<a id="updateCount" 
class="layui-btn layui-btn-radius layui-btn-normal"
href="javascript:void(0)" onclick="updateItem(${item.value.id},${item.value.count})">修改</a>

a标签单击事件后跳转 href要先设置无跳转
再用 location.href 重写href跳转

function updateItem(itemId,count) {
        layer.prompt(
            {
                value:count
            },
            function (value, index, elem) {
                location.href = "cartServlet?action=updateCount&id=" + itemId + "&count=" + value;
                layer.close(index);
            });
    }