messaging in service workers

sw-messaging.js

// index.html -- Sending message TO service worker, FROM page:
navigator.serviceWorker.controller.postMessage(message);

// service-worker.js -- Handling message event in service worker by posting a message back to the page:
self.addEventListener('message', function(event) {
  self.clients.matchAll().then(function(client) { 
    client[0].postMessage({
      command: 'logMessage',
      error: null,
      message: 'hi there message here!!'
    });
  });
});