处理后台返回的文件流，base64码实现文件导出功能

借鉴文章：https://blog.csdn.net/developer_qi/article/details/87803950 

一、 处理文件流

    ①  请求接口时，声明responseType: 'blob', 告诉后台需要返回的的报文是文件

export function writeWithHead(params) {
  return request({
    url: window.CONFIG.restIp + '/workOrderExcelOutDto/writeWithHead',
    method: 'post',
    data: params,
    responseType: 'blob',   // 请求的时候必须添加
    timeout: 200000
  })
}

 　②  处理后台返回的流数据

writeWithHead(params).then(res => {
        if (res.status === 200) {
          this.$message({
            type: "success",
            message: '导出成功'
          })
          const blob = new Blob([res.data]);
          const fileName = '导出文件.xlsx';//下载文件名称
          const elink = document.createElement('a');
          elink.download = fileName;
          elink.style.display = 'none';
          elink.href = URL.createObjectURL(blob);
          document.body.appendChild(elink);
          elink.click();
          URL.revokeObjectURL(elink.href); // 释放URL 对象
          document.body.removeChild(elink);
        } else {
          this.$message({
            type: "error",
            message: '导出失败'
          });
        }
      });

　二、 base64码  （原理一样，多一步操作，需要将base64码转换成文件流后在操纵）

writeWithHead(params).then(res => {
   if (res.status === 200) {
     this.$message({
        type: "success",
        message: '导出成功'
       })
     this.downloadFileByBase64(res.data,'导出列表')
　} else { 
　　　this.$message({ type: "error", message: '导出失败' });
　}});
dataURLtoBlob (dataurl, filename) {
      let arr = dataurl.split(",");
      let mime = arr[0].match(/:(.*?);/)[1];
      let bstr = atob(arr[1]);
      let n = bstr.length;
      let u8arr = new Uint8Array(n);
      while (n--) {
        u8arr[n] = bstr.charCodeAt(n);
      }
      return new Blob([u8arr], { type: mime });
    },
  downloadFile (url, name = "导出列表") {
      var a = document.createElement("a");
      a.setAttribute("href", url);
      a.setAttribute("download", name);
      a.setAttribute("target", "_blank");
      let clickEvent = document.createEvent("MouseEvents");
      clickEvent.initEvent("click", true, true);
      a.dispatchEvent(clickEvent);
    },
 downloadFileByBase64 (base64, name) {
      let myBlob = this.dataURLtoBlob(base64);
      let myUrl = URL.createObjectURL(myBlob);
      this.downloadFile(myUrl, name);
  },

　有不足之处，还望大佬多指教