intersectionObserverDemo.tsx

const element = document.querySelector(ELEMENT_SELECTOR);
const parentElement = document.querySelector(PARENT_ELEMENT_SELECTOR);
const observer = new IntersectionObserver(
  (entries: any) => {
    /**
     * Callback will run when observed element intersect with given root
     */
  },
  {
    root: parentElement, // if passed null take document as root 
    rootMargin: "0px", // give 0px margin to container value is given like CSS margin but it is not CSS
    threshold: 0.5, // when 50% of the observed element is intersected
  }
);
observer.observe(element);

// if you want to remove observer
observer.unobserve(element);