Mongodb + express: typeerror: dbcollection is not a function

See more:

database

server

MongoDB

Node.js

I'm trying to follow this tutorial to send form data to mongodb:
Save form data to MongoDB with NodeJS · programming mentor[^]

This is how he connects to mongodb:

var dbConn = mongodb.MongoClient.connect('mongodb://localhost:27017');

This is what he does later in the code:

app.post('/post-feedback', function (req, res) {
    dbConn.then(function(db) {
        delete req.body._id; // for safety reasons
        db.collection('feedbacks').insertOne(req.body);
    });    
    res.send('Data received:\n' + JSON.stringify(req.body));
});

which gives the following error:

TypeError: dbcollection is not a function

It seems that since I'm using version 3.6 of mongo, I need to connect in another way for the code to work. But how?

What I have tried:

I tried to connect like this:

var dbConn = MongoClient.connect('mongodb://localhost', function (err, client) {
  if (err) throw err;
  var db = client.db('mytestingdb');
});

But that just gives me another error (though in a different part of the code):

TypeError: Cannot read property then of undefined