In the small, Rescript generates smaller code than Scala.js, that is true. It is mostly a question of the interdependencies in the standard Scala collection library, though, not so much the semantics of the language. And Scala.js also has its share of "tricks" (aka optimizations) to reduce indirections and overhead.

Regarding interop and the link you posted, here is the equivalent documentation page in Scala.js: https://www.scala-js.org/doc/interoperability/types.html

Yes, Rescript has more built-in types that map straightforwardly, but that is at the cost of some correctness. For example, using a JavaScript array means that JavaScript can resize it under your feet. Using `undefined` to represent `None` means that you cannot tell the difference between `None` and `Some(None)`. It's a fine trade-off to make. Scala.js happens to make the other trade-off, and offers separate types for JavaScript interop.

I like the concepts and design of Rescript, really. It's very interesting because they have all the essential requirements right (IMO) like comprehensive JavaScript interop, while making all the opposite design decisions on nonessential trade-offs compared to Scala.js.