useLogin.test.tsx

it("should return the same user-information upon rerender", async () => {
  // given: a successful login
  const { result, rerender } = renderHook(() => useLogin());

  act(() => result.current.onLogin("admin", "admin"));

  await waitFor(() => expect(result.current.isLoginError).toBeFalsy());
  await waitFor(() => expect(result.current.loggedInUsername).toEqual("admin"));

  // when: re-rendering the component
  rerender();

  // then: the user-information is saved
  expect(result.current.loggedInUsername).toBe("admin");
  expect(result.current.isLoginError).toBeFalsy();
});