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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>createEvent MDN</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} #box{border:1px solid #ccc;padding:1rem;} pre{border:1px solid #ccc;padding:0.75rem;margin-top:0.75rem;}</style>
</head>
<body>
  <div id="box">Target element</div>
  <button id="btn" type="button">Fire build (legacy createEvent)</button>
  <pre id="out">Click the button</pre>
  <script>
    var elem = document.getElementById("box");
    elem.addEventListener("build", function () {
      document.getElementById("out").textContent = "build fired on " + elem.id;
    });
    document.getElementById("btn").addEventListener("click", function () {
      var event = document.createEvent("Event");
      event.initEvent("build", true, true);
      elem.dispatchEvent(event);
    });
  </script>
</body>
</html>

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Document.createEvent — MDN legacy (Try It)