[ PROMPT_NODE_25165 ]

React Best Practices – Agents

[ SKILL_DOCUMENTATION ]

# React Best Practices **Version 0.1.0** Vercel Engineering January 2026 > **Note:** > This document is mainly for agents and LLMs to follow when maintaining, > generating, or refactoring React and Next.js codebases at Vercel. Humans > may also find it useful, but guidance here is optimized for automation > and consistency by AI-assisted workflows. --- ## Abstract Comprehensive performance optimization guide for React and Next.js applications, designed for AI agents and LLMs. Contains 40+ rules across 8 categories, prioritized by impact from critical (eliminating waterfalls, reducing bundle size) to incremental (advanced patterns). Each rule includes detailed explanations, real-world examples comparing incorrect vs. correct implementations, and specific impact metrics to guide automated refactoring and code generation. --- ## Table of Contents 1. [Eliminating Waterfalls](#1-eliminating-waterfalls) — **CRITICAL** - 1.1 [Defer Await Until Needed](#11-defer-await-until-needed) - 1.2 [Dependency-Based Parallelization](#12-dependency-based-parallelization) - 1.3 [Prevent Waterfall Chains in API Routes](#13-prevent-waterfall-chains-in-api-routes) - 1.4 [Promise.all() for Independent Operations](#14-promiseall-for-independent-operations) - 1.5 [Strategic Suspense Boundaries](#15-strategic-suspense-boundaries) 2. [Bundle Size Optimization](#2-bundle-size-optimization) — **CRITICAL** - 2.1 [Avoid Barrel File Imports](#21-avoid-barrel-file-imports) - 2.2 [Conditional Module Loading](#22-conditional-module-loading) - 2.3 [Defer Non-Critical Third-Party Libraries](#23-defer-non-critical-third-party-libraries) - 2.4 [Dynamic Imports for Heavy Components](#24-dynamic-imports-for-heavy-components) - 2.5 [Preload Based on User Intent](#25-preload-based-on-user-intent) 3. [Server-Side Performance](#3-server-side-performance) — **HIGH** - 3.1 [Cross-Request LRU Caching](#31-cross-request-lru-caching) - 3.2 [Minimize Serialization at RSC Boundaries](#32-minimize-serialization-at-rsc-boundaries) - 3.3 [Parallel Data Fetching with Component Composition](#33-parallel-data-fetching-with-component-composition) - 3.4 [Per-Request Deduplication with React.cache()](#34-per-request-deduplication-with-reactcache) - 3.5 [Use after() for Non-Blocking Operations](#35-use-after-for-non-blocking-operations) 4. [Client-Side Data Fetching](#4-client-side-data-fetching) — **MEDIUM-HIGH** - 4.1 [Deduplicate Global Event Listeners](#41-deduplicate-global-event-listeners) - 4.2 [Use SWR for Automatic Deduplication](#42-use-swr-for-automatic-deduplication) 5. [Re-render Optimization](#5-re-render-optimization) — **MEDIUM** - 5.1 [Defer State Reads to Usage Point](#51-defer-state-reads-to-usage-point) - 5.2 [Extract to Memoized Components](#52-extract-to-memoized-components) - 5.3 [Narrow Effect Dependencies](#53-narrow-effect-dependencies) - 5.4 [Subscribe to Derived State](#54-subscribe-to-derived-state) - 5.5 [Use Functional setState Updates](#55-use-functional-setstate-updates) - 5.6 [Use Lazy State Initialization](#56-use-lazy-state-initialization) - 5.7 [Use Transitions for Non-Urgent Updates](#57-use-transitions-for-non-urgent-updates) 6. [Rendering Performance](#6-rendering-performance) — **MEDIUM** - 6.1 [Animate SVG Wrapper Instead of SVG Element](#61-animate-svg-wrapper-instead-of-svg-element) - 6.2 [CSS content-visibility for Long Lists](#62-css-content-visibility-for-long-lists) - 6.3 [Hoist Static JSX Elements](#63-hoist-static-jsx-elements) - 6.4 [Optimize SVG Precision](#64-optimize-svg-precision) - 6.5 [Prevent Hydration Mismatch Without Flickering](#65-prevent-hydration-mismatch-without-flickering) - 6.6 [Use Activity Component for Show/Hide](#66-use-activity-component-for-showhide) - 6.7 [Use Explicit Conditional Rendering](#67-use-explicit-conditional-rendering) 7. [JavaScript Performance](#7-javascript-performance) — **LOW-MEDIUM** - 7.1 [Batch DOM CSS Changes](#71-batch-dom-css-changes) - 7.2 [Build Index Maps for Repeated Lookups](#72-build-index-maps-for-repeated-lookups) - 7.3 [Cache Property Access in Loops](#73-cache-property-access-in-loops) - 7.4 [Cache Repeated Function Calls](#74-cache-repeated-function-calls) - 7.5 [Cache Storage API Calls](#75-cache-storage-api-calls) - 7.6 [Combine Multiple Array Iterations](#76-combine-multiple-array-iterations) - 7.7 [Early Length Check for Array Comparisons](#77-early-length-check-for-array-comparisons) - 7.8 [Early Return from Functions](#78-early-return-from-functions) - 7.9 [Hoist RegExp Creation](#79-hoist-regexp-creation) - 7.10 [Use Loop for Min/Max Instead of Sort](#710-use-loop-for-minmax-instead-of-sort) - 7.11 [Use Set/Map for O(1) Lookups](#711-use-setmap-for-o1-lookups) - 7.12 [Use toSorted() Instead of sort() for Immutability](#712-use-tosorted-instead-of-sort-for-immutability) 8. [Advanced Patterns](#8-advanced-patterns) — **LOW** - 8.1 [Store Event Handlers in Refs](#81-store-event-handlers-in-refs) - 8.2 [useLatest for Stable Callback Refs](#82-uselatest-for-stable-callback-refs) --- ## 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** ```typescript 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** ```typescript 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** ```typescript // Incorrect: always fetches permissions async function updateResource(resourceId: string, userId: string) { const permissions = await fetchPermissions(userId) const resource = await getResource(resourceId) if (!resource) { return { error: 'Not found' } } if (!permissions.canEdit) { return { error: 'Forbidden' } } return await updateResourceData(resource, permissions) } // Correct: fetches only when needed async function updateResource(resourceId: string, userId: string) { const resource = await getResource(resourceId) if (!resource) { return { error: 'Not found' } } const permissions = await fetchPermissions(userId) if (!permissions.canEdit) { return { error: 'Forbidden' } } return await updateResourceData(resource, permissions) } ``` This optimization is especially valuable when the skipped branch is frequently taken, or when the deferred operation is expensive. ### 1.2 Dependency-Based Parallelization **Impact: CRITICAL (2-10× improvement)** For operations with partial dependencies, use `better-all` to maximize parallelism. It automatically starts each task at the earliest possible moment. **Incorrect: profile waits for config unnecessarily** ```typescript const [user, config] = await Promise.all([ fetchUser(), fetchConfig() ]) const profile = await fetchProfile(user.id) ``` **Correct: config and profile run in parallel** ```typescript import { all } from 'better-all' const { user, config, profile } = await all({ async user() { return fetchUser() }, async config() { return fetchConfig() }, async profile() { return fetchProfile((await this.$.user).id) } }) ``` Reference: [https://github.com/shuding/better-all](https://github.com/shuding/better-all) ### 1.3 Prevent Waterfall Chains in API Routes **Impact: CRITICAL (2-10× improvement)** In API routes and Server Actions, start independent operations immediately, even if you don't await them yet. **Incorrect: config waits for auth, data waits for both** ```typescript 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** ```typescript export async function GET(request: Request) { const sessionPromise = auth() const configPromise = fetchConfig() const session = await sessionPromise const [config, data] = await Promise.all([ configPromise, fetchData(session.user.id) ]) return Response.json({ data, config }) } ``` For operations with more complex dependency chains, use `better-all` to automatically maximize parallelism (see Dependency-Based Parallelization). ### 1.4 Promise.all() for Independent Operations **Impact: CRITICAL (2-10× improvement)** When async operations have no interdependencies, execute them concurrently using `Promise.all()`. **Incorrect: sequential execution, 3 round trips** ```typescript const user = await fetchUser() const posts = await fetchPosts() const comments = await fetchComments() ``` **Correct: parallel execution, 1 round trip** ```typescript const [user, posts, comments] = await Promise.all([ fetchUser(), fetchPosts(), fetchComments() ]) ``` ### 1.5 Strategic Suspense Boundaries **Impact: HIGH (faster initial paint)** Instead of awaiting data in async components before returning JSX, use Suspense boundaries to show the wrapper UI faster while data loads. **Incorrect: wrapper blocked by data fetching** ```tsx async function Page() { const data = await fetchData() // Blocks entire page return (

