I'm a full stack Web Developer at daytime and I'd argue that I probably do speak JavaScript more fluently than I do English. So for me it really was a no brainer. The custom conversion tools needed for the graphics / map / sounds are also written in JS, so it's a bonus to have good interop with those too.

In addition to that, getting the compiler up to speed (takes < 90ms on my old core i5) was a nice exercise in getting more familiar with v8's performance analysis tools :)

Some options include: He is familiar with it, the Z80 is so simple it doesn't really matter, and someone else already wrote one in JavaScript. Lastly does it matter?