How to track changes in environment variables in nodejs?

Question

JavaScript

There is a script that should look at a certain environment variable and, depending on its value, perform some action.
So how do you get the new value of a variable if it changes?
UPD:
Perhaps it is important to describe what it is for.
It is necessary to react to certain changes in the system without restarting app.js. In particular, after performing a certain task, a variable is set in the system that I would like to monitor from app.js.
UPD 2:
It may be possible to dodge somehow, of course, through exec
But for now I will not put my mind to it.

Answer 1

Watcher can be written.