QUESTION: Что такое Intersection Observer?

The Intersection Observer API in JavaScript is a powerful and efficient tool designed for asynchronously observing the intersection of a target element with its parent container or with the browser's own viewport. The main advantage of this approach is that the code runs not on every mouse movement or scroll pixel, but only at moments when the visibility state changes, which dramatically reduces CPU load and improves web page performance.

To create an observer, the new IntersectionObserver(callback, options) constructor is used, where the callback function fires upon each intersection, and the options object allows you to configure the root element, margins, and thresholds. After creating the observer, you need to start tracking a specific DOM node using the observer.observe(element) method, passing the desired element to it.

Classic examples of the practical application of Intersection Observer are two common tasks: lazy loading of images and infinite scrolling. In the first case, an image starts loading from the server only when the user approaches it during scrolling. In the second case, a special marker element at the very bottom of the page triggers the loading of the next portion of content right at the moment it enters the user's field of view, creating a smooth and seamless interaction experience.