React Development Skill
This skill provides comprehensive guidance for building modern React applications using hooks, components, state management, context, effects, and performance optimization techniques based on official React documentation from react.dev.
When to Use This Skill
Use this skill when:
- Building single-page applications (SPAs) with React
- Creating reusable UI components and component libraries
- Managing complex application state with hooks and context
- Implementing forms, data fetching, and side effects
- Optimizing React application performance
- Building interactive user interfaces with dynamic data
- Migrating class components to functional components with hooks
- Implementing global state management without external libraries
- Creating custom hooks for reusable logic
- Building accessible and performant web applications
Core Concepts
Components
Components are the building blocks of React applications. They let you split the UI into independent, reusable pieces.
Functional Components (Modern Approach):
function Welcome(props) {
return <h1>Hello, {props.name}</h1>;
}
// Arrow function syntax
const Greeting = ({ name, age }) => {
return (
<div>
<h2>{name}</h2>
<p>Age: {age}</p>
</div>
);
};Component Composition:
function App() {
return (
<div>
<Welcome name="Sara" />
<Welcome name="Cahal" />
<Welcome name="Edite" />
</div>
);
}JSX
JSX is a syntax extension for JavaScript that looks similar to HTML. It produces React elements.
JSX Fundamentals:
// Embedding expressions
const name = 'Josh Perez';
const element = <h1>Hello, {name}</h1>;
// JSX attributes
const image = <img src={user.avatarUrl} alt={user.name} />;
// JSX children
const container = (
<div>
<h1>Welcome</h1>
<p>Get started with React</p>
</div>
);
// Conditional rendering
const greeting = (
<div>
{isLoggedIn ? <UserGreeting /> : <GuestGreeting />}
</div>
);
// Lists and keys
const numbers = [1, 2, 3, 4, 5];
const listItems = numbers.map((number) =>
<li key={number.toString()}>{number}</li>
);Props
Props are arguments passed into React components. They are passed to components via HTML attributes.
Passing and Using Props:
function Product({ name, price, inStock }) {
return (
<div className="product">
<h3>{name}</h3>
<p>${price}</p>
{inStock ? <span>In Stock</span> : <span>Out of Stock</span>}
</div>
);
}
// Usage
<Product name="Laptop" price={999} inStock={true} />Props with Children:
function Card({ title, children }) {
return (
<div className="card">
<h2>{title}</h2>
<div className="card-content">
{children}
</div>
</div>
);
}
// Usage
<Card title="Welcome">
<p>This is the card content</p>
<button>Click me</button>
</Card>Default Props:
function Button({ text = 'Click me', variant = 'primary' }) {
return <button className={variant}>{text}</button>;
}State
State is a component's memory. It lets components remember information and respond to user interactions.
Local Component State:
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(count - 1)}>Decrement</button>
<button onClick={() => setCount(0)}>Reset</button>
</div>
);
}React Hooks
Hooks let you use state and other React features in functional components.
useState
The useState hook lets you add state to functional components.
Basic Usage:
import { useState } from 'react';
function Form() {
const [name, setName] = useState('');
const [email, setEmail] = useState('');
const handleSubmit = (e) => {
e.preventDefault();
console.log({ name, email });
};
return (
<form onSubmit={handleSubmit}>
<input
value={name}
onChange={(e) => setName(e.target.value)}
placeholder="Name"
/>
<input
value={email}
onChange={(e) => setEmail(e.target.value)}
placeholder="Email"
type="email"
/>
<button type="submit">Submit</button>
</form>
);
}State with Objects:
function UserProfile() {
const [user, setUser] = useState({
name: '',
age: 0,
email: ''
});
const updateField = (field, value) => {
setUser(prev => ({
...prev,
[field]: value
}));
};
return (
<div>
<input
value={user.name}
onChange={(e) => updateField('name', e.target.value)}
/>
<input
type="number"
value={user.age}
onChange={(e) => updateField('age', parseInt(e.target.value))}
/>
<input
type="email"
value={user.email}
onChange={(e) => updateField('email', e.target.value)}
/>
</div>
);
}State with Arrays:
function TodoList() {
const [todos, setTodos] = useState([]);
const [input, setInput] = useState('');
const addTodo = () => {
setTodos(prev => [...prev, { id: Date.now(), text: input }]);
setInput('');
};
const removeTodo = (id) => {
setTodos(prev => prev.filter(todo => todo.id !== id));
};
return (
<div>
<input value={input} onChange={(e) => setInput(e.target.value)} />
<button onClick={addTodo}>Add</button>
<ul>
{todos.map(todo => (
<li key={todo.id}>
{todo.text}
<button onClick={() => removeTodo(todo.id)}>Delete</button>
</li>
))}
</ul>
</div>
);
}useEffect
The useEffect hook lets you perform side effects in functional components.
