FCM

importScripts('https://www.gstatic.com/firebasejs/3.5.2/firebase-app.js');
importScripts('https://www.gstatic.com/firebasejs/3.5.2/firebase-messaging.js');

firebase.initializeApp({
  'messagingSenderId': '509824733250'
});

const messaging = firebase.messaging();

messaging.setBackgroundMessageHandler(function(payload) {
  console.log('[firebase-messaging-sw.js] Received background message ', payload);

  const notificationTitle = 'Background Message Title';
  const notificationOptions = {
    body: 'Background Message body.'
  };

  return self.registration.showNotification(notificationTitle,
      notificationOptions);
});