I have server.js and db.js The db.js file interacts with my database using Mongoose and I use server.js to call functions from 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}`)

                })

            }

        }

    })

}

And my server.js calls three functions from db.js :

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

var db = DB()

db.getPostsAll()

db.newPost()

I don't understand why I get this error :

connection error: { MongoNetworkError: connection 4 to black-test-shard-00-01-ewyaf.mongodb.net:27017 closed

at TLSSocket.<anonymous> (E:HTMLblack-boxnode_modulesmongodb-corelibconnectionconnection.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)]: {} }

What am I doing wrong? I found an article but can't make anything of it.