# 075 Agents Be525726

> React Best Practices

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- Category: Web & Frontend
- Author: tools-only (https://skillmd.com/u/tools-only)
- Updated: 2026-09-22
- Page: https://skillmd.com/skills/tools-only/075-agents-be525726

---

# 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](#1-eliminating-waterfalls) — **CRITICAL**
   - 1.1 [Defer Await Until Needed](#11-defer-await-until-needed)
   - 1.2 [Dependency-Based Parallelization](#12-dependency-based-parallelization)
   - 1.3 [Prevent Waterfall Chains in API Routes](#13-prevent-waterfall-chains-in-api-routes)
   - 1.4 [Promise.all() for Independent Operations](#14-promiseall-for-independent-operations)
   - 1.5 [Strategic Suspense Boundaries](#15-strategic-suspense-boundaries)
2. [Bundle Size Optimization](#2-bundle-size-optimization) — **CRITICAL**
   - 2.1 [Avoid Barrel File Imports](#21-avoid-barrel-file-imports)
   - 2.2 [Conditional Module Loading](#22-conditional-module-loading)
   - 2.3 [Defer Non-Critical Third-Party Libraries](#23-defer-non-critical-third-party-libraries)
   - 2.4 [Dynamic Imports for Heavy Components](#24-dynamic-imports-for-heavy-components)
   - 2.5 [Preload Based on User Intent](#25-preload-based-on-user-intent)
3. [Server-Side Performance](#3-server-side-performance) — **HIGH**
   - 3.1 [Authenticate Server Actions Like API Routes](#31-authenticate-server-actions-like-api-routes)
   - 3.2 [Avoid Duplicate Serialization in RSC Props](#32-avoid-duplicate-serialization-in-rsc-props)
   - 3.3 [Cross-Request LRU Caching](#33-cross-request-lru-caching)
   - 3.4 [Minimize Serialization at RSC Boundaries](#34-minimize-serialization-at-rsc-boundaries)
   - 3.5 [Parallel Data Fetching with Component Composition](#35-parallel-data-fetching-with-component-composition)
   - 3.6 [Per-Request Deduplication with React.cache()](#36-per-request-deduplication-with-reactcache)
   - 3.7 [Use after() for Non-Blocking Operations](#37-use-after-for-non-blocking-operations)
4. [Client-Side Data Fetching](#4-client-side-data-fetching) — **MEDIUM-HIGH**
   - 4.1 [Deduplicate Global Event Listeners](#41-deduplicate-global-event-listeners)
   - 4.2 [Use Passive Event Listeners for Scrolling Performance](#42-use-passive-event-listeners-for-scrolling-performance)
   - 4.3 [Use SWR for Automatic Deduplication](#43-use-swr-for-automatic-deduplication)
   - 4.4 [Version and Minimize localStorage Data](#44-version-and-minimize-localstorage-data)
5. [Re-render Optimization](#5-re-render-optimization) — **MEDIUM**
   - 5.1 [Calculate Derived State During Rendering](#51-calculate-derived-state-during-rendering)
   - 5.2 [Defer State Reads to Usage Point](#52-defer-state-reads-to-usage-point)
   - 5.3 [Do not wrap a simple expression with a primitive result type in useMemo](#53-do-not-wrap-a-simple-expression-with-a-primitive-result-type-in-usememo)
   - 5.4 [Extract Default Non-primitive Parameter Value from Memoized Component to Constant](#54-extract-default-non-primitive-parameter-value-from-memoized-component-to-constant)
   - 5.5 [Extract to Memoized Components](#55-extract-to-memoized-components)
   - 5.6 [Narrow Effect Dependencies](#56-narrow-effect-dependencies)
   - 5.7 [Put Interaction Logic in Event Handlers](#57-put-interaction-logic-in-event-handlers)
   - 5.8 [Subscribe to Derived State](#58-subscribe-to-derived-state)
   - 5.9 [Use Functional setState Updates](#59-use-functional-setstate-updates)
   - 5.10 [Use Lazy State Initialization](#510-use-lazy-state-initialization)
   - 5.11 [Use Transitions for Non-Urgent Updates](#511-use-transitions-for-non-urgent-updates)
   - 5.12 [Use useRef for Transient Values](#512-use-useref-for-transient-values)
6. [Rendering Performance](#6-rendering-performance) — **MEDIUM**
   - 6.1 [Animate SVG Wrapper Instead of SVG Element](#61-animate-svg-wrapper-instead-of-svg-element)
   - 6.2 [CSS content-visibility for Long Lists](#62-css-content-visibility-for-long-lists)
   - 6.3 [Hoist Static JSX Elements](#63-hoist-static-jsx-elements)
   - 6.4 [Optimize SVG Precision](#64-optimize-svg-precision)
   - 6.5 [Prevent Hydration Mismatch Without Flickering](#65-prevent-hydration-mismatch-without-flickering)
   - 6.6 [Suppress Expected Hydration Mismatches](#66-suppress-expected-hydration-mismatches)
   - 6.7 [Use Activity Component for Show/Hide](#67-use-activity-component-for-showhide)
   - 6.8 [Use Explicit Conditional Rendering](#68-use-explicit-conditional-rendering)
   - 6.9 [Use useTransition Over Manual Loading States](#69-use-usetransition-over-manual-loading-states)
7. [JavaScript Performance](#7-javascript-performance) — **LOW-MEDIUM**
   - 7.1 [Avoid Layout Thrashing](#71-avoid-layout-thrashing)
   - 7.2 [Build Index Maps for Repeated Lookups](#72-build-index-maps-for-repeated-lookups)
   - 7.3 [Cache Property Access in Loops](#73-cache-property-access-in-loops)
   - 7.4 [Cache Repeated Function Calls](#74-cache-repeated-function-calls)
   - 7.5 [Cache Storage API Calls](#75-cache-storage-api-calls)
   - 7.6 [Combine Multiple Array Iterations](#76-combine-multiple-array-iterations)
   - 7.7 [Early Length Check for Array Comparisons](#77-early-length-check-for-array-comparisons)
   - 7.8 [Early Return from Functions](#78-early-return-from-functions)
   - 7.9 [Hoist RegExp Creation](#79-hoist-regexp-creation)
   - 7.10 [Use Loop for Min/Max Instead of Sort](#710-use-loop-for-minmax-instead-of-sort)
   - 7.11 [Use Set/Map for O(1) Lookups](#711-use-setmap-for-o1-lookups)
   - 7.12 [Use toSorted() Instead of sort() for Immutability](#712-use-tosorted-instead-of-sort-for-immutability)
8. [Advanced Patterns](#8-advanced-patterns) — **LOW**
   - 8.1 [Initialize App Once, Not Per Mount](#81-initialize-app-once-not-per-mount)
   - 8.2 [Store Event Handlers in Refs](#82-store-event-handlers-in-refs)
   - 8.3 [useEffectEvent for Stable Callback Refs](#83-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 Defer Await Until Needed

