JavaScript Is Way Too Slow – for What?

There's more to mobile web apps than JavaScript

I keep finding that programmers—even web programmers—frequently think “web application” means “JavaScript application.” Programmers are, of course, used to working with programming languages, and often see application environments from the perspective of the programming language in use.

These blinders derail Drew Crawford’s detailed rant on Why mobile web apps are slow. It turns out that “slow for what?” is a key part of the question, as Crawford reveals near the very end:

JavaScript is too slow for mobile app use in 2013 (e.g., for photo editing, etc.).

Do we need to run Photoshop on our mobile phones using JavaScript underneath? I agree that cramming everything into an interpreted language on a memory-, processing-, and bandwidth-constrained device is a stupid idea—but “photo editing” is an extreme case, to put it mildly. Perhaps critics will move on to video editing when we eventually get to the next round.

Despite that flaw, if the rant had been titled “Why mobile JavaScript apps are slow,” it would have done better. His lists of benchmarks, discussions of processor challenges, and recognition that the easy work in JavaScript optimization is already done, are all tremendously useful. There is little reason to expect JavaScript performance to improve in the future at the rate is has achieved for the last decade.

It may also be true that the browser vendors have optimized their performance as far as they can, at least in the relatively stable fields of HTML parsing and processing, and CSS selectors and formatting. Adding querySelector to the DOM was a massive one-time speed upgrade, letting JavaScript developers drop most of their tree walking in favor of a more declarative approach relying on native code in which vendors had already invested heavily.

So how can we optimize mobile web development?

The first answer is pretty simple: don’t try to use JavaScript for tasks (like photo or video editing) that push it past its limits. As far as JavaScript has come, it wasn’t built for that. Native apps are indeed good for more than building browsers.

The second answer is trickier: it means recognizing that web applications include much more than JavaScript. JavaScript began as a scripting language for gluing together objects written in other languages, and it still excels at that. Build your application from the HTML—the easiest stuff to process!—up. (If you want graphics, SVG might be a better path.) Then layer CSS on top of that. Rely on APIs built into the browser whenever you can, using JavaScript to connect with their much faster processing. HTML5 keeps making that a better and better option.

When I was programming our HTML 4 & 5: The Complete Reference, I marveled regularly at how tasks that seemed huge, like loading a new CSS stylesheet, moved rapidly compared to implementing smaller changes using JavaScript. More broadly, Estelle Weyl’s JavaScript: You Don’t Need a Framework For That (slides) resonates both because it shows how to use smaller chunks of JavaScript and because it explores how to use CSS creatively to avoid using JavaScript at all.

Building great applications using JavaScript, especially great mobile web applications, often means using JavaScript itself as sparingly as possible. That’s always been a feature of JavaScript, not a bug.

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