wehub-resource-sync
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React Best Practices - Complete Document
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.
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
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
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
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
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
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.
2. Bundle Size Optimization
Impact: CRITICAL
Reducing initial bundle size improves Time to Interactive and Largest Contentful Paint.
2.1 Avoid Barrel File Imports
Import directly from source files instead of barrel files to avoid loading thousands of unused modules. Barrel files are entry points that re-export multiple modules (e.g., index.js that does export * from './module').
Popular icon and component libraries can have up to 10,000 re-exports in their entry file. For many React packages, it takes 200-800ms just to import them, affecting both development speed and production cold starts.
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
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
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
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
Preload heavy bundles before they're needed to reduce perceived latency.
Example: preload on hover/focus
function EditorButton({ onClick }: { onClick: () => void }) {
const preload = () => {
if (typeof window !== 'undefined') {
void import('./monaco-editor')
}
}
return (
<button
onMouseEnter={preload}
onFocus={preload}
onClick={onClick}
>
Open Editor
</button>
)
}
Example: preload when feature flag is enabled
function FlagsProvider({ children, flags }: Props) {
useEffect(() => {
if (flags.editorEnabled && typeof window !== 'undefined') {
void import('./monaco-editor').then(mod => mod.init())
}
}, [flags.editorEnabled])
return <FlagsContext.Provider value={flags}>
{children}
</FlagsContext.Provider>
}
The typeof window !== 'undefined' check prevents bundling preloaded modules for SSR, optimizing server bundle size and build speed.
3. Server-Side Performance
Impact: HIGH
Optimizing server-side rendering and data fetching eliminates server-side waterfalls and reduces response times.
3.1 Cross-Request LRU Caching
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. In serverless, consider Redis for cross-process caching.
Reference: https://github.com/isaacs/node-lru-cache
3.2 Minimize Serialization at RSC Boundaries
The React Server/Client boundary serializes all object properties. 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
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()
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.
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
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
SWR enables request deduplication, caching, and revalidation across component instances.
Incorrect: no deduplication, each instance fetches
function UserList() {
const [users, setUsers] = useState([])
useEffect(() => {
fetch('/api/users')
.then(r => r.json())
.then(setUsers)
}, [])
}
Correct: multiple instances share one request
import useSWR from 'swr'
function UserList() {
const { data: users } = useSWR('/api/users', fetcher)
}
For immutable data:
import { useImmutableSWR } from '@/lib/swr'
function StaticContent() {
const { data } = useImmutableSWR('/api/config', fetcher)
}
For mutations:
import { useSWRMutation } from 'swr/mutation'
function UpdateButton() {
const { trigger } = useSWRMutation('/api/user', updateUser)
return <button onClick={() => trigger()}>Update</button>
}
Reference: https://swr.vercel.app
5. Re-render Optimization
Impact: MEDIUM
Reducing unnecessary re-renders minimizes wasted computation and improves UI responsiveness.
5.1 Defer State Reads to Usage Point
Don't subscribe to dynamic state (searchParams, localStorage) if you only read it inside callbacks.
Incorrect: subscribes to all searchParams changes
function ShareButton({ chatId }: { chatId: string }) {
const searchParams = useSearchParams()
const handleShare = () => {
const ref = searchParams.get('ref')
shareChat(chatId, { ref })
}
return <button onClick={handleShare}>Share</button>
}
Correct: reads on demand, no subscription
function ShareButton({ chatId }: { chatId: string }) {
const handleShare = () => {
const params = new URLSearchParams(window.location.search)
const ref = params.get('ref')
shareChat(chatId, { ref })
}
return <button onClick={handleShare}>Share</button>
}
5.2 Extract to Memoized Components
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>
)
}
5.3 Narrow Effect Dependencies
Specify primitive dependencies instead of objects to minimize effect re-runs.
Incorrect: re-runs on any user field change
useEffect(() => {
console.log(user.id)
}, [user])
Correct: re-runs only when id changes
useEffect(() => {
console.log(user.id)
}, [user.id])
For derived state, compute outside effect:
// Incorrect: runs on width=767, 766, 765...
useEffect(() => {
if (width < 768) {
enableMobileMode()
}
}, [width])
// Correct: runs only on boolean transition
const isMobile = width < 768
useEffect(() => {
if (isMobile) {
enableMobileMode()
}
}, [isMobile])
5.4 Subscribe to Derived State
Subscribe to derived boolean state instead of continuous values to reduce re-render frequency.
