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.

var options = {
  attributeOldValue: true | false
}

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.

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.