React hooks - useState, useEffect, and custom hooks

                  import React, { useState, useEffect, useCallback, useMemo, useRef, useContext } from 'react';

// 1. useState - managing component state
function Counter() {
  const [count, setCount] = useState(0);
  const [name, setName] = useState('');

  // Update functions can use previous state
  const increment = () => setCount(prev => prev + 1);
  const decrement = () => setCount(prev => prev - 1);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={increment}>+</button>
      <button onClick={decrement}>-</button>

      <input
        value={name}
        onChange={(e) => setName(e.target.value)}
        placeholder="Enter name"
      />
    </div>
  );
}

// 2. useEffect - side effects
function UserProfile({ userId }) {
  const [user, setUser] = useState(null);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);

  useEffect(() => {
    // Runs after render
    console.log('Component mounted or userId changed');

    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();

    // Cleanup function
    return () => {
      cancelled = true;
      console.log('Cleanup on unmount or before next effect');
    };
  }, [userId]); // Re-run when userId changes

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

  return <div>{user.name}</div>;
}

// 3. useCallback - memoize functions
function TodoList({ todos, onToggle }) {
  const [filter, setFilter] = useState('all');

  // Memoize callback to prevent child re-renders
  const handleToggle = useCallback((id) => {
    onToggle(id);
  }, [onToggle]);

  const filteredTodos = useMemo(() => {
    if (filter === 'active') return todos.filter(t => !t.completed);
    if (filter === 'completed') return todos.filter(t => t.completed);
    return todos;
  }, [todos, filter]);

  return (
    <div>
      <div>
        <button onClick={() => setFilter('all')}>All</button>
        <button onClick={() => setFilter('active')}>Active</button>
        <button onClick={() => setFilter('completed')}>Completed</button>
      </div>
      {filteredTodos.map(todo => (
        <TodoItem key={todo.id} todo={todo} onToggle={handleToggle} />
      ))}
    </div>
  );
}

const TodoItem = React.memo(({ todo, onToggle }) => {
  console.log('TodoItem rendered:', todo.id);
  return (
    <div onClick={() => onToggle(todo.id)}>
      {todo.text}
    </div>
  );
});

// 4. useRef - mutable references
function TextInputWithFocus() {
  const inputRef = useRef(null);
  const renderCount = useRef(0);

  useEffect(() => {
    renderCount.current += 1;
  });

  const focusInput = () => {
    inputRef.current?.focus();
  };

  return (
    <div>
      <input ref={inputRef} type="text" />
      <button onClick={focusInput}>Focus Input</button>
      <p>Render count: {renderCount.current}</p>
    </div>
  );
}

// 5. useContext - accessing context
const ThemeContext = React.createContext('light');

function ThemedButton() {
  const theme = useContext(ThemeContext);

  return (
    <button className={`btn-${theme}`}>
      Themed Button
    </button>
  );
}

// 6. Custom hooks - reusable logic
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 App() {
  const [name, setName] = useLocalStorage('name', 'Anonymous');

  return (
    <input
      value={name}
      onChange={(e) => setName(e.target.value)}
    />
  );
}

// 7. Custom hook - API data fetching
function useFetch(url) {
  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);
        const json = await response.json();

        if (!cancelled) {
          setData(json);
          setError(null);
        }
      } catch (err) {
        if (!cancelled) {
          setError(err);
        }
      } finally {
        if (!cancelled) {
          setLoading(false);
        }
      }
    }

    fetchData();

    return () => {
      cancelled = true;
    };
  }, [url]);

  return { data, loading, error };
}

// Usage
function Users() {
  const { data: users, loading, error } = useFetch('/api/users');

  if (loading) return <div>Loading...</div>;
  if (error) return <div>Error!</div>;

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

// 8. Custom hook - window size
function useWindowSize() {
  const [size, setSize] = useState({
    width: window.innerWidth,
    height: window.innerHeight,
  });

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

    window.addEventListener('resize', handleResize);
    return () => window.removeEventListener('resize', handleResize);
  }, []);

  return size;
}

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

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