Caesar Cipher algorithm, encrypt string wrap back around to letters

caesar-cipher.js

process.stdin.resume();
process.stdin.setEncoding('ascii');

var input_stdin = "";
var input_stdin_array = "";
var input_currentline = 0;

process.stdin.on('data', function (data) {
    input_stdin += data;
});

process.stdin.on('end', function () {
    input_stdin_array = input_stdin.split("\n");
    main();    
});

function readLine() {
    return input_stdin_array[input_currentline++];
}

/////////////// ignore above this line ////////////////////

function main() {
    var n = parseInt(readLine());
    var s = readLine();
    var k = parseInt(readLine());
    var result = '';
    
    for (var i=0; i<s.length; i++) {
        var char = s[i].charCodeAt();
        var ch = s[i];
        
        if ((char >= 'A'.charCodeAt() && char <= 'Z'.charCodeAt())) {
            char += k;
            
            while (char > 'Z'.charCodeAt()) {
                char = 'A'.charCodeAt() + (char - 'Z'.charCodeAt() - 1);
            }
            
            ch = String.fromCharCode(char);
        }
        else if (char >= 'a'.charCodeAt() && char <= 'z'.charCodeAt()) {
            char += k;
            
            while (char > 'z'.charCodeAt()) {
                char = 'a'.charCodeAt() + (char - 'z'.charCodeAt() - 1);
            }
            
            ch = String.fromCharCode(char);
        }

        result += ch;
    }
    
    console.log(result);
}

zexample.txt

Julius Caesar protected his confidential information by encrypting it in a cipher. Caesar's cipher rotated every letter in a string by a fixed number, , making it unreadable by his enemies. Given a string, , and a number, , encrypt  and print the resulting string.

Note: The cipher only encrypts letters; symbols, such as -, remain unencrypted.

Input Format

The first line contains an integer, , which is the length of the unencrypted string. 
The second line contains the unencrypted string, . 
The third line contains the integer encryption key, , which is the number of letters to rotate.

Constraints 
 
 
 is a valid ASCII string and doesn't contain any spaces.

Output Format

For each test case, print the encoded string.

Sample Input

11
middle-Outz
2
Sample Output

okffng-Qwvb
Explanation

Each unencrypted letter is replaced with the letter occurring  spaces after it when listed alphabetically. Think of the alphabet as being both case-sensitive and circular; if  rotates past the end of the alphabet, it loops back to the beginning (i.e.: the letter after  is , and the letter after  is ).

Selected Examples: 
 (ASCII 109) becomes  (ASCII 111). 
 (ASCII 105) becomes  (ASCII 107). 
 remains the same, as symbols are not encoded. 
 (ASCII 79) becomes  (ASCII 81). 
 (ASCII 122) becomes  (ASCII 98); because  is the last letter of the alphabet,  (ASCII 97) is the next letter after it in lower-case rotation.