如何计算mongoose中的平均值？

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/mydatabase', { useNewUrlParser: true });

const mySchema = new mongoose.Schema({
  name: String,
  age: Number,
  score: Number
});

const MyModel = mongoose.model('MyModel', mySchema);

MyModel.aggregate([
  {
    $group: {
      _id: null,
      averageScore: { $avg: '$score' }
    }
  }
]).exec((err, result) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log('Average score:', result[0].averageScore);
});