如何从Mongoose对象中查询数据？

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

const mongoose = require('mongoose');
const Schema = mongoose.Schema;

const userSchema = new Schema({
  name: String,
  age: Number
});

const User = mongoose.model('User', userSchema);

User.find({}, (err, users) => {
  if (err) {
    console.error(err);
  } else {
    console.log(users);
  }
});

User.find({ age: { $gte: 18 } }, (err, users) => {
  if (err) {
    console.error(err);
  } else {
    console.log(users);
  }
});

User.find({ age: { $gte: 18 } }).select('name').exec((err, users) => {
  if (err) {
    console.error(err);
  } else {
    console.log(users);
  }
});

User.find({ age: { $gte: 18 } }).sort('-age').exec((err, users) => {
  if (err) {
    console.error(err);
  } else {
    console.log(users);
  }
});