swap.js

/*
12590 = 10592 (sort array. ignore 0. 1 is next smallest and already in front. pay attention to 0. swap with number after 1 )
21045 = 12045 (sort array. ignore 0. 1 is next smallest but not in front. swap with number in front)
21450 = 12450 (sort array. ignore 0. 1 is next smallest but not in front. swap with number in front)
10390 = 10039 (sort array. ignore 0. 1 is next smallest and already in front. pay attention to 0. we have a 0 after 1 already. move to next 0, swap with third number)
20305 = 20035 (sort array. ignore 0. 2 is next smallest and already in front. pay attention to 0. we have a 0 after 2 already. move to next 0, swap with third number)
21352 = 12352 (sort array. no zero. 1 is smallest but not in front. swap with number in front)
16289 = 12689 (sort array. no zero. 1 is smallest and already in front. move to next)
*/


var input = "21450";
var arr = input.split('');
var sorted_arr = JSON.parse(JSON.stringify(arr));
sorted_arr.sort();

console.log(arr);
console.log(sorted_arr);

var j = 0;
var zeroIgnored = false;
for (i = 0; i < arr.length; i++) {
    //check if first element is a zero
    if (i == 0) {
        if (sorted_arr[0] == '0') {
           j++;
           zeroIgnored = true;
           console.log('ignored zero');
       }
    }
    console.log(i);
    console.log(j);
    if (arr[i] !== sorted_arr[j]) {
        //smallest number isn't in front.
        var swapIndex = arr.indexOf(sorted_arr[j]);
        var buffer = arr[i];
        arr[i] = arr[swapIndex];
        arr[swapIndex] = buffer;
        break;
    } else {
        //smallest is already in front
        if (!zeroIgnored) {
            j++;
        } else {
            //pay attention to zero
            j = 0;
        }
    }
}

//unfortunately, this doesn't handle a case of multiple zeros.
console.log(arr);