Tags: domscripting

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A tiny lesson in query selection

We have a saying at Clearleft:

Everything is a tiny lesson.

I bet you learn something new every day, even if it’s something small. These small tips and techniques can easily get lost. They seem almost not worth sharing. But it’s the small stuff that takes the least effort to share, and often provides the most reward for someone else out there. Take for example, this great tip for getting assets out of Sketch that Cassie shared with me.

Cassie was working on a piece of JavaScript yesterday when we spotted a tiny lesson that tripped up both of us. The script was a fairly straightforward piece of DOM scripting. As a general rule, we do a sort of feature detection near the start of the script. Let’s say you’re using querySelector to get a reference to an element in the DOM:

var someElement = document.querySelector('.someClass');

Before going any further, check to make sure that the reference isn’t falsey (in other words, make sure that DOM node actually exists):

if (!someElement) return;

That will exit the script if there’s no element with a class of someClass on the page.

The situation that tripped us up was like this:

var myLinks = document.querySelectorAll('a.someClass');

if (!myLinks) return;

That should exit the script if there are no A elements with a class of someClass, right?

As it turns out, querySelectorAll is subtly different to querySelector. If you give querySelector a reference to non-existent element, it will return a value of null (I think). But querySelectorAll always returns an array (well, technically it’s a NodeList but same difference mostly). So if the selector you pass to querySelectorAll doesn’t match anything, it still returns an array, but the array is empty. That means instead of just testing for its existence, you need to test that it’s not empty by checking its length property:

if (!myLinks.length) return;

That’s a tiny lesson.

DOM Scripting, second edition

You may have noticed that there’s a second edition of DOM Scripting out. I can’t take any credit for it; I had very little to do with it. But I’m happy to report that the additions meet with my approval.

I’ve written about it on the DOM Scripting blog if you want all the details on what’s new. In short, all the updates are good ones for a book that’s now five years old.

If you’ve already got the first edition, you probably don’t need this update, but if you’re looking for a good introduction to JavaScript with a good smattering of Ajax and HTML5, and an additional dollop of jQuery, the second edition of DOM Scripting will serve you well.

The cover of the second edition, alas, is considerably shittier than the first edition. So don’t judge the second edition of a book by its cover.

Ajax workshop in NYC

On July 6th I’ll be presenting an all day Ajax and DOM Scripting workshop in New York with Carson Workshops.

A few days later, on the 10th and 11th of July, An Event Apart NYC comes to town. Why not make a week of it? If you’re coming along to AEA, you might want to arrive a few days early for the workshop.

The Ajax workshop costs $495 and will be held at the Digital Sandbox. Registration for An Event Apart costs $1095. It will be held at Scandinavia House.

My previous workshops in London and Manchester were a lot of fun and garnered plenty of praise so I’m really excited about taking the show to New York. If you live in or near New York city, come along for a day of Ajaxy goodness and come away with a Neo-like “I know Kung-Fu” awareness of DOM Scripting.

Oh, and If you sign up now, you’ll also get a copy of my book.