I had this difficulty and it took me a while to figure out how to install nodejs modules offline.
I always imagined that it would be simple …. In the back of my head I thought that the gap is as simple as this:
First download the github module into a folder;
Then run the code replacing the path to the folder the module was saved to (when you downloaded from github) and the modulo name;
npm install modulo_name –offline-installers=”C:\your\folder”
Very simple!! 🙂