HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>cancelBubble migrate</title>
  <style>
    #parent { padding: 16px; border: 2px solid #333; }
    #child { padding: 12px; border: 2px dashed #666; margin-top: 8px; }
  </style>
</head>
<body>
  <div id="parent">Parent
    <div id="child">Child</div>
  </div>
  <button id="legacy" type="button">Legacy cancelBubble</button>
  <button id="modern" type="button">Modern stopPropagation</button>
  <pre id="out">Waiting...</pre>
  <script>
    var parent = document.getElementById("parent");
    var child = document.getElementById("child");
    var out = document.getElementById("out");

    function run(mode) {
      var log = [];
      function onParent() { log.push("parent"); }
      function onChild(e) {
        log.push("child");
        if (mode === "legacy") e.cancelBubble = true;
        else e.stopPropagation();
      }
      parent.addEventListener("ping", onParent);
      child.addEventListener("ping", onChild);
      child.dispatchEvent(new Event("ping", { bubbles: true }));
      parent.removeEventListener("ping", onParent);
      child.removeEventListener("ping", onChild);
      out.textContent = mode + ": " + log.join(" → ") + " (parent blocked)";
    }

    document.getElementById("legacy").onclick = function () { run("legacy"); };
    document.getElementById("modern").onclick = function () { run("modern"); };
  </script>
</body>
</html>

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Event.cancelBubble — migrate side-by-side (Try It)