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: And then attach it to a DOM node: 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
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
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: 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: 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: 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: …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: 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.
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:
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.
let observer = new MutationObserver(callback);
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