a标签下载跨域文件，解决download失效问题

downloadFile(url, fileName) {//跨域文件路径、下载到本地的文件名
        var x = new XMLHttpRequest();
        x.open("GET", url, true);
        x.responseType = 'blob';
        x.onload=function(e) {
          var url = window.URL.createObjectURL(x.response)
          var a = document.createElement('a');
          a.href = url
          a.download = fileName;
          a.click()
        }
        x.send();
      },