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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>ariaPlaceholder vs attribute</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    var el = document.createElement("div");
    el.setAttribute("role", "textbox");
    el.setAttribute("aria-placeholder", "5-digit zip code");
    el.setAttribute("aria-label", "Zip code");
    document.body.insertBefore(el, document.getElementById("out"));
    el.ariaPlaceholder = "12345";
    document.getElementById("out").textContent = JSON.stringify({
      fromProperty: el.ariaPlaceholder,
      fromAttribute: el.getAttribute("aria-placeholder"),
      same: el.ariaPlaceholder === el.getAttribute("aria-placeholder")
    }, null, 2);
  </script>
</body>
</html>

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Element.ariaPlaceholder — vs attribute (Try It)