div>{data.content}</div>
}

function DataSummary({ dataPromise }: { dataPromise: Promise<Data> }) {
  const data = use(dataPromise) // Reuses the same promise
  return <div>{data.summary}</div>
}

Both components share the same promise, so only one fetch occurs. Layout renders immediately while both components wait together.

When NOT to use this pattern:

• Critical data needed for layout decisions (affects positioning)
• SEO-critical content above the fold
• Small, fast queries where suspense overhead isn't worth it
• When you want to avoid layout shift (loading → content jump)

Trade-off: Faster initial paint vs potential layout shift. Choose based on your UX priorities.

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):

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):

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+):

// 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

Conditional Module Loading

Load large data or modules only when a feature is activated.

Example (lazy-load animation frames):

function AnimationPlayer({ enabled }: { enabled: boolean }) {
  const [frames, setFrames] = useState<Frame[] | null>(null)

  useEffect(() => {
    if (enabled && !frames && typeof window !== 'undefined') {
      import('./animation-frames.js')
        .then(mod => setFrames(mod.frames))
        .catch(() => setEnabled(false))
    }
  }, [enabled, frames])

  if (!frames) return <Skeleton />
  return <Canvas frames={frames} />
}

The typeof window !== 'undefined' check prevents bundling this module for SSR, optimizing server bundle size and build speed.

Defer Non-Critical Third-Party Libraries

Analytics, logging, and error tracking don't block user interaction. Load them after hydration.

Incorrect (blocks initial bundle):

import { Analytics } from '@vercel/analytics/react'

export default function RootLayout({ children }) {
  return (
    <html>
      <body>
        {children}
        <Analytics />
      </body>
    </html>
  )
}

Correct (loads after hydration):

import dynamic from 'next/dynamic'

const Analytics = dynamic(
  () => import('@vercel/analytics/react').then(m => m.Analytics),
  { ssr: false }
)

export default function RootLayout({ children }) {
  return (
    <html>
      <body>
        {children}
        <Analytics />
      </body>
    </html>
  )
}

Dynamic Imports for Heavy Components

Use next/dynamic to lazy-load large components not needed on initial render.

Incorrect (Monaco bundles with main chunk ~300KB):

import { MonacoEditor } from './monaco-editor'

function CodePanel({ code }: { code: string }) {
  return <MonacoEditor value={code} />
}

Correct (Monaco loads on demand):

import dynamic from 'next/dynamic'

const MonacoEditor = dynamic(
  () => import('./monaco-editor').then(m => m.MonacoEditor),
  { ssr: false }
)

function CodePanel({ code }: { code: string }) {
  return <MonacoEditor value={code} />
}

Preload Based on User Intent

Preload heavy bundles before they're needed to reduce perceived latency.

Example (preload on hover/focus):

function EditorButton({ onClick }: { onClick: () => void }) {
  const preload = () => {
    if (typeof window !== 'undefined') {
      void import('./monaco-editor')
    }
  }

  return (
    <button
      onMouseEnter={preload}
      onFocus={preload}
      onClick={onClick}
    >
      Open Editor
    </button>
  )
}

Example (preload when feature flag is enabled):

function FlagsProvider({ children, flags }: Props) {
  useEffect(() => {
    if (flags.editorEnabled && typeof window !== 'undefined') {
      void import('./monaco-editor').then(mod => mod.init())
    }
  }, [flags.editorEnabled])

  return <FlagsContext.Provider value={flags}>
    {children}
  </FlagsContext.Provider>
}

The typeof window !== 'undefined' check prevents bundling preloaded modules for SSR, optimizing server bundle size and build speed.

Deduplicate Global Event Listeners

Use useSWRSubscription() to share global event listeners across component instances.

Incorrect (N instances = N listeners):

function useKeyboardShortcut(key: string, callback: () => void) {
  useEffect(() => {
    const handler = (e: KeyboardEvent) => {
      if (e.metaKey && e.key === key) {
        callback()
      }
    }
    window.addEventListener('keydown', handler)
    return () => window.removeEventListener('keydown', handler)
  }, [key, callback])
}

When using the useKeyboardShortcut hook multiple times, each instance will register a new listener.

