Clean conditional loading

It’s December. That means it’s time for the geek advent calendars to get revved up again:

• PHP Advent 2011,
• Adfont Calendar 2011 and
• 24 Ways 2011.

For every day until Christmas Eve, you can find a tasty geek treat on each of those sites.

Today’s offering on 24 Ways is a little something I wrote called Conditional Loading for Responsive Designs. It expands on the technique I’m using on Huffduffer to conditionally load inessential content into a sidebar with Ajax where the layout is wide enough to accommodate it:

if (document.documentElement.clientWidth > 640) {
// Use Ajax to retrieve content here.
}

In that example, the Ajax only kicks in if the viewport is wider than 640 pixels. Assuming I’ve got a media query that also kicks in at 640 pixels, everything is hunky-dory.

But …it doesn’t feel very DRY to have that 640 pixel number repeated in two places: once in the CSS and again in the JavaScript. It feels particularly icky if I’m using ems for my media query breakpoints (as I often do) while using pixels in JavaScript.

At my recent responsive enhancement workshop in Düsseldorf, Andreas Nebiker pointed out an elegant solution: instead of testing the width of the viewport in JavaScript, why not check for a style change that would have been executed within a media query instead?

So, say for example I’ve got some CSS like this:

@media all and (min-width: 640px) {
    [role="complementary"] {
        width: 30%;
        float: right;
    }
}

Then in my JavaScript I could test to see if that element has the wide-screen layout or not:

var sidebar = document.querySelector('[role="complementary"]'),
floating = window.getComputedStyle(sidebar,null).getPropertyValue('float');
if (floating == 'right') {
// Use Ajax to retrieve content here.
}

Or something like that. The breakpoint is only ever specified once so I ever change it from 640 pixels to something else (like 40 ems) then I only have to make that change in one place. Feel free to grab the example and play around with it.

By the way, you’ll notice that in the original 24 Ways article and also in this updated example, I’m only testing the layout on page load, not on page resize. It would be fairly easy to add in an onResize test as well, but I’m increasingly coming to the conclusion that—apart from the legitimate case of orientation change on tablets—the only people resizing their browser windows after the page loads are web designers testing responsive designs. Still, it would be nice to get some more data to test that hypothesis.