HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>message vs error</title>
</head>
<body>
  <button id="ok" type="button">Healthy ping</button>
  <button id="bad" type="button">Broken worker</button>
  <pre id="out">Waiting...</pre>
  <script>
    var out = document.getElementById("out");
    var lines = [];

    function makeWorker(source) {
      var url = URL.createObjectURL(
        new Blob([source], { type: "application/javascript" })
      );
      var worker = new Worker(url);
      worker.onmessage = function (e) {
        lines.push("ok: " + e.data);
        out.textContent = lines.join("\n");
        worker.terminate();
        URL.revokeObjectURL(url);
      };
      worker.onerror = function (e) {
        lines.push("fail: " + (e.message || "error"));
        out.textContent = lines.join("\n");
        e.preventDefault();
        worker.terminate();
        URL.revokeObjectURL(url);
      };
      return worker;
    }

    document.getElementById("ok").addEventListener("click", function () {
      var w = makeWorker("self.onmessage = function () { self.postMessage('pong'); };");
      w.postMessage(null);
    });

    document.getElementById("bad").addEventListener("click", function () {
      makeWorker("throw new Error('boom');");
    });
  </script>
</body>
</html>

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Worker message vs error (Try It)