**Impact: HIGH (avoids blocking unused code paths)**

Move `await` operations into the branches where they're actually used to avoid blocking code paths that don't need them.

**Incorrect: blocks both branches**

```typescript
async function handleRequest(userId: string, skipProcessing: boolean) {
  const userData = await fetchUserData(userId)
  
  if (skipProcessing) {
    // Returns immediately but still waited for userData
    return { skipped: true }
  }
  
  // Only this branch uses userData
  return processUserData(userData)
}
```

**Correct: only blocks when needed**

```typescript
async function handleRequest(userId: string, skipProcessing: boolean) {
  if (skipProcessing) {
    // Returns immediately without waiting
    return { skipped: true }
  }
  
  // Fetch only when needed
  const userData = await fetchUserData(userId)
  return processUserData(userData)
}
```

**Another example: early return optimization**

```typescript
// Incorrect: always fetches permissions
async function updateResource(resourceId: string, userId: string) {
  const permissions = await fetchPermissions(userId)
  const resource = await getResource(resourceId)
  
  if (!resource) {
    return { error: 'Not found' }
  }
  
  if (!permissions.canEdit) {
    return { error: 'Forbidden' }
  }
  
  return await updateResourceData(resource, permissions)
}

// Correct: fetches only when needed
async function updateResource(resourceId: string, userId: string) {
  const resource = await getResource(resourceId)
  
  if (!resource) {
    return { error: 'Not found' }
  }
  
  const permissions = await fetchPermissions(userId)
  
  if (!permissions.canEdit) {
    return { error: 'Forbidden' }
  }
  
  return await updateResourceData(resource, permissions)
}
```

This optimization is especially valuable when the skipped branch is frequently taken, or when the deferred operation is expensive.

