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I am building a REST API service with Node, Express and MongoDB. I installed MongoDB and it runs normally on my PC on localhost:27017. I can add collections and read them. In my app.js file I have this setup

var express = require('express');
var mongoose = require('mongoose');
var bodyParser = require('body-parser');
mongoose.connect('mongodb://127.0.0.1:27017/bookAPI');
var db = mongoose.connection;
db.on('error', console.error.bind(console, 'connection error:'));
db.once('open', function callback () {
    console.log("h");
var app = express();
app.use(bodyParser.urlencoded({ extended: true }));
app.use(bodyParser.json());
var port = process.env.PORT || 3000;
app.use('/api', require('./routes/api.js'));
app.listen(port, function() {
    console.log('Running on port ' + port);
I always get an error- MongoError - cannot connect UNKNOWN.
I've searched for hours and didn't find any solution. How can I fix it so it can connect to MongoDB, which is working properly...?
                Do you have the mongod instance running on the same machine? Also, try mongodb://localhost:27017/bookAPI
– snozza
                Jul 11, 2015 at 22:04
                I do have mongod running and also inserted some dummy data. I tested the same instanse on a different PC with Win7 installed and it worked. I found out later that XP is not supported by mongodb in the new releases.
– Georgi Arnaudov
                Jul 12, 2015 at 13:47
// Data connection
mongoose.connect(dbpath, {user: 'username', pass: 'password', useUnifiedTopology: true , useNewUrlParser: true })
  .then(()=> console.log("Now connected to MongoDB!"))
  .catch(err=> console.error("Something went wrong", err));
I was using Compass and this piece of code works perfectly fine.
                Found the issue!  Mongoose docs have you put the username and password into the dbpath, adding them into options worked like a charm.
– Mark
                Dec 18, 2019 at 21:21
var db = mongoose.connection;
db.on('error', console.error.bind(console, 'connection error:'));
db.once('open', function callback () {
  console.log("h");
Into:
var appConnection = mongoose.createConnection('mongodb://127.0.0.1:27017/bookAPI');
appConnection.on('error', console.error.bind(console, 'connection error:'));
appConnection.once('open', function callback () {
  console.log("h");
var connectionString = "mongodb://" + host + ":" + dport + "/" + dbName;
mongoose.connect(connectionString, function(err) {
    if (err) {
        console.log(err)
    } else {
        console.log('Connected to database ' +dbName);
        
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