> 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.
- 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)
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'
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) {
### 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.
**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.
**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**
**Correct: loads once at module initialization**
**Alternative: synchronous file reads with Node.js fs**
**General Node.js example: loading config or templates**
**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](https://vercel.com/docs/fluid-compute):** Module-level caching is especially effective because multiple concurrent requests share the same function instance. The static assets stay loaded in memory across requests without cold start penalties.
**In traditional serverless:** Each cold start re-executes module-level code, but subsequent warm invocations reuse the loaded assets until the instance is recycled.
### 3.5 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 <Profileuser={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 <Profilename={user.name}/>
}
'use client'
function Profile({ name }: { name: string }) {
return <div>{name}</div>
}
```
### 3.6 Parallel Data Fetching with Component Composition
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.
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.
// 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.
// 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.
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.
### 5.3 Do not wrap a simple expression with a primitive result type in useMemo
**Impact: LOW-MEDIUM (wasted computation on every render)**
When an expression is simple (few logical or arithmetical operators) and has a primitive result type (boolean, number, string), do not wrap it in `useMemo`.
Calling `useMemo` and comparing hook dependencies may consume more resources than the expression itself.
### 5.4 Extract Default Non-primitive Parameter Value from Memoized Component to Constant
**Impact: MEDIUM (restores memoization by using a constant for default value)**
When memoized component has a default value for some non-primitive optional parameter, such as an array, function, or object, calling the component without that parameter results in broken memoization. This is because new value instances are created on every rerender, and they do not pass strict equality comparison in `memo()`.
To address this issue, extract the default value into a constant.
**Incorrect: `onClick` has different values on every rerender**
Extract expensive work into memoized components to enable early returns before computation.
**Incorrect: computes avatar even when loading**
```tsx
function Profile({ user, loading }: Props) {
const avatar = useMemo(() => {
const id = computeAvatarId(user)
return <Avatarid={id}/>
}, [user])
if (loading) return <Skeleton/>
return <div>{avatar}</div>
}
```
**Correct: skips computation when loading**
```tsx
const UserAvatar = memo(function UserAvatar({ user }: { user: User }) {
const id = useMemo(() => computeAvatarId(user), [user])
return <Avatarid={id}/>
})
function Profile({ user, loading }: Props) {
if (loading) return <Skeleton/>
return (
<div>
<UserAvataruser={user}/>
</div>
)
}
```
**Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, manual memoization with `memo()` and `useMemo()` is not necessary. The compiler automatically optimizes re-renders.
### 5.6 Narrow Effect Dependencies
**Impact: LOW (minimizes effect re-runs)**
Specify primitive dependencies instead of objects to minimize effect re-runs.
**Incorrect: re-runs on any user field change**
```tsx
useEffect(() => {
console.log(user.id)
}, [user])
```
**Correct: re-runs only when id changes**
```tsx
useEffect(() => {
console.log(user.id)
}, [user.id])
```
**For derived state, compute outside effect:**
```tsx
// Incorrect: runs on width=767, 766, 765...
useEffect(() => {
if (width <768){
enableMobileMode()
}
}, [width])
// Correct: runs only on boolean transition
const isMobile = width <768
useEffect(() => {
if (isMobile) {
enableMobileMode()
}
}, [isMobile])
```
### 5.7 Put Interaction Logic in Event Handlers
**Impact: MEDIUM (avoids effect re-runs and duplicate side effects)**
If a side effect is triggered by a specific user action (submit, click, drag), run it in that event handler. Do not model the action as state + effect; it makes effects re-run on unrelated changes and can duplicate the action.
**Impact: MEDIUM (prevents stale closures and unnecessary callback recreations)**
When updating state based on the current state value, use the functional update form of setState instead of directly referencing the state variable. This prevents stale closures, eliminates unnecessary dependencies, and creates stable callback references.
