Stubs are functions that simulate the behavior of another component.

In the context of our tests, we don't really care about the returned value of res.json() – we only care that our checkEmptyPayload middleware function relays this value back faithfully. Therefore, we can turn our res.json spy into a stub, and make it return a reference to an object:

resJsonReturnValue = {};
res = {
  status: spy(),
  set: spy(),
  json: stub().returns(resJsonReturnValue),
};

We can then add another assertion step to compare the value returned by the checkEmptyPayload function, and the value returned by our res.json stub; they should be strictly identical:

describe('and the content-length header is "0"', function () {
  let resJsonReturnValue;
  let returnedValue;

  beforeEach(function () {
    ...
    returnedValue = checkEmptyPayload(req, res, next);
  });

  ...

  it('should return whatever res.json() returns', function () {
    assert.strictEqual(returnedValue, resJsonReturnValue);
  });

  ...
});

Run the unit tests by executing yarn run test:unit, fix any errors that cause the tests to fail, and then commit the unit tests to the repository:

$ git add -A && git commit -m "Add unit tests for checkEmptyPayload middleware"