jQuery custom filter problem: element stack is way too much

I am trying to implement ":random" selector which selects a random element from a stack of elements.

Here's the code: http://jsfiddle.net/nuSWF/

The problem is the selector sometimes tries to select an element which does not exist(out of index). So I prepared the demo code which highlights the cause but I don't understand why. I seems it is a bug or something.

P.S: I know I could select a random element with other methods but this time I have to implement this, also wondering what's going on inside.

What you're seeing with the stack length is normal, it's just evaluating the :random selector on all <a> elements in mydiv1 before the > child selector, so the length at that point is 4.

For example, this would produce the result you're expecting:

var elements2 = $('#mydiv1>a').filter(':random');

You can test it out here.

So what's happening overall is your selector is indeed filtering to random <a> elements...but those may or may not be further filtered out by the > child selector later (if they're under a <span>, they get filtered). This is true of all selectors if you think about it...any filter you perform just reduces the set of elements...they may further filtered later by more selectors.