Incorrect: re-renders on every pixel change
function Sidebar() {
const width = useWindowWidth() // updates continuously
const isMobile = width < 768
return <nav className={isMobile ? 'mobile' : 'desktop'}>
}
Correct: re-renders only when boolean changes
function Sidebar() {
const isMobile = useMediaQuery('(max-width: 767px)')
return <nav className={isMobile ? 'mobile' : 'desktop'}>
}
5.5 Use Lazy State Initialization
Pass a function to useState for expensive initial values. Without the function form, the initializer runs on every render even though the value is only used once.
Incorrect: runs on every render
function FilteredList({ items }: { items: Item[] }) {
// buildSearchIndex() runs on EVERY render, even after initialization
const [searchIndex, setSearchIndex] = useState(buildSearchIndex(items))
const [query, setQuery] = useState('')
// When query changes, buildSearchIndex runs again unnecessarily
return <SearchResults index={searchIndex} query={query} />
}
function UserProfile() {
// JSON.parse runs on every render
const [settings, setSettings] = useState(
JSON.parse(localStorage.getItem('settings') || '{}')
)
return <SettingsForm settings={settings} onChange={setSettings} />
}
Correct: runs only once
function FilteredList({ items }: { items: Item[] }) {
// buildSearchIndex() runs ONLY on initial render
const [searchIndex, setSearchIndex] = useState(() => buildSearchIndex(items))
const [query, setQuery] = useState('')
return <SearchResults index={searchIndex} query={query} />
}
function UserProfile() {
// JSON.parse runs only on initial render
const [settings, setSettings] = useState(() => {
const stored = localStorage.getItem('settings')
return stored ? JSON.parse(stored) : {}
})
return <SettingsForm settings={settings} onChange={setSettings} />
}
Use lazy initialization when computing initial values from localStorage/sessionStorage, building data structures (indexes, maps), reading from the DOM, or performing heavy transformations.
For simple primitives (useState(0)), direct references (useState(props.value)), or cheap literals (useState({})), the function form is unnecessary.
5.6 Use Transitions for Non-Urgent Updates
Mark frequent, non-urgent state updates as transitions to maintain UI responsiveness.
Incorrect: blocks UI on every scroll
function ScrollTracker() {
const [scrollY, setScrollY] = useState(0)
useEffect(() => {
const handler = () => setScrollY(window.scrollY)
window.addEventListener('scroll', handler, { passive: true })
return () => window.removeEventListener('scroll', handler)
}, [])
}
Correct: non-blocking updates
import { startTransition } from 'react'
function ScrollTracker() {
const [scrollY, setScrollY] = useState(0)
useEffect(() => {
const handler = () => {
startTransition(() => setScrollY(window.scrollY))
}
window.addEventListener('scroll', handler, { passive: true })
return () => window.removeEventListener('scroll', handler)
}, [])
}
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
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
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
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.
6.4 Optimize SVG Precision
Reduce SVG coordinate precision to decrease file size. The optimal precision depends on the viewBox size, but in general reducing precision should be considered.
Incorrect: excessive precision
<path d="M 10.293847 20.847362 L 30.938472 40.192837" />
Correct: 1 decimal place
<path d="M 10.3 20.8 L 30.9 40.2" />
Automate with SVGO:
npx svgo --precision=1 --multipass icon.svg
6.5 Prevent Hydration Mismatch Without Flickering
When rendering content that depends on client-side storage (localStorage, cookies), avoid both SSR breakage and post-hydration flickering by injecting a synchronous script that updates the DOM before React hydrates.
Incorrect: breaks SSR
function ThemeWrapper({ children }: { children: ReactNode }) {
// localStorage is not available on server - throws error
const theme = localStorage.getItem('theme') || 'light'
return (
<div className={theme}>
{children}
</div>
)
}
Server-side rendering will fail because localStorage is undefined.
Incorrect: visual flickering
function ThemeWrapper({ children }: { children: ReactNode }) {
const [theme, setTheme] = useState('light')
useEffect(() => {
// Runs after hydration - causes visible flash
const stored = localStorage.getItem('theme')
if (stored) {
setTheme(stored)
}
}, [])
return (
<div className={theme}>
{children}
</div>
)
}
Component first renders with default value (light), then updates after hydration, causing a visible flash of incorrect content.
