service-worker.js

self.addEventListener("push", (event) => {
  message_object = JSON.parse(event.data.text());

  let title = message_object.title;
  let options = {
    body: message_object.body,
    tag: "push-simple-demo-notification-tag",
    icon: message_object.icon,
    link: message_object.link
  }

  const testDataObject = {
    name: "waitUntil",
    favorite_drink: "Fire Ball",
    favorite_food: "Steak"
  }

  event.waitUntil(
    self.registration.showNotification(title, options)
      .then(postsendTestDataData(testDataObject))   
  )
});

self.addEventListener('notificationclick', function (event) {
  event.notification.close();
  console.log('self in click event Listener', self)
  event.waitUntil(
    clients.openWindow(event.currentTarget.message_object.link)
  );
});

function postsendTestDataData(data) {
  let domain;
  const local = location.origin.includes("127");
  local ? domain = "http://localhost:3000/" : "https://arcane-stream-87798.herokuapp.com/"

  return fetch(`${domain}/test_data`, {
    body: JSON.stringify(data), // must match 'Content-Type' header
    method: 'POST',
    headers: {
      'user-agent': 'Mozilla/4.0 MDN Example',
      'content-type': 'application/json'
    },
  })
    .then(response => console.log(response)) // parses response to JSON
}