13th September 2020

Mutation observer

MutationObserver is a built-in object that observes a DOM element and fires a callback when it detects a change.

We’ll first take a look at the syntax, and then explore a real-world use case, to see where such thing may be useful.

Syntax

MutationObserver is easy to use.

First, we create an observer with a callback-function:

let observer = new MutationObserver(callback);

And then attach it to a DOM node:

observer.observe(node, config);

config is an object with boolean options “what kind of changes to react on”:

  • childList – changes in the direct children of node,
  • subtree – in all descendants of node,
  • attributes – attributes of node,
  • attributeFilter – an array of attribute names, to observe only selected ones.
  • characterData – whether to observe node.data (text content),

Few other options:

  • attributeOldValue – if true, pass both the old and the new value of attribute to callback (see below), otherwise only the new one (needs attributes option),
  • characterDataOldValue – if true, pass both the old and the new value of node.data to callback (see below), otherwise only the new one (needs characterData option).

Then after any changes, the callback is executed: changes are passed in the first argument as a list of MutationRecord objects, and the observer itself as the second argument.

MutationRecord objects have properties:

  • type – mutation type, one of
    • "attributes": attribute modified
    • "characterData": data modified, used for text nodes,
    • "childList": child elements added/removed,
  • target – where the change occurred: an element for "attributes", or text node for "characterData", or an element for a "childList" mutation,
  • addedNodes/removedNodes – nodes that were added/removed,
  • previousSibling/nextSibling – the previous and next sibling to added/removed nodes,
  • attributeName/attributeNamespace – the name/namespace (for XML) of the changed attribute,
  • oldValue – the previous value, only for attribute or text changes, if the corresponding option is set attributeOldValue/characterDataOldValue.

For example, here’s a <div> with a contentEditable attribute. That attribute allows us to focus on it and edit.

<div contentEditable id="elem">Click and <b>edit</b>, please</div>

<script>
let observer = new MutationObserver(mutationRecords => {
  console.log(mutationRecords); // console.log(the changes)
});

// observe everything except attributes
observer.observe(elem, {
  childList: true, // observe direct children
  subtree: true, // and lower descendants too
  characterDataOldValue: true // pass old data to callback
});
</script>

If we run this code in the browser, then focus on the given <div> and change the text inside <b>edit</b>, console.log will show one mutation:

mutationRecords = [{
  type: "characterData",
  oldValue: "edit",
  target: <text node>,
  // other properties empty
}];

If we make more complex editing operations, e.g. remove the <b>edit</b>, the mutation event may contain multiple mutation records:

mutationRecords = [{
  type: "childList",
  target: <div#elem>,
  removedNodes: [<b>],
  nextSibling: <text node>,
  previousSibling: <text node>
  // other properties empty
}, {
  type: "characterData"
  target: <text node>
  // ...mutation details depend on how the browser handles such removal
  // it may coalesce two adjacent text nodes "edit " and ", please" into one node
  // or it may leave them separate text nodes
}];

So, MutationObserver allows to react on any changes within DOM subtree.

Usage for integration

When such thing may be useful?

Imagine the situation when you need to add a third-party script that contains useful functionality, but also does something unwanted, e.g. shows ads <div class="ads">Unwanted ads</div>.

Naturally, the third-party script provides no mechanisms to remove it.

Using MutationObserver, we can detect when the unwanted element appears in our DOM and remove it.

There are other situations when a third-party script adds something into our document, and we’d like to detect, when it happens, to adapt our page, dynamically resize something etc.

MutationObserver allows to implement this.

Usage for architecture

There are also situations when MutationObserver is good from architectural standpoint.

Let’s say we’re making a website about programming. Naturally, articles and other materials may contain source code snippets.

Such snippet in an HTML markup looks like this:

...
<pre class="language-javascript"><code>
  // here's the code
  let hello = "world";
</code></pre>
...

For better readability and at the same time, to beautify it, we’ll be using a JavaScript syntax highlighting library on our site, like Prism.js. To get syntax highlighting for above snippet in Prism, Prism.highlightElem(pre) is called, which examines the contents of such pre elements and adds special tags and styles for colored syntax highlighting into those elements, similar to what you see in examples here, on this page.

