How to detect if executing on main thread vs worker

function main() {
  // create worker from this same file
  const worker = new Worker(import.meta.resolve("./mod.ts"), {
      name: "hello-worker",
      type: "module",
  });
  worker.onmessage = (e: MessageEvent<{ message: string }>) => {
      console.log(e.data.message);
  };
  worker.postMessage({
      message: "hello",
  });
}

if (import.meta.main) {
    if (self.window) {
        main();
    } else {
        const worker = self as unknown as Worker & { name?: string };
        worker.onmessage = (e: MessageEvent<{ message: string }>) => {
            worker.postMessage({
                message: `${e.data.message} from ${worker.name}`,
            });
            self.close();
        };
    }
}