I think we are limited but what modules are included in nwjs. Unless you know how to add more?
Running regedit to find the documents folder is one option. But seems messy.
There is a winapi function that you can use to get the location of the documents folder, but it's only accessible from c. Or we could get it with the node ffi module if it was included with nwjs.
Anyways here is the source of a simple c program that just prints the users documents folder location.
getDocumentsFolder.c
To build that with the tiny c compiler
Then to run it you need to run basically this javascript:
Basically it synchronously runs a file and returns the output of the program as utf8 text. I had to modify it slightly as it needs the full path of the program.
Anyways here is my test of it working in nwjs from c2. It only works on export at the moment though. It includes that exe compiled from the source above.
dropbox.com/s/cpwuxmmj7ekocxx/nwjs_getDocumentsFolder.capx