function createWorker() {
  if (!window.Worker) {
    resultOutput.textContent = "Web Workers are not supported in this browser.";
    return null;
  }

  const worker = new Worker("worker.js", { name: "sum-worker" });
  writeLog("main: worker created");

  worker.addEventListener("message", (event) => {
    const message = event.data;

    if (message.type === "progress") {
      writeLog(message.text);
      return;
    }

    if (message.type === "result") {
      window.lastWorkerResult = message;
      resultOutput.textContent = `Worker total: ${message.total}`;
      writeLog(`main: total ${message.total} from ${message.count} numbers`);
      worker.terminate();

      if (activeWorker === worker) {
        activeWorker = null;
      }

      writeLog("main: worker terminated");
    }
  });

  worker.addEventListener("error", (event) => {
    resultOutput.textContent = `Worker failed: ${event.message}`;
    writeLog(`main: worker error at ${event.filename}:${event.lineno}`);

    if (activeWorker === worker) {
      activeWorker = null;
    }
  });

  return worker;
}