react-patterns

React Patterns - Modern Development Guide

A comprehensive skill for mastering modern React development patterns, including React 18+ features, hooks, component composition, state management strategies, and performance optimization techniques for building scalable web applications.

When to Use This Skill

Use this skill when:

Building modern React applications with functional components and hooks
Implementing complex state management with useReducer and Context API
Optimizing React application performance with memoization techniques
Creating reusable custom hooks for shared logic
Working with Server Components and React Server Components (RSC)
Managing forms, side effects, and asynchronous operations
Refactoring class components to modern functional patterns
Building type-safe React applications with proper patterns
Implementing advanced component composition patterns
Debugging React performance issues and unnecessary re-renders

Core Concepts

React Philosophy

Modern React emphasizes:

Functional Components: Pure functions that return JSX
Hooks: Composable state and side effect management
Declarative UI: Describe what the UI should look like, React handles updates
Component Composition: Build complex UIs from simple, reusable components
Unidirectional Data Flow: Props flow down, events flow up
Immutability: Never mutate state directly, always create new objects/arrays

Component Types

Presentational Components: Focus on UI, receive data via props
Container Components: Handle logic, state, and side effects
Server Components: Render on the server, no client JavaScript
Client Components: Interactive components marked with "use client"
Async Components: Server components that can await data

React Hooks Reference

Built-in State Hooks

useState

Manage local component state for simple values.

Syntax:

const [state, setState] = useState(initialValue);

Basic Example:

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(prev => prev + 1)}>Increment (functional)</button> </div> ); }

Best Practices:

Use functional updates when new state depends on previous state
Keep state as local as possible
Don't store derived values in state
Initialize with functions for expensive computations

Multiple State Variables:

function Form() { const [name, setName] = useState(''); const [email, setEmail] = useState(''); const [age, setAge] = useState(0);

return ( <form> <input value={name} onChange={e => setName(e.target.value)} /> <input value={email} onChange={e => setEmail(e.target.value)} /> <input type="number" value={age} onChange={e => setAge(Number(e.target.value))} /> </form> ); }

Lazy Initialization:

function ExpensiveComponent() { // Function only runs once on initial render const [state, setState] = useState(() => { const initialValue = expensiveComputation(); return initialValue; });

return <div>{state}</div>; }

useReducer

Manage complex state logic with a reducer pattern (similar to Redux).

Syntax:

const [state, dispatch] = useReducer(reducer, initialState, init?);

Task Manager Example:

import { useReducer } from 'react';

function TaskApp() { const [tasks, dispatch] = useReducer(tasksReducer, []);

function handleAddTask(text) { dispatch({ type: 'added', id: Date.now(), text: text, }); }

function handleChangeTask(task) { dispatch({ type: 'changed', task: task }); }

function handleDeleteTask(taskId) { dispatch({ type: 'deleted', id: taskId }); }

return ( <> <AddTask onAddTask={handleAddTask} /> <TaskList tasks={tasks} onChangeTask={handleChangeTask} onDeleteTask={handleDeleteTask} /> </> ); }

When to use useReducer:

Multiple related state values
Complex state update logic
State transitions follow predictable patterns
Need to optimize performance with many updates
Want to separate state logic from component

useContext

Access context values without prop drilling.

Syntax:

const value = useContext(SomeContext);

Theme Context Example:

import { createContext, useContext, useState } from 'react';

const ThemeContext = createContext(null);

export function ThemeProvider({ children }) { const [theme, setTheme] = useState('light');

const toggleTheme = () => { setTheme(prev => prev === '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 Button() { const { theme, toggleTheme } = useTheme();

return ( <button onClick={toggleTheme} className={theme === 'dark' ? 'btn-dark' : 'btn-light'} > Toggle Theme </button> ); }

Combining useReducer + useContext:

import { createContext, useContext, useReducer } from 'react';

const TasksContext = createContext(null); 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); }

// Usage in components function TaskList() { const tasks = useTasks(); const dispatch = useTasksDispatch();

return ( <ul> {tasks.map(task => ( <li key={task.id}> {task.text} <button onClick={() => dispatch({ type: 'deleted', id: task.id })}> Delete </button> </li> ))} </ul> ); }

Effect Hooks

useEffect

Synchronize component with external systems (APIs, DOM, subscriptions).