**Incorrect: requires state as dependency**
```tsx
function TodoList() {
const [items, setItems] = useState(initialItems)
// Callback must depend on items, recreated on every items change
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.
1.**Stable callback references** - Callbacks don't need to be recreated when state changes
2.**No stale closures** - Always operates on the latest state value
3.**Fewer dependencies** - Simplifies dependency arrays and reduces memory leaks
4.**Prevents bugs** - Eliminates the most common source of React closure bugs
**When to use functional updates:**
- Any setState that depends on the current state value
- Inside useCallback/useMemo when state is needed
- Event handlers that reference state
- Async operations that update state
**When direct updates are fine:**
- Setting state to a static value: `setCount(0)`
- Setting state from props/arguments only: `setName(newName)`
- State doesn't depend on previous value
**Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, the compiler can automatically optimize some cases, but functional updates are still recommended for correctness and to prevent stale closure bugs.
### 5.10 Use Lazy State Initialization
**Impact: MEDIUM (wasted computation on every render)**
Pass a function to `useState` for expensive initial values. Without the function form, the initializer runs on every render even though the value is only used once.
**Incorrect: runs on every render**
```tsx
function FilteredList({ items }: { items: Item[] }) {
// buildSearchIndex() runs on EVERY render, even after initialization
Use lazy initialization when computing initial values from localStorage/sessionStorage, building data structures (indexes, maps), reading from the DOM, or performing heavy transformations.
For simple primitives (`useState(0)`), direct references (`useState(props.value)`), or cheap literals (`useState({})`), the function form is unnecessary.
### 5.11 Use Transitions for Non-Urgent Updates
**Impact: MEDIUM (maintains UI responsiveness)**
Mark frequent, non-urgent state updates as transitions to maintain UI responsiveness.
**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.
This applies to all CSS transforms and transitions (`transform`, `opacity`, `translate`, `scale`, `rotate`). The wrapper div allows browsers to use GPU acceleration for smoother animations.
### 6.2 CSS content-visibility for Long Lists
**Impact: HIGH (faster initial render)**
Apply `content-visibility: auto` to defer off-screen rendering.
**CSS:**
```css
.message-item {
content-visibility: auto;
contain-intrinsic-size: 0 80px;
}
```
**Example:**
```tsx
function MessageList({ messages }: { messages: Message[] }) {
return (
<divclassName="overflow-y-auto h-screen">
{messages.map(msg => (
<divkey={msg.id}className="message-item">
<Avataruser={msg.author}/>
<div>{msg.content}</div>
</div>
))}
</div>
)
}
```
For 1000 messages, browser skips layout/paint for ~990 off-screen items (10× faster initial render).
### 6.3 Hoist Static JSX Elements
**Impact: LOW (avoids re-creation)**
Extract static JSX outside components to avoid re-creation.
This is especially helpful for large and static SVG nodes, which can be expensive to recreate on every render.
**Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, the compiler automatically hoists static JSX elements and optimizes component re-renders, making manual hoisting unnecessary.
### 6.4 Optimize SVG Precision
**Impact: LOW (reduces file size)**
Reduce SVG coordinate precision to decrease file size. The optimal precision depends on the viewBox size, but in general reducing precision should be considered.
**Incorrect: excessive precision**
```svg
<pathd="M 10.293847 20.847362 L 30.938472 40.192837"/>
```
**Correct: 1 decimal place**
```svg
<pathd="M 10.3 20.8 L 30.9 40.2"/>
```
**Automate with SVGO:**
```bash
npx svgo --precision=1 --multipass icon.svg
```
### 6.5 Prevent Hydration Mismatch Without Flickering
**Impact: MEDIUM (avoids visual flicker and hydration errors)**
When rendering content that depends on client-side storage (localStorage, cookies), avoid both SSR breakage and post-hydration flickering by injecting a synchronous script that updates the DOM before React hydrates.