Basic Side Effects:
import { useState, useEffect } from 'react';
function DocumentTitle() {
const [count, setCount] = useState(0);
useEffect(() => {
document.title = `Count: ${count}`;
}, [count]);
return (
<button onClick={() => setCount(count + 1)}>
Clicked {count} times
</button>
);
}Data Fetching:
function UserData({ userId }) {
const [user, setUser] = useState(null);
const [loading, setLoading] = useState(true);
const [error, setError] = useState(null);
useEffect(() => {
let cancelled = false;
async function fetchUser() {
try {
setLoading(true);
const response = await fetch(`/api/users/${userId}`);
const data = await response.json();
if (!cancelled) {
setUser(data);
setError(null);
}
} catch (err) {
if (!cancelled) {
setError(err.message);
}
} finally {
if (!cancelled) {
setLoading(false);
}
}
}
fetchUser();
return () => {
cancelled = true;
};
}, [userId]);
if (loading) return <div>Loading...</div>;
if (error) return <div>Error: {error}</div>;
return <div>{user?.name}</div>;
}Event Listeners and Cleanup:
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);
// Cleanup function
return () => {
window.removeEventListener('resize', handleResize);
};
}, []); // Empty dependency array = run once on mount
return <div>{size.width} x {size.height}</div>;
}Timers and Intervals:
function Timer() {
const [seconds, setSeconds] = useState(0);
const [isRunning, setIsRunning] = useState(false);
useEffect(() => {
if (!isRunning) return;
const interval = setInterval(() => {
setSeconds(s => s + 1);
}, 1000);
return () => clearInterval(interval);
}, [isRunning]);
return (
<div>
<p>Seconds: {seconds}</p>
<button onClick={() => setIsRunning(!isRunning)}>
{isRunning ? 'Pause' : 'Start'}
</button>
<button onClick={() => setSeconds(0)}>Reset</button>
</div>
);
}useContext
The useContext hook lets you read and subscribe to context from your component.
Creating and Using Context:
import { createContext, useContext, useState } from 'react';
const ThemeContext = createContext('light');
function App() {
const [theme, setTheme] = useState('light');
return (
<ThemeContext.Provider value={theme}>
<Toolbar />
<button onClick={() => setTheme(theme === 'light' ? 'dark' : 'light')}>
Toggle Theme
</button>
</ThemeContext.Provider>
);
}
function Toolbar() {
return (
<div>
<ThemedButton />
</div>
);
}
function ThemedButton() {
const theme = useContext(ThemeContext);
return (
<button className={theme}>
I am styled by {theme} theme
</button>
);
}Multiple Contexts:
const ThemeContext = createContext('light');
const UserContext = createContext(null);
function App() {
const [theme, setTheme] = useState('light');
const [currentUser, setCurrentUser] = useState({ name: 'John', role: 'admin' });
return (
<ThemeContext.Provider value={theme}>
<UserContext.Provider value={currentUser}>
<Dashboard />
</UserContext.Provider>
</ThemeContext.Provider>
);
}
function Dashboard() {
const theme = useContext(ThemeContext);
const user = useContext(UserContext);
return (
<div className={theme}>
<h1>Welcome, {user.name}</h1>
<p>Role: {user.role}</p>
</div>
);
}useReducer
The useReducer hook is an alternative to useState for managing complex state logic.
