Question

I’m looking for a solution on how to use addClass for every 4 LI element, like this:

<ul>
<li>Element 1</li>
<li>Element 2</li>
<li>Element 3</li>
<li class="newclass">Element 4</li>
<li>Element 5</li>
<li>Element 6</li>
<li>Element 8</li>
<li class="newclass">Element 8</li>
</ul>

Answer 1

two ways.

CSS selector: http://jsfiddle.net/at9CJ/1/

document.getElements('ul > li:nth-child(4n+4)').addClass('new');

mod children:

document.getElements('ul > li').each(function(el, i){
    (i+1) % 4 === 0 && el.addClass('new');
});