**Incorrect: breaks SSR**
```tsx
function ThemeWrapper({ children }: { children: ReactNode }) {
// localStorage is not available on server - throws error
Server-side rendering will fail because `localStorage` is undefined.
**Incorrect: visual flickering**
```tsx
function ThemeWrapper({ children }: { children: ReactNode }) {
const [theme, setTheme] = useState('light')
useEffect(() => {
// Runs after hydration - causes visible flash
const stored = localStorage.getItem('theme')
if (stored) {
setTheme(stored)
}
}, [])
return (
<divclassName={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**
```tsx
function ThemeWrapper({ children }: { children: ReactNode }) {
return (
<>
<divid="theme-wrapper">
{children}
</div>
<script
dangerouslySetInnerHTML={{
__html: `
(function() {
try {
var theme = localStorage.getItem('theme') || 'light';
var el = document.getElementById('theme-wrapper');
if (el) el.className = theme;
} catch (e) {}
})();
`,
}}
/>
</>
)
}
```
The inline script executes synchronously before showing the element, ensuring the DOM already has the correct value. No flickering, no hydration mismatch.
This pattern is especially useful for theme toggles, user preferences, authentication states, and any client-only data that should render immediately without flashing default values.
### 6.6 Suppress Expected Hydration Mismatches
**Impact: LOW-MEDIUM (avoids noisy hydration warnings for known differences)**
In SSR frameworks (e.g., Next.js), some values are intentionally different on server vs client (random IDs, dates, locale/timezone formatting). For these *expected* mismatches, wrap the dynamic text in an element with `suppressHydrationWarning` to prevent noisy warnings. Do not use this to hide real bugs. Don’t overuse it.
**Incorrect: known mismatch warnings**
```tsx
function Timestamp() {
return <span>{new Date().toLocaleString()}</span>
}
```
**Correct: suppress expected mismatch only**
```tsx
function Timestamp() {
return (
<spansuppressHydrationWarning>
{new Date().toLocaleString()}
</span>
)
}
```
### 6.7 Use Activity Component for Show/Hide
**Impact: MEDIUM (preserves state/DOM)**
Use React's `<Activity>` to preserve state/DOM for expensive components that frequently toggle visibility.
**Usage:**
```tsx
import { Activity } from 'react'
function Dropdown({ isOpen }: Props) {
return (
<Activitymode={isOpen?'visible':'hidden'}>
<ExpensiveMenu/>
</Activity>
)
}
```
Avoids expensive re-renders and state loss.
### 6.8 Use Explicit Conditional Rendering
**Impact: LOW (prevents rendering 0 or NaN)**
Use explicit ternary operators (`? :`) instead of `&&` for conditional rendering when the condition can be `0`, `NaN`, or other falsy values that render.
**Incorrect: renders "0" when count is 0**
```tsx
function Badge({ count }: { count: number }) {
return (
<div>
{count &&<spanclassName="badge">{count}</span>}
</div>
)
}
// When count = 0, renders: <div>0</div>
// When count = 5, renders: <div><spanclass="badge">5</span></div>
Micro-optimizations for hot paths can add up to meaningful improvements.
### 7.1 Avoid Layout Thrashing
**Impact: MEDIUM (prevents forced synchronous layouts and reduces performance bottlenecks)**
Avoid interleaving style writes with layout reads. When you read a layout property (like `offsetWidth`, `getBoundingClientRect()`, or `getComputedStyle()`) between style changes, the browser is forced to trigger a synchronous reflow.
**This is OK: browser batches style changes**
```typescript
function updateElementStyles(element: HTMLElement) {
// Each line invalidates style, but browser batches the recalculation
element.style.width = '100px'
element.style.height = '200px'
element.style.backgroundColor = 'blue'
element.style.border = '1px solid black'
}
```
**Incorrect: interleaved reads and writes force reflows**
Prefer CSS classes over inline styles when possible. CSS files are cached by the browser, and classes provide better separation of concerns and are easier to maintain.
