test-two.js

// The expensive animation is causing performance issues. Write the function 'yourFunction' that reduces calls to the animation.

function scrollHandler() {
  var items = document.querySelectorAll('.list-item');
  var i = 0;
  var len = items.length;

  for(; i < len; i++) {
    // perform expensive animation
  }
}

window.addEventListener('scroll', yourFunction(scrollHandler));