process.env.POST

In Nodejs, you can see the code like this

the port is either process.env.PORT or 3000.
The "process.evn.PORT" can be passed in by command line argument. For example:

now the port is 4444.

Sometimes the setting may be env specific, e.g. database URL. Here is one solution.
https://gist.github.com/wilsonbalderrama/0ad2b64b5ab6287c7318

config.js