Flatten Firebase & Firestore JSON in Google Sheets

function FLATTEN_JSON(range, delimiter) {
  delimiter = delimiter || ".";
  var raw = Array.isArray(range) ? range.flat().filter(Boolean).map(String) : (range ? [String(range)] : []);
  if (!raw.length) return [["No JSON provided"]];
  var rows = [], allCols = {}, colOrder = [];
  for (var i = 0; i < raw.length; i++) {
    try {
      var parsed = JSON.parse(raw[i]);
      var items = Array.isArray(parsed) ? parsed : [parsed];
      for (var j = 0; j < items.length; j++) {
        var flat = _pfFlat(items[j], "", delimiter);
        rows.push(flat);
        for (var k in flat) { if (!allCols[k]) { allCols[k] = true; colOrder.push(k); } }
      }
    } catch(e) { rows.push({"error": e.message}); if (!allCols["error"]) { allCols["error"] = true; colOrder.push("error"); } }
  }
  if (!rows.length || !colOrder.length) return [["Empty JSON"]];
  var res = [colOrder];
  for (var r = 0; r < rows.length; r++) {
    res.push(colOrder.map(function(c) { var v = rows[r][c]; return v === undefined ? "" : (typeof v === "object" ? JSON.stringify(v) : v); }));
  }
  return res;
}
function _pfFlat(obj, pfx, dlm) {
  var out = {};
  if (obj === null || typeof obj !== "object") { out[pfx || "value"] = obj; return out; }
  if (Array.isArray(obj)) {
    if (!obj.length) { out[pfx] = "[]"; return out; }
    var isP = obj.every(function(x) { return x === null || typeof x !== "object"; });
    if (isP) { out[pfx] = obj.join(", "); return out; }
    for (var i = 0; i < obj.length; i++) {
      var sf = _pfFlat(obj[i], pfx ? pfx + dlm + i : String(i), dlm);
      for (var k in sf) out[k] = sf[k];
    }
    return out;
  }
  for (var k in obj) {
    if (obj.hasOwnProperty(k)) {
      var pk = pfx ? pfx + dlm + k : k;
      var v = obj[k];
      if (v !== null && typeof v === "object") {
        var sf2 = _pfFlat(v, pk, dlm);
        for (var k2 in sf2) out[k2] = sf2[k2];
      } else { out[pk] = v; }
    }
  }
  return out;
}