vercel-react-best-practices

Vercel React best practices for rendering performance, bundle efficiency, and scalable component architecture.

View source React Next.js Performance Systems

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npx skills add https://github.com/vercel-labs/agent-skills --skill vercel-react-best-practices

React Best Practices

Version 1.0.0
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. 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.

Eliminating Waterfalls — CRITICAL
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 Prefer Statically Analyzable Paths
- 2.6 Preload Based on User Intent
Server-Side Performance — HIGH
Client-Side Data Fetching — MEDIUM-HIGH
Re-render Optimization — MEDIUM
Rendering Performance — MEDIUM
JavaScript Performance — LOW-MEDIUM
- 7.1 Avoid Layout Thrashing
- 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 Defer Non-Critical Work with requestIdleCallback
- 7.8 Early Length Check for Array Comparisons
- 7.9 Early Return from Functions
- 7.10 Hoist RegExp Creation
- 7.11 Use flatMap to Map and Filter in One Pass
- 7.12 Use Loop for Min/Max Instead of Sort
- 7.13 Use Set/Map for O(1) Lookups
- 7.14 Use toSorted() Instead of sort() for Immutability
Advanced Patterns — LOW

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 Check Cheap Conditions Before Async Flags

Impact: HIGH (avoids unnecessary async work when a synchronous guard already fails)

When a branch uses await for a flag or remote value and also requires a cheap synchronous condition (local props, request metadata, already-loaded state), evaluate the cheap condition first. Otherwise you pay for the async call even when the compound condition can never be true.

This is a specialization of Defer Await Until Needed for flag && cheapCondition style checks.

Incorrect:

Correct:

This matters when getFlag hits the network, a feature-flag service, or React.cache / DB work: skipping it when someCondition is false removes that cost on the cold path.

Keep the original order if someCondition is expensive, depends on the flag, or you must run side effects in a fixed order.

1.2 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

typescript

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

Correct: only blocks when needed

typescript

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

Another example: early return optimization

typescript

// Incorrect: always fetches permissionsasync 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 neededasync 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.

For await getFlag() combined with a cheap synchronous guard (flag && someCondition), see Check Cheap Conditions Before Async Flags.

1.3 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

Correct: config and profile run in parallel

typescript

import { all } from 'better-all'
const { user, config, profile } = await all({  async user() { return fetchUser() },  async config() { return fetchConfig() },  async profile() {    return fetchProfile((await this.$.user).id)  }})

Alternative without extra dependencies:

typescript

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

We can also create all the promises first, and do Promise.all() at the end.

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

1.4 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

typescript

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

typescript

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).

1.5 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

Correct: parallel execution, 1 round trip

1.6 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

tsx

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

tsx

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

tsx

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

tsx

import { Check, X, Menu } from 'lucide-react'// Loads 1,583 modules, takes ~2.8s extra in dev// Runtime cost: 200-800ms on every cold start
import { Button, TextField } from '@mui/material'// Loads 2,225 modules, takes ~4.2s extra in dev

Correct - Next.js 13.5+ (recommended):

This is the recommended approach because it preserves TypeScript type safety and editor autocompletion while still eliminating the barrel import cost.

Correct - Direct imports (non-Next.js projects):

TypeScript warning: Some libraries (notably lucide-react) don't ship .d.ts files for their deep import paths. Importing from lucide-react/dist/esm/icons/check resolves to an implicit any type, causing errors under strict or noImplicitAny. Prefer optimizePackageImports when available, or verify the library exports types for its subpaths before using direct imports.

These optimizations 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

tsx

function AnimationPlayer({ enabled, setEnabled }: { enabled: boolean; setEnabled: React.Dispatch<React.SetStateAction<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, setEnabled])
  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

tsx

import { Analytics } from '@vercel/analytics/react'
export default function RootLayout({ children }) {  return (    <html>      <body>        {children}        <Analytics />      </body>    </html>  )}

Correct: loads after hydration

tsx

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

Correct: Monaco loads on demand

tsx

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 Prefer Statically Analyzable Paths

Impact: HIGH (avoids accidental broad bundles and file traces)

Build tools work best when import and file-system paths are obvious at build time. If you hide the real path inside a variable or compose it too dynamically, the tool either has to include a broad set of possible files, warn that it cannot analyze the import, or widen file tracing to stay safe.

Prefer explicit maps or literal paths so the set of reachable files stays narrow and predictable. This is the same rule whether you are choosing modules with import() or reading files in server/build code.

When analysis becomes too broad, the cost is real:

Larger server bundles
Slower builds
Worse cold starts
More memory use

Incorrect: the bundler cannot tell what may be imported

Correct: use an explicit map of allowed modules

Incorrect: a 2-value enum still hides the final path from static analysis

Correct: make each final path literal at the callsite

In Next.js server code, this matters for output file tracing too. path.join(process.cwd(), someVar) can widen the traced file set because Next.js statically analyze import, require, and fs usage.

Reference: https://nextjs.org/docs/app/api-reference/config/next-config-js/output, https://nextjs.org/learn/seo/dynamic-imports, https://vite.dev/guide/features.html, https://esbuild.github.io/api/, https://www.npmjs.com/package/@rollup/plugin-dynamic-import-vars, https://webpack.js.org/guides/dependency-management/

2.6 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

tsx

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

tsx

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 Authenticate Server Actions Like API Routes

Impact: CRITICAL (prevents unauthorized access to server mutations)

Server Actions (functions with "use server") are exposed as public endpoints, just like API routes. Always verify authentication and authorization inside each Server Action—do not rely solely on middleware, layout guards, or page-level checks, as Server Actions can be invoked directly.