Correct (N instances = 1 listener):

import useSWRSubscription from 'swr/subscription'

// Module-level Map to track callbacks per key
const keyCallbacks = new Map<string, Set<() => void>>()

function useKeyboardShortcut(key: string, callback: () => void) {
  // Register this callback in the Map
  useEffect(() => {
    if (!keyCallbacks.has(key)) {
      keyCallbacks.set(key, new Set())
    }
    keyCallbacks.get(key)!.add(callback)

    return () => {
      const set = keyCallbacks.get(key)
      if (set) {
        set.delete(callback)
        if (set.size === 0) {
          keyCallbacks.delete(key)
        }
      }
    }
  }, [key, callback])

  useSWRSubscription('global-keydown', () => {
    const handler = (e: KeyboardEvent) => {
      if (e.metaKey && keyCallbacks.has(e.key)) {
        keyCallbacks.get(e.key)!.forEach(cb => cb())
      }
    }
    window.addEventListener('keydown', handler)
    return () => window.removeEventListener('keydown', handler)
  })
}

function Profile() {
  // Multiple shortcuts will share the same listener
  useKeyboardShortcut('p', () => { /* ... */ }) 
  useKeyboardShortcut('k', () => { /* ... */ })
  // ...
}

Use SWR for Automatic Deduplication

SWR enables request deduplication, caching, and revalidation across component instances.

Incorrect (no deduplication, each instance fetches):

function UserList() {
  const [users, setUsers] = useState([])
  useEffect(() => {
    fetch('/api/users')
      .then(r => r.json())
      .then(setUsers)
  }, [])
}

Correct (multiple instances share one request):

import useSWR from 'swr'

function UserList() {
  const { data: users } = useSWR('/api/users', fetcher)
}

For immutable data:

import { useImmutableSWR } from '@/lib/swr'

function StaticContent() {
  const { data } = useImmutableSWR('/api/config', fetcher)
}

For mutations:

import { useSWRMutation } from 'swr/mutation'

function UpdateButton() {
  const { trigger } = useSWRMutation('/api/user', updateUser)
  return <button onClick={() => trigger()}>Update</button>
}

Reference: https://swr.vercel.app

Batch DOM CSS Changes

Avoid changing styles one property at a time. Group multiple CSS changes together via classes or cssText to minimize browser reflows.

Incorrect (multiple reflows):

function updateElementStyles(element: HTMLElement) {
  // Each line triggers a reflow
  element.style.width = '100px'
  element.style.height = '200px'
  element.style.backgroundColor = 'blue'
  element.style.border = '1px solid black'
}

Correct (add class - single reflow):

// CSS file
.highlighted-box {
  width: 100px;
  height: 200px;
  background-color: blue;
  border: 1px solid black;
}

// JavaScript
function updateElementStyles(element: HTMLElement) {
  element.classList.add('highlighted-box')
}

Correct (change cssText - single reflow):

function updateElementStyles(element: HTMLElement) {
  element.style.cssText = `
    width: 100px;
    height: 200px;
    background-color: blue;
    border: 1px solid black;
  `
}

React example:

// Incorrect: changing styles one by one
function Box({ isHighlighted }: { isHighlighted: boolean }) {
  const ref = useRef<HTMLDivElement>(null)
  
  useEffect(() => {
    if (ref.current && isHighlighted) {
      ref.current.style.width = '100px'
      ref.current.style.height = '200px'
      ref.current.style.backgroundColor = 'blue'
    }
  }, [isHighlighted])
  
  return <div ref={ref}>Content</div>
}

// Correct: toggle class
function Box({ isHighlighted }: { isHighlighted: boolean }) {
  return (
    <div className={isHighlighted ? 'highlighted-box' : ''}>
      Content
    </div>
  )
}

Prefer CSS classes over inline styles when possible. Classes are cached by the browser and provide better separation of concerns.

Cache Repeated Function Calls

Use a module-level Map to cache function results when the same function is called repeatedly with the same inputs during render.

Incorrect (redundant computation):

function ProjectList({ projects }: { projects: Project[] }) {
  return (
    <div>
      {projects.map(project => {
        // slugify() called 100+ times for same project names
        const slug = slugify(project.name)
        
        return <ProjectCard key={project.id} slug={slug} />
      })}
    </div>
  )
}

Correct (cached results):

// Module-level cache
const slugifyCache = new Map<string, string>()

function cachedSlugify(text: string): string {
  if (slugifyCache.has(text)) {
    return slugifyCache.get(text)!
  }
  const result = slugify(text)
  slugifyCache.set(text, result)
  return result
}

function ProjectList({ projects }: { projects: Project[] }) {
  return (
    <div>
      {projects.map(project => {
        // Computed only once per unique project name
        const slug = cachedSlugify(project.name)
        
        return <ProjectCard key={project.id} slug={slug} />
      })}
    </div>
  )
}

Simpler pattern for single-value functions:

let isLoggedInCache: boolean | null = null

function isLoggedIn(): boolean {
  if (isLoggedInCache !== null) {
    return isLoggedInCache
  }
  
  isLoggedInCache = document.cookie.includes('auth=')
  return isLoggedInCache
}

// Clear cache when auth changes
function onAuthChange() {
  isLoggedInCache = null
}

Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components.

