ReferenceError onTouchStart is not defined jquery.flexslider.js

29 Apr 2015

I was supposed to write this blog post like a year ago, but have been very busy in the last 12 months not writing this blog post. But yet here we are.

Doing some compatibility research on top Japanese sites, I ran into my old nemesis: ReferenceError: onTouchStart is not defined jquery.flexslider.js:397:12.

I first ran into this in January of 2014 in its more spooky form ReferenceError: g is not defined. Eventually I figured out it was a problem in a WooThemes jQuery plugin called FlexSlider, the real bug being the undefined behavior of function declaration hoisting in conditions (everyone just nod like that makes sense).

In JavaScript-is-his-co-pilot Juriy's words,

Another important trait of function declarations is that declaring them conditionally is non-standardized and varies across different environments. You should never rely on functions being declared conditionally and use function expressions instead.

In this case, they were conditionally declaring a function, but referencing it before said declaration in the if block, as an event handler, i.e.,

if (boop) {
  blah.addEventListener("touchstart", wowowow);

  function wowowow() {}

No big deal, easy fix. I wrote a patch. Some people manually patched their sites and moved on with their lives. I tried to.

We ran into this a few more times in Bugzilla and land.

TC-39 bosom buddies Brendan and Allen noted that ES6 specifies things in such a way that these sites will eventually work in all ES62015 compliant browsers. Here's the bug to track that work in SpiderMonkey.

Cool! Until then, my lonely pull request is still hanging out at (16 months later). The good news is FlexSlider is open source, so you're allowed to fix their bugs by manually applying that patch on your site. Then your touch-enabled slider widget stuff will work in Mobile Firefox browsers.