When I tried /Dropbox/think/development/js/YSlow_example/ everything worked well, but I tried to do the same here, and npm install yslowjs --save didn't work, so when I copied the module in YSlow_example and did npm install --save yslowjs at the same time, everything worked fine, so not sure if was the module copied or the npm install with the --save before yslowjs that worked.