Capturing a custom event in node js

I know how promises and callbacks work in node js but I am very confused about how to capture custom events.
Let's say I have a global variable x

 var x = 10 

Some asynchronus functions in the event loop update value of x. How can I capture variable x having a particular value? Like what should I do when I want to run a function

    foo()

when variable x attain a value of let's say 50?

via smart.Waqar