I was really impressed with the questions from the interviewer.

The questions were open-ended and involved a discussion about my experience, as well as my opinions on technology and patterns in frontend frameworks.

The questions fed off of my answers as we meandered around React, state management, MobX, functional programming, and reactive programming.

The code problem even seemed to stem from my answers, which I thought was a really nice touch.

Given an animation library in an object-oriented fashion (i.e., a class Animation with subscribe() and unsubscribe() methods that attach event handlers to some animating element), how would you rewrite this behavior in a functional style?