Basic Reducer Pattern:
import { useReducer } from 'react';
function counterReducer(state, action) {
switch (action.type) {
case 'increment':
return { count: state.count + 1 };
case 'decrement':
return { count: state.count - 1 };
case 'reset':
return { count: 0 };
default:
throw new Error(`Unknown action: ${action.type}`);
}
}
function Counter() {
const [state, dispatch] = useReducer(counterReducer, { count: 0 });
return (
<div>
<p>Count: {state.count}</p>
<button onClick={() => dispatch({ type: 'increment' })}>+</button>
<button onClick={() => dispatch({ type: 'decrement' })}>-</button>
<button onClick={() => dispatch({ type: 'reset' })}>Reset</button>
</div>
);
}Complex State Management (Task List Pattern from Context7):
function tasksReducer(tasks, action) {
switch (action.type) {
case 'added': {
return [...tasks, {
id: action.id,
text: action.text,
done: false
}];
}
case 'changed': {
return tasks.map(t => {
if (t.id === action.task.id) {
return action.task;
} else {
return t;
}
});
}
case 'deleted': {
return tasks.filter(t => t.id !== action.id);
}
default: {
throw Error('Unknown action: ' + action.type);
}
}
}
function TaskApp() {
const [tasks, dispatch] = useReducer(tasksReducer, initialTasks);
function handleAddTask(text) {
dispatch({
type: 'added',
id: nextId++,
text: text,
});
}
function handleChangeTask(task) {
dispatch({
type: 'changed',
task: task
});
}
function handleDeleteTask(taskId) {
dispatch({
type: 'deleted',
id: taskId
});
}
return (
<>
<h1>Prague itinerary</h1>
<AddTask onAddTask={handleAddTask} />
<TaskList
tasks={tasks}
onChangeTask={handleChangeTask}
onDeleteTask={handleDeleteTask}
/>
</>
);
}
let nextId = 3;
const initialTasks = [
{ id: 0, text: 'Visit Kafka Museum', done: true },
{ id: 1, text: 'Watch a puppet show', done: false },
{ id: 2, text: 'Lennon Wall pic', done: false }
];useMemo
The useMemo hook lets you cache the result of expensive calculations.
Memoizing Expensive Calculations:
import { useMemo, useState } from 'react';
function ProductList({ products, category }) {
const [sortOrder, setSortOrder] = useState('asc');
const filteredAndSortedProducts = useMemo(() => {
console.log('Filtering and sorting products...');
const filtered = products.filter(p => p.category === category);
return filtered.sort((a, b) => {
if (sortOrder === 'asc') {
return a.price - b.price;
}
return b.price - a.price;
});
}, [products, category, sortOrder]);
return (
<div>
<button onClick={() => setSortOrder(sortOrder === 'asc' ? 'desc' : 'asc')}>
Sort: {sortOrder}
</button>
<ul>
{filteredAndSortedProducts.map(product => (
<li key={product.id}>{product.name} - ${product.price}</li>
))}
</ul>
</div>
);
}Preventing Object Recreation:
function SearchResults({ query }) {
const searchOptions = useMemo(() => ({
query,
limit: 10,
caseSensitive: false
}), [query]);
// searchOptions object only recreated when query changes
const results = useSearch(searchOptions);
return <ResultsList results={results} />;
}useCallback
The useCallback hook lets you cache a function definition between re-renders.
Memoizing Event Handlers:
import { useCallback, useState } from 'react';
function ProductPage({ productId }) {
const [items, setItems] = useState([]);
const handleAddToCart = useCallback(() => {
setItems(prevItems => [...prevItems, productId]);
}, [productId]);
return <AddToCartButton onAdd={handleAddToCart} />;
}
// Memoized child component
const AddToCartButton = memo(({ onAdd }) => {
console.log('Button rendered');
return <button onClick={onAdd}>Add to Cart</button>;
});Optimizing Child Components:
function TodoList() {
const [todos, setTodos] = useState(initialTodos);
const handleToggle = useCallback((id) => {
setTodos(prevTodos =>
prevTodos.map(todo =>
todo.id === id ? { ...todo, done: !todo.done } : todo
)
);
}, []);
const handleDelete = useCallback((id) => {
setTodos(prevTodos => prevTodos.filter(todo => todo.id !== id));
}, []);
return (
<ul>
{todos.map(todo => (
<TodoItem
key={todo.id}
todo={todo}
onToggle={handleToggle}
onDelete={handleDelete}
/>
))}
</ul>
);
}useRef
The useRef hook lets you reference a value that's not needed for rendering.
Accessing DOM Elements:
import { useRef } from 'react';
function TextInput() {
const inputRef = useRef(null);
function handleClick() {
inputRef.current.focus();
}
return (
<>
<input ref={inputRef} />
<button onClick={handleClick}>Focus input</button>
</>
);
}Storing Mutable Values:
function Stopwatch() {
const [time, setTime] = useState(0);
const intervalRef = useRef(null);
function handleStart() {
intervalRef.current = setInterval(() => {
setTime(t => t + 1);
}, 10);
}
function handleStop() {
clearInterval(intervalRef.current);
}
return (
<div>
<p>Time: {(time / 100).toFixed(2)}s</p>
<button onClick={handleStart}>Start</button>
<button onClick={handleStop}>Stop</button>
</div>
);
}Video Player Control (Context7 Pattern):
import { useRef, useState } from 'react';
function VideoPlayer({ src, isPlaying }) {
const ref = useRef(null);
useEffect(() => {
if (isPlaying) {
ref.current.play();
} else {
ref.current.pause();
}
}, [isPlaying]);
return <video ref={ref} src={src} loop playsInline />;
}
function App() {
const [isPlaying, setIsPlaying] = useState(false);
return (
<>
<button onClick={() => setIsPlaying(!isPlaying)}>
{isPlaying ? 'Pause' : 'Play'}
</button>
<VideoPlayer
isPlaying={isPlaying}
src="https://interactive-examples.mdn.mozilla.net/media/cc0-videos/flower.mp4"
/>
</>
);
}State Management Patterns
Local State Pattern
Use local state for component-specific data that doesn't need to be shared.
