Blob 转base64

blobToBase64(blob) {
       return new Promise((resolve, reject) => {
         const fileReader = new FileReader();
         fileReader.onload = (e) => {
           resolve(e.target.result);
         };
         // readAsDataURL
         fileReader.readAsDataURL(blob);
         fileReader.onerror = () => {
           reject(new Error('文件流异常'));
         };
       });
   }