Some of the stuff is kind of questionable to me in the sense that I could believe you could probably make some kind of sufficiently wonky JS that this would do the "wrong" thing.
But it's super cool that they are able to do this as I think it shows that claims of JS complexity based on the size of JS engines is overlooking just how much of that size/complexity comes from the "make it fast" drive vs. what the language requires. Here you have a Mad props to them for even attempting it as well - it simply would not have ever occurred to me to say "let's just write a small JS engine" and I would have spent stupid amounts of time attempting to use JSC* from python instead.
[* JSC appears to be the only JS engine with a pure C API, and the API and ABI are stable so on iOS/macOS at least you can just use the system one which reduces binary size+build annoyance. The downside is that C is terrible, and C++ (differently terrible? :D) APIs make for much more pleasant interfaces to the VM - constructors+destructors mean that you get automatic lifetime management so handles to objects aren't miserable, you can have templates that allow your API to provide handles that have real type information. JSC only has JSValueRef and JSObjectRef, and as a JSObjectRef is a JSValueRef it's actually just a typedef to const JSValueRef :D OTOH other hand I do thing JSC's partially conservative GC is better for stack/temporary variables is superior to Handles for the most part, but it's also absolutely necessary to have an API that isn't absolutely wretched. The real problem with JSC's API is that it has not got any love for many many many .... many years so it doesn't have any way to handle or interact with many modern features without some kludgy wrappers where you push your API objects into JS and have the JS code wrap them up. The API objects are also super slow, as they basically get treated as "oh ffs" objects that obey no rules. I really do wish it would get updated to something more pleasant and really usable.]