When exactly should we run that highlighting method? Well, we can do it on DOMContentLoaded event, or put the script at the bottom of the page. The moment our DOM is ready, we can search for elements pre[class*="language"] and call Prism.highlightElem on them:

// highlight all code snippets on the page
document.querySelectorAll('pre[class*="language"]').forEach(Prism.highlightElem);

Everything’s simple so far, right? We find code snippets in HTML and highlight them.

Now let’s go on. Let’s say we’re going to dynamically fetch materials from a server. We’ll study methods for that later in the tutorial. For now it only matters that we fetch an HTML article from a webserver and display it on demand:

let article = /* fetch new content from server */
articleElem.innerHTML = article;

The new article HTML may contain code snippets. We need to call Prism.highlightElem on them, otherwise they won’t get highlighted.

Where and when to call Prism.highlightElem for a dynamically loaded article?

We could append that call to the code that loads an article, like this:

let article = /* fetch new content from server */
articleElem.innerHTML = article;

let snippets = articleElem.querySelectorAll('pre[class*="language-"]');
snippets.forEach(Prism.highlightElem);

…But, imagine if we have many places in the code where we load our content – articles, quizzes, forum posts, etc. Do we need to put the highlighting call everywhere, to highlight the code in content after loading? That’s not very convenient.

And what if the content is loaded by a third-party module? For example, we have a forum written by someone else, that loads content dynamically, and we’d like to add syntax highlighting to it. No one likes patching third-party scripts.

Luckily, there’s another option.

We can use MutationObserver to automatically detect when code snippets are inserted into the page and highlight them.

So we’ll handle the highlighting functionality in one place, relieving us from the need to integrate it.

Dynamic highlight demo

Here’s the working example.

If you run this code, it starts observing the element below and highlighting any code snippets that appear there:

let observer = new MutationObserver(mutations => {

  for(let mutation of mutations) {
    // examine new nodes, is there anything to highlight?

    for(let node of mutation.addedNodes) {
      // we track only elements, skip other nodes (e.g. text nodes)
      if (!(node instanceof HTMLElement)) continue;

      // check the inserted element for being a code snippet
      if (node.matches('pre[class*="language-"]')) {
        Prism.highlightElement(node);
      }

      // or maybe there's a code snippet somewhere in its subtree?
      for(let elem of node.querySelectorAll('pre[class*="language-"]')) {
        Prism.highlightElement(elem);
      }
    }
  }

});

let demoElem = document.getElementById('highlight-demo');

observer.observe(demoElem, {childList: true, subtree: true});

Here, below, there’s an HTML-element and JavaScript that dynamically fills it using innerHTML.

Please run the previous code (above, observes that element), and then the code below. You’ll see how MutationObserver detects and highlights the snippet.

A demo-element with id="highlight-demo", run the code above to observe it.

The following code populates its innerHTML, that causes the MutationObserver to react and highlight its contents:

let demoElem = document.getElementById('highlight-demo');

// dynamically insert content with code snippets
demoElem.innerHTML = `A code snippet is below:
  <pre class="language-javascript"><code> let hello = "world!"; </code></pre>
  <div>Another one:</div>
  <div>
    <pre class="language-css"><code>.class { margin: 5px; } </code></pre>
  </div>
`;

Now we have MutationObserver that can track all highlighting in observed elements or the whole document. We can add/remove code snippets in HTML without thinking about it.

Additional methods

There’s a method to stop observing the node:

  • observer.disconnect() – stops the observation.

When we stop the observing, it might be possible that some changes were not yet processed by the observer. In such cases, we use

  • observer.takeRecords() – gets a list of unprocessed mutation records – those that happened, but the callback has not handled them.

These methods can be used together, like this:

// get a list of unprocessed mutations
// should be called before disconnecting,
// if you care about possibly unhandled recent mutations
let mutationRecords = observer.takeRecords();

// stop tracking changes
observer.disconnect();
...
Records returned by observer.takeRecords() are removed from the processing queue

The callback won’t be called for records, returned by observer.takeRecords().

Garbage collection interaction

Observers use weak references to nodes internally. That is, if a node is removed from the DOM, and becomes unreachable, then it can be garbage collected.

The mere fact that a DOM node is observed doesn’t prevent the garbage collection.

Summary

MutationObserver can react to changes in DOM – attributes, text content and adding/removing elements.

We can use it to track changes introduced by other parts of our code, as well as to integrate with third-party scripts.

MutationObserver can track any changes. The config “what to observe” options are used for optimizations, not to spend resources on unneeded callback invocations.

Tutorial map

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