NamedNodeMap.setNamedItemNS()

setNamedItemNS(attr);

<span ob:one="one"></span>
<pre></pre>

const parser = new DOMParser();
// ob:one in <span> is not in a namespace, while ob:one in <warning>, is.
const xmlString = '<warning ob:one="test" xmlns:ob="http://www.example.com/ob">Beware!</warning>';
const doc = parser.parseFromString(xmlString, "application/xml");

const span = document.getElementsByTagName("span")[0];
const pre = document.getElementsByTagName("pre")[0];
const warning = doc.getElementsByTagName("warning")[0];
const attrMap = span.attributes;

let result = "The `<span>` element initially contains " + attrMap.length + " attribute.\n\n";

result += "We remove `one` from `<span>` and adds it to `<pre>`.\n";
const one = warning.attributes.removeNamedItemNS("http://www.example.com/ob", "one");
attrMap.setNamedItemNS(one);
result += "The `<span>` element now contains " + span.attributes.length + " attributes:\n\n";
result += "Prefix\tLocal name\tQualified name\n";
result += "=========================================\n";
for(let attr of attrMap) {
  result += attr.prefix + "\t" + attr.localName + "\t\t" + attr.name + "\n";
}

pre.textContent = result;