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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>ariaHasPopup vs attribute</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    var el = document.createElement("input");
    el.setAttribute("role", "combobox");
    el.setAttribute("aria-haspopup", "true");
    document.body.insertBefore(el, document.getElementById("out"));
    el.ariaHasPopup = "listbox";
    document.getElementById("out").textContent = JSON.stringify({
      fromProperty: el.ariaHasPopup,
      fromAttribute: el.getAttribute("aria-haspopup"),
      same: el.ariaHasPopup === el.getAttribute("aria-haspopup")
    }, null, 2);
  </script>
</body>
</html>

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Element.ariaHasPopup — vs attribute (Try It)