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HTML

              
                <h2>MutationObserver Example - Attributes</h2>

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

<p id="myParagraph" class="paragraph">This paragraph element will be observed for changes to its attributes.</p>

<p><button class="btnStart">Start Observing for Mutations</button> 
  <button class="btnStop" disabled>Stop Observing for Mutations</button></p>

<p><button class="btnAttr">Toggle a Class</button></p>


              
            
!

CSS

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

.paragraph {
  border: dashed 3px #aaa;
  padding: 10px;
  font-size: 20px;
}

button {
  margin: 3px;
}
              
            
!

JS

              
                let mPar = document.getElementById('myParagraph'),
    btnAttr = document.querySelector('.btnAttr'),
    btnStart = document.querySelector('.btnStart'),
    btnStop = document.querySelector('.btnStop'),
    options = {
      attributes: true
    },
    observer = new MutationObserver(mCallback);

function mCallback (mutations) {
  for (let mutation of mutations) {
    if (mutation.type === 'attributes') {
      console.log(`Mutation Detected: An attribute has been added or removed. The attribute name is: ${mutation.attributeName}.`);
    }
  }
}

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

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

btnAttr.addEventListener('click', function () {
  mPar.classList.toggle('paragraph');
}, false);

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

Console