Correct: no flicker, no hydration mismatch
function ThemeWrapper({ children }: { children: ReactNode }) {
return (
<>
<div id="theme-wrapper">
{children}
</div>
<script
dangerouslySetInnerHTML={{
__html: `
(function() {
try {
var theme = localStorage.getItem('theme') || 'light';
var el = document.getElementById('theme-wrapper');
if (el) el.className = theme;
} catch (e) {}
})();
`,
}}
/>
</>
)
}
The inline script executes synchronously before showing the element, ensuring the DOM already has the correct value. No flickering, no hydration mismatch.
This pattern is especially useful for theme toggles, user preferences, authentication states, and any client-only data that should render immediately without flashing default values.
6.6 Use Activity Component for Show/Hide
Use React's <Activity> to preserve state/DOM for expensive components that frequently toggle visibility.
Usage:
import { Activity } from 'react'
function Dropdown({ isOpen }: Props) {
return (
<Activity mode={isOpen ? 'visible' : 'hidden'}>
<ExpensiveMenu />
</Activity>
)
}
Avoids expensive re-renders and state loss.
6.7 Use Explicit Conditional Rendering
Use explicit ternary operators (? :) instead of && for conditional rendering when the condition can be 0, NaN, or other falsy values that render.
Incorrect: renders "0" when count is 0
function Badge({ count }: { count: number }) {
return (
<div>
{count && <span className="badge">{count}</span>}
</div>
)
}
// When count = 0, renders: <div>0</div>
// When count = 5, renders: <div><span class="badge">5</span></div>
Correct: renders nothing when count is 0
function Badge({ count }: { count: number }) {
return (
<div>
{count > 0 ? <span className="badge">{count}</span> : null}
</div>
)
}
// When count = 0, renders: <div></div>
// When count = 5, renders: <div><span class="badge">5</span></div>
7. JavaScript Performance
Impact: LOW-MEDIUM
Micro-optimizations for hot paths can add up to meaningful improvements.
7.1 Batch DOM CSS Changes
Avoid changing styles one property at a time. Group multiple CSS changes together via classes or cssText to minimize browser reflows.
Incorrect: multiple reflows
function updateElementStyles(element: HTMLElement) {
// Each line triggers a reflow
element.style.width = '100px'
element.style.height = '200px'
element.style.backgroundColor = 'blue'
element.style.border = '1px solid black'
}
Correct: add class - single reflow
// CSS file
.highlighted-box {
width: 100px;
height: 200px;
background-color: blue;
border: 1px solid black;
}
// JavaScript
function updateElementStyles(element: HTMLElement) {
element.classList.add('highlighted-box')
}
Correct: change cssText - single reflow
function updateElementStyles(element: HTMLElement) {
element.style.cssText = `
width: 100px;
height: 200px;
background-color: blue;
border: 1px solid black;
`
}
React example:
// Incorrect: changing styles one by one
function Box({ isHighlighted }: { isHighlighted: boolean }) {
const ref = useRef<HTMLDivElement>(null)
useEffect(() => {
if (ref.current && isHighlighted) {
ref.current.style.width = '100px'
ref.current.style.height = '200px'
ref.current.style.backgroundColor = 'blue'
}
}, [isHighlighted])
return <div ref={ref}>Content</div>
}
// Correct: toggle class
function Box({ isHighlighted }: { isHighlighted: boolean }) {
return (
<div className={isHighlighted ? 'highlighted-box' : ''}>
Content
</div>
)
}
Prefer CSS classes over inline styles when possible. Classes are cached by the browser and provide better separation of concerns.
7.2 Build Index Maps for Repeated Lookups
Multiple .find() calls by the same key should use a Map.
Incorrect (O(n) per lookup):
function processOrders(orders: Order[], users: User[]) {
return orders.map(order => ({
...order,
user: users.find(u => u.id === order.userId)
}))
}
Correct (O(1) per lookup):
function processOrders(orders: Order[], users: User[]) {
const userById = new Map(users.map(u => [u.id, u]))
return orders.map(order => ({
...order,
user: userById.get(order.userId)
}))
}
Build map once (O(n)), then all lookups are O(1).
For 1000 orders × 1000 users: 1M ops → 2K ops.
7.3 Cache Property Access in Loops
Cache object property lookups in hot paths.
Incorrect: 3 lookups × N iterations
for (let i = 0; i < arr.length; i++) {
process(obj.config.settings.value)
}
Correct: 1 lookup total
const value = obj.config.settings.value
const len = arr.length
for (let i = 0; i < len; i++) {
process(value)
}
7.4 Cache Repeated Function Calls
Use a module-level Map to cache function results when the same function is called repeatedly with the same inputs during render.
