React Best Practices

React Best Practices

Performance optimization guide for client-side React applications. Contains 47 rules across 8 categories, prioritized by impact. Adapted from Vercel Engineering's guidelines, filtered for client-only React + Vite (no Next.js / SSR).

When to Apply

Reference these guidelines when:

• Writing new React components or hooks
• Reviewing code for performance issues
• Refactoring existing React code
• Optimizing bundle size or render performance
• Writing or reviewing useEffect usage

Rule Categories by Priority

Quick Reference

1. Eliminating Waterfalls (CRITICAL)

• async-defer-await — Move await into branches where actually used
• async-parallel — Use Promise.all() for independent operations
• async-dependencies — Use promise chaining for partial dependencies
• async-api-routes — Start promises early, await late

2. Bundle Size Optimization (CRITICAL)

• bundle-barrel-imports — Import directly, avoid barrel files
• bundle-conditional — Load modules only when feature is activated
• bundle-defer-third-party — Load analytics/logging after mount
• bundle-preload — Preload on hover/focus for perceived speed

3. Client-Side Data Fetching (MEDIUM-HIGH)

• client-event-listeners — Deduplicate global event listeners
• client-passive-event-listeners — Use passive listeners for scroll/touch
• client-swr-dedup — Use SWR for automatic request deduplication
• client-localstorage-schema — Version and minimize localStorage data

4. Re-render Optimization (MEDIUM)

• rerender-derived-state-no-effect — Derive state during render, not effects
• rerender-defer-reads — Don't subscribe to state only used in callbacks
• rerender-simple-expression-in-memo — Avoid memo for simple primitives
• rerender-memo-with-default-value — Hoist default non-primitive props
• rerender-memo — Extract expensive work into memoized components
• rerender-dependencies — Use primitive dependencies in effects
• rerender-move-effect-to-event — Put interaction logic in event handlers
• rerender-derived-state — Subscribe to derived booleans, not raw values
• rerender-functional-setstate — Use functional setState for stable callbacks
• rerender-lazy-state-init — Pass function to useState for expensive values
• rerender-transitions — Use startTransition for non-urgent updates
• rerender-use-ref-transient-values — Use refs for transient frequent values

5. useEffect Patterns (MEDIUM)

• useeffect-anti-patterns — Common mistakes: derived state in effects, effect chains, notifying parents
• useeffect-alternatives — Decision tree and alternatives: derived state, key prop, event handlers, useSyncExternalStore

6. Rendering Performance (MEDIUM)

• rendering-animate-svg-wrapper — Animate div wrapper, not SVG element
• rendering-content-visibility — Use content-visibility for long lists
• rendering-hoist-jsx — Extract static JSX outside components
• rendering-svg-precision — Reduce SVG coordinate precision
• rendering-conditional-render — Use ternary, not && for conditionals
• rendering-usetransition-loading — Prefer useTransition for loading state

7. JavaScript Performance (LOW-MEDIUM)

• js-batch-dom-css — Batch DOM reads/writes to avoid layout thrashing
• js-index-maps — Build Map for repeated lookups
• js-cache-property-access — Cache object properties in loops
• js-cache-function-results — Cache function results in module-level Map
• js-cache-storage — Cache localStorage/sessionStorage reads
• js-combine-iterations — Combine multiple filter/map into one loop
• js-length-check-first — Check array length before expensive comparison
• js-early-exit — Return early from functions
• js-hoist-regexp — Hoist RegExp creation outside loops
• js-min-max-loop — Use loop for min/max instead of sort
• js-set-map-lookups — Use Set/Map for O(1) lookups
• js-tosorted-immutable — Use toSorted() for immutability

8. Advanced Patterns (LOW)

• advanced-init-once — Initialize app once per app load
• advanced-event-handler-refs — Store event handlers in refs
• advanced-use-latest — useEffectEvent for stable callback refs

How to Use

Read individual reference files for detailed explanations and code examples:

references/async-parallel.md
references/rerender-derived-state-no-effect.md
references/useeffect-anti-patterns.md

Each reference file contains:

• Brief explanation of why it matters
• Incorrect code example with explanation
• Correct code example with explanation
• Additional context and references

Sections

This file defines all sections, their ordering, impact levels, and descriptions. The section ID (in parentheses) is the filename prefix used to group references.

Adapted from Vercel Engineering's React Best Practices for client-only React + Vite + Tauri. Server-side / Next.js-specific rules have been removed.

---

1. Eliminating Waterfalls (async)

Impact: CRITICAL Description: Waterfalls are the #1 performance killer. Each sequential await adds full network latency. Eliminating them yields the largest gains.

