firebase-messaging-sw.js

importScripts("https://www.gstatic.com/firebasejs/5.7.1/firebase-app.js");
importScripts("https://www.gstatic.com/firebasejs/5.7.1/firebase-messaging.js");

const config = {
  messagingSenderId: "<MESSAGING_SENDER_ID>"
}

firebase.initializeApp(config);

const messaging = firebase.messaging();

messaging.setBackgroundMessageHandler(function(payload) {
  const notification = JSON.parse(payload.data.notification);

  return self.registration.showNotification(notification.title, notification);
});