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MutationObserver: MutationObserver() constructor - Web APIs

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The DOM MutationObserver() constructor — part of the MutationObserver interface — creates and returns a new observer which invokes a specified callback when DOM events occur.

DOM observation does not begin immediately; the observe() method must be called first to establish which portion of the DOM to watch and what kinds of changes to watch for.

new MutationObserver(callback)

callback

A function which will be called on each DOM change that qualifies given the observed node or subtree and options.

The callback function takes as input two parameters:

1. An array of MutationRecord objects, describing each change that occurred.
2. The MutationObserver which invoked the callback. This is most often used to disconnect the observer using MutationObserver.disconnect().

See the examples below for more details.

A new MutationObserver object, configured to call the specified callback when DOM mutations occur.

This example has buttons to add an <li> element to a list, and to remove the first <li> element from the list.

We use a MutationObserver to be notified about changes to the list. In the callback, we log additions and removals, and as soon as the list is empty, we disconnect the observer.

The "Reset example" button resets the example to its original state.

<button id="add">Add child</button>
<button id="remove">Remove child</button>
<button id="reset">Reset example</button>

<ul id="container"></ul>

<pre id="log"></pre>

#container,
#log {
  height: 150px;
  overflow: scroll;
}

#container li {
  background-color: paleturquoise;
  margin: 0.5rem;
}

const add = document.querySelector("#add");
const remove = document.querySelector("#remove");
const reset = document.querySelector("#reset");
const container = document.querySelector("#container");
const log = document.querySelector("#log");

let namePrefix = 0;

add.addEventListener("click", () => {
  const newItem = document.createElement("li");
  newItem.textContent = `item ${namePrefix}`;
  container.appendChild(newItem);
  namePrefix++;
});

remove.addEventListener("click", () => {
  const itemToRemove = document.querySelector("li");
  if (itemToRemove) {
    itemToRemove.parentNode.removeChild(itemToRemove);
  }
});

reset.addEventListener("click", () => {
  document.location.reload();
});

function logChanges(records, observer) {
  for (const record of records) {
    for (const addedNode of record.addedNodes) {
      log.textContent = `Added: ${addedNode.textContent}\n${log.textContent}`;
    }
    for (const removedNode of record.removedNodes) {
      log.textContent = `Removed: ${removedNode.textContent}\n${log.textContent}`;
    }
    if (record.target.childNodes.length === 0) {
      log.textContent = `Disconnected\n${log.textContent}`;
      observer.disconnect();
    }
    console.log(record.target.childNodes.length);
  }
}

const observerOptions = {
  childList: true,
  subtree: true,
};

const observer = new MutationObserver(logChanges);
observer.observe(container, observerOptions);

Try clicking "Add child" to add list items, and "Remove child" to remove them. The observer callback logs additions and removals. As soon as the list is empty, the observer logs a "Disconnected" message and disconnects the observer.

The "Reset example" button reloads the example so you can try it again.

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