How to connect Node.js to a MongoDB droplet

Hello, I just created a MongoDB database following this tutorial: https://www.digitalocean.com/community/tutorials/how-to-install-and-secure-mongodb-on-ubuntu-16-04 and everything works fine when I’m accessing a database from the mongo shell but I can’t connect to the database using node.js

This is my code:

import mongoose from 'mongoose';
mongoose.connect('mongodb://user:pass@droplet_ip:27017/my_database', (err, res) => {
  if(err) {
    console.log(err);
  }
  else {
    console.log('connected');
  }
})

And this is the error:

{ MongoError: failed to connect to server [droplet_ip:27017] on first connect [MongoError: connection 0 to droplet_ip:27017 timed out]
    at Pool.<anonymous> (/node_modules/mongodb-core/lib/topologies/server.js:336:35)
    at emitOne (events.js:96:13)
    at Pool.emit (events.js:189:7)
    at Connection.<anonymous> (/node_modules/mongodb-core/lib/connection/pool.js:280:12)
    at Object.onceWrapper (events.js:291:19)
    at emitTwo (events.js:106:13)
    at Connection.emit (events.js:192:7)
    at Socket.<anonymous> (/node_modules/mongodb-core/lib/connection/connection.js:197:10)
    at Object.onceWrapper (events.js:291:19)
    at emitNone (events.js:86:13)
    at Socket.emit (events.js:186:7)
    at Socket._onTimeout (net.js:342:8)
    at ontimeout (timers.js:365:14)
    at tryOnTimeout (timers.js:237:5)
    at Timer.listOnTimeout (timers.js:207:5)
  name: 'MongoError',
  message: 'failed to connect to server [droplet_ip:27017] on first connect [MongoError: connection 0 to droplet_ip:27017 timed out]' }

What am I doing wrong?