Can't understand the use of window.onscroll = null in React

Question

What's the use of window.onscroll = null here? It shows no difference even when i comment that last line out.

const [isScrolled, setIsScrolled] = useState(false);

window.onscroll = () => {
    setIsScrolled(window.pageYOffset === 0? false : true)
    console.log(isScrolled);

    return () => (window.onscroll = null);
};

Answer 1

Removes the event handler added to onScroll event. But better to move it inside useEffect hook.

Answer 2

As written, the window.onscroll = null is useless. It will never be executed. However, this code is suspiciously close to code that is useful. The following code is what i would expect to see:

const [isScrolled, setIsScrolled] = useState(false);
useEffect(() => {
  window.onscroll = () => {
    setIsScrolled(window.pageYOffset === 0? false : true)
  };
  return () => (window.onscroll = null);
}, []);

In this modified version, return () => (window.onscroll = null) is returning a cleanup function for the effect. When the component unmounts, this function runs and removes the event listener. This is important to do so that you avoid memory leaks.