16 sections · 105 entries
function ComponentName(props) {
return <JSX />;
}
// or arrow function
const ComponentName = (props) => <JSX />;function Greeting({ name }) {
return <h1>Welcome, {name}!</h1>;
}
// Usage
<Greeting name="Amara" />Renders: <h1>Welcome, Amara!</h1>Note Component names must start with a capital letter. React treats lowercase tags as native HTML elements.
function Component({ propA, propB }) { ... }
// or
function Component(props) {
const { propA, propB } = props;
}function UserCard({ username, role, isActive }) {
return (
<div>
<strong>{username}</strong>
<span>{role}</span>
{isActive && <span>Online</span>}
</div>
);
}Renders the user card with username, role, and optional online badge.Note Props are read-only. Never mutate a prop directly inside a component. Destructuring in the parameter list is the most common pattern.
function Wrapper({ children }) {
return <div className="wrapper">{children}</div>;
}function Card({ children, title }) {
return (
<section className="card">
<h2>{title}</h2>
<div className="card-body">{children}</div>
</section>
);
}
// Usage
<Card title="Profile">
<p>This content is passed as children.</p>
</Card>Renders a card wrapper around the paragraph.Note children can be any renderable content: text, elements, arrays, or even functions (for render props).
function Component({ propA = defaultValue }) { ... }function Badge({ label = "Member", color = "gray" }) {
return (
<span style={{ backgroundColor: color }}>
{label}
</span>
);
}
// Uses defaults
<Badge />
// Overrides
<Badge label="Admin" color="crimson" />First renders "Member" with gray background. Second renders "Admin" with crimson.Note Use default parameter values in the destructuring rather than the legacy defaultProps static property, which is deprecated in React 19.
function Parent() {
return (
<Layout>
<Header />
<Content />
<Footer />
</Layout>
);
}function PageLayout({ sidebar, content }) {
return (
<div className="layout">
<aside>{sidebar}</aside>
<main>{content}</main>
</div>
);
}
function Dashboard() {
return (
<PageLayout
sidebar={<NavMenu />}
content={<StatsPanel />}
/>
);
}Note Composition via props is preferred over inheritance. Passing components as props gives you flexible "slot" patterns without tightly coupling parent and child.
{condition && <Component />}
{condition ? <A /> : <B />}
if (condition) return <A />; return <B />;function Notification({ message, type }) {
if (!message) return null;
return (
<div className={`alert alert-${type}`}>
{type === "error" ? "Error: " : "Info: "}
{message}
</div>
);
}Returns null when no message. Otherwise renders an alert with type prefix.Note Returning null renders nothing. Watch out with &&: if the left side is 0, React renders "0" because it is a valid JSX child. Use Boolean(count) && <Component /> or count > 0 && <Component /> instead.
{items.map(item => (
<Component key={item.uniqueId} {...item} />
))}function TaskList({ tasks }) {
return (
<ul>
{tasks.map(task => (
<li key={task.id}>
<span>{task.title}</span>
{task.done && <span> (completed)</span>}
</li>
))}
</ul>
);
}Note Keys must be stable, unique among siblings, and derived from data (like a database ID). Never use array index as key if the list order can change -- it causes subtle rendering bugs and broken state.
<React.Fragment>...</React.Fragment>
// Short syntax
<>...</>function UserInfo({ name, email }) {
return (
<>
<dt>{name}</dt>
<dd>{email}</dd>
</>
);
}
// With key (long syntax required)
function GlossaryList({ items }) {
return (
<dl>
{items.map(item => (
<React.Fragment key={item.id}>
<dt>{item.term}</dt>
<dd>{item.definition}</dd>
</React.Fragment>
))}
</dl>
);
}Note Fragments let you group elements without adding extra DOM nodes. Use the long syntax <React.Fragment key={...}> when mapping a list, since the short syntax does not support the key attribute.
// Must return single root element
return (
<div>
<Child />
</div>
);
// Self-closing tags required for void elements
<img src={url} alt="desc" />function ProductCard({ product }) {
return (
<article className="product-card">
<img src={product.imageUrl} alt={product.name} />
<h3>{product.name}</h3>
<p>{product.description}</p>
<strong>${product.price.toFixed(2)}</strong>
</article>
);
}Note JSX is syntactic sugar for function calls. Every tag must be closed. Use className instead of class, htmlFor instead of for. Attribute names follow camelCase convention.
{expression}
// Any valid JavaScript expression inside curly bracesfunction PriceDisplay({ price, discount }) {
const finalPrice = price * (1 - discount);
const savings = price - finalPrice;
return (
<div>
<p>Original: ${price.toFixed(2)}</p>
<p>You save: ${savings.toFixed(2)}</p>
<p className="total">Total: ${finalPrice.toFixed(2)}</p>
<small>Applied {(discount * 100).toFixed(0)}% off</small>
</div>
);
}Note You can embed any JavaScript expression inside curly braces: variables, function calls, math, ternaries. Statements (if, for, while) cannot appear inside JSX directly -- use them before the return.
// AND pattern
{isLoggedIn && <Dashboard />}
// Ternary
{isLoggedIn ? <Dashboard /> : <LoginForm />}
// IIFE for complex logic
{(() => { if (x) return <A />; return <B />; })()}function StatusBadge({ status }) {
return (
<span>
{status === "active" && <span className="dot green" />}
{status === "inactive" ? (
<span className="label">Away</span>
) : (
<span className="label">{status}</span>
)}
</span>
);
}Note For more than two branches, extract the logic into a variable or helper function above the return statement rather than nesting ternaries. Readable code wins over clever one-liners.
<Component {...propsObject} />function FormField({ label, ...inputProps }) {
return (
<div className="field">
<label>{label}</label>
<input {...inputProps} />
</div>
);
}
// Usage: all standard input attributes are forwarded
<FormField
label="Email"
type="email"
placeholder="you@example.com"
required
/>Note Spread is great for forwarding props. Be careful: it can pass unintended attributes to DOM elements. Destructure known props first, then spread the rest.
// JSX (compiled to createElement)
<Button color="blue">Save</Button>
// Equivalent createElement call
React.createElement(Button, { color: "blue" }, "Save")// This JSX:
const element = (
<div className="container">
<h1>Title</h1>
<p>Paragraph</p>
</div>
);
// Compiles to:
const element = React.createElement(
"div",
{ className: "container" },
React.createElement("h1", null, "Title"),
React.createElement("p", null, "Paragraph")
);Note You almost never need to call createElement directly. JSX is syntactically cleaner and the standard in the ecosystem. The JSX transform in modern setups does not require importing React at the top of every file.
<div style={{ property: value }} />function ProgressBar({ percent }) {
return (
<div
style={{
width: `${percent}%`,
height: "8px",
backgroundColor: percent > 70 ? "#4caf50" : "#ff9800",
borderRadius: "4px",
transition: "width 0.3s ease",
}}
/>
);
}Note Style properties use camelCase (backgroundColor, not background-color). Values that need units must include them as strings ("8px"), except for unitless properties like opacity and zIndex which accept numbers.
const [state, setState] = useState(initialValue);import { useState } from 'react';
function Counter() {
const [count, setCount] = useState(0);
return (
<div>
<p>Count: {count}</p>
<button onClick={() => setCount(count + 1)}>Increment</button>
<button onClick={() => setCount(0)}>Reset</button>
</div>
);
}Displays count starting at 0. Clicking increment adds 1 each time.Note setState triggers a re-render. The state value remains the same within the current render cycle even after calling setState -- the new value appears on the next render.
setState(prevState => newState);function StepCounter() {
const [count, setCount] = useState(0);
function incrementThree() {
// Each updater receives the latest pending state
setCount(prev => prev + 1);
setCount(prev => prev + 1);
setCount(prev => prev + 1);
}
return (
<button onClick={incrementThree}>
Count: {count}
</button>
);
}Each click adds 3 to the count, because each updater builds on the previous pending value.Note Always use the updater form when the next state depends on the previous state. Writing setCount(count + 1) three times in a row would only add 1 because count is a stale snapshot from the current render.
const [state, setState] = useState(() => computeInitialState());function FilteredList() {
// Expensive computation runs only on mount, not every render
const [sortedItems, setSortedItems] = useState(() => {
const saved = localStorage.getItem('cachedItems');
return saved ? JSON.parse(saved) : [];
});
return <ItemList items={sortedItems} />;
}Note Pass a function (not the result of calling it) to avoid running expensive initialization logic on every render. React only calls the initializer function during the first render and ignores it on subsequent renders.
setState(prev => ({ ...prev, key: newValue }));function ProfileEditor() {
const [profile, setProfile] = useState({
firstName: '',
lastName: '',
bio: '',
});
function handleChange(field, value) {
setProfile(prev => ({
...prev,
[field]: value,
}));
}
return (
<>
<input
value={profile.firstName}
onChange={e => handleChange('firstName', e.target.value)}
placeholder="First name"
/>
<input
value={profile.lastName}
onChange={e => handleChange('lastName', e.target.value)}
placeholder="Last name"
/>
</>
);
}Note Always spread the previous object to create a new copy. Directly mutating state (profile.firstName = 'X') will NOT trigger a re-render because React compares by reference.