Next.js documentation explicitly states: "Treat Server Actions with the same security considerations as public-facing API endpoints, and verify if the user is allowed to perform a mutation."

Incorrect: no authentication check

Correct: authentication inside the action

typescript

'use server'
import { verifySession } from '@/lib/auth'import { unauthorized } from '@/lib/errors'
export async function deleteUser(userId: string) {  // Always check auth inside the action  const session = await verifySession()    if (!session) {    throw unauthorized('Must be logged in')  }    // Check authorization too  if (session.user.role !== 'admin' && session.user.id !== userId) {    throw unauthorized('Cannot delete other users')  }    await db.user.delete({ where: { id: userId } })  return { success: true }}

With input validation:

typescript

'use server'
import { verifySession } from '@/lib/auth'import { z } from 'zod'
const updateProfileSchema = z.object({  userId: z.string().uuid(),  name: z.string().min(1).max(100),  email: z.string().email()})
export async function updateProfile(data: unknown) {  // Validate input first  const validated = updateProfileSchema.parse(data)    // Then authenticate  const session = await verifySession()  if (!session) {    throw new Error('Unauthorized')  }    // Then authorize  if (session.user.id !== validated.userId) {    throw new Error('Can only update own profile')  }    // Finally perform the mutation  await db.user.update({    where: { id: validated.userId },    data: {      name: validated.name,      email: validated.email    }  })    return { success: true }}

Reference: https://nextjs.org/docs/app/guides/authentication

3.2 Avoid Duplicate Serialization in RSC Props

Impact: LOW (reduces network payload by avoiding duplicate serialization)

RSC→client serialization deduplicates by object reference, not value. Same reference = serialized once; new reference = serialized again. Do transformations (.toSorted(), .filter(), .map()) in client, not server.

Incorrect: duplicates array

Correct: sends 3 strings

Nested deduplication behavior:

tsx

// string[] - duplicates everythingusernames={['a','b']} sorted={usernames.toSorted()} // sends 4 strings
// object[] - duplicates array structure onlyusers={[{id:1},{id:2}]} sorted={users.toSorted()} // sends 2 arrays + 2 unique objects (not 4)

Deduplication works recursively. Impact varies by data type:

string[], number[], boolean[]: HIGH impact - array + all primitives fully duplicated
object[]: LOW impact - array duplicated, but nested objects deduplicated by reference

Operations breaking deduplication: create new references

Arrays: .toSorted(), .filter(), .map(), .slice(), [...arr]
Objects: {...obj}, Object.assign(), structuredClone(), JSON.parse(JSON.stringify())

More examples:

tsx

// ❌ Bad<C users={users} active={users.filter(u => u.active)} /><C product={product} productName={product.name} />
// ✅ Good<C users={users} /><C product={product} />// Do filtering/destructuring in client

Exception: Pass derived data when transformation is expensive or client doesn't need original.

3.3 Avoid Shared Module State for Request Data

Impact: HIGH (prevents concurrency bugs and request data leaks)

For React Server Components and client components rendered during SSR, avoid using mutable module-level variables to share request-scoped data. Server renders can run concurrently in the same process. If one render writes to shared module state and another render reads it, you can get race conditions, cross-request contamination, and security bugs where one user's data appears in another user's response.

Treat module scope on the server as process-wide shared memory, not request-local state.

Incorrect: request data leaks across concurrent renders

If two requests overlap, request A can set currentUser, then request B overwrites it before request A finishes rendering Dashboard.

Correct: keep request data local to the render tree

Safe exceptions:

Immutable static assets or config loaded once at module scope
Shared caches intentionally designed for cross-request reuse and keyed correctly
Process-wide singletons that do not store request- or user-specific mutable data

For static assets and config, see Hoist Static I/O to Module Level.

3.4 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:

typescript

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: 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

3.5 Hoist Static I/O to Module Level

Impact: HIGH (avoids repeated file/network I/O per request)

When loading static assets (fonts, logos, images, config files) in route handlers or server functions, hoist the I/O operation to module level. Module-level code runs once when the module is first imported, not on every request. This eliminates redundant file system reads or network fetches that would otherwise run on every invocation.

Incorrect: reads font file on every request

typescript

// app/api/og/route.tsximport { ImageResponse } from 'next/og'
export async function GET(request: Request) {  // Runs on EVERY request - expensive!  const fontData = await fetch(    new URL('./fonts/Inter.ttf', import.meta.url)  ).then(res => res.arrayBuffer())
  const logoData = await fetch(    new URL('./images/logo.png', import.meta.url)  ).then(res => res.arrayBuffer())
  return new ImageResponse(    <div style={{ fontFamily: 'Inter' }}>      <img src={logoData} />      Hello World    </div>,    { fonts: [{ name: 'Inter', data: fontData }] }  )}

Correct: loads once at module initialization

typescript

// app/api/og/route.tsximport { ImageResponse } from 'next/og'
// Module-level: runs ONCE when module is first importedconst fontData = fetch(  new URL('./fonts/Inter.ttf', import.meta.url)).then(res => res.arrayBuffer())
const logoData = fetch(  new URL('./images/logo.png', import.meta.url)).then(res => res.arrayBuffer())
export async function GET(request: Request) {  // Await the already-started promises  const [font, logo] = await Promise.all([fontData, logoData])
  return new ImageResponse(    <div style={{ fontFamily: 'Inter' }}>      <img src={logo} />      Hello World    </div>,    { fonts: [{ name: 'Inter', data: font }] }  )}

