{user.name}
// uses 1 field
+}
+```
+
+**Correct: serializes only 1 field**
+
+```tsx
+async function Page() {
+ const user = await fetchUser()
+ return {name}
+}
+```
+
+### 3.6 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 (
+
+
+ )
+}
+
+async function Sidebar() {
+ const items = await fetchSidebarItems()
+ return
+}
+```
+
+**Correct: both fetch simultaneously**
+
+```tsx
+async function Header() {
+ const data = await fetchHeader()
+ return {header}
+ {data}
+}
+
+async function Sidebar() {
+ const items = await fetchSidebarItems()
+ return
+}
+
+export default function Page() {
+ return (
+
+
+ )
+}
+```
+
+**Alternative with children prop:**
+
+```tsx
+async function Header() {
+ const data = await fetchHeader()
+ return {data}
+}
+
+async function Sidebar() {
+ const items = await fetchSidebarItems()
+ return
+}
+
+function Layout({ children }: { children: ReactNode }) {
+ return (
+
+
+ )
+}
+
+export default function Page() {
+ return (
+ {fullName}
+} +``` + +**Correct: derive during render** + +```tsx +function Form() { + const [firstName, setFirstName] = useState('First') + const [lastName, setLastName] = useState('Last') + const fullName = firstName + ' ' + lastName + + return{fullName}
+} +``` + +Reference: [https://react.dev/learn/you-might-not-need-an-effect](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 +} +``` + +**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 +} +``` + +### 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
+ {user.followers} followers
+ {user.posts} posts
+
+ )
+
+ return (
+
+
+ )
+}
+```
+
+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 (
+
+ {followers} followers
+ {posts} posts
+
+ )
+}
+
+function UserProfile({ user, theme }) {
+ return (
+
+
+ )
+}
+```
+
+**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**
+
+```tsx
+const UserAvatar = memo(function UserAvatar({ onClick = () => {} }: { onClick?: () => void }) {
+ // ...
+})
+
+// Used without optional onClick
+{avatar}
+}
+```
+
+**Correct: skips computation when loading**
+
+```tsx
+const UserAvatar = memo(function UserAvatar({ user }: { user: User }) {
+ const id = useMemo(() => computeAvatarId(user), [user])
+ return
+
+ )
+}
+```
+
+**Note:** If your project has [React Compiler](https://react.dev/learn/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**
+
+```tsx
+useEffect(() => {
+ console.log(user.id)
+}, [user])
+```
+
+**Correct: re-runs only when id changes**
+
+```tsx
+useEffect(() => {
+ console.log(user.id)
+}, [user.id])
+```
+
+**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 transition
+const isMobile = width < 768
+useEffect(() => {
+ 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
+}
+```
+
+**Correct: do it in the handler**
+
+```tsx
+function Form() {
+ const theme = useContext(ThemeContext)
+
+ function handleSubmit() {
+ post('/api/register')
+ showToast('Registered', theme)
+ }
+
+ return
+}
+```
+
+Reference: [https://react.dev/learn/removing-effect-dependencies#should-this-code-move-to-an-event-handler](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**
+
+```tsx
+useEffect(() => {
+ analytics.trackPageView(pathname)
+ document.title = `${pageTitle} | My App`
+}, [pathname, pageTitle])
+```
+
+**Correct: effects run independently**
+
+```tsx
+useEffect(() => {
+ analytics.trackPageView(pathname)
+}, [pathname])
+
+useEffect(() => {
+ document.title = `${pageTitle} | My App`
+}, [pageTitle])
+```
+
+**Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, it automatically optimizes dependency tracking and may handle some of these cases for you.
