But the Reason example you shared is almost valid TypeScript already!

https://www.typescriptlang.org/play/?#code/DYUwLgBAHgXBAUBDO...

I've seen JS using lambdas instead of classic functions like that in the wild. And that Reason can be made more JS-idiomatic without changes to the syntax:

    let x = (a: int, b: int): int => a + b;