How do I create a simple server component that can fetch data in NextJS 13?

I am learning NextJS. Currently, I am using NextJS 13 with the new app directory instead of the old pages directory. I have tried various ways of fetching data, but I have not gotten any of them to work. It should be as simple as

src/app/page.tsx:

export interface ToDo {
  userId: number;
  id: number;
  title: string;
  completed: boolean;
}

const getToDos = async (): Promise<ToDo[]> => {
  const res = await fetch('https://jsonplaceholder.typicode.com/todos/1');
  console.log(`Status: ${res.status}`); // prints 'Status: 200'
  return res.json();
}

const Home = async () => {
  const toDos = await getToDos();
  return (
    <ul>
      {toDos.map((todo) => (
        <li key={todo.id}>{todo.title}</li>
      ))}
    </ul>
  );
}

export default Home;

The JSON returned by the endpoint is

{
  "userId": 1,
  "id": 1,
  "title": "delectus aut autem",
  "completed": false
}

This is the output. Notice how I am getting both a 200 OK status and an error message:

Warning: Only plain objects can be passed to Client Components from Server Components. Classes or other objects with methods are not supported.
  <... client={{queryCache: ..., mutationCache: ..., logger: ..., defaultOptions: ..., queryDefaults: ..., mutationDefaults: ..., mountCount: ...}} children=...>
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Warning: Only plain objects can be passed to Client Components from Server Components. Classes or other objects with methods are not supported.
  {queryCache: {listeners: Set, subscribe: function, config: ..., queries: ..., queriesMap: ...}, mutationCache: ..., logger: ..., defaultOptions: ..., queryDefaults: ..., mutationDefaults: ..., mountCount: ...}
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Status: 200

-  ┌ GET / 200 in 606ms
   │
   └──── GET https://jsonplaceholder.typicode.com/todos/1 200 in 67ms (cache: HIT)

- error node_modules/next/dist/compiled/react-server-dom-webpack/cjs/react-server-dom-webpack-server.edge.development.js (1863:12) @ resolveModelToJSON
- error Error: Functions cannot be passed directly to Client Components unless you explicitly expose it by marking it with "use server".
  {listeners: Set, subscribe: function, config: ..., queries: ..., queriesMap: ...}
                              ^^^^^^^^
    at stringify (<anonymous>)
null
- error node_modules/next/dist/compiled/react-server-dom-webpack/cjs/react-server-dom-webpack-server.edge.development.js (1863:12) @ resolveModelToJSON
- error Error: Functions cannot be passed directly to Client Components unless you explicitly expose it by marking it with "use server".
  {listeners: Set, subscribe: function, config: ..., queries: ..., queriesMap: ...}
                              ^^^^^^^^
    at stringify (<anonymous>)
digest: "501055159"
null

If I add "use server" to the beginning of page.tsx, I get a compilation error:

Server Actions require `experimental.serverActions` option to be enabled in your Next.js config: https://nextjs.org/docs/app/building-your-application/data-fetching/server-actions

Ideally I would like to statically generate HTML, but for now I would be grateful for any working fetching code.

Update

One of the previous solutions I tried was to use Tanstack Query. I have now removed all of that code from my layout.tsx. Now I get a different error for this code:

export default async function Home(): Promise<JSX.Element> {
  const todos = await getToDos();
  console.log(todos);
  return (
    <ul>
      {todos.map((todo) => (
        <li key={todo.id}>{todo.title}</li>
      ))}
    </ul>
  );
}

saying

error TypeError: todos.map is not a function

It seems you are trying to pass a function from a server component to a client component, this is not possible unless it is a server action, since server actions are still a feature currently in alpha, you need to enable them by setting experimental.serverActions in your next.config.js file:

module.exports = {
  experimental: {
    serverActions: true,
  },
};

Here is an example of a server side component:

import "server-only";

interface ToDo {
  userId: number;
  id: number;
  title: string;
  completed: boolean;
}

async function getToDos(): Promise<ToDo[]> {
  const res = await fetch("https://jsonplaceholder.typicode.com/todos");
  return await res.json(); // by the way, you were missing an await here
}

export default async function Home(): Promise<JSX.Element> {
  const todos = await getToDos();
  return (
    <ul>
      {todos.map((todo) => (
        <li key={todo.id}>{todo.title}</li>
      ))}
    </ul>
  );
}

I strongly recommend you install the server-only package, so an error is thrown if you try to import a server component into the client side.

As @FabioNettis correctly pointed out, I was trying to fetch a single object, which is not iterable. I had accidentally tried to fetch from the path /todos/1 rather than /todos. So the fetching code was actually correct, although not great. Good error handling would have made debugging easier.