Correct: synchronous fs at module level

typescript

// app/api/og/route.tsximport { ImageResponse } from 'next/og'import { readFileSync } from 'fs'import { join } from 'path'
// Synchronous read at module level - blocks only during module initconst fontData = readFileSync(  join(process.cwd(), 'public/fonts/Inter.ttf'))
const logoData = readFileSync(  join(process.cwd(), 'public/images/logo.png'))
export async function GET(request: Request) {  return new ImageResponse(    <div style={{ fontFamily: 'Inter' }}>      <img src={logoData} />      Hello World    </div>,    { fonts: [{ name: 'Inter', data: fontData }] }  )}

Incorrect: reads config on every call

typescript

import fs from 'node:fs/promises'
export async function processRequest(data: Data) {  const config = JSON.parse(    await fs.readFile('./config.json', 'utf-8')  )  const template = await fs.readFile('./template.html', 'utf-8')
  return render(template, data, config)}

Correct: hoists config and template to module level

typescript

import fs from 'node:fs/promises'
const configPromise = fs  .readFile('./config.json', 'utf-8')  .then(JSON.parse)const templatePromise = fs.readFile('./template.html', 'utf-8')
export async function processRequest(data: Data) {  const [config, template] = await Promise.all([    configPromise,    templatePromise,  ])
  return render(template, data, config)}

When to use this pattern:

Loading fonts for OG image generation
Loading static logos, icons, or watermarks
Reading configuration files that don't change at runtime
Loading email templates or other static templates
Any static asset that's the same across all requests

When not to use this pattern:

Assets that vary per request or user
Files that may change during runtime (use caching with TTL instead)
Large files that would consume too much memory if kept loaded
Sensitive data that shouldn't persist in memory

With Vercel's Fluid Compute, module-level caching is especially effective because multiple concurrent requests share the same function instance. The static assets stay loaded in memory across requests without cold start penalties.

In traditional serverless, each cold start re-executes module-level code, but subsequent warm invocations reuse the loaded assets until the instance is recycled.

3.6 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

tsx

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

3.7 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

tsx

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

tsx

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:

tsx

async function Header() {  const data = await fetchHeader()  return <div>{data}</div>}
async function Sidebar() {  const items = await fetchSidebarItems()  return <nav>{items.map(renderItem)}</nav>}
function Layout({ children }: { children: ReactNode }) {  return (    <div>      <Header />      {children}    </div>  )}
export default function Page() {  return (    <Layout>      <Sidebar />    </Layout>  )}

3.8 Parallel Nested Data Fetching

Impact: CRITICAL (eliminates server-side waterfalls)

When fetching nested data in parallel, chain dependent fetches within each item's promise so a slow item doesn't block the rest.

Incorrect: a single slow item blocks all nested fetches

If one getChat(id) out of 100 is extremely slow, the authors of the other 99 chats can't start loading even though their data is ready.

Correct: each item chains its own nested fetch

Each item independently chains getChat → getUser, so a slow chat doesn't block author fetches for the others.

3.9 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:

typescript

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.

Avoid inline objects as arguments:

React.cache() uses shallow equality (Object.is) to determine cache hits. Inline objects create new references each call, preventing cache hits.

Incorrect: always cache miss

typescript

const getUser = cache(async (params: { uid: number }) => {  return await db.user.findUnique({ where: { id: params.uid } })})
// Each call creates new object, never hits cachegetUser({ uid: 1 })getUser({ uid: 1 })  // Cache miss, runs query again

Correct: cache hit

If you must pass objects, pass the same reference:

Next.js-Specific Note:

In Next.js, the fetch API is automatically extended with request memoization. Requests with the same URL and options are automatically deduplicated within a single request, so you don't need React.cache() for fetch calls. However, React.cache() is still essential for other async tasks:

Database queries (Prisma, Drizzle, etc.)
Heavy computations
Authentication checks
File system operations
Any non-fetch async work

Use React.cache() to deduplicate these operations across your component tree.

Reference: https://react.dev/reference/react/cache

3.10 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

tsx

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

tsx

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

tsx

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

tsx

import useSWRSubscription from 'swr/subscription'
// Module-level Map to track callbacks per keyconst 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 Passive Event Listeners for Scrolling Performance

Impact: MEDIUM (eliminates scroll delay caused by event listeners)

Add { passive: true } to touch and wheel event listeners to enable immediate scrolling. Browsers normally wait for listeners to finish to check if preventDefault() is called, causing scroll delay.

Incorrect:

typescript

useEffect(() => {  const handleTouch = (e: TouchEvent) => console.log(e.touches[0].clientX)  const handleWheel = (e: WheelEvent) => console.log(e.deltaY)    document.addEventListener('touchstart', handleTouch)  document.addEventListener('wheel', handleWheel)    return () => {    document.removeEventListener('touchstart', handleTouch)    document.removeEventListener('wheel', handleWheel)  }}, [])

Correct:

typescript

useEffect(() => {  const handleTouch = (e: TouchEvent) => console.log(e.touches[0].clientX)  const handleWheel = (e: WheelEvent) => console.log(e.deltaY)    document.addEventListener('touchstart', handleTouch, { passive: true })  document.addEventListener('wheel', handleWheel, { passive: true })    return () => {    document.removeEventListener('touchstart', handleTouch)    document.removeEventListener('wheel', handleWheel)  }}, [])

Use passive when: tracking/analytics, logging, any listener that doesn't call preventDefault().

Don't use passive when: implementing custom swipe gestures, custom zoom controls, or any listener that needs preventDefault().

4.3 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

Correct: multiple instances share one request

For immutable data:

For mutations:

Reference: https://swr.vercel.app

4.4 Version and Minimize localStorage Data

Impact: MEDIUM (prevents schema conflicts, reduces storage size)

Add version prefix to keys and store only needed fields. Prevents schema conflicts and accidental storage of sensitive data.

