vercel-best-practices

name	vercel-best-practices
description	React best practices per Vercel Engineering
license	MIT
metadata	{"author":"vercel","version":"1.0.0"}

React Best Practices

Version 1.0.0 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. 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 — CRITICAL
   • 1.1 Check Cheap Conditions Before Async Flags
   • 1.2 Defer Await Until Needed
   • 1.3 Dependency-Based Parallelization
   • 1.4 Prevent Waterfall Chains in API Routes
   • 1.5 Promise.all() for Independent Operations
   • 1.6 Strategic Suspense Boundaries
2. Bundle Size Optimization — CRITICAL
   • 2.1 Avoid Barrel File Imports
   • 2.2 Conditional Module Loading
   • 2.3 Defer Non-Critical Third-Party Libraries
   • 2.4 Dynamic Imports for Heavy Components
   • 2.5 Prefer Statically Analyzable Paths
   • 2.6 Preload Based on User Intent
3. Server-Side Performance — HIGH
   • 3.1 Authenticate Server Actions Like API Routes
   • 3.2 Avoid Duplicate Serialization in RSC Props
   • 3.3 Avoid Shared Module State for Request Data
   • 3.4 Cross-Request LRU Caching
   • 3.5 Hoist Static I/O to Module Level
   • 3.6 Minimize Serialization at RSC Boundaries
   • 3.7 Parallel Data Fetching with Component Composition
   • 3.8 Parallel Nested Data Fetching
   • 3.9 Per-Request Deduplication with React.cache()
   • 3.10 Use after() for Non-Blocking Operations
4. Client-Side Data Fetching — MEDIUM-HIGH
   • 4.1 Deduplicate Global Event Listeners
   • 4.2 Use Passive Event Listeners for Scrolling Performance
   • 4.3 Use SWR for Automatic Deduplication
   • 4.4 Version and Minimize localStorage Data
5. Re-render Optimization — MEDIUM
   • 5.1 Calculate Derived State During Rendering
   • 5.2 Defer State Reads to Usage Point
   • 5.3 Do not wrap a simple expression with a primitive result type in useMemo
   • 5.4 Don't Define Components Inside Components
   • 5.5 Extract Default Non-primitive Parameter Value from Memoized Component to Constant
   • 5.6 Extract to Memoized Components
   • 5.7 Narrow Effect Dependencies
   • 5.8 Put Interaction Logic in Event Handlers
   • 5.9 Split Combined Hook Computations
   • 5.10 Subscribe to Derived State
   • 5.11 Use Functional setState Updates
   • 5.12 Use Lazy State Initialization
   • 5.13 Use Transitions for Non-Urgent Updates
   • 5.14 Use useDeferredValue for Expensive Derived Renders
   • 5.15 Use useRef for Transient Values
6. Rendering Performance — MEDIUM
   • 6.1 Animate SVG Wrapper Instead of SVG Element
   • 6.2 CSS content-visibility for Long Lists
   • 6.3 Hoist Static JSX Elements
   • 6.4 Optimize SVG Precision
   • 6.5 Prevent Hydration Mismatch Without Flickering
   • 6.6 Suppress Expected Hydration Mismatches
   • 6.7 Use Activity Component for Show/Hide
   • 6.8 Use defer or async on Script Tags
   • 6.9 Use Explicit Conditional Rendering
   • 6.10 Use React DOM Resource Hints
   • 6.11 Use useTransition Over Manual Loading States
7. JavaScript Performance — LOW-MEDIUM
   • 7.1 Avoid Layout Thrashing
   • 7.2 Build Index Maps for Repeated Lookups
   • 7.3 Cache Property Access in Loops
   • 7.4 Cache Repeated Function Calls
   • 7.5 Cache Storage API Calls
   • 7.6 Combine Multiple Array Iterations
   • 7.7 Defer Non-Critical Work with requestIdleCallback
   • 7.8 Early Length Check for Array Comparisons
   • 7.9 Early Return from Functions
   • 7.10 Hoist RegExp Creation
   • 7.11 Use flatMap to Map and Filter in One Pass
   • 7.12 Use Loop for Min/Max Instead of Sort
   • 7.13 Use Set/Map for O(1) Lookups
   • 7.14 Use toSorted() Instead of sort() for Immutability
8. Advanced Patterns — LOW
   • 8.1 Do Not Put Effect Events in Dependency Arrays
   • 8.2 Initialize App Once, Not Per Mount
   • 8.3 Store Event Handlers in Refs
   • 8.4 useEffectEvent 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 Check Cheap Conditions Before Async Flags

Impact: HIGH (avoids unnecessary async work when a synchronous guard already fails)

When a branch uses await for a flag or remote value and also requires a cheap synchronous condition (local props, request metadata, already-loaded state), evaluate the cheap condition first. Otherwise you pay for the async call even when the compound condition can never be true.

This is a specialization of Defer Await Until Needed for flag && cheapCondition style checks.