function LoginForm() {
const [formData, setFormData] = useState({
username: '',
password: '',
rememberMe: false
});
const [errors, setErrors] = useState({});
const [isSubmitting, setIsSubmitting] = useState(false);
const handleChange = (field) => (e) => {
const value = e.target.type === 'checkbox' ? e.target.checked : e.target.value;
setFormData(prev => ({ ...prev, [field]: value }));
};
const validate = () => {
const newErrors = {};
if (!formData.username) newErrors.username = 'Required';
if (!formData.password) newErrors.password = 'Required';
return newErrors;
};
const handleSubmit = async (e) => {
e.preventDefault();
const newErrors = validate();
if (Object.keys(newErrors).length > 0) {
setErrors(newErrors);
return;
}
setIsSubmitting(true);
try {
await login(formData);
} catch (error) {
setErrors({ submit: error.message });
} finally {
setIsSubmitting(false);
}
};
return (
<form onSubmit={handleSubmit}>
<input
value={formData.username}
onChange={handleChange('username')}
placeholder="Username"
/>
{errors.username && <span>{errors.username}</span>}
<input
type="password"
value={formData.password}
onChange={handleChange('password')}
placeholder="Password"
/>
{errors.password && <span>{errors.password}</span>}
<label>
<input
type="checkbox"
checked={formData.rememberMe}
onChange={handleChange('rememberMe')}
/>
Remember me
</label>
<button type="submit" disabled={isSubmitting}>
{isSubmitting ? 'Logging in...' : 'Log in'}
</button>
{errors.submit && <div>{errors.submit}</div>}
</form>
);
}Context API Pattern
Use Context for global or widely-shared state like themes, user authentication, or preferences.
Theme Context with Provider:
import { createContext, useContext, useState } from 'react';
const ThemeContext = createContext(null);
export function ThemeProvider({ children }) {
const [theme, setTheme] = useState('light');
const toggleTheme = () => {
setTheme(prevTheme => prevTheme === 'light' ? 'dark' : 'light');
};
return (
<ThemeContext.Provider value={{ theme, toggleTheme }}>
{children}
</ThemeContext.Provider>
);
}
export function useTheme() {
const context = useContext(ThemeContext);
if (!context) {
throw new Error('useTheme must be used within ThemeProvider');
}
return context;
}
// Usage
function App() {
return (
<ThemeProvider>
<Page />
</ThemeProvider>
);
}
function Page() {
const { theme, toggleTheme } = useTheme();
return (
<div className={`page-${theme}`}>
<button onClick={toggleTheme}>Toggle Theme</button>
</div>
);
}Reducer + Context Pattern (Context7 Best Practice)
Combine useReducer with Context for scalable state management.
