Edit in JSFiddle

var list = document.getElementById("parentElement");

var MutationObserver = window.MutationObserver ||
        window.WebKitMutationObserver || 
        window.MozMutationObserver;
 
  var observer = new MutationObserver(function(mutations) {  
    mutations.forEach(function(mutation) {
      if (mutation.type === 'childList') {
            var count = list.children.length;
          list.children[count-1].innerHTML = 
              "Element " + count + " has been injected!";
      }
    });
  });
 
  observer.observe(list, {
  	attributes: true, 
  	childList: true, 
  	characterData: true 
   });

setInterval(
    function(){	
       list.appendChild(
           document.createElement("li")
       )
    }, 
2000);

<ul id="parentElement"></ul>

#parentElement > li {
	padding: 10px;
	background: #FF6A6A;
	margin-bottom: 10px;
    list-style: none outside none;
}