通过JavaScript下载文件到本地的方法（单文件）

 const elt = document.createElement('a');
 elt.setAttribute('href', url);
 elt.setAttribute('download', 'file.png');
 elt.style.display = 'none';
 document.body.appendChild(elt);
 elt.click();
 document.body.removeChild(elt);

function downloadWithBlob(url) {
 fetch(url).then(res => res.blob().then(blob => {
  var a = document.createElement('a');
  var url = window.URL.createObjectURL(blob);
  var filename = 'file.png';
  a.href = url;
  a.download = filename;
  a.click();
  window.URL.revokeObjectURL(url);
 }));
}

function downloadPic(url) {
 const img = new Image;
 const canvas = document.createElement('canvas');
 const ctx = canvas.getContext('2d');
 img.onload = function() {
  canvas.width = this.width;
  canvas.height = this.height;
  ctx.drawImage(this, 0, 0);

  const elt = document.createElement('a');
  elt.setAttribute('href', canvas.toDataURL('image/png'));
  elt.setAttribute('download', 'file.png');
  elt.style.display = 'none';
  document.body.appendChild(elt);
  elt.click();
  document.body.removeChild(elt);
 };
 img.crossOrigin = 'anonymous';
 img.src = url;
}

 const iframe = document.createElement('iframe');
 iframe.src = url;
 iframe.style.display = 'none';
 document.body.appendChild(iframe);

 const downloadUrl = 'http://exampale.com/download?url=' + encodeURIComponent(url) + '&name=xxx';
 const elt = document.createElement('a');
 elt.setAttribute('href', downloadUrl);
 elt.setAttribute('download', 'file.png');
 ... 

const url = decodeURIComponent(req.query.url);
http.get(url, (response) => {
 res.setHeader('Content-disposition', 'attachment;filename=' + req.query.name);
 res.setHeader('Content-type', 'application/octet-stream');
 response.pipe(res);
});