// Add
setItems(prev => [...prev, newItem]);
// Remove
setItems(prev => prev.filter(i => i.id !== id));
// Update one
setItems(prev => prev.map(i => i.id === id ? { ...i, done: true } : i));function TodoList() {
const [todos, setTodos] = useState([]);
const [text, setText] = useState('');
function addTodo() {
if (!text.trim()) return;
setTodos(prev => [
...prev,
{ id: crypto.randomUUID(), text, done: false },
]);
setText('');
}
function removeTodo(id) {
setTodos(prev => prev.filter(t => t.id !== id));
}
function toggleTodo(id) {
setTodos(prev =>
prev.map(t => t.id === id ? { ...t, done: !t.done } : t)
);
}
return (
<div>
<input value={text} onChange={e => setText(e.target.value)} />
<button onClick={addTodo}>Add</button>
<ul>
{todos.map(t => (
<li key={t.id}>
<span
style={{ textDecoration: t.done ? 'line-through' : 'none' }}
onClick={() => toggleTodo(t.id)}
>
{t.text}
</span>
<button onClick={() => removeTodo(t.id)}>Remove</button>
</li>
))}
</ul>
</div>
);
}Note Use filter to remove, map to update, and spread to add. Never use push, splice, or direct index assignment on state arrays -- those mutate the existing array instead of creating a new one.
const [name, setName] = useState('');
const [age, setAge] = useState(0);
const [isActive, setIsActive] = useState(false);function RegistrationForm() {
const [email, setEmail] = useState('');
const [password, setPassword] = useState('');
const [acceptTerms, setAcceptTerms] = useState(false);
const isValid = email.includes('@') && password.length >= 8 && acceptTerms;
return (
<form>
<input
type="email"
value={email}
onChange={e => setEmail(e.target.value)}
/>
<input
type="password"
value={password}
onChange={e => setPassword(e.target.value)}
/>
<label>
<input
type="checkbox"
checked={acceptTerms}
onChange={e => setAcceptTerms(e.target.checked)}
/>
I accept the terms
</label>
<button disabled={!isValid}>Register</button>
</form>
);
}Note Separate state variables are preferred when values change independently. Combine into an object only when values logically belong together and always change at the same time.
useEffect(() => {
// side effect
return () => { /* cleanup */ };
}, [dependencies]);import { useState, useEffect } from 'react';
function DocumentTitle({ title }) {
useEffect(() => {
document.title = title;
}, [title]);
return <h1>{title}</h1>;
}Note The effect runs after the component renders. The dependency array controls when it re-runs. Omit the array: runs after every render. Empty array []: runs once on mount. With values: runs when any dependency changes.
useEffect(effectFn, [dep1, dep2]);
// Empty array: mount only
useEffect(effectFn, []);
// No array: every render
useEffect(effectFn);function SearchResults({ query, page }) {
const [results, setResults] = useState([]);
useEffect(() => {
// Runs whenever query or page changes
let cancelled = false;
async function search() {
const data = await fetchResults(query, page);
if (!cancelled) setResults(data);
}
search();
return () => { cancelled = true; };
}, [query, page]);
return <ResultsList items={results} />;
}Note React uses Object.is to compare dependencies. Objects and arrays create new references each render, so they will always trigger the effect. Extract primitive values or memoize objects before listing them as dependencies.
useEffect(() => {
// setup
return () => {
// cleanup runs before next effect and on unmount
};
}, [deps]);function ChatConnection({ roomId }) {
useEffect(() => {
const connection = connectToRoom(roomId);
connection.open();
return () => {
connection.close();
};
}, [roomId]);
return <ChatWindow />;
}Note The cleanup function runs before the effect re-executes (when dependencies change) and when the component unmounts. This prevents memory leaks from subscriptions, timers, and open connections.
useEffect(() => {
let ignore = false;
async function load() {
const data = await fetchData();
if (!ignore) setState(data);
}
load();
return () => { ignore = true; };
}, [deps]);function UserProfile({ userId }) {
const [user, setUser] = useState(null);
const [loading, setLoading] = useState(true);
useEffect(() => {
let ignore = false;
setLoading(true);
async function loadUser() {
const response = await fetch(`/api/users/${userId}`);
const data = await response.json();
if (!ignore) {
setUser(data);
setLoading(false);
}
}
loadUser();
return () => { ignore = true; };
}, [userId]);
if (loading) return <p>Loading...</p>;
return <div>{user.name}</div>;
}Note The ignore flag prevents setting state on an unmounted component or after a stale request. In React 19, consider using the use() hook with Suspense for data fetching instead of this pattern.
useEffect(() => {
const handler = (e) => { ... };
window.addEventListener('event', handler);
return () => window.removeEventListener('event', handler);
}, []);function WindowSize() {
const [size, setSize] = useState({
width: window.innerWidth,
height: window.innerHeight,
});
useEffect(() => {
function handleResize() {
setSize({
width: window.innerWidth,
height: window.innerHeight,
});
}
window.addEventListener('resize', handleResize);
return () => window.removeEventListener('resize', handleResize);
}, []);
return <p>{size.width} x {size.height}</p>;
}Note Always remove event listeners in the cleanup to avoid duplicate handlers and memory leaks. The handler reference must be the same function for addEventListener and removeEventListener.
useEffect(() => {
const id = setInterval(() => { ... }, delay);
return () => clearInterval(id);
}, [delay]);function Stopwatch() {
const [seconds, setSeconds] = useState(0);
const [running, setRunning] = useState(false);
useEffect(() => {
if (!running) return;
const intervalId = setInterval(() => {
setSeconds(prev => prev + 1);
}, 1000);
return () => clearInterval(intervalId);
}, [running]);
return (
<div>
<p>{seconds}s</p>
<button onClick={() => setRunning(r => !r)}>
{running ? 'Pause' : 'Start'}
</button>
<button onClick={() => { setRunning(false); setSeconds(0); }}>
Reset
</button>
</div>
);
}Note Always clear intervals and timeouts in cleanup. Use the updater form (prev => prev + 1) inside intervals to avoid stale closure issues with the state value.
// Derived state: compute during render
const fullName = firstName + ' ' + lastName;
// Event response: handle in the event handler
function handleClick() { doSomething(); }// BAD: useEffect to sync derived data
const [firstName, setFirstName] = useState('Kai');
const [lastName, setLastName] = useState('Chen');
const [fullName, setFullName] = useState('');
useEffect(() => {
setFullName(firstName + ' ' + lastName);
}, [firstName, lastName]);
// GOOD: compute directly during render
const fullName = firstName + ' ' + lastName;
// BAD: useEffect for event-driven logic
useEffect(() => {
if (submitted) sendAnalytics();
}, [submitted]);
// GOOD: call directly in the event handler
function handleSubmit() {
submitForm();
sendAnalytics();
}Note Overusing useEffect is one of the most common React mistakes. If you can calculate something from existing props or state, do it during render. If something should happen in response to a user action, put it in the event handler.
const ref = useRef(null);
// Attach to element
<element ref={ref} />
// Access
ref.currentimport { useRef } from 'react';
function AutoFocusInput() {
const inputRef = useRef(null);
function handleClick() {
inputRef.current.focus();
inputRef.current.select();
}
return (
<div>
<input ref={inputRef} defaultValue="Edit me" />
<button onClick={handleClick}>Focus & Select</button>
</div>
);
}Note ref.current is null during the first render and gets assigned after the DOM node mounts. Never read or write ref.current during rendering -- only inside effects or event handlers.
const valueRef = useRef(initialValue);
valueRef.current = newValue; // does NOT trigger re-renderfunction ClickTracker() {
const clickCount = useRef(0);
function handleClick() {
clickCount.current += 1;
console.log(`Clicked ${clickCount.current} times`);
}
return <button onClick={handleClick}>Click me</button>;
}
// Common use: storing previous value
function usePrevious(value) {
const prevRef = useRef(undefined);
const previous = prevRef.current;
prevRef.current = value;
return previous;
}Note useRef is like an instance variable for function components. Updating .current does not cause a re-render. Use it for values you need to persist across renders without triggering UI updates: timer IDs, previous values, flags.
// React 19: ref is a regular prop
function CustomInput({ ref, ...props }) {
return <input ref={ref} {...props} />;
}
// Pre-React 19 (still works):
const CustomInput = forwardRef((props, ref) => {
return <input ref={ref} {...props} />;
});// React 19 style: ref is just a prop
function FancyInput({ ref, placeholder, className }) {
return (
<input
ref={ref}
placeholder={placeholder}
className={`fancy-input ${className}`}
/>
);
}
// Parent component
function SearchBar() {
const inputRef = useRef(null);
return (
<div>
<FancyInput ref={inputRef} placeholder="Search..." />
<button onClick={() => inputRef.current.focus()}>Focus</button>
</div>
);
}Note In React 19, ref is passed as a regular prop to function components. forwardRef is no longer necessary and is considered deprecated. Existing code using forwardRef still works but should be migrated over time.
<element ref={(node) => {
// node is the DOM element or null on unmount
}} />function MeasuredBox() {
const [height, setHeight] = useState(0);
const measureRef = (node) => {
if (node !== null) {
setHeight(node.getBoundingClientRect().height);
}
};
return (
<div>
<div ref={measureRef} style={{ padding: '20px' }}>
<p>This box has dynamic content.</p>
<p>Its height is measured via a callback ref.</p>
</div>
<p>Measured height: {height}px</p>
</div>
);
}Note Callback refs fire whenever the ref attaches or detaches. In React 19, callback refs can return a cleanup function that runs when the element is removed, similar to useEffect cleanup. Useful for measuring elements or integrating third-party DOM libraries.
useImperativeHandle(ref, () => ({
customMethod() { ... },
}), [deps]);import { useRef, useImperativeHandle } from 'react';
function VideoPlayer({ ref, src }) {
const videoRef = useRef(null);
useImperativeHandle(ref, () => ({
play() {
videoRef.current.play();
},
pause() {
videoRef.current.pause();
},
restart() {
videoRef.current.currentTime = 0;
videoRef.current.play();
},
}));
return <video ref={videoRef} src={src} />;
}
// Parent controls the player
function MediaPage() {
const playerRef = useRef(null);
return (
<div>
<VideoPlayer ref={playerRef} src="/video.mp4" />
<button onClick={() => playerRef.current.play()}>Play</button>
<button onClick={() => playerRef.current.restart()}>Restart</button>
</div>
);
}Note useImperativeHandle restricts what the parent can access through the ref. Expose only the methods the parent needs instead of the entire DOM node. Combine with ref-as-prop in React 19 (no forwardRef needed).
const timerRef = useRef(null);
// Start
timerRef.current = setTimeout(fn, ms);
// Clear
clearTimeout(timerRef.current);function DebouncedSearch({ onSearch }) {
const [query, setQuery] = useState('');
const timerRef = useRef(null);
function handleChange(e) {
const value = e.target.value;
setQuery(value);
clearTimeout(timerRef.current);
timerRef.current = setTimeout(() => {
onSearch(value);
}, 400);
}
useEffect(() => {
return () => clearTimeout(timerRef.current);
}, []);
return <input value={query} onChange={handleChange} />;
}Note Store timer IDs in a ref so they survive re-renders without causing them. Always clear the timer on unmount via useEffect cleanup to prevent firing after the component is gone.
import { createContext } from 'react';
const MyContext = createContext(defaultValue);// theme-context.js
import { createContext } from 'react';
export const ThemeContext = createContext({
mode: 'light',
toggleTheme: () => {},
});Note The default value is used only when a component reads the context but there is no matching Provider above it in the tree. Pass a realistic shape as the default so TypeScript inference and fallback behavior work correctly.
