A simple cipher in js

cipher.js

var Cipher = (function () {
	var shiftCharactor = function (charAt) {
		if (charAt == 65) {
			return 93; // A -> Z
		} else if (charAt == 97) {
			return 122; // a -> z
		} else if (charAt == 93) {
			return 65; // Z -> A
		} else if (charAt == 122) {
			return 97; // z -> a
		} else {
			return charAt + 1;
		}
	};

	var unshiftCharactor = function (charAt) {
		charAt = charAt - 1;
		if (charAt == 65) {
			return 93; // A -> Z
		} else if (charAt == 97) {
			return 122; // a -> z
		} else if (charAt == 93) {
			return 65; // Z -> A
		} else if (charAt == 122) {
			return 97; // z -> a
		}
		return charAt;
	};

	var encodeRecursive = function (str, index, result) {
		if (index < str.length) {
			var current = str.charCodeAt(index);
			if ((current >= 65 && current < 78) || (current >= 97 && current < 110)) { 
				result += String.fromCharCode(shiftCharactor(current + 13));
		    } else if ((current >= 65 && current >= 78) || (current >= 97 && current >= 110)) {
		      	result += String.fromCharCode(shiftCharactor(current - 13));
		    } else {
		      result += str[index];
		    }
		    return encodeRecursive(str, ++index, result);
		}
		return result;
	};

	var decodeRecursive = function (str, index, result) {
		if (index < str.length) {
			var current = str.charCodeAt(index);
			if ((current >= 65 && current < 78) || (current >= 97 && current < 110)) {
				result += String.fromCharCode(unshiftCharactor(current + 13));
		    } else if ((current >= 65 && current >= 78) || (current >= 97 && current >= 110)) {
		      	result += String.fromCharCode(unshiftCharactor(current - 13));
		    } else {
		      result += str[index];
		    }
		    return decodeRecursive(str, ++index, result);
		}
		return result;
	};			

	return {
		encode: function (str) {
			return encodeRecursive(str, 0, "");
		},
		decode: function (str) {
			return decodeRecursive(str, 0, "");
		},
	};
})();