Sidebar

Header

Footer

) } ``` The entire layout waits for data even though only the middle section needs it. **Correct: wrapper shows immediately, data streams in** ```tsx function Page() { return (

Sidebar

Header

Footer

) } async function DataDisplay() { const data = await fetchData() // Only blocks this component return

{data.content}

} ``` Sidebar, Header, and Footer render immediately. Only DataDisplay waits for data. **Alternative: share promise across components** ```tsx function Page() { // Start fetch immediately, but don't await const dataPromise = fetchData() return (

Sidebar

Header

Footer

) } function DataDisplay({ dataPromise }: { dataPromise: Promise }) { const data = use(dataPromise) // Unwraps the promise return

{data.content}

} function DataSummary({ dataPromise }: { dataPromise: Promise }) { const data = use(dataPromise) // Reuses the same promise return

{data.summary}

} ``` Both components share the same promise, so only one fetch occurs. Layout renders immediately while both components wait together. **When NOT to use this pattern:** - Critical data needed for layout decisions (affects positioning) - SEO-critical content above the fold - Small, fast queries where suspense overhead isn't worth it - When you want to avoid layout shift (loading → content jump) **Trade-off:** Faster initial paint vs potential layout shift. Choose based on your UX priorities. --- ## 2. Bundle Size Optimization **Impact: CRITICAL** Reducing initial bundle size improves Time to Interactive and Largest Contentful Paint. ### 2.1 Avoid Barrel File Imports **Impact: CRITICAL (200-800ms import cost, slow builds)** Import directly from source files instead of barrel files to avoid loading thousands of unused modules. **Barrel files** are entry points that re-export multiple modules (e.g., `index.js` that does `export * from './module'`). Popular icon and component libraries can have **up to 10,000 re-exports** in their entry file. For many React packages, **it takes 200-800ms just to import them**, affecting both development speed and production cold starts. **Why tree-shaking doesn't help:** When a library is marked as external (not bundled), the bundler can't optimize it. If you bundle it to enable tree-shaking, builds become substantially slower analyzing the entire module graph. **Incorrect: imports entire library** ```tsx 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** ```tsx import Check from 'lucide-react/dist/esm/icons/check' import X from 'lucide-react/dist/esm/icons/x' import Menu from 'lucide-react/dist/esm/icons/menu' // Loads only 3 modules (~2KB vs ~1MB) import Button from '@mui/material/Button' import TextField from '@mui/material/TextField' // Loads only what you use ``` **Alternative: Next.js 13.5+** ```js // next.config.js - use optimizePackageImports module.exports = { experimental: { optimizePackageImports: ['lucide-react', '@mui/material'] } } // Then you can keep the ergonomic barrel imports: import { Check, X, Menu } from 'lucide-react' // Automatically transformed to direct imports at build time ``` Direct imports provide 15-70% faster dev boot, 28% faster builds, 40% faster cold starts, and significantly faster HMR. Libraries commonly affected: `lucide-react`, `@mui/material`, `@mui/icons-material`, `@tabler/icons-react`, `react-icons`, `@headlessui/react`, `@radix-ui/react-*`, `lodash`, `ramda`, `date-fns`, `rxjs`, `react-use`. Reference: [https://vercel.com/blog/how-we-optimized-package-imports-in-next-js](https://vercel.com/blog/how-we-optimized-package-imports-in-next-js) ### 2.2 Conditional Module Loading **Impact: HIGH (loads large data only when needed)** Load large data or modules only when a feature is activated. **Example: lazy-load animation frames** ```tsx function AnimationPlayer({ enabled }: { enabled: boolean }) { const [frames, setFrames] = useState(null) useEffect(() => { if (enabled && !frames && typeof window !== 'undefined') { import('./animation-frames.js') .then(mod => setFrames(mod.frames)) .catch(() => setEnabled(false)) } }, [enabled, frames]) if (!frames) return return } ``` The `typeof window !== 'undefined'` check prevents bundling this module for SSR, optimizing server bundle size and build speed. ### 2.3 Defer Non-Critical Third-Party Libraries **Impact: MEDIUM (loads after hydration)** Analytics, logging, and error tracking don't block user interaction. Load them after hydration. **Incorrect: blocks initial bundle** ```tsx import { Analytics } from '@vercel/analytics/react' export default function RootLayout({ children }) { return ( {children} ) } ``` **Correct: loads after hydration** ```tsx import dynamic from 'next/dynamic' const Analytics = dynamic( () => import('@vercel/analytics/react').then(m => m.Analytics), { ssr: false } ) export default function RootLayout({ children }) { return ( {children} ) } ``` ### 2.4 Dynamic Imports for Heavy Components **Impact: CRITICAL (directly affects TTI and LCP)** Use `next/dynamic` to lazy-load large components not needed on initial render. **Incorrect: Monaco bundles with main chunk ~300KB** ```tsx import { MonacoEditor } from './monaco-editor' function CodePanel({ code }: { code: string }) { return } ``` **Correct: Monaco loads on demand** ```tsx import dynamic from 'next/dynamic' const MonacoEditor = dynamic( () => import('./monaco-editor').then(m => m.MonacoEditor), { ssr: false } ) function CodePanel({ code }: { code: string }) { return } ``` ### 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** ```tsx function EditorButton({ onClick }: { onClick: () => void }) { const preload = () => { if (typeof window !== 'undefined') { void import('./monaco-editor') } } return ( ) } ``` **Example: preload when feature flag is enabled** ```tsx function FlagsProvider({ children, flags }: Props) { useEffect(() => { if (flags.editorEnabled && typeof window !== 'undefined') { void import('./monaco-editor').then(mod => mod.init()) } }, [flags.editorEnabled]) return {children} } ``` The `typeof window !== 'undefined'` check prevents bundling preloaded modules for SSR, optimizing server bundle size and build speed. --- ## 3. Server-Side Performance **Impact: HIGH** Optimizing server-side rendering and data fetching eliminates server-side waterfalls and reduces response times. ### 3.1 Cross-Request LRU Caching **Impact: HIGH (caches across requests)** `React.cache()` only works within one request. For data shared across sequential requests (user clicks button A then button B), use an LRU cache. **Implementation:** ```typescript import { LRUCache } from 'lru-cache' const cache = new LRUCache({ max: 1000, ttl: 5 * 60 * 1000 // 5 minutes }) export async function getUser(id: string) { const cached = cache.get(id) if (cached) return cached const user = await db.user.findUnique({ where: { id } }) cache.set(id, user) return user } // Request 1: DB query, result cached // Request 2: cache hit, no DB query ``` Use when sequential user actions hit multiple endpoints needing the same data within seconds. **With Vercel's [Fluid Compute](https://vercel.com/docs/fluid-compute):** LRU caching is especially effective because multiple concurrent requests can share the same function instance and cache. This means the cache persists across requests without needing external storage like Redis. **In traditional serverless:** Each invocation runs in isolation, so consider Redis for cross-process caching. Reference: [https://github.com/isaacs/node-lru-cache](https://github.com/isaacs/node-lru-cache) ### 3.2 Minimize Serialization at RSC Boundaries **Impact: HIGH (reduces data transfer size)** The React Server/Client boundary serializes all object properties into strings and embeds them in the HTML response and subsequent RSC requests. This serialized data directly impacts page weight and load time, so **size matters a lot**. Only pass fields that the client actually uses. **Incorrect: serializes all 50 fields** ```tsx async function Page() { const user = await fetchUser() // 50 fields return } 'use client' function Profile({ user }: { user: User }) { return

{user.name}

// uses 1 field } ``` **Correct: serializes only 1 field** ```tsx async function Page() { const user = await fetchUser() return } 'use client' function Profile({ name }: { name: string }) { return

{name}

} ``` ### 3.3 Parallel Data Fetching with Component Composition **Impact: CRITICAL (eliminates server-side waterfalls)** React Server Components execute sequentially within a tree. Restructure with composition to parallelize data fetching. **Incorrect: Sidebar waits for Page's fetch to complete** ```tsx export default async function Page() { const header = await fetchHeader() return (