<MyContext.Provider value={contextValue}>
{children}
</MyContext.Provider>import { useState } from 'react';
import { ThemeContext } from './theme-context';
function ThemeProvider({ children }) {
const [mode, setMode] = useState('light');
const contextValue = {
mode,
toggleTheme: () => setMode(m => m === 'light' ? 'dark' : 'light'),
};
return (
<ThemeContext.Provider value={contextValue}>
{children}
</ThemeContext.Provider>
);
}Note Wrap the Provider at the highest point where descendant components need the value. If the value is a new object on every render, wrap it with useMemo to prevent unnecessary re-renders of consumers.
import { useContext } from 'react';
const value = useContext(MyContext);import { useContext } from 'react';
import { ThemeContext } from './theme-context';
function ThemeToggleButton() {
const { mode, toggleTheme } = useContext(ThemeContext);
return (
<button
onClick={toggleTheme}
style={{
background: mode === 'dark' ? '#333' : '#fff',
color: mode === 'dark' ? '#fff' : '#333',
}}
>
Current: {mode}
</button>
);
}Note useContext always looks for the nearest Provider above the calling component. If none is found, it uses the default value from createContext. The component re-renders whenever the provided value changes.
<AuthContext.Provider value={auth}>
<ThemeContext.Provider value={theme}>
{children}
</ThemeContext.Provider>
</AuthContext.Provider>function AppProviders({ children }) {
const auth = useAuthState();
const theme = useThemeState();
const locale = useLocaleState();
return (
<AuthContext.Provider value={auth}>
<ThemeContext.Provider value={theme}>
<LocaleContext.Provider value={locale}>
{children}
</LocaleContext.Provider>
</ThemeContext.Provider>
</AuthContext.Provider>
);
}
// Usage at app root
<AppProviders>
<App />
</AppProviders>Note Split unrelated data into separate contexts. A single monolithic context forces every consumer to re-render when any part of the value changes, even if they only use a small slice.
// Encapsulate context + hook in a module
const Ctx = createContext(null);
export function useMyContext() {
const ctx = useContext(Ctx);
if (!ctx) throw new Error('Missing Provider');
return ctx;
}import { createContext, useContext, useState } from 'react';
const CartContext = createContext(null);
export function CartProvider({ children }) {
const [items, setItems] = useState([]);
function addItem(product) {
setItems(prev => [...prev, { ...product, qty: 1 }]);
}
function removeItem(id) {
setItems(prev => prev.filter(item => item.id !== id));
}
const totalItems = items.reduce((sum, i) => sum + i.qty, 0);
return (
<CartContext.Provider value={{ items, addItem, removeItem, totalItems }}>
{children}
</CartContext.Provider>
);
}
export function useCart() {
const context = useContext(CartContext);
if (!context) {
throw new Error('useCart must be used within a CartProvider');
}
return context;
}Note Exporting a custom hook that throws when the Provider is missing catches mistakes early and gives a clear error message instead of undefined values that silently break downstream.
// Memoize the value object
const value = useMemo(() => ({ data, handler }), [data]);
// Split read-only data from dispatch
<DataContext.Provider value={data}>
<DispatchContext.Provider value={dispatch}>import { createContext, useContext, useMemo, useState } from 'react';
const CountContext = createContext(null);
const CountDispatchContext = createContext(null);
function CountProvider({ children }) {
const [count, setCount] = useState(0);
// dispatch never changes identity
const increment = useMemo(
() => () => setCount(c => c + 1),
[]
);
return (
<CountContext.Provider value={count}>
<CountDispatchContext.Provider value={increment}>
{children}
</CountDispatchContext.Provider>
</CountContext.Provider>
);
}
// Components reading only dispatch won't re-render when count changes
function IncrementButton() {
const increment = useContext(CountDispatchContext);
return <button onClick={increment}>+1</button>;
}Note Splitting context into a data context and a dispatch context is a powerful pattern. Components that only call actions (dispatch) will not re-render when the data changes, because the dispatch reference stays stable.
const memoizedValue = useMemo(() => computeValue(a, b), [a, b]);import { useMemo, useState } from 'react';
function ProductFilter({ products }) {
const [query, setQuery] = useState('');
const [sortBy, setSortBy] = useState('name');
const filtered = useMemo(() => {
const matched = products.filter(p =>
p.name.toLowerCase().includes(query.toLowerCase())
);
return matched.sort((a, b) =>
a[sortBy] > b[sortBy] ? 1 : -1
);
}, [products, query, sortBy]);
return (
<div>
<input value={query} onChange={e => setQuery(e.target.value)} />
<select value={sortBy} onChange={e => setSortBy(e.target.value)}>
<option value="name">Name</option>
<option value="price">Price</option>
</select>
<ul>{filtered.map(p => <li key={p.id}>{p.name}</li>)}</ul>
</div>
);
}Note useMemo caches the result of a computation. React may discard the cache under memory pressure, so never rely on it for correctness -- only for performance. Do not memoize everything by default; profile first.
const memoizedFn = useCallback((args) => {
doSomething(args, dep);
}, [dep]);import { useCallback, useState } from 'react';
function ParentList({ items }) {
const [selectedId, setSelectedId] = useState(null);
const handleSelect = useCallback((id) => {
setSelectedId(id);
}, []);
return (
<ul>
{items.map(item => (
<ListItem
key={item.id}
item={item}
onSelect={handleSelect}
isSelected={item.id === selectedId}
/>
))}
</ul>
);
}Note useCallback(fn, deps) is identical to useMemo(() => fn, deps). Its primary use case is passing stable function references to memoized child components (wrapped with React.memo) to prevent their re-renders.
// All values from component scope used inside must be listed
useCallback(() => fn(a, b), [a, b]);
useMemo(() => compute(x), [x]);function ChatRoom({ roomId, serverUrl }) {
// Recreated only when roomId or serverUrl change
const connect = useCallback(() => {
const ws = new WebSocket(`${serverUrl}/rooms/${roomId}`);
ws.onopen = () => console.log('Connected to', roomId);
return ws;
}, [roomId, serverUrl]);
useEffect(() => {
const ws = connect();
return () => ws.close();
}, [connect]);
return <div>Chat: {roomId}</div>;
}Note Missing a dependency leads to stale closures. Including an unnecessary one causes pointless recalculations. Use the exhaustive-deps lint rule (eslint-plugin-react-hooks) to catch mistakes automatically.
const MemoizedComponent = React.memo(Component);
// With custom comparison
const MemoizedComponent = React.memo(Component, (prevProps, nextProps) => {
return prevProps.id === nextProps.id;
});import { memo, useState, useCallback } from 'react';
const ExpensiveRow = memo(function ExpensiveRow({ item, onToggle }) {
// This component only re-renders if item or onToggle change
return (
<tr onClick={() => onToggle(item.id)}>
<td>{item.name}</td>
<td>{item.value.toFixed(2)}</td>
</tr>
);
});
function DataTable({ rows }) {
const [selected, setSelected] = useState(new Set());
const handleToggle = useCallback((id) => {
setSelected(prev => {
const next = new Set(prev);
next.has(id) ? next.delete(id) : next.add(id);
return next;
});
}, []);
return (
<table>
<tbody>
{rows.map(row => (
<ExpensiveRow key={row.id} item={row} onToggle={handleToggle} />
))}
</tbody>
</table>
);
}Note React.memo does a shallow comparison of props. It only helps if the component re-renders with the same props frequently. Pair it with useCallback for function props and useMemo for object/array props.
// DO memoize when:
// - Computation is genuinely expensive (sorting large arrays, complex math)
// - Passing callbacks/objects to React.memo children
// - Value is a dependency of another hook
// DON'T memoize when:
// - The computation is trivial
// - The component rarely re-renders
// - No downstream consumer cares about reference stability// GOOD: expensive computation
const sortedTransactions = useMemo(
() => transactions.slice().sort(compareDates),
[transactions]
);
// GOOD: stable reference for memoized child
const handleDelete = useCallback(
(id) => dispatch({ type: 'DELETE', id }),
[dispatch]
);
// UNNECESSARY: trivial computation
// const label = useMemo(() => `Hello ${name}`, [name]);
// Just write: const label = `Hello ${name}`;Note Memoization has a cost: it uses memory to store results and adds comparison overhead. Only apply it where profiling shows a measurable benefit. Premature memoization adds complexity without improving performance.
const options = useMemo(() => ({ key: value }), [value]);
useEffect(() => { doSomething(options); }, [options]);function DataFetcher({ endpoint, filters }) {
// Without useMemo, a new object is created every render,
// causing the effect to re-run endlessly
const requestConfig = useMemo(
() => ({
url: endpoint,
params: filters,
headers: { 'Accept': 'application/json' },
}),
[endpoint, filters]
);
useEffect(() => {
const controller = new AbortController();
fetch(requestConfig.url, {
...requestConfig,
signal: controller.signal,
}).then(r => r.json()).then(setData);
return () => controller.abort();
}, [requestConfig]);
}Note When an object or array is a dependency of useEffect, memoize it to prevent infinite effect loops. Every render creates a new reference, and useEffect sees a 'change' even when the values inside are identical.
const [state, dispatch] = useReducer(reducer, initialState);
// reducer: (state, action) => newStateimport { useReducer } from 'react';
function cartReducer(state, action) {
switch (action.type) {
case 'ADD_ITEM':
return { ...state, items: [...state.items, action.payload] };
case 'REMOVE_ITEM':
return {
...state,
items: state.items.filter(i => i.id !== action.payload),
};
case 'CLEAR':
return { ...state, items: [] };
default:
throw new Error(`Unknown action: ${action.type}`);
}
}
function ShoppingCart() {
const [cart, dispatch] = useReducer(cartReducer, { items: [] });
return (
<div>
<p>{cart.items.length} items</p>
<button onClick={() =>
dispatch({ type: 'ADD_ITEM', payload: { id: 1, name: 'Notebook' } })
}>
Add Notebook
</button>
<button onClick={() => dispatch({ type: 'CLEAR' })}>
Clear Cart
</button>
</div>
);
}Note Prefer useReducer over useState when you have complex state logic with multiple sub-values, or when the next state depends on the previous state in varied ways. The reducer function must be pure -- no side effects.
const id = useId();import { useId } from 'react';
function LabeledInput({ label, type = 'text' }) {
const id = useId();
return (
<div>
<label htmlFor={id}>{label}</label>
<input id={id} type={type} />
</div>
);
}
// Each instance gets a unique, stable ID
<LabeledInput label="Username" />
<LabeledInput label="Email" type="email" />Note useId generates IDs that are stable across server and client renders, preventing hydration mismatches. Never use it for keys in a list. It is meant for accessibility attributes (htmlFor, aria-describedby, etc.).
const deferredValue = useDeferredValue(value);import { useState, useDeferredValue, useMemo } from 'react';
function HeavySearch({ allItems }) {
const [query, setQuery] = useState('');
const deferredQuery = useDeferredValue(query);
const results = useMemo(
() => allItems.filter(item =>
item.name.toLowerCase().includes(deferredQuery.toLowerCase())
),
[allItems, deferredQuery]
);
const isStale = query !== deferredQuery;
return (
<div>
<input
value={query}
onChange={e => setQuery(e.target.value)}
placeholder="Search..."
