IntersectionObserver.observe()

The observe() method of the IntersectionObserver interface adds an element to the set of target elements being watched by the IntersectionObserver. One observer has one set of thresholds and one root, but can watch multiple target elements for visibility changes in keeping with those.

To stop observing the element, call IntersectionObserver.unobserve().

When the visibility of the specified element crosses over one of the observer’s visibility thresholds (as listed in IntersectionObserver.thresholds), the observer’s callback is executed with an array of IntersectionObserverEntry objects representing the intersection changes which occurred. Note that this design allows multiple elements’ intersection changes to be processed by a single call to the callback.