Incorrect:

const someFlag = await getFlag();

if (someFlag && someCondition) {
	// ...
}

Correct:

if (someCondition) {
	const someFlag = await getFlag();
	if (someFlag) {
		// ...
	}
}

This matters when getFlag hits the network, a feature-flag service, or React.cache / DB work: skipping it when someCondition is false removes that cost on the cold path.

Keep the original order if someCondition is expensive, depends on the flag, or you must run side effects in a fixed order.

1.2 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.

For await getFlag() combined with a cheap synchronous guard (flag && someCondition), see Check Cheap Conditions Before Async Flags.

1.3 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

1.4 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).

1.5 Promise.all() for Independent Operations

Impact: CRITICAL (2-10× improvement)

When async operations have no interdependencies, execute them concurrently using Promise.all().

Incorrect: sequential execution, 3 round trips

const user = await fetchUser();
const posts = await fetchPosts();
const comments = await fetchComments();

Correct: parallel execution, 1 round trip

const [user, posts, comments] = await Promise.all([fetchUser(), fetchPosts(), fetchComments()]);

1.6 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

async function Page() {
	const data = await fetchData(); // Blocks entire page

	return (
		<div>
			<div>Sidebar</div>
			<div>Header</div>
			<div>
				<DataDisplay data={data} />
			</div>
			<div>Footer</div>
		</div>
	);
}

The entire layout waits for data even though only the middle section needs it.

Correct: wrapper shows immediately, data streams in

function Page() {
	return (
		<div>
			<div>Sidebar</div>
			<div>Header</div>
			<div>
				<Suspense fallback={<Skeleton />}>
					<DataDisplay />
				</Suspense>
			</div>
			<div>Footer</div>
		</div>
	);
}

async function DataDisplay() {
	const data = await fetchData(); // Only blocks this component
	return <div>{data.content}</div>;
}

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

Alternative: share promise across components

function Page() {
	// Start fetch immediately, but don't await
	const dataPromise = fetchData();

	return (
		<div>
			<div>Sidebar</div>
			<div>Header</div>
			<Suspense fallback={<Skeleton />}>
				<DataDisplay dataPromise={dataPromise} />
				<DataSummary dataPromise={dataPromise} />
			</Suspense>
			<div>Footer</div>
		</div>
	);
}

function DataDisplay({ dataPromise }: { dataPromise: Promise<Data> }) {
	const data = use(dataPromise); // Unwraps the promise
	return <div>{data.content}</div>;
}

function DataSummary({ dataPromise }: { dataPromise: Promise<Data> }) {
	const data = use(dataPromise); // Reuses the same promise
	return <div>{data.summary}</div>;
}

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

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 - Next.js 13.5+ (recommended):

// Keep the standard imports - Next.js transforms them to direct imports
import { Check, X, Menu } from 'lucide-react';
// Full TypeScript support, no manual path wrangling

This is the recommended approach because it preserves TypeScript type safety and editor autocompletion while still eliminating the barrel import cost.

Correct - Direct imports (non-Next.js projects):

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

TypeScript warning: Some libraries (notably lucide-react) don't ship .d.ts files for their deep import paths. Importing from lucide-react/dist/esm/icons/check resolves to an implicit any type, causing errors under strict or noImplicitAny. Prefer optimizePackageImports when available, or verify the library exports types for its subpaths before using direct imports.

These optimizations provide 15-70% faster dev boot, 28% faster builds, 40% faster cold starts, and significantly faster HMR.

Libraries commonly affected: lucide-react, @mui/material, @mui/icons-material, @tabler/icons-react, react-icons, @headlessui/react, @radix-ui/react-*, lodash, ramda, date-fns, rxjs, react-use.

Reference: https://vercel.com/blog/how-we-optimized-package-imports-in-next-js

2.2 Conditional Module Loading

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.

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

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

export default function RootLayout({ children }) {
	return (
		<html>
			<body>
				{children}
				<Analytics />
			</body>
		</html>
	);
}

Correct: loads after hydration

import dynamic from 'next/dynamic';

const Analytics = dynamic(() => import('@vercel/analytics/react').then((m) => m.Analytics), {
	ssr: false,
});

export default function RootLayout({ children }) {
	return (
		<html>
			<body>
				{children}
				<Analytics />
			</body>
		</html>
	);
}

2.4 Dynamic Imports for Heavy Components

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

import { MonacoEditor } from './monaco-editor';

function CodePanel({ code }: { code: string }) {
	return <MonacoEditor value={code} />;
}

Correct: Monaco loads on demand

import dynamic from 'next/dynamic';

const MonacoEditor = dynamic(() => import('./monaco-editor').then((m) => m.MonacoEditor), {
	ssr: false,
});

function CodePanel({ code }: { code: string }) {
	return <MonacoEditor value={code} />;
}

2.5 Prefer Statically Analyzable Paths

Impact: HIGH (avoids accidental broad bundles and file traces)

Build tools work best when import and file-system paths are obvious at build time. If you hide the real path inside a variable or compose it too dynamically, the tool either has to include a broad set of possible files, warn that it cannot analyze the import, or widen file tracing to stay safe.

Prefer explicit maps or literal paths so the set of reachable files stays narrow and predictable. This is the same rule whether you are choosing modules with import() or reading files in server/build code.

