<script>
  var observer = new IntersectionObserver(function(changes) {
    console.log(changes);
    changes.forEach(function(element,index) {
      // statements
      if (element.intersectionRatio > 0 && element.intersectionRatio <= 1) {
        element.target.src = element.target.dataset.src;
      }
    });
  });


  function addObserver() {
    var listItems = document.querySelectorAll('.list-item-img');
    listItems.forEach(function(item) {
      observer.observe(item);
    });
  }

  addObserver();
</script>