I'm working on Solace¹, a programming language that targets JavaScript, but is not a superset, like Typescript.
The idea is, to allow zero-runtime-cost safeguards, like Optional Values and Error Unions (syntactically they look like Zig's), decent pattern matching and Traits. There's still a lot of work to do, though, because some of the approaches I went for, don't hold in all cases (generics are tricky).
1) https://git.koehr.ing/n/solace