Moddable is GPL though, which might be problematic for some use cases.

Yes, it's well worth understanding the licensing before trying it.

Their commercial license is 25¢/unit (no minimum), and their FOSS license is LGPL for the runtime, GPL for the development tools, and Creative Commons for their example code.

For anyone interested in XS specifically or embedded software licensing in general, they do a good job of explaining their rationale here: https://www.moddable.com/license

Moddable isn't:

https://github.com/Moddable-OpenSource/moddable/tree/public/...

duktape is MIT.

moddable is GPL and LGPL.

How difficult would it be to port Firefox's javascript engine to embedded platforms?

I've never tried, but Mozilla's example doesn't look that hard.

https://github.com/mozilla-spidermonkey/spidermonkey-embeddi...

The big issue (like with v8) is binary size. Double-digit megabytes before you even start running code is unacceptable in a lot of environments.

It's a giant pain in the behind, if you can even get it to compile correctly.

In my experience (having embedded both spidermonkey and duktape) is that if embedding is not a primary use case for the engine it can get messy and complicated to update (lots of api breakage etc). This was the reason I switched from spidermonkey to duktape for my app. That said, it was probably five or six years ago. Perhaps things have changed since. I guess the takeaway is just that. If the engine isn’t maintained for embedding. Don’t use it as such.

Yeah, I guess JIT compilation to native code can also be a difficult issue.

I have not tried SpiderMonkey, but embedding V8 is pretty easy.