JavaScript Increment string function based on @dandoescode php inc function for Scrapyrd. Original inc function can be found here https://github.com/dandoescode/scrapyrd/blob/master/fuel/app/classes/scrapyrd.php

increment-string.js

function setCharAt(str,index,chr) {
    if(index > str.length-1) return str;
    return str.substr(0,index) + chr + str.substr(index+1);
}

function incString(n, pos) {

    if(!pos) {pos = 0};

    var set = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'
    , setmax = 61;

    if(n.length==0){
        return set[0];
    }

    var nindex = n.length - 1 - pos
    , char = n[nindex]
    , setindex = set.indexOf(char);

    if (nindex < 0) { return set[0]+n };

    if(setindex == setmax) {
        return incString(setCharAt(n,nindex,set[0]), ++pos);
    } else {
        return setCharAt(n,nindex,set[++setindex]);
    }
}