MutationObserver

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

MutationObserver 提供開發人員一個方法，來對 DOM tree 的變動來作反應，這被設計用來替換在 DOM3 事件規範中的 Mutation Events。

建構式

`MutationObserver()`

以下舉例為一個新的 DOM mutation observers 建構式。

new MutationObserver(
  function callback
);

參數

callback: 這個函式會在 DOM 有變化時被呼叫，observer 會用兩個參數來呼叫它，第一個是 MutationRecord 物件陣列，而第二個參數則是觀察者目標本身。

Instance methods

`void observe( Node target, MutationObserverInit options );`
`void disconnect();`
`Array takeRecords();`

備註： Node target should not be confused with NodeJS.

`observe()`

Registers the MutationObserver instance to receive notifications of DOM mutations on the specified node.

void observe(
  Node target,
  MutationObserverInit options
);

Parameters

target: The Node on which to observe DOM mutations.
options: A MutationObserverInit object, specifies which DOM mutations should be reported.

備註：Adding an observer to an element is just like addEventListener, if you observe the element multiple times it does not make a difference. Meaning if you observe element twice, the observe callback does not fire twice, nor will you have to run disconnect() twice. In other words, once an element is observed, observing it again with the same observer instance will do nothing. However if the callback object is different it will of course add another observer to it.

`disconnect()`

Stops the MutationObserver instance from receiving notifications of DOM mutations. Until the observe() method is used again, observer's callback will not be invoked.

void disconnect();

`takeRecords()`

Empties the MutationObserver instance's record queue and returns what was in there.

Array takeRecords();

Return value

Returns an Array of MutationRecords.

`MutationObserverInit`

MutationObserverInit is an object which can specify the following properties:

備註：At the very least, childList, attributes, or characterData must be set to true. Otherwise, "An invalid or illegal string was specified" error is thrown.

Property	Description
`childList`	Set to `true` if additions and removals of the target node's child elements (including text nodes) are to be observed.
`attributes`	Set to `true` if mutations to target's attributes are to be observed.
`characterData`	Set to `true` if mutations to target's data are to be observed.
`subtree`	Set to `true` if mutations to not just target, but also target's descendants are to be observed.
`attributeOldValue`	Set to `true` if `attributes` is set to `true` and target's attribute value before the mutation needs to be recorded.
`characterDataOldValue`	Set to `true` if `characterData` is set to `true` and target's data before the mutation needs to be recorded.
`attributeFilter`	Set to an array of attribute local names (without namespace) if not all attribute mutations need to be observed.

`MutationRecord`

MutationRecord is the object that will be passed to the observer's callback. It has the following properties:

Property	Type	Description
`type`	`String`	Returns `attributes` if the mutation was an attribute mutation, `characterData` if it was a mutation to a `CharacterData` node, and `childList` if it was a mutation to the tree of nodes.
`target`	`Node`	Returns the node the mutation affected, depending on the `type`. For `attributes`, it is the element whose attribute changed. For `characterData`, it is the `CharacterData` node. For `childList`, it is the node whose children changed.
`addedNodes`	`NodeList`	Return the nodes added. Will be an empty `NodeList` if no nodes were added.
`removedNodes`	`NodeList`	Return the nodes removed. Will be an empty `NodeList` if no nodes were removed.
`previousSibling`	`Node`	Return the previous sibling of the added or removed nodes, or `null`.
`nextSibling`	`Node`	Return the next sibling of the added or removed nodes, or `null`.
`attributeName`	`String`	Returns the local name of the changed attribute, or `null`.
`attributeNamespace`	`String`	Returns the namespace of the changed attribute, or `null`.
`oldValue`	`String`	The return value depends on the `type`. For `attributes`, it is the value of the changed attribute before the change. For `characterData`, it is the data of the changed node before the change. For `childList`, it is `null`.

Example usage

The following example was taken from this blog post.

// select the target node
var target = document.querySelector("#some-id");

// create an observer instance
var observer = new MutationObserver(function (mutations) {
  mutations.forEach(function (mutation) {
    console.log(mutation.type);
  });
});

// configuration of the observer:
var config = { attributes: true, childList: true, characterData: true };

// pass in the target node, as well as the observer options
observer.observe(target, config);

// later, you can stop observing
observer.disconnect();

Additional reading

A brief overview
A more in-depth discussion
A screencast by Chromium developer Rafael Weinstein
The mutation summary library
The DOM standard which defines the MutationObserver interface

Browser compatibility

BCD tables only load in the browser