Incorrect:

Correct:

typescript

const VERSION = 'v2'
function saveConfig(config: { theme: string; language: string }) {  try {    localStorage.setItem(`userConfig:${VERSION}`, JSON.stringify(config))  } catch {    // Throws in incognito/private browsing, quota exceeded, or disabled  }}
function loadConfig() {  try {    const data = localStorage.getItem(`userConfig:${VERSION}`)    return data ? JSON.parse(data) : null  } catch {    return null  }}
// Migration from v1 to v2function migrate() {  try {    const v1 = localStorage.getItem('userConfig:v1')    if (v1) {      const old = JSON.parse(v1)      saveConfig({ theme: old.darkMode ? 'dark' : 'light', language: old.lang })      localStorage.removeItem('userConfig:v1')    }  } catch {}}

Store minimal fields from server responses:

typescript

// User object has 20+ fields, only store what UI needsfunction cachePrefs(user: FullUser) {  try {    localStorage.setItem('prefs:v1', JSON.stringify({      theme: user.preferences.theme,      notifications: user.preferences.notifications    }))  } catch {}}

Always wrap in try-catch: getItem() and setItem() throw in incognito/private browsing (Safari, Firefox), when quota exceeded, or when disabled.

Benefits: Schema evolution via versioning, reduced storage size, prevents storing tokens/PII/internal flags.

5. Re-render Optimization

Impact: MEDIUM

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

5.1 Calculate Derived State During Rendering

Impact: MEDIUM (avoids redundant renders and state drift)

If a value can be computed from current props/state, do not store it in state or update it in an effect. Derive it during render to avoid extra renders and state drift. Do not set state in effects solely in response to prop changes; prefer derived values or keyed resets instead.

Incorrect: redundant state and effect

tsx

function Form() {  const [firstName, setFirstName] = useState('First')  const [lastName, setLastName] = useState('Last')  const [fullName, setFullName] = useState('')
  useEffect(() => {    setFullName(firstName + ' ' + lastName)  }, [firstName, lastName])
  return <p>{fullName}</p>}

Correct: derive during render

Reference: https://react.dev/learn/you-might-not-need-an-effect

5.2 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

tsx

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

tsx

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>}

5.3 Do not wrap a simple expression with a primitive result type in useMemo

Impact: LOW-MEDIUM (wasted computation on every render)

When an expression is simple (few logical or arithmetical operators) and has a primitive result type (boolean, number, string), do not wrap it in useMemo.

Calling useMemo and comparing hook dependencies may consume more resources than the expression itself.

Incorrect:

tsx

function Header({ user, notifications }: Props) {  const isLoading = useMemo(() => {    return user.isLoading || notifications.isLoading  }, [user.isLoading, notifications.isLoading])
  if (isLoading) return <Skeleton />  // return some markup}

Correct:

5.4 Don't Define Components Inside Components

Impact: HIGH (prevents remount on every render)

Defining a component inside another component creates a new component type on every render. React sees a different component each time and fully remounts it, destroying all state and DOM.

A common reason developers do this is to access parent variables without passing props. Always pass props instead.

Incorrect: remounts on every render

tsx

function UserProfile({ user, theme }) {  // Defined inside to access `theme` - BAD  const Avatar = () => (    <img      src={user.avatarUrl}      className={theme === 'dark' ? 'avatar-dark' : 'avatar-light'}    />  )
  // Defined inside to access `user` - BAD  const Stats = () => (    <div>      <span>{user.followers} followers</span>      <span>{user.posts} posts</span>    </div>  )
  return (    <div>      <Avatar />      <Stats />    </div>  )}

Every time UserProfile renders, Avatar and Stats are new component types. React unmounts the old instances and mounts new ones, losing any internal state, running effects again, and recreating DOM nodes.

Correct: pass props instead

tsx

function Avatar({ src, theme }: { src: string; theme: string }) {  return (    <img      src={src}      className={theme === 'dark' ? 'avatar-dark' : 'avatar-light'}    />  )}
function Stats({ followers, posts }: { followers: number; posts: number }) {  return (    <div>      <span>{followers} followers</span>      <span>{posts} posts</span>    </div>  )}
function UserProfile({ user, theme }) {  return (    <div>      <Avatar src={user.avatarUrl} theme={theme} />      <Stats followers={user.followers} posts={user.posts} />    </div>  )}

Symptoms of this bug:

Input fields lose focus on every keystroke
Animations restart unexpectedly
useEffect cleanup/setup runs on every parent render
Scroll position resets inside the component

5.5 Extract Default Non-primitive Parameter Value from Memoized Component to Constant

Impact: MEDIUM (restores memoization by using a constant for default value)

When memoized component has a default value for some non-primitive optional parameter, such as an array, function, or object, calling the component without that parameter results in broken memoization. This is because new value instances are created on every rerender, and they do not pass strict equality comparison in memo().

To address this issue, extract the default value into a constant.

Incorrect: onClick has different values on every rerender

Correct: stable default value

5.6 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

tsx

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

tsx

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.

5.7 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

Correct: re-runs only when id changes

For derived state, compute outside effect:

tsx

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

5.8 Put Interaction Logic in Event Handlers

Impact: MEDIUM (avoids effect re-runs and duplicate side effects)

If a side effect is triggered by a specific user action (submit, click, drag), run it in that event handler. Do not model the action as state + effect; it makes effects re-run on unrelated changes and can duplicate the action.

