使用Mongoose查找所有不具有未定义元素的文档

const mongoose = require('mongoose');

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

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

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

User.find({ name: { $exists: true }, age: { $exists: true }, email: { $exists: true } }, (err, users) => {
  if (err) {
    console.error(err);
  } else {
    console.log(users);
  }
});

mongoose.disconnect();