2. Bundle Size Optimization (bundle)

Impact: CRITICAL Description: Reducing initial bundle size improves Time to Interactive and Largest Contentful Paint.

3. Client-Side Data Fetching (client)

Impact: MEDIUM-HIGH Description: Automatic deduplication and efficient data fetching patterns reduce redundant network requests.

4. Re-render Optimization (rerender)

Impact: MEDIUM Description: Reducing unnecessary re-renders minimizes wasted computation and improves UI responsiveness.

5. useEffect Patterns (useeffect)

Impact: MEDIUM Description: Effects are an escape hatch. Most state updates, derived values, and user interactions should be handled without effects.

6. Rendering Performance (rendering)

Impact: MEDIUM Description: Optimizing the rendering process reduces the work the browser needs to do.

7. JavaScript Performance (js)

Impact: LOW-MEDIUM Description: Micro-optimizations for hot paths can add up to meaningful improvements.

8. Advanced Patterns (advanced)

Impact: LOW Description: Advanced patterns for specific cases that require careful implementation.

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: (e) => 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: (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.

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

function Comp() {
  useEffect(() => {
    loadFromStorage()
    checkAuthToken()
  }, [])

  // ...
}

Correct: once per app load

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

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

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

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])
}

Prevent Waterfall Chains in API Routes

Impact: CRITICAL (2-10× improvement)

In API routes and Server Actions, start independent operations immediately, even if you don't await them yet.

Incorrect: config waits for auth, data waits for both

export async function GET(request: Request) {
  const session = await auth()
  const config = await fetchConfig()
  const data = await fetchData(session.user.id)
  return Response.json({ data, config })
}

Correct: auth and config start immediately

export async function GET(request: Request) {
  const sessionPromise = auth()
  const configPromise = fetchConfig()
  const session = await sessionPromise
  const [config, data] = await Promise.all([
    configPromise,
    fetchData(session.user.id)
  ])
  return Response.json({ data, config })
}

For operations with more complex dependency chains, use better-all to automatically maximize parallelism (see Dependency-Based Parallelization).

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.

Dependency-Based Parallelization

Impact: CRITICAL (2-10× improvement)

For operations with partial dependencies, use better-all to maximize parallelism. It automatically starts each task at the earliest possible moment.

Incorrect: profile waits for config unnecessarily

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

Correct: config and profile run in parallel

import { all } from 'better-all'

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

Alternative without extra dependencies:

const userPromise = fetchUser()
const profilePromise = userPromise.then(user => fetchProfile(user.id))

const [user, config, profile] = await Promise.all([
  userPromise,
  fetchConfig(),
  profilePromise
])

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

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

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()
])

Avoid Barrel File Imports

Impact: CRITICAL (200-800ms import cost, slow builds)

Import directly from source files instead of barrel files to avoid loading thousands of unused modules. Barrel files are entry points that re-export multiple modules (e.g., index.js that does export * from './module').

Popular icon and component libraries can have up to 10,000 re-exports in their entry file. For many React packages, it takes 200-800ms just to import them, affecting both development speed and production cold starts.

Why tree-shaking doesn't help: When a library is marked as external (not bundled), the bundler can't optimize it. If you bundle it to enable tree-shaking, builds become substantially slower analyzing the entire module graph.

Incorrect: imports entire library

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

import { Button, TextField } from '@mui/material'
// Loads 2,225 modules, takes ~4.2s extra in dev

Correct: imports only what you need

import Check from 'lucide-react/dist/esm/icons/check'
import X from 'lucide-react/dist/esm/icons/x'
import Menu from 'lucide-react/dist/esm/icons/menu'
// Loads only 3 modules (~2KB vs ~1MB)

import Button from '@mui/material/Button'
import TextField from '@mui/material/TextField'
// Loads only what you use

Alternative: Vite

Vite handles tree-shaking well for bundled dependencies, but direct imports are still preferred for:

• Faster dev server startup (fewer modules to pre-bundle)
• Faster HMR updates (smaller dependency graphs)
• Libraries marked as external or loaded via CDN

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

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, setEnabled }: { enabled: boolean; setEnabled: React.Dispatch<React.SetStateAction<boolean>> }) {
  const [frames, setFrames] = useState<Frame[] | null>(null)

  useEffect(() => {
    if (enabled && !frames && typeof window !== 'undefined') {
      import('./animation-frames.js')
        .then(mod => setFrames(mod.frames))
        .catch(() => setEnabled(false))
    }
  }, [enabled, frames, setEnabled])

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

The typeof window !== 'undefined' check prevents bundling this module for SSR, optimizing server bundle size and build speed.

Defer Non-Critical Third-Party Libraries

Impact: MEDIUM (loads after initial render)

Analytics, logging, and error tracking don't block user interaction. Load them after the initial render.

