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HTML

              
                <h2>MutationObserver Example - Using the takeRecords() Method</h2>

<p>In this demo, the MutationObserver is watching for changes to the list. But in this case, the <code>takeRecords()</code> method is used to get info from and otherwise deal with the mutation record array.</p>

<p>When the <code>takeRecords()</code> method is called, the mutation queue is emptied before it's delivered to the callback function, so the callback function won't do anything in this intance. A console message will display for start/stop and to indicate the change that took place via the <code>takeRecords()</code> method.</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 Element</button></p>


              
            
!

CSS

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

.list {
  min-height: 80px;
}

button {
  margin: 3px;
}
              
            
!

JS

              
                let mList = document.getElementById('myList'),
    btnChild = document.querySelector('.btnChild'),
    btnStart = document.querySelector('.btnStart'),
    btnStop = document.querySelector('.btnStop'),
    options = {
      childList: true
    },
    observer = new MutationObserver(mCallback),
    myRecords;

function mCallback(mutations) {
  for (let mutation of mutations) {
    if (mutation.type === 'childList') {
      console.log('Mutation Detected: A child element has been added or removed.');
    }
  }
}

function doLogAndBtn(msg) {
  console.log(msg);
  btnStart.disabled = !btnStart.disabled;
  btnStop.disabled = !btnStop.disabled;
}

btnStart.addEventListener('click', function () {
  observer.observe(mList, options);
  doLogAndBtn('Observing for mutations: STARTED');
}, false);

btnChild.addEventListener('click', function () {
  if (document.querySelector('.child')) {
    mList.removeChild(document.querySelector('.child'));
  } else {
    mList.insertAdjacentHTML('beforeend', '\n<li class="child">Peaches</li>');
  }
  myRecords = observer.takeRecords();
}, false);

btnStop.addEventListener('click', function () {
  observer.disconnect();
  if (myRecords) {
    console.log(`${myRecords[0].target} was changed using the ${myRecords[0].type} option.`);
  }
  doLogAndBtn('Observing for mutations: STOPPED');
}, false);
              
            
!
999px

Console