When analysis becomes too broad, the cost is real:

• Larger server bundles

• Slower builds

• Worse cold starts

• More memory use

Incorrect: the bundler cannot tell what may be imported

const PAGE_MODULES = {
	home: './pages/home',
	settings: './pages/settings',
} as const;

const Page = await import(PAGE_MODULES[pageName]);

Correct: use an explicit map of allowed modules

const PAGE_MODULES = {
	home: () => import('./pages/home'),
	settings: () => import('./pages/settings'),
} as const;

const Page = await PAGE_MODULES[pageName]();

Incorrect: a 2-value enum still hides the final path from static analysis

const baseDir = path.join(process.cwd(), 'content/' + contentKind);

Correct: make each final path literal at the callsite

const baseDir =
	kind === ContentKind.Blog
		? path.join(process.cwd(), 'content/blog')
		: path.join(process.cwd(), 'content/docs');

In Next.js server code, this matters for output file tracing too. path.join(process.cwd(), someVar) can widen the traced file set because Next.js statically analyze import, require, and fs usage.

Reference: https://nextjs.org/docs/app/api-reference/config/next-config-js/output, https://nextjs.org/learn/seo/dynamic-imports, https://vite.dev/guide/features.html, https://esbuild.github.io/api/, https://www.npmjs.com/package/@rollup/plugin-dynamic-import-vars, https://webpack.js.org/guides/dependency-management/

2.6 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.

---

3. Server-Side Performance

Impact: HIGH

Optimizing server-side rendering and data fetching eliminates server-side waterfalls and reduces response times.

3.1 Authenticate Server Actions Like API Routes

Impact: CRITICAL (prevents unauthorized access to server mutations)

Server Actions (functions with "use server") are exposed as public endpoints, just like API routes. Always verify authentication and authorization inside each Server Action—do not rely solely on middleware, layout guards, or page-level checks, as Server Actions can be invoked directly.

Next.js documentation explicitly states: "Treat Server Actions with the same security considerations as public-facing API endpoints, and verify if the user is allowed to perform a mutation."

Incorrect: no authentication check

'use server';

export async function deleteUser(userId: string) {
	// Anyone can call this! No auth check
	await db.user.delete({ where: { id: userId } });
	return { success: true };
}

Correct: authentication inside the action

'use server';

import { verifySession } from '@/lib/auth';
import { unauthorized } from '@/lib/errors';

export async function deleteUser(userId: string) {
	// Always check auth inside the action
	const session = await verifySession();

	if (!session) {
		throw unauthorized('Must be logged in');
	}

	// Check authorization too
	if (session.user.role !== 'admin' && session.user.id !== userId) {
		throw unauthorized('Cannot delete other users');
	}

	await db.user.delete({ where: { id: userId } });
	return { success: true };
}

With input validation:

'use server';

import { verifySession } from '@/lib/auth';
import { z } from 'zod';

const updateProfileSchema = z.object({
	userId: z.string().uuid(),
	name: z.string().min(1).max(100),
	email: z.string().email(),
});

export async function updateProfile(data: unknown) {
	// Validate input first
	const validated = updateProfileSchema.parse(data);

	// Then authenticate
	const session = await verifySession();
	if (!session) {
		throw new Error('Unauthorized');
	}

	// Then authorize
	if (session.user.id !== validated.userId) {
		throw new Error('Can only update own profile');
	}

	// Finally perform the mutation
	await db.user.update({
		where: { id: validated.userId },
		data: {
			name: validated.name,
			email: validated.email,
		},
	});

	return { success: true };
}

Reference: https://nextjs.org/docs/app/guides/authentication

3.2 Avoid Duplicate Serialization in RSC Props

Impact: LOW (reduces network payload by avoiding duplicate serialization)

RSC→client serialization deduplicates by object reference, not value. Same reference = serialized once; new reference = serialized again. Do transformations (.toSorted(), .filter(), .map()) in client, not server.

Incorrect: duplicates array

// RSC: sends 6 strings (2 arrays × 3 items)
<ClientList usernames={usernames} usernamesOrdered={usernames.toSorted()} />

Correct: sends 3 strings

// RSC: send once
<ClientList usernames={usernames} />;

// Client: transform there
('use client');
const sorted = useMemo(() => [...usernames].sort(), [usernames]);

Nested deduplication behavior:

// string[] - duplicates everything
usernames={['a','b']} sorted={usernames.toSorted()} // sends 4 strings

// object[] - duplicates array structure only
users={[{id:1},{id:2}]} sorted={users.toSorted()} // sends 2 arrays + 2 unique objects (not 4)

Deduplication works recursively. Impact varies by data type:

• string[], number[], boolean[]: HIGH impact - array + all primitives fully duplicated

• object[]: LOW impact - array duplicated, but nested objects deduplicated by reference

Operations breaking deduplication: create new references

• Arrays: .toSorted(), .filter(), .map(), .slice(), [...arr]

• Objects: {...obj}, Object.assign(), structuredClone(), JSON.parse(JSON.stringify())

More examples:

// ❌ Bad
<C users={users} active={users.filter(u => u.active)} />
<C product={product} productName={product.name} />

// ✅ Good
<C users={users} />
<C product={product} />
// Do filtering/destructuring in client

Exception: Pass derived data when transformation is expensive or client doesn't need original.

