Scala is a JVM language, with explicit design for good interoperability with Java.
Scala.Net came to life in 2011 and died around 2013, despite support from EPFL.
Scala.js arrived in November 2013 and now has sufficient traction that important libraries have explicit support. I found this story describing compiling to JS because on AWS Lamba node has faster startup than the JVM! It is no longer a refuge for people who hate having to use JS in the browser.
The latest addition is scala.native, using LLVM to compile directly to native code. It is rather amusing to see the reference to “ahead-of-time” compiling, as if that is something unusual.