[marijn]

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.

[torb-xyz]

Microsoft _do_ have the research project Static TypeScript that does try to do this with a subset: https://www.microsoft.com/en-us/research/publication/static-...

But your point stands I think this subset will be incompatible with how many (including myself) writes TypeScript.