Express, req, res, and chaining

Given that Express is my favorite Node.js web application framework, very often I find myself having to unit test handler functions. Normally these functions are simply returning the send or the render methods invocation on the res object. Sometimes a status method is invoked right before the ones mentioned above, to explicitly set the HTTP status code (e.g.: res.status(500).send({ message: ‘Oh Noes!’ });). To be able to test this I’m using the following jest mock:

const res = {
  send: jest.fn(),
  status: jest.fn(() => res)
};

The above enables me to use the req mock and be able to assert against send whether or not status has been invoked before.

I’ve also put together a GitHub Gist that hopefully explains this technique better.

Thanks so much to Scott Lepich for the original idea and to Nick Balestra for inspiring a more compact version of the mock.