How to test javascript code?

Question

Software testing

Good evening dear users and readers!
I ran into such a problem that I don’t understand how to test the controller function - getTracks. The code for this function is as follows:

var Promise  = require('bluebird');
  var Errors = require('./errors');
  var pageConditions = require('./sequelize').getPageConditions;

  function getTracks(req, res, next) {

    var conditions = pageConditions(req.params.page, 10);
    req.requestedUser.getTracksAndCount(conditions)
      .then(function(result) {
        res.set('X-Total-Count', result.count);
        res.set('X-Offset', conditions.offset);
        res.set('X-Limit', conditions.limit);
        res.json({
          status: 'OK',
          data: result.rows
        });
      })
      .catch(function(err) {
        next(err);
      });
  }

Test code below:

var assert = require('assert');
var sinon = require('sinon');
var Promise = require('bluebird');
var appPath = './app';
var mock = require('mock-require');
var Controller = require(appPath + '/users');

describe('User controller', function() {

  describe('method `getTracks`', function() {

    var controller;
    var getPageConditions = sinon.stub();

    beforeEach(function() {
      getPageConditions = sinon.stub();

      mock(appPath + '/helpers/sequelize', {
        getPageConditions: getPageConditions
      });
      controller = Controller();
    });

    it('response track list if user have tracks by condition', function() {
      var req = {
        params: {
          page: 'fakeValue'
        },
        requestedUser: {
          getTracksAndCount: sinon.stub()
        }
      };
      var res = {
        set: sinon.spy(),
        json: sinon.spy(),
      };
      var next;
      var conditions = {
        offset: 'fakeValueOffset',
        limit: 'fakeValueLimit'
      };
      var result = {
        count: 'fakeValueCount',
        rows: 'fakeValueRows'
      };

      getPageConditions.returns(conditions);
      req.requestedUser.getTracksAndCount.returns(Promise.resolve(result));

      controller.getTracks(req, res, next);
        assert(getPageConditions.called);
        assert(req.requestedUser.getTracksAndCount.calledWith(conditions));
        assert(res.set.calledWith('X-Total-Count', result.count));
        assert(res.set.calledWith('X-Offset', conditions.offset));
        assert(res.set.calledWith('X-Limit', conditions.limit));
    });
  });
});

The test is executed if commented out

assert(res.set.calledWith('X-Total-Count', result.count));
        assert(res.set.calledWith('X-Offset', conditions.offset));
        assert(res.set.calledWith('X-Limit', conditions.limit));

Tell me, how is it possible to test that res was called with these values? I understand that res.set does not work, because

req.requestedUser.getTracksAndCount.returns(Promise.resolve(result));

returns a promise with the result - result and all subsequent actions

res.set('X-Total-Count', result.count);
        res.set('X-Offset', conditions.offset);
        res.set('X-Limit', conditions.limit);
        res.json({
          status: 'OK',
          data: result.rows
        });

are not committed in functions