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PublishedSeptember 30, 2026 at 10:36 PM

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version: "1.0.1" name: react-patterns description: Modern React development with hooks, component patterns, state management, and performance optimization for building scalable applications

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:

javascript

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

Basic Example:

javascript

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:

javascript

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:

javascript

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:

javascript

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

Task Manager Example:

javascript

import { useReducer } from 'react';
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 =>
        t.id === action.task.id ? action.task : 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, []);
  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:

javascript

const value = useContext(SomeContext);

Theme Context Example:

javascript

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:

javascript

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:

javascript

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

Data Fetching Example:

javascript

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:

javascript

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:

javascript

// 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:

javascript

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:

javascript

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

Example:

javascript

import { useMemo } from 'react';
function TodoList({ todos, filter }) {
  // Only recompute when todos or filter changes
  const visibleTodos = useMemo(() => {
    console.log('Filtering todos...');
    return todos.filter(todo => {
      if (filter === 'all') return true;
      if (filter === 'active') return !todo.done;
      if (filter === 'completed') return todo.done;
      return true;
    });
  }, [todos, filter]);
  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):

javascript

// ❌ 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:

javascript

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

Example:

javascript

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:

javascript

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

Example:

javascript

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:

javascript

const ref = useRef(initialValue);

DOM Reference Example:

javascript

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:

javascript

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:

javascript

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:

javascript

const [isPending, startTransition] = useTransition();

Example:

javascript

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:

javascript

const deferredValue = useDeferredValue(value);

Example:

javascript

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:

javascript

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

Example:

javascript

"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):

javascript

"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:

javascript

useCustomHook // ✅ Correct
customHook    // ❌ Wrong

Data Fetching Hook

javascript

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

javascript

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

javascript

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

javascript

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

javascript

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

javascript

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.

javascript

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>
      <TabPanels>
        <TabPanel value="tab1">Content for Tab 1</TabPanel>
        <TabPanel value="tab2">Content for Tab 2</TabPanel>
        <TabPanel value="tab3">Content for Tab 3</TabPanel>
      </TabPanels>
    </Tabs>
  );
}

Render Props

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

javascript

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.

javascript

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 <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.

javascript

// 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

1. Memoization

javascript

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>
  );
}

2. Code Splitting

javascript

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>
  );
}

3. Virtualization

javascript

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>
  );
}

4. Avoid Anonymous Functions

javascript

// ❌ 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

javascript

// 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

javascript

// 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

javascript

// 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

javascript

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

javascript

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)

javascript

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

javascript

const StateContext = createContext();
const DispatchContext = createContext();
function reducer(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) };
    default:
      return state;
  }
}
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

1. 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

2. 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

3. 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

4. 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

5. 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

1. Mutating State

javascript

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

2. Missing Dependencies

javascript

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

3. Conditional Hooks

javascript

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

4. Creating Components Inside Components

javascript

// ❌ 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

← v1.0.0 All versions