Syntax:

useEffect(() => { // Effect logic return () => { // Cleanup logic }; }, [dependencies]);

Data Fetching Example:

import { useState, useEffect } from 'react';

function UserProfile({ userId }) { const [user, setUser] = useState(null); const [loading, setLoading] = useState(true); const [error, setError] = useState(null);

useEffect(() => { let ignore = false; // Prevent race conditions

async function fetchUser() {
  try {
    setLoading(true);
    const response = await fetch(`/api/users/${userId}`);
    const data = await response.json();

    if (!ignore) {
      setUser(data);
    }
  } catch (err) {
    if (!ignore) {
      setError(err.message);
    }
  } finally {
    if (!ignore) {
      setLoading(false);
    }
  }
}

fetchUser();

return () => {
  ignore = true; // Cleanup
};

}, [userId]);

if (loading) return <div>Loading...</div>; if (error) return <div>Error: {error}</div>; return <div>User: {user?.name}</div>; }

Subscription Example:

function ChatRoom({ roomId }) { const [messages, setMessages] = useState([]);

useEffect(() => { const connection = createConnection(roomId);

connection.on('message', (msg) => {
  setMessages(prev => [...prev, msg]);
});

connection.connect();

return () => {
  connection.disconnect(); // Cleanup on unmount or roomId change
};

}, [roomId]);

return ( <div> {messages.map((msg, i) => ( <div key={i}>{msg}</div> ))} </div> ); }

Common Effect Patterns:

// Run once on mount useEffect(() => { console.log('Component mounted'); }, []); // Empty dependency array

// Run on every render useEffect(() => { console.log('Component rendered'); }); // No dependency array

// Run when specific values change useEffect(() => { console.log('userId changed:', userId); }, [userId]); // Dependency array with values

// Cleanup on unmount useEffect(() => { const timer = setTimeout(() => { console.log('Delayed action'); }, 1000);

return () => clearTimeout(timer); }, []);

useLayoutEffect

Synchronous version of useEffect, runs before browser paint.

Use Cases:

Measuring DOM elements
Synchronous DOM mutations
Preventing visual flickering

Example:

import { useLayoutEffect, useRef, useState } from 'react';

function Tooltip() { const ref = useRef(null); const [tooltipHeight, setTooltipHeight] = useState(0);

useLayoutEffect(() => { const { height } = ref.current.getBoundingClientRect(); setTooltipHeight(height); // Synchronous update before paint }, []);

return ( <div ref={ref}> Tooltip content (height: {tooltipHeight}px) </div> ); }

Performance Hooks

useMemo

Cache expensive computations between renders.

Syntax:

const cachedValue = useMemo(() => computeExpensiveValue(a, b), [a, b]);

Example:

import { useMemo } from 'react';

return ( <ul> {visibleTodos.map(todo => ( <li key={todo.id}>{todo.text}</li> ))} </ul> ); }

When to use useMemo:

Expensive calculations (filtering, sorting large arrays)
Preventing unnecessary re-renders of child components
Stabilizing object/array references passed as dependencies

Anti-pattern (unnecessary):

// ❌ Don't use for simple calculations const sum = useMemo(() => a + b, [a, b]); // Overkill

// ✅ Just compute directly const sum = a + b;

useCallback

Memoize function references between renders.

Syntax:

const cachedFn = useCallback(() => { // Function logic }, [dependencies]);

Example:

import { useState, useCallback, memo } from 'react';

const TodoItem = memo(function TodoItem({ todo, onChange, onDelete }) { console.log('TodoItem rendered:', todo.id);

return ( <li> <input type="checkbox" checked={todo.done} onChange={() => onChange(todo)} /> {todo.text} <button onClick={() => onDelete(todo.id)}>Delete</button> </li> ); });

function TodoList() { const [todos, setTodos] = useState([]);

// Memoize handlers to prevent TodoItem re-renders const handleChange = useCallback((todo) => { setTodos(prev => prev.map(t => t.id === todo.id ? { ...t, done: !t.done } : t )); }, []);

const handleDelete = useCallback((id) => { setTodos(prev => prev.filter(t => t.id !== id)); }, []);

return ( <ul> {todos.map(todo => ( <TodoItem key={todo.id} todo={todo} onChange={handleChange} onDelete={handleDelete} /> ))} </ul> ); }

When to use useCallback:

Passing callbacks to memoized child components
Function is a dependency of useEffect or other hooks
Optimizing expensive event handlers

React.memo

Memoize entire component to prevent unnecessary re-renders.

