Connect a database to an API

This is the shape of nearly every backend you'll be asked to debug: a route receives a request, a controller talks to the model, and the response carries the right status code.

POST /todos  →  createTodo(req, res)  →  Todo.create(...)  →  201 + todo
GET /todos/:id  →  getTodo(req, res)  →  Todo.findById(id)  →  200 / 404 / 400

A controller, step by step

export async function getTodo(req, res) {
  const { id } = req.params;
  if (!mongoose.isValidObjectId(id))          // 1. reject garbage early
    return res.status(400).json({ error: "bad id" });
  const todo = await Todo.findById(id);       // 2. ask the database (await!)
  if (!todo)                                  // 3. nothing there
    return res.status(404).json({ error: "Todo not found" });
  res.json(todo);                             // 4. happy path, 200
}

Notice the return before each early response. Without it the function keeps going and tries to answer twice.

Which status?

How the tests talk to your API

The tests use supertest: request(app).post("/todos").send({ title: "x" }) fires a real request at your Express app without starting a server, then check res.status and res.body.