pas2js/packages/firebase/js/firebase-messaging-sw.js

importScripts('https://www.gstatic.com/firebasejs/9.2.0/firebase-app-compat.js');
importScripts('https://www.gstatic.com/firebasejs/9.2.0/firebase-messaging-compat.js');
importScripts('config.js');

firebase.initializeApp(firebaseConfig);

const messaging = firebase.messaging();

// 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.
// Keep in mind that FCM will still show notification messages automatically 
// and you should use data messages for custom notifications.
// For more info see: 
// https://firebase.google.com/docs/cloud-messaging/concept-options
messaging.onBackgroundMessage(function(payload) {
  console.log('[firebase-messaging-sw.js] Received background message ', payload);
  // Customize notification here
  const notificationTitle = 'We received a message';
  self.registration.showNotification(notificationTitle,payload.notification);
});