{header}

) } async function Sidebar() { const items = await fetchSidebarItems() return } ``` **Correct: both fetch simultaneously** ```tsx async function Header() { const data = await fetchHeader() return

{data}

} async function Sidebar() { const items = await fetchSidebarItems() return } export default function Page() { return (

) } ``` **Alternative with children prop:** ```tsx async function Layout({ children }: { children: ReactNode }) { const header = await fetchHeader() return (

{header}

{children}

) } async function Sidebar() { const items = await fetchSidebarItems() return } export default function Page() { return ( ) } ``` ### 3.4 Per-Request Deduplication with React.cache() **Impact: MEDIUM (deduplicates within request)** Use `React.cache()` for server-side request deduplication. Authentication and database queries benefit most. **Usage:** ```typescript import { cache } from 'react' export const getCurrentUser = cache(async () => { const session = await auth() if (!session?.user?.id) return null return await db.user.findUnique({ where: { id: session.user.id } }) }) ``` Within a single request, multiple calls to `getCurrentUser()` execute the query only once. ### 3.5 Use after() for Non-Blocking Operations **Impact: MEDIUM (faster response times)** Use Next.js's `after()` to schedule work that should execute after a response is sent. This prevents logging, analytics, and other side effects from blocking the response. **Incorrect: blocks response** ```tsx import { logUserAction } from '@/app/utils' export async function POST(request: Request) { // Perform mutation await updateDatabase(request) // Logging blocks the response const userAgent = request.headers.get('user-agent') || 'unknown' await logUserAction({ userAgent }) return new Response(JSON.stringify({ status: 'success' }), { status: 200, headers: { 'Content-Type': 'application/json' } }) } ``` **Correct: non-blocking** ```tsx import { after } from 'next/server' import { headers, cookies } from 'next/headers' import { logUserAction } from '@/app/utils' export async function POST(request: Request) { // Perform mutation await updateDatabase(request) // Log after response is sent after(async () => { const userAgent = (await headers()).get('user-agent') || 'unknown' const sessionCookie = (await cookies()).get('session-id')?.value || 'anonymous' logUserAction({ sessionCookie, userAgent }) }) return new Response(JSON.stringify({ status: 'success' }), { status: 200, headers: { 'Content-Type': 'application/json' } }) } ``` The response is sent immediately while logging happens in the background. **Common use cases:** - Analytics tracking - Audit logging - Sending notifications - Cache invalidation - Cleanup tasks **Important notes:** - `after()` runs even if the response fails or redirects - Works in Server Actions, Route Handlers, and Server Components Reference: [https://nextjs.org/docs/app/api-reference/functions/after](https://nextjs.org/docs/app/api-reference/functions/after) --- ## 4. Client-Side Data Fetching **Impact: MEDIUM-HIGH** Automatic deduplication and efficient data fetching patterns reduce redundant network requests. ### 4.1 Deduplicate Global Event Listeners **Impact: LOW (single listener for N components)** Use `useSWRSubscription()` to share global event listeners across component instances. **Incorrect: N instances = N listeners** ```tsx 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** ```tsx import useSWRSubscription from 'swr/subscription' // Module-level Map to track callbacks per key const keyCallbacks = new Map<string, Set void>>() function useKeyboardShortcut(key: string, callback: () => void) { // Register this callback in the Map useEffect(() => { if (!keyCallbacks.has(key)) { keyCallbacks.set(key, new Set()) } keyCallbacks.get(key)!.add(callback) return () => { const set = keyCallbacks.get(key) if (set) { set.delete(callback) if (set.size === 0) { keyCallbacks.delete(key) } } } }, [key, callback]) useSWRSubscription('global-keydown', () => { const handler = (e: KeyboardEvent) => { if (e.metaKey && keyCallbacks.has(e.key)) { keyCallbacks.get(e.key)!.forEach(cb => cb()) } } window.addEventListener('keydown', handler) return () => window.removeEventListener('keydown', handler) }) } function Profile() { // Multiple shortcuts will share the same listener useKeyboardShortcut('p', () => { /* ... */ }) useKeyboardShortcut('k', () => { /* ... */ }) // ... } ``` ### 4.2 Use SWR for Automatic Deduplication **Impact: MEDIUM-HIGH (automatic deduplication)** SWR enables request deduplication, caching, and revalidation across component instances. **Incorrect: no deduplication, each instance fetches** ```tsx function UserList() { const [users, setUsers] = useState([]) useEffect(() => { fetch('/api/users') .then(r => r.json()) .then(setUsers) }, []) } ``` **Correct: multiple instances share one request** ```tsx import useSWR from 'swr' function UserList() { const { data: users } = useSWR('/api/users', fetcher) } ``` **For immutable data:** ```tsx import { useImmutableSWR } from '@/lib/swr' function StaticContent() { const { data } = useImmutableSWR('/api/config', fetcher) } ``` **For mutations:** ```tsx import { useSWRMutation } from 'swr/mutation' function UpdateButton() { const { trigger } = useSWRMutation('/api/user', updateUser) return } ``` Reference: [https://swr.vercel.app](https://swr.vercel.app) --- ## 5. Re-render Optimization **Impact: MEDIUM** Reducing unnecessary re-renders minimizes wasted computation and improves UI responsiveness. ### 5.1 Defer State Reads to Usage Point **Impact: MEDIUM (avoids unnecessary subscriptions)** Don't subscribe to dynamic state (searchParams, localStorage) if you only read it inside callbacks. **Incorrect: subscribes to all searchParams changes** ```tsx function ShareButton({ chatId }: { chatId: string }) { const searchParams = useSearchParams() const handleShare = () => { const ref = searchParams.get('ref') shareChat(chatId, { ref }) } return } ``` **Correct: reads on demand, no subscription** ```tsx function ShareButton({ chatId }: { chatId: string }) { const handleShare = () => { const params = new URLSearchParams(window.location.search) const ref = params.get('ref') shareChat(chatId, { ref }) } return } ``` ### 5.2 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** ```tsx function Profile({ user, loading }: Props) { const avatar = useMemo(() => { const id = computeAvatarId(user) return }, [user]) if (loading) return return

