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MutationObserver

MutationObserver provides developers a way to react to changes in a DOM. It is designed as a replacement for Mutation Events defined in the DOM3 Events specification.

Constructor

MutationObserver()

Constructor for instantiating new DOM mutation observers.

MutationObserver(
  function callback
);
Parameters
callback
The function which will be called on each DOM mutation. The observer will call this function with two arguments. The first is an array of objects, each of type MutationRecord. The second is this MutationObserver instance.

Instance methods

void observe( Node target, MutationObserverInit options );
void disconnect();
Array takeRecords();

observe()

Registers the MutationObserver instance to receive notifications of DOM mutations on the specified node.

void observe(
  Node target,
  MutationObserverInit options
);
Parameters
target
The Node on which to observe DOM mutations.
options
A MutationObserverInit object, specifies which DOM mutations should be reported.
NOTE: Adding an observer to an element is just like addEventListener, if you observe the element multiple times it does not make a difference. Meaning if you observe element twice, the observe callback does not fire twice, nor will you have to run disconnect() twice. In other words, once an element is observed, observing it again with the same observer instance will do nothing. However if the callback object is different it will of course add another observer to it.

disconnect()

Stops the MutationObserver instance from receiving notifications of DOM mutations. Until the observe() method is used again, observer's callback will not be invoked.

void disconnect();

takeRecords()

Empties the MutationObserver instance's record queue and returns what was in there.

Array takeRecords();
Return value

Returns an Array of MutationRecords.

MutationObserverInit

MutationObserverInit is an object which can specify the following properties:

NOTE: At the very least, childList, attributes, or characterData must be set to true. Otherwise, "An invalid or illegal string was specified" error is thrown.
Property Description
childList Set to true if additions and removals of the target node's child elements (including text nodes) are to be observed.
attributes Set to true if mutations to target's attributes are to be observed.
characterData Set to true if mutations to target's data are to be observed.
subtree Set to true if mutations to not just target, but also target's descendants are to be observed.
attributeOldValue Set to true if attributes is set to true and target's attribute value before the mutation needs to be recorded.
characterDataOldValue Set to true if characterData is set to true and target's data before the mutation needs to be recorded.
attributeFilter Set to an array of attribute local names (without namespace) if not all attribute mutations need to be observed.

MutationRecord

MutationRecord is the object that will be passed to the observer's callback. It has the following properties:

Property Type Description
type String Returns attributes if the mutation was an attribute mutation, characterData if it was a mutation to a CharacterData node, and childList if it was a mutation to the tree of nodes.
target Node Returns the node the mutation affected, depending on the type. For attributes, it is the element whose attribute changed. For characterData, it is the CharacterData node. For childList, it is the node whose children changed.
addedNodes NodeList Return the nodes added. Will be an empty NodeList if no nodes were added.
removedNodes NodeList Return the nodes removed. Will be an empty NodeList if no nodes were removed.
previousSibling Node Return the previous sibling of the added or removed nodes, or null.
nextSibling Node Return the next sibling of the added or removed nodes, or null.
attributeName String Returns the local name of the changed attribute, or null.
attributeNamespace String Returns the namespace of the changed attribute, or null.
oldValue String The return value depends on the type. For attributes, it is the value of the changed attribute before the change. For characterData, it is the data of the changed node before the change. For childList, it is null.

Example usage

The following example was taken from this blog post.

// select the target node
var target = document.querySelector('#some-id');
 
// create an observer instance
var observer = new MutationObserver(function(mutations) {
  mutations.forEach(function(mutation) {
    console.log(mutation.type);
  });    
});
 
// configuration of the observer:
var config = { attributes: true, childList: true, characterData: true };
 
// pass in the target node, as well as the observer options
observer.observe(target, config);
 
// later, you can stop observing
observer.disconnect();

Additional reading

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 18 -webkit
26
14 (14) 11 15 6.0 -webkit
Feature Android Chrome for Android Firefox Mobile (Gecko) IE  (Windows Phone)  Opera Mobile Safari Mobile
Basic support ? 18 -webkit
26
14.0 (14) 11 (8.1) 15

6 -webkit

7