My summary of Scala.js’s good points that this article mentions (because its table of contents repeats some points):

• static typing with type inference (for error detection without boilerplate)
• IDE and refactoring support
• an ecosystem of editor plugins
• a large ecosystem of libraries and bindings
• relatively easy JVM/JS platform interop and easy writing of bindings
• a library for building web apps server-side as well as client-side, so as to share code

Some good points that Scala.js does not have:

• quick compile times
• simplicity
• purity from side effects in the type system
• in my experience, Scala lacks good documentation for its tooling (it’s hard to set up a project and import it into my IDE so everything works)

Some close compile-to-JS competitors and their biggest weaknesses compared to Scala.js:

• ClojureScript – not statically typed
• TypeScript – weak type system, less-ergonomic server-side libraries
• Kotlin – unstable JS tooling, slightly worse type system than Scala, less-ergonomic server-side libraries
• Elm/PureScript – no server-side support
• Haskell with GHCJS – incomplete platform compatibility (or so I heard a while ago), difficult interop and poor library ecosystem
• OCaml with Js_of_ocaml – unstable tooling, difficult interop and poor library ecosystem
• Haxe – worse type system; I don’t know much about its server-side libraries or its ecosystem