Task Management with Reducer + Context:
import { createContext, useContext, useReducer } from 'react';
// Context for tasks data
const TasksContext = createContext(null);
// Context for dispatch function
const TasksDispatchContext = createContext(null);
export function TasksProvider({ children }) {
const [tasks, dispatch] = useReducer(tasksReducer, initialTasks);
return (
<TasksContext.Provider value={tasks}>
<TasksDispatchContext.Provider value={dispatch}>
{children}
</TasksDispatchContext.Provider>
</TasksContext.Provider>
);
}
export function useTasks() {
return useContext(TasksContext);
}
export function useTasksDispatch() {
return useContext(TasksDispatchContext);
}
function tasksReducer(tasks, action) {
switch (action.type) {
case 'added': {
return [...tasks, {
id: action.id,
text: action.text,
done: false
}];
}
case 'changed': {
return tasks.map(t => {
if (t.id === action.task.id) {
return action.task;
}
return t;
});
}
case 'deleted': {
return tasks.filter(t => t.id !== action.id);
}
default: {
throw Error('Unknown action: ' + action.type);
}
}
}
const initialTasks = [
{ id: 0, text: 'Philosopher's Path', done: true },
{ id: 1, text: 'Visit the temple', done: false },
{ id: 2, text: 'Drink matcha', done: false }
];
// Component using the pattern
function AddTask() {
const [text, setText] = useState('');
const dispatch = useTasksDispatch();
return (
<>
<input
placeholder="Add task"
value={text}
onChange={e => setText(e.target.value)}
/>
<button onClick={() => {
setText('');
dispatch({
type: 'added',
id: nextId++,
text: text,
});
}}>Add</button>
</>
);
}
function TaskList() {
const tasks = useTasks();
return (
<ul>
{tasks.map(task => (
<Task key={task.id} task={task} />
))}
</ul>
);
}
function Task({ task }) {
const [isEditing, setIsEditing] = useState(false);
const dispatch = useTasksDispatch();
let taskContent;
if (isEditing) {
taskContent = (
<>
<input
value={task.text}
onChange={e => {
dispatch({
type: 'changed',
task: {
...task,
text: e.target.value
}
});
}} />
<button onClick={() => setIsEditing(false)}>
Save
</button>
</>
);
} else {
taskContent = (
<>
{task.text}
<button onClick={() => setIsEditing(true)}>
Edit
</button>
</>
);
}
return (
<label>
<input
type="checkbox"
checked={task.done}
onChange={e => {
dispatch({
type: 'changed',
task: {
...task,
done: e.target.checked
}
});
}}
/>
{taskContent}
<button onClick={() => {
dispatch({
type: 'deleted',
id: task.id
});
}}>
Delete
</button>
</label>
);
}
let nextId = 3;Custom Hooks
Custom hooks let you extract component logic into reusable functions.
Basic Custom Hook
import { useState, useEffect } from 'react';
function useWindowSize() {
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 size;
}
// Usage
function Component() {
const { width, height } = useWindowSize();
return <div>{width} x {height}</div>;
}Online Status Hook (Context7 Pattern)
import { useState, useEffect } from 'react';
function useOnlineStatus() {
const [isOnline, setIsOnline] = useState(true);
useEffect(() => {
function handleOnline() {
setIsOnline(true);
}
function handleOffline() {
setIsOnline(false);
}
window.addEventListener('online', handleOnline);
window.addEventListener('offline', handleOffline);
return () => {
window.removeEventListener('online', handleOnline);
window.removeEventListener('offline', handleOffline);
};
}, []);
return isOnline;
}
// Usage
function StatusBar() {
const isOnline = useOnlineStatus();
return <h1>{isOnline ? '✅ Online' : '❌ Disconnected'}</h1>;
}Form Hook
function useForm(initialValues, onSubmit) {
const [values, setValues] = useState(initialValues);
const [errors, setErrors] = useState({});
const [isSubmitting, setIsSubmitting] = useState(false);
const handleChange = (name, value) => {
setValues(prev => ({ ...prev, [name]: value }));
// Clear error when user starts typing
if (errors[name]) {
setErrors(prev => ({ ...prev, [name]: null }));
}
};
const handleSubmit = async (e) => {
e.preventDefault();
setIsSubmitting(true);
try {
await onSubmit(values);
} catch (error) {
setErrors({ submit: error.message });
} finally {
setIsSubmitting(false);
}
};
const reset = () => {
setValues(initialValues);
setErrors({});
};
return {
values,
errors,
isSubmitting,
handleChange,
handleSubmit,
setErrors,
reset
};
}
// Usage
function ContactForm() {
const { values, errors, isSubmitting, handleChange, handleSubmit } = useForm(
{ name: '', email: '', message: '' },
async (data) => {
await fetch('/api/contact', {
method: 'POST',
body: JSON.stringify(data)
});
}
);
return (
<form onSubmit={handleSubmit}>
<input
value={values.name}
onChange={(e) => handleChange('name', e.target.value)}
/>
{errors.name && <span>{errors.name}</span>}
<input
value={values.email}
onChange={(e) => handleChange('email', e.target.value)}
/>
{errors.email && <span>{errors.email}</span>}
<textarea
value={values.message}
onChange={(e) => handleChange('message', e.target.value)}
/>
{errors.message && <span>{errors.message}</span>}
