JavaScript Document createNSResolver() Method

const node = document.body;
const resolved = document.createNSResolver(node);

console.log(resolved === node);                // true
console.log(resolved.nodeName);                // "BODY"
console.log(typeof document.createNSResolver); // "function"

// Legacy-looking sample (do not copy into new apps):
const contextNode = document.documentElement;
const resolver = document.createNSResolver(contextNode);

const result = document.evaluate(
  "//*",
  document,
  resolver, // today this is just contextNode again
  XPathResult.ORDERED_NODE_SNAPSHOT_TYPE,
  null
);

console.log(result.snapshotLength);
console.log(resolver === contextNode); // true

const svgNS = "http://www.w3.org/2000/svg";

function nsResolver(prefix) {
  const map = { svg: svgNS };
  return map[prefix] || null;
}

const result = document.evaluate(
  "count(//svg:circle)",
  document,
  nsResolver,
  XPathResult.NUMBER_TYPE,
  null
);

console.log(result.numberValue);