TypeError: Router.use() requires middleware function but got a Object in recent Express

Question

First I am aware that the same exact questions appears here in Stackoverflow, so please bear with me.

So I am using node.js, express and mongoose and I get this error whenever I try to start the server using

node app

The error is

TypeError: Router.use() requires middleware function but got a Object

And tracing the error leads me to line 32 in app.js

TraceMSG

at Object.<anonymous> (C:\Proj-Phase2\app.js:32:5)

Line32:

let authRouter = require('./server/routes/authRoutes');

Now there is a similar question on stackoverflow

TypeError: Router.use() requires middleware function but got a Object

however the solution suggests a replacment of my line to be

app.use(app.router);
routes.initialize(app);

However this is already deprecated.

I printed my express version by printing the command

npm list express

And the result was

    ├── express@4.13.3
└─┬ lite-server@1.3.2
  └─┬ browser-sync@2.10.1
    └─┬ browser-sync-ui@0.5.16
      └─┬ weinre@2.0.0-pre-I0Z7U9OV
        └── express@2.5.11

AND here my app.js for Your reference

    "use strict";

let express = require('express'),
    mongoose = require('mongoose'),
    bodyParser = require('body-parser'),
    open = require('open');

let dbConnection = mongoose.connect('mongodb://localhost/eRubric', function(err) {
    if(!err) {
        let instructorRepository = require('./server/models/instructorRepository');
        instructorRepository.initDb();
    }
});

//Initialize App
let app = express();
//Allow serving static files
app.use(express.static(__dirname));

//Define port
let port = process.env.PORT || 9080;

app.use(bodyParser.urlencoded({extended:true}));
//aut-deserialize the body of incoming request to a json object
app.use(bodyParser.json());


//Router Config
let authRouter = require('./server/routes/authRoutes');

//Replacement for recent express versions
app.use('/api/instructor/auth', authRouter);
//app.use(app.router);
//authRouter.initialize(app);

app.listen(port, function(){
    console.log('Hero App is running my app on http://localhost:' + port);
    //open('http://localhost:' + port);
});

AND HERE IS MY ROUTES file

"use strict";
let express = require('express');
let authController = require('../controllers/authController');

let authRouter = express.Router();

authRouter.route('/')
    .post((req, res) => authController.authInstrucotr(req, res));