Web/API/MutationObserverInit/attributeOldValue

The MutationObserverInit dictionary's optional attributeOldValue property is used to specify whether or not to record the prior value of the altered attribute in MutationRecord objects denoting attribute value changes.

Syntax

var options = {
  attributeOldValue: true | false
}

Value

A Boolean value indicating whether or not the prior value of a changed attribute should be included in the MutationObserver.oldValue property when reporting attribute value changes. If true, oldValue is set accordingly. If false, it is not.

When using attributeOldValue, setting the attributes option to true is optional.

Example

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(function(mutation) {
    switch(mutation.type) {
      case "attributes":
        notifyUser("Attribute name " + mutation.attributeName +
            " changed to " + mutation.target[mutation.attributeName] +
            " (was " + mutation.oldValue + ")");
        break;
    }
  });
}

var targetNode = document.querySelector("#target");

var observer = new MutationObserver(callback);
observer.observe(targetNode, {
  attributes: true,
  attributeOldValue: true
});

The callback() function—which will be passed into the observe() method when starting the observer, looks at each item in the list of MutationRecord objects. For any items representing an attribute change (which can be detected by the value of MutationRecord.type being "attributes"), a function called notifyUser() is used to tell the user the name of the attribute that changed as well as the attribute's new value (mutation.target[mutation.attributeName]) and its old value (mutation.oldValue).

When observe() is called, the specified options are attributes and attributeOldValue, which means that changes to attribute values will be reported, and each mutation record will include the oldValue property specifying the attribute's previous value.

Specifications

Specification	Status	Comment
DOMThe definition of 'MutationObserverInit.attributeOldValue' in that specification.	Living Standard

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`attributeOldValue`	Chrome Full support 26 Full support 26 No support 18 — 26 Prefixed' Prefixed' Implemented with the vendor prefix: webkit	Edge Full support 12	Firefox Full support 14 Notes' Full support 14 Notes' Notes' Starting in Firefox 36, `attributeOldValue` has no default value; previously, its default value was `false`.	IE Full support 11	Opera Full support 15	Safari Full support 7 Full support 7 No support 6 — 7 Prefixed' Prefixed' Implemented with the vendor prefix: webkit	WebView Android ?	Chrome Android Full support 26 Full support 26 No support 18 — 26 Prefixed' Prefixed' Implemented with the vendor prefix: webkit	Firefox Android Full support 14 Notes' Full support 14 Notes' Notes' Starting in Firefox 36, `attributeOldValue` has no default value; previously, its default value was `false`.	Opera Android Full support 14	Safari iOS Full support 7 Full support 7 No support 6 — 7 Prefixed' Prefixed' Implemented with the vendor prefix: webkit	Samsung Internet Android Full support 1.5 Full support 1.5 No support 1.0 — 1.5 Prefixed' Prefixed' Implemented with the vendor prefix: webkit

Legend

Full support: Full support
Compatibility unknown: Compatibility unknown
See implementation notes.': See implementation notes.
Requires a vendor prefix or different name for use.': Requires a vendor prefix or different name for use.

MutationObserverInit.attributeOldValue by Mozilla Contributors is licensed under CC-BY-SA 2.5.