You may be coding java, but none of the tangible benefits of java over javascript actually translate, because you're actually writing javascript, with all of the warts and limitations and weirdness therein.

The javascript exported will not be truly type safe, because javascript is not, and cannot be. It may appear to act like it, within the context of the code when run as predicted, but this isn't the same thing as an actual strict language in a runtime which expects and enforces those rules. Highly efficient? Doubtless. Optimized to avoid as many pitfalls of the language as possible? Certainly. But anything more capable than bog-standard javascript? No. Because it can't be.