String#succ_prec.js

String#succ_prec.js

String.prototype.succ = function () {
  if (!/^[a-z]+$/.test(this)) return '';

  var parts = this.split(''),
      i = parts.length - 1;

  do {
    parts[i] = String.fromCharCode(97 + (parts[i].charCodeAt(0) - 70) % 26);
    if (parts[i] !== 'a') break;
    else if (i === 0) parts.unshift('a');
  } while (i--);

  return parts.join('');
};

String.prototype.prec = function () {
  if (!/^[a-z]+$/.test(this)) return '';

  var parts = this.split(''),
      i = parts.length - 1;

  do {
    parts[i] = String.fromCharCode(97 + (parts[i].charCodeAt(0) - 72) % 26);
    if (parts[i] !== 'z') break;
    else if (i === 0) parts.shift();
  } while (i--);

  return parts.join('');
};