数组单项上下移动

down(arr, index) {
        if(index == arr.length -1) {
            return;
        }
        sItems(arr, index, index + 1);
    },
up (arr, index) {
        if(index == 0) {
            return;
        }
        sItems(arr, index, index - 1);
    },
sItems (arr, index1, index2) {
        arr[index1] = arr.splice(index2, 1, arr[index1])[0];
        return arr;
    }