how to test fetch

function mockFetch(data) {
  return jest.fn().mockImplementation(() =>
    Promise.resolve({
      ok: true,
      json: () => data
    })
  );
}

test('fetchPerson()', async () => {
  fetch = mockFetch(someJson); // or window.fetch

  const person = await fetchPerson('whatever id');
  expect(person).toEqual(someJson);

  // Make sure fetch has been called exactly once
  expect(fetch).toHaveBeenCalledTimes(1);
});