Javascript to create a cipher?

Hi all, I would like to create an encoding Storyline in which the user specifies the Caesar cipher (+N) to encode a word. For example, typing MARK in a +2 cipher work output OCTM. Does that make sense? I was hoping to use JS to pull apart the input variable and encode it, but I can't seem to make it work.

Here's the JS that I am currently using:

var player = GetPlayer();
var str = player.GetVar("TextEntry");
var amount = player.Getvar("Cipher");
var output = '';
for (var i = 0; i < str.length; i ++) {
var c = str[i];
if (c.match(/[a-z]/i)) {
var code = str.charCodeAt(i);
if ((code >= 65) && (code <= 90))
c = String.fromCharCode(((code - 65 + amount) % 26) + 65);
else if ((code >= 97) && (code <= 122))
c = String.fromCharCode(((code - 97 + amount) % 26) + 97);
};
output += c;
};

player.SetVar("returned",output);
player.SetVar("YouTyped",str);