mongoose.connect not invoking callback after connection to mongodb

I'm trying to make a connection to MongoDB using Mongoose. But it does neither throws a error nor connects to the database. Following is my code.

const express = require('express');
const app = express();
const port = process.env.PORT || 8080;
const mongoose = require('mongoose');

console.log('Hi, there!!');    

mongoose.connect('mongodb://localhost:27017/db_name', (err) => {
 console.log('Callback called');
 if(err) throw err; 
 console.log('Connected to database');
})

In the above code none of the console.log inside the callback do happen. But any placed outside the mongoose connect do work like console.log('Hi, there!!')

via Abhinav Vudya