HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>createEvent bubbles</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} #parent{border:1px solid #ccc;padding:1rem;} #child{border:1px dashed #999;padding:0.75rem;margin-top:0.5rem;} pre{border:1px solid #ccc;padding:0.75rem;margin-top:0.75rem;}</style>
</head>
<body>
  <div id="parent">Parent
    <div id="child">Child</div>
  </div>
  <button id="btn" type="button">Dispatch ping from child</button>
  <pre id="out">Click the button</pre>
  <script>
    var parent = document.getElementById("parent");
    var child = document.getElementById("child");
    var lines = [];
    parent.addEventListener("ping", function () {
      lines.push("parent heard ping");
      document.getElementById("out").textContent = lines.join("\n");
    });
    document.getElementById("btn").addEventListener("click", function () {
      lines = [];
      var event = document.createEvent("Event");
      event.initEvent("ping", true, false);
      child.dispatchEvent(event);
      lines.push(event.bubbles + " " + event.cancelable);
      document.getElementById("out").textContent = lines.join("\n");
    });
  </script>
</body>
</html>

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Document.createEvent — bubbling flags (Try It)