문제.

const getHeaders = () => {
  const resultType = XPathResult.ORDERED_NODE_ITERATOR_TYPE;
  return document.evaluate('//h1', document, null, resultType, null);
};

const xPathResult1 = getHeaders();
for (const node of xPathResult1) console.log(node);
// Uncaught TypeError: xPathResult is not iterable

const xPathResult2 = getHeaders();
Array.from(xPathResult2);
// []

const xPathResult3 = getHeaders();
[...xPathResult3]
// Uncaught TypeError: object is not iterable (cannot read property Symbol(Symbol.iterator))

해결책

const getHeaders = () => {
  const resultType = XPathResult.ORDERED_NODE_ITERATOR_TYPE;
  return document.evaluate('//h1', document, null, resultType, null);
};

// イテレータ型の XPathResult オブジェクトを Generator オブジェクトに変換する。
const makeIterable = function*(xPathResult) {
  while (true) {
    const node = xPathResult.iterateNext();
    if (!node) break;
    yield node;
  }
};

const xPathResult1 = getHeaders();
for (const node of makeIterable(xPathResult1)) console.log(node);
// h1
// h1
// h1

const xPathResult2 = getHeaders();
Array.from(makeIterable(xPathResult2));
// [h1, h1, h1]

const xPathResult3 = getHeaders();
[...makeIterable(xPathResult3)];
// [h1, h1, h1]

참고 자료