I found this which is for desktop but is likely the same for documents. On windows at least, any language will have a "desktop" folder which points to the same folder with the translated version.
stackoverflow.com/questions/57742157/node-js-how-to-find-the-desktop-path
So for windows at least it would be this. Which will work for any language.
NWJS.UserFolder&"Documents\"
or you can also do a check to see if that folder exists and use this if it doesn't.
NWJS.UserFolder&"my documents\"
For mac and linux you can try. Notice the other slash. Actually you could probably use this for windows too. Windows doesn't care what slash you use.
NWJS.UserFolder&"Documents/"
I'm not sure if mac and linux do the same thing with different languages as windows. It looks like those are valid folder on those platforms like windows too.
Worst case you can first see if the folder exists, and create it if it doesn't.