how to mock this CORS call in nodejs express function

I've added CORS module to a google cloud function. I want to be able to mock the request & response to support unit testing it.

So, how would I do either of these?

1. mock the request/response objects better so they don't break the CORS module?
2. Otherwise is there a way to stub the CORS module and still have that stub call the next middleware (using Sinon)? (which is my main code)

Libraries I'm using are sinon and jasmine.

Test Code

fit('should connect to the elastic server', (done) => {
    const req = { query: { text: 'input' }, method: 'GET' };
    const res = {
      statusCode: 200,
      status: function (code) {
        fail(code);
        this.statusCode = code;
        return this;
      },
      send: function (json) {
        console.log(`Responding with ${json} and ${this.statusCode}`);
        done();
      }
    };
    search.pingServer(req, res);
  });

Function under test

import * as cors from 'cors';

export let pingServer = functions.https.onRequest((req, res) => {
  console.log('pinging the server');

  // workaround to enable CORS requests.
  const corsFn = cors({ origin: true });
  corsFn(req, res, function () {
    if (req.method !== 'GET') {
      console.error(new Error(`Invalid request method ${req.method}`));
      res.status(500).send('/errors/invalidRequestMethod');
    }

    Promise.resolve(env.esClient().ping({})
      .then((response) => {
        console.log('Connected');
        res.send(response);
      }))
      .catch((err) => {
        console.error(err);
        res.status(500).send('/errors/esRequestError');
      });
  });
});

via Richard G