An implementation of Ruby's string.succ method in JavaScript

succ.js

/*
 * An implementation of Ruby's string.succ method.
 * By Devon Govett
 *
 * Returns the successor to str. The successor is calculated by incrementing characters starting 
 * from the rightmost alphanumeric (or the rightmost character if there are no alphanumerics) in the
 * string. Incrementing a digit always results in another digit, and incrementing a letter results in
 * another letter of the same case.
 *
 * If the increment generates a carry, the character to the left of it is incremented. This 
 * process repeats until there is no carry, adding an additional character if necessary.
 *
 * succ("abcd")      == "abce"
 * succ("THX1138")   == "THX1139"
 * succ("<<koala>>") == "<<koalb>>"
 * succ("1999zzz")   == "2000aaa"
 * succ("ZZZ9999")   == "AAAA0000"
 */

function succ(input) {
    var alphabet = 'abcdefghijklmnopqrstuvwxyz',
        length = alphabet.length,
        result = input,
        i = input.length;
        
    while(i >= 0) {
        var last = input.charAt(--i),
            next = '',
            carry = false;
        
        if (isNaN(last)) {
            index = alphabet.indexOf(last.toLowerCase());
            
            if (index === -1) {
                next = last;
                carry = true;
            }
            else {
                var isUpperCase = last === last.toUpperCase();
                next = alphabet.charAt((index + 1) % length);
                if (isUpperCase) {
                    next = next.toUpperCase();
                }
                
                carry = index + 1 >= length;
                if (carry && i === 0) {
                    var added = isUpperCase ? 'A' : 'a';
                    result = added + next + result.slice(1);
                    break;
                }
            }
        }
        else {
            next = +last + 1;
            if(next > 9) {
                next = 0;
                carry = true
            }
            
            if (carry && i === 0) {
                result = '1' + next + result.slice(1);
                break;
            }
        }
        
        result = result.slice(0, i) + next + result.slice(i + 1);
        if (!carry) {
            break;
        }
    }
    return result;
}

thanks @devongovett this saved me a nice chunk of time. Much appreciated.