/>
<div style={{ opacity: isStale ? 0.6 : 1 }}>
{results.map(item => <p key={item.id}>{item.name}</p>)}
</div>
</div>
);
}Note useDeferredValue lets the UI stay responsive by deferring re-rendering of expensive parts. It is similar to debouncing but integrated with React's rendering scheduler. Compare the deferred and current value to show a stale indicator.
const [isPending, startTransition] = useTransition();import { useState, useTransition } from 'react';
function TabContainer() {
const [tab, setTab] = useState('home');
const [isPending, startTransition] = useTransition();
function selectTab(nextTab) {
startTransition(() => {
setTab(nextTab);
});
}
return (
<div>
<nav>
<button onClick={() => selectTab('home')}>Home</button>
<button onClick={() => selectTab('analytics')}>Analytics</button>
<button onClick={() => selectTab('settings')}>Settings</button>
</nav>
{isPending && <p>Loading...</p>}
<TabPanel activeTab={tab} />
</div>
);
}Note startTransition marks state updates as non-urgent, allowing urgent updates (like typing) to interrupt them. The isPending flag lets you show loading indicators. Unlike useDeferredValue, useTransition wraps the state update itself.
const snapshot = useSyncExternalStore(
subscribe,
getSnapshot,
getServerSnapshot? // optional, for SSR
);import { useSyncExternalStore } from 'react';
function useOnlineStatus() {
return useSyncExternalStore(
(notify) => {
window.addEventListener('online', notify);
window.addEventListener('offline', notify);
return () => {
window.removeEventListener('online', notify);
window.removeEventListener('offline', notify);
};
},
() => navigator.onLine, // client snapshot
() => true // server snapshot (assume online)
);
}
function StatusBar() {
const isOnline = useOnlineStatus();
return <p>{isOnline ? 'Connected' : 'Offline'}</p>;
}Note Designed for subscribing to external data sources (browser APIs, third-party stores) in a way that is compatible with concurrent rendering. The subscribe function must accept a callback and return an unsubscribe function.
useDebugValue(value);
useDebugValue(value, formatFn); // lazy formattingimport { useDebugValue, useSyncExternalStore } from 'react';
function useMediaQuery(query) {
const matches = useSyncExternalStore(
(notify) => {
const mql = window.matchMedia(query);
mql.addEventListener('change', notify);
return () => mql.removeEventListener('change', notify);
},
() => window.matchMedia(query).matches,
() => false
);
// Shows in React DevTools next to the hook
useDebugValue(matches ? `Matches: ${query}` : `No match: ${query}`);
return matches;
}Note useDebugValue only affects the React DevTools display for custom hooks. It adds no runtime behavior. Use the format function (second argument) to defer expensive string formatting until the hook is actually inspected in DevTools.
const value = use(resource);
// resource can be a Promise or a Contextimport { use, Suspense } from 'react';
function UserGreeting({ userPromise }) {
// React suspends until the promise resolves
const user = use(userPromise);
return <h2>Hello, {user.name}!</h2>;
}
function App() {
const userPromise = fetchCurrentUser();
return (
<Suspense fallback={<p>Loading user...</p>}>
<UserGreeting userPromise={userPromise} />
</Suspense>
);
}Note Unlike other hooks, use() can be called inside if statements and loops. When reading a promise, wrap the component in Suspense. The promise should be created outside the component or cached -- do not create new promises inside render.
const value = use(SomeContext);
// Can be called conditionally, unlike useContextimport { use, createContext } from 'react';
const FeatureFlagContext = createContext({ darkMode: false });
function Banner({ showThemed }) {
// Reading context conditionally -- not possible with useContext
if (showThemed) {
const { darkMode } = use(FeatureFlagContext);
return (
<div style={{ background: darkMode ? '#222' : '#eee' }}>
Themed banner
</div>
);
}
return <div>Standard banner</div>;
}Note use(Context) works like useContext but can be placed after early returns or inside conditionals. This is useful when you only need the context value in some code paths.
<form action={actionFn}>
...
</form>
// actionFn receives FormData automaticallyfunction ContactForm() {
async function submitContact(formData) {
const name = formData.get('name');
const email = formData.get('email');
await sendContactRequest({ name, email });
}
return (
<form action={submitContact}>
<input name="name" placeholder="Your name" required />
<input name="email" type="email" placeholder="Email" required />
<button type="submit">Send</button>
</form>
);
}Note Actions are async functions passed to the action prop of <form>. They receive FormData as an argument. React handles the pending state and error transitions automatically. Actions work with useActionState and useFormStatus for richer patterns.
const { pending, data, method, action } = useFormStatus();import { useFormStatus } from 'react-dom';
function SubmitButton() {
const { pending } = useFormStatus();
return (
<button type="submit" disabled={pending}>
{pending ? 'Saving...' : 'Save Changes'}
</button>
);
}
function EditProfile() {
async function updateProfile(formData) {
await saveProfile(Object.fromEntries(formData));
}
return (
<form action={updateProfile}>
<input name="displayName" placeholder="Display name" />
<input name="location" placeholder="Location" />
<SubmitButton />
</form>
);
}Note useFormStatus must be called from a component rendered inside a <form>. It will not work if called in the same component that renders the form -- it reads from the nearest parent form. Import it from 'react-dom', not 'react'.
const [optimisticState, setOptimistic] = useOptimistic(
actualState,
updateFn? // (currentState, optimisticValue) => newState
);import { useOptimistic, useState } from 'react';
function MessageThread({ initialMessages }) {
const [messages, setMessages] = useState(initialMessages);
const [optimisticMessages, addOptimistic] = useOptimistic(
messages,
(current, newMsg) => [...current, { ...newMsg, sending: true }]
);
async function sendMessage(formData) {
const text = formData.get('message');
const tempMsg = { id: Date.now(), text, sending: true };
addOptimistic(tempMsg);
const saved = await postMessage(text);
setMessages(prev => [...prev, saved]);
}
return (
<div>
{optimisticMessages.map(msg => (
<p key={msg.id} style={{ opacity: msg.sending ? 0.6 : 1 }}>
{msg.text}
</p>
))}
<form action={sendMessage}>
<input name="message" />
<button type="submit">Send</button>
</form>
</div>
);
}Note useOptimistic shows a temporary state while an async action is in progress. When the action completes (or fails), React reverts to the actual state. The optimistic value is automatically rolled back if the action throws.
const [state, formAction, isPending] = useActionState(
actionFn,
initialState
);
// actionFn: (previousState, formData) => newStateimport { useActionState } from 'react';
function LoginForm() {
const [state, loginAction, isPending] = useActionState(
async (prevState, formData) => {
const email = formData.get('email');
const password = formData.get('password');
try {
await authenticateUser(email, password);
return { error: null, success: true };
} catch (err) {
return { error: err.message, success: false };
}
},
{ error: null, success: false }
);
return (
<form action={loginAction}>
<input name="email" type="email" required />
<input name="password" type="password" required />
{state.error && <p className="error">{state.error}</p>}
<button disabled={isPending}>
{isPending ? 'Signing in...' : 'Sign In'}
</button>
</form>
);
}Note useActionState combines action handling with state management. The action function receives the previous state as its first argument and FormData as its second. The third return value (isPending) tells you if the action is running.
// React 19: no forwardRef needed
function MyComponent({ ref, ...props }) {
return <div ref={ref} {...props} />;
}function TextArea({ ref, label, ...rest }) {
return (
<div className="field">
<label>{label}</label>
<textarea ref={ref} {...rest} />
</div>
);
}
function CommentForm() {
const textareaRef = useRef(null);
function handleReset() {
textareaRef.current.value = '';
textareaRef.current.focus();
}
return (
<div>
<TextArea ref={textareaRef} label="Comment" rows={4} />
<button onClick={handleReset}>Clear</button>
</div>
);
}Note React 19 passes ref as a regular prop to function components. forwardRef is deprecated -- migrate by moving ref into the props destructuring. Existing forwardRef code continues to work but will show deprecation warnings.
// Render <title>, <meta>, <link> directly in components
function Page() {
return (
<>
<title>Page Title</title>
<meta name="description" content="..." />
{/* page content */}
</>
);
}function ProductPage({ product }) {
return (
<article>
<title>{product.name} | ShopApp</title>
<meta name="description" content={product.summary} />
<link rel="canonical" href={`/products/${product.slug}`} />
<h1>{product.name}</h1>
<p>{product.description}</p>
<strong>${product.price}</strong>
</article>
);
}
// React hoists these tags into the <head> automaticallyNote In React 19, metadata tags rendered inside components are automatically hoisted into the document <head>. This eliminates the need for third-party helmet libraries for basic metadata management. Works with SSR and client rendering.
<input value={state} onChange={e => setState(e.target.value)} />function EmailInput() {
const [email, setEmail] = useState('');
const isValid = email.includes('@') && email.includes('.');
return (
<div>
<input
type="email"
value={email}
onChange={e => setEmail(e.target.value)}
className={isValid ? 'valid' : 'invalid'}
/>
{!isValid && email.length > 0 && (
<small>Please enter a valid email address</small>
)}
</div>
);
}Note Controlled inputs keep React as the single source of truth. The value prop locks the input to the state. If you set value without onChange, the input becomes read-only.