<button type="submit" disabled={isSubmitting}>
{isSubmitting ? 'Sending...' : 'Send'}
</button>
</form>
);
}Fetch Hook
function useFetch(url, options = {}) {
const [data, setData] = useState(null);
const [loading, setLoading] = useState(true);
const [error, setError] = useState(null);
useEffect(() => {
let cancelled = false;
async function fetchData() {
try {
setLoading(true);
const response = await fetch(url, options);
if (!response.ok) {
throw new Error(`HTTP error! status: ${response.status}`);
}
const result = await response.json();
if (!cancelled) {
setData(result);
setError(null);
}
} catch (err) {
if (!cancelled) {
setError(err.message);
setData(null);
}
} finally {
if (!cancelled) {
setLoading(false);
}
}
}
fetchData();
return () => {
cancelled = true;
};
}, [url, JSON.stringify(options)]);
return { data, loading, error };
}
// Usage
function UserProfile({ userId }) {
const { data, loading, error } = useFetch(`/api/users/${userId}`);
if (loading) return <div>Loading...</div>;
if (error) return <div>Error: {error}</div>;
return <div>{data.name}</div>;
}Local Storage Hook
function useLocalStorage(key, initialValue) {
const [storedValue, setStoredValue] = useState(() => {
try {
const item = window.localStorage.getItem(key);
return item ? JSON.parse(item) : initialValue;
} catch (error) {
console.error(error);
return initialValue;
}
});
const setValue = (value) => {
try {
const valueToStore = value instanceof Function ? value(storedValue) : value;
setStoredValue(valueToStore);
window.localStorage.setItem(key, JSON.stringify(valueToStore));
} catch (error) {
console.error(error);
}
};
return [storedValue, setValue];
}
// Usage
function Settings() {
const [theme, setTheme] = useLocalStorage('theme', 'light');
const [fontSize, setFontSize] = useLocalStorage('fontSize', 16);
return (
<div>
<select value={theme} onChange={(e) => setTheme(e.target.value)}>
<option value="light">Light</option>
<option value="dark">Dark</option>
</select>
<input
type="number"
value={fontSize}
onChange={(e) => setFontSize(parseInt(e.target.value))}
/>
</div>
);
}Performance Optimization
React.memo
Memoize components to prevent unnecessary re-renders.
import { memo } from 'react';
const ExpensiveComponent = memo(function ExpensiveComponent({ data, onAction }) {
console.log('Rendering expensive component');
return (
<div>
{data.map(item => (
<div key={item.id}>{item.name}</div>
))}
<button onClick={onAction}>Action</button>
</div>
);
});
// With custom comparison
const CustomMemoComponent = memo(
function Component({ user }) {
return <div>{user.name}</div>;
},
(prevProps, nextProps) => {
// Return true if props are equal (skip re-render)
return prevProps.user.id === nextProps.user.id;
}
);Lazy Loading
Load components on demand to reduce initial bundle size.
import { lazy, Suspense } from 'react';
const HeavyComponent = lazy(() => import('./HeavyComponent'));
const AdminPanel = lazy(() => import('./AdminPanel'));
function App() {
return (
<div>
<Suspense fallback={<div>Loading...</div>}>
<HeavyComponent />
</Suspense>
</div>
);
}
// Lazy loading with routes
function Dashboard() {
const [showAdmin, setShowAdmin] = useState(false);
return (
<div>
<button onClick={() => setShowAdmin(true)}>
Show Admin Panel
</button>
{showAdmin && (
<Suspense fallback={<Spinner />}>
<AdminPanel />
</Suspense>
)}
</div>
);
}Code Splitting
Split your code into smaller chunks for better performance.
import { lazy, Suspense } from 'react';
import { BrowserRouter, Routes, Route } from 'react-router-dom';
const Home = lazy(() => import('./pages/Home'));
const About = lazy(() => import('./pages/About'));
const Dashboard = lazy(() => import('./pages/Dashboard'));
function App() {
return (
<BrowserRouter>
<Suspense fallback={<div>Loading page...</div>}>
<Routes>
<Route path="/" element={<Home />} />
<Route path="/about" element={<About />} />
<Route path="/dashboard" element={<Dashboard />} />
</Routes>
</Suspense>
</BrowserRouter>
);
}Virtual Scrolling
Render only visible items in large lists.
function VirtualList({ items, height, itemHeight }) {
const [scrollTop, setScrollTop] = useState(0);
const startIndex = Math.floor(scrollTop / itemHeight);
const endIndex = Math.min(
startIndex + Math.ceil(height / itemHeight),
items.length
);
const visibleItems = items.slice(startIndex, endIndex);
const offsetY = startIndex * itemHeight;
const totalHeight = items.length * itemHeight;
return (
<div
style={{ height, overflow: 'auto' }}
onScroll={(e) => setScrollTop(e.target.scrollTop)}
>
<div style={{ height: totalHeight, position: 'relative' }}>
<div style={{ transform: `translateY(${offsetY}px)` }}>
{visibleItems.map((item, index) => (
<div
key={startIndex + index}
style={{ height: itemHeight }}
>
{item}
</div>
))}
</div>
</div>
</div>
);
}Best Practices from Context7 Research
1. Proper Dependency Arrays
Always include all dependencies in useEffect, useMemo, and useCallback.
