Caesar cipher algorithm in javascript

caesarCipher.js

String.prototype.caesarCipher = function(level) {
  function nextLetter(s){
    return s.replace(/([a-zA-Z])[^a-zA-Z]*$/, function(a){
        var c= a.charCodeAt(0);
        switch(c){
            case 90: return 'A';
            case 122: return 'a';
            default: return String.fromCharCode(++c);
        }
    });
  }
  var res = "", temp;
  for(var i = 0; i < this.length; i++) {
    temp = this[i];
    for(var y = 0; y < level; y++) temp = nextLetter(temp);
    res += temp;
  }
  return res;
}