Mutation events

Mutation events provide a mechanism for a web page or an extension to get notified about changes made to the DOM. Use Mutation Observers instead if possible.

Preface

The mutation events have been marked as deprecated in the DOM Events specification, as the API's design is flawed (see details in the "DOM Mutation Events Replacement: The Story So Far / Existing Points of Consensus" post to public-webapps).

Mutation Observers are the proposed replacement for mutation events in DOM4. They are expected to be included in Firefox 14 and Chrome 18.

The practical reasons to avoid the mutation events are performance issues and cross-browser support.

Performance

Adding DOM mutation listeners to a document profoundly degrades the performance of further DOM modifications to that document (making them 1.5 - 7 times slower!). Moreover, removing the listeners does not reverse the damage.

The performance effect is limited to the documents that have the mutation event listeners.

Cross-browser support

These events are not implemented consistently across different browsers, for example:

• IE prior to version 9 didn't support the mutation events at all and does not implement some of them correctly in version 9 (for example, DOMNodeInserted)
• WebKit doesn't support DOMAttrModified (see webkit bug 8191 and the workaround)
• "mutation name events", i.e. DOMElementNameChanged and DOMAttributeNameChanged are not supported in Firefox (as of version 11), and probably in other browsers as well.
• ...

Dottoro documents browser support for mutation events.

Mutation events list

The following is a list of all mutation events, as defined in DOM Level 3 Events specification:

• DOMAttrModified
• DOMAttributeNameChanged
• DOMCharacterDataModified
• DOMElementNameChanged
• DOMNodeInserted
• DOMNodeInsertedIntoDocument
• DOMNodeRemoved
• DOMNodeRemovedFromDocument
• DOMSubtreeModified

Usage

You can register a listener for mutation events using element.addEventListener as follows:

element.addEventListener("DOMNodeInserted", function (ev) {
  // ...
}, false);

The event object is passed to the listener in a MutationEvent (see its definition in the specification) for most events, and MutationNameEvent for DOMAttributeNameChanged and DOMElementNameChanged.