Syntax:

const MemoizedComponent = memo(function Component(props) { // Component logic }, arePropsEqual?);

Example:

import { memo } from 'react';

const ExpensiveComponent = memo(function ExpensiveComponent({ data, onClick }) { console.log('ExpensiveComponent rendered');

return ( <div> {data.map(item => ( <div key={item.id} onClick={() => onClick(item)}> {item.name} </div> ))} </div> ); });

// With custom comparison const CustomMemoComponent = memo( function CustomMemoComponent({ user }) { return <div>{user.name}</div>; }, (prevProps, nextProps) => { // Return true if props are equal (skip re-render) return prevProps.user.id === nextProps.user.id; } );

Ref Hooks

useRef

Create mutable reference that persists across renders.

Syntax:

const ref = useRef(initialValue);

DOM Reference Example:

import { useRef } from 'react';

function VideoPlayer({ src, isPlaying }) { const videoRef = useRef(null);

useEffect(() => { if (isPlaying) { videoRef.current.play(); } else { videoRef.current.pause(); } }, [isPlaying]);

return <video ref={videoRef} src={src} />; }

Storing Mutable Values:

function Timer() { const intervalRef = useRef(null); const [count, setCount] = useState(0);

function start() { if (intervalRef.current) return; // Already running

intervalRef.current = setInterval(() => {
  setCount(c => c + 1);
}, 1000);

}

function stop() { if (intervalRef.current) { clearInterval(intervalRef.current); intervalRef.current = null; } }

return ( <div> <p>Count: {count}</p> <button onClick={start}>Start</button> <button onClick={stop}>Stop</button> </div> ); }

Previous Value Pattern:

function usePrevious(value) { const ref = useRef();

useEffect(() => { ref.current = value; }, [value]);

return ref.current; }

// Usage function Counter({ count }) { const prevCount = usePrevious(count);

return ( <div> <p>Current: {count}</p> <p>Previous: {prevCount}</p> </div> ); }

Transition Hooks (React 18+)

useTransition

Mark state updates as non-urgent, keeping UI responsive.

Syntax:

const [isPending, startTransition] = useTransition();

Example:

import { useState, useTransition } from 'react';

function SearchResults() { const [query, setQuery] = useState(''); const [results, setResults] = useState([]); const [isPending, startTransition] = useTransition();

function handleChange(e) { const value = e.target.value; setQuery(value); // Urgent update (input value)

startTransition(() => {
  // Non-urgent update (search results)
  const filtered = performExpensiveSearch(value);
  setResults(filtered);
});

}

return ( <div> <input value={query} onChange={handleChange} /> {isPending && <div>Searching...</div>} <ul> {results.map(result => ( <li key={result.id}>{result.name}</li> ))} </ul> </div> ); }

useDeferredValue

Defer updating part of the UI.

Syntax:

const deferredValue = useDeferredValue(value);

Example:

import { useState, useDeferredValue, memo } from 'react';

const SlowList = memo(function SlowList({ items }) { // Intentionally slow rendering return ( <ul> {items.map(item => ( <li key={item.id}>{item.name}</li> ))} </ul> ); });

function App() { const [text, setText] = useState(''); const deferredText = useDeferredValue(text);

return ( <> <input value={text} onChange={e => setText(e.target.value)} /> <SlowList items={filterItems(deferredText)} /> </> ); }

Server Action Hooks (RSC)

useActionState

Manage server action state and pending status.