<input defaultValue={initial} ref={inputRef} />
// Read with: inputRef.current.valuefunction QuickSearch() {
const searchRef = useRef(null);
function handleSubmit(e) {
e.preventDefault();
const query = searchRef.current.value;
performSearch(query);
}
return (
<form onSubmit={handleSubmit}>
<input ref={searchRef} defaultValue="" placeholder="Search..." />
<button type="submit">Go</button>
</form>
);
}Note Use defaultValue (not value) for uncontrolled inputs. The DOM holds the state. Uncontrolled inputs are simpler for cases where you only need the value at submission time. For dynamic validation or conditional logic, controlled inputs are better.
// Traditional
<form onSubmit={handleSubmit}>
// React 19 Actions
<form action={actionFn}>// Traditional approach
function FeedbackForm() {
const [message, setMessage] = useState('');
const [submitted, setSubmitted] = useState(false);
async function handleSubmit(e) {
e.preventDefault();
await sendFeedback(message);
setSubmitted(true);
setMessage('');
}
if (submitted) return <p>Thank you for your feedback!</p>;
return (
<form onSubmit={handleSubmit}>
<textarea
value={message}
onChange={e => setMessage(e.target.value)}
required
/>
<button type="submit">Submit</button>
</form>
);
}Note With onSubmit, always call e.preventDefault() to stop the browser from reloading the page. With React 19 form actions, preventDefault is handled automatically. Both patterns are valid -- actions are newer and reduce boilerplate.
// Validate on change, blur, or submit
const errors = {};
if (!value) errors.field = 'Required';function SignupForm() {
const [fields, setFields] = useState({ name: '', email: '', age: '' });
const [errors, setErrors] = useState({});
const [touched, setTouched] = useState({});
function validate(data) {
const errs = {};
if (!data.name.trim()) errs.name = 'Name is required';
if (!data.email.includes('@')) errs.email = 'Invalid email';
if (Number(data.age) < 18) errs.age = 'Must be at least 18';
return errs;
}
function handleChange(field, value) {
const updated = { ...fields, [field]: value };
setFields(updated);
if (touched[field]) {
setErrors(validate(updated));
}
}
function handleBlur(field) {
setTouched(prev => ({ ...prev, [field]: true }));
setErrors(validate(fields));
}
function handleSubmit(e) {
e.preventDefault();
const errs = validate(fields);
setErrors(errs);
setTouched({ name: true, email: true, age: true });
if (Object.keys(errs).length === 0) {
submitSignup(fields);
}
}
return (
<form onSubmit={handleSubmit}>
<input
value={fields.name}
onChange={e => handleChange('name', e.target.value)}
onBlur={() => handleBlur('name')}
/>
{touched.name && errors.name && <span>{errors.name}</span>}
{/* repeat for other fields */}
<button type="submit">Sign Up</button>
</form>
);
}Note Validate on blur for a good UX: users see errors after leaving a field, not while typing. On submit, validate everything and mark all fields as touched. For complex forms, consider a form library or useReducer.
function handleChange(e) {
const { name, value } = e.target;
setForm(prev => ({ ...prev, [name]: value }));
}function AddressForm() {
const [address, setAddress] = useState({
street: '',
city: '',
state: '',
zip: '',
});
function handleChange(e) {
const { name, value } = e.target;
setAddress(prev => ({ ...prev, [name]: value }));
}
return (
<form>
<input name="street" value={address.street} onChange={handleChange} placeholder="Street" />
<input name="city" value={address.city} onChange={handleChange} placeholder="City" />
<input name="state" value={address.state} onChange={handleChange} placeholder="State" />
<input name="zip" value={address.zip} onChange={handleChange} placeholder="ZIP" />
</form>
);
}Note Use the name attribute on inputs and a computed property name ([name]) to handle many fields with one function. This scales well and avoids writing separate handlers for each field.
// Select
<select value={state} onChange={handler}>...
// Checkbox
<input type="checkbox" checked={state} onChange={handler} />
// Radio
<input type="radio" value="opt" checked={state === 'opt'} onChange={handler} />function PreferencesForm() {
const [color, setColor] = useState('blue');
const [newsletter, setNewsletter] = useState(false);
const [plan, setPlan] = useState('free');
return (
<form>
{/* Select */}
<label>Favorite Color</label>
<select value={color} onChange={e => setColor(e.target.value)}>
<option value="blue">Blue</option>
<option value="green">Green</option>
<option value="red">Red</option>
</select>
{/* Checkbox */}
<label>
<input
type="checkbox"
checked={newsletter}
onChange={e => setNewsletter(e.target.checked)}
/>
Subscribe to newsletter
</label>
{/* Radio */}
{['free', 'pro', 'enterprise'].map(option => (
<label key={option}>
<input
type="radio"
name="plan"
value={option}
checked={plan === option}
onChange={e => setPlan(e.target.value)}
/>
{option}
</label>
))}
</form>
);
}Note For checkboxes, read e.target.checked (boolean), not e.target.value. For selects, value goes on the <select> element, not on individual <option> elements. Radio buttons in the same group share the same name attribute.
<button onClick={handler}>...</button>
// With argument
<button onClick={() => handler(arg)}>...</button>function ColorPicker({ colors, onSelect }) {
return (
<div className="palette">
{colors.map(color => (
<button
key={color}
style={{ backgroundColor: color, width: 40, height: 40 }}
onClick={() => onSelect(color)}
aria-label={`Select ${color}`}
/>
))}
</div>
);
}Note Pass a function reference, not a function call. Writing onClick={handler()} would call the function during render. Use an arrow function when you need to pass arguments: onClick={() => handler(id)}.
<input onChange={(e) => handleChange(e.target.value)} />
<select onChange={(e) => handleSelect(e.target.value)}>function TemperatureConverter() {
const [celsius, setCelsius] = useState('');
const fahrenheit = celsius !== '' ? (Number(celsius) * 9/5 + 32).toFixed(1) : '';
return (
<div>
<label>
Celsius:
<input
type="number"
value={celsius}
onChange={e => setCelsius(e.target.value)}
/>
</label>
<p>Fahrenheit: {fahrenheit}</p>
</div>
);
}Note React's onChange fires on every keystroke, not just when the field loses focus (which is the native DOM behavior). This makes it ideal for real-time validation and derived values.
<form onSubmit={handleSubmit}>
...
<button type="submit">Submit</button>
</form>function SearchForm({ onSearch }) {
const [query, setQuery] = useState('');
function handleSubmit(e) {
e.preventDefault();
if (query.trim()) {
onSearch(query.trim());
}
}
return (
<form onSubmit={handleSubmit}>
<input
value={query}
onChange={e => setQuery(e.target.value)}
placeholder="Search articles..."
/>
<button type="submit">Search</button>
</form>
);
}Note Always call e.preventDefault() in onSubmit to prevent the browser from performing a full page reload. The submit event fires when pressing Enter inside an input or clicking a submit button.
function handler(event) {
event.target // the DOM element
event.currentTarget // element the handler is attached to
event.preventDefault()
event.stopPropagation()
event.nativeEvent // underlying browser event
}function LinkInterceptor({ children }) {
function handleClick(e) {
// Check if it's an internal link
const href = e.target.closest('a')?.getAttribute('href');
if (href?.startsWith('/')) {
e.preventDefault();
navigate(href); // client-side navigation
}
}
return <div onClick={handleClick}>{children}</div>;
}Note React wraps native events in SyntheticEvent objects that work identically across browsers. As of React 17+, events are no longer pooled, so you can access event properties asynchronously without calling e.persist().
e.preventDefault(); // stop default browser action
e.stopPropagation(); // stop bubbling to parent handlersfunction DropZone({ onFileDrop }) {
function handleDragOver(e) {
e.preventDefault(); // Required to allow drop
}
function handleDrop(e) {
e.preventDefault();
e.stopPropagation();
const files = Array.from(e.dataTransfer.files);
onFileDrop(files);
}
return (
<div
onDragOver={handleDragOver}
onDrop={handleDrop}
className="drop-zone"
>
Drop files here
</div>
);
}Note Use preventDefault for links, form submissions, and drag-and-drop. Use stopPropagation sparingly -- it makes debugging harder. Prefer checking the event target to decide whether to act instead of stopping propagation.
// Capture phase (fires before children)
<div onClickCapture={handler}>...</div>
// Event delegation: one handler on parent
<ul onClick={handleItemClick}>
<li data-id="1">A</li>
<li data-id="2">B</li>
</ul>function ItemGrid({ items, onItemAction }) {
function handleClick(e) {
const itemEl = e.target.closest('[data-item-id]');
if (!itemEl) return;
const id = itemEl.dataset.itemId;
const action = e.target.dataset.action;
if (action === 'delete') {
onItemAction(id, 'delete');
} else if (action === 'edit') {
onItemAction(id, 'edit');
}
}
return (
<div onClick={handleClick}>
{items.map(item => (
<div key={item.id} data-item-id={item.id}>
<span>{item.name}</span>
<button data-action="edit">Edit</button>
<button data-action="delete">Delete</button>
</div>
))}
</div>
);
}Note Event delegation (one handler on a parent) reduces the number of event listeners and works well for dynamic lists. Use data-* attributes or closest() to identify which child triggered the event. React internally uses delegation at the root already.
function useCustomHook(params) {
// use built-in hooks inside
return result;
}function useLocalStorage(key, initialValue) {
const [value, setValue] = useState(() => {
const stored = localStorage.getItem(key);
return stored !== null ? JSON.parse(stored) : initialValue;
});
useEffect(() => {
localStorage.setItem(key, JSON.stringify(value));
}, [key, value]);
return [value, setValue];
}
// Usage
function Settings() {
const [theme, setTheme] = useLocalStorage('theme', 'light');
return (
<button onClick={() => setTheme(t => t === 'light' ? 'dark' : 'light')}>
Theme: {theme}
</button>
);
}Note Custom hooks must start with 'use' so React can enforce the rules of hooks. They let you extract and share stateful logic between components without changing the component hierarchy.
