Document: scroll event

The scroll event fires when the document view has been scrolled. For element scrolling, see Element: scroll event.

Since scroll events can fire at a high rate, the event handler shouldn't execute computationally expensive operations such as DOM modifications. Instead, it is recommended to throttle the event using requestAnimationFrame(), setTimeout(), or a CustomEvent, as follows.

Note, however, that input events and animation frames are fired at about the same rate, and therefore the optimization below is often unnecessary. This example optimizes thescroll event for requestAnimationFrame.

// Reference: http://www.html5rocks.com/en/tutorials/speed/animations/

let lastKnownScrollPosition = 0;
let ticking = false;

function doSomething(scrollPos) {
  // Do something with the scroll position
}

document.addEventListener('scroll', function(e) {
  lastKnownScrollPosition = window.scrollY;

  if (!ticking) {
    window.requestAnimationFrame(function() {
      doSomething(lastKnownScrollPosition);
      ticking = false;
    });

    ticking = true;
  }
});

See more, similar examples on the resize event page.

• Element: scroll event