Incorrect: blocks initial bundle

import { Analytics } from '@vercel/analytics/react'

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

Correct: lazy-loaded after initial render

import { lazy, Suspense } from 'react'

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

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

Preload Based on User Intent

Impact: MEDIUM (reduces perceived latency)

Preload heavy bundles before they're needed to reduce perceived latency.

Example: preload on hover/focus

function EditorButton({ onClick }: { onClick: () => void }) {
  const preload = () => {
    if (typeof window !== 'undefined') {
      void import('./monaco-editor')
    }
  }

  return (
    <button
      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.

Deduplicate Global Event Listeners

Impact: LOW (single listener for N components)

Use useSWRSubscription() to share global event listeners across component instances.

Incorrect: N instances = N listeners

function useKeyboardShortcut(key: string, callback: () => void) {
  useEffect(() => {
    const handler = (e: KeyboardEvent) => {
      if (e.metaKey && e.key === key) {
        callback()
      }
    }
    window.addEventListener('keydown', handler)
    return () => window.removeEventListener('keydown', handler)
  }, [key, callback])
}

When using the useKeyboardShortcut hook multiple times, each instance will register a new listener.

Correct: N instances = 1 listener

import useSWRSubscription from 'swr/subscription'

// Module-level Map to track callbacks per key
const keyCallbacks = new Map<string, Set<() => void>>()

function useKeyboardShortcut(key: string, callback: () => void) {
  // Register this callback in the Map
  useEffect(() => {
    if (!keyCallbacks.has(key)) {
      keyCallbacks.set(key, new Set())
    }
    keyCallbacks.get(key)!.add(callback)

    return () => {
      const set = keyCallbacks.get(key)
      if (set) {
        set.delete(callback)
        if (set.size === 0) {
          keyCallbacks.delete(key)
        }
      }
    }
  }, [key, callback])

  useSWRSubscription('global-keydown', () => {
    const handler = (e: KeyboardEvent) => {
      if (e.metaKey && keyCallbacks.has(e.key)) {
        keyCallbacks.get(e.key)!.forEach(cb => cb())
      }
    }
    window.addEventListener('keydown', handler)
    return () => window.removeEventListener('keydown', handler)
  })
}

function Profile() {
  // Multiple shortcuts will share the same listener
  useKeyboardShortcut('p', () => { /* ... */ }) 
  useKeyboardShortcut('k', () => { /* ... */ })
  // ...
}

Version and Minimize localStorage Data

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

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

Incorrect:

// No version, stores everything, no error handling
localStorage.setItem('userConfig', JSON.stringify(fullUserObject))
const data = localStorage.getItem('userConfig')

Correct:

const VERSION = 'v2'

function saveConfig(config: { theme: string; language: string }) {
  try {
    localStorage.setItem(`userConfig:${VERSION}`, JSON.stringify(config))
  } catch {
    // Throws in incognito/private browsing, quota exceeded, or disabled
  }
}

function loadConfig() {
  try {
    const data = localStorage.getItem(`userConfig:${VERSION}`)
    return data ? JSON.parse(data) : null
  } catch {
    return null
  }
}

// Migration from v1 to v2
function migrate() {
  try {
    const v1 = localStorage.getItem('userConfig:v1')
    if (v1) {
      const old = JSON.parse(v1)
      saveConfig({ theme: old.darkMode ? 'dark' : 'light', language: old.lang })
      localStorage.removeItem('userConfig:v1')
    }
  } catch {}
}

Store minimal fields from server responses:

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

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

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

Use Passive Event Listeners for Scrolling Performance

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

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

Incorrect:

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

Correct:

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

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

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

Use SWR for Automatic Deduplication

Impact: MEDIUM-HIGH (automatic deduplication)

SWR enables request deduplication, caching, and revalidation across component instances.

Incorrect: no deduplication, each instance fetches

function UserList() {
  const [users, setUsers] = useState([])
  useEffect(() => {
    fetch('/api/users')
      .then(r => r.json())
      .then(setUsers)
  }, [])
}

Correct: multiple instances share one request

import useSWR from 'swr'

function UserList() {
  const { data: users } = useSWR('/api/users', fetcher)
}

For immutable data:

import { useImmutableSWR } from '@/lib/swr'

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

For mutations:

import { useSWRMutation } from 'swr/mutation'

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

Reference: https://swr.vercel.app

Avoid Layout Thrashing

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

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

This is OK: browser batches style changes

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

Incorrect: interleaved reads and writes force reflows

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

Correct: batch writes, then read once

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

Correct: batch reads, then writes

function updateElementStyles(element: HTMLElement) {
  element.classList.add('highlighted-box')
  
  const { width, height } = element.getBoundingClientRect()
}

Better: use CSS classes

React example:

