found drama

Today, while exploring Larry Myers’ jasmine-reporters extensions for Jasmine, I encountered something that I’d never seen before:

	// bail when all pages have been processed
	setInterval(function(){
	var exit_code = 0;
	for (i = 0, l = pages.length; i < l; i++) {
	page = pages[i];
	if (page.__exit_code === null) {
	// wait until later
	return;
	}
	exit_code |= page.__exit_code;
	}
	phantom.exit(exit_code);
	}, 100);

view raw jasmine-reporters-bitwise-or-assignment.js hosted with by GitHub

(Source.)

Did you see it, too? I saw it because my editor didn’t highlight it as an operator like I was expecting it to. What is this |=? I wondered. I scratched my head. Is that right? Is this some kind of error? But the script ran, and it ran fine. Surely this was a gap in my knowledge.

I took a closer look at it. Was this |= some kind of bitwise operator?

I turned to MDN’s page on assignment operators for my answer. ¹ Sure enough, |= was indeed a real thing: the bitwise OR assignment operator. ²

It works just like you think it would, which is just like the other compound assignment operators. In other words, these statements are equivalent:

x = x | 1

And:

x |= 1

Of course, the question that I asked here ³ was: Was this the most idiomatic way to do this?

I thought maybe it wasn’t–if only because I don’t know too many JS devs that are intimately familiar with ⁴ bitwise operators. Knowing what we know about JavaScript’s truthy/falsy values for numbers, this line:

exit_code |= page.__exit_code

…could just as easily have been this:

exit_code = page.__exit_code || exit_code

…but that’s not as succinct, and not necessarily as clear.

Granted, “not necessarily as clear” assumes that you know what | is good for in the first place. And I’ll be honest here: I’m still bitwise-naïve, so don’t look at me for deep and rigorous answers there. But if I was able to figure out this much in fifteen minutes this afternoon, then it’s at least not a completely foreign entity. And that’s good enough for me.

1. MSDN has a good explanation as well.[]
2. If you want to be a pedantic jerk, it’s a “bitwise inclusive OR assignment operator”.[]
3. Aside from Well how exactly does the bitwise OR work again?[]
4. Or even just “well-versed in”.[]

About Rob Friesel