function Form() {
  const [firstName, setFirstName] = useState('Taylor');
  const [lastName, setLastName] = useState('Swift');

  // Runs every render - that's fine and intentional
  const fullName = firstName + ' ' + lastName;
  const isValid = firstName.length > 0 && lastName.length > 0;
}

import { useMemo } from 'react';

function TodoList({ todos, filter }) {
  const visibleTodos = useMemo(
    () => getFilteredTodos(todos, filter),
    [todos, filter]
  );
}

console.time('filter');
const visibleTodos = getFilteredTodos(todos, filter);
console.timeEnd('filter');
// If > 1ms, consider memoizing

// Parent passes userId as key
function ProfilePage({ userId }) {
  return (
    <Profile
      userId={userId}
      key={userId}  // Different userId = different component instance
    />
  );
}

function Profile({ userId }) {
  // All state here resets when userId changes
  const [comment, setComment] = useState('');
  const [likes, setLikes] = useState([]);
}

// BAD: Storing object that needs Effect to "adjust"
function List({ items }) {
  const [selection, setSelection] = useState(null);

  useEffect(() => {
    setSelection(null); // Reset when items change
  }, [items]);
}

// GOOD: Store ID, derive object
function List({ items }) {
  const [selectedId, setSelectedId] = useState(null);

  // Derived - no Effect needed
  const selection = items.find(item => item.id === selectedId) ?? null;
}

// Event handler knows exactly what happened
function ProductPage({ product, addToCart }) {
  function handleBuyClick() {
    addToCart(product);
    showNotification(`Added ${product.name}!`);
    analytics.track('product_added', { id: product.id });
  }

  function handleCheckoutClick() {
    addToCart(product);
    showNotification(`Added ${product.name}!`);
    navigateTo('/checkout');
  }
}

function buyProduct() {
  addToCart(product);
  showNotification(`Added ${product.name}!`);
}

function handleBuyClick() { buyProduct(); }
function handleCheckoutClick() { buyProduct(); navigateTo('/checkout'); }

// Instead of manual Effect subscription
function useOnlineStatus() {
  const [isOnline, setIsOnline] = useState(true);

  useEffect(() => {
    function update() { setIsOnline(navigator.onLine); }
    window.addEventListener('online', update);
    window.addEventListener('offline', update);
    return () => {
      window.removeEventListener('online', update);
      window.removeEventListener('offline', update);
    };
  }, []);

  return isOnline;
}

// Use purpose-built hook
import { useSyncExternalStore } from 'react';

function subscribe(callback) {
  window.addEventListener('online', callback);
  window.addEventListener('offline', callback);
  return () => {
    window.removeEventListener('online', callback);
    window.removeEventListener('offline', callback);
  };
}

function useOnlineStatus() {
  return useSyncExternalStore(
    subscribe,
    () => navigator.onLine,      // Client value
    () => true                   // Server value (SSR)
  );
}

// Instead of syncing via Effects between siblings
function Parent() {
  const [value, setValue] = useState('');

  return (
    <>
      <Input value={value} onChange={setValue} />
      <Preview value={value} />
    </>
  );
}

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

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

    fetch(url)
      .then(res => res.json())
      .then(json => {
        if (!ignore) {
          setData(json);
          setError(null);
        }
      })
      .catch(err => {
        if (!ignore) setError(err);
      })
      .finally(() => {
        if (!ignore) setLoading(false);
      });

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

  return { data, error, loading };
}

// Usage
function SearchResults({ query }) {
  const { data, error, loading } = useData(`/api/search?q=${query}`);
}