Building a new JIT for a dynamic language, if it is supposed to be competitive with JS compilers, is a project several orders of magnitude bigger than building a runtime like this.
TypeScript's design is very much that of a typing layer over a dynamic language. It's unsoundness and various loopholes to escape typing mean it's not much of a help for typed compilation.
TypeScript's design is very much that of a typing layer over a dynamic language. It's unsoundness and various loopholes to escape typing mean it's not much of a help for typed compilation.