gecko-dev/dom/workers/test/timeoutTracing_worker.js

/**
 * Any copyright is dedicated to the Public Domain.
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

onmessage = function(event) {
  throw "No messages should reach me!";
}

setInterval(function() { postMessage("Still alive!"); }, 20);
setInterval(";", 20);

postMessage("Begin!");