Next String (using counting sort)

nextLex.js

function nextLex(str) {
    var breakIndex = -1;
    var chars = str.split("");
    for (var i=chars.length-1; i>0; i--) {
        if (chars[i] > chars[i-1]) {
            breakIndex = i-1;
            break;
        }
    }

    if (breakIndex > -1) {
        var swapIndex = breakIndex + 1;
        for (var i=swapIndex+1; i<chars.length; i++) {
            if (chars[i] > str[breakIndex] && chars[i] < chars[swapIndex]) {
                swapIndex = i;
            }
        }
        swap(breakIndex, swapIndex, chars);
        chars = countingSort(chars, breakIndex+1, chars.length-1);
    }
    return chars.join("");
}

function countingSort(array, start, end) {
    var b = [];
    for (var i=0;i<26;i++) {
        b.push(0);
    }
    var aCode = 'a'.charCodeAt(0);
	for (var i=start; i<=end; i++) {
        var idx =  (array[i].charCodeAt(0) - aCode)%26;
        b[idx]++;
    }
    var right = [];
    for (var i=0;i<b.length;i++){
        var x = b[i];
        while (x>0){
            right.push(String.fromCharCode(aCode+i));
            x--;
        }
    }
    array.splice(start);
    array = array.concat(right);
    return array;
}

function swap(i, j, array) {
	var temp = array[i];
	array[i] = array[j];
	array[j] = temp;
}

nextLex("xyzafedcb");

The use of counting sort will be much faster.