So, it turns out we're not the only ones with a JavaScript-based game engine:
http://atomicgameengine.com/

And it uses Duktape!

You can use JS in Unity too, but we all know that's not FOSS.

I was most intrigued about the 3D module, but that seems to cost money to use? Oh well.

Unity's JS interface is also really...inelegant.

Yeah, the source to this is available, but it's definitely not free software:
https://github.com/AtomicGameEngine/AtomicGameEngine/blob/master/LICENSE.md

My favorite part of the EULA has to be this bit (emphasis mine):

Custom builds of the Atomic Game Engine Editor require product activation and platform deployment licenses. These can also be acquired at our website.

If you're building from source, couldn't you easily take the activation code out?

So yeah, we still have the benefit of being FOSS.  I just found it out interesting there was another game engine out there using Duktape.

I have heard of and seen this engine. I think I first saw it on steam. At the time they had little to show and nothing much of value, a basic scene node system that resembled GameMaker, but now I think they've gone much further along.

Sounds like a good editor/engine combo to use to finally make my games. /kidding /sphereforlife