JS+CSS3响应式垂直滚动时间轴动画特效

JS代码部分：

var items = document.querySelectorAll(".timeline li"); 
 
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])) { 
      if(!items[i].classList.contains("in-view")){ 
        items[i].classList.add("in-view"); 
      } 
    } else if(items[i].classList.contains("in-view")) { 
        items[i].classList.remove("in-view"); 
    } 
  } 
} 
  
window.addEventListener("load", callbackFunc); 
window.addEventListener("scroll", callbackFunc);