See [this gist](https://gist.github.com/paulirish/5d52fb081b3570c81e3a) and [CSS Triggers](https://csstriggers.com/) for more information on layout-forcing operations.
### 7.2 Build Index Maps for Repeated Lookups
**Impact: LOW-MEDIUM (1M ops to 2K ops)**
Multiple `.find()` calls by the same key should use a Map.
**Incorrect (O(n) per lookup):**
```typescript
function processOrders(orders: Order[], users: User[]) {
return orders.map(order => ({
...order,
user: users.find(u => u.id === order.userId)
}))
}
```
**Correct (O(1) per lookup):**
```typescript
function processOrders(orders: Order[], users: User[]) {
const userById = new Map(users.map(u => [u.id, u]))
return orders.map(order => ({
...order,
user: userById.get(order.userId)
}))
}
```
Build map once (O(n)), then all lookups are O(1).
For 1000 orders × 1000 users: 1M ops → 2K ops.
### 7.3 Cache Property Access in Loops
**Impact: LOW-MEDIUM (reduces lookups)**
Cache object property lookups in hot paths.
**Incorrect: 3 lookups × N iterations**
```typescript
for (let i = 0; i <arr.length;i++){
process(obj.config.settings.value)
}
```
**Correct: 1 lookup total**
```typescript
const value = obj.config.settings.value
const len = arr.length
for (let i = 0; i <len;i++){
process(value)
}
```
### 7.4 Cache Repeated Function Calls
**Impact: MEDIUM (avoid redundant computation)**
Use a module-level Map to cache function results when the same function is called repeatedly with the same inputs during render.
**Incorrect: redundant computation**
```typescript
function ProjectList({ projects }: { projects: Project[] }) {
return (
<div>
{projects.map(project => {
// slugify() called 100+ times for same project names
const slug = slugify(project.name)
return <ProjectCardkey={project.id}slug={slug}/>
})}
</div>
)
}
```
**Correct: cached results**
```typescript
// Module-level cache
const slugifyCache = new Map<string,string>()
function cachedSlugify(text: string): string {
if (slugifyCache.has(text)) {
return slugifyCache.get(text)!
}
const result = slugify(text)
slugifyCache.set(text, result)
return result
}
function ProjectList({ projects }: { projects: Project[] }) {
If storage can change externally (another tab, server-set cookies), invalidate cache:
### 7.6 Combine Multiple Array Iterations
**Impact: LOW-MEDIUM (reduces iterations)**
Multiple `.filter()` or `.map()` calls iterate the array multiple times. Combine into one loop.
**Incorrect: 3 iterations**
```typescript
const admins = users.filter(u => u.isAdmin)
const testers = users.filter(u => u.isTester)
const inactive = users.filter(u => !u.isActive)
```
**Correct: 1 iteration**
```typescript
const admins: User[] = []
const testers: User[] = []
const inactive: User[] = []
for (const user of users) {
if (user.isAdmin) admins.push(user)
if (user.isTester) testers.push(user)
if (!user.isActive) inactive.push(user)
}
```
### 7.7 Early Length Check for Array Comparisons
**Impact: MEDIUM-HIGH (avoids expensive operations when lengths differ)**
When comparing arrays with expensive operations (sorting, deep equality, serialization), check lengths first. If lengths differ, the arrays cannot be equal.
In real-world applications, this optimization is especially valuable when the comparison runs in hot paths (event handlers, render loops).
**Incorrect: always runs expensive comparison**
```typescript
function hasChanges(current: string[], original: string[]) {
// Always sorts and joins, even when lengths differ
Two O(n log n) sorts run even when `current.length` is 5 and `original.length` is 100. There is also overhead of joining the arrays and comparing the strings.