Reference: How we made the Vercel Dashboard twice as fast

Cache Property Access in Loops

Cache object property lookups in hot paths.

Incorrect (3 lookups × N iterations):

for (let i = 0; i < arr.length; i++) {
  process(obj.config.settings.value)
}

Correct (1 lookup total):

const value = obj.config.settings.value
const len = arr.length
for (let i = 0; i < len; i++) {
  process(value)
}

Cache Storage API Calls

localStorage, sessionStorage, and document.cookie are synchronous and expensive. Cache reads in memory.

Incorrect (reads storage on every call):

function getTheme() {
  return localStorage.getItem('theme') ?? 'light'
}
// Called 10 times = 10 storage reads

Correct (Map cache):

const storageCache = new Map<string, string | null>()

function getLocalStorage(key: string) {
  if (!storageCache.has(key)) {
    storageCache.set(key, localStorage.getItem(key))
  }
  return storageCache.get(key)
}

function setLocalStorage(key: string, value: string) {
  localStorage.setItem(key, value)
  storageCache.set(key, value)  // keep cache in sync
}

Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components.

Cookie caching:

let cookieCache: Record<string, string> | null = null

function getCookie(name: string) {
  if (!cookieCache) {
    cookieCache = Object.fromEntries(
      document.cookie.split('; ').map(c => c.split('='))
    )
  }
  return cookieCache[name]
}

Important (invalidate on external changes):

If storage can change externally (another tab, server-set cookies), invalidate cache:

window.addEventListener('storage', (e) => {
  if (e.key) storageCache.delete(e.key)
})

document.addEventListener('visibilitychange', () => {
  if (document.visibilityState === 'visible') {
    storageCache.clear()
  }
})

Combine Multiple Array Iterations

Multiple .filter() or .map() calls iterate the array multiple times. Combine into one loop.

Incorrect (3 iterations):

const admins = users.filter(u => u.isAdmin)
const testers = users.filter(u => u.isTester)
const inactive = users.filter(u => !u.isActive)

Correct (1 iteration):

const admins: User[] = []
const testers: User[] = []
const inactive: User[] = []

for (const user of users) {
  if (user.isAdmin) admins.push(user)
  if (user.isTester) testers.push(user)
  if (!user.isActive) inactive.push(user)
}

Early Return from Functions

Return early when result is determined to skip unnecessary processing.

Incorrect (processes all items even after finding answer):

function validateUsers(users: User[]) {
  let hasError = false
  let errorMessage = ''
  
  for (const user of users) {
    if (!user.email) {
      hasError = true
      errorMessage = 'Email required'
    }
    if (!user.name) {
      hasError = true
      errorMessage = 'Name required'
    }
    // Continues checking all users even after error found
  }
  
  return hasError ? { valid: false, error: errorMessage } : { valid: true }
}

Correct (returns immediately on first error):

function validateUsers(users: User[]) {
  for (const user of users) {
    if (!user.email) {
      return { valid: false, error: 'Email required' }
    }
    if (!user.name) {
      return { valid: false, error: 'Name required' }
    }
  }

  return { valid: true }
}

Hoist RegExp Creation

Don't create RegExp inside render. Hoist to module scope or memoize with useMemo().

Incorrect (new RegExp every render):

function Highlighter({ text, query }: Props) {
  const regex = new RegExp(`(${query})`, 'gi')
  const parts = text.split(regex)
  return <>{parts.map((part, i) => ...)}</>
}

Correct (memoize or hoist):

const EMAIL_REGEX = /^[^\s@]+@[^\s@]+\.[^\s@]+$/

function Highlighter({ text, query }: Props) {
  const regex = useMemo(
    () => new RegExp(`(${escapeRegex(query)})`, 'gi'),
    [query]
  )
  const parts = text.split(regex)
  return <>{parts.map((part, i) => ...)}</>
}

Warning (global regex has mutable state):

Global regex (/g) has mutable lastIndex state:

const regex = /foo/g
regex.test('foo')  // true, lastIndex = 3
regex.test('foo')  // false, lastIndex = 0

Build Index Maps for Repeated Lookups

Multiple .find() calls by the same key should use a Map.

