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React best practices per Vercel Engineering
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| name | vercel-best-practices |
| description | React best practices per Vercel Engineering |
| license | MIT |
| metadata | {"author":"vercel","version":"1.0.0"} |
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.
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.
Impact: CRITICAL
Waterfalls are the #1 performance killer. Each sequential await adds full network latency. Eliminating them yields the largest gains.
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.
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.
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
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).
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()]);
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.
Impact: CRITICAL
Reducing initial bundle size improves Time to Interactive and Largest Contentful Paint.
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.tsfiles for their deep import paths. Importing fromlucide-react/dist/esm/icons/checkresolves to an implicitanytype, causing errors understrictornoImplicitAny. PreferoptimizePackageImportswhen 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
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.
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>
);
}
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} />;
}
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/
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.
Impact: HIGH
Optimizing server-side rendering and data fetching eliminates server-side waterfalls and reduces response times.
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
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.
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.
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
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.
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>;
}
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>
);
}
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.
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
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
Impact: MEDIUM-HIGH
Automatic deduplication and efficient data fetching patterns reduce redundant network requests.
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', () => {
/* ... */
});
// ...
}
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().
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
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.
Impact: MEDIUM
Reducing unnecessary re-renders minimizes wasted computation and improves UI responsiveness.
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
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>;
}
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
}
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
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 />
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.
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]);
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
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.
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'} />;
}
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:
Stable callback references - Callbacks don't need to be recreated when state changes
No stale closures - Always operates on the latest state value
Fewer dependencies - Simplifies dependency arrays and reduces memory leaks
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.
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.
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);
}, []);
}
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
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)',
}}
/>
);
}
Impact: MEDIUM
Optimizing the rendering process reduces the work the browser needs to do.
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.
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).
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.
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
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.
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>;
}
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.
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
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>
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 |