let download = (res)=>{
            // res 为blob对象
            let downloadElement = document.createElement('a');
            let href = window.URL.createObjectURL(res); //创建下载的链接
            // let type = res.type.substr(res.type.indexOf("/") + 1);
            downloadElement.href = href;
            downloadElement.download = fileName; //下载后文件名
            document.body.appendChild(downloadElement);
            downloadElement.click(); //点击下载
            document.body.removeChild(downloadElement); //下载完成移除元素
            window.URL.revokeObjectURL(href); //释放掉blob对象
          }