Incorrect (O(n) per lookup):

function processOrders(orders: Order[], users: User[]) {
  return orders.map(order => ({
    ...order,
    user: users.find(u => u.id === order.userId)
  }))
}

Correct (O(1) per lookup):

function processOrders(orders: Order[], users: User[]) {
  const userById = new Map(users.map(u => [u.id, u]))

  return orders.map(order => ({
    ...order,
    user: userById.get(order.userId)
  }))
}

Build map once (O(n)), then all lookups are O(1). For 1000 orders × 1000 users: 1M ops → 2K ops.

Early Length Check for Array Comparisons

When comparing arrays with expensive operations (sorting, deep equality, serialization), check lengths first. If lengths differ, the arrays cannot be equal.

In real-world applications, this optimization is especially valuable when the comparison runs in hot paths (event handlers, render loops).

Incorrect (always runs expensive comparison):

function hasChanges(current: string[], original: string[]) {
  // Always sorts and joins, even when lengths differ
  return current.sort().join() !== original.sort().join()
}

Two O(n log n) sorts run even when current.length is 5 and original.length is 100. There is also overhead of joining the arrays and comparing the strings.

Correct (O(1) length check first):

function hasChanges(current: string[], original: string[]) {
  // Early return if lengths differ
  if (current.length !== original.length) {
    return true
  }
  // Only sort/join when lengths match
  const currentSorted = current.toSorted()
  const originalSorted = original.toSorted()
  for (let i = 0; i < currentSorted.length; i++) {
    if (currentSorted[i] !== originalSorted[i]) {
      return true
    }
  }
  return false
}

This new approach is more efficient because:

• It avoids the overhead of sorting and joining the arrays when lengths differ
• It avoids consuming memory for the joined strings (especially important for large arrays)
• It avoids mutating the original arrays
• It returns early when a difference is found

Use Loop for Min/Max Instead of Sort

Finding the smallest or largest element only requires a single pass through the array. Sorting is wasteful and slower.

Incorrect (O(n log n) - sort to find latest):

interface Project {
  id: string
  name: string
  updatedAt: number
}

function getLatestProject(projects: Project[]) {
  const sorted = [...projects].sort((a, b) => b.updatedAt - a.updatedAt)
  return sorted[0]
}

Sorts the entire array just to find the maximum value.

Incorrect (O(n log n) - sort for oldest and newest):

function getOldestAndNewest(projects: Project[]) {
  const sorted = [...projects].sort((a, b) => a.updatedAt - b.updatedAt)
  return { oldest: sorted[0], newest: sorted[sorted.length - 1] }
}

Still sorts unnecessarily when only min/max are needed.

Correct (O(n) - single loop):

function getLatestProject(projects: Project[]) {
  if (projects.length === 0) return null
  
  let latest = projects[0]
  
  for (let i = 1; i < projects.length; i++) {
    if (projects[i].updatedAt > latest.updatedAt) {
      latest = projects[i]
    }
  }
  
  return latest
}

function getOldestAndNewest(projects: Project[]) {
  if (projects.length === 0) return { oldest: null, newest: null }
  
  let oldest = projects[0]
  let newest = projects[0]
  
  for (let i = 1; i < projects.length; i++) {
    if (projects[i].updatedAt < oldest.updatedAt) oldest = projects[i]
    if (projects[i].updatedAt > newest.updatedAt) newest = projects[i]
  }
  
  return { oldest, newest }
}

Single pass through the array, no copying, no sorting.

Alternative (Math.min/Math.max for small arrays):

const numbers = [5, 2, 8, 1, 9]
const min = Math.min(...numbers)
const max = Math.max(...numbers)

This works for small arrays but can be slower for very large arrays due to spread operator limitations. Use the loop approach for reliability.

Use Set/Map for O(1) Lookups

Convert arrays to Set/Map for repeated membership checks.

Incorrect (O(n) per check):

const allowedIds = ['a', 'b', 'c', ...]
items.filter(item => allowedIds.includes(item.id))

Correct (O(1) per check):

const allowedIds = new Set(['a', 'b', 'c', ...])
items.filter(item => allowedIds.has(item.id))

Use toSorted() Instead of sort() for Immutability

.sort() mutates the array in place, which can cause bugs with React state and props. Use .toSorted() to create a new sorted array without mutation.

