Comment by ajb

The only language I'm aware of that's a bit like that is rpython, but it's the other way round: designed for python to compile to it. If you think about it, you get more benefit from the typed language being the base one, as the compiler or JIT can make more assumptions, producing faster code . Typescript had no alternative but to do it the other way, since it's a lot harder to get things adopted into the browser than to ship them independently.