Nodejs - writing and saving files

I am investigating how to download files to a user's local machine but I'm not quite sure what I need in order to do this. I'm using Nodejs and Express with Angularjs on the front-end.

User's can write text into a textarea and it's this text that will be written to the file.

To do this I have:

...
fs = require('fs');
fs.writeFile('filename.txt', textarea.text, function (err) {
  if (err) return console.log(err);
  res.send(200);
});
...

Once the file is created how do I get it to download on the user's machinea?

Use res.download

res.download('filename.txt');

http://expressjs.com/4x/api.html#res.download

If you don't need to store the file on the server, you could just sent it back to the user directly:

res.attachment('filename.txt');
res.set('Content-Type', 'text/plain');
res.send(textarea.text);

This is not only simpler but also improves performance (no disk i/o) and more secure (no untrusted files on your server).