// ❌ Bad - missing dependencies
useEffect(() => {
fetchData(userId);
}, []);
// ✅ Good - all dependencies included
useEffect(() => {
fetchData(userId);
}, [userId]);2. Cleanup Functions
Always cleanup side effects to prevent memory leaks.
useEffect(() => {
const subscription = api.subscribe(id);
return () => {
subscription.unsubscribe();
};
}, [id]);3. Separate Concerns
Split context for data and dispatch to optimize re-renders.
// ✅ Good - separate contexts
const TasksContext = createContext(null);
const TasksDispatchContext = createContext(null);
// Components that only dispatch won't re-render when tasks change
function AddTask() {
const dispatch = useTasksDispatch(); // No re-render when tasks change
// ...
}4. Avoid Inline Object Creation
Use useMemo or useCallback to prevent unnecessary re-renders.
// ❌ Bad - new object on every render
<Component style={{ margin: 10 }} />
// ✅ Good - memoized object
const style = useMemo(() => ({ margin: 10 }), []);
<Component style={style} />5. State Updater Functions
Use updater functions when new state depends on previous state.
// ❌ Bad - may use stale state
setCount(count + 1);
// ✅ Good - uses current state
setCount(prevCount => prevCount + 1);6. Extract Complex Logic
Move complex state logic to reducers or custom hooks.
// ✅ Good - complex logic in reducer
function cartReducer(state, action) {
switch (action.type) {
case 'add_item':
// Complex logic here
return newState;
case 'remove_item':
// Complex logic here
return newState;
default:
return state;
}
}7. Key Props for Lists
Always provide unique keys for list items.
// ❌ Bad - using index as key
{items.map((item, index) => <div key={index}>{item}</div>)}
// ✅ Good - using unique ID
{items.map(item => <div key={item.id}>{item.text}</div>)}8. Controlled Components
Prefer controlled components for form inputs.
function Form() {
const [value, setValue] = useState('');
return (
<input
value={value}
onChange={(e) => setValue(e.target.value)}
/>
);
}9. Error Boundaries
Implement error boundaries to catch and handle errors gracefully.
class ErrorBoundary extends React.Component {
constructor(props) {
super(props);
this.state = { hasError: false, error: null };
}
static getDerivedStateFromError(error) {
return { hasError: true, error };
}
componentDidCatch(error, errorInfo) {
console.error('Error caught:', error, errorInfo);
}
render() {
if (this.state.hasError) {
return <h1>Something went wrong.</h1>;
}
return this.props.children;
}
}
// Usage
<ErrorBoundary>
<MyComponent />
</ErrorBoundary>10. Prop Types or TypeScript
Use PropTypes or TypeScript for type checking.