Incorrect: event modeled as state + effect

tsx

function Form() {  const [submitted, setSubmitted] = useState(false)  const theme = useContext(ThemeContext)
  useEffect(() => {    if (submitted) {      post('/api/register')      showToast('Registered', theme)    }  }, [submitted, theme])
  return <button onClick={() => setSubmitted(true)}>Submit</button>}

Correct: do it in the handler

tsx

function Form() {  const theme = useContext(ThemeContext)
  function handleSubmit() {    post('/api/register')    showToast('Registered', theme)  }
  return <button onClick={handleSubmit}>Submit</button>}

Reference: https://react.dev/learn/removing-effect-dependencies#should-this-code-move-to-an-event-handler

5.9 Split Combined Hook Computations

Impact: MEDIUM (avoids recomputing independent steps)

When a hook contains multiple independent tasks with different dependencies, split them into separate hooks. A combined hook reruns all tasks when any dependency changes, even if some tasks don't use the changed value.

Incorrect: changing sortOrder recomputes filtering

tsx

const sortedProducts = useMemo(() => {  const filtered = products.filter((p) => p.category === category)  const sorted = filtered.toSorted((a, b) =>    sortOrder === "asc" ? a.price - b.price : b.price - a.price  )  return sorted}, [products, category, sortOrder])

Correct: filtering only recomputes when products or category change

tsx

const filteredProducts = useMemo(  () => products.filter((p) => p.category === category),  [products, category])
const sortedProducts = useMemo(  () =>    filteredProducts.toSorted((a, b) =>      sortOrder === "asc" ? a.price - b.price : b.price - a.price    ),  [filteredProducts, sortOrder])

This pattern also applies to useEffect when combining unrelated side effects:

Incorrect: both effects run when either dependency changes

Correct: effects run independently

Note: If your project has React Compiler enabled, it automatically optimizes dependency tracking and may handle some of these cases for you.

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

Correct: re-renders only when boolean changes

5.11 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

tsx

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

tsx

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:

Stable callback references - Callbacks don't need to be recreated when state changes
No stale closures - Always operates on the latest state value
Fewer dependencies - Simplifies dependency arrays and reduces memory leaks
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.

5.12 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

tsx

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

tsx

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.

5.13 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

tsx

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

tsx

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)  }, [])}

5.14 Use useDeferredValue for Expensive Derived Renders

Impact: MEDIUM (keeps input responsive during heavy computation)

When user input triggers expensive computations or renders, use useDeferredValue to keep the input responsive. The deferred value lags behind, allowing React to prioritize the input update and render the expensive result when idle.

Incorrect: input feels laggy while filtering

tsx

function Search({ items }: { items: Item[] }) {  const [query, setQuery] = useState('')  const filtered = items.filter(item => fuzzyMatch(item, query))
  return (    <>      <input value={query} onChange={e => setQuery(e.target.value)} />      <ResultsList results={filtered} />    </>  )}

Correct: input stays snappy, results render when ready

tsx

function Search({ items }: { items: Item[] }) {  const [query, setQuery] = useState('')  const deferredQuery = useDeferredValue(query)  const filtered = useMemo(    () => items.filter(item => fuzzyMatch(item, deferredQuery)),    [items, deferredQuery]  )  const isStale = query !== deferredQuery
  return (    <>      <input value={query} onChange={e => setQuery(e.target.value)} />      <div style={{ opacity: isStale ? 0.7 : 1 }}>        <ResultsList results={filtered} />      </div>    </>  )}

When to use:

Filtering/searching large lists
Expensive visualizations (charts, graphs) reacting to input
Any derived state that causes noticeable render delays

Note: Wrap the expensive computation in useMemo with the deferred value as a dependency, otherwise it still runs on every render.

Reference: https://react.dev/reference/react/useDeferredValue

5.15 Use useRef for Transient Values

Impact: MEDIUM (avoids unnecessary re-renders on frequent updates)

When a value changes frequently and you don't want a re-render on every update (e.g., mouse trackers, intervals, transient flags), store it in useRef instead of useState. Keep component state for UI; use refs for temporary DOM-adjacent values. Updating a ref does not trigger a re-render.

Incorrect: renders every update

tsx

function Tracker() {  const [lastX, setLastX] = useState(0)
  useEffect(() => {    const onMove = (e: MouseEvent) => setLastX(e.clientX)    window.addEventListener('mousemove', onMove)    return () => window.removeEventListener('mousemove', onMove)  }, [])
  return (    <div      style={{        position: 'fixed',        top: 0,        left: lastX,        width: 8,        height: 8,        background: 'black',      }}    />  )}

Correct: no re-render for tracking

tsx

function Tracker() {  const lastXRef = useRef(0)  const dotRef = useRef<HTMLDivElement>(null)
  useEffect(() => {    const onMove = (e: MouseEvent) => {      lastXRef.current = e.clientX      const node = dotRef.current      if (node) {        node.style.transform = `translateX(${e.clientX}px)`      }    }    window.addEventListener('mousemove', onMove)    return () => window.removeEventListener('mousemove', onMove)  }, [])
  return (    <div      ref={dotRef}      style={{        position: 'fixed',        top: 0,        left: 0,        width: 8,        height: 8,        background: 'black',        transform: 'translateX(0px)',      }}    />  )}

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

tsx

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

tsx

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:

Example:

tsx

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

Correct: reuses same element

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

Correct: 1 decimal place

Automate with SVGO:

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

tsx

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

tsx

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

tsx

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 Suppress Expected Hydration Mismatches

Impact: LOW-MEDIUM (avoids noisy hydration warnings for known differences)

In SSR frameworks (e.g., Next.js), some values are intentionally different on server vs client (random IDs, dates, locale/timezone formatting). For these expected mismatches, wrap the dynamic text in an element with suppressHydrationWarning to prevent noisy warnings. Do not use this to hide real bugs. Don’t overuse it.