Incorrect: redundant computation
function ProjectList({ projects }: { projects: Project[] }) {
return (
<div>
{projects.map(project => {
// slugify() called 100+ times for same project names
const slug = slugify(project.name)
return <ProjectCard key={project.id} slug={slug} />
})}
</div>
)
}
Correct: cached results
// Module-level cache
const cachedSlugify = new Map<string, any>()
function getCachedSlug(text: string): string {
if (cachedSlugify.has(text)) {
return cachedSlugify.get(text)
}
const result = slugify(text)
cachedSlugify.set(text, result)
return result
}
function ProjectList({ projects }: { projects: Project[] }) {
return (
<div>
{projects.map(project => {
// Computed only once per unique project name
const slug = getCachedSlug(project.name)
return <ProjectCard key={project.id} slug={slug} />
})}
</div>
)
}
Simpler pattern for single-value functions:
let isLoggedInCache: boolean | null = null
function isLoggedIn(): boolean {
if (isLoggedInCache !== null) {
return isLoggedInCache
}
isLoggedInCache = document.cookie.includes('auth=')
return isLoggedInCache
}
// Clear cache when auth changes
function onAuthChange() {
isLoggedInCache = null
}
Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components.
Reference: https://vercel.com/blog/how-we-made-the-vercel-dashboard-twice-as-fast
7.5 Cache Storage API Calls
localStorage, sessionStorage, and document.cookie are synchronous and expensive. Cache reads in memory.
Incorrect: reads storage on every call
function getTheme() {
return localStorage.getItem('theme') ?? 'light'
}
// Called 10 times = 10 storage reads
Correct: Map cache
const storageCache = new Map<string, string | null>()
function getLocalStorage(key: string) {
if (!storageCache.has(key)) {
storageCache.set(key, localStorage.getItem(key))
}
return storageCache.get(key)
}
function setLocalStorage(key: string, value: string) {
localStorage.setItem(key, value)
storageCache.set(key, value) // keep cache in sync
}
Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components.
Cache invalidation:
window.addEventListener('storage', (e) => {
if (e.key) storageCache.delete(e.key)
})
document.addEventListener('visibilitychange', () => {
if (document.visibilityState === 'visible') {
storageCache.clear()
}
})
7.6 Combine Multiple Array Iterations
Multiple .filter() or .map() calls iterate the array multiple times. Combine into one loop.
Incorrect: 3 iterations
const admins = users.filter(u => u.isAdmin)
const testers = users.filter(u => u.isTester)
const inactive = users.filter(u => !u.isActive)
Correct: 1 iteration
const admins: User[] = []
const testers: User[] = []
const inactive: User[] = []
for (const user of users) {
if (user.isAdmin) admins.push(user)
if (user.isTester) testers.push(user)
if (!user.isActive) inactive.push(user)
}
7.7 Early Length Check for Array Comparisons
When comparing arrays with expensive operations (sorting, deep equality, serialization), check lengths first. If lengths differ, the arrays cannot be equal.
In real-world applications, this optimization is especially valuable when the comparison runs in hot paths (event handlers, render loops).
Incorrect: always runs expensive comparison
function hasChanges(current: string[], original: string[]) {
// Always sorts and joins, even when lengths differ
return current.sort().join() !== original.sort().join()
}
Two O(n log n) sorts run even when current.length is 5 and original.length is 100.
Correct (O(1) length check first):
function hasChanges(current: string[], original: string[]) {
// Early return if lengths differ
if (current.length !== original.length) {
return true
}
// Only sort 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 approach avoids sorting when lengths differ, avoiding both computational overhead and memory consumption for joined strings.
7.8 Early Return from Functions
Return early when result is determined to skip unnecessary processing.
Incorrect: processes all items even after finding answer
function validateUsers(users: User[]) {
let hasError = false
let errorMessage = ''
for (const user of users) {
if (!user.email) {
hasError = true
errorMessage = 'Email required'
}
if (!user.name) {
hasError = true
errorMessage = 'Name required'
}
// Continues checking all users even after error found
}
return hasError ? { valid: false, error: errorMessage } : { valid: true }
}
Correct: returns immediately on first error
function validateUsers(users: User[]) {
for (const user of users) {
if (!user.email) {
return { valid: false, error: 'Email required' }
}
if (!user.name) {
return { valid: false, error: 'Name required' }
}
}
return { valid: true }
}
7.9 Hoist RegExp Creation
Don't create RegExp inside render. Hoist to module scope or memoize with useMemo().
