background-sync/service_workers/sw.js

// The source to post setup and completion results to.
let source = null;

function sendMessageToDocument(msg) {
  source.postMessage(msg);
}

// Notify the document that the SW is registered and ready.
self.addEventListener('message', event => {
  source = event.source;
  sendMessageToDocument('ready');
});

self.addEventListener('sync', event => {
  sendMessageToDocument('sync event received!');
});