Incorrect (mutates original array):

function UserList({ users }: { users: User[] }) {
  // Mutates the users prop array!
  const sorted = useMemo(
    () => users.sort((a, b) => a.name.localeCompare(b.name)),
    [users]
  )
  return <div>{sorted.map(renderUser)}</div>
}

Correct (creates new array):

function UserList({ users }: { users: User[] }) {
  // Creates new sorted array, original unchanged
  const sorted = useMemo(
    () => users.toSorted((a, b) => a.name.localeCompare(b.name)),
    [users]
  )
  return <div>{sorted.map(renderUser)}</div>
}

Why this matters in React:

1. Props/state mutations break React's immutability model - React expects props and state to be treated as read-only 2. Causes stale closure bugs - Mutating arrays inside closures (callbacks, effects) can lead to unexpected behavior

Browser support (fallback for older browsers):

.toSorted() is available in all modern browsers (Chrome 110+, Safari 16+, Firefox 115+, Node.js 20+). For older environments, use spread operator:

// Fallback for older browsers
const sorted = [...items].sort((a, b) => a.value - b.value)

Other immutable array methods:

• .toSorted() - immutable sort
• .toReversed() - immutable reverse
• .toSpliced() - immutable splice
• .with() - immutable element replacement

Use Activity Component for Show/Hide

Use React's <Activity> to preserve state/DOM for expensive components that frequently toggle visibility.

Usage:

import { Activity } from 'react'

function Dropdown({ isOpen }: Props) {
  return (
    <Activity mode={isOpen ? 'visible' : 'hidden'}>
      <ExpensiveMenu />
    </Activity>
  )
}

Avoids expensive re-renders and state loss.

Animate SVG Wrapper Instead of SVG Element

Many browsers don't have hardware acceleration for CSS3 animations on SVG elements. Wrap SVG in a <div> and animate the wrapper instead.

Incorrect (animating SVG directly - no hardware acceleration):

function LoadingSpinner() {
  return (
    <svg 
      className="animate-spin"
      width="24" 
      height="24" 
      viewBox="0 0 24 24"
    >
      <circle cx="12" cy="12" r="10" stroke="currentColor" />
    </svg>
  )
}

Correct (animating wrapper div - hardware accelerated):

function LoadingSpinner() {
  return (
    <div className="animate-spin">
      <svg 
        width="24" 
        height="24" 
        viewBox="0 0 24 24"
      >
        <circle cx="12" cy="12" r="10" stroke="currentColor" />
      </svg>
    </div>
  )
}

This applies to all CSS transforms and transitions (transform, opacity, translate, scale, rotate). The wrapper div allows browsers to use GPU acceleration for smoother animations.

Use Explicit Conditional Rendering

Use explicit ternary operators (? :) instead of && for conditional rendering when the condition can be 0, NaN, or other falsy values that render.

Incorrect (renders "0" when count is 0):

function Badge({ count }: { count: number }) {
  return (
    <div>
      {count && <span className="badge">{count}</span>}
    </div>
  )
}

// When count = 0, renders: <div>0</div>
// When count = 5, renders: <div><span class="badge">5</span></div>

Correct (renders nothing when count is 0):

function Badge({ count }: { count: number }) {
  return (
    <div>
      {count > 0 ? <span className="badge">{count}</span> : null}
    </div>
  )
}

// When count = 0, renders: <div></div>
// When count = 5, renders: <div><span class="badge">5</span></div>

CSS content-visibility for Long Lists

Apply content-visibility: auto to defer off-screen rendering.

CSS:

.message-item {
  content-visibility: auto;
  contain-intrinsic-size: 0 80px;
}

Example:

function MessageList({ messages }: { messages: Message[] }) {
  return (
    <div className="overflow-y-auto h-screen">
      {messages.map(msg => (
        <div key={msg.id} className="message-item">
          <Avatar user={msg.author} />
          <div>{msg.content}</div>
        </div>
      ))}
    </div>
  )
}

For 1000 messages, browser skips layout/paint for ~990 off-screen items (10× faster initial render).

Hoist Static JSX Elements

Extract static JSX outside components to avoid re-creation.

Incorrect (recreates element every render):

function LoadingSkeleton() {
  return <div className="animate-pulse h-20 bg-gray-200" />
}

function Container() {
  return (
    <div>
      {loading && <LoadingSkeleton />}
    </div>
  )
}

Correct (reuses same element):

const loadingSkeleton = (
  <div className="animate-pulse h-20 bg-gray-200" />
)

function Container() {
  return (
    <div>
      {loading && loadingSkeleton}
    </div>
  )
}

This is especially helpful for large and static SVG nodes, which can be expensive to recreate on every render.

Note: If your project has React Compiler enabled, the compiler automatically hoists static JSX elements and optimizes component re-renders, making manual hoisting unnecessary.

Prevent Hydration Mismatch Without Flickering

When rendering content that depends on client-side storage (localStorage, cookies), avoid both SSR breakage and post-hydration flickering by injecting a synchronous script that updates the DOM before React hydrates.