### 1.2 Dependency-Based Parallelization

**Impact: CRITICAL (2-10× improvement)**

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

**Incorrect: profile waits for config unnecessarily**

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

**Correct: config and profile run in parallel**

```typescript
import { all } from 'better-all'

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

**Alternative without extra dependencies:**

```typescript
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](https://github.com/shuding/better-all)

### 1.3 Prevent Waterfall Chains in API Routes

**Impact: CRITICAL (2-10× improvement)**

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

**Incorrect: config waits for auth, data waits for both**

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

**Correct: auth and config start immediately**

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

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

### 1.4 Promise.all() for Independent Operations

**Impact: CRITICAL (2-10× improvement)**

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

**Incorrect: sequential execution, 3 round trips**

```typescript
const user = await fetchUser()
const posts = await fetchPosts()
const comments = await fetchComments()
```

**Correct: parallel execution, 1 round trip**

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

### 1.5 Strategic Suspense Boundaries

**Impact: HIGH (faster initial paint)**

Instead of awaiting data in async components before returning JSX, use Suspense boundaries to show the wrapper UI faster while data loads.

**Incorrect: wrapper blocked by data fetching**

```tsx
async function Page() {
  const data = await fetchData() // Blocks entire page
  
  return (
    <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**

```tsx
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**

```tsx
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**

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

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

**Correct: imports only what you need**

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

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

**Alternative: Next.js 13.5+**

```js
// next.config.js - use optimizePackageImports
module.exports = {
  experimental: {
    optimizePackageImports: ['lucide-react', '@mui/material']
  }
}

// Then you can keep the ergonomic barrel imports:
import { Check, X, Menu } from 'lucide-react'
// Automatically transformed to direct imports at build time
```

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

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

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

### 2.2 Conditional Module Loading

**Impact: HIGH (loads large data only when needed)**

Load large data or modules only when a feature is activated.

**Example: lazy-load animation frames**

```tsx
function AnimationPlayer({ enabled, 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**

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

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

**Correct: loads after hydration**

```tsx
import dynamic from 'next/dynamic'

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

export default function RootLayout({ children }) {
  return (
    <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**

```tsx
import { MonacoEditor } from './monaco-editor'

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

**Correct: Monaco loads on demand**

```tsx
import dynamic from 'next/dynamic'

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

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

### 2.5 Preload Based on User Intent

**Impact: MEDIUM (reduces perceived latency)**

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

**Example: preload on hover/focus**

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

  return (
    <button
      onMouseEnter={preload}
      onFocus={preload}
      onClick={onClick}
    >
      Open Editor
    </button>
  )
}
```

**Example: preload when feature flag is enabled**

```tsx
function FlagsProvider({ children, flags }: Props) {
  useEffect(() => {
    if (flags.editorEnabled && typeof window !== 'undefined') {
      void import('./monaco-editor').then(mod => mod.init())
    }
  }, [flags.editorEnabled])

  return <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**

```typescript
'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**

```typescript
'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:**

```typescript
'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](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**

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

**Correct: sends 3 strings**

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

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

**Nested deduplication behavior:**

```tsx
// 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:**

```tsx
// ❌ 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 Cross-Request LRU Caching

**Impact: HIGH (caches across requests)**

`React.cache()` only works within one request. For data shared across sequential requests (user clicks button A then button B), use an LRU cache.

