viewport.js

/*
 * Check if elements are visible in the current viewport
 * If is visible add a class in-view else remove
*/
function isElementInViewport(el) {
  var rect = el.getBoundingClientRect();
  return (
    rect.top >= 0 &&
    rect.left >= 0 &&
    rect.bottom <= (window.innerHeight || document.documentElement.clientHeight) &&
    rect.right <= (window.innerWidth || document.documentElement.clientWidth)
  );
}

function callbackFunc() {
  for (var i = 0; i < items.length; i++) {
    if (isElementInViewport(items[i])) {
      items[i].classList.add("in-view");
    } else {
      items[i].classList.remove("in-view");
    }
  }
}

// listen for events
window.addEventListener("load", callbackFunc);
window.addEventListener("resize", callbackFunc);
window.addEventListener("scroll", callbackFunc);