MutationObserver.observe()

The MutationObserver method observe() configures the MutationObserver callback to begin receiving notifications of changes to the DOM that match the given options. Depending on the configuration, the observer may watch a single Node in the DOM tree, or that node and some or all of its descendant nodes.

To stop the MutationObserver (so that none of its callbacks will be triggered any longer), call MutationObserver.disconnect().

Syntax

mutationObserver.observe(target, options)

Parameters

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
MutationObserverInit 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 MutationObserverInit 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 in MutationObserver for details on watching for attribute changes and value recording. 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. For details and an example, see Monitoring text content changes in MutationObserver. The default value is false.

Return value

undefined.

Exceptions

TypeError
Thrown in any of the following circumstances:

Usage notes

Reusing MutationObservers

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.

Observation follows nodes when disconnected

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.

Example

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 a MutationObserverInit 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(function() {
    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});

Specifications

Specification Status Comment
DOM
The definition of 'MutationObserver.observe()' in that specification.
Living Standard

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
observeChrome Full support 18Edge Full support 12Firefox Full support 14IE Full support 11Opera Full support 15Safari Full support 6WebView Android Full support ≤37Chrome Android Full support 18Firefox Android Full support 14Opera Android Full support 14Safari iOS Full support 6Samsung Internet Android Full support 1.0

Legend

Full support  
Full support