Alternative to DOMNodeInserted

DOMNodeInserted is known to make dynamic pages slow, MDN even recommends not using it altogether, but doesn't provide any alternatives.

I'm not interested in the element inserted, I just need to know when some script modifies the DOM. Is there a better alternative to mutation event listeners (maybe getElementsByTagName inside an nsiTimer)?

If you are creating a web app that targets recent mobile phones and newer versions of browsers (Firefox 5+, Chrome 4+, Safari 4+, iOS Safari 3+, Android 2.1+), you can use the following code to create an awesome event for the insertion of dom nodes, and it even runs on the nodes initially part of the page's static mark-up!

Here's the link to the full post with and example: http://www.backalleycoder.com/2012/04/25/i-want-a-damnodeinserted/

Note on Mutation Observers: while the newer Mutation Observers features in recent browsers are great for monitoring simple insertions and changes to the DOM, do understand that this method can be used to do far more as it allows you to monitor for any CSS rule match you can thing of. This is super powerful for many use-cases, so I wrapped this up in a library here: https://github.com/csuwildcat/SelectorListener

You'll need to add the appropriate prefixes to the CSS and animationstart event name if you want to target various browsers. You can read more about that in the post linked to above.

The basic node insertion case

CSS:

@keyframes nodeInserted {  
    from {  
        outline-color: #fff; 
    }
    to {  
        outline-color: #000;
    }  
}

div.some-control {
    animation-duration: 0.01s;
    animation-name: nodeInserted;
}

JavaScript:

document.addEventListener('animationstart', function(event){
    if (event.animationName == 'nodeInserted'){
        // Do something here
    }
}, true);

Listening for more complex selector matches:

This enables things that are almost impossible to do with Mutation Observers

CSS:

@keyframes adjacentFocusSequence {  
    from {  
        outline-color: #fff; 
    }
    to {  
        outline-color: #000;
    }  
}

.one + .two + .three:focus {
    animation-duration: 0.01s;
    animation-name: adjacentFocusSequence;
}

JavaScript:

document.addEventListener('animationstart', function(event){
    if (event.animationName == 'adjacentFocusSequence'){
        // Do something here when '.one + .two + .three' are 
        // adjacent siblings AND node '.three' is focused
    }
}, true);

One new alternative that @naugtur briefly mentioned is MutationObserver. It's designed as a replacement for the deprecated mutation events, if the browser(s) you're developing for supports it (like if you're developing a browser extension).