Syntax:

const [state, formAction, isPending] = useActionState(serverAction, initialState);

Example:

"use client";

import { useActionState } from 'react'; import { updateName } from './actions';

function UpdateNameForm() { const [state, submitAction, isPending] = useActionState( updateName, { error: null } );

return ( <form action={submitAction}> <input type="text" name="name" disabled={isPending} /> {state.error && <span className="error">{state.error}</span>} <button type="submit" disabled={isPending}> {isPending ? 'Updating...' : 'Update'} </button> </form> ); }

Server Action (actions.js):

"use server";

export async function updateName(prevState, formData) { const name = formData.get('name');

if (!name) { return { error: 'Name is required' }; }

try { await db.users.updateName(name); return { error: null }; } catch (err) { return { error: 'Failed to update name' }; } }

Custom Hooks Patterns

Custom hooks let you extract and reuse stateful logic across components.

Naming Convention

Always prefix custom hooks with use :

useCustomHook // ✅ Correct customHook // ❌ Wrong

Data Fetching Hook

import { useState, useEffect } from 'react';

export function useFetch(url) { const [data, setData] = useState(null); const [loading, setLoading] = useState(true); const [error, setError] = useState(null);

useEffect(() => { let ignore = false;

async function fetchData() {
  try {
    setLoading(true);
    setError(null);

    const response = await fetch(url);
    if (!response.ok) {
      throw new Error(`HTTP error! status: ${response.status}`);
    }

    const result = await response.json();

    if (!ignore) {
      setData(result);
    }
  } catch (err) {
    if (!ignore) {
      setError(err.message);
    }
  } finally {
    if (!ignore) {
      setLoading(false);
    }
  }
}

fetchData();

return () => {
  ignore = true;
};

}, [url]);

return { data, loading, error }; }

// Usage function UserProfile({ userId }) { const { data: user, loading, error } = useFetch(/api/users/${userId});

if (loading) return <div>Loading...</div>; if (error) return <div>Error: {error}</div>; return <div>{user.name}</div>; }

Form Input Hook

import { useState } from 'react';

export function useFormInput(initialValue = '') { const [value, setValue] = useState(initialValue);

function handleChange(e) { setValue(e.target.value); }

function reset() { setValue(initialValue); }

return { value, onChange: handleChange, reset }; }

// Usage function LoginForm() { const email = useFormInput(''); const password = useFormInput('');

function handleSubmit(e) { e.preventDefault(); console.log('Email:', email.value); console.log('Password:', password.value); email.reset(); password.reset(); }

return ( <form onSubmit={handleSubmit}> <input type="email" {...email} placeholder="Email" /> <input type="password" {...password} placeholder="Password" /> <button type="submit">Login</button> </form> ); }

Local Storage Hook

import { useState, useEffect } from 'react';

export function useLocalStorage(key, initialValue) { const [value, setValue] = useState(() => { try { const item = window.localStorage.getItem(key); return item ? JSON.parse(item) : initialValue; } catch (error) { console.error(error); return initialValue; } });

useEffect(() => { try { window.localStorage.setItem(key, JSON.stringify(value)); } catch (error) { console.error(error); } }, [key, value]);

return [value, setValue]; }

// Usage function Settings() { const [theme, setTheme] = useLocalStorage('theme', 'light'); const [language, setLanguage] = useLocalStorage('language', 'en');

return ( <div> <select value={theme} onChange={e => setTheme(e.target.value)}> <option value="light">Light</option> <option value="dark">Dark</option> </select> <select value={language} onChange={e => setLanguage(e.target.value)}> <option value="en">English</option> <option value="es">Spanish</option> </select> </div> ); }

Debounce Hook

import { useState, useEffect } from 'react';

export function useDebounce(value, delay = 500) { const [debouncedValue, setDebouncedValue] = useState(value);

useEffect(() => { const handler = setTimeout(() => { setDebouncedValue(value); }, delay);

return () => {
  clearTimeout(handler);
};

}, [value, delay]);

return debouncedValue; }

// Usage function SearchComponent() { const [searchTerm, setSearchTerm] = useState(''); const debouncedSearchTerm = useDebounce(searchTerm, 500);

useEffect(() => { if (debouncedSearchTerm) { // Perform search console.log('Searching for:', debouncedSearchTerm); } }, [debouncedSearchTerm]);

return ( <input type="text" value={searchTerm} onChange={e => setSearchTerm(e.target.value)} placeholder="Search..." /> ); }

