HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>cancelable prevent</title>
</head>
<body>
  <button id="run" type="button">Dispatch cancelable save</button>
  <pre id="out">Waiting...</pre>
  <script>
    var target = new EventTarget();
    var out = document.getElementById("out");

    target.addEventListener("save", function (event) {
      if (event.cancelable) {
        event.preventDefault();
      }
      out.textContent =
        "cancelable=" + event.cancelable +
        "\ndefaultPrevented=" + event.defaultPrevented;
    });

    document.getElementById("run").onclick = function () {
      var ok = target.dispatchEvent(new Event("save", { cancelable: true }));
      out.textContent += "\ndispatchEvent returned " + ok;
    };
  </script>
</body>
</html>

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Event.cancelable — preventDefault (Try It)