{avatar}

} ``` **Correct: skips computation when loading** ```tsx const UserAvatar = memo(function UserAvatar({ user }: { user: User }) { const id = useMemo(() => computeAvatarId(user), [user]) return }) function Profile({ user, loading }: Props) { if (loading) return return (

) } ``` **Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, manual memoization with `memo()` and `useMemo()` is not necessary. The compiler automatically optimizes re-renders. ### 5.3 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** ```tsx useEffect(() => { console.log(user.id) }, [user]) ``` **Correct: re-runs only when id changes** ```tsx useEffect(() => { console.log(user.id) }, [user.id]) ``` **For derived state, compute outside effect:** ```tsx // Incorrect: runs on width=767, 766, 765... useEffect(() => { if (width < 768) { enableMobileMode() } }, [width]) // Correct: runs only on boolean transition const isMobile = width { if (isMobile) { enableMobileMode() } }, [isMobile]) ``` ### 5.4 Subscribe to Derived State **Impact: MEDIUM (reduces re-render frequency)** Subscribe to derived boolean state instead of continuous values to reduce re-render frequency. **Incorrect: re-renders on every pixel change** ```tsx function Sidebar() { const width = useWindowWidth() // updates continuously const isMobile = width < 768 return

} ``` **Correct: re-renders only when boolean changes** ```tsx function Sidebar() { const isMobile = useMediaQuery('(max-width: 767px)') return

} ``` ### 5.5 Use Functional setState Updates **Impact: MEDIUM (prevents stale closures and unnecessary callback recreations)** When updating state based on the current state value, use the functional update form of setState instead of directly referencing the state variable. This prevents stale closures, eliminates unnecessary dependencies, and creates stable callback references. **Incorrect: requires state as dependency** ```tsx function TodoList() { const [items, setItems] = useState(initialItems) // Callback must depend on items, recreated on every items change const addItems = useCallback((newItems: Item[]) => { setItems([...items, ...newItems]) }, [items]) // ❌ items dependency causes recreations // Risk of stale closure if dependency is forgotten const removeItem = useCallback((id: string) => { setItems(items.filter(item => item.id !== id)) }, []) // ❌ Missing items dependency - will use stale items! return } ``` 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** ```tsx 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 } ``` **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](https://react.dev/learn/react-compiler) enabled, the compiler can automatically optimize some cases, but functional updates are still recommended for correctness and to prevent stale closure bugs. ### 5.6 Use Lazy State Initialization **Impact: MEDIUM (wasted computation on every render)** Pass a function to `useState` for expensive initial values. Without the function form, the initializer runs on every render even though the value is only used once. **Incorrect: runs on every render** ```tsx 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 } function UserProfile() { // JSON.parse runs on every render const [settings, setSettings] = useState( JSON.parse(localStorage.getItem('settings') || '{}') ) return } ``` **Correct: runs only once** ```tsx function FilteredList({ items }: { items: Item[] }) { // buildSearchIndex() runs ONLY on initial render const [searchIndex, setSearchIndex] = useState(() => buildSearchIndex(items)) const [query, setQuery] = useState('') return } function UserProfile() { // JSON.parse runs only on initial render const [settings, setSettings] = useState(() => { const stored = localStorage.getItem('settings') return stored ? JSON.parse(stored) : {} }) return } ``` Use lazy initialization when computing initial values from localStorage/sessionStorage, building data structures (indexes, maps), reading from the DOM, or performing heavy transformations. For simple primitives (`useState(0)`), direct references (`useState(props.value)`), or cheap literals (`useState({})`), the function form is unnecessary. ### 5.7 Use Transitions for Non-Urgent Updates **Impact: MEDIUM (maintains UI responsiveness)** Mark frequent, non-urgent state updates as transitions to maintain UI responsiveness. **Incorrect: blocks UI on every scroll** ```tsx 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** ```tsx import { startTransition } from 'react' function ScrollTracker() { const [scrollY, setScrollY] = useState(0) useEffect(() => { const handler = () => { startTransition(() => setScrollY(window.scrollY)) } window.addEventListener('scroll', handler, { passive: true }) return () => window.removeEventListener('scroll', handler) }, []) } ``` --- ## 6. Rendering Performance **Impact: MEDIUM** Optimizing the rendering process reduces the work the browser needs to do. ### 6.1 Animate SVG Wrapper Instead of SVG Element **Impact: LOW (enables hardware acceleration)** Many browsers don't have hardware acceleration for CSS3 animations on SVG elements. Wrap SVG in a `

` and animate the wrapper instead. **Incorrect: animating SVG directly - no hardware acceleration** ```tsx function LoadingSpinner() { return ( ) } ``` **Correct: animating wrapper div - hardware accelerated** ```tsx function LoadingSpinner() { return (

) } ``` This applies to all CSS transforms and transitions (`transform`, `opacity`, `translate`, `scale`, `rotate`). The wrapper div allows browsers to use GPU acceleration for smoother animations. ### 6.2 CSS content-visibility for Long Lists **Impact: HIGH (faster initial render)** Apply `content-visibility: auto` to defer off-screen rendering. **CSS:** ```css .message-item { content-visibility: auto; contain-intrinsic-size: 0 80px; } ``` **Example:** ```tsx function MessageList({ messages }: { messages: Message[] }) { return (

