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// 用fetch发送请求
fetch('/upload/user.png').then((res) => {
  res.blob().then((blob) => {
    const blobUrl = window.URL.createObjectURL(blob);
    // 这里的文件名根据实际情况从响应头或者url里获取
    const filename = 'user.jpg';
    const a = document.createElement('a');
    a.href = blobUrl;
    a.download = filename;;
    a.click();
    window.URL.revokeObjectURL(blobUrl);
  });
});

const xhr = new XMLHttpRequest();
xhr.open('GET', '/upload/user.png', true);
xhr.responseType = 'blob';
xhr.onload = function() {
  if (this.status === 200) {
  const fileName = 'test.jpg';
    const blob = new Blob([this.response]);
    const blobUrl = window.URL.createObjectURL(blob);
    const a = document.createElement('a');
    a.href = blobUrl;
    a.download = fileName;
    a.click();
    window.URL.revokeObjectURL(blobUrl);
  }
};
xhr.send();