 Knowing what is and what is not visible can be very useful information. You could lazy load images once they come into view. You could stop playing a video once it goes out of view, or you could save to your analytics how much of your blog post was actually read by the user. Intersection Observer are a performance API that notify you when an element enters or leaves the viewport. Intersection Observer are very efficient to work across iframes and are perfect to react to scrolling. To use Intersection Observer, create a new Intersection Observer by passing it a callback function for each element that you're interested in, call.observe on the Intersection Observer. From now on, whenever any of the observed elements enter or leave the viewport, your callback will be called with details about what happened and to which element. Intersection Observer offer a couple of options, one of them being the Thresholds option. You can set the Threshold option to an array of numbers between zero and one to get multiple notifications per element. For example, if you set the Threshold option to zero, point three, three, point six, six, and one, you'll get notification not only on entrance and exit, but also when one third or two thirds of the elements area are in view. For more information on the use cases and options, read my article in the description below. Bye. I am a true artist and my work just convinced you of that. So if you want more, you should subscribe to this channel. And if you wanna watch the latest supercharged live stream, you can do that over there.