async function fetchRecursive(base_url, arr, level) {
  const result = [];
  if(level == 0) return result;
  if (Array.isArray(arr))
    for (const n of arr) {
      if (n.children) {
        let ch_arr;
        try {
          result.push({
            id: n.id,
            externalId: n.externalId,
            text: n.text,
            children: await fetch(base_url + n.id).then((r) => {
              ch_arr = { ...r };
              return r.json();
            }),
            productCount: n.productCount,
          });
        } catch (error) {
          result.push({ id: n.id, error });
        }
        result.push(...(await fetchRecursive(base_url, ch_arr, level - 1)));
      } else {
        result.push(n);
      }
    }
  return result;
}
данные такого вида
{id: 13661, externalId: "КЗ000000000000002853", text: "J-Y (ST) Y", children: false, productCount: 3}