Incorrect: new RegExp every render
function Highlighter({ text, query }: Props) {
const regex = new RegExp(`(${query})`, 'gi')
const parts = text.split(regex)
return <>{parts.map((part, i) => ...)}</>
}
Correct: memoize or hoist
const regex = /foo/g
Note: RegExp with global flag (g) has stateful lastIndex. Cache and reset as needed.
7.10 Use Loop for Min/Max Instead of Sort
Finding the smallest or largest element only requires a single pass. Sorting is wasteful.
Incorrect (O(n log n) - sort to find latest):
interface Project {
id: string
name: string
updatedAt: number
}
function getLatestProject(projects: Project[]) {
const sorted = [...projects].sort((a, b) => b.updatedAt - a.updatedAt)
return sorted[0]
}
Correct (O(n) - single loop):
function getLatestProject(projects: Project[]) {
if (projects.length === 0) return null
let latest = projects[0]
for (let i = 1; i < projects.length; i++) {
if (projects[i].updatedAt > latest.updatedAt) {
latest = projects[i]
}
}
return latest
}
function getOldestAndNewest(projects: Project[]) {
if (projects.length === 0) return { oldest: null, newest: null }
let oldest = projects[0]
let newest = projects[0]
for (let i = 1; i < projects.length; i++) {
if (projects[i].updatedAt < oldest.updatedAt) oldest = projects[i]
if (projects[i].updatedAt > newest.updatedAt) newest = projects[i]
}
return { oldest, newest }
}
Single pass through the array, no copying, no sorting.
7.11 Use Set/Map for O(1) Lookups
Convert arrays to Set/Map for repeated membership checks.
Incorrect (O(n) per check):
const allowedIds = ['a', 'b', 'c', ...]
items.filter(item => allowedIds.includes(item.id))
Correct (O(1) per check):
const allowedIds = new Set(['a', 'b', 'c', ...])
items.filter(item => allowedIds.has(item.id))
7.12 Use toSorted() Instead of sort() for Immutability
.sort() mutates the array in place, which can cause bugs with React state and props. Use .toSorted() to create a new sorted array without mutation.
Incorrect: mutates original array
function UserList({ users }: { users: User[] }) {
// Mutates the users prop array!
const sorted = useMemo(
() => users.sort((a, b) => a.name.localeCompare(b.name)),
[users]
)
return <div>{sorted.map(renderUser)}</div>
}
Correct: creates new array
function UserList({ users }: { users: User[] }) {
// Creates new sorted array, original unchanged
const sorted = useMemo(
() => users.toSorted((a, b) => a.name.localeCompare(b.name)),
[users]
)
return <div>{sorted.map(renderUser)}</div>
}
Browser support fallback:
const sorted = [...items].sort((a, b) => a.value - b.value)
8. Advanced Patterns
Impact: LOW
Advanced patterns for specific cases that require careful implementation.
8.1 Store Event Handlers in Refs
Store callbacks in refs when used in effects that shouldn't re-subscribe on callback changes.
Incorrect: re-subscribes on every render
function useWindowEvent(event: string, handler: () => void) {
useEffect(() => {
window.addEventListener(event, handler)
return () => window.removeEventListener(event, handler)
}, [event, handler])
}
Correct: stable subscription
function useWindowEvent(event: string, handler: () => void) {
const handlerRef = useRef(handler)
useEffect(() => {
handlerRef.current = handler
}, [handler])
useEffect(() => {
const listener = () => handlerRef.current()
window.addEventListener(event, listener)
return () => window.removeEventListener(event, listener)
}, [event])
}
8.2 useLatest for Stable Callback Refs
Access latest values in callbacks without adding them to dependency arrays. Prevents effect re-runs while avoiding stale closures.
Implementation:
function useLatest<T>(value: T) {
const ref = useRef(value)
useEffect(() => {
ref.current = value
}, [value])
return ref
}
Incorrect: effect re-runs on every callback change
function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {
const [query, setQuery] = useState('')
useEffect(() => {
const timeout = setTimeout(() => onSearch(query), 300)
return () => clearTimeout(timeout)
}, [query, onSearch])
}
Correct: stable effect, fresh callback
function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {
const [query, setQuery] = useState('')
const onSearchRef = useLatest(onSearch)
useEffect(() => {
const timeout = setTimeout(() => onSearchRef.current(query), 300)
return () => clearTimeout(timeout)
}, [query])
}