Connect to database

const connect_to_db = () => {
        const mongoose = require('mongoose');
        mongoose.connect('mongodb://localhost:27017/<DB_NAME>', {useNewUrlParser: true, useUnifiedTopology: true});
        const db = mongoose.connection;
        db.on('error', console.error.bind(console, 'connection error:'));
        db.once('open', console.log.bind(console, 'Connected to db'));
    }