MutationObserver()
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.
Syntax
const observer = new MutationObserver(callback)
Parameters
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:- An array of
MutationRecord
objects, describing each change that occurred; and - the
MutationObserver
which invoked thecallback
.
See the example below for more details.
- An array of
Return value
A new MutationObserver
object, configured to call the specified callback
when DOM mutations occur.
Example
This example creates a new MutationObserver
configured to watch a node and all of its children for additions and removals of elements to the tree, as well as any changes to attributes on any of the elements in the tree.
The callback function
function callback(mutationList, observer) { mutationList.forEach( (mutation) => { switch(mutation.type) { case 'childList': /* One or more children have been added to and/or removed from the tree. (See mutation.addedNodes and mutation.removedNodes.) */ break; case 'attributes': /* An attribute value changed on the element in mutation.target. The attribute name is in mutation.attributeName, and its previous value is in mutation.oldValue. */ break; } }); }
The callback()
function is invoked when the observer sees changes matching the configuration of the observation request specified when calling observe()
to begin watching the DOM.
The kind of change that took place (either a change to the list of children, or a change to an attribute) is detected by looking at the mutation.type
property.
Creating and starting the observer
This code actually sets up the observation process.
const targetNode = document.querySelector("#someElement"); const observerOptions = { childList: true, attributes: true, // Omit (or set to false) to observe only changes to the parent node subtree: true } const observer = new MutationObserver(callback); observer.observe(targetNode, observerOptions);
The desired subtree is located by finding an element with the ID someElement
. A set of options for the observer is also established in the observerOptions
record. In it, we specify values of true
for both childList
and attributes
, so we get the information we want.
Then the observer is instantiated, specifying the callback()
function. We begin observing the DOM nodes of interest by calling observe()
, specifying the target
node and the options
object.
From this point until disconnect()
is called, callback()
will be called each time an element is added to or removed from the DOM tree rooted at targetNode
, or any of those elements' attributes are changed.
Specifications
Browser compatibility
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | |
MutationObserver |
26
18
|
12 |
14 |
11 |
15 |
7
6
|
≤37
≤37
|
26
18
|
14 |
14 |
7
6
|
1.5
1.0
|
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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver/MutationObserver