mock service worker, sad path test failure

Fixing the mock server

The problem is that the collect function is expecting a JSON response even in case of an error, but your mock server doesn't return that. So, when you do res.json() in your collect function you get an error.

Update your response resolver to return a json response.

return res(ctx.json({message: "error"}), ctx.status(401));

Fixing the test

toThrow is not the correct matcher here, because async functions always return promise and in your case the collect function returns a promise that gets rejected with the thrown data.
So, you can use the toEqual matcher instead.

Also you need to update the way the error is tested. You can opt any of the following options:

Using rejects matcher:

it("handles failure", () => {
  server.use(
    rest.get(
      "http://api.openweathermap.org/data/2.5/weather",
      (req, res, ctx) => {
        return res(ctx.json({message: "error"}), ctx.status(401));
      }
    )
  );
  return expect(collect()).rejects.toEqual({ message: "error", status: 401 });
});

Using async/await syntax:

it("handles failure", async () => {
  server.use(
    rest.get(
      "http://api.openweathermap.org/data/2.5/weather",
      (req, res, ctx) => {
        return res(ctx.status(401));
      }
    )
  );
  try {
    await collect();
  } catch (err) {
    expect(err).toEqual({ message: "error", status: 401 });
  }
});

Using .catch

But in this approach you need to explicitly check that your catch assertion has been called, otherwise a fulfilled promise will not fail your test.

it("handles failure", async () => {
  server.use(
    rest.get(
      "http://api.openweathermap.org/data/2.5/weather",
      (req, res, ctx) => {
        return res(ctx.status(401));
      }
    )
  );
  expect.assertions(1);
  return collect().catch((err) =>
    expect(err).toEqual({ message: "error", status: 401 })
  );
});

Fixing the collect function

In your collect function the status should be res.status and not data.code.

And you can also clean the following code a bit by moving the res.json() call out of the conditional.

require("dotenv").config();

export const collect = async () => {
  const key = process.env.REACT_APP_API_KEY;
  const res = await fetch(
    `http://api.openweathermap.org/data/2.5/weather?q=london&appid=${key}`
  );
  const data = await res.json();
  if (res.status !== 200) {
    throw { message: data.message, status: res.status };
  }
  return data;
};

And also you shouldn't be storing secrets in react environment variables, that would be exposed. Docs