Environment variables are really handy to pass value to a process. It really helps you to easily set the environment like development or production or pass a port number to use, so you don’t want change the code to set a temporarily value.
You can read the environment variables in nodejs using
process.env.VARIABLE_NAME
For example if you want to pass the port number, you can use
var port = process.env.PORT || 9000;
so it will take 9000 as the default port and if you want change the port temporarily, you can run like
PORT=9002 node app.js
Hope that helped you.