Instruction file imported from DataCovey/nornweave (.cursor/rules/react.mdc). Copyright stays with the author.

React Best Practices

Comprehensive performance optimization guide for React applications. Contains 35+ rules across 7 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.

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 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()
])

1.3 Dependency-Based Parallelization

Impact: CRITICAL (2-10× improvement)

For operations with partial dependencies, start independent operations immediately, even if you don't await them yet.

Incorrect: config waits for auth, data waits for both

async function fetchData() {
  const session = await auth()
  const config = await fetchConfig()
  const data = await fetchUserData(session.user.id)
  return { data, config }
}

Correct: auth and config start immediately

async function fetchData() {
  const sessionPromise = auth()
  const configPromise = fetchConfig()
  const session = await sessionPromise
  const [config, data] = await Promise.all([
    configPromise,
    fetchUserData(session.user.id)
  ])
  return { data, config }
}

1.4 Strategic Suspense Boundaries

Impact: HIGH (faster initial paint)

Use Suspense boundaries to show the wrapper UI faster while data loads.

Incorrect: wrapper blocked by data fetching

function Page() {
  const [data, setData] = useState(null)
  const [loading, setLoading] = useState(true)
  
  useEffect(() => {
    fetchData().then(d => {
      setData(d)
      setLoading(false)
    })
  }, [])
  
  if (loading) return <Skeleton />
  
  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>
  )
}

function DataDisplay() {
  const data = use(fetchData()) // Using React's use() hook
  return <div>{data.content}</div>
}

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

When NOT to use this pattern:

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

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.

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

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.

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) {
      import('./animation-frames.js')
        .then(mod => setFrames(mod.frames))
        .catch(() => setEnabled(false))
    }
  }, [enabled, frames])

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

2.3 Defer Non-Critical Third-Party Libraries

Impact: MEDIUM (loads after initial render)

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

Incorrect: blocks initial bundle

import { Analytics } from '@analytics/react'

export default function App({ children }) {
  return (
    <div>
      {children}
      <Analytics />
    </div>
  )
}

Correct: loads lazily

import { lazy, Suspense } from 'react'

const Analytics = lazy(() => import('@analytics/react').then(m => ({ default: m.Analytics })))

export default function App({ children }) {
  return (
    <div>
      {children}
      <Suspense fallback={null}>
        <Analytics />
      </Suspense>
    </div>
  )
}

2.4 Dynamic Imports for Heavy Components

Impact: CRITICAL (directly affects TTI and LCP)

Use React.lazy() 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 { lazy, Suspense } from 'react'

const MonacoEditor = lazy(() => import('./monaco-editor').then(m => ({ default: m.MonacoEditor })))

function CodePanel({ code }: { code: string }) {
  return (
    <Suspense fallback={<div>Loading editor...</div>}>
      <MonacoEditor value={code} />
    </Suspense>
  )
}

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 = () => {
    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) {
      void import('./monaco-editor').then(mod => mod.init())
    }
  }, [flags.editorEnabled])

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

3. Client-Side Data Fetching

Impact: MEDIUM-HIGH

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

3.1 Deduplicate Global Event Listeners

Impact: LOW (single listener for N components)

Use useSWRSubscription() or similar patterns 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)
  })
}

3.2 Use SWR or React Query for Automatic Deduplication

Impact: MEDIUM-HIGH (automatic deduplication)

SWR/React Query enable 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 useSWRImmutable from 'swr/immutable'

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

For mutations:

import useSWRMutation from 'swr/mutation'

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

4. Re-render Optimization

Impact: MEDIUM

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

4.1 Defer State Reads to Usage Point

Impact: MEDIUM (avoids unnecessary subscriptions)

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

Incorrect: subscribes to all searchParams changes

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

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

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

Correct: reads on demand, no subscription

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

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

4.2 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.

4.3 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])

4.4 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'}>
}

4.5 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.

Incorrect: requires state as dependency

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

Correct: stable callbacks, no stale closures

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

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

4.6 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} />
}

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} />
}

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

4.7 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)
  }, [])
}

5. Rendering Performance

Impact: MEDIUM

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

5.1 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>
  )
}

5.2 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).

5.3 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.

5.4 Optimize SVG Precision

Impact: LOW (reduces file size)

Reduce SVG coordinate precision to decrease file size.

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

5.5 Use Activity Component for Show/Hide

Impact: MEDIUM (preserves state/DOM)

Use React's <Activity> (React 19+) 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.

5.6 Use Explicit Conditional Rendering

Impact: LOW (prevents rendering 0 or NaN)

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

Incorrect: renders "0" when count is 0

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

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

Correct: renders nothing when count is 0

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

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

6. JavaScript Performance

Impact: LOW-MEDIUM

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

6.1 Batch DOM CSS Changes

Impact: MEDIUM (reduces reflows/repaints)

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

Incorrect: multiple reflows

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

Correct: add class - single reflow

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

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

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>
  )
}

6.2 Build Index Maps for Repeated Lookups

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

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

Incorrect (O(n) per lookup):

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

Correct (O(1) per lookup):

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

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

6.3 Cache Property Access in Loops

Impact: LOW-MEDIUM (reduces lookups)

Cache object property lookups in hot paths.

Incorrect: 3 lookups × N iterations

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

Correct: 1 lookup total

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

6.4 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.

Incorrect: redundant computation

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

Correct: cached results

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

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

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

6.5 Cache Storage API Calls

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

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

Incorrect: reads storage on every call

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

Correct: Map cache

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

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

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

Important: invalidate on external changes

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

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

6.6 Combine Multiple Array Iterations

Impact: LOW-MEDIUM (reduces iterations)

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

Incorrect: 3 iterations

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

Correct: 1 iteration

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

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

6.7 Early Return from Functions

Impact: LOW-MEDIUM (avoids unnecessary computation)

Return early when result is determined to skip unnecessary processing.

Incorrect: processes all items even after finding answer

function validateUsers(users: User[]) {
  let hasError = false
  let errorMessage = ''
  
  for (const user of users) {
    if (!user.email) {
      hasError = true
      errorMessage = 'Email required'
    }
    if (!user.name) {
      hasError = true
      errorMessage = 'Name required'
    }
  }
  
  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 }
}

6.8 Hoist RegExp Creation

Impact: LOW-MEDIUM (avoids recreation)

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

Incorrect: new RegExp every render

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

Correct: memoize or hoist

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

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

6.9 Use Set/Map for O(1) Lookups

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

Convert arrays to Set/Map for repeated membership checks.

Incorrect (O(n) per check):

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

Correct (O(1) per check):

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

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

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

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

Incorrect: mutates original array

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

Correct: creates new array

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

Browser support: fallback for older browsers

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

Other immutable array methods:

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

7. Advanced Patterns

Impact: LOW

Advanced patterns for specific cases that require careful implementation.

7.1 Store Event Handlers in Refs

Impact: LOW (stable subscriptions)

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

Incorrect: re-subscribes on every render

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

Correct: stable subscription

import { useEffectEvent } from 'react'

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

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

7.2 useLatest for Stable Callback Refs

Impact: LOW (prevents effect re-runs)

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

Implementation:

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

Incorrect: effect re-runs on every callback change

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

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

Correct: stable effect, fresh callback

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

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

References

1. https://react.dev
2. https://swr.vercel.app
3. https://tanstack.com/query