Storing application config in the environment variables helps separate development, testing and production configuration.
I've been using dotenv to store my local development settings in a .env file (which I don't copy to production)
Simply put
require('dotenv').load();
at the top of your app.js file, then create a .env file with
KEY=VALUE_PAIRS
like so.
Now KEY is accessable in your application using process.env.KEY