Detect DOM Node Insertions with JavaScript and CSS Animations

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I work with an awesome cast of developers at Mozilla, and one of them in Daniel Buchner. Daniel's shared with me an awesome strategy for detecting when nodes have been injected into a parent node without using the deprecated DOM Events API. This hack uses JavaScript, as you would expect, but another technology you wouldn't expect: CSS animations. Let me prove to you that it works!


All that's required is a parent element with which we'd like to listen to node insertions within:

<ul id="parentElement"></ul>

You can use any selector helper you'd like, but I've chosen an ID here.


In order to get a handle on node insertion detection, we need to set up a series of keyframe animations which will start when the node is inserted. The clip property is used since it has no effect on the node itself:

/* set up the keyframes; remember to create prefixed keyframes too! */
@keyframes nodeInserted {  
	from { opacity: 0.99; }
	to { opacity: 1; }  

With the keyframes created, the animation needs to be applied on the elements you'd like to listen for. Note the tiny duration; that relaxes the animation footprint on the browser.

#parentElement > li {
    animation-duration: 0.001s;
    animation-name: nodeInserted;

Add the animation to the child nodes you are listening for. When the animation ends, the insertion event will fire!

The JavaScript

The first step is creating an function which will act as the event listener callback. Within the function, an initial event.animationName check must be made to ensure it's the animation name we want to listen for in this specific case:

var insertListener = function(event){
	if (event.animationName == "nodeInserted") {
		// This is the debug for knowing our listener worked!
		// event.target is the new node!
		console.warn("Another node has been inserted! ", event, event.target);

If the animation name matches the desired animation, we know a DOM node has been injected. Now it's time to add the event listener to the parent:

document.addEventListener("animationstart", insertListener, false); // standard + firefox
document.addEventListener("MSAnimationStart", insertListener, false); // IE
document.addEventListener("webkitAnimationStart", insertListener, false); // Chrome + Safari

How awesomely simple is that?!

Daniel created this solution to aid in his forthcoming web components initiative, an initiative I'll be covering more in depth very soon. This node insertion hack is useful and uses no framework, so it's an incredible mechanism that can be used by anyone. Well done to Daniel!

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  1. Rolf

    Nice hack.. especially with all the vendor prefixes in the css :)

    You mention the DOM events API is deprecated, but not all? https://developer.mozilla.org/en/Mozilla_event_reference shows DOMContentLoaded is not deprecated?

    Cool post.

    • Fairly certain DOMContentLoaded will stay; the others are taxing on the browser.

    • To be specific: DOM Level 3 Mutation events are the ones that got the deprecation ax ;)

  2. Don’t know much about jquery. Can’t think this is possible using jquery! Gonna to try today, yeahhh :)

    • Daniel

      Yeah, this is just vanilla JS, it works all by itself, no framework required – and for the record, jQuery is not magical-unicorn-pixie code, it can’t do a single thing JavaScript itself can’t do…because it IS JavaScript.

  3. sasklacz

    why shouldn’t it be ? Whole javascript here is just adding event listeners.

  4. Me

    Very-very sneaky, sir :)

  5. Clever. A couple of questions:

    1. What is the overhead of this technique?
    2. I would assume that browsers consider animations as optional, so they can be skipped under heavy load. Has this hack been rigorously tested for any missed insertions?

  6. Might be useful to also include a link to Daniel’s original post in the article.

  7. Pablo P.

    How to apply this to $(document).on("nodeinserted",".selector",handler); ?

    use nodeinserted just like any other event that works with jQuery on?

    It could be very useful to target elements that were inserted, and its event being insertion rather than interactions such as click or focus. Thank you

  8. I just came to say that this is brilliant. You’re a genius. It’s a shame that IE9 doesn’t support CSS Keyframes as this technique would open the possibility to make a proper CSS Keyframes polyfill.

    Keep up the good work!

  9. Just want to say thanks man, this is very helpful to our team and a beautiful solution. Previously I was just polling the DOM… =p

    Thanks again!

  10. Clever approach. With regards to the CSS, and if I’m not mistaken, it’s typically best to place the unprefixed properties after the prefixed properties so that the unprefixed implementation overrules the older prefixed implementation in browsers that support both for backward-compat.

    Fantastic solution to a very common problem though.

  11. This technique is very useful in browser extensions, for example adding a widget to gmail (something like rapportive). In gmail, nearly the entire environment is rendered via javascript and well after window.onload event. Your extension environment is also completely sandboxed so you can’t bind to any of gmail’s javascript events.

    The only thing you have access to is the DOM and this technique works beautifully! The response time is near instant — very well done, thank you!

  12. Jesse

    pretty neat. i left console up on your demo and came back to the page and nodes were still getting inserted even when on another page?!

  13. @Jesse: the setInterval (or mootools or javascript equivalent; im a jquery kinda guy) function still fires even when you leave the tab, as long as the page is open in the browser the event will fire, because you were inspecting that page it showed up in console even though your active tab was back here.

  14. Why are mutation events deprecated anyway, when there’s nothing but hacks to replace them?

    btw DOMContentLoaded is not a mutation event, it just sounds like one because of its name

  15. Did you notice that you have used following for IE:

    document.addEventListener("MSAnimationStart", insertListener, false); 

    addEventListener ain’t for IE

    Nice trick however mate. I think we can have a growl notification with a bit CSS3 animation for DOM node insertion update

  16. Captain Dingleberry

    Anyone got this to work in IE10? Looks like it does not support animation of the clip property (based on this : http://stackoverflow.com/questions/11431439/msanimationstart-event-doesnt-work-on-ie10) , so the animation start event does not fire. Any other properties one could animate that does not force a reflow of the page?

  17. I got it to work in IE10. And made it a library some time ago.


    It also allows selectors etc.

  18. joan

    IE 9 doesn’t support CSS animations. And as I know there’re also mutation events and mutation observers designed for DOM mutation events.

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