Only if it's been ported to Scala.js; in fact, same goes for Scala to Scala.js, all libs need to be cross compiled.
The only thing you get for "free" in Scala.js is Scala, which is of course no small accomplishment, but the JVM ecosystem as a whole does not come with it.
That's what I thought, and why I was confused by the original comment. Scala.js doesn't seem to have any benefits over js_of_ocaml, unless you prefer the language itself.
I was expressing myself sloppily, sorry. You still can use many things in the JVM ecosystem because you can compile (most of) scala.js not only to Javascript but also to the JVM, and then analyse the code with JVM specific tools like fuzzers, debuggers, profilers etc. This is powerful. Making sure your code runs on two platform will expose additional problems that one platform alone doesn't do.
I see. Well, I suppose that's true of OCaml as well. Its ecosystem is smaller, but it does have some pretty powerful tools, including a time-traveling debugger.
The only thing you get for "free" in Scala.js is Scala, which is of course no small accomplishment, but the JVM ecosystem as a whole does not come with it.