3.3 Avoid Shared Module State for Request Data

Impact: HIGH (prevents concurrency bugs and request data leaks)

For React Server Components and client components rendered during SSR, avoid using mutable module-level variables to share request-scoped data. Server renders can run concurrently in the same process. If one render writes to shared module state and another render reads it, you can get race conditions, cross-request contamination, and security bugs where one user's data appears in another user's response.

Treat module scope on the server as process-wide shared memory, not request-local state.

Incorrect: request data leaks across concurrent renders

let currentUser: User | null = null;

export default async function Page() {
	currentUser = await auth();
	return <Dashboard />;
}

async function Dashboard() {
	return <div>{currentUser?.name}</div>;
}

If two requests overlap, request A can set currentUser, then request B overwrites it before request A finishes rendering Dashboard.

Correct: keep request data local to the render tree

export default async function Page() {
	const user = await auth();
	return <Dashboard user={user} />;
}

function Dashboard({ user }: { user: User | null }) {
	return <div>{user?.name}</div>;
}

Safe exceptions:

• Immutable static assets or config loaded once at module scope

• Shared caches intentionally designed for cross-request reuse and keyed correctly

• Process-wide singletons that do not store request- or user-specific mutable data

For static assets and config, see Hoist Static I/O to Module Level.

3.4 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:

import { LRUCache } from 'lru-cache';

const cache = new LRUCache<string, any>({
	max: 1000,
	ttl: 5 * 60 * 1000, // 5 minutes
});

export async function getUser(id: string) {
	const cached = cache.get(id);
	if (cached) return cached;

	const user = await db.user.findUnique({ where: { id } });
	cache.set(id, user);
	return user;
}

// Request 1: DB query, result cached
// Request 2: cache hit, no DB query

Use when sequential user actions hit multiple endpoints needing the same data within seconds.

With Vercel's 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

3.5 Hoist Static I/O to Module Level

Impact: HIGH (avoids repeated file/network I/O per request)

When loading static assets (fonts, logos, images, config files) in route handlers or server functions, hoist the I/O operation to module level. Module-level code runs once when the module is first imported, not on every request. This eliminates redundant file system reads or network fetches that would otherwise run on every invocation.

Incorrect: reads font file on every request

// app/api/og/route.tsx
import { ImageResponse } from 'next/og'

export async function GET(request: Request) {
  // Runs on EVERY request - expensive!
  const fontData = await fetch(
    new URL('./fonts/Inter.ttf', import.meta.url)
  ).then(res => res.arrayBuffer())

  const logoData = await fetch(
    new URL('./images/logo.png', import.meta.url)
  ).then(res => res.arrayBuffer())

  return new ImageResponse(
    <div style={{ fontFamily: 'Inter' }}>
      <img src={logoData} />
      Hello World
    </div>,
    { fonts: [{ name: 'Inter', data: fontData }] }
  )
}

Correct: loads once at module initialization

// app/api/og/route.tsx
import { ImageResponse } from 'next/og'

// Module-level: runs ONCE when module is first imported
const fontData = fetch(
  new URL('./fonts/Inter.ttf', import.meta.url)
).then(res => res.arrayBuffer())

const logoData = fetch(
  new URL('./images/logo.png', import.meta.url)
).then(res => res.arrayBuffer())

export async function GET(request: Request) {
  // Await the already-started promises
  const [font, logo] = await Promise.all([fontData, logoData])

  return new ImageResponse(
    <div style={{ fontFamily: 'Inter' }}>
      <img src={logo} />
      Hello World
    </div>,
    { fonts: [{ name: 'Inter', data: font }] }
  )
}

Correct: synchronous fs at module level

// app/api/og/route.tsx
import { ImageResponse } from 'next/og'
import { readFileSync } from 'fs'
import { join } from 'path'

// Synchronous read at module level - blocks only during module init
const fontData = readFileSync(
  join(process.cwd(), 'public/fonts/Inter.ttf')
)

const logoData = readFileSync(
  join(process.cwd(), 'public/images/logo.png')
)

export async function GET(request: Request) {
  return new ImageResponse(
    <div style={{ fontFamily: 'Inter' }}>
      <img src={logoData} />
      Hello World
    </div>,
    { fonts: [{ name: 'Inter', data: fontData }] }
  )
}

Incorrect: reads config on every call

import fs from 'node:fs/promises';

export async function processRequest(data: Data) {
	const config = JSON.parse(await fs.readFile('./config.json', 'utf-8'));
	const template = await fs.readFile('./template.html', 'utf-8');

	return render(template, data, config);
}

Correct: hoists config and template to module level

import fs from 'node:fs/promises';

const configPromise = fs.readFile('./config.json', 'utf-8').then(JSON.parse);
const templatePromise = fs.readFile('./template.html', 'utf-8');

export async function processRequest(data: Data) {
	const [config, template] = await Promise.all([configPromise, templatePromise]);

	return render(template, data, config);
}

When to use this pattern:

• Loading fonts for OG image generation

• Loading static logos, icons, or watermarks

• Reading configuration files that don't change at runtime

• Loading email templates or other static templates

• Any static asset that's the same across all requests

When not to use this pattern:

• Assets that vary per request or user

• Files that may change during runtime (use caching with TTL instead)

• Large files that would consume too much memory if kept loaded

• Sensitive data that shouldn't persist in memory

With Vercel's Fluid Compute, module-level caching is especially effective because multiple concurrent requests share the same function instance. The static assets stay loaded in memory across requests without cold start penalties.