{messages.map(msg => (

{msg.content}

))}

) } ``` For 1000 messages, browser skips layout/paint for ~990 off-screen items (10× faster initial render). ### 6.3 Hoist Static JSX Elements **Impact: LOW (avoids re-creation)** Extract static JSX outside components to avoid re-creation. **Incorrect: recreates element every render** ```tsx function LoadingSkeleton() { return

} function Container() { return (

{loading && }

) } ``` **Correct: reuses same element** ```tsx const loadingSkeleton = (

) function Container() { return (

{loading && loadingSkeleton}

) } ``` This is especially helpful for large and static SVG nodes, which can be expensive to recreate on every render. **Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, the compiler automatically hoists static JSX elements and optimizes component re-renders, making manual hoisting unnecessary. ### 6.4 Optimize SVG Precision **Impact: LOW (reduces file size)** Reduce SVG coordinate precision to decrease file size. The optimal precision depends on the viewBox size, but in general reducing precision should be considered. **Incorrect: excessive precision** ```svg ``` **Correct: 1 decimal place** ```svg ``` **Automate with SVGO:** ```bash npx svgo --precision=1 --multipass icon.svg ``` ### 6.5 Prevent Hydration Mismatch Without Flickering **Impact: MEDIUM (avoids visual flicker and hydration errors)** When rendering content that depends on client-side storage (localStorage, cookies), avoid both SSR breakage and post-hydration flickering by injecting a synchronous script that updates the DOM before React hydrates. **Incorrect: breaks SSR** ```tsx function ThemeWrapper({ children }: { children: ReactNode }) { // localStorage is not available on server - throws error const theme = localStorage.getItem('theme') || 'light' return (

{children}

) } ``` Server-side rendering will fail because `localStorage` is undefined. **Incorrect: visual flickering** ```tsx function ThemeWrapper({ children }: { children: ReactNode }) { const [theme, setTheme] = useState('light') useEffect(() => { // Runs after hydration - causes visible flash const stored = localStorage.getItem('theme') if (stored) { setTheme(stored) } }, []) return (

{children}

) } ``` Component first renders with default value (`light`), then updates after hydration, causing a visible flash of incorrect content. **Correct: no flicker, no hydration mismatch** ```tsx function ThemeWrapper({ children }: { children: ReactNode }) { return (

{children}

` to preserve state/DOM for expensive components that frequently toggle visibility. **Usage:** ```tsx import { Activity } from 'react' function Dropdown({ isOpen }: Props) { return ( ) } ``` Avoids expensive re-renders and state loss. ### 6.7 Use Explicit Conditional Rendering **Impact: LOW (prevents rendering 0 or NaN)** Use explicit ternary operators (`? :`) instead of `&&` for conditional rendering when the condition can be `0`, `NaN`, or other falsy values that render. **Incorrect: renders "0" when count is 0** ```tsx function Badge({ count }: { count: number }) { return (

{count && {count}}

) } // When count = 0, renders:

// When count = 5, renders:

``` **Correct: renders nothing when count is 0** ```tsx function Badge({ count }: { count: number }) { return (

{count > 0 ? {count} : null}

) } // When count = 0, renders:

// When count = 5, renders:

``` --- ## 7. JavaScript Performance **Impact: LOW-MEDIUM** Micro-optimizations for hot paths can add up to meaningful improvements. ### 7.1 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** ```typescript 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** ```typescript // CSS file .highlighted-box { width: 100px; height: 200px; background-color: blue; border: 1px solid black; } // JavaScript function updateElementStyles(element: HTMLElement) { element.classList.add('highlighted-box') } ``` **Correct: change cssText - single reflow** ```typescript function updateElementStyles(element: HTMLElement) { element.style.cssText = ` width: 100px; height: 200px; background-color: blue; border: 1px solid black; ` } ``` **React example:** ```tsx // Incorrect: changing styles one by one function Box({ isHighlighted }: { isHighlighted: boolean }) { const ref = useRef(null) useEffect(() => { if (ref.current && isHighlighted) { ref.current.style.width = '100px' ref.current.style.height = '200px' ref.current.style.backgroundColor = 'blue' } }, [isHighlighted]) return

Content

} // Correct: toggle class function Box({ isHighlighted }: { isHighlighted: boolean }) { return (

Content

) } ``` Prefer CSS classes over inline styles when possible. Classes are cached by the browser and provide better separation of concerns. ### 7.2 Build Index Maps for Repeated Lookups **Impact: LOW-MEDIUM (1M ops to 2K ops)** Multiple `.find()` calls by the same key should use a Map. **Incorrect (O(n) per lookup):** ```typescript function processOrders(orders: Order[], users: User[]) { return orders.map(order => ({ ...order, user: users.find(u => u.id === order.userId) })) } ``` **Correct (O(1) per lookup):** ```typescript function processOrders(orders: Order[], users: User[]) { const userById = new Map(users.map(u => [u.id, u])) return orders.map(order => ({ ...order, user: userById.get(order.userId) })) } ``` Build map once (O(n)), then all lookups are O(1). For 1000 orders × 1000 users: 1M ops → 2K ops. ### 7.3 Cache Property Access in Loops **Impact: LOW-MEDIUM (reduces lookups)** Cache object property lookups in hot paths. **Incorrect: 3 lookups × N iterations** ```typescript for (let i = 0; i < arr.length; i++) { process(obj.config.settings.value) } ``` **Correct: 1 lookup total** ```typescript const value = obj.config.settings.value const len = arr.length for (let i = 0; i < len; i++) { process(value) } ``` ### 7.4 Cache Repeated Function Calls **Impact: MEDIUM (avoid redundant computation)** Use a module-level Map to cache function results when the same function is called repeatedly with the same inputs during render. **Incorrect: redundant computation** ```typescript function ProjectList({ projects }: { projects: Project[] }) { return (

