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                <h2>MutationObserver Example - Recording an Old Attribute Value</h2>

<p>This page uses the MutationObserver API to watch for changes to the <code>data-name</code> attribute on the specified paragraph element. When the <code>data-name</code> attribute is changed by the user, the <code>mutation.oldValue</code> property is used to record the previous value of the attribute each time it's changed.</p>

<p id="myParagraph" data-name="Example" class="paragraph">This paragraph element will be observed for changes to a <code>data-*</code> attribute.</p>

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

<p><input type="text" placeholder="Enter some text">
  <button class="btnAttr">Change the Data Attribute Value</button></p>



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

.paragraph {
  border: dashed 3px #aaa;
  padding: 7px;

button {
  margin: 3px;


                let mPar = document.getElementById('myParagraph'),
    btnAttr = document.querySelector('.btnAttr'),
    btnStart = document.querySelector('.btnStart'),
    btnStop = document.querySelector('.btnStop'),
    input = document.querySelector('input'),
    options = {
      attributes: true,
      attributeOldValue: 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}, old value is ${mutation.oldValue}.`);

function doLogAndBtn(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 () { = input.value;
}, false);

btnStop.addEventListener('click', function () {
  doLogAndBtn('Observing for mutations: STOPPED');
}, false);