import PropTypes from 'prop-types';
function User({ name, age, email }) {
return <div>{name} ({age})</div>;
}
User.propTypes = {
name: PropTypes.string.isRequired,
age: PropTypes.number.isRequired,
email: PropTypes.string
};
User.defaultProps = {
email: 'no-email@example.com'
};Additional Examples
Example 1: Multi-Step Form
function MultiStepForm() {
const [step, setStep] = useState(1);
const [formData, setFormData] = useState({
personalInfo: {},
address: {},
preferences: {}
});
const updateFormData = (section, data) => {
setFormData(prev => ({
...prev,
[section]: { ...prev[section], ...data }
}));
};
const nextStep = () => setStep(s => s + 1);
const prevStep = () => setStep(s => s - 1);
return (
<div>
{step === 1 && (
<PersonalInfoStep
data={formData.personalInfo}
onNext={(data) => {
updateFormData('personalInfo', data);
nextStep();
}}
/>
)}
{step === 2 && (
<AddressStep
data={formData.address}
onNext={(data) => {
updateFormData('address', data);
nextStep();
}}
onPrev={prevStep}
/>
)}
{step === 3 && (
<PreferencesStep
data={formData.preferences}
onSubmit={(data) => {
updateFormData('preferences', data);
submitForm({ ...formData, preferences: data });
}}
onPrev={prevStep}
/>
)}
</div>
);
}Example 2: Infinite Scroll
function InfiniteScroll() {
const [items, setItems] = useState([]);
const [page, setPage] = useState(1);
const [loading, setLoading] = useState(false);
const [hasMore, setHasMore] = useState(true);
const loadMore = useCallback(async () => {
if (loading || !hasMore) return;
setLoading(true);
try {
const newItems = await fetchItems(page);
setItems(prev => [...prev, ...newItems]);
setHasMore(newItems.length > 0);
setPage(p => p + 1);
} finally {
setLoading(false);
}
}, [page, loading, hasMore]);
useEffect(() => {
const handleScroll = () => {
if (
window.innerHeight + window.scrollY >=
document.documentElement.scrollHeight - 500
) {
loadMore();
}
};
window.addEventListener('scroll', handleScroll);
return () => window.removeEventListener('scroll', handleScroll);
}, [loadMore]);
return (
<div>
{items.map(item => <Item key={item.id} data={item} />)}
{loading && <div>Loading...</div>}
{!hasMore && <div>No more items</div>}
</div>
);
}Example 3: Debounced Search
function useDebounce(value, delay) {
const [debouncedValue, setDebouncedValue] = useState(value);
useEffect(() => {
const handler = setTimeout(() => {
setDebouncedValue(value);
}, delay);
return () => {
clearTimeout(handler);
};
}, [value, delay]);
return debouncedValue;
}
function SearchComponent() {
const [searchTerm, setSearchTerm] = useState('');
const debouncedSearchTerm = useDebounce(searchTerm, 500);
const [results, setResults] = useState([]);
useEffect(() => {
if (debouncedSearchTerm) {
searchAPI(debouncedSearchTerm).then(setResults);
} else {
setResults([]);
}
}, [debouncedSearchTerm]);
return (
<div>
<input
value={searchTerm}
onChange={(e) => setSearchTerm(e.target.value)}
placeholder="Search..."
/>
<ul>
{results.map(result => (
<li key={result.id}>{result.title}</li>
))}
</ul>
</div>
);
}Example 4: Modal with Portal
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-button" onClick={onClose}>×</button>
{children}
</div>
</div>,
document.body
);
}
// Usage
function App() {
const [isModalOpen, setIsModalOpen] = useState(false);
return (
<div>
<button onClick={() => setIsModalOpen(true)}>Open Modal</button>
<Modal isOpen={isModalOpen} onClose={() => setIsModalOpen(false)}>
<h2>Modal Content</h2>
<p>This is a modal dialog</p>
</Modal>
</div>
);
}Example 5: Drag and Drop
function DragDropList() {
const [items, setItems] = useState([
{ id: 1, text: 'Item 1' },
{ id: 2, text: 'Item 2' },
{ id: 3, text: 'Item 3' }
]);
const [draggedItem, setDraggedItem] = useState(null);
const handleDragStart = (item) => {
setDraggedItem(item);
};
const handleDragOver = (e) => {
e.preventDefault();
};
const handleDrop = (targetItem) => {
if (!draggedItem || draggedItem.id === targetItem.id) return;
const draggedIndex = items.findIndex(i => i.id === draggedItem.id);
const targetIndex = items.findIndex(i => i.id === targetItem.id);
const newItems = [...items];
newItems.splice(draggedIndex, 1);
newItems.splice(targetIndex, 0, draggedItem);
setItems(newItems);
setDraggedItem(null);
};
return (
<ul>
{items.map(item => (
<li
key={item.id}
draggable
onDragStart={() => handleDragStart(item)}
onDragOver={handleDragOver}
onDrop={() => handleDrop(item)}
>
{item.text}
</li>
))}
</ul>
);
}Summary
This React development skill covers:
- Core Concepts: Components, JSX, Props, State
- Essential Hooks: useState, useEffect, useContext, useReducer, useMemo, useCallback, useRef
- State Management: Local state, Context API, Reducer + Context pattern
- Custom Hooks: Reusable logic extraction patterns
- Performance: Memoization, lazy loading, code splitting, virtual scrolling
- Best Practices: From Context7 research including proper dependencies, cleanup, separation of concerns
- Real-world Examples: Forms, infinite scroll, search, modals, drag-and-drop
The patterns and examples are based on official React documentation (Trust Score: 10) and represent modern React development practices focusing on functional components and hooks.