// Incorrect: interleaving style changes with layout queries
function Box({ isHighlighted }: { isHighlighted: boolean }) {
  const ref = useRef<HTMLDivElement>(null)
  
  useEffect(() => {
    if (ref.current && isHighlighted) {
      ref.current.style.width = '100px'
      const width = ref.current.offsetWidth // Forces layout
      ref.current.style.height = '200px'
    }
  }, [isHighlighted])
  
  return <div ref={ref}>Content</div>
}

// Correct: toggle class
function Box({ isHighlighted }: { isHighlighted: boolean }) {
  return (
    <div className={isHighlighted ? 'highlighted-box' : ''}>
      Content
    </div>
  )
}

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

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

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

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>
  )
}

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

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)
}

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
}

Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components.

Cookie caching:

let cookieCache: Record<string, string> | null = null

function getCookie(name: string) {
  if (!cookieCache) {
    cookieCache = Object.fromEntries(
      document.cookie.split('; ').map(c => c.split('='))
    )
  }
  return cookieCache[name]
}

Important: invalidate on external changes

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:

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)
}

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'
    }
    // 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 }
}

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) => ...)}</>
}

Warning: global regex has mutable state

const regex = /foo/g
regex.test('foo')  // true, lastIndex = 3
regex.test('foo')  // false, lastIndex = 0

Global regex (/g) has mutable lastIndex state:

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)
  }))
}

Build map once (O(n)), then all lookups are O(1).

For 1000 orders × 1000 users: 1M ops → 2K ops.

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

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):

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

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):

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):

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):

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

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. Use the loop approach for reliability.

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))

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>
}

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

// 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

Animate SVG Wrapper Instead of SVG Element

Impact: LOW (enables hardware acceleration)

Many browsers don't have hardware acceleration for CSS3 animations on SVG elements. Wrap SVG in a <div> and animate the wrapper instead.

Incorrect: animating SVG directly - no hardware acceleration

function LoadingSpinner() {
  return (
    <svg 
      className="animate-spin"
      width="24" 
      height="24" 
      viewBox="0 0 24 24"
    >
      <circle cx="12" cy="12" r="10" stroke="currentColor" />
    </svg>
  )
}

Correct: animating wrapper div - hardware accelerated

function LoadingSpinner() {
  return (
    <div className="animate-spin">
      <svg 
        width="24" 
        height="24" 
        viewBox="0 0 24 24"
      >
        <circle cx="12" cy="12" r="10" stroke="currentColor" />
      </svg>
    </div>
  )
}

This applies to all CSS transforms and transitions (transform, opacity, translate, scale, rotate). The wrapper div allows browsers to use GPU acceleration for smoother animations.

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>

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).

Hoist Static JSX Elements

Impact: LOW (avoids re-creation)

Extract static JSX outside components to avoid re-creation.

Incorrect: recreates element every render

function LoadingSkeleton() {
  return <div className="animate-pulse h-20 bg-gray-200" />
}

function Container() {
  return (
    <div>
      {loading && <LoadingSkeleton />}
    </div>
  )
}

Correct: reuses same element

const loadingSkeleton = (
  <div className="animate-pulse h-20 bg-gray-200" />
)

function Container() {
  return (
    <div>
      {loading && loadingSkeleton}
    </div>
  )
}

This is especially helpful for large and static SVG nodes, which can be expensive to recreate on every render.

Note: If your project has React Compiler enabled, the compiler automatically hoists static JSX elements and optimizes component re-renders, making manual hoisting unnecessary.

Optimize SVG Precision

Impact: LOW (reduces file size)

Reduce SVG coordinate precision to decrease file size. The optimal precision depends on the viewBox size, but in general reducing precision should be considered.

Incorrect: excessive precision

<path d="M 10.293847 20.847362 L 30.938472 40.192837" />

Correct: 1 decimal place

<path d="M 10.3 20.8 L 30.9 40.2" />

Automate with SVGO:

pnpx svgo --precision=1 --multipass icon.svg

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

function SearchResults() {
  const [query, setQuery] = useState('')
  const [results, setResults] = useState([])
  const [isLoading, setIsLoading] = useState(false)

  const handleSearch = async (value: string) => {
    setIsLoading(true)
    setQuery(value)
    const data = await fetchResults(value)
    setResults(data)
    setIsLoading(false)
  }

  return (
    <>
      <input onChange={(e) => handleSearch(e.target.value)} />
      {isLoading && <Spinner />}
      <ResultsList results={results} />
    </>
  )
}

