If you want to convert to es6 with using babel:
npm install -D babel-cli babel-preset-es2015

create a new file in your root directory called .babelrc and use this code:
{ "presets": ["es2015"] }

in your package.json, change your start script to : ``"start": "nodemon app.js --exec babel-node"

and if you want to add a build & serve command for running on production:
"build": "babel app.js -d dist", "serve": "node dist/app.js"

Your package is missing nodemon so when running npm start, it fails.
run this command and commit:
npm install -D nodemon

Also, a side note, you might want to separate your dependancies and dev dependancies in your package as well. Besides that, great example of passport with mongojs.