tomevault-io 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
- Eliminating Waterfalls — CRITICAL
- Bundle Size Optimization — CRITICAL
- Server-Side Performance — HIGH
- Client-Side Data Fetching — MEDIUM-HIGH
- Re-render Optimization — MEDIUM
- Rendering Performance — MEDIUM
- 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
- 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 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
>
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: 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 redirectsWorks 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 => 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
}
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
}
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} />
}
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} />
}
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.
5.6 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} />
}
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} />
}
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
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