连接MongoDB出错

[LongCM]

我用作者的代码连接了MongoDB数据库的时候，报错出了“Cannot read property 'on' of undefined” 错误，发现是connection.on监听连接成功或失败的回调时出错了，懵逼ing........查了一下，别人是这样写的，

const mongoose = require('mongoose')
const Schema = mongoose.Schema
mongoose.Promise = global.Promise;
mongoose.connect('mongodb://127.0.0.1:27017/books'); //先连接的
// var db = mongoose.connect('mongodb://127.0.0.1:27017/books');
var db = mongoose.connection;
db.on("error", function (error) {
console.log("数据库连接失败：" + error)
})

db.on("open", function () {
console.log("数据库连接成功")
})

这样才连接成功，作为一个初学者，难道是作者的写法已经是过时的写法吗？为啥作者写法会报错的，求大神给我解惑，谢谢大佬！

[LongCM]

作者是这样写的：

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

mongoose.Promise = global.Promise
const db = mongoose.connect('mongodb://localhost:27017/books')

db.connection.on("error", function (error) {
console.log("数据库连接失败：" + error)
})

db.connection.on("open", function () {
console.log("数据库连接成功")
})

上面写法会报“TypeError: Cannot read property 'on' of undefined”错误，我改了一下，如下：

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

mongoose.Promise = global.Promise;
mongoose.connect('mongodb://localhost:27017/books');
const db = mongoose.connection;

db.on("error", function (error) {
console.log("数据库连接失败：" + error)
})

db.on("open", function () {
console.log("数据库连接成功")
})

就可以运行成功了！！！