In traditional serverless, each cold start re-executes module-level code, but subsequent warm invocations reuse the loaded assets until the instance is recycled.

3.6 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

async function Page() {
	const user = await fetchUser(); // 50 fields
	return <Profile user={user} />;
}

('use client');
function Profile({ user }: { user: User }) {
	return <div>{user.name}</div>; // uses 1 field
}

Correct: serializes only 1 field

async function Page() {
	const user = await fetchUser();
	return <Profile name={user.name} />;
}

('use client');
function Profile({ name }: { name: string }) {
	return <div>{name}</div>;
}

3.7 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

export default async function Page() {
	const header = await fetchHeader();
	return (
		<div>
			<div>{header}</div>
			<Sidebar />
		</div>
	);
}

async function Sidebar() {
	const items = await fetchSidebarItems();
	return <nav>{items.map(renderItem)}</nav>;
}

Correct: both fetch simultaneously

async function Header() {
	const data = await fetchHeader();
	return <div>{data}</div>;
}

async function Sidebar() {
	const items = await fetchSidebarItems();
	return <nav>{items.map(renderItem)}</nav>;
}

export default function Page() {
	return (
		<div>
			<Header />
			<Sidebar />
		</div>
	);
}

Alternative with children prop:

async function Header() {
	const data = await fetchHeader();
	return <div>{data}</div>;
}

async function Sidebar() {
	const items = await fetchSidebarItems();
	return <nav>{items.map(renderItem)}</nav>;
}

function Layout({ children }: { children: ReactNode }) {
	return (
		<div>
			<Header />
			{children}
		</div>
	);
}

export default function Page() {
	return (
		<Layout>
			<Sidebar />
		</Layout>
	);
}

3.8 Parallel Nested Data Fetching

Impact: CRITICAL (eliminates server-side waterfalls)

When fetching nested data in parallel, chain dependent fetches within each item's promise so a slow item doesn't block the rest.

Incorrect: a single slow item blocks all nested fetches

const chats = await Promise.all(chatIds.map((id) => getChat(id)));

const chatAuthors = await Promise.all(chats.map((chat) => getUser(chat.author)));

If one getChat(id) out of 100 is extremely slow, the authors of the other 99 chats can't start loading even though their data is ready.

Correct: each item chains its own nested fetch

const chatAuthors = await Promise.all(
	chatIds.map((id) => getChat(id).then((chat) => getUser(chat.author)))
);

Each item independently chains getChat → getUser, so a slow chat doesn't block author fetches for the others.

3.9 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:

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.

Avoid inline objects as arguments:

React.cache() uses shallow equality (Object.is) to determine cache hits. Inline objects create new references each call, preventing cache hits.

Incorrect: always cache miss

const getUser = cache(async (params: { uid: number }) => {
	return await db.user.findUnique({ where: { id: params.uid } });
});

// Each call creates new object, never hits cache
getUser({ uid: 1 });
getUser({ uid: 1 }); // Cache miss, runs query again

Correct: cache hit

const params = { uid: 1 };
getUser(params); // Query runs
getUser(params); // Cache hit (same reference)

If you must pass objects, pass the same reference:

Next.js-Specific Note:

In Next.js, the fetch API is automatically extended with request memoization. Requests with the same URL and options are automatically deduplicated within a single request, so you don't need React.cache() for fetch calls. However, React.cache() is still essential for other async tasks:

• Database queries (Prisma, Drizzle, etc.)

• Heavy computations

• Authentication checks

• File system operations

• Any non-fetch async work

Use React.cache() to deduplicate these operations across your component tree.

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

3.10 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

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

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

---

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

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

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

Correct: N instances = 1 listener

import useSWRSubscription from 'swr/subscription';

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

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

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

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

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

4.2 Use 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().

4.3 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

4.4 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.

---

5. Re-render Optimization

Impact: MEDIUM

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

5.1 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

5.2 Defer State Reads to Usage Point

Impact: MEDIUM (avoids unnecessary subscriptions)

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

Incorrect: subscribes to all searchParams changes

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

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

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

Correct: reads on demand, no subscription

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

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

5.3 Do not wrap a simple expression with a primitive result type in useMemo

Impact: LOW-MEDIUM (wasted computation on every render)

When an expression is simple (few logical or arithmetical operators) and has a primitive result type (boolean, number, string), do not wrap it in useMemo.

Calling useMemo and comparing hook dependencies may consume more resources than the expression itself.

