Have you ