Get every nth item in array
<p>I have an array of HTML elements. I'm checking whether a previous object in the array exists like this:</p>

<pre><code>var boxes = $('.box'); // creating the array

boxes.each(function(i){ // going through the array
    var prevBox = i&gt;0?$(boxes[i-1]):false; // check whether a previous box exists
    if (prevBox) { } // do something
    else { } // do something else
});
</code></pre>

<p>This works well. But I would also need to check the existence of every fourth object (box) in the array, or more precisely whether an object three objects before the current one exists.</p>

<p>This doesn't work:</p>

<pre><code>var prevBox = i&gt;0?$(boxes[i-4]):false;
</code></pre>

<p>I believe using <code>jQuery.grep()</code> and checking if <code>(i % 4) == 0</code> might be the answer, but with my limited knowledge of Javascript, I don't know how to apply it to what I have now.</p>

<p>Anyone can help? Thanks!</p>
