Express helper, res.download

Express has a helper for this:

app.get('/xxx', function(req, res){
  var file = __dirname + '/xxx';
  res.download(file); // Set disposition and send it.
});

http file download header

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Disposition

res.setHeader('Content-disposition', 'attachment; filename=xxx');