{projects.map(project => { // slugify() called 100+ times for same project names const slug = slugify(project.name) return })}

) } ``` **Correct: cached results** ```typescript // Module-level cache const slugifyCache = new Map() 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 (

{projects.map(project => { // Computed only once per unique project name const slug = cachedSlugify(project.name) return })}

) } ``` **Simpler pattern for single-value functions:** ```typescript let isLoggedInCache: boolean | null = null function isLoggedIn(): boolean { if (isLoggedInCache !== null) { return isLoggedInCache } isLoggedInCache = document.cookie.includes('auth=') return isLoggedInCache } // Clear cache when auth changes function onAuthChange() { isLoggedInCache = null } ``` Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components. Reference: [https://vercel.com/blog/how-we-made-the-vercel-dashboard-twice-as-fast](https://vercel.com/blog/how-we-made-the-vercel-dashboard-twice-as-fast) ### 7.5 Cache Storage API Calls **Impact: LOW-MEDIUM (reduces expensive I/O)** `localStorage`, `sessionStorage`, and `document.cookie` are synchronous and expensive. Cache reads in memory. **Incorrect: reads storage on every call** ```typescript function getTheme() { return localStorage.getItem('theme') ?? 'light' } // Called 10 times = 10 storage reads ``` **Correct: Map cache** ```typescript const storageCache = new Map() function getLocalStorage(key: string) { if (!storageCache.has(key)) { storageCache.set(key, localStorage.getItem(key)) } return storageCache.get(key) } function setLocalStorage(key: string, value: string) { localStorage.setItem(key, value) storageCache.set(key, value) // keep cache in sync } ``` Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components. **Cookie caching:** ```typescript let cookieCache: Record | null = null function getCookie(name: string) { if (!cookieCache) { cookieCache = Object.fromEntries( document.cookie.split('; ').map(c => c.split('=')) ) } return cookieCache[name] } ``` **Important: invalidate on external changes** ```typescript window.addEventListener('storage', (e) => { if (e.key) storageCache.delete(e.key) }) document.addEventListener('visibilitychange', () => { if (document.visibilityState === 'visible') { storageCache.clear() } }) ``` If storage can change externally (another tab, server-set cookies), invalidate cache: ### 7.6 Combine Multiple Array Iterations **Impact: LOW-MEDIUM (reduces iterations)** Multiple `.filter()` or `.map()` calls iterate the array multiple times. Combine into one loop. **Incorrect: 3 iterations** ```typescript const admins = users.filter(u => u.isAdmin) const testers = users.filter(u => u.isTester) const inactive = users.filter(u => !u.isActive) ``` **Correct: 1 iteration** ```typescript const admins: User[] = [] const testers: User[] = [] const inactive: User[] = [] for (const user of users) { if (user.isAdmin) admins.push(user) if (user.isTester) testers.push(user) if (!user.isActive) inactive.push(user) } ``` ### 7.7 Early Length Check for Array Comparisons **Impact: MEDIUM-HIGH (avoids expensive operations when lengths differ)** When comparing arrays with expensive operations (sorting, deep equality, serialization), check lengths first. If lengths differ, the arrays cannot be equal. In real-world applications, this optimization is especially valuable when the comparison runs in hot paths (event handlers, render loops). **Incorrect: always runs expensive comparison** ```typescript function hasChanges(current: string[], original: string[]) { // Always sorts and joins, even when lengths differ return current.sort().join() !== original.sort().join() } ``` Two O(n log n) sorts run even when `current.length` is 5 and `original.length` is 100. There is also overhead of joining the arrays and comparing the strings. **Correct (O(1) length check first):** ```typescript function hasChanges(current: string[], original: string[]) { // Early return if lengths differ if (current.length !== original.length) { return true } // Only sort/join when lengths match const currentSorted = current.toSorted() const originalSorted = original.toSorted() for (let i = 0; i < currentSorted.length; i++) { if (currentSorted[i] !== originalSorted[i]) { return true } } return false } ``` This new approach is more efficient because: - It avoids the overhead of sorting and joining the arrays when lengths differ - It avoids consuming memory for the joined strings (especially important for large arrays) - It avoids mutating the original arrays - It returns early when a difference is found ### 7.8 Early Return from Functions **Impact: LOW-MEDIUM (avoids unnecessary computation)** Return early when result is determined to skip unnecessary processing. **Incorrect: processes all items even after finding answer** ```typescript function validateUsers(users: User[]) { let hasError = false let errorMessage = '' for (const user of users) { if (!user.email) { hasError = true errorMessage = 'Email required' } if (!user.name) { hasError = true errorMessage = 'Name required' } // Continues checking all users even after error found } return hasError ? { valid: false, error: errorMessage } : { valid: true } } ``` **Correct: returns immediately on first error** ```typescript function validateUsers(users: User[]) { for (const user of users) { if (!user.email) { return { valid: false, error: 'Email required' } } if (!user.name) { return { valid: false, error: 'Name required' } } } return { valid: true } } ``` ### 7.9 Hoist RegExp Creation **Impact: LOW-MEDIUM (avoids recreation)** Don't create RegExp inside render. Hoist to module scope or memoize with `useMemo()`. **Incorrect: new RegExp every render** ```tsx function Highlighter({ text, query }: Props) { const regex = new RegExp(`(${query})`, 'gi') const parts = text.split(regex) return {parts.map((part, i) => ...)} new RegExp(`(${escapeRegex(query)})`, 'gi'), [query] ) const parts = text.split(regex) return {parts.map((part, i) => ...)} b.updatedAt - a.updatedAt) return sorted[0] } ``` Sorts the entire array just to find the maximum value. **Incorrect (O(n log n) - sort for oldest and newest):** ```typescript function getOldestAndNewest(projects: Project[]) { const sorted = [...projects].sort((a, b) => a.updatedAt - b.updatedAt) return { oldest: sorted[0], newest: sorted[sorted.length - 1] } } ``` Still sorts unnecessarily when only min/max are needed. **Correct (O(n) - single loop):** ```typescript function getLatestProject(projects: Project[]) { if (projects.length === 0) return null let latest = projects[0] for (let i = 1; i 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 newest.updatedAt) newest = projects[i] } return { oldest, newest } } ``` Single pass through the array, no copying, no sorting. **Alternative: Math.min/Math.max for small arrays** ```typescript const numbers = [5, 2, 8, 1, 9] const min = Math.min(...numbers) const max = Math.max(...numbers) ``` This works for small arrays but can be slower for very large arrays due to spread operator limitations. Use the loop approach for reliability. ### 7.11 Use Set/Map for O(1) Lookups **Impact: LOW-MEDIUM (O(n) to O(1))** Convert arrays to Set/Map for repeated membership checks. **Incorrect (O(n) per check):** ```typescript const allowedIds = ['a', 'b', 'c', ...] items.filter(item => allowedIds.includes(item.id)) ``` **Correct (O(1) per check):** ```typescript const allowedIds = new Set(['a', 'b', 'c', ...]) items.filter(item => allowedIds.has(item.id)) ``` ### 7.12 Use toSorted() Instead of sort() for Immutability **Impact: MEDIUM-HIGH (prevents mutation bugs in React state)** `.sort()` mutates the array in place, which can cause bugs with React state and props. Use `.toSorted()` to create a new sorted array without mutation. **Incorrect: mutates original array** ```typescript function UserList({ users }: { users: User[] }) { // Mutates the users prop array! const sorted = useMemo( () => users.sort((a, b) => a.name.localeCompare(b.name)), [users] ) return

