database: {
    mongoURL: 'mongodb://localhost:27017/example',
  },

 database: {
    mongoURL: 'mongodb://localhost:27017/',
    name: 'databaseName',
  },

// Create a MonboDB connection pool and start the Node.js app
MongoClient.connect(config.database.mongoURL, { promiseLibrary: Promise })
  .catch(err => console.error(err.stack))
  .then((db) => {
    app.locals.db = db; // See http://expressjs.com/en/4x/api.html#app.locals
    app.listen(port);
     
     ...

  });

MongoClient.connect(config.database.mongoURL, { promiseLibrary: Promise })
  .catch(err => console.error(err.stack))
  .then((client) => {
    app.locals.db = client.db(config.database.name);
    app.listen(port);

    ...

  });