nik13kaydash

const getNestedItems = (data, test) =>
  data instanceof Object
    ? Object.values(data).flatMap(n => getNestedItems(n, test))
    : test(data) ? [ data ] : [];


const result = getNestedItems(obj, x => /^[A-Z]+$/.test(x)).join('');

function* getNestedItems(data, test) {
  if (Object(data) === data) {
    for (const k in data) if (Object.hasOwn(data, k)) {
      yield* getNestedItems(data[k], test);
    }
  } else if (test(data)) {
    yield data;
  }
}


const result = ''.concat(...getNestedItems(obj, x => x?.toUpperCase?.() === x));