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/7.5.0/firebase-app.js');
importScripts('https://www.gstatic.com/firebasejs/7.5.0/firebase-messaging.js');

// Initialize the Firebase app in the service worker by passing in the
// messagingSenderId.
firebase.initializeApp({
    'messagingSenderId': '439884301632'
});

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

messaging.setBackgroundMessageHandler(function(payload) {
    console.log('[firebase-messaging-sw.js] Received background message ', payload);
    // Customize notification here
    const {title, body, icon} = payload.data;
    const notificationTitle = title;
    const notificationOptions = {
        body,
        icon
    };

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