Intersection Observer is a JavaScript API that we can use to do a lot of really useful things that we used to have to rely on scroll event listeners for. Instead of firing off over and over and over again on tons of items, they sit there and wait for their target intersect with the page before firing.
We can use them to watch for when things enter or leave the viewport, and we can play with the options to change how much of the item has entered (the threshold), or how far into the page it is (the rootMargin).
In this video we're exploring the fundamentals of how they work so that in the following weeks we can start having fun with them!