Correct: useTransition with built-in pending state

import { useTransition, useState } from 'react'

function SearchResults() {
  const [query, setQuery] = useState('')
  const [results, setResults] = useState([])
  const [isPending, startTransition] = useTransition()

  const handleSearch = (value: string) => {
    setQuery(value) // Update input immediately
    
    startTransition(async () => {
      // Fetch and update results
      const data = await fetchResults(value)
      setResults(data)
    })
  }

  return (
    <>
      <input onChange={(e) => handleSearch(e.target.value)} />
      {isPending && <Spinner />}
      <ResultsList results={results} />
    </>
  )
}

Benefits:

• Automatic pending state: No need to manually manage setIsLoading(true/false)

• Error resilience: Pending state correctly resets even if the transition throws

• Better responsiveness: Keeps the UI responsive during updates

• Interrupt handling: New transitions automatically cancel pending ones

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

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>
}

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])

Calculate Derived State During Rendering

Impact: MEDIUM (avoids redundant renders and state drift)

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

Incorrect: redundant state and effect

function Form() {
  const [firstName, setFirstName] = useState('First')
  const [lastName, setLastName] = useState('Last')
  const [fullName, setFullName] = useState('')

  useEffect(() => {
    setFullName(firstName + ' ' + lastName)
  }, [firstName, lastName])

  return <p>{fullName}</p>
}

Correct: derive during render

function Form() {
  const [firstName, setFirstName] = useState('First')
  const [lastName, setLastName] = useState('Last')
  const fullName = firstName + ' ' + lastName

  return <p>{fullName}</p>
}

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

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'} />
}

Use Functional setState Updates

Impact: MEDIUM (prevents stale closures and unnecessary callback recreations)

When updating state based on the current state value, use the functional update form of setState instead of directly referencing the state variable. This prevents stale closures, eliminates unnecessary dependencies, and creates stable callback references.

Incorrect: requires state as dependency

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

The first callback is recreated every time items changes, which can cause child components to re-render unnecessarily. The second callback has a stale closure bug—it will always reference the initial items value.

Correct: stable callbacks, no stale closures

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

Benefits:

1. Stable callback references - Callbacks don't need to be recreated when state changes

2. No stale closures - Always operates on the latest state value

3. Fewer dependencies - Simplifies dependency arrays and reduces memory leaks

4. Prevents bugs - Eliminates the most common source of React closure bugs

When to use functional updates:

• Any setState that depends on the current state value

• Inside useCallback/useMemo when state is needed

• Event handlers that reference state

• Async operations that update state

When direct updates are fine:

• Setting state to a static value: setCount(0)

• Setting state from props/arguments only: setName(newName)

• State doesn't depend on previous value

Note: If your project has React Compiler enabled, the compiler can automatically optimize some cases, but functional updates are still recommended for correctness and to prevent stale closure bugs.

Use Lazy State Initialization

Impact: MEDIUM (wasted computation on every render)

Pass a function to useState for expensive initial values. Without the function form, the initializer runs on every render even though the value is only used once.

Incorrect: runs on every render

function FilteredList({ items }: { items: Item[] }) {
  // buildSearchIndex() runs on EVERY render, even after initialization
  const [searchIndex, setSearchIndex] = useState(buildSearchIndex(items))
  const [query, setQuery] = useState('')
  
  // When query changes, buildSearchIndex runs again unnecessarily
  return <SearchResults index={searchIndex} query={query} />
}

function UserProfile() {
  // JSON.parse runs on every render
  const [settings, setSettings] = useState(
    JSON.parse(localStorage.getItem('settings') || '{}')
  )
  
  return <SettingsForm settings={settings} 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.

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

const UserAvatar = memo(function UserAvatar({ onClick = () => {} }: { onClick?: () => void }) {
  // ...
})

// Used without optional onClick
<UserAvatar />

Correct: stable default value

const NOOP = () => {};

const UserAvatar = memo(function UserAvatar({ onClick = NOOP }: { onClick?: () => void }) {
  // ...
})

// Used without optional onClick
<UserAvatar />

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.

