Client: type property

// service worker client (e.g. a document)
function sendMessage(message) {
  return new Promise((resolve, reject) => {
    // note that this is the ServiceWorker.postMessage version
    navigator.serviceWorker.controller.postMessage(message);
    window.serviceWorker.onMessage = (e) => {
      resolve(e.data);
    };
  });
}

// controlling service worker
self.addEventListener("message", (e) => {
  // e.source is a client object
  e.source.postMessage(`Hello! Your message was: ${e.data}`);
  // Let's also post the type value back to the client
  e.source.postMessage(e.source.type);
});