Inverting Selected Ranges on Google Spreadsheet using Google Apps Script

// Ref: https://stackoverflow.com/a/21231012/7108653
const columnToLetter_ = (column) => {
  let temp,
    letter = "";
  while (column > 0) {
    temp = (column - 1) % 26;
    letter = String.fromCharCode(temp + 65) + letter;
    column = (column - temp - 1) / 26;
  }
  return letter;
};
const letterToColumn_ = (letter) => {
  let column = 0,
    length = letter.length;
  for (let i = 0; i < length; i++) {
    column += (letter.charCodeAt(i) - 64) * Math.pow(26, length - i - 1);
  }
  return column;
};

// This is from https://tanaikech.github.io/2020/04/04/updated-expanding-a1notations-using-google-apps-script/
function expandA1Notations_(a1Notations, maxRow, maxColumn) {
  maxRow = maxRow || "1000";
  maxColumn = maxColumn || "Z";
  const reg1 = new RegExp("^([A-Z]+)([0-9]+)$");
  const reg2 = new RegExp("^([A-Z]+)$");
  const reg3 = new RegExp("^([0-9]+)$");
  return a1Notations.map((e) => {
    const a1 = e.split("!");
    const r = a1.length > 1 ? a1[1] : a1[0];
    const [r1, r2] = r.split(":");
    if (!r2) return [r1];
    let rr;
    if (reg1.test(r1) && reg1.test(r2)) {
      rr = [r1.toUpperCase().match(reg1), r2.toUpperCase().match(reg1)];
    } else if (reg2.test(r1) && reg2.test(r2)) {
      rr = [
        [null, r1, 1],
        [null, r2, maxRow],
      ];
    } else if (reg1.test(r1) && reg2.test(r2)) {
      rr = [r1.toUpperCase().match(reg1), [null, r2, maxRow]];
    } else if (reg2.test(r1) && reg1.test(r2)) {
      rr = [[null, r1, maxRow], r2.toUpperCase().match(reg1)];
    } else if (reg3.test(r1) && reg3.test(r2)) {
      rr =
        Number(r1) > Number(r2)
          ? [
              [null, "A", r2],
              [null, maxColumn, r1],
            ]
          : [
              [null, "A", r1],
              [null, maxColumn, r2],
            ];
    } else if (reg1.test(r1) && reg3.test(r2)) {
      rr = [r1.toUpperCase().match(reg1), [null, maxColumn, r2]];
    } else if (reg3.test(r1) && reg1.test(r2)) {
      let temp = r2.toUpperCase().match(reg1);
      rr =
        Number(temp[2]) > Number(r1)
          ? [
              [null, temp[1], r1],
              [null, maxColumn, temp[2]],
            ]
          : [temp, [null, maxColumn, r1]];
    } else {
      throw new Error(`Wrong a1Notation: ${r}`);
    }
    const obj = {
      startRowIndex: Number(rr[0][2]),
      endRowIndex: rr.length == 1 ? Number(rr[0][2]) + 1 : Number(rr[1][2]) + 1,
      startColumnIndex: letterToColumn_(rr[0][1]),
      endColumnIndex:
        rr.length == 1
          ? letterToColumn_(rr[0][1]) + 1
          : letterToColumn_(rr[1][1]) + 1,
    };
    let temp = [];
    for (let i = obj.startRowIndex; i < obj.endRowIndex; i++) {
      for (let j = obj.startColumnIndex; j < obj.endColumnIndex; j++) {
        temp.push(columnToLetter_(j) + i);
      }
    }
    return temp;
  });
}

// Please run this function after the cells were selected.
// By this, the selected cells are reversed.
function main() {
  const activeRanges = SpreadsheetApp.getActiveRangeList().getRanges();
  if (activeRanges.length == 0) return;
  const a1Notations = activeRanges.map((e) => e.getA1Notation());
  const res = expandA1Notations_(a1Notations);
  const obj = res.reduce((o, r) => {
    r.forEach((c) => (o[c] = true));
    return o;
  }, {});
  const sheet = SpreadsheetApp.getActiveSheet();
  const rows = sheet.getMaxRows();
  const cols = sheet.getMaxColumns();
  const allA1Notations = [...Array(rows)].flatMap((_, r) =>
    [...Array(cols)].map((_, c) => `${columnToLetter_(c + 1)}${r + 1}`)
  );
  const reverseA1Notations = allA1Notations.filter((r) => !obj[r]);
  if (reverseA1Notations.length == 0) return;
  sheet.getRangeList(reverseA1Notations).activate();
}