+
+### 5.10 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**
+
+```tsx
+function Sidebar() {
+ const width = useWindowWidth() // updates continuously
+ const isMobile = width < 768
+ return
+}
+```
+
+**Correct: re-renders only when boolean changes**
+
+```tsx
+function Sidebar() {
+ const isMobile = useMediaQuery('(max-width: 767px)')
+ return
+}
+```
+
+### 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
+
+
+ )
+}
+```
+
+**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](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 (
+
+ )
+}
+```
+
+**Correct: no re-render for tracking**
+
+```tsx
+function Tracker() {
+ const lastXRef = useRef(0)
+ const dotRef = useRef` and animate the wrapper instead.
+
+**Incorrect: animating SVG directly - no hardware acceleration**
+
+```tsx
+function LoadingSpinner() {
+ return (
+
+ )
+}
+```
+
+**Correct: animating wrapper div - hardware accelerated**
+
+```tsx
+function LoadingSpinner() {
+ return (
+ ` to preserve state/DOM for expensive components that frequently toggle visibility.
+
+**Usage:**
+
+```tsx
+import { Activity } from 'react'
+
+function Dropdown({ isOpen }: Props) {
+ return (
+
+
+ )
+}
+```
+
+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 (
+
+
+
+
+
+ {/* content */}
+
+ )
+}
+```
+
+**Correct: non-blocking**
+
+```tsx
+import Script from 'next/script'
+
+export default function Page() {
+ return (
+ <>
+
+
+
+ )
+}
+```
+
+**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](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 (
+ {/* content */}
+}
+```
+
+**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 (
+
+ {children}
+
+ )
+}
+```
+
+**Example: preload modules for code-split routes**
+
+```tsx
+import { preloadModule, preinitModule } from 'react-dom'
+
+function Navigation() {
+ const preloadDashboard = () => {
+ preloadModule('/dashboard.js', { as: 'script' })
+ }
+
+ return (
+
+ )
+}
+```
+
+**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](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 (
+ <>
+ handleSearch(e.target.value)} />
+ {isLoading && (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 ()
+
+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 (
+ ()
+
+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 | 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**
+
+```typescript
+const admins = users.filter(u => u.isAdmin)
+const testers = users.filter(u => u.isTester)
+const inactive = users.filter(u => !u.isActive)
+```
+
+**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:**
+
+```typescript
+// Ensure analytics fires within 2 seconds even if browser stays busy
+requestIdleCallback(
+ () => analytics.track('page_view', { path: location.pathname }),
+ { timeout: 2000 }
+)
+```
+
+**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:**
+
+```typescript
+const scheduleIdleWork = window.requestIdleCallback ?? ((cb: () => void) => setTimeout(cb, 1))
+
+scheduleIdleWork(() => {
+ // Non-critical work
+})
+```
+
+**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**
+
+```typescript
+function hasChanges(current: string[], original: string[]) {
+ // Always sorts and joins, even when lengths differ
+ return current.sort().join() !== original.sort().join()
+}
+```
+
+Two O(n log n) sorts run even when `current.length` is 5 and `original.length` is 100. There is also overhead of joining the arrays and comparing the strings.
+
+**Correct (O(1) length check first):**
+
+```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**
+
+```tsx
+function Highlighter({ text, query }: Props) {
+ const regex = new RegExp(`(${query})`, 'gi')
+ const parts = text.split(regex)
+ return <>{parts.map((part, i) => ...)}
+}
+```
+
+**Correct: memoize or hoist**
+
+```tsx
+const EMAIL_REGEX = /^[^\s@]+@[^\s@]+\.[^\s@]+$/
+
+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**
+
+```typescript
+const regex = /foo/g
+regex.test('foo') // true, lastIndex = 3
+regex.test('foo') // false, lastIndex = 0
+```
+
+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**
+
+```typescript
+const userNames = users
+ .map(user => user.isActive ? user.name : null)
+ .filter(Boolean)
+```
+
+**Correct: 1 iteration, no intermediate array**
+
+```typescript
+const userNames = users.flatMap(user =>
+ user.isActive ? [user.name] : []
+)
+```
+
+**More examples:**
+
+```typescript
+// Extract valid emails from responses
+// Before
+const emails = responses
+ .map(r => r.success ? r.data.email : null)
+ .filter(Boolean)
+
+// After
+const emails = responses.flatMap(r =>
+ r.success ? [r.data.email] : []
+)
+
+// Parse and filter valid numbers
+// Before
+const numbers = strings
+ .map(s => parseInt(s, 10))
+ .filter(n => !isNaN(n))
+
+// After
+const 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):**
+
+```typescript
+function getOldestAndNewest(projects: Project[]) {
+ const sorted = [...projects].sort((a, b) => a.updatedAt - b.updatedAt)
+ return { oldest: sorted[0], newest: sorted[sorted.length - 1] }
+}
+```
+
+Still sorts unnecessarily when only min/max are needed.
