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HTML

              
                <h2>MutationObserver Example - childList</h2>

<p>This page uses the MutationObserver API to watch for changes to the DOM tree. In this case, the MutationObserver is looking for child nodes to be changed on the targeted element (the list).</p>

<p> A "Start..." and a "Stop..." button are used to start and stop the MutationObserver. Console messages are displayed specifying which change took place and whether the MutationObserver is started or stopped.</p>

<ul id="myList" class="list">
  <li>Apples</li>
  <li>Oranges</li>
  <li>Bananas</li>
  <li class="child">Peaches</li>
</ul>

<p><button class="btnStart">Start Observing for Mutations</button> 
  <button class="btnStop" disabled>Stop Observing for Mutations</button></p>
<p><button class="btnChild">Add/Remove a Child Node</button></p>


              
            
!

CSS

              
                body {
  padding: 0 20px;
  font-size: 1.2em;
}

.list {
  min-height: 90px;
}

button {
  margin: 3px;
}
              
            
!

JS

              
                let mList = document.getElementById('myList'),
    btnChild = document.querySelector('.btnChild'),
    btnStart = document.querySelector('.btnStart'),
    btnStop = document.querySelector('.btnStop'),
    // The options object, later passed into observe()
    options = {
      childList: true
    },
    // Grab a reference to the MutationObserver
    // created with the constructor
    observer = new MutationObserver(mCallback);

// This is the callback passed into the constructor
function mCallback(mutations) {
  // loop through the mutations record to see
  // if any match what we want
  for (let mutation of mutations) {
    if (mutation.type === 'childList') {
      console.log('Mutation Detected: A child node has been added or removed.');
    }
  }
}

// Just a utility function to simplify some btn/log code
function doLogAndBtn(msg) {
  console.log(msg);
  btnStart.disabled = !btnStart.disabled;
  btnStop.disabled = !btnStop.disabled;
}

btnStart.addEventListener('click', function () {
  // Start observing the list element using
  // the passed in options as qualifiers
  observer.observe(mList, options);
  doLogAndBtn('Observing for mutations: STARTED');
}, false);

// This button interactively modifies the DOM
// which meets the specified observe criteria
btnChild.addEventListener('click', function () {
  if (document.querySelector('.child')) {
    mList.removeChild(document.querySelector('.child'));
  } else {
    mList.insertAdjacentHTML('beforeend', '\n<li class="child">Peaches</li>');
  }
}, false);

btnStop.addEventListener('click', function () {
  // This stops the MutationObserver
  observer.disconnect();
  doLogAndBtn('Observing for mutations: STOPPED');
}, false);
              
            
!
999px

Console