Incorrect (breaks SSR):

function ThemeWrapper({ children }: { children: ReactNode }) {
  // localStorage is not available on server - throws error
  const theme = localStorage.getItem('theme') || 'light'
  
  return (
    <div className={theme}>
      {children}
    </div>
  )
}

Server-side rendering will fail because localStorage is undefined.

Incorrect (visual flickering):

function ThemeWrapper({ children }: { children: ReactNode }) {
  const [theme, setTheme] = useState('light')
  
  useEffect(() => {
    // Runs after hydration - causes visible flash
    const stored = localStorage.getItem('theme')
    if (stored) {
      setTheme(stored)
    }
  }, [])
  
  return (
    <div className={theme}>
      {children}
    </div>
  )
}

Component first renders with default value (light), then updates after hydration, causing a visible flash of incorrect content.

Correct (no flicker, no hydration mismatch):

function ThemeWrapper({ children }: { children: ReactNode }) {
  return (
    <>
      <div id="theme-wrapper">
        {children}
      </div>
      <script
        dangerouslySetInnerHTML={{
          __html: `
            (function() {
              try {
                var theme = localStorage.getItem('theme') || 'light';
                var el = document.getElementById('theme-wrapper');
                if (el) el.className = theme;
              } catch (e) {}
            })();
          `,
        }}
      />
    </>
  )
}

The inline script executes synchronously before showing the element, ensuring the DOM already has the correct value. No flickering, no hydration mismatch.

This pattern is especially useful for theme toggles, user preferences, authentication states, and any client-only data that should render immediately without flashing default values.

Optimize SVG Precision

Reduce SVG coordinate precision to decrease file size. The optimal precision depends on the viewBox size, but in general reducing precision should be considered.

Incorrect (excessive precision):

<path d="M 10.293847 20.847362 L 30.938472 40.192837" />

Correct (1 decimal place):

<path d="M 10.3 20.8 L 30.9 40.2" />

Automate with SVGO:

npx svgo --precision=1 --multipass icon.svg

Defer State Reads to Usage Point

Don't subscribe to dynamic state (searchParams, localStorage) if you only read it inside callbacks.

Incorrect (subscribes to all searchParams changes):

function ShareButton({ chatId }: { chatId: string }) {
  const searchParams = useSearchParams()

  const handleShare = () => {
    const ref = searchParams.get('ref')
    shareChat(chatId, { ref })
  }

  return <button onClick={handleShare}>Share</button>
}

Correct (reads on demand, no subscription):

function ShareButton({ chatId }: { chatId: string }) {
  const handleShare = () => {
    const params = new URLSearchParams(window.location.search)
    const ref = params.get('ref')
    shareChat(chatId, { ref })
  }

  return <button onClick={handleShare}>Share</button>
}

Narrow Effect Dependencies

Specify primitive dependencies instead of objects to minimize effect re-runs.

Incorrect (re-runs on any user field change):

useEffect(() => {
  console.log(user.id)
}, [user])

Correct (re-runs only when id changes):

useEffect(() => {
  console.log(user.id)
}, [user.id])

For derived state, compute outside effect:

// Incorrect: runs on width=767, 766, 765...
useEffect(() => {
  if (width < 768) {
    enableMobileMode()
  }
}, [width])

// Correct: runs only on boolean transition
const isMobile = width < 768
useEffect(() => {
  if (isMobile) {
    enableMobileMode()
  }
}, [isMobile])

Subscribe to Derived State

Subscribe to derived boolean state instead of continuous values to reduce re-render frequency.

Incorrect (re-renders on every pixel change):

function Sidebar() {
  const width = useWindowWidth()  // updates continuously
  const isMobile = width < 768
  return <nav className={isMobile ? 'mobile' : 'desktop'}>
}

Correct (re-renders only when boolean changes):

function Sidebar() {
  const isMobile = useMediaQuery('(max-width: 767px)')
  return <nav className={isMobile ? 'mobile' : 'desktop'}>
}

Use Functional setState Updates

When updating state based on the current state value, use the functional update form of setState instead of directly referencing the state variable. This prevents stale closures, eliminates unnecessary dependencies, and creates stable callback references.

Incorrect (requires state as dependency):

function TodoList() {
  const [items, setItems] = useState(initialItems)
  
  // Callback must depend on items, recreated on every items change
  const addItems = useCallback((newItems: Item[]) => {
    setItems([...items, ...newItems])
  }, [items])  // ❌ items dependency causes recreations
  
  // Risk of stale closure if dependency is forgotten
  const removeItem = useCallback((id: string) => {
    setItems(items.filter(item => item.id !== id))
  }, [])  // ❌ Missing items dependency - will use stale items!
  
  return <ItemsEditor items={items} onAdd={addItems} onRemove={removeItem} />
}

The first callback is recreated every time items changes, which can cause child components to re-render unnecessarily. The second callback has a stale closure bug—it will always reference the initial items value.