Incorrect:

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
}

5.4 Don't Define Components Inside Components

Impact: HIGH (prevents remount on every render)

Defining a component inside another component creates a new component type on every render. React sees a different component each time and fully remounts it, destroying all state and DOM.

A common reason developers do this is to access parent variables without passing props. Always pass props instead.

Incorrect: remounts on every render

function UserProfile({ user, theme }) {
	// Defined inside to access `theme` - BAD
	const Avatar = () => (
		<img src={user.avatarUrl} className={theme === 'dark' ? 'avatar-dark' : 'avatar-light'} />
	);

	// Defined inside to access `user` - BAD
	const Stats = () => (
		<div>
			<span>{user.followers} followers</span>
			<span>{user.posts} posts</span>
		</div>
	);

	return (
		<div>
			<Avatar />
			<Stats />
		</div>
	);
}

Every time UserProfile renders, Avatar and Stats are new component types. React unmounts the old instances and mounts new ones, losing any internal state, running effects again, and recreating DOM nodes.

Correct: pass props instead

function Avatar({ src, theme }: { src: string; theme: string }) {
	return <img src={src} className={theme === 'dark' ? 'avatar-dark' : 'avatar-light'} />;
}

function Stats({ followers, posts }: { followers: number; posts: number }) {
	return (
		<div>
			<span>{followers} followers</span>
			<span>{posts} posts</span>
		</div>
	);
}

function UserProfile({ user, theme }) {
	return (
		<div>
			<Avatar src={user.avatarUrl} theme={theme} />
			<Stats followers={user.followers} posts={user.posts} />
		</div>
	);
}

Symptoms of this bug:

• Input fields lose focus on every keystroke

• Animations restart unexpectedly

• useEffect cleanup/setup runs on every parent render

• Scroll position resets inside the component

5.5 Extract Default Non-primitive Parameter Value from Memoized Component to Constant

Impact: MEDIUM (restores memoization by using a constant for default value)

When memoized component has a default value for some non-primitive optional parameter, such as an array, function, or object, calling the component without that parameter results in broken memoization. This is because new value instances are created on every rerender, and they do not pass strict equality comparison in memo().

To address this issue, extract the default value into a constant.

Incorrect: onClick has different values on every rerender

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 />

5.6 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.

5.7 Narrow Effect Dependencies

Impact: LOW (minimizes effect re-runs)

Specify primitive dependencies instead of objects to minimize effect re-runs.

Incorrect: re-runs on any user field change

useEffect(() => {
	console.log(user.id);
}, [user]);

Correct: re-runs only when id changes

useEffect(() => {
	console.log(user.id);
}, [user.id]);

For derived state, compute outside effect:

// Incorrect: runs on width=767, 766, 765...
useEffect(() => {
	if (width < 768) {
		enableMobileMode();
	}
}, [width]);

// Correct: runs only on boolean transition
const isMobile = width < 768;
useEffect(() => {
	if (isMobile) {
		enableMobileMode();
	}
}, [isMobile]);

5.8 Put Interaction Logic in Event Handlers

Impact: MEDIUM (avoids effect re-runs and duplicate side effects)

If a side effect is triggered by a specific user action (submit, click, drag), run it in that event handler. Do not model the action as state + effect; it makes effects re-run on unrelated changes and can duplicate the action.

Incorrect: event modeled as state + effect

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

5.9 Split Combined Hook Computations

Impact: MEDIUM (avoids recomputing independent steps)

When a hook contains multiple independent tasks with different dependencies, split them into separate hooks. A combined hook reruns all tasks when any dependency changes, even if some tasks don't use the changed value.

Incorrect: changing sortOrder recomputes filtering

const sortedProducts = useMemo(() => {
	const filtered = products.filter((p) => p.category === category);
	const sorted = filtered.toSorted((a, b) =>
		sortOrder === 'asc' ? a.price - b.price : b.price - a.price
	);
	return sorted;
}, [products, category, sortOrder]);

Correct: filtering only recomputes when products or category change

const filteredProducts = useMemo(
	() => products.filter((p) => p.category === category),
	[products, category]
);

const sortedProducts = useMemo(
	() =>
		filteredProducts.toSorted((a, b) =>
			sortOrder === 'asc' ? a.price - b.price : b.price - a.price
		),
	[filteredProducts, sortOrder]
);

This pattern also applies to useEffect when combining unrelated side effects:

Incorrect: both effects run when either dependency changes

useEffect(() => {
	analytics.trackPageView(pathname);
	document.title = `${pageTitle} | My App`;
}, [pathname, pageTitle]);

Correct: effects run independently

useEffect(() => {
	analytics.trackPageView(pathname);
}, [pathname]);

useEffect(() => {
	document.title = `${pageTitle} | My App`;
}, [pageTitle]);

Note: If your project has React Compiler enabled, it automatically optimizes dependency tracking and may handle some of these cases for you.

5.10 Subscribe to Derived State

Impact: MEDIUM (reduces re-render frequency)

Subscribe to derived boolean state instead of continuous values to reduce re-render frequency.

