denoland-deno/tests/testdata/workers/worker_event_handlers.js

self.onmessage = (evt) => {
  console.log("Target from self.onmessage:", String(evt.target));
};

self.addEventListener("message", (evt) => {
  console.log("Target from message event listener:", String(evt.target));

  // Throw an error here so the global's error event will fire.
  throw new Error("Some error message");
});

self.onerror = (...args) => {
  // Take the last 100 characters so the filename doesn't get truncated
  // depending on the dev's FS structure.
  args = args.map((v) => typeof v == "string" ? v.slice(-100) : v);
  console.log("Arguments from self.onerror:", args);
  return true;
};

self.addEventListener("error", (evt) => {
  // Returning true from self.onerror means that subsequent event listeners
  // should see the event as canceled.
  console.log("Is event canceled?:", evt.defaultPrevented);

  self.close();
});