What is a server action in Next.js, and what problem does it solve?

A server action is a function marked with a use server directive that runs only on the server but can be called directly from a client component or a form, letting you update data, such as saving a form submission, without needing to manually create a separate API route just to handle that one operation.

'use server';

export async function addTodo(formData) {
  const text = formData.get('text');
  await db.todo.create({ data: { text } });
}