code: function (num) {
        var s = '';
  
        while (num >= 0) {
          s = String.fromCharCode(num % 26 + 97) + s;
          num = Math.floor(num / 26) - 1;
        }
  
        return s.toUpperCase();
      }
docs: convert number to string char, e.g 0 => A, 25 => Z, 26 => AA
  @param {Number} num
  @returns {String}