Yandex interview tasks

Yandex_task.js

// [1, 3, 5, 7, 8, 11, 12, 13, 20] -> 1,3,5,7-8,11-13,20
function short(arr) {
    var sorted = arr.sort(function (a, b) {
        if (a < b) { return -1; }
        else if (a > b) { return 1; }
        else { return 0; }
    });

    function add(res, newValue) {
        var last = res[res.length-1];
        if (!last) {
            res.push([newValue]);
            return;
        }

        var [firstItem, lastItem] = last;
        if (firstItem && lastItem) {
            if (lastItem+1 === newValue) {
                last[last.length-1] = newValue;
            } else {
                res.push([newValue]);   
            }
        } else if (firstItem) {
            if (firstItem + 1 === newValue) {
                last.push(newValue);
            } else {
                res.push([newValue]);
            }
        } else {
            last.push(newValue);
        }
    }

    return sorted
        .reduce(function (res, currentItem) {
            add(res, currentItem);

            return res;
        }, [])
        .map(function (minMax) {
            return minMax.join('-');
        })
        .join(',');
}

short([1, 3, 5, 7, 8, 11, 12, 13, 20]);