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.

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.

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