This tutorial explains how to use the new IntersectionObserver API from HTML5. It allows you to determine when elements are entering or leaving the screen. You can figure out when elements are touching each other. You can define areas of the screen which will act as triggers for other code.
Sample Code GIST: https://gist.github.com/prof3ssorSt3v3/c567586a9a36da47df0e27a1d26334cc