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

function Form() {
  const [submitted, setSubmitted] = useState(false)
  const theme = useContext(ThemeContext)

  useEffect(() => {
    if (submitted) {
      post('/api/register')
      showToast('Registered', theme)
    }
  }, [submitted, theme])

  return <button onClick={() => setSubmitted(true)}>Submit</button>
}

Correct: do it in the handler

function Form() {
  const theme = useContext(ThemeContext)

  function handleSubmit() {
    post('/api/register')
    showToast('Registered', theme)
  }

  return <button onClick={handleSubmit}>Submit</button>
}

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

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:

function Header({ user, notifications }: Props) {
  const isLoading = useMemo(() => {
    return user.isLoading || notifications.isLoading
  }, [user.isLoading, notifications.isLoading])

  if (isLoading) return <Skeleton />
  // return some markup
}

Correct:

function Header({ user, notifications }: Props) {
  const isLoading = user.isLoading || notifications.isLoading

  if (isLoading) return <Skeleton />
  // return some markup
}

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)
  }, [])
}

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

function Tracker() {
  const [lastX, setLastX] = useState(0)

  useEffect(() => {
    const onMove = (e: MouseEvent) => setLastX(e.clientX)
    window.addEventListener('mousemove', onMove)
    return () => window.removeEventListener('mousemove', onMove)
  }, [])

  return (
    <div
      style={{
        position: 'fixed',
        top: 0,
        left: lastX,
        width: 8,
        height: 8,
        background: 'black',
      }}
    />
  )
}

Correct: no re-render for tracking

function Tracker() {
  const lastXRef = useRef(0)
  const dotRef = useRef<HTMLDivElement>(null)

  useEffect(() => {
    const onMove = (e: MouseEvent) => {
      lastXRef.current = e.clientX
      const node = dotRef.current
      if (node) {
        node.style.transform = `translateX(${e.clientX}px)`
      }
    }
    window.addEventListener('mousemove', onMove)
    return () => window.removeEventListener('mousemove', onMove)
  }, [])

  return (
    <div
      ref={dotRef}
      style={{
        position: 'fixed',
        top: 0,
        left: 0,
        width: 8,
        height: 8,
        background: 'black',
        transform: 'translateX(0px)',
      }}
    />
  )
}

Better Alternatives to useEffect

Table of Contents

• Quick Reference
• When You DO Need Effects
• When You DON'T Need Effects
• Decision Tree
• 1. Calculate During Render (Derived State)
• 2. useMemo for Expensive Calculations
• 3. Key Prop to Reset State
• 4. Store ID Instead of Object
• 5. Event Handlers for User Actions
• 6. useSyncExternalStore for External Stores
• 7. Lifting State Up
• 8. Custom Hooks for Data Fetching
• Summary: When to Use What

Quick Reference

When You DO Need Effects

• Synchronizing with external systems (non-React widgets, browser APIs)
• Subscriptions to external stores (use useSyncExternalStore when possible)
• Analytics/logging that runs because component displayed
• Data fetching with proper cleanup (or use framework's built-in mechanism)

When You DON'T Need Effects

1. Transforming data for rendering - Calculate at top level, re-runs automatically 2. Handling user events - Use event handlers, you know exactly what happened 3. Deriving state - Just compute it: const fullName = firstName + ' ' + lastName 4. Chaining state updates - Calculate all next state in the event handler

Decision Tree

Need to respond to something?
├── User interaction (click, submit, drag)?
│   └── Use EVENT HANDLER
├── Component appeared on screen?
│   └── Use EFFECT (external sync, analytics)
├── Props/state changed and need derived value?
│   └── CALCULATE DURING RENDER
│       └── Expensive? Use useMemo
└── Need to reset state when prop changes?
    └── Use KEY PROP on component

1. Calculate During Render (Derived State)

For values derived from props or state, just compute them:

function Form() {
  const [firstName, setFirstName] = useState('Taylor')
  const [lastName, setLastName] = useState('Swift')

  // Runs every render - that's fine and intentional
  const fullName = firstName + ' ' + lastName
  const isValid = firstName.length > 0 && lastName.length > 0
}

When to use: The value can be computed from existing props/state.

2. useMemo for Expensive Calculations

When computation is expensive, memoize it:

import { useMemo } from 'react'

function TodoList({ todos, filter }) {
  const visibleTodos = useMemo(
    () => getFilteredTodos(todos, filter),
    [todos, filter]
  )
}

How to know if it's expensive:

console.time('filter')
const visibleTodos = getFilteredTodos(todos, filter)
console.timeEnd('filter')
// If > 1ms, consider memoizing

3. Key Prop to Reset State

To reset ALL state when a prop changes, use key:

// Parent passes userId as key
function ProfilePage({ userId }) {
  return (
    <Profile
      userId={userId}
      key={userId}  // Different userId = different component instance
    />
  )
}

function Profile({ userId }) {
  // All state here resets when userId changes
  const [comment, setComment] = useState('')
  const [likes, setLikes] = useState([])
}

When to use: You want a "fresh start" when an identity prop changes.

