Setting Environment Variables for Node to retrieve

Environment variables (in this case) are being used to pass credentials to your application. USER_ID and USER_KEY can both be accessed from process.env.USER_ID and process.env.USER_KEY respectively. You don't need to edit them, just access their contents.

It looks like they are simply giving you the choice between loading your USER_ID and USER_KEY from either process.env or some specificed file on disk.

Now, the magic happens when you run the application.

USER_ID=239482 USER_KEY=foobar node app.js

That will pass the user id 239482 and the user key as foobar. This is suitable for testing, however for production, you will probably be configuring some bash scripts to export variables.

I highly recommend looking into the dotenv package.

https://github.com/motdotla/dotenv

It's kind of similar to the library suggested within the answer from @Benxamin, but it's a lot cleaner and doesn't require any bash scripts. Also worth noting that the code base is popular and well maintained.

Basically you need a .env file (which I highly recommend be ignored from your git/mercurial/etc):

FOO=bar
BAZ=bob

Then in your application entry file put the following line in as early as possible:

require('dotenv').config();

Boom. Done. 'process.env' will now contain the variables above:

console.log(process.env.FOO);
// bar

The '.env' file isn't required so you don't need to worry about your app falling over in it's absence.