text: importScripts('https://www.gstatic.com/firebasejs/4.2.0/firebase-app.js');
  importScripts('https://www.gstatic.com/firebasejs/4.2.0/firebase-messaging.js');
  
  var config = {
    apiKey: 'AIzaSyD8cVGW-e4zCLa98ivZmBp42VpFP53_dvg',
    authDomain: 'natural-broker-164116.firebaseapp.com',
    databaseURL: 'https://natural-broker-164116.firebaseio.com',
    projectId: 'natural-broker-164116',
    storageBucket: 'natural-broker-164116.appspot.com',
    messagingSenderId: '492763510386',
  };
  firebase.initializeApp(config);
  
  const messaging = firebase.messaging();
  messaging.setBackgroundMessageHandler(function(payload) {
    const title = payload.notification.title;
    const option = {
      body: payload.notification.body,
    };
    return self.registration.showNotification(title, options);
  });
  