Unable to send chrome notification

I'm trying to send a Chrome notification from my server, but I'm getting this message from the node sdk:

Promise { <pending> }

This is my code:

const webpush = require('web-push');

var public_key = 'xxx';
var private_key = 'xxx';


webpush.setVapidDetails(
  'mailto:example@gmail.com',
  public_key,
  private_key
);


var Subscription = {
  endpoint: 'XXXX',
  keys: {
    p256dh: 'XXX',
    auth: 'XXX=='
  }
};


var options = "Yay it works."


webpush.setGCMAPIKey('XXXX');
webpush.sendNotification(Subscription, options);

And this is my service worker code:

'use strict';


self.addEventListener('push', function(event) {
  console.log('[Service Worker] Push Received.');
  console.log(`[Service Worker] Push had this data: "${event.data.text()}"`);

  const title = 'Push Codelab';
  const options = {
    body: 'Yay it works.',
    icon: '../images/icon.png',
    badge: '../images/badge.png'
  };

  const notificationPromise = self.registration.showNotification(title, options);
  event.waitUntil(notificationPromise);
});

via Filipe Ferminiano