Window Size Hook

import { useState, useEffect } from 'react';

export function useWindowSize() { const [windowSize, setWindowSize] = useState({ width: undefined, height: undefined, });

useEffect(() => { function handleResize() { setWindowSize({ width: window.innerWidth, height: window.innerHeight, }); }

handleResize(); // Set initial size
window.addEventListener('resize', handleResize);

return () => window.removeEventListener('resize', handleResize);

}, []);

return windowSize; }

// Usage function ResponsiveComponent() { const { width, height } = useWindowSize();

return ( <div> <p>Window width: {width}px</p> <p>Window height: {height}px</p> {width < 768 ? <MobileView /> : <DesktopView />} </div> ); }

Online Status Hook

import { useState, useEffect } from 'react';

export function useOnlineStatus() { const [isOnline, setIsOnline] = useState(navigator.onLine);

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 ( <div className={isOnline ? 'online' : 'offline'}> {isOnline ? '✅ Online' : '❌ Offline'} </div> ); }

Component Patterns

Compound Components

Build components that work together while sharing implicit state.

import { createContext, useContext, useState } from 'react';

const TabsContext = createContext();

export function Tabs({ children, defaultValue }) { const [activeTab, setActiveTab] = useState(defaultValue);

return ( <TabsContext.Provider value={{ activeTab, setActiveTab }}> <div className="tabs">{children}</div> </TabsContext.Provider> ); }

export function TabList({ children }) { return <div className="tab-list">{children}</div>; }

export function Tab({ value, children }) { const { activeTab, setActiveTab } = useContext(TabsContext); const isActive = activeTab === value;

return ( <button className={tab ${isActive ? 'active' : ''}} onClick={() => setActiveTab(value)} > {children} </button> ); }

export function TabPanels({ children }) { return <div className="tab-panels">{children}</div>; }

export function TabPanel({ value, children }) { const { activeTab } = useContext(TabsContext);

if (activeTab !== value) return null;

return <div className="tab-panel">{children}</div>; }

// Usage function App() { return ( <Tabs defaultValue="tab1"> <TabList> <Tab value="tab1">Tab 1</Tab> <Tab value="tab2">Tab 2</Tab> <Tab value="tab3">Tab 3</Tab> </TabList>

  &#x3C;TabPanels>
    &#x3C;TabPanel value="tab1">Content for Tab 1&#x3C;/TabPanel>
    &#x3C;TabPanel value="tab2">Content for Tab 2&#x3C;/TabPanel>
    &#x3C;TabPanel value="tab3">Content for Tab 3&#x3C;/TabPanel>
  &#x3C;/TabPanels>
&#x3C;/Tabs>

); }

Render Props

Share code between components using a prop whose value is a function.

function DataFetcher({ url, render }) { const [data, setData] = useState(null); const [loading, setLoading] = useState(true);

useEffect(() => { fetch(url) .then(res => res.json()) .then(data => { setData(data); setLoading(false); }); }, [url]);

return render({ data, loading }); }

// Usage function App() { return ( <DataFetcher url="/api/users" render={({ data, loading }) => ( loading ? <div>Loading...</div> : ( <ul> {data?.map(user => ( <li key={user.id}>{user.name}</li> ))} </ul> ) )} /> ); }

Higher-Order Components (HOC)

Wrap components to enhance functionality.

import { useEffect, useState } from 'react';

// HOC for data fetching function withData(Component, url) { return function WithDataComponent(props) { const [data, setData] = useState(null); const [loading, setLoading] = useState(true);

useEffect(() => {
  fetch(url)
    .then(res => res.json())
    .then(data => {
      setData(data);
      setLoading(false);
    });
}, []);

return &#x3C;Component {...props} data={data} loading={loading} />;

}; }

// Original component function UserList({ data, loading }) { if (loading) return <div>Loading...</div>;

return ( <ul> {data?.map(user => ( <li key={user.id}>{user.name}</li> ))} </ul> ); }

// Enhanced component const UserListWithData = withData(UserList, '/api/users');

// Usage function App() { return <UserListWithData />; }

Container/Presenter Pattern

Separate logic from presentation.