Incorrect: re-renders on every pixel change

function Sidebar() {
	const width = useWindowWidth(); // updates continuously
	const isMobile = width < 768;
	return <nav className={isMobile ? 'mobile' : 'desktop'} />;
}

Correct: re-renders only when boolean changes

function Sidebar() {
	const isMobile = useMediaQuery('(max-width: 767px)');
	return <nav className={isMobile ? 'mobile' : 'desktop'} />;
}

5.11 Use Functional setState Updates

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

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

Incorrect: requires state as dependency

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.

5.12 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.

5.13 Use Transitions for Non-Urgent Updates

Impact: MEDIUM (maintains UI responsiveness)

Mark frequent, non-urgent state updates as transitions to maintain UI responsiveness.

Incorrect: blocks UI on every scroll

function ScrollTracker() {
	const [scrollY, setScrollY] = useState(0);
	useEffect(() => {
		const handler = () => setScrollY(window.scrollY);
		window.addEventListener('scroll', handler, { passive: true });
		return () => window.removeEventListener('scroll', handler);
	}, []);
}

Correct: non-blocking updates

import { startTransition } from 'react';

function ScrollTracker() {
	const [scrollY, setScrollY] = useState(0);
	useEffect(() => {
		const handler = () => {
			startTransition(() => setScrollY(window.scrollY));
		};
		window.addEventListener('scroll', handler, { passive: true });
		return () => window.removeEventListener('scroll', handler);
	}, []);
}

5.14 Use useDeferredValue for Expensive Derived Renders

Impact: MEDIUM (keeps input responsive during heavy computation)

When user input triggers expensive computations or renders, use useDeferredValue to keep the input responsive. The deferred value lags behind, allowing React to prioritize the input update and render the expensive result when idle.

Incorrect: input feels laggy while filtering

function Search({ items }: { items: Item[] }) {
	const [query, setQuery] = useState('');
	const filtered = items.filter((item) => fuzzyMatch(item, query));

	return (
		<>
			<input value={query} onChange={(e) => setQuery(e.target.value)} />
			<ResultsList results={filtered} />
		</>
	);
}

Correct: input stays snappy, results render when ready

function Search({ items }: { items: Item[] }) {
	const [query, setQuery] = useState('');
	const deferredQuery = useDeferredValue(query);
	const filtered = useMemo(
		() => items.filter((item) => fuzzyMatch(item, deferredQuery)),
		[items, deferredQuery]
	);
	const isStale = query !== deferredQuery;

	return (
		<>
			<input value={query} onChange={(e) => setQuery(e.target.value)} />
			<div style={{ opacity: isStale ? 0.7 : 1 }}>
				<ResultsList results={filtered} />
			</div>
		</>
	);
}

When to use:

• Filtering/searching large lists

• Expensive visualizations (charts, graphs) reacting to input

• Any derived state that causes noticeable render delays

Note: Wrap the expensive computation in useMemo with the deferred value as a dependency, otherwise it still runs on every render.

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

5.15 Use useRef for Transient Values

Impact: MEDIUM (avoids unnecessary re-renders on frequent updates)

When a value changes frequently and you don't want a re-render on every update (e.g., mouse trackers, intervals, transient flags), store it in useRef instead of useState. Keep component state for UI; use refs for temporary DOM-adjacent values. Updating a ref does not trigger a re-render.

Incorrect: renders every update

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)',
			}}
		/>
	);
}

---

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 <div> and animate the wrapper instead.

Incorrect: animating SVG directly - no hardware acceleration

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

Correct: animating wrapper div - hardware accelerated

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

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

6.2 CSS content-visibility for Long Lists

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="h-screen overflow-y-auto">
			{messages.map((msg) => (
				<div key={msg.id} className="message-item">
					<Avatar user={msg.author} />
					<div>{msg.content}</div>
				</div>
			))}
		</div>
	);
}

For 1000 messages, browser skips layout/paint for ~990 off-screen items (10× faster initial render).

6.3 Hoist Static JSX Elements

Impact: LOW (avoids re-creation)

Extract static JSX outside components to avoid re-creation.

Incorrect: recreates element every render

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

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

Correct: reuses same element

const loadingSkeleton = <div className="h-20 animate-pulse 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.

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

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

Correct: 1 decimal place

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

Automate with SVGO:

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

6.5 Prevent Hydration Mismatch Without Flickering

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

function ThemeWrapper({ children }: { children: ReactNode }) {
	// localStorage is not available on server - throws error
	const theme = localStorage.getItem('theme') || 'light';

	return <div className={theme}>{children}</div>;
}

Server-side rendering will fail because localStorage is undefined.

Incorrect: visual flickering

function ThemeWrapper({ children }: { children: ReactNode }) {
	const [theme, setTheme] = useState('light');

	useEffect(() => {
		// Runs after hydration - causes visible flash
		const stored = localStorage.getItem('theme');
		if (stored) {
			setTheme(stored);
		}
	}, []);

	return <div className={theme}>{children}</div>;
}

Component first renders with default value (light), then updates after hydration, causing a visible flash of incorrect content.