+
+**Correct (O(n) - single loop):**
+
+```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**
+
+```typescript
+const numbers = [5, 2, 8, 1, 9]
+const min = Math.min(...numbers)
+const max = Math.max(...numbers)
+```
+
+This works for small arrays, but can be slower 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](https://jsfiddle.net/qw1jabsx/4/). 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):**
+
+```typescript
+const allowedIds = ['a', 'b', 'c', ...]
+items.filter(item => allowedIds.includes(item.id))
+```
+
+**Correct (O(1) per check):**
+
+```typescript
+const allowedIds = new Set(['a', 'b', 'c', ...])
+items.filter(item => allowedIds.has(item.id))
+```
+
+### 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
+
+
+ )
+}
+```
+
+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:**
+
+```css
+.message-item {
+ content-visibility: auto;
+ contain-intrinsic-size: 0 80px;
+}
+```
+
+**Example:**
+
+```tsx
+function MessageList({ messages }: { messages: Message[] }) {
+ return (
+
+ {messages.map(msg => (
+
+ )
+}
+```
+
+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**
+
+```tsx
+function LoadingSkeleton() {
+ return
+}
+
+function Container() {
+ return (
+
+
+ ))}
+ {msg.content}
+
+ {loading &&
+ )
+}
+```
+
+**Correct: reuses same element**
+
+```tsx
+const loadingSkeleton = (
+
+)
+
+function Container() {
+ return (
+
+ {loading && loadingSkeleton}
+
+ )
+}
+```
+
+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](https://react.dev/learn/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**
+
+```svg
+
+ {children}
+
+ )
+}
+```
+
+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 (
+
+ {children}
+
+ )
+}
+```
+
+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 (
+ <>
+
+ {children}
+
+
+
+ )
+}
+```
+
+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**
+
+```tsx
+function Timestamp() {
+ return {new Date().toLocaleString()}
+}
+```
+
+**Correct: suppress expected mismatch only**
+
+```tsx
+function Timestamp() {
+ return (
+
+ {new Date().toLocaleString()}
+
+ )
+}
+```
+
+### 6.7 Use Activity Component for Show/Hide
+
+**Impact: MEDIUM (preserves state/DOM)**
+
+Use React's `
+ {count && {count}}
+
+ )
+}
+
+// When count = 0, renders: 0
+// When count = 5, renders: 5
+```
+
+**Correct: renders nothing when count is 0**
+
+```tsx
+function Badge({ count }: { count: number }) {
+ return (
+
+ {count > 0 ? {count} : null}
+
+ )
+}
+
+// When count = 0, renders:
+// When count = 5, renders: 5
+```
+
+### 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 Content
+}
+
+// Correct: toggle class
+function Box({ isHighlighted }: { isHighlighted: boolean }) {
+ return (
+
+ Content
+
+ )
+}
+```
+
+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](https://gist.github.com/paulirish/5d52fb081b3570c81e3a) and [CSS Triggers](https://csstriggers.com/) 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):**
+
+```typescript
+function processOrders(orders: Order[], users: User[]) {
+ return orders.map(order => ({
+ ...order,
+ user: users.find(u => u.id === order.userId)
+ }))
+}
+```
+
+**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**
+
+```typescript
+for (let i = 0; i < arr.length; i++) {
+ process(obj.config.settings.value)
+}
+```
+
+**Correct: 1 lookup total**
+
+```typescript
+const value = obj.config.settings.value
+const len = arr.length
+for (let i = 0; i < len; i++) {
+ process(value)
+}
+```
+
+### 7.4 Cache Repeated Function Calls
+
+**Impact: MEDIUM (avoid redundant computation)**
+
+Use a module-level Map to cache function results when the same function is called repeatedly with the same inputs during render.
