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
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
attributeOldValue | Chrome
Full support
26
| Edge Full support 12 | Firefox
Full support
14
| IE Full support 11 | Opera Full support 15 | Safari
Full support
7
| WebView Android ? | Chrome Android
Full support
26
| Firefox Android
Full support
14
| Opera Android Full support 14 | Safari iOS
Full support
7
| Samsung Internet Android
Full support
1.5
|
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.