//base64轉(zhuǎn)png
    base64ImgtoFile(dataurl, filename = 'file') {
      const arr = dataurl.split(',')
      const mime = arr[0].match(/:(.*?);/)[1]
      const suffix = mime.split('/')[1]
      const bstr = atob(arr[1])
      let n = bstr.length
      const u8arr = new Uint8Array(n)
      while (n--) {
        u8arr[n] = bstr.charCodeAt(n)
      }
      return new File([u8arr], `${filename}.${suffix}`, {
        type: mime
      })
    },
    //圖片下載
    downloadBlob(blob, filename) {
      const url = URL.createObjectURL(blob);
      const a = document.createElement('a');
      a.href = url;
      a.download = filename;
      a.click();
      URL.revokeObjectURL(url);
    }

    var base64Img = 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACAAAAAgCAYAAABzenr0AAAAAXNSR0IArs4c6QAABAFJREFU
..........................DlzUWu+L9NALAI4D+JOIkszc5PuWNABcBbAbwGGt9ewmAFLKJ5n5s5iXEhZCvBQEgQ3YEdd1p4UQR9PpdH11dXWRmV8motPJZPJgIpH4bWVl5dDw8PAR3/ftE2B9BuzX6Ojo/kajYZvp/i5vhAYRXbBPs3YTdcuYlPIuZj7tOM4r2+n+vx6nPdR/x1T/Aau20TA//wiSAAAAAElFTkSuQmCC';
    var imgFile = this.base64ImgtoFile(base64Img);
    this.downloadBlob(imgFile, 'test.png')

 getBase64(file) {
      //把圖片轉(zhuǎn)成base64編碼
      return new Promise(function (resolve, reject) {
        let reader = new FileReader();
        let imgResult = "";
        reader.readAsDataURL(file);
        reader.onload = function () {
          imgResult = reader.result;
        };
        reader.onerror = function (error) {
          reject(error);
        };
        reader.onloadend = function () {
          resolve(imgResult);
        };
      });
    },