jQuery: counting visible elements - efficiency/speed problems

Question

I have some code that works fine but it's become too slow:

HTML:

I have a container that contains about 50 ul elements. Each ul element has a h4 heading followed by a series of li elements. The function hides the heading if no line elements are visible.

Javascript/jQuery:

            function show_or_hide_headings() {
                $('#container').children('ul').each(function (i) {
                    var $this = $(this),
                        $h4 = $this.children(':first');
                    if ($this.children('li:visible').length) {
                        $h4.show();
                    } else {
                        $h4.hide();
                    }
                }); 
            }

It was working quite acceptably until I changed the nature of the li elements. Each li is now a mini table comprising <table><tr><td>icon</td><td>text</td></tr></table>. It now takes 2 seconds to process, whereas it previously worked in less than half a second. (The table is there to stop the text wrapping under the icon.)

I confess I can't quite understand why adding the additional elements into each li should slow down the DOM processing so much because I've used the .children selector to only go one DOM layer deep.

I've also tried:

                $('#container').find('h4').each(function (i) {
                    var $this = $(this);
                    if ($this.siblings('li:visible').length) {
                       $this.show();
                    } else {
                       $this.hide();
                    }
                }); 

and $('#container').children().children('h4') for good measure.

What is notable, too, is that when there are many li elements visible, it is much slower than when few are visible. There are no more lines now, however, than when it worked quite quickly (i.e., before the table was put into each line).

Any advice greatly appreciated, but please don't request I post more code than I have :)

Thanks.

Answer 1

I suspect that determining if an element is visible or not is quite expensive. Consider instead adding and deleting a class to hide or show elements. Then you can select them directly based on the class, which will mostly be supported by a host getElementsByClassName or querySelectorAll method.