Node/Express: File not downloading using fs.pipe() - node.js

Question

I'm having an issue downloading files to my local machine using fs.pipe() in my Node.js/Express cloud foundry application.
Basically, a POST request is sent to my server side code containing the file name my user wants to download. I access the file using the GET command from the npm module ssh2-sftp-client. Finally, this file gets saved to the users local downloads folder using the npm module downloads-folder to identify this location. The code looks like this:
app.post('/download-file', function(req, res) {
// Declare the files remote and local path as a variable.
const remoteFilename = 'invoice/csv/' + req.body.file;
const localFilename = downloadsFolder() + '/' + req.body.file;
// Use the SFTP GET command to get the file passing its remote path variable.
sftp.get(remoteFilename).then((stream) => {
// Download the file to the users machine.
stream.pipe(fs.createWriteStream(localFilename));
// Redirect user.
res.redirect('/invoice')
});
})
This works perfectly when running locally and the file gets downloaded with no issues. As this screenshot shows, the output for the destination file path is this:
However, when I push this to our cloud foundry provider using cf push, the application still works fine but when I want to download the file it fails. I get no errors when error catching, the only thing thats changed is that the output for the destination file path has changed to:
I have no idea why this is, this code works fine in Chrome, Safari when running locally but when hosted doesn't do anything. Can anyone explain what's going wrong here?
Many thanks,
G

Answer 1

// Download the file to the users machine.
It doesn't do that, though: it downloads the file to the machine on which the server code is running. That's why it seems to work when you run the server on localhost, because the server machine and the user machine are the same.
Offering the file as a download involves streaming the file through the response object, making sure that you set the correct header. Something like this:
sftp.get(remoteFilename).then((stream) => {
res.set('content-disposition', `attachment; filename="${ req.body.file }"`);
stream.pipe(res);
});
When offering a file as "attachment", it typically opens the download window of the browser and the browser stays on the same page. You can't both stream and perform a redirect, but because the browser doesn't change the URL, that should be okay.
Also, you have no control over where the user will download the file to. It may be the downloads folder, but they are free to chose another folder.