How can I detect window scroll ended in javascript?

You could use requestAnimationFrame to detect when the scrollTop is greater than y

requestAnimationFrame is way better than setting both an interval and an event listener on scroll, for a matter of performance.

const element = document.getElementById('element');
const y = element.getBoundingClientRect().top + window.scrollY;


function checkScrollEnd() {
  if ((window.scrollY || document.body.scrollTop || document.documentElement.scrollTop) < y) {
    window.requestAnimationFrame(checkScrollEnd);
  }
  else {
    alert('end of scroll')   
  }
}

window.requestAnimationFrame(checkScrollEnd);


window.scroll({
  top: y,
  behavior: 'smooth'  
});

Example fiddle