4. Store ID Instead of Object

To preserve selection when list changes:

// BAD: Storing object that needs Effect to "adjust"
function List({ items }) {
  const [selection, setSelection] = useState(null)

  useEffect(() => {
    setSelection(null) // Reset when items change
  }, [items])
}

// GOOD: Store ID, derive object
function List({ items }) {
  const [selectedId, setSelectedId] = useState(null)

  // Derived - no Effect needed
  const selection = items.find(item => item.id === selectedId) ?? null
}

Benefit: If item with selectedId exists in new list, selection preserved.

5. Event Handlers for User Actions

User clicks/submits/drags should be handled in event handlers, not Effects:

// Event handler knows exactly what happened
function ProductPage({ product, addToCart }) {
  function handleBuyClick() {
    addToCart(product)
    showNotification(`Added ${product.name}!`)
    analytics.track('product_added', { id: product.id })
  }

  function handleCheckoutClick() {
    addToCart(product)
    showNotification(`Added ${product.name}!`)
    navigateTo('/checkout')
  }
}

Shared logic: Extract a function, call from both handlers:

function buyProduct() {
  addToCart(product)
  showNotification(`Added ${product.name}!`)
}

function handleBuyClick() { buyProduct() }
function handleCheckoutClick() { buyProduct(); navigateTo('/checkout') }

6. useSyncExternalStore for External Stores

For subscribing to external data (browser APIs, third-party stores):

import { useSyncExternalStore } from 'react'

function subscribe(callback) {
  window.addEventListener('online', callback)
  window.addEventListener('offline', callback)
  return () => {
    window.removeEventListener('online', callback)
    window.removeEventListener('offline', callback)
  }
}

function useOnlineStatus() {
  return useSyncExternalStore(
    subscribe,
    () => navigator.onLine
  )
}

7. Lifting State Up

When two components need synchronized state, lift it to common ancestor:

// Instead of syncing via Effects between siblings
function Parent() {
  const [value, setValue] = useState('')

  return (
    <>
      <Input value={value} onChange={setValue} />
      <Preview value={value} />
    </>
  )
}

8. Custom Hooks for Data Fetching

Extract fetch logic with proper cleanup:

function useData(url) {
  const [data, setData] = useState(null)
  const [error, setError] = useState(null)
  const [loading, setLoading] = useState(true)

  useEffect(() => {
    let ignore = false
    setLoading(true)

    fetch(url)
      .then(res => res.json())
      .then(json => {
        if (!ignore) {
          setData(json)
          setError(null)
        }
      })
      .catch(err => {
        if (!ignore) setError(err)
      })
      .finally(() => {
        if (!ignore) setLoading(false)
      })

    return () => { ignore = true }
  }, [url])

  return { data, error, loading }
}

Summary: When to Use What

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

useEffect Anti-Patterns

Table of Contents

• 1. Redundant State for Derived Values
• 2. Filtering/Transforming Data in Effect
• 3. Resetting State on Prop Change
• 4. Event-Specific Logic in Effect
• 5. Chains of Effects
• 6. Notifying Parent via Effect
• 7. Passing Data Up to Parent
• 8. Fetching Without Cleanup (Race Condition)
• 9. App Initialization in Effect

Effects are an escape hatch from React. They let you synchronize with external systems. If there is no external system involved, you shouldn't need an Effect.

1. Redundant State for Derived Values

// BAD: Extra state + Effect for derived value
function Form() {
  const [firstName, setFirstName] = useState('Taylor')
  const [lastName, setLastName] = useState('Swift')
  const [fullName, setFullName] = useState('')

  useEffect(() => {
    setFullName(firstName + ' ' + lastName)
  }, [firstName, lastName])
}

// GOOD: Calculate during rendering
function Form() {
  const [firstName, setFirstName] = useState('Taylor')
  const [lastName, setLastName] = useState('Swift')
  const fullName = firstName + ' ' + lastName // Just compute it
}

Why it's bad: Causes extra render pass with stale value, then re-renders with updated value.

2. Filtering/Transforming Data in Effect

// BAD: Effect to filter list
function TodoList({ todos, filter }) {
  const [visibleTodos, setVisibleTodos] = useState([])

  useEffect(() => {
    setVisibleTodos(getFilteredTodos(todos, filter))
  }, [todos, filter])
}

// GOOD: Filter during render (memoize if expensive)
function TodoList({ todos, filter }) {
  const visibleTodos = useMemo(
    () => getFilteredTodos(todos, filter),
    [todos, filter]
  )
}

3. Resetting State on Prop Change

// BAD: Effect to reset state
function ProfilePage({ userId }) {
  const [comment, setComment] = useState('')

  useEffect(() => {
    setComment('')
  }, [userId])
}

// GOOD: Use key prop
function ProfilePage({ userId }) {
  return <Profile userId={userId} key={userId} />
}

function Profile({ userId }) {
  const [comment, setComment] = useState('') // Resets automatically
}

Why key works: React treats components with different keys as different components, recreating state.