Correct (stable callbacks, no stale closures):

function TodoList() {
  const [items, setItems] = useState(initialItems)
  
  // Stable callback, never recreated
  const addItems = useCallback((newItems: Item[]) => {
    setItems(curr => [...curr, ...newItems])
  }, [])  // ✅ No dependencies needed
  
  // Always uses latest state, no stale closure risk
  const removeItem = useCallback((id: string) => {
    setItems(curr => curr.filter(item => item.id !== id))
  }, [])  // ✅ Safe and stable
  
  return <ItemsEditor items={items} onAdd={addItems} onRemove={removeItem} />
}

Benefits:

1. Stable callback references - Callbacks don't need to be recreated when state changes 2. No stale closures - Always operates on the latest state value 3. Fewer dependencies - Simplifies dependency arrays and reduces memory leaks 4. Prevents bugs - Eliminates the most common source of React closure bugs

When to use functional updates:

• Any setState that depends on the current state value
• Inside useCallback/useMemo when state is needed
• Event handlers that reference state
• Async operations that update state

When direct updates are fine:

• Setting state to a static value: setCount(0)
• Setting state from props/arguments only: setName(newName)
• State doesn't depend on previous value

Note: If your project has React Compiler enabled, the compiler can automatically optimize some cases, but functional updates are still recommended for correctness and to prevent stale closure bugs.

Use Lazy State Initialization

Pass a function to useState for expensive initial values. Without the function form, the initializer runs on every render even though the value is only used once.

Incorrect (runs on every render):

function FilteredList({ items }: { items: Item[] }) {
  // buildSearchIndex() runs on EVERY render, even after initialization
  const [searchIndex, setSearchIndex] = useState(buildSearchIndex(items))
  const [query, setQuery] = useState('')
  
  // When query changes, buildSearchIndex runs again unnecessarily
  return <SearchResults index={searchIndex} query={query} />
}

function UserProfile() {
  // JSON.parse runs on every render
  const [settings, setSettings] = useState(
    JSON.parse(localStorage.getItem('settings') || '{}')
  )
  
  return <SettingsForm settings={settings} onChange={setSettings} />
}

Correct (runs only once):

function FilteredList({ items }: { items: Item[] }) {
  // buildSearchIndex() runs ONLY on initial render
  const [searchIndex, setSearchIndex] = useState(() => buildSearchIndex(items))
  const [query, setQuery] = useState('')
  
  return <SearchResults index={searchIndex} query={query} />
}

function UserProfile() {
  // JSON.parse runs only on initial render
  const [settings, setSettings] = useState(() => {
    const stored = localStorage.getItem('settings')
    return stored ? JSON.parse(stored) : {}
  })
  
  return <SettingsForm settings={settings} onChange={setSettings} />
}

Use lazy initialization when computing initial values from localStorage/sessionStorage, building data structures (indexes, maps), reading from the DOM, or performing heavy transformations.

For simple primitives (useState(0)), direct references (useState(props.value)), or cheap literals (useState({})), the function form is unnecessary.

Extract to Memoized Components

Extract expensive work into memoized components to enable early returns before computation.

Incorrect (computes avatar even when loading):

function Profile({ user, loading }: Props) {
  const avatar = useMemo(() => {
    const id = computeAvatarId(user)
    return <Avatar id={id} />
  }, [user])

  if (loading) return <Skeleton />
  return <div>{avatar}</div>
}

Correct (skips computation when loading):

const UserAvatar = memo(function UserAvatar({ user }: { user: User }) {
  const id = useMemo(() => computeAvatarId(user), [user])
  return <Avatar id={id} />
})

function Profile({ user, loading }: Props) {
  if (loading) return <Skeleton />
  return (
    <div>
      <UserAvatar user={user} />
    </div>
  )
}

Note: If your project has React Compiler enabled, manual memoization with memo() and useMemo() is not necessary. The compiler automatically optimizes re-renders.

Use Transitions for Non-Urgent Updates

Mark frequent, non-urgent state updates as transitions to maintain UI responsiveness.