{sorted.map(renderUser)}

} ``` **Correct: creates new array** ```typescript function UserList({ users }: { users: User[] }) { // Creates new sorted array, original unchanged const sorted = useMemo( () => users.toSorted((a, b) => a.name.localeCompare(b.name)), [users] ) return

{sorted.map(renderUser)}

} ``` **Why this matters in React:** 1. Props/state mutations break React's immutability model - React expects props and state to be treated as read-only 2. Causes stale closure bugs - Mutating arrays inside closures (callbacks, effects) can lead to unexpected behavior **Browser support: fallback for older browsers** ```typescript // Fallback for older browsers const sorted = [...items].sort((a, b) => a.value - b.value) ``` `.toSorted()` is available in all modern browsers (Chrome 110+, Safari 16+, Firefox 115+, Node.js 20+). For older environments, use spread operator: **Other immutable array methods:** - `.toSorted()` - immutable sort - `.toReversed()` - immutable reverse - `.toSpliced()` - immutable splice - `.with()` - immutable element replacement --- ## 8. Advanced Patterns **Impact: LOW** Advanced patterns for specific cases that require careful implementation. ### 8.1 Store Event Handlers in Refs **Impact: LOW (stable subscriptions)** Store callbacks in refs when used in effects that shouldn't re-subscribe on callback changes. **Incorrect: re-subscribes on every render** ```tsx function useWindowEvent(event: string, handler: () => void) { useEffect(() => { window.addEventListener(event, handler) return () => window.removeEventListener(event, handler) }, [event, handler]) } ``` **Correct: stable subscription** ```tsx import { useEffectEvent } from 'react' function useWindowEvent(event: string, handler: () => void) { const onEvent = useEffectEvent(handler) useEffect(() => { window.addEventListener(event, onEvent) return () => window.removeEventListener(event, onEvent) }, [event]) } ``` **Alternative: use `useEffectEvent` if you're on latest React:** `useEffectEvent` provides a cleaner API for the same pattern: it creates a stable function reference that always calls the latest version of the handler. ### 8.2 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:** ```typescript function useLatest(value: T) { const ref = useRef(value) useEffect(() => { ref.current = value }, [value]) return ref } ``` **Incorrect: effect re-runs on every callback change** ```tsx function SearchInput({ onSearch }: { onSearch: (q: string) => void }) { const [query, setQuery] = useState('') useEffect(() => { const timeout = setTimeout(() => onSearch(query), 300) return () => clearTimeout(timeout) }, [query, onSearch]) } ``` **Correct: stable effect, fresh callback** ```tsx 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](https://react.dev) 2. [https://nextjs.org](https://nextjs.org) 3. [https://swr.vercel.app](https://swr.vercel.app) 4. [https://github.com/shuding/better-all](https://github.com/shuding/better-all) 5. [https://github.com/isaacs/node-lru-cache](https://github.com/isaacs/node-lru-cache) 6. [https://vercel.com/blog/how-we-optimized-package-imports-in-next-js](https://vercel.com/blog/how-we-optimized-package-imports-in-next-js) 7. [https://vercel.com/blog/how-we-made-the-vercel-dashboard-twice-as-fast](https://vercel.com/blog/how-we-made-the-vercel-dashboard-twice-as-fast)

Source: claude-code-templates (MIT). See About Us for full credits.

BAGUA AI