Jquery :eq() selector not working for me

Here is what I'm trying to do:

$(document).ready(function(){
    $('ul li:eq(3n+1)').css({backgroundColor:'#f00'});
});

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<ul>
    <li>Item 1</li>
    <li>Item 1</li>
    <li>Item 1</li>
    <li>Item 1</li>
    <li>Item 1</li>
    <li>Item 1</li>
    <li>Item 1</li>
    <li>Item 1</li>
</ul>

http://jsfiddle.net/mD6Hf/2/

---

Because you seem to have confused jQuery's eq() selector with CSS' :nth-child() pseudo-selector.

To use a CSS selector for this (albeit in the context of jQuery):

$(document).ready(function(){
    $('ul li:nth-child(3n+1)').css({backgroundColor:'#f00'});
});

JS Fiddle demo

Using filter() you can use:

$('li').filter(
    function(i){
        if (i%3 + 1 == 1){
            return $(this);
        }
}).css('background-color','#f00');

JS Fiddle demo.

But I can't see how to do it with eq() since, so far as I can see, that's meant to return only one result (though I've not exhaustively read the whole jQuery API).

References:

• :nth-child selector().
• filter().

---

Or if you want to use :eq() then try it this way:

$('ul li:eq(3)').css({backgroundColor:'#f00'});

:eq() is 0-based. Means that index 3 is the fourth element.

---

Here's one solution that uses a little more jQuery to do the job:

$(document).ready(function(){
    $('ul li').each(function(i,el) {
        if (i%3==0)
        $(this).css({backgroundColor:'#f00'});
    })
});

http://jsfiddle.net/mD6Hf/6/

This will work even if the browser doesn't support CSS3.