function stringAt(index) {
  let str = '';
  let cindex = index;
  while (cindex >= alphabets.length) {
    cindex /= alphabets.length;
    cindex -= 1;
    str += alphabets[parseInt(cindex, 10) % alphabets.length];
  }
  const last = index % alphabets.length;
  str += alphabets[last];
  return str;
}
index number 2 letters
@example stringAt(26) ==> 'AA'
@date 2019-10-10
@export
@param {number} index
@returns {string}