Creating test doubles

There are more tests we could write for the games service, but let's look at a different module for now. How would we go about testing our users middleware? The following code is from middleware/users.js:

module.exports = function(req, res, next) {
    let userId = req.cookies.userId;
    if (!userId) {
        userId = uuid.v4();
        res.cookie('userId', userId);
    }
    req.user = {
        id: userId
    };
    next();
};

In order to test this class, we will need to pass in arguments for the req, res, and next parameters with which our code interacts. We don't have a real request, response, or middleware pipeline available, so we need to create some stand-in values instead. Stand-in values such as this are generally called test doubles. Our code reads an attribute from the request and calls the cookie method on the response. We can create test doubles for these as follows, in a new test script under test/middleware/users.js:

'use strict';

const middleware = require...