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)
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.
HTML
<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>
CSS
#container,
#log {
height: 150px;
overflow: scroll;
}
#container li {
background-color: paleturquoise;
margin: 0.5rem;
}
JavaScript
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);
Result
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.