当属性为空时如何触发mongoose默认值

const mongoose = require('mongoose');

const schema = new mongoose.Schema({
  name: {
    type: String,
    default: 'John Doe' // 当name属性为空时，默认值为'John Doe'
  },
  age: {
    type: Number,
    default: function() {
      return Math.floor(Math.random() * 100); // 当age属性为空时，默认值为一个随机数
    }
  }
});

const Model = mongoose.model('Model', schema);

const doc = new Model({});

console.log(doc.name); // 输出: 'John Doe'
console.log(doc.age); // 输出: 一个随机数