HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Worker error message</title>
</head>
<body>
  <pre id="out">Waiting...</pre>
  <script>
    var code = "throw new Error('boom');";
    var url = URL.createObjectURL(new Blob([code], { type: "application/javascript" }));
    var worker = new Worker(url);
    worker.onerror = function (event) {
      document.getElementById("out").textContent =
        event.message || "unknown worker error";
      event.preventDefault();
      worker.terminate();
      URL.revokeObjectURL(url);
    };
  </script>
</body>
</html>

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Worker error — event.message (Try It)