HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>hostname is localhost</title>
</head>
<body>
  <pre id="out">Waiting...</pre>
  <script>
    var code =
      "var name = location.hostname;" +
      "self.postMessage({ hostname: name, isLocalhost: name === 'localhost' });";
    var url = URL.createObjectURL(new Blob([code], { type: "application/javascript" }));
    var worker = new Worker(url);
    worker.onmessage = function (event) {
      document.getElementById("out").textContent =
        "hostname: " + JSON.stringify(event.data.hostname) +
        "\nisLocalhost: " + event.data.isLocalhost +
        "\npage hostname: " + location.hostname;
      worker.terminate();
      URL.revokeObjectURL(url);
    };
  </script>
</body>
</html>

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WorkerLocation.hostname — is localhost? (Try It)