问如何在forEach循环中使用JavaScript FileReader？

EN

 let base64Data: string;
 let attachment: Attachment;  
 let blob: Blob;
 docList.forEach(([pdfDoc, title]) => {
            blob = pdfDoc.output('blob'); 
            var reader = new FileReader();
            reader.readAsDataURL(blob);
            reader.onloadend = function() {
              base64data = reader.result;
              attachment = new Attachment();
              attachment.setFilename(title);
              attachment.setContent(base64data);
              attachment.setType('application/pdf');
              attachments.push(attachment);
            }
         });