Correct: no flicker, no hydration mismatch

function ThemeWrapper({ children }: { children: ReactNode }) {
	return (
		<>
			<div id="theme-wrapper">{children}</div>
			<script
				dangerouslySetInnerHTML={{
					__html: `
            (function() {
              try {
                var theme = localStorage.getItem('theme') || 'light';
                var el = document.getElementById('theme-wrapper');
                if (el) el.className = theme;
              } catch (e) {}
            })();
          `,
				}}
			/>
		</>
	);
}

The inline script executes synchronously before showing the element, ensuring the DOM already has the correct value. No flickering, no hydration mismatch.

This pattern is especially useful for theme toggles, user preferences, authentication states, and any client-only data that should render immediately without flashing default values.

6.6 Suppress Expected Hydration Mismatches

Impact: LOW-MEDIUM (avoids noisy hydration warnings for known differences)

In SSR frameworks (e.g., Next.js), some values are intentionally different on server vs client (random IDs, dates, locale/timezone formatting). For these expected mismatches, wrap the dynamic text in an element with suppressHydrationWarning to prevent noisy warnings. Do not use this to hide real bugs. Don’t overuse it.

Incorrect: known mismatch warnings

function Timestamp() {
	return <span>{new Date().toLocaleString()}</span>;
}

Correct: suppress expected mismatch only

function Timestamp() {
	return <span suppressHydrationWarning>{new Date().toLocaleString()}</span>;
}

6.7 Use Activity Component for Show/Hide

Impact: MEDIUM (preserves state/DOM)

Use React's <Activity> to preserve state/DOM for expensive components that frequently toggle visibility.

Usage:

import { Activity } from 'react';

function Dropdown({ isOpen }: Props) {
	return (
		<Activity mode={isOpen ? 'visible' : 'hidden'}>
			<ExpensiveMenu />
		</Activity>
	);
}

Avoids expensive re-renders and state loss.

6.8 Use defer or async on Script Tags

Impact: HIGH (eliminates render-blocking)

Script tags without defer or async block HTML parsing while the script downloads and executes. This delays First Contentful Paint and Time to Interactive.

• defer: Downloads in parallel, executes after HTML parsing completes, maintains execution order

• async: Downloads in parallel, executes immediately when ready, no guaranteed order

Use defer for scripts that depend on DOM or other scripts. Use async for independent scripts like analytics.

Incorrect: blocks rendering

export default function Document() {
	return (
		<html>
			<head>
				<script src="https://example.com/analytics.js" />
				<script src="/scripts/utils.js" />
			</head>
			<body>{/* content */}</body>
		</html>
	);
}

Correct: non-blocking

import Script from 'next/script';

export default function Page() {
	return (
		<>
			<Script src="https://example.com/analytics.js" strategy="afterInteractive" />
			<Script src="/scripts/utils.js" strategy="beforeInteractive" />
		</>
	);
}

Note: In Next.js, prefer the next/script component with strategy prop instead of raw script tags:

Reference: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script#defer

6.9 Use Explicit Conditional Rendering

Impact: LOW (prevents rendering 0 or NaN)

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

Incorrect: renders "0" when count is 0

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

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

Correct: renders nothing when count is 0

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

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

6.10 Use React DOM Resource Hints

Impact: HIGH (reduces load time for critical resources)

React DOM provides APIs to hint the browser about resources it will need. These are especially useful in server components to start loading resources before the client even receives the HTML.

• prefetchDNS(href): Resolve DNS for a domain you expect to connect to

• preconnect(href): Establish connection (DNS + TCP + TLS) to a server

• preload(href, options): Fetch a resource (stylesheet, font, script, image) you'll use soon

• preloadModule(href): Fetch an ES module you'll use soon

• preinit(href, options): Fetch and evaluate a stylesheet or script

• preinitModule(href): Fetch and evaluate an ES module

Example: preconnect to third-party APIs

import { preconnect, prefetchDNS } from 'react-dom';

export default function App() {
	prefetchDNS('https://analytics.example.com');
	preconnect('https://api.example.com');

	return <main>{/* content */}</main>;
}

Example: preload critical fonts and styles

import { preload, preinit } from 'react-dom';

export default function RootLayout({ children }) {
	// Preload font file
	preload('/fonts/inter.woff2', { as: 'font', type: 'font/woff2', crossOrigin: 'anonymous' });

	// Fetch and apply critical stylesheet immediately
	preinit('/styles/critical.css', { as: 'style' });

	return (
		<html>
			<body>{children}</body>
		</html>
	);
}

Example: preload modules for code-split routes

import { preloadModule, preinitModule } from 'react-dom';

function Navigation() {
	const preloadDashboard = () => {
		preloadModule('/dashboard.js', { as: 'script' });
	};

	return (
		<nav>
			<a href="/dashboard" onMouseEnter={preloadDashboard}>
				Dashboard
			</a>
		</nav>
	);
}

When to use each:

| API | Use case |

|-----|----------|

| prefetchDNS | Third-party domains you'll connect to later |

| preconnect | APIs or CDNs you'll fetch from immediately |