当新的子元素出现在html元素中时触发事件[重复]

function addNew() {
  var targetNode = document.getElementById('chat-wrapper');
  targetNode.innerHTML += '<span> Message' + targetNode.querySelectorAll('span').length + '</span>';
}

window.onload = function() {

  var targetNode = document.getElementById('chat-wrapper');


  var config = {
    attributes: true,
    childList: true,
    subtree: true
  };

  var observer = new MutationObserver(callback);


  observer.observe(targetNode, config);
}



// Callback function to execute when mutations are observed
var callback = function(mutationsList) {
  for (var mutation of mutationsList) {
    if (mutation.type == 'childList') {
      console.log('A child node has been added or removed.');
    } else if (mutation.type == 'attributes') {
      console.log('The ' + mutation.attributeName + ' attribute was modified.');
    }
  }
};

<div id="chat-wrapper">
</div>

<button onclick='addNew()'>Add New</button>