On Script Loaders


Two recent projects have come out that attempt to address the “dynamic script loader” use case: HeadJS and ControlJS. Since I’m the creator of LABjs, a general, all-purpose, performance-oriented dynamic script loader that’s been around for about a year and a half now, and is well-known enough to be in use on several major sites, including Twitter, Vimeo, and Zappos, many people ask my opinions when new entries into this space arise.

I’ve been hesitant to rush to negative judgement on either one, because I believe it’s important to encourage experimentation and progress in this area, for the sake of the greater web. But I do think it’s important to shed a little bit of light onto both projects and explain some concerns I have with their approach. I respect the authors of both libraries, so I hope they will take my ramblings here as constructive criticism rather than an attack.

ControlJS: backstory

ControlJS comes from the immensely experienced and talented Steve Souders, who leads performance efforts for Google. For readers who aren’t aware, when I first was building LABjs back in 2009, Steve stepped in and offered some very helpful and timely advice and collaboration, and I credit much of LABjs’ success since to Steve’s wisdom and experience.

Recently, though, Steve has focused in a different direction than was necessarily the focus back then. His focus is now more aimed at delaying all script loadings until after the rest of the page content has loaded, whereas LABjs’ goal was to simply allow easy parallel loading of scripts (instead of blocking behavior) right along side the rest of the page content. In my opinion, there are some scripts which are less important, like Google Analytics, social sharing buttons, etc. And I whole heartedly agree with “deferring” that code’s loading until “later”, to not take up precious valuable bandwidth/connections/cpu.

But there’s also a lot of JavaScript that is just as important as the content it decorates, and to me, the idea of delaying that code by a noticeable amount will lead, in general, to proliferation of a distasteful visual effect I call “FUBC” (Flash of Un-Behaviored Content). In other words, we’ll see pages that flash up raw text content, only to swap out a moment or two later into the widgetized and JavaScript stylized version of the page, where the content is in fancy tab-sets, modal dialogs, etc.

To be clear, any dynamic script loader can create this effect unintentionally, including LABjs. But what ControlJS appears to be intentionally doing is delaying scripts even longer past when content is visible/useable, which will exacerbate this FUBC problem quite a bit.

There are of course ways to mitigate this, using default/inline CSS rules and <noscript> tags (a technique I talk about on the LABjs site), but if you hide all the raw content with CSS and don’t re-display it until the JavaScript is present, you lose ALL of the benefit of deferring that JavaScript logic to let the content load quicker. The goal is admirable, but I think it will end up being neutral or worse UX for most sites.

This is a tenuous and difficult UX balance that sites need to consider carefully. I do not endorse Steve’s suggestions, that basically all sites should move to this model. It makes sense for some of them, if they intentionally design their UX that way. But it’s by far not optimal for a lot of sites, and using his suggestions will trade out performance for really sub-optimal user experience during page-load if not done with caution and reserve.

So, that is the context under which Steve presents us ControlJS. It’s an attempt to create a script loader that more closely models his view of how page-loads should work (that scripts should all defer loading and execution until all content is finished). If you agree with him, and aren’t worried about FUBC (or have already carefully thought about and designed around it), ControlJS is something to at least consider. But if you’re just planning to drop in ControlJS to an existing site, I think this is possibly a big mistake and the wrong direction for the web to head in.

So, I simply chose to disagree with Steve on this point, and we’re now focusing on different goals.

ControlJS: approach

In addition to my UX concerns with Steve’s approach to page-load optimization that’s embodied in ControlJS, I have some problems with the functional approach he’s taken as well.


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