4. Event-Specific Logic in Effect

// BAD: Effect for button click result
function ProductPage({ product, addToCart }) {
  useEffect(() => {
    if (product.isInCart) {
      showNotification(`Added ${product.name}!`)
    }
  }, [product])

  function handleBuyClick() {
    addToCart(product)
  }
}

// GOOD: Handle in event handler
function ProductPage({ product, addToCart }) {
  function handleBuyClick() {
    addToCart(product)
    showNotification(`Added ${product.name}!`)
  }
}

Why it's bad: Effect fires on page refresh (isInCart is true), showing notification unexpectedly.

5. Chains of Effects

// BAD: Effects triggering each other
function Game() {
  const [card, setCard] = useState(null)
  const [goldCardCount, setGoldCardCount] = useState(0)
  const [round, setRound] = useState(1)
  const [isGameOver, setIsGameOver] = useState(false)

  useEffect(() => {
    if (card?.gold) setGoldCardCount(c => c + 1)
  }, [card])

  useEffect(() => {
    if (goldCardCount > 3) {
      setRound(r => r + 1)
      setGoldCardCount(0)
    }
  }, [goldCardCount])

  useEffect(() => {
    if (round > 5) setIsGameOver(true)
  }, [round])
}

// GOOD: Calculate in event handler
function Game() {
  const [card, setCard] = useState(null)
  const [goldCardCount, setGoldCardCount] = useState(0)
  const [round, setRound] = useState(1)
  const isGameOver = round > 5 // Derived!

  function handlePlaceCard(nextCard) {
    if (isGameOver) throw Error('Game ended')

    setCard(nextCard)
    if (nextCard.gold) {
      if (goldCardCount < 3) {
        setGoldCardCount(goldCardCount + 1)
      } else {
        setGoldCardCount(0)
        setRound(round + 1)
        if (round === 5) alert('Good game!')
      }
    }
  }
}

Why it's bad: Multiple re-renders (setCard -> setGoldCardCount -> setRound -> setIsGameOver). Also fragile for features like history replay.

6. Notifying Parent via Effect

// BAD: Effect to notify parent
function Toggle({ onChange }) {
  const [isOn, setIsOn] = useState(false)

  useEffect(() => {
    onChange(isOn)
  }, [isOn, onChange])

  function handleClick() {
    setIsOn(!isOn)
  }
}

// GOOD: Notify in same event
function Toggle({ onChange }) {
  const [isOn, setIsOn] = useState(false)

  function updateToggle(nextIsOn) {
    setIsOn(nextIsOn)
    onChange(nextIsOn) // Same event, batched render
  }

  function handleClick() {
    updateToggle(!isOn)
  }
}

// BEST: Fully controlled component
function Toggle({ isOn, onChange }) {
  function handleClick() {
    onChange(!isOn)
  }
}

7. Passing Data Up to Parent

// BAD: Child fetches, passes up via Effect
function Parent() {
  const [data, setData] = useState(null)
  return <Child onFetched={setData} />
}

function Child({ onFetched }) {
  const data = useSomeAPI()

  useEffect(() => {
    if (data) onFetched(data)
  }, [onFetched, data])
}

// GOOD: Parent fetches, passes down
function Parent() {
  const data = useSomeAPI()
  return <Child data={data} />
}

Why: Data should flow down. Upward flow via Effects makes debugging hard.

8. Fetching Without Cleanup (Race Condition)

// BAD: No cleanup - race condition
function SearchResults({ query }) {
  const [results, setResults] = useState([])

  useEffect(() => {
    fetchResults(query).then(json => {
      setResults(json) // "hello" response may arrive after "hell"
    })
  }, [query])
}

// GOOD: Cleanup ignores stale responses
function SearchResults({ query }) {
  const [results, setResults] = useState([])

  useEffect(() => {
    let ignore = false

    fetchResults(query).then(json => {
      if (!ignore) setResults(json)
    })

    return () => { ignore = true }
  }, [query])
}

9. App Initialization in Effect

// BAD: Runs twice in dev, may break auth
function App() {
  useEffect(() => {
    loadDataFromLocalStorage()
    checkAuthToken() // May invalidate token on second call!
  }, [])
}

// GOOD: Module-level guard
let didInit = false

function App() {
  useEffect(() => {
    if (!didInit) {
      didInit = true
      loadDataFromLocalStorage()
      checkAuthToken()
    }
  }, [])
}

// ALSO GOOD: Module-level execution
if (typeof window !== 'undefined') {
  checkAuthToken()
  loadDataFromLocalStorage()
}

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