+
+**Incorrect: redundant computation**
+
+```typescript
+function ProjectList({ projects }: { projects: Project[] }) {
+ return (
+
+ {projects.map(project => {
+ // slugify() called 100+ times for same project names
+ const slug = slugify(project.name)
+
+ return
+ )
+}
+```
+
+**Correct: cached results**
+
+```typescript
+// Module-level cache
+const slugifyCache = new Map
+ {projects.map(project => {
+ // Computed only once per unique project name
+ const slug = cachedSlugify(project.name)
+
+ return
+ )
+}
+```
+
+**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 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](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**
+
+```typescript
+function getTheme() {
+ return localStorage.getItem('theme') ?? 'light'
+}
+// Called 10 times = 10 storage reads
+```
+
+**Correct: Map cache**
+
+```typescript
+const storageCache = new Map{sorted.map(renderUser)}
+}
+```
+
+**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 {sorted.map(renderUser)}
+}
+```
+
+**Why this matters in React:**
+
+1. Props/state mutations break React's immutability model - React expects props and state to be treated as read-only
+
+2. Causes stale closure bugs - Mutating arrays inside closures (callbacks, effects) can lead to unexpected behavior
+
+**Browser support: fallback for older browsers**
+
+```typescript
+// Fallback for older browsers
+const sorted = [...items].sort((a, b) => a.value - b.value)
+```
+
+`.toSorted()` is available in all modern browsers (Chrome 110+, Safari 16+, Firefox 115+, Node.js 20+). For older environments, use spread operator:
+
+**Other immutable array methods:**
+
+- `.toSorted()` - immutable sort
+
+- `.toReversed()` - immutable reverse
+
+- `.toSpliced()` - immutable splice
+
+- `.with()` - immutable element replacement
+
+---
+
+## 8. Advanced Patterns
+
+**Impact: LOW**
+
+Advanced patterns for specific cases that require careful implementation.
+
+### 8.1 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**
+
+```tsx
+function Comp() {
+ useEffect(() => {
+ loadFromStorage()
+ checkAuthToken()
+ }, [])
+
+ // ...
+}
+```
+
+**Correct: once per app load**
+
+```tsx
+let didInit = false
+
+function Comp() {
+ useEffect(() => {
+ if (didInit) return
+ didInit = true
+ loadFromStorage()
+ checkAuthToken()
+ }, [])
+
+ // ...
+}
+```
+
+Reference: [https://react.dev/learn/you-might-not-need-an-effect#initializing-the-application](https://react.dev/learn/you-might-not-need-an-effect#initializing-the-application)
+
+### 8.2 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.
+
+### 8.3 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])
+}
+```
+
+---
+
+## References
+
+1. [https://react.dev](https://react.dev)
+2. [https://nextjs.org](https://nextjs.org)
+3. [https://swr.vercel.app](https://swr.vercel.app)
+4. [https://github.com/shuding/better-all](https://github.com/shuding/better-all)
+5. [https://github.com/isaacs/node-lru-cache](https://github.com/isaacs/node-lru-cache)
+6. [https://vercel.com/blog/how-we-optimized-package-imports-in-next-js](https://vercel.com/blog/how-we-optimized-package-imports-in-next-js)
+7. [https://vercel.com/blog/how-we-made-the-vercel-dashboard-twice-as-fast](https://vercel.com/blog/how-we-made-the-vercel-dashboard-twice-as-fast)