jQuery's Child Selector

January 25, 2011

Good Things to Come (I Swear)

It has been quite some time since I posted on this blog. I’ve still been coding a lot but I got lazy and couldn’t come up with anything i wanted to write about. I promise, however, that I’ll soon be doing a writeup on some experiments I did with node.js and backbone.js so keep an eye out for that.

On jQuery Selectors

Now on to the topic at hand. If you are at all familiar with CSS2 and CSS3 selectors you’ll know just how powerful and expressive they can be despite their rather terse syntax. If you take a look at jQuery selectors you can see the influence. In the places where jQuery selectors innovate, they still try to make the code analogous things like CSS pseudo-classes. The :first selector is a perfect example. At work we have been developing a jQuery plugin which facilitates navigating a table full of inputs in a pretty clean way using only the semantics of HTML tables. I’m going to try to get that open-sourced so keep an eye out for that plugin in the near future under the name “focusgrid”. While I was working on this plugin, I found the need to operate on direct descendant. Thats where jQuery’s parent child operator comes in (parent > child). There are some good examples in the documentation. If you are familiar with the CSS implementation, it works just the same.

Child Selector Example

Say you had the following HTML and you wanted to find all images which were direct descendents of elements with the class image_container

<img src="decoy.jpg" />
<div class="image_container">
  <img src="bingo.jpg" />
  <span class="image_container">
    <img src="alsobingo.jpg" />

Your jQuery would look like;

$('.image_container > img')

This would return you the images for bingo.jpg and alsobingo.jpg, omitting decoy.jpg.

Using Child Selector With Implicit Parent

In my case, I was in a nested loop and was to be selecting from an element rather than from the global scope. In my case I was scoped to a table and looking for TR tags at the first level, potentially nested underneath a TBODY, THEAD or TFOOT tag, but not in a nested table. As it turns out jQuery’s selector will use the currently scoped element(s) as an implicit parent in the parent > child selector syntax, allowing you to leave the parent out entirely.

    <tr class="findme">
    <tr class="findme">
          <tr class="dontfindme"></tr>

Your jQuery would look like:

var $table = $('table:first'); // Say we had to be scoped to the table
$table.find('> tr, > thead > tr, > tbody > tr, > tfoot > tr'); // This will get both findme's and skip the dontfindme

The left hand operand of the parent/child operator is implicit in this case so you won’t have to do another (expensive) document scope find with an even more complicated selector. Pretty neat.