MutationObserver: observe() method

target

A DOM Node (which may be an Element) within the DOM tree to watch for changes, or to be the root of a subtree of nodes to be watched.

options

An object providing options that describe which DOM mutations should be reported to mutationObserver's callback. At a minimum, one of childList, attributes, and/or characterData must be true when you call observe(). Otherwise, a TypeError exception will be thrown.

Options are as follows:

subtree Optional

Set to true to extend monitoring to the entire subtree of nodes rooted at target. All of the other properties are then extended to all of the nodes in the subtree instead of applying solely to the target node. The default value is false.

childList Optional

Set to true to monitor the target node (and, if subtree is true, its descendants) for the addition of new child nodes or removal of existing child nodes. The default value is false.

attributes Optional

Set to true to watch for changes to the value of attributes on the node or nodes being monitored. The default value is true if either of attributeFilter or attributeOldValue is specified, otherwise the default value is false.

attributeFilter Optional

An array of specific attribute names to be monitored. If this property isn't included, changes to all attributes cause mutation notifications.

attributeOldValue Optional

Set to true to record the previous value of any attribute that changes when monitoring the node or nodes for attribute changes; See Monitoring attribute values for an example of watching for attribute changes and recording values. The default value is false.

characterData Optional

Set to true to monitor the specified target node (and, if subtree is true, its descendants) for changes to the character data contained within the node or nodes. The default value is true if characterDataOldValue is specified, otherwise the default value is false.

characterDataOldValue Optional

Set to true to record the previous value of a node's text whenever the text changes on nodes being monitored. The default value is false.

None (undefined).

TypeError

Thrown in any of the following circumstances:

• The options are configured such that nothing will actually be monitored. (For example, if childList, attributes, and characterData are all false.)
• The value of options.attributes is false (indicating that attribute changes are not to be monitored), but attributeOldValue is true and/or attributeFilter is present.
• The characterDataOldValue option is true but characterData is false (indicating that character changes are not to be monitored).

You can call observe() multiple times on the same MutationObserver to watch for changes to different parts of the DOM tree and/or different types of changes. There are some caveats to note:

• If you call observe() on a node that's already being observed by the same MutationObserver, all existing observers are automatically removed from all targets being observed before the new observer is activated.
• If the same MutationObserver is not already in use on the target, then the existing observers are left alone and the new one is added.

Mutation observers are intended to let you be able to watch the desired set of nodes over time, even if the direct connections between those nodes are severed. If you begin watching a subtree of nodes, and a portion of that subtree is detached and moved elsewhere in the DOM, you continue to watch the detached segment of nodes, receiving the same callbacks as before the nodes were detached from the original subtree.

In other words, until you've been notified that nodes are being split off from your monitored subtree, you'll get notifications of changes to that split-off subtree and its nodes. This prevents you from missing changes that occur after the connection is severed and before you have a chance to specifically begin monitoring the moved node or subtree for changes.

Theoretically, this means that if you keep track of the MutationRecord objects describing the changes that occur, you should be able to "undo" the changes, rewinding the DOM back to its initial state.

In this example, we demonstrate how to call the method observe() on an instance of MutationObserver, once it has been set up, passing it a target element and an options object.

// identify an element to observe
const elementToObserve = document.querySelector("#targetElementId");

// create a new instance of `MutationObserver` named `observer`,
// passing it a callback function
const observer = new MutationObserver(() => {
  console.log("callback that runs when observer is triggered");
});

// call `observe()` on that MutationObserver instance,
// passing it the element to observe, and the options object
observer.observe(elementToObserve, { subtree: true, childList: true });

In this example, a Mutation Observer is set up to watch for changes to the status and username attributes in any elements contained within a subtree that displays the names of users in a chat room. This lets the code, for example, reflect changes to users' nicknames, or to mark them as away from keyboard (AFK) or offline.

function callback(mutationList) {
  mutationList.forEach((mutation) => {
    switch (mutation.type) {
      case "attributes":
        switch (mutation.attributeName) {
          case "status":
            userStatusChanged(mutation.target.username, mutation.target.status);
            break;
          case "username":
            usernameChanged(mutation.oldValue, mutation.target.username);
            break;
        }
        break;
    }
  });
}

const userListElement = document.querySelector("#userlist");

const observer = new MutationObserver(callback);
observer.observe(userListElement, {
  attributeFilter: ["status", "username"],
  attributeOldValue: true,
  subtree: true,
});

In this example we observe an element for attribute value changes, and add a button which toggles the element's dir attribute between "ltr" and "rtl". Inside the observer's callback, we log the old value of the attribute.

HTML

<button id="toggle">Toggle direction</button><br />
<div id="container">
  <input type="text" id="rhubarb" dir="ltr" value="Tofu" />
</div>
<pre id="output"></pre>

CSS

body {
  background-color: paleturquoise;
}

button,
input,
pre {
  margin: 0.5rem;
}

JavaScript

const toggle = document.querySelector("#toggle");
const rhubarb = document.querySelector("#rhubarb");
const observerTarget = document.querySelector("#container");
const output = document.querySelector("#output");

toggle.addEventListener("click", () => {
  rhubarb.dir = rhubarb.dir === "ltr" ? "rtl" : "ltr";
});

const config = {
  subtree: true,
  attributeOldValue: true,
};

const callback = (mutationList) => {
  for (const mutation of mutationList) {
    if (mutation.type === "attributes") {
      output.textContent = `The ${mutation.attributeName} attribute was modified from "${mutation.oldValue}".`;
    }
  }
};

const observer = new MutationObserver(callback);
observer.observe(observerTarget, config);

Result