// Container (logic) function UserListContainer() { const [users, setUsers] = useState([]); const [loading, setLoading] = useState(true); const [searchTerm, setSearchTerm] = useState('');

useEffect(() => { fetchUsers().then(data => { setUsers(data); setLoading(false); }); }, []);

const filteredUsers = users.filter(user => user.name.toLowerCase().includes(searchTerm.toLowerCase()) );

return ( <UserListPresenter users={filteredUsers} loading={loading} searchTerm={searchTerm} onSearchChange={setSearchTerm} /> ); }

// Presenter (UI) function UserListPresenter({ users, loading, searchTerm, onSearchChange }) { if (loading) return <div>Loading...</div>;

return ( <div> <input type="text" value={searchTerm} onChange={e => onSearchChange(e.target.value)} placeholder="Search users..." /> <ul> {users.map(user => ( <li key={user.id}>{user.name}</li> ))} </ul> </div> ); }

Performance Optimization

Identifying Performance Issues

Use React DevTools Profiler to identify:

Components re-rendering unnecessarily
Expensive render operations
State update cascades

Optimization Strategies

Memoization

import { memo, useMemo, useCallback } from 'react';

const ExpensiveList = memo(function ExpensiveList({ items, onItemClick }) { console.log('Rendering ExpensiveList');

return ( <ul> {items.map(item => ( <li key={item.id} onClick={() => onItemClick(item.id)}> {item.name} </li> ))} </ul> ); });

function App() { const [items, setItems] = useState([]); const [count, setCount] = useState(0);

// Memoize expensive computation const sortedItems = useMemo(() => { console.log('Sorting items...'); return [...items].sort((a, b) => a.name.localeCompare(b.name)); }, [items]);

// Memoize callback const handleItemClick = useCallback((id) => { console.log('Item clicked:', id); }, []);

return ( <div> <button onClick={() => setCount(c => c + 1)}>Count: {count}</button> <ExpensiveList items={sortedItems} onItemClick={handleItemClick} /> </div> ); }

Code Splitting

import { lazy, Suspense } from 'react';

// Lazy load components const HeavyComponent = lazy(() => import('./HeavyComponent')); const Dashboard = lazy(() => import('./Dashboard'));

function App() { return ( <Suspense fallback={<div>Loading...</div>}> <HeavyComponent /> </Suspense> ); }

// Route-based splitting import { BrowserRouter, Routes, Route } from 'react-router-dom';

function App() { return ( <BrowserRouter> <Suspense fallback={<div>Loading...</div>}> <Routes> <Route path="/" element={<Home />} /> <Route path="/dashboard" element={<Dashboard />} /> </Routes> </Suspense> </BrowserRouter> ); }

Virtualization

import { FixedSizeList } from 'react-window';

function VirtualizedList({ items }) { const Row = ({ index, style }) => ( <div style={style}> {items[index].name} </div> );

return ( <FixedSizeList height={500} itemCount={items.length} itemSize={50} width="100%" > {Row} </FixedSizeList> ); }

Avoid Anonymous Functions

// ❌ Bad - creates new function on every render function List({ items }) { return ( <ul> {items.map(item => ( <li key={item.id} onClick={() => console.log(item.id)}> {item.name} </li> ))} </ul> ); }

// ✅ Good - use useCallback function List({ items }) { const handleClick = useCallback((id) => { console.log(id); }, []);

return ( <ul> {items.map(item => ( <li key={item.id} onClick={() => handleClick(item.id)}> {item.name} </li> ))} </ul> ); }

Server Components (RSC)

React Server Components render on the server, reducing client bundle size.

Server Component

// app/users/page.js (Server Component by default) async function UsersPage() { // Fetch data directly on server const users = await db.users.findAll();

return ( <div> <h1>Users</h1> <ul> {users.map(user => ( <li key={user.id}>{user.name}</li> ))} </ul> </div> ); }

export default UsersPage;

Client Component

// components/Counter.js "use client"; // Mark as client component

import { useState } from 'react';

export default function Counter() { const [count, setCount] = useState(0);

return ( <button onClick={() => setCount(c => c + 1)}> Count: {count} </button> ); }

Composition Pattern

// app/page.js (Server Component) import Counter from '@/components/Counter'; // Client Component

async function HomePage() { const data = await fetchData(); // Server-side data fetching

return ( <div> <h1>{data.title}</h1> <p>{data.description}</p> <Counter /> {/* Interactive client component */} </div> ); }

export default HomePage;

