Caesar Cipher

encrypt.js

function encrypt(input){
  var alphabet = "abcdefghijklmnopqrstuvwxyz";
  var fullAlphabet = alphabet + alphabet + alphabet;
  var offset = 3;
  offset = (offset % alphabet.length);
  var result = '';

  for(i=0; i < input.length; i++){
	 var letter = input[i];
	 var upper = (letter == letter.toUpperCase());
	 letter = letter.toLowerCase();
	
	 var index = alphabet.indexOf(letter);
	 if(index == -1){
	   result += letter;
	 } else {
	   index = (index+offset) > (alphabet.length-1) ? ((index+offset) - (alphabet.length-1)-1) : index+offset;
	   var nextLetter = alphabet[index];
	   if(upper) nextLetter = nextLetter.toUpperCase();
	   result += nextLetter;
	 }
  }
	
  return result;
}