// Server Component (default) — async, fetches data
async function ProductPage({ id }: { id: string }) {
  const product = await db.product.findUnique({ where: { id } });

  return (
    <div>
      <h1>{product.name}</h1>
      <AddToCartButton productId={product.id} />
    </div>
  );
}

// Client Component — handles interactivity
'use client';
function AddToCartButton({ productId }: { productId: string }) {
  const [isPending, startTransition] = useTransition();

  const handleAdd = () => {
    startTransition(async () => {
      await addToCart(productId);
    });
  };

  return (
    <button onClick={handleAdd} disabled={isPending}>
      {isPending ? 'Adding...' : 'Add to Cart'}
    </button>
  );
}

'use client';
import { useOptimistic } from 'react';

function TodoList({ todos, addTodo }: { todos: Todo[]; addTodo: (t: Todo) => Promise<void> }) {
  const [optimisticTodos, addOptimisticTodo] = useOptimistic(
    todos,
    (state, newTodo: Todo) => [...state, { ...newTodo, pending: true }]
  );

  const handleSubmit = async (formData: FormData) => {
    const newTodo = { id: Date.now(), text: formData.get('text') as string };
    addOptimisticTodo(newTodo);  // Immediate UI update
    await addTodo(newTodo);      // Actual backend call
  };

  return (
    <form action={handleSubmit}>
      {optimisticTodos.map(todo => (
        <div key={todo.id} style={{ opacity: todo.pending ? 0.5 : 1 }}>
          {todo.text}
        </div>
      ))}
      <input type="text" name="text" />
      <button type="submit">Add</button>
    </form>
  );
}

// app/actions.ts
'use server';
import { z } from 'zod';
import { revalidatePath } from 'next/cache';

const schema = z.object({
  title: z.string().min(5),
  content: z.string().min(10),
});

export async function createPost(prevState: any, formData: FormData) {
  const parsed = schema.safeParse({
    title: formData.get('title'),
    content: formData.get('content'),
  });

  if (!parsed.success) {
    return { errors: parsed.error.flatten().fieldErrors };
  }

  await db.post.create({ data: parsed.data });
  revalidatePath('/posts');
  return { success: true };
}

// app/blog/new/page.tsx
'use client';
import { useActionState } from 'react';
import { createPost } from '../actions';

export default function NewPostPage() {
  const [state, formAction, pending] = useActionState(createPost, {});

  return (
    <form action={formAction}>
      <input name="title" placeholder="Title" />
      {state.errors?.title && <span>{state.errors.title[0]}</span>}
      <textarea name="content" placeholder="Content" />
      <button type="submit" disabled={pending}>
        {pending ? 'Publishing...' : 'Publish'}
      </button>
    </form>
  );
}

export 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;
}

function SearchableList({ items }: { items: Item[] }) {
  const [query, setQuery] = useState('');
  const [isPending, startTransition] = useTransition();
  const [filteredItems, setFilteredItems] = useState(items);

  const handleChange = (e: React.ChangeEvent<HTMLInputElement>) => {
    setQuery(e.target.value);
    startTransition(() => {
      setFilteredItems(items.filter(i => i.name.toLowerCase().includes(e.target.value.toLowerCase())));
    });
  };

  return (
    <div>
      <input value={query} onChange={handleChange} />
      {isPending && <span>Filtering...</span>}
      <ul>{filteredItems.map(i => <li key={i.id}>{i.name}</li>)}</ul>
    </div>
  );
}