One who has authorization set up for MongoDB (as described here) might then want to write a Node/Express application that somehow involves the use of Mongoose. This requires an extra step not documented in a number of guides on this subject, but is easy to remedy. Connection should look something like this:
const dbURI = 'mongodb://admin:password@localhost/db?authSource=admin';
mongoose.connect(dbURI, {
useMongoClient: true
});