MutationObserver not picking up on child nodes

Question

Fiddle: https://jsfiddle.net/7gj26hqu/

I would like to have one MutationObserver that can detect all new nodes within itself. In the example, I have set {childList: true, subtree: true}, but div#nested does not appear in the MutationObserver (shown in the console).

How can I have an observer detects any depth of child nodes?

const domObserver = new MutationObserver((records) => {
  records.forEach((record) => {
    console.log(record)
  })
})

domObserver.observe(document.querySelector('#frame'), {childList: true, subtree: true})

// copy child nodes out of #template (as a string) and inject them into #frame for the observer to detect
document.querySelector('#frame').innerHTML = document.querySelector('#template').innerHTML

<div id="frame"></div>

<div id="template" style="display: none;">
   <div class="level-1">
    <div class="level-2">

      <div id="nested">
        I exist in the DOM but am not being seen by the MutationObserver
      </div>

    </div>
  </div>

  <div class="level-1">
    <div class="level-2">

    </div>
  </div>

  <div class="level-1">
    <div class="level-2">

    </div>
  </div>
</div>

Answer 1

It looks like when a container that's being observed has its innerHTML set, the children of the container are emptied, and then new children get added fully intact. Using a synchronous observer (just so you can see what's happening), see how child nodes exist when the element is being appended:

// Look at results in browser console:

window.addEventListener('DOMNodeInserted', (e) => {
  console.log(e.path[0].childNodes.length);
});

document.querySelector('#frame').innerHTML = document.querySelector('#template').innerHTML

<div id="frame"></div>

<div id="template" style="display: none;">
   <div class="level-1">
    <div class="level-2">
    
      <div id="nested">
        I exist in the DOM but am not being seen by the MutationObserver
      </div>
      
    </div>
  </div>
  
  <div class="level-1">
    <div class="level-2">
    
    </div>
  </div>
  
  <div class="level-1">
    <div class="level-2">
    
    </div>
  </div>
</div>

The grandchild of the container being observed is attached to the child before the child gets attached to the container, so the subtree: true doesn't see the attachment action.

To detect all child nodes inserted this way, you'll have to recursively iterate through all elements in the MutationRecord manually, despite the subtree: true.

const recurse = (parent) => {
  
  console.log(parent);
  if (parent.childNodes) {
    [...parent.childNodes].forEach(recurse);
  }
};
const domObserver = new MutationObserver((records) => {
  for (const record of records) {
    for (const node of record.addedNodes) {
      recurse(node);
    }
  }
})

domObserver.observe(document.querySelector('#frame'), {childList: true, subtree: true})

// copy child nodes out of #template (as a string) and inject them into #frame for the observer to detect
document.querySelector('#frame').innerHTML = document.querySelector('#template').innerHTML

<div id="frame"></div>

<div id="template" style="display: none;">
   <div class="level-1">
    <div class="level-2">

      <div id="nested">
        I exist in the DOM but am not being seen by the MutationObserver
      </div>

    </div>
  </div>

  <div class="level-1">
    <div class="level-2">

    </div>
  </div>

  <div class="level-1">
    <div class="level-2">

    </div>
  </div>
</div>

Result:

If you want to use a TreeWalker instead:

const frame = document.querySelector('#frame');
const domObserver = new MutationObserver(() => {
  // If the container's innerHTML was assigned to, iterate over all descendants:
  const treeWalker = document.createTreeWalker(frame);
  const nodes = [];
  let currentNode = treeWalker.currentNode;
  while (currentNode) {
    nodes.push(currentNode);
    currentNode = treeWalker.nextNode();
  }
  console.log(nodes);
});

domObserver.observe(frame, {
  childList: true,
  subtree: true
})

// copy child nodes out of #template (as a string) and inject them into #frame for the observer to detect
document.querySelector('#frame').innerHTML = document.querySelector('#template').innerHTML

<div id="frame"></div>

<div id="template" style="display: none;">
  <div class="level-1">
    <div class="level-2">

      <div id="nested">
        I exist in the DOM but am not being seen by the MutationObserver
      </div>

    </div>
  </div>

  <div class="level-1">
    <div class="level-2">

    </div>
  </div>

  <div class="level-1">
    <div class="level-2">

    </div>
  </div>
</div>