login-test.js

describe('when user was successfully logged in', () => {
    const externalResponseHeaders = {
      'set-cookie': 'valid-cookie-from-external-endpoint',
    };

    beforeEach(async () => {
      simulateSucessfulLoginResponse(externalResponseHeaders);  // implementation should not be relevant for this demonstration
      await executeAction();  // implementation should not be relevant for this demonstration
    });

    it('should send OK with correct headers', () => {
      expect(response.status).toHaveBeenCalledWith(200);
      expect(response.header).toHaveBeenCalledWith('set-cookie', externalResponseHeaders['set-cookie']);
      expect(response.send).toHaveBeenCalledTimes(1);
    });

});