Change global variables programmatically using Heroku?

I currently have an application built with Express.js that runs via Heroku. I need to define a global variable that I can decrement each time a specific function is called, but I'm not sure how to do that programatically. I have a file of constants called consts.js and was trying to just set the variable (REMAINING) as follows: CONST.REMAINING -= 1 whenever the function is called, but unfortunately that doesn't work. What is a better way to go about this?

via user3802348