[sjrd]

A lot of the mentioned languages also allow deep interoperability between their heap and the JavaScript heap, e.g., circular references between objects of the "two heaps", and free access to fields and methods of objects of the other language.

That's very hard (if not impossible) to achieve without leak and performance degradation if the two languages have their own GC, with their own heaps.

Compiling a language to JS is not about making it work. That's easy (it becomes hard to cite a language that does not do it). It's about designing the language to interoperate with JS, and making that work. That is the real challenge.

[lomnakkus]

> Compiling a language to JS is not about making it work. That's easy (it becomes hard to cite a language that does not do it). It's about designing the language to interoperate with JS, and making that work. That is the real challenge.

It's very interesting that Scala and Scala.js have such a relatively painless interaction, but in general I'd say interoperation is "technically" simple by just employing an FFI?

Obviously, words like "seamless" and "effortless" start to enter the vocabulary here, but I'm not entirely these targets are worth it. Are they, do you think?

(I mean, obviously, Scala.js must have seamless 'interop' to Scala, but is 'seamless' introp with JS worth it, or should you require explicit FFI? I'm not sure, but I think you ultimately chose FFI-via-annotations, but there's a lot of fuzziness wrt. js.Dynamic.)