**Correct (O(1) length check first):**
```typescript
function hasChanges(current: string[], original: string[]) {
// Early return if lengths differ
if (current.length !== original.length) {
return true
}
// Only sort when lengths match
const currentSorted = current.toSorted()
const originalSorted = original.toSorted()
for (let i = 0; i <currentSorted.length;i++){
if (currentSorted[i] !== originalSorted[i]) {
return true
}
}
return false
}
```
This new approach is more efficient because:
- It avoids the overhead of sorting and joining the arrays when lengths differ
- It avoids consuming memory for the joined strings (especially important for large arrays)
if (projects[i].updatedAt <oldest.updatedAt)oldest =projects[i]
if (projects[i].updatedAt > newest.updatedAt) newest = projects[i]
}
return { oldest, newest }
}
```
Single pass through the array, no copying, no sorting.
**Alternative: Math.min/Math.max for small arrays**
```typescript
const numbers = [5, 2, 8, 1, 9]
const min = Math.min(...numbers)
const max = Math.max(...numbers)
```
This works for small arrays, but can be slower or just throw an error for very large arrays due to spread operator limitations. Maximal array length is approximately 124000 in Chrome 143 and 638000 in Safari 18; exact numbers may vary - see [the fiddle](https://jsfiddle.net/qw1jabsx/4/). Use the loop approach for reliability.
### 7.11 Use Set/Map for O(1) Lookups
**Impact: LOW-MEDIUM (O(n) to O(1))**
Convert arrays to Set/Map for repeated membership checks.
### 7.12 Use toSorted() Instead of sort() for Immutability
**Impact: MEDIUM-HIGH (prevents mutation bugs in React state)**
`.sort()` mutates the array in place, which can cause bugs with React state and props. Use `.toSorted()` to create a new sorted array without mutation.
**Incorrect: mutates original array**
```typescript
function UserList({ users }: { users: User[] }) {
// Mutates the users prop array!
const sorted = useMemo(
() => users.sort((a, b) => a.name.localeCompare(b.name)),
[users]
)
return <div>{sorted.map(renderUser)}</div>
}
```
**Correct: creates new array**
```typescript
function UserList({ users }: { users: User[] }) {
// Creates new sorted array, original unchanged
const sorted = useMemo(
() => users.toSorted((a, b) => a.name.localeCompare(b.name)),
[users]
)
return <div>{sorted.map(renderUser)}</div>
}
```
**Why this matters in React:**
1. Props/state mutations break React's immutability model - React expects props and state to be treated as read-only
2. Causes stale closure bugs - Mutating arrays inside closures (callbacks, effects) can lead to unexpected behavior
**Browser support: fallback for older browsers**
```typescript
// Fallback for older browsers
const sorted = [...items].sort((a, b) => a.value - b.value)
```
`.toSorted()` is available in all modern browsers (Chrome 110+, Safari 16+, Firefox 115+, Node.js 20+). For older environments, use spread operator:
**Other immutable array methods:**
-`.toSorted()` - immutable sort
-`.toReversed()` - immutable reverse
-`.toSpliced()` - immutable splice
-`.with()` - immutable element replacement
---
## 8. Advanced Patterns
**Impact: LOW**
Advanced patterns for specific cases that require careful implementation.
### 8.1 Initialize App Once, Not Per Mount
**Impact: LOW-MEDIUM (avoids duplicate init in development)**
Do not put app-wide initialization that must run once per app load inside `useEffect([])` of a component. Components can remount and effects will re-run. Use a module-level guard or top-level init in the entry module instead.
**Incorrect: runs twice in dev, re-runs on remount**
**Alternative: use `useEffectEvent` if you're on latest React:**
`useEffectEvent` provides a cleaner API for the same pattern: it creates a stable function reference that always calls the latest version of the handler.
### 8.3 useEffectEvent for Stable Callback Refs
**Impact: LOW (prevents effect re-runs)**
Access latest values in callbacks without adding them to dependency arrays. Prevents effect re-runs while avoiding stale closures.
**Incorrect: effect re-runs on every callback change**
