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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>setNamedItemNS MDN</title>
</head>
<body>
  <span ob:one="one">Span</span>
  <pre id="out">Waiting...</pre>
  <script>
    var doc = new DOMParser().parseFromString('<warning ob:one="test" xmlns:ob="http://www.example.com/ob">Beware!</warning>', "application/xml");
    var warning = doc.querySelector("warning");
    var span = document.querySelector("span");
    var one = warning.attributes.removeNamedItemNS("http://www.example.com/ob", "one");
    span.attributes.setNamedItemNS(one);
    document.getElementById("out").textContent = span.attributes.length + " " + one.value;
  </script>
</body>
</html>

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NamedNodeMap.setNamedItemNS() — MDN move ob:one (Try It)