Callback hell?

const mongoose = require('mongoose');

mongoose.Promise = global.Promise;
mongoose.connect('mongodb://localhost:27017/treehouse');

const db = mongoose.connection;

db.on('error', (err) => { console.log('Connection Error: ', err); });

// Once the connection opens, the callback will be called
db.once('open', () => {
  console.log('Database connection successful!');

  const Schema = mongoose.Schema;
  const AnimalSchema = new Schema({
    type: { type: String, default: 'goldfish' },
    size: { type: String, default: 'small' },
    color: { type: String, default: 'gold' },
    mass: { type: Number, default: 0.01 },
    name: { type: String, default: 'Angela' }
  });

  const Animal = mongoose.model('Animal', AnimalSchema);

  const elephant = new Animal({
    type: 'elephant',
    size: 'big',
    color: 'gray',
    mass: 6000,
    name: 'Lawrence'
  });

  // Color supplied by defaults
  const whale = new Animal({
    type: 'whale',
    size: 'big',
    mass: 190000,
    name: 'Fig'
  });

  // All values supplied by defaults
  const animal = new Animal({});

  (async function () {
    try {
      // Remove all documents
      console.log('Deleting existing documents...');
      await Animal.remove({});

      // Save all animals
      console.log('Saving new documents...');
      await Promise.all([elephant.save(), whale.save(), animal.save()]);

      // Log each animal with a size of 'big'
      console.log('\n' + 'Big animals:');
      const animals = await Animal.find({ size: 'big' });
      animals.forEach((item) => { console.log(item); });

      // Close the database connection
      db.close();
      console.log('\n' + 'Database connection closed.');

    // Catch errors here
    } catch (err) {
      console.log(err);
      db.close();
    }
  }());
});