Incorrect: known mismatch warnings

Correct: suppress expected mismatch only

6.7 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:

Avoids expensive re-renders and state loss.

6.8 Use defer or async on Script Tags

Impact: HIGH (eliminates render-blocking)

Script tags without defer or async block HTML parsing while the script downloads and executes. This delays First Contentful Paint and Time to Interactive.

defer: Downloads in parallel, executes after HTML parsing completes, maintains execution order
async: Downloads in parallel, executes immediately when ready, no guaranteed order

Use defer for scripts that depend on DOM or other scripts. Use async for independent scripts like analytics.

Incorrect: blocks rendering

tsx

export default function Document() {  return (    <html>      <head>        <script src="https://example.com/analytics.js" />        <script src="/scripts/utils.js" />      </head>      <body>{/* content */}</body>    </html>  )}

Correct: non-blocking

tsx

import Script from 'next/script'
export default function Page() {  return (    <>      <Script src="https://example.com/analytics.js" strategy="afterInteractive" />      <Script src="/scripts/utils.js" strategy="beforeInteractive" />    </>  )}

Note: In Next.js, prefer the next/script component with strategy prop instead of raw script tags:

Reference: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script#defer

6.9 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

tsx

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

tsx

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>

6.10 Use React DOM Resource Hints

Impact: HIGH (reduces load time for critical resources)

React DOM provides APIs to hint the browser about resources it will need. These are especially useful in server components to start loading resources before the client even receives the HTML.

prefetchDNS(href): Resolve DNS for a domain you expect to connect to
preconnect(href): Establish connection (DNS + TCP + TLS) to a server
preload(href, options): Fetch a resource (stylesheet, font, script, image) you'll use soon
preloadModule(href): Fetch an ES module you'll use soon
preinit(href, options): Fetch and evaluate a stylesheet or script
preinitModule(href): Fetch and evaluate an ES module

Example: preconnect to third-party APIs

tsx

import { preconnect, prefetchDNS } from 'react-dom'
export default function App() {  prefetchDNS('https://analytics.example.com')  preconnect('https://api.example.com')
  return <main>{/* content */}</main>}

Example: preload critical fonts and styles

tsx

import { preload, preinit } from 'react-dom'
export default function RootLayout({ children }) {  // Preload font file  preload('/fonts/inter.woff2', { as: 'font', type: 'font/woff2', crossOrigin: 'anonymous' })
  // Fetch and apply critical stylesheet immediately  preinit('/styles/critical.css', { as: 'style' })
  return (    <html>      <body>{children}</body>    </html>  )}

Example: preload modules for code-split routes

tsx

import { preloadModule, preinitModule } from 'react-dom'
function Navigation() {  const preloadDashboard = () => {    preloadModule('/dashboard.js', { as: 'script' })  }
  return (    <nav>      <a href="/dashboard" onMouseEnter={preloadDashboard}>        Dashboard      </a>    </nav>  )}

When to use each:

| API | Use case |

|-----|----------|

| prefetchDNS | Third-party domains you'll connect to later |

| preconnect | APIs or CDNs you'll fetch from immediately |

| preload | Critical resources needed for current page |

| preloadModule | JS modules for likely next navigation |

| preinit | Stylesheets/scripts that must execute early |

| preinitModule | ES modules that must execute early |

Reference: https://react.dev/reference/react-dom#resource-preloading-apis

6.11 Use useTransition Over Manual Loading States

Impact: LOW (reduces re-renders and improves code clarity)

Use useTransition instead of manual useState for loading states. This provides built-in isPending state and automatically manages transitions.

Incorrect: manual loading state

tsx

function SearchResults() {  const [query, setQuery] = useState('')  const [results, setResults] = useState([])  const [isLoading, setIsLoading] = useState(false)
  const handleSearch = async (value: string) => {    setIsLoading(true)    setQuery(value)    const data = await fetchResults(value)    setResults(data)    setIsLoading(false)  }
  return (    <>      <input onChange={(e) => handleSearch(e.target.value)} />      {isLoading && <Spinner />}      <ResultsList results={results} />    </>  )}

Correct: useTransition with built-in pending state

tsx

import { useTransition, useState } from 'react'
function SearchResults() {  const [query, setQuery] = useState('')  const [results, setResults] = useState([])  const [isPending, startTransition] = useTransition()
  const handleSearch = (value: string) => {    setQuery(value) // Update input immediately        startTransition(async () => {      // Fetch and update results      const data = await fetchResults(value)      setResults(data)    })  }
  return (    <>      <input onChange={(e) => handleSearch(e.target.value)} />      {isPending && <Spinner />}      <ResultsList results={results} />    </>  )}

Benefits:

Automatic pending state: No need to manually manage setIsLoading(true/false)
Error resilience: Pending state correctly resets even if the transition throws
Better responsiveness: Keeps the UI responsive during updates
Interrupt handling: New transitions automatically cancel pending ones

Reference: https://react.dev/reference/react/useTransition

7. JavaScript Performance

Impact: LOW-MEDIUM

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

7.1 Avoid Layout Thrashing

Impact: MEDIUM (prevents forced synchronous layouts and reduces performance bottlenecks)

Avoid interleaving style writes with layout reads. When you read a layout property (like offsetWidth, getBoundingClientRect(), or getComputedStyle()) between style changes, the browser is forced to trigger a synchronous reflow.