Incorrect (blocks UI on every scroll):

function ScrollTracker() {
  const [scrollY, setScrollY] = useState(0)
  useEffect(() => {
    const handler = () => setScrollY(window.scrollY)
    window.addEventListener('scroll', handler, { passive: true })
    return () => window.removeEventListener('scroll', handler)
  }, [])
}

Correct (non-blocking updates):

import { startTransition } from 'react'

function ScrollTracker() {
  const [scrollY, setScrollY] = useState(0)
  useEffect(() => {
    const handler = () => {
      startTransition(() => setScrollY(window.scrollY))
    }
    window.addEventListener('scroll', handler, { passive: true })
    return () => window.removeEventListener('scroll', handler)
  }, [])
}

Use after() for Non-Blocking Operations

Use Next.js's after() to schedule work that should execute after a response is sent. This prevents logging, analytics, and other side effects from blocking the response.

Incorrect (blocks response):

import { logUserAction } from '@/app/utils'

export async function POST(request: Request) {
  // Perform mutation
  await updateDatabase(request)
  
  // Logging blocks the response
  const userAgent = request.headers.get('user-agent') || 'unknown'
  await logUserAction({ userAgent })
  
  return new Response(JSON.stringify({ status: 'success' }), {
    status: 200,
    headers: { 'Content-Type': 'application/json' }
  })
}

Correct (non-blocking):

import { after } from 'next/server'
import { headers, cookies } from 'next/headers'
import { logUserAction } from '@/app/utils'

export async function POST(request: Request) {
  // Perform mutation
  await updateDatabase(request)
  
  // Log after response is sent
  after(async () => {
    const userAgent = (await headers()).get('user-agent') || 'unknown'
    const sessionCookie = (await cookies()).get('session-id')?.value || 'anonymous'
    
    logUserAction({ sessionCookie, userAgent })
  })
  
  return new Response(JSON.stringify({ status: 'success' }), {
    status: 200,
    headers: { 'Content-Type': 'application/json' }
  })
}

The response is sent immediately while logging happens in the background.

Common use cases:

• Analytics tracking
• Audit logging
• Sending notifications
• Cache invalidation
• Cleanup tasks

Important notes:

• after() runs even if the response fails or redirects
• Works in Server Actions, Route Handlers, and Server Components

Reference: https://nextjs.org/docs/app/api-reference/functions/after

Cross-Request LRU Caching

React.cache() only works within one request. For data shared across sequential requests (user clicks button A then button B), use an LRU cache.

Implementation:

import { LRUCache } from 'lru-cache'

const cache = new LRUCache<string, any>({
  max: 1000,
  ttl: 5 * 60 * 1000  // 5 minutes
})

export async function getUser(id: string) {
  const cached = cache.get(id)
  if (cached) return cached

  const user = await db.user.findUnique({ where: { id } })
  cache.set(id, user)
  return user
}

// Request 1: DB query, result cached
// Request 2: cache hit, no DB query

Use when sequential user actions hit multiple endpoints needing the same data within seconds.

With Vercel's [Fluid Compute](https://vercel.com/docs/fluid-compute): LRU caching is especially effective because multiple concurrent requests can share the same function instance and cache. This means the cache persists across requests without needing external storage like Redis.

In traditional serverless: Each invocation runs in isolation, so consider Redis for cross-process caching.

Reference: https://github.com/isaacs/node-lru-cache

Per-Request Deduplication with React.cache()

Use React.cache() for server-side request deduplication. Authentication and database queries benefit most.

Usage:

import { cache } from 'react'

export const getCurrentUser = cache(async () => {
  const session = await auth()
  if (!session?.user?.id) return null
  return await db.user.findUnique({
    where: { id: session.user.id }
  })
})

Within a single request, multiple calls to getCurrentUser() execute the query only once.

Parallel Data Fetching with Component Composition

React Server Components execute sequentially within a tree. Restructure with composition to parallelize data fetching.

Incorrect (Sidebar waits for Page's fetch to complete):

export default async function Page() {
  const header = await fetchHeader()
  return (
    <div>
      <div>{header}</div>
      <Sidebar />
    </div>
  )
}

async function Sidebar() {
  const items = await fetchSidebarItems()
  return <nav>{items.map(renderItem)}</nav>
}

Correct (both fetch simultaneously):

async function Header() {
  const data = await fetchHeader()
  return <div>{data}</div>
}

async function Sidebar() {
  const items = await fetchSidebarItems()
  return <nav>{items.map(renderItem)}</nav>
}

export default function Page() {
  return (
    <div>
      <Header />
      <Sidebar />
    </div>
  )
}

Alternative with children prop:

async function Layout({ children }: { children: ReactNode }) {
  const header = await fetchHeader()
  return (
    <div>
      <div>{header}</div>
      {children}
    </div>
  )
}

async function Sidebar() {
  const items = await fetchSidebarItems()
  return <nav>{items.map(renderItem)}</nav>
}

export default function Page() {
  return (
    <Layout>
      <Sidebar />
    </Layout>
  )
}