firebase-messaging-sw.js

// [START initialize_firebase_in_sw]
 // 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': '13971250415'});
 // Retrieve an instance of Firebase Messaging so that it can handle background
 // messages.
 const messaging = firebase.messaging();
 // [END initialize_firebase_in_sw]


// If you would like to customize notifications that are received in the
// background (Web app is closed or not in browser focus) then you should
// implement this optional method.
// [START background_handler]
messaging.setBackgroundMessageHandler((payload) => {
  console.log('[firebase-messaging-sw.js] Received background message ', payload);
  // Customize notification here
  const notificationTitle = 'Background Message Title';
  const notificationOptions = {
    body: 'Background Message body.'
    // icon: '/firebase-logo.png'
  };

  return self.registration.showNotification(notificationTitle,
      notificationOptions);
});
// [END background_handler]