This is OK: browser batches style changes

typescript

function updateElementStyles(element: HTMLElement) {  // Each line invalidates style, but browser batches the recalculation  element.style.width = '100px'  element.style.height = '200px'  element.style.backgroundColor = 'blue'  element.style.border = '1px solid black'}

Incorrect: interleaved reads and writes force reflows

typescript

function layoutThrashing(element: HTMLElement) {  element.style.width = '100px'  const width = element.offsetWidth  // Forces reflow  element.style.height = '200px'  const height = element.offsetHeight  // Forces another reflow}

Correct: batch writes, then read once

typescript

function updateElementStyles(element: HTMLElement) {  // Batch all writes together  element.style.width = '100px'  element.style.height = '200px'  element.style.backgroundColor = 'blue'  element.style.border = '1px solid black'    // Read after all writes are done (single reflow)  const { width, height } = element.getBoundingClientRect()}

Correct: batch reads, then writes

Better: use CSS classes

React example:

tsx

// Incorrect: interleaving style changes with layout queriesfunction Box({ isHighlighted }: { isHighlighted: boolean }) {  const ref = useRef<HTMLDivElement>(null)    useEffect(() => {    if (ref.current && isHighlighted) {      ref.current.style.width = '100px'      const width = ref.current.offsetWidth // Forces layout      ref.current.style.height = '200px'    }  }, [isHighlighted])    return <div ref={ref}>Content</div>}
// Correct: toggle classfunction Box({ isHighlighted }: { isHighlighted: boolean }) {  return (    <div className={isHighlighted ? 'highlighted-box' : ''}>      Content    </div>  )}

Prefer CSS classes over inline styles when possible. CSS files are cached by the browser, and classes provide better separation of concerns and are easier to maintain.

See this gist and CSS Triggers for more information on layout-forcing operations.

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):

Correct (O(1) per lookup):

typescript

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

Correct: 1 lookup total

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

typescript

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

typescript

// Module-level cacheconst 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:

typescript

let isLoggedInCache: boolean | null = null
function isLoggedIn(): boolean {  if (isLoggedInCache !== null) {    return isLoggedInCache  }    isLoggedInCache = document.cookie.includes('auth=')  return isLoggedInCache}
// Clear cache when auth changesfunction 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

Correct: Map cache

typescript

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:

typescript

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

typescript

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

Correct: 1 iteration

typescript

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 Defer Non-Critical Work with requestIdleCallback

Impact: MEDIUM (keeps UI responsive during background tasks)

Use requestIdleCallback() to schedule non-critical work during browser idle periods. This keeps the main thread free for user interactions and animations, reducing jank and improving perceived performance.

Incorrect: blocks main thread during user interaction

typescript

function handleSearch(query: string) {  const results = searchItems(query)  setResults(results)
  // These block the main thread immediately  analytics.track('search', { query })  saveToRecentSearches(query)  prefetchTopResults(results.slice(0, 3))}

Correct: defers non-critical work to idle time

typescript

function handleSearch(query: string) {  const results = searchItems(query)  setResults(results)
  // Defer non-critical work to idle periods  requestIdleCallback(() => {    analytics.track('search', { query })  })
  requestIdleCallback(() => {    saveToRecentSearches(query)  })
  requestIdleCallback(() => {    prefetchTopResults(results.slice(0, 3))  })}

With timeout for required work:

Chunking large tasks:

typescript

function processLargeDataset(items: Item[]) {  let index = 0
  function processChunk(deadline: IdleDeadline) {    // Process items while we have idle time (aim for <50ms chunks)    while (index < items.length && deadline.timeRemaining() > 0) {      processItem(items[index])      index++    }
    // Schedule next chunk if more items remain    if (index < items.length) {      requestIdleCallback(processChunk)    }  }
  requestIdleCallback(processChunk)}

With fallback for unsupported browsers:

When to use:

Analytics and telemetry
Saving state to localStorage/IndexedDB
Prefetching resources for likely next actions
Processing non-urgent data transformations
Lazy initialization of non-critical features

When NOT to use:

User-initiated actions that need immediate feedback
Rendering updates the user is waiting for
Time-sensitive operations

7.8 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

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):

typescript

function hasChanges(current: string[], original: string[]) {  // Early return if lengths differ  if (current.length !== original.length) {    return true  }  // Only sort 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

7.9 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

typescript

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

typescript

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 }}

7.10 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

Correct: memoize or hoist

tsx

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:

7.11 Use flatMap to Map and Filter in One Pass

Impact: LOW-MEDIUM (eliminates intermediate array)

Chaining .map().filter(Boolean) creates an intermediate array and iterates twice. Use .flatMap() to transform and filter in a single pass.

Incorrect: 2 iterations, intermediate array

Correct: 1 iteration, no intermediate array

More examples:

typescript

// Extract valid emails from responses// Beforeconst emails = responses  .map(r => r.success ? r.data.email : null)  .filter(Boolean)
// Afterconst emails = responses.flatMap(r =>  r.success ? [r.data.email] : [])
// Parse and filter valid numbers// Beforeconst numbers = strings  .map(s => parseInt(s, 10))  .filter(n => !isNaN(n))
// Afterconst numbers = strings.flatMap(s => {  const n = parseInt(s, 10)  return isNaN(n) ? [] : [n]})

When to use:

Transforming items while filtering some out
Conditional mapping where some inputs produce no output
Parsing/validating where invalid inputs should be skipped

7.12 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):

typescript

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):

Still sorts unnecessarily when only min/max are needed.

Correct (O(n) - single loop):

typescript

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

This works for small arrays, but can be slower or just throw an error for very large arrays due to spread operator limitations. Maximal array length is approximately 124000 in Chrome 143 and 638000 in Safari 18; exact numbers may vary - see the fiddle. Use the loop approach for reliability.