**Implementation:**

```typescript
import { LRUCache } from 'lru-cache'

const cache = new LRUCache<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](https://vercel.com/docs/fluid-compute):** LRU caching is especially effective because multiple concurrent requests can share the same function instance and cache. This means the cache persists across requests without needing external storage like Redis.

**In traditional serverless:** Each invocation runs in isolation, so consider Redis for cross-process caching.

Reference: [https://github.com/isaacs/node-lru-cache](https://github.com/isaacs/node-lru-cache)

### 3.4 Minimize Serialization at RSC Boundaries

**Impact: HIGH (reduces data transfer size)**

The React Server/Client boundary serializes all object properties into strings and embeds them in the HTML response and subsequent RSC requests. This serialized data directly impacts page weight and load time, so **size matters a lot**. Only pass fields that the client actually uses.

**Incorrect: serializes all 50 fields**

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

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

**Correct: serializes only 1 field**

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

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

### 3.5 Parallel Data Fetching with Component Composition

**Impact: CRITICAL (eliminates server-side waterfalls)**

React Server Components execute sequentially within a tree. Restructure with composition to parallelize data fetching.

**Incorrect: Sidebar waits for Page's fetch to complete**

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

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

**Correct: both fetch simultaneously**

```tsx
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:**

```tsx
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>
  )
}
```

### 3.6 Per-Request Deduplication with React.cache()

**Impact: MEDIUM (deduplicates within request)**

Use `React.cache()` for server-side request deduplication. Authentication and database queries benefit most.

**Usage:**

```typescript
import { cache } from 'react'

export const getCurrentUser = cache(async () => {
  const session = await auth()
  if (!session?.user?.id) return null
  return await db.user.findUnique({
    where: { id: session.user.id }
  })
})
```

Within a single request, multiple calls to `getCurrentUser()` execute the query only once.

**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**

```typescript
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**

```typescript
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](https://react.dev/reference/react/cache)

### 3.7 Use after() for Non-Blocking Operations

**Impact: MEDIUM (faster response times)**

Use Next.js's `after()` to schedule work that should execute after a response is sent. This prevents logging, analytics, and other side effects from blocking the response.

**Incorrect: blocks response**

```tsx
import { logUserAction } from '@/app/utils'

export async function POST(request: Request) {
  // Perform mutation
  await updateDatabase(request)
  
  // Logging blocks the response
  const userAgent = request.headers.get('user-agent') || 'unknown'
  await logUserAction({ userAgent })
  
  return new Response(JSON.stringify({ status: 'success' }), {
    status: 200,
    headers: { 'Content-Type': 'application/json' }
  })
}
```

**Correct: non-blocking**

```tsx
import { after } from 'next/server'
import { headers, cookies } from 'next/headers'
import { logUserAction } from '@/app/utils'

export async function POST(request: Request) {
  // Perform mutation
  await updateDatabase(request)
  
  // Log after response is sent
  after(async () => {
    const userAgent = (await headers()).get('user-agent') || 'unknown'
    const sessionCookie = (await cookies()).get('session-id')?.value || 'anonymous'
    
    logUserAction({ sessionCookie, userAgent })
  })
  
  return new Response(JSON.stringify({ status: 'success' }), {
    status: 200,
    headers: { 'Content-Type': 'application/json' }
  })
}
```

The response is sent immediately while logging happens in the background.

**Common use cases:**

- Analytics tracking

- Audit logging

- Sending notifications

- Cache invalidation

- Cleanup tasks

**Important notes:**

- `after()` runs even if the response fails or redirects

- Works in Server Actions, Route Handlers, and Server Components

Reference: [https://nextjs.org/docs/app/api-reference/functions/after](https://nextjs.org/docs/app/api-reference/functions/after)

---

## 4. Client-Side Data Fetching

**Impact: MEDIUM-HIGH**

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

### 4.1 Deduplicate Global Event Listeners

**Impact: LOW (single listener for N components)**

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

**Incorrect: N instances = N listeners**

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

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

**Correct: N instances = 1 listener**

```tsx
import useSWRSubscription from 'swr/subscription'

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

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

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

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

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

### 4.2 Use 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:**

```typescript
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:**

```typescript
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**

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

**Correct: multiple instances share one request**

```tsx
import useSWR from 'swr'

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

**For immutable data:**

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

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

**For mutations:**

```tsx
import { useSWRMutation } from 'swr/mutation'

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

Reference: [https://swr.vercel.app](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:**

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

**Correct:**

```typescript
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:**

```typescript
// 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**

```tsx
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**

```tsx
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](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**

```tsx
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**

```tsx
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 computat

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