firebase-messaging-sw.js

// Give the service worker access to Firebase Messaging.
// Note that you can only use Firebase Messaging here, other Firebase libraries
// are not available in the service worker.
importScripts("https://www.gstatic.com/firebasejs/4.8.1/firebase-app.js");
importScripts("https://www.gstatic.com/firebasejs/4.8.1/firebase-messaging.js");

// Initialize the Firebase app in the service worker by passing in the
// messagingSenderId.
firebase.initializeApp({
  messagingSenderId: '940866661647'
});

// Retrieve an instance of Firebase Messaging so that it can handle background
// messages.
const messaging = firebase.messaging();