State Management Strategies

Local State (useState)

Best for: Component-specific state

function Counter() { const [count, setCount] = useState(0); return <button onClick={() => setCount(c => c + 1)}>{count}</button>; }

Lifted State

Best for: Shared state between sibling components

function Parent() { const [sharedValue, setSharedValue] = useState('');

return ( <> <ChildA value={sharedValue} onChange={setSharedValue} /> <ChildB value={sharedValue} /> </> ); }

Context API

Best for: App-wide state (theme, auth, language)

const AuthContext = createContext();

function AuthProvider({ children }) { const [user, setUser] = useState(null);

const login = async (credentials) => { const user = await api.login(credentials); setUser(user); };

const logout = () => setUser(null);

return ( <AuthContext.Provider value={{ user, login, logout }}> {children} </AuthContext.Provider> ); }

useReducer + Context

Best for: Complex state logic with multiple actions

const StateContext = createContext(); const DispatchContext = createContext();

function StoreProvider({ children }) { const [state, dispatch] = useReducer(reducer, { items: [] });

return ( <StateContext.Provider value={state}> <DispatchContext.Provider value={dispatch}> {children} </DispatchContext.Provider> </StateContext.Provider> ); }

Best Practices

Component Design

Keep components small and focused (Single Responsibility)
Extract reusable logic into custom hooks
Prefer composition over inheritance
Use prop spreading sparingly
Define PropTypes or TypeScript types

State Management

Keep state as local as possible
Lift state only when necessary
Avoid unnecessary state (derive from props when possible)
Use reducers for complex state logic
Batch state updates when possible

Performance

Don't optimize prematurely - measure first
Use React DevTools Profiler
Memoize expensive computations
Avoid creating objects/arrays in render
Use key prop correctly for lists

Effects

Keep effects focused (one concern per effect)
Always specify dependencies
Clean up effects (return cleanup function)
Use AbortController for fetch requests
Avoid race conditions with ignore flags

Code Organization

src/ ├── components/ │ ├── common/ # Reusable UI components │ ├── features/ # Feature-specific components │ └── layout/ # Layout components ├── hooks/ # Custom hooks ├── context/ # Context providers ├── utils/ # Utility functions ├── services/ # API services └── types/ # TypeScript types

Common Anti-Patterns to Avoid

Mutating State

// ❌ Wrong const handleAdd = () => { items.push(newItem); setItems(items); };

// ✅ Correct const handleAdd = () => { setItems([...items, newItem]); };

Missing Dependencies

// ❌ Wrong useEffect(() => { console.log(userId); }, []); // Missing userId dependency

// ✅ Correct useEffect(() => { console.log(userId); }, [userId]);

Conditional Hooks

// ❌ Wrong if (condition) { const [state, setState] = useState(0); }

// ✅ Correct const [state, setState] = useState(0); if (condition) { // Use state }

Creating Components Inside Components

// ❌ Wrong function Parent() { function Child() { // Re-created on every render return <div>Child</div>; } return <Child />; }

// ✅ Correct function Child() { return <div>Child</div>; }

function Parent() { return <Child />; }

Troubleshooting

Common Issues

Issue: Infinite re-render loop

Check useEffect dependencies
Avoid setting state directly in render
Use functional updates: setState(prev => prev + 1)

Issue: Stale closure

Use functional updates in setState
Add missing dependencies to useEffect
Use useRef for mutable values

Issue: Component not re-rendering

Check if state is being mutated (use immutable updates)
Verify React.memo comparison function
Ensure new reference for objects/arrays

Issue: Memory leak warning

Clean up effects (return cleanup function)
Cancel ongoing requests on unmount
Clear timers/intervals

Resources

React Documentation: https://react.dev
React Hooks: https://react.dev/reference/react
React Server Components: https://react.dev/reference/rsc
React DevTools: https://react.dev/learn/react-developer-tools
Patterns: https://patterns.dev/react

Skill Version: 1.0.0 Last Updated: October 2025 Skill Category: Frontend Development, React, JavaScript Compatible With: React 18+, Next.js 13+, TypeScript

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