function Parent({ children }) { ... }
Parent.Child = function Child(props) { ... };const AccordionContext = createContext(null);
function Accordion({ children }) {
const [openIndex, setOpenIndex] = useState(null);
return (
<AccordionContext.Provider value={{ openIndex, setOpenIndex }}>
<div className="accordion">{children}</div>
</AccordionContext.Provider>
);
}
function AccordionItem({ index, title, children }) {
const { openIndex, setOpenIndex } = useContext(AccordionContext);
const isOpen = openIndex === index;
return (
<div>
<button onClick={() => setOpenIndex(isOpen ? null : index)}>
{title} {isOpen ? '−' : '+'}
</button>
{isOpen && <div className="panel">{children}</div>}
</div>
);
}
Accordion.Item = AccordionItem;
// Usage
<Accordion>
<Accordion.Item index={0} title="Section A">Content A</Accordion.Item>
<Accordion.Item index={1} title="Section B">Content B</Accordion.Item>
</Accordion>Note Compound components share implicit state via context, letting you create flexible APIs like <Select> + <Select.Option>. The parent manages state and children consume it without prop drilling.
function DataProvider({ render }) {
const data = useData();
return render(data);
}
// or via children
<DataProvider>{(data) => <View data={data} />}</DataProvider>function MouseTracker({ children }) {
const [position, setPosition] = useState({ x: 0, y: 0 });
useEffect(() => {
function handleMove(e) {
setPosition({ x: e.clientX, y: e.clientY });
}
window.addEventListener('mousemove', handleMove);
return () => window.removeEventListener('mousemove', handleMove);
}, []);
return children(position);
}
// Usage
<MouseTracker>
{({ x, y }) => (
<div>
Cursor is at ({x}, {y})
</div>
)}
</MouseTracker>Note Render props are largely replaced by custom hooks, which are simpler. Still useful when you need to control what renders based on shared logic without creating a custom hook for every case.
// Error boundaries must be class components (as of React 19)
class ErrorBoundary extends React.Component {
state = { hasError: false };
static getDerivedStateFromError(error) {
return { hasError: true };
}
componentDidCatch(error, info) { logError(error, info); }
render() {
if (this.state.hasError) return <Fallback />;
return this.props.children;
}
}class AppErrorBoundary extends React.Component {
constructor(props) {
super(props);
this.state = { hasError: false, error: null };
}
static getDerivedStateFromError(error) {
return { hasError: true, error };
}
componentDidCatch(error, errorInfo) {
reportToService(error, errorInfo.componentStack);
}
render() {
if (this.state.hasError) {
return (
<div className="error-screen">
<h1>Something went wrong</h1>
<button onClick={() => this.setState({ hasError: false })}>
Try again
</button>
</div>
);
}
return this.props.children;
}
}
// Wrap sections of the UI
<AppErrorBoundary>
<Dashboard />
</AppErrorBoundary>Note Error boundaries catch errors during rendering, in lifecycle methods, and in constructors of the tree below them. They do NOT catch errors in event handlers, async code, or server-side rendering. Use try/catch for those.
import { createPortal } from 'react-dom';
createPortal(children, domNode);import { createPortal } from 'react-dom';
function Modal({ isOpen, onClose, children }) {
if (!isOpen) return null;
return createPortal(
<div className="modal-overlay" onClick={onClose}>
<div className="modal-content" onClick={e => e.stopPropagation()}>
<button className="close-btn" onClick={onClose}>X</button>
{children}
</div>
</div>,
document.getElementById('modal-root')
);
}
// Usage
function App() {
const [showModal, setShowModal] = useState(false);
return (
<div>
<button onClick={() => setShowModal(true)}>Open</button>
<Modal isOpen={showModal} onClose={() => setShowModal(false)}>
<h2>Modal Title</h2>
<p>Modal body content here.</p>
</Modal>
</div>
);
}Note Portals render children into a different DOM node outside the parent hierarchy, but events still bubble through the React tree (not the DOM tree). Perfect for modals, tooltips, and dropdowns that need to escape overflow:hidden or z-index contexts.
const LazyComponent = React.lazy(() => import('./Component'));
<Suspense fallback={<Loading />}>
<LazyComponent />
</Suspense>import { lazy, Suspense, useState } from 'react';
const AdminPanel = lazy(() => import('./AdminPanel'));
const UserDashboard = lazy(() => import('./UserDashboard'));
function App({ role }) {
return (
<Suspense fallback={<div className="spinner">Loading...</div>}>
{role === 'admin' ? <AdminPanel /> : <UserDashboard />}
</Suspense>
);
}Note React.lazy only works with default exports. For named exports, create an intermediate module that re-exports as default. Suspense can also wrap components that use the use() hook to read promises in React 19.
const StateCtx = createContext(null);
const DispatchCtx = createContext(null);
function Provider({ children }) {
const [state, dispatch] = useReducer(reducer, init);
return (
<StateCtx.Provider value={state}>
<DispatchCtx.Provider value={dispatch}>
{children}
</DispatchCtx.Provider>
</StateCtx.Provider>
);
}const TodoStateContext = createContext(null);
const TodoDispatchContext = createContext(null);
function todoReducer(state, action) {
switch (action.type) {
case 'ADD':
return [...state, { id: crypto.randomUUID(), text: action.text, done: false }];
case 'TOGGLE':
return state.map(t =>
t.id === action.id ? { ...t, done: !t.done } : t
);
case 'DELETE':
return state.filter(t => t.id !== action.id);
default:
throw new Error(`Unhandled: ${action.type}`);
}
}
function TodoProvider({ children }) {
const [todos, dispatch] = useReducer(todoReducer, []);
return (
<TodoStateContext.Provider value={todos}>
<TodoDispatchContext.Provider value={dispatch}>
{children}
</TodoDispatchContext.Provider>
</TodoStateContext.Provider>
);
}
function useTodos() {
return useContext(TodoStateContext);
}
function useTodoDispatch() {
return useContext(TodoDispatchContext);
}Note This pattern provides Redux-like state management with built-in React APIs. Splitting state and dispatch into separate contexts prevents components that only dispatch actions from re-rendering when state changes.
function withFeature(WrappedComponent) {
return function EnhancedComponent(props) {
const data = useFeatureData();
return <WrappedComponent {...props} data={data} />;
};
}function withAuth(Component) {
return function AuthenticatedComponent(props) {
const { user, loading } = useAuth();
if (loading) return <p>Authenticating...</p>;
if (!user) return <LoginRedirect />;
return <Component {...props} user={user} />;
};
}
// Usage
const ProtectedDashboard = withAuth(Dashboard);
function App() {
return <ProtectedDashboard showStats />;
}Note HOCs were the primary code reuse pattern before hooks. Custom hooks are now preferred for most cases because they are simpler and avoid the wrapper component nesting. HOCs are still useful for cross-cutting concerns like auth guards.
// Move shared state to the nearest common ancestor
function Parent() {
const [shared, setShared] = useState(init);
return (
<>
<ChildA value={shared} />
<ChildB onChange={setShared} />
</>
);
}function TemperatureCalculator() {
const [temp, setTemp] = useState('');
const [scale, setScale] = useState('c');
const celsius = scale === 'f' ? ((Number(temp) - 32) * 5/9).toFixed(1) : temp;
const fahrenheit = scale === 'c' ? ((Number(temp) * 9/5) + 32).toFixed(1) : temp;
return (
<div>
<TemperatureInput
label="Celsius"
value={celsius}
onChange={val => { setTemp(val); setScale('c'); }}
/>
<TemperatureInput
label="Fahrenheit"
value={fahrenheit}
onChange={val => { setTemp(val); setScale('f'); }}
/>
</div>
);
}
function TemperatureInput({ label, value, onChange }) {
return (
<label>
{label}:
<input value={value} onChange={e => onChange(e.target.value)} />
</label>
);
}Note When two components need the same data, move the state to their closest shared parent and pass it down as props. This is the fundamental React data flow pattern.
// 1. Context
// 2. Composition (pass components, not data)
// 3. Component composition with children// PROBLEM: drilling "user" through many layers
// <App user={user}> -> <Layout user> -> <Header user> -> <Avatar user>
// SOLUTION 1: Context
const UserContext = createContext(null);
// provide at top, consume at Avatar
// SOLUTION 2: Composition -- pass the rendered element
function App({ user }) {
const avatar = <Avatar user={user} />;
return <Layout header={<Header avatar={avatar} />} />;
}
// Now Layout and Header don't need to know about "user" at all
function Layout({ header }) {
return (
<div>
<nav>{header}</nav>
<main>...</main>
</div>
);
}
function Header({ avatar }) {
return <div className="header">{avatar}</div>;
}Note Before reaching for context, try component composition. Passing pre-rendered elements as props skips intermediate layers entirely and keeps the data flow explicit. Context is better for truly global data (theme, auth, locale).
// Create -> Provide -> Consume
const Ctx = createContext(defaultValue);
<Ctx.Provider value={value}>...
const value = useContext(Ctx);// Locale context for internationalization
const LocaleContext = createContext('en');
function LocaleProvider({ children }) {
const [locale, setLocale] = useState('en');
const translations = useMemo(
() => loadTranslations(locale),
[locale]
);
const value = useMemo(
() => ({ locale, setLocale, t: (key) => translations[key] || key }),
[locale, translations]
);
return (
<LocaleContext.Provider value={value}>
{children}
</LocaleContext.Provider>
);
}
function useLocale() {
return useContext(LocaleContext);
}
// Usage in any component
function WelcomeBanner() {
const { t, locale } = useLocale();
return <h1>{t('welcome_message')} ({locale})</h1>;
}Note Context is ideal for low-frequency updates like theme, locale, and auth. For high-frequency updates (e.g., mouse position), context can cause excessive re-renders -- consider useSyncExternalStore or a dedicated state library.
// Consider external state management when:
// - Many components read/write the same state
// - State updates are frequent and complex
// - You need middleware (logging, persistence, devtools)
// - Server state caching is needed (React Query, SWR)// Signs you might need a state library:
// 1. Context re-renders are a bottleneck
// function App() {
// return (
// <StoreProvider> {/* every consumer re-renders on any change */}
// <BigComponentTree />
// </StoreProvider>
// );
// }
// 2. You need selectors (subscribe to a slice)
// const count = useStore(state => state.count);
// 3. You need server state cache with stale-while-revalidate
// const { data, isLoading } = useQuery('users', fetchUsers);
// Popular choices:
// - Zustand: lightweight, hook-based, selectors
// - Jotai: atomic state, bottom-up approach
// - React Query / TanStack Query: server state management
// - Redux Toolkit: complex client state with devtoolsNote Start with useState and useReducer + context. Only add an external library when you hit a concrete pain point: performance issues from context re-renders, complex async logic, or the need for devtools and middleware.
