我有server.js和db.js该db.js文件使用Mongoose与我的数据库交互，并且我习惯于server.js从中调用函数db.js：

var mongoose = require('mongoose');
mongoose.connect('', { useNewUrlParser: true })
var Schema = mongoose.Schema;

module.exports = function () {
    var db = mongoose.connection;
    db.on('error', console.error.bind(console, 'connection error:'));
    return db.once('open', function() {
        console.log("Connected to DB")
        var postschema = new Schema({
            title: String,
            intro: String,
            body: String,
            author: String,
            timestamp: { type: Date, default: Date.now }
        });

        var post = mongoose.model('post', postschema);

        return {
            newPost(title, intro, body, author) {
                var newpost = new post({
                    title: title,
                    intro: intro,
                    body: body,
                    author: author
                })
            },
            getPostsAll() {
                post.find({}, function (err, res) {
                    return (`Error:${err} Posts:${res}`)
                })
            }
        }
    })
}

我的server.js从中调用了三个函数db.js：

var DB = require('./db.js')
var db = DB()
db.getPostsAll()
db.newPost()

我不明白为什么会收到此错误：

connection error: { MongoNetworkError: connection 4 to black-test-shard-00-01-ewyaf.mongodb.net:27017 closed
at TLSSocket.<anonymous> (E:\HTML\black-box\node_modules\mongodb-core\lib\connection\connection.js:276:9)
at Object.onceWrapper (events.js:272:13)
at TLSSocket.emit (events.js:185:15)
at _handle.close (net.js:541:12)
at TCP.done [as _onclose] (_tls_wrap.js:379:7)
  name: 'MongoNetworkError',
  errorLabels: [ 'TransientTransactionError' ],
  [Symbol(mongoErrorContextSymbol)]: {} }

我究竟做错了什么？我找到了一篇文章，但什么也做不了。