const mongo = require('mongodb').MongoClient;
const goodUrl = 'mongodb://localhost:27017/constructor1';
const badUrl = 'mongodb://localhost:27017/constructor';

mongo.connect(goodUrl, {useNewUrlParser : true})
.then(client => {
   let db = client.db();
   console.log('good:', typeof db); // 'good: object'
});

mongo.connect(badUrl, {useNewUrlParser : true})
.then(client => {
   let db = client.db();
   console.log('bad:', typeof db); // 'bad: function'
});