7.13 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):

Correct (O(1) per check):

7.14 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

typescript

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

typescript

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:

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

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 Do Not Put Effect Events in Dependency Arrays

Impact: LOW (avoids unnecessary effect re-runs and lint errors)

Effect Event functions do not have a stable identity. Their identity intentionally changes on every render. Do not include the function returned by useEffectEvent in a useEffect dependency array. Keep the actual reactive values as dependencies and call the Effect Event from inside the effect body or subscriptions created by that effect.

Incorrect: Effect Event added as a dependency

tsx

import { useEffect, useEffectEvent } from 'react'
function ChatRoom({ roomId, onConnected }: {  roomId: string  onConnected: () => void}) {  const handleConnected = useEffectEvent(onConnected)
  useEffect(() => {    const connection = createConnection(roomId)    connection.on('connected', handleConnected)    connection.connect()
    return () => connection.disconnect()  }, [roomId, handleConnected])}

Including the Effect Event in dependencies makes the effect re-run every render and triggers the React Hooks lint rule.

Correct: depend on reactive values, not the Effect Event

tsx

import { useEffect, useEffectEvent } from 'react'
function ChatRoom({ roomId, onConnected }: {  roomId: string  onConnected: () => void}) {  const handleConnected = useEffectEvent(onConnected)
  useEffect(() => {    const connection = createConnection(roomId)    connection.on('connected', handleConnected)    connection.connect()
    return () => connection.disconnect()  }, [roomId])}

Reference: https://react.dev/reference/react/useEffectEvent#effect-event-in-deps

8.2 Initialize App Once, Not Per Mount

Impact: LOW-MEDIUM (avoids duplicate init in development)

Do not put app-wide initialization that must run once per app load inside useEffect([]) of a component. Components can remount and effects will re-run. Use a module-level guard or top-level init in the entry module instead.

Incorrect: runs twice in dev, re-runs on remount

Correct: once per app load

Reference: https://react.dev/learn/you-might-not-need-an-effect#initializing-the-application

8.3 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

tsx

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

Correct: stable subscription

tsx

import { useEffectEvent } from 'react'
function useWindowEvent(event: string, handler: (e) => 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.

8.4 useEffectEvent 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.

Incorrect: effect re-runs on every callback change

tsx

function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {  const [query, setQuery] = useState('')
  useEffect(() => {    const timeout = setTimeout(() => onSearch(query), 300)    return () => clearTimeout(timeout)  }, [query, onSearch])}

Correct: using React's useEffectEvent

tsx

import { useEffectEvent } from 'react';
function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {  const [query, setQuery] = useState('')  const onSearchEvent = useEffectEvent(onSearch)
  useEffect(() => {    const timeout = setTimeout(() => onSearchEvent(query), 300)    return () => clearTimeout(timeout)  }, [query])}

vercel-react-best-practices

Install

React Best Practices

Abstract

Table of Contents

1. Eliminating Waterfalls

1.1 Check Cheap Conditions Before Async Flags

1.2 Defer Await Until Needed

1.3 Dependency-Based Parallelization

1.4 Prevent Waterfall Chains in API Routes

1.5 Promise.all() for Independent Operations

1.6 Strategic Suspense Boundaries

2. Bundle Size Optimization

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 Prefer Statically Analyzable Paths

2.6 Preload Based on User Intent

3. Server-Side Performance

3.1 Authenticate Server Actions Like API Routes

3.2 Avoid Duplicate Serialization in RSC Props

3.3 Avoid Shared Module State for Request Data

3.4 Cross-Request LRU Caching

3.5 Hoist Static I/O to Module Level

3.6 Minimize Serialization at RSC Boundaries

3.7 Parallel Data Fetching with Component Composition

3.8 Parallel Nested Data Fetching

3.9 Per-Request Deduplication with React.cache()

3.10 Use after() for Non-Blocking Operations

4. Client-Side Data Fetching

4.1 Deduplicate Global Event Listeners

4.2 Use Passive Event Listeners for Scrolling Performance

4.3 Use SWR for Automatic Deduplication

4.4 Version and Minimize localStorage Data

5. Re-render Optimization

5.1 Calculate Derived State During Rendering

5.2 Defer State Reads to Usage Point

5.3 Do not wrap a simple expression with a primitive result type in useMemo

5.4 Don't Define Components Inside Components

5.5 Extract Default Non-primitive Parameter Value from Memoized Component to Constant

5.6 Extract to Memoized Components

5.7 Narrow Effect Dependencies

5.8 Put Interaction Logic in Event Handlers

5.9 Split Combined Hook Computations

5.10 Subscribe to Derived State

5.11 Use Functional setState Updates

5.12 Use Lazy State Initialization

5.13 Use Transitions for Non-Urgent Updates

5.14 Use useDeferredValue for Expensive Derived Renders

5.15 Use useRef for Transient Values

6. Rendering Performance

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 Suppress Expected Hydration Mismatches

6.7 Use Activity Component for Show/Hide

6.8 Use defer or async on Script Tags

6.9 Use Explicit Conditional Rendering

6.10 Use React DOM Resource Hints

6.11 Use useTransition Over Manual Loading States

7. JavaScript Performance

7.1 Avoid Layout Thrashing

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 Defer Non-Critical Work with requestIdleCallback

7.8 Early Length Check for Array Comparisons

7.9 Early Return from Functions

7.10 Hoist RegExp Creation

7.11 Use flatMap to Map and Filter in One Pass

7.12 Use Loop for Min/Max Instead of Sort

7.13 Use Set/Map for O(1) Lookups

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

8. Advanced Patterns

8.1 Do Not Put Effect Events in Dependency Arrays