// Compute from existing state during render
const derivedValue = computeFrom(stateA, stateB);
// Do NOT store derived values in separate statefunction ShoppingCart({ items }) {
// GOOD: derive during render
const subtotal = items.reduce((sum, item) => sum + item.price * item.qty, 0);
const tax = subtotal * 0.08;
const total = subtotal + tax;
const itemCount = items.reduce((sum, item) => sum + item.qty, 0);
return (
<div>
<p>{itemCount} items</p>
<p>Subtotal: ${subtotal.toFixed(2)}</p>
<p>Tax: ${tax.toFixed(2)}</p>
<p>Total: ${total.toFixed(2)}</p>
</div>
);
// BAD: storing derived data in state
// const [total, setTotal] = useState(0);
// useEffect(() => setTotal(subtotal + tax), [subtotal, tax]);
}Note If you can compute a value from existing state or props, derive it inline during render instead of syncing it with useState + useEffect. Derived state in useEffect causes an extra render cycle and introduces potential bugs.
// Keep state as close to where it is used as possible
// Move state DOWN if only one child needs it
// Move state UP only when siblings need to share it// BAD: state lives too high
function App() {
const [searchQuery, setSearchQuery] = useState('');
return (
<Layout>
<Header />
<SearchPanel query={searchQuery} setQuery={setSearchQuery} />
<Footer />
</Layout>
);
}
// GOOD: state lives in the component that uses it
function App() {
return (
<Layout>
<Header />
<SearchPanel />
<Footer />
</Layout>
);
}
function SearchPanel() {
const [searchQuery, setSearchQuery] = useState('');
const results = useSearchResults(searchQuery);
return (
<div>
<input value={searchQuery} onChange={e => setSearchQuery(e.target.value)} />
<ResultsList results={results} />
</div>
);
}Note Colocating state reduces unnecessary re-renders and makes components easier to understand. Only lift state when it genuinely needs to be shared. Global state for local concerns is an anti-pattern that hurts performance and readability.
<div style={{ cssProperty: value }} />function AlertBanner({ type, message }) {
const styles = {
padding: '12px 16px',
borderRadius: '6px',
fontWeight: 'bold',
backgroundColor: type === 'error' ? '#fde8e8' : '#e8f5e9',
color: type === 'error' ? '#c62828' : '#2e7d32',
border: `1px solid ${type === 'error' ? '#ef9a9a' : '#a5d6a7'}`,
};
return <div style={styles}>{message}</div>;
}Note Inline styles use camelCase property names and string/number values. They cannot handle pseudo-classes (:hover), media queries, or keyframe animations. Best for truly dynamic values that change at runtime.
import styles from './Component.module.css';
<div className={styles.container} />// Button.module.css
// .button { padding: 8px 16px; border-radius: 4px; }
// .primary { background: #1976d2; color: white; }
// .secondary { background: #e0e0e0; color: #333; }
import styles from './Button.module.css';
function Button({ variant = 'primary', children, ...rest }) {
const className = `${styles.button} ${styles[variant]}`;
return (
<button className={className} {...rest}>
{children}
</button>
);
}Note CSS Modules scope class names locally by default, preventing naming collisions. The imported styles object maps your authored class names to unique generated ones. Supported out of the box by most React build tools.
<div className="static-class" />
<div className={dynamicVariable} />
<div className={`base ${condition ? 'active' : ''}`} />function NavLink({ href, isActive, children }) {
// Template literal approach
const className = `nav-link ${isActive ? 'nav-link--active' : ''}`;
return <a href={href} className={className}>{children}</a>;
}
// Helper function for multiple conditional classes
function classNames(...classes) {
return classes.filter(Boolean).join(' ');
}
function Card({ featured, compact, className }) {
return (
<div
className={classNames(
'card',
featured && 'card--featured',
compact && 'card--compact',
className
)}
>
{/* ... */}
</div>
);
}Note Use a small classNames helper or the popular 'clsx' package to compose conditional classes cleanly. Avoid complex ternary chains inside className attributes -- extract the logic to keep JSX readable.
// With clsx or classnames library
import clsx from 'clsx';
<div className={clsx('base', { active: isActive, disabled: isDisabled })} />// Using the clsx library
import clsx from 'clsx';
function StatusChip({ status }) {
return (
<span
className={clsx('chip', {
'chip--success': status === 'active',
'chip--warning': status === 'pending',
'chip--danger': status === 'error',
'chip--muted': status === 'inactive',
})}
>
{status}
</span>
);
}
// Without a library
function StatusChipManual({ status }) {
const chipClass = [
'chip',
status === 'active' && 'chip--success',
status === 'error' && 'chip--danger',
].filter(Boolean).join(' ');
return <span className={chipClass}>{status}</span>;
}Note The clsx library (or classnames) accepts strings, objects, and arrays. Object keys are included when their value is truthy. It is lightweight (~228 bytes) and widely used in the React ecosystem.
// styled-components / Emotion example pattern
const StyledButton = styled.button`
background: ${props => props.primary ? 'blue' : 'gray'};
color: white;
`;// Using a CSS-in-JS library (styled-components pattern)
import styled from 'styled-components';
const Container = styled.div`
max-width: 800px;
margin: 0 auto;
padding: 20px;
`;
const Title = styled.h1`
font-size: ${props => props.large ? '2.5rem' : '1.5rem'};
color: ${props => props.muted ? '#888' : '#111'};
margin-bottom: 16px;
`;
function ArticlePage({ article }) {
return (
<Container>
<Title large>{article.title}</Title>
<p>{article.body}</p>
</Container>
);
}Note CSS-in-JS co-locates styles with components and supports dynamic props, theming, and automatic vendor prefixing. The tradeoff is runtime overhead and larger bundle size. Consider CSS Modules or Tailwind for better performance in critical paths.
<div className="flex items-center gap-4 p-4 bg-white rounded-lg shadow" />function UserCard({ user }) {
return (
<div className="flex items-center gap-4 p-4 bg-white rounded-lg shadow-sm border">
<img
src={user.avatarUrl}
alt={user.name}
className="w-12 h-12 rounded-full object-cover"
/>
<div>
<h3 className="font-semibold text-gray-900">{user.name}</h3>
<p className="text-sm text-gray-500">{user.role}</p>
</div>
<span
className={`ml-auto px-2 py-1 text-xs rounded-full ${
user.active
? 'bg-green-100 text-green-800'
: 'bg-gray-100 text-gray-600'
}`}
>
{user.active ? 'Active' : 'Offline'}
</span>
</div>
);
}Note Utility-first CSS like Tailwind works well with React's component model -- styles are scoped per component by nature. Use clsx for conditional classes. Extract repeated patterns into components rather than creating custom CSS utility classes.
const Memoized = React.memo(Component);
// Re-renders only when props change (shallow comparison)import { memo } from 'react';
const ChatMessage = memo(function ChatMessage({ author, text, timestamp }) {
// This only re-renders when author, text, or timestamp change
return (
<div className="message">
<strong>{author}</strong>
<p>{text}</p>
<time>{new Date(timestamp).toLocaleTimeString()}</time>
</div>
);
});
// Parent re-renders frequently (new messages arrive)
// but existing ChatMessage components skip re-rendering
function ChatFeed({ messages }) {
return (
<div>
{messages.map(msg => (
<ChatMessage
key={msg.id}
author={msg.author}
text={msg.text}
timestamp={msg.timestamp}
/>
))}
</div>
);
}Note React.memo only does a shallow prop comparison. If you pass objects or functions that are recreated each render, memo has no effect. Pair with useMemo/useCallback for those props.
const Page = lazy(() => import('./Page'));
<Suspense fallback={<Spinner />}>
<Page />
</Suspense>import { lazy, Suspense } from 'react';
const Home = lazy(() => import('./pages/Home'));
const Products = lazy(() => import('./pages/Products'));
const Account = lazy(() => import('./pages/Account'));
function AppRouter() {
const [route, setRoute] = useState('home');
return (
<nav>
<button onClick={() => setRoute('home')}>Home</button>
<button onClick={() => setRoute('products')}>Products</button>
<button onClick={() => setRoute('account')}>Account</button>
<Suspense fallback={<div className="page-loader">Loading page...</div>}>
{route === 'home' && <Home />}
{route === 'products' && <Products />}
{route === 'account' && <Account />}
</Suspense>
</nav>
);
}Note Route-based code splitting gives the biggest bundle size reduction because entire page components and their dependencies are loaded on demand. Place the Suspense boundary around the lazy components, not the entire app.
// 1. Move state down (closer to where it is used)
// 2. Lift content up (pass components as children)
// 3. Memoize with React.memo + useCallback/useMemo// PROBLEM: typing in input re-renders the expensive list
function BadPage() {
const [query, setQuery] = useState('');
return (
<div>
<input value={query} onChange={e => setQuery(e.target.value)} />
<ExpensiveList /> {/* re-renders on every keystroke */}
</div>
);
}
// FIX 1: Move state down into its own component
function GoodPage() {
return (
<div>
<SearchInput /> {/* state lives here now */}
<ExpensiveList /> {/* no longer re-renders on keystrokes */}
</div>
);
}
function SearchInput() {
const [query, setQuery] = useState('');
return <input value={query} onChange={e => setQuery(e.target.value)} />;
}
// FIX 2: Lift content up via children
function InputWrapper({ children }) {
const [query, setQuery] = useState('');
return (
<div>
<input value={query} onChange={e => setQuery(e.target.value)} />
{children} {/* children don't re-render */}
</div>
);
}
function Page() {
return (
<InputWrapper>
<ExpensiveList />
</InputWrapper>
);
}Note Before reaching for React.memo, try restructuring your component tree. Moving state down and lifting content up are free optimizations that often eliminate the problem entirely.
// Stable keys help React identify which items changed
<Item key={item.id} />
// Reset component state by changing the key
<Form key={selectedUserId} />// Using key to reset component state
function UserEditor({ userId }) {
// When userId changes, React destroys the old Form
// and mounts a fresh one with clean state
return <ProfileForm key={userId} userId={userId} />;
}
function ProfileForm({ userId }) {
const [name, setName] = useState('');
const [bio, setBio] = useState('');
useEffect(() => {
fetchUser(userId).then(user => {
setName(user.name);
setBio(user.bio);
});
}, [userId]);
return (
<form>
<input value={name} onChange={e => setName(e.target.value)} />
<textarea value={bio} onChange={e => setBio(e.target.value)} />
</form>
);
}Note Changing a component's key forces React to unmount and remount it, resetting all internal state. This is a clean way to 'reset' a form or component when switching between items without manually clearing each state variable.
<Profiler id="name" onRender={callback}>
<Component />
</Profiler>
// callback(id, phase, actualDuration, baseDuration, startTime, commitTime)import { Profiler } from 'react';
function onRenderCallback(
id,
phase, // 'mount' or 'update'
actualDuration, // time spent rendering
baseDuration, // estimated time without memoization
startTime,
commitTime
) {
console.log(`${id} [${phase}]: ${actualDuration.toFixed(1)}ms`);
}
function App() {
return (
<Profiler id="ProductList" onRender={onRenderCallback}>
<ProductList />
</Profiler>
);
}Note Use the Profiler component or React DevTools Profiler tab to identify slow renders. Focus on components with high actualDuration. The baseDuration shows the worst-case cost without any memoization.
// Render only visible items in long lists
// Use react-window or react-virtuoso// Concept: only render items in the viewport
import { FixedSizeList } from 'react-window';
function VirtualizedUserList({ users }) {
const Row = ({ index, style }) => (
<div style={style} className="user-row">
<span>{users[index].name}</span>
<span>{users[index].email}</span>
</div>
);
return (
<FixedSizeList
height={500}
itemCount={users.length}
itemSize={50}
width="100%"
>
{Row}
</FixedSizeList>
);
}
// Renders ~10-15 visible rows instead of 10,000Note For lists with hundreds or thousands of items, virtualization dramatically improves performance by only rendering visible rows. react-window is lightweight; react-virtuoso supports dynamic heights. Measure first -- small lists do not need this.
// PROBLEM: handler captures old state value
// FIX: use updater function or reffunction BrokenCounter() {
const [count, setCount] = useState(0);
useEffect(() => {
const id = setInterval(() => {
// BUG: count is always 0 (captured from initial render)
setCount(count + 1);
}, 1000);
return () => clearInterval(id);
}, []); // empty deps = closure captures initial count
return <p>{count}</p>; // stuck at 1
}
function FixedCounter() {
const [count, setCount] = useState(0);
useEffect(() => {
const id = setInterval(() => {
// FIX: updater function always has the latest value
setCount(prev => prev + 1);
}, 1000);
return () => clearInterval(id);
}, []);
return <p>{count}</p>; // increments correctly
}Note Stale closures happen when a callback captures a state value that never updates. Common in setInterval, setTimeout, and event listeners set up in useEffect. Use the updater form of setState or store values in a ref.
// BAD: mutating state directly
state.items.push(newItem);
setState(state);
// GOOD: create a new reference
setState({ ...state, items: [...state.items, newItem] });function BrokenTags() {
const [tags, setTags] = useState(['react', 'hooks']);
function addTag(tag) {
// BUG: push mutates the existing array
tags.push(tag);
setTags(tags); // same reference, React sees no change
}
return (
<div>
{tags.map(t => <span key={t}>{t}</span>)}
<button onClick={() => addTag('new')}>Add</button>
</div>
);
}
function FixedTags() {
const [tags, setTags] = useState(['react', 'hooks']);
function addTag(tag) {
// CORRECT: spread creates a new array
setTags(prev => [...prev, tag]);
}
return (
<div>
{tags.map(t => <span key={t}>{t}</span>)}
<button onClick={() => addTag('new')}>Add</button>
</div>
);
}Note React uses reference equality (Object.is) to decide whether to re-render. Mutating an object or array in place keeps the same reference, so React skips the update. Always produce a new object/array via spread, map, filter, or slice.
// BAD: index as key with reorderable list
{items.map((item, i) => <Item key={i} />)}
// GOOD: stable unique identifier
{items.map(item => <Item key={item.id} />)}// BUG: using index as key with a removable list
function BrokenList() {
const [items, setItems] = useState([
{ id: 'a', text: 'First' },
{ id: 'b', text: 'Second' },
{ id: 'c', text: 'Third' },
]);
function removeFirst() {
setItems(prev => prev.slice(1));
}
return (
<div>
{/* Using index: after removing first item,
React thinks items shifted, inputs lose state */}
{items.map((item, index) => (
<div key={index}>
<input defaultValue={item.text} />
</div>
))}
<button onClick={removeFirst}>Remove First</button>
</div>
);
}
// FIX: use item.id as key
// {items.map(item => <div key={item.id}>...)}Note Index keys cause broken behavior when items are added, removed, or reordered. React associates state (including uncontrolled input values) with the key. When keys shift, state gets attached to the wrong item.
// MISSING DEPS: effect uses stale values
useEffect(() => { fn(a, b); }, []); // a, b missing
// OBJECT DEPS: infinite loop
useEffect(() => { ... }, [{ x: 1 }]); // new ref each render// BUG: missing dependency
function Greeting({ name }) {
const [message, setMessage] = useState('');
useEffect(() => {
// name is used but not in deps -- stale value
setMessage(`Hello, ${name}!`);
}, []); // should be [name]
return <p>{message}</p>;
}
// BUG: object dependency causes infinite loop
function DataLoader({ filters }) {
useEffect(() => {
fetchData(filters);
}, [filters]); // if parent recreates filters object each render, infinite loop!
// FIX: destructure primitive values
const { category, minPrice } = filters;
useEffect(() => {
fetchData({ category, minPrice });
}, [category, minPrice]);
}Note Enable the eslint-plugin-react-hooks exhaustive-deps rule to catch missing dependencies. For object dependencies, extract primitive values or memoize the object in the parent. Never suppress the lint rule without understanding why.
// CAUSE 1: setState during render
function Bad() {
const [x, setX] = useState(0);
setX(1); // triggers re-render during render!
}
// CAUSE 2: object/array in useEffect deps
useEffect(() => { ... }, [{ a: 1 }]); // new ref every render// BUG: calling setState unconditionally during render
function BrokenComponent({ data }) {
const [processed, setProcessed] = useState(null);
// This runs on every render, which triggers a new render...
setProcessed(transform(data));
return <Display data={processed} />;
}
// FIX: derive the value without state
function FixedComponent({ data }) {
const processed = transform(data); // compute inline
return <Display data={processed} />;
}
// BUG: onClick={handler()} calls immediately
function BrokenButton() {
const [count, setCount] = useState(0);
// handler() executes during render, calling setCount, looping
return <button onClick={setCount(count + 1)}>Click</button>;
}
// FIX: pass function reference
function FixedButton() {
const [count, setCount] = useState(0);
return <button onClick={() => setCount(count + 1)}>Click</button>;
}Note Infinite re-render loops crash your app. The most common causes: (1) calling setState directly in the render body, (2) onClick={fn()} instead of onClick={fn}, (3) useEffect with an object/array literal in the dependency array that creates a new reference every render.
// ANTI-PATTERN: effect to sync derived state
useEffect(() => { setB(computeFrom(a)); }, [a]);
// BETTER: compute inline
const b = computeFrom(a);// BAD: useEffect chain
function PriceCalculator({ items }) {
const [subtotal, setSubtotal] = useState(0);
const [tax, setTax] = useState(0);
const [total, setTotal] = useState(0);
useEffect(() => {
setSubtotal(items.reduce((s, i) => s + i.price, 0));
}, [items]);
useEffect(() => {
setTax(subtotal * 0.1);
}, [subtotal]);
useEffect(() => {
setTotal(subtotal + tax);
}, [subtotal, tax]);
// 3 extra renders for what should be 0!
return <p>Total: ${total.toFixed(2)}</p>;
}
// GOOD: calculate everything inline
function PriceCalculator({ items }) {
const subtotal = items.reduce((s, i) => s + i.price, 0);
const tax = subtotal * 0.1;
const total = subtotal + tax;
return <p>Total: ${total.toFixed(2)}</p>;
}Note Each unnecessary useEffect + setState causes an extra render cycle. If a value can be derived from props or state, compute it during render. Reserve useEffect for genuine side effects: API calls, subscriptions, DOM manipulation, and third-party library integration.
// PROBLEM: async callback sets state on unmounted component
useEffect(() => {
fetchData().then(data => setState(data)); // may be unmounted!
}, []);
// FIX: track mounted status
useEffect(() => {
let active = true;
fetchData().then(data => { if (active) setState(data); });
return () => { active = false; };
}, []);function UserProfile({ userId }) {
const [user, setUser] = useState(null);
useEffect(() => {
let cancelled = false;
async function loadUser() {
const response = await fetch(`/api/users/${userId}`);
const data = await response.json();
// Only update state if this effect is still active
if (!cancelled) {
setUser(data);
}
}
loadUser();
return () => {
cancelled = true;
};
}, [userId]);
if (!user) return <p>Loading...</p>;
return <h1>{user.name}</h1>;
}Note While React 18+ no longer warns about this, it is still a logic bug. Without the cancelled flag, a slow request for user A could resolve after switching to user B, overwriting user B's data with user A's data. Always use a cleanup flag or AbortController.
// React 18+ batches ALL state updates automatically
function handleClick() {
setA(1); // does not trigger render
setB(2); // does not trigger render
setC(3); // single render with all three updates
}function StatusPanel() {
const [loading, setLoading] = useState(false);
const [data, setData] = useState(null);
const [error, setError] = useState(null);
async function loadData() {
setLoading(true);
setError(null);
// React batches these two -- one render
try {
const result = await fetchDashboardData();
setData(result);
setLoading(false);
// React batches these two -- one render
} catch (err) {
setError(err.message);
setLoading(false);
// React batches these two -- one render
}
}
return (
<div>
{loading && <p>Loading...</p>}
{error && <p>Error: {error}</p>}
{data && <Dashboard data={data} />}
</div>
);
}Note Since React 18, automatic batching applies to all updates, including those in promises, setTimeout, and native event handlers. You rarely need to worry about batching anymore. If you ever need to force a synchronous render, use flushSync (imported from react-dom) as a last resort.