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 |
---|---|---|
DOM The definition of 'MutationObserverInit.attributeOldValue' in that specification. |
Living Standard |
Browser compatibility
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