I am running my nodejs app by npm start
I just installed nodemon by
sudo npm install -g nodemon so that i can get my server restarted when i s
You're running express 4, which has the app.listen call in a different file than app.js. The command you're looking for is nodemon bin/www (localhost and 3000 are not needed in this scenario).
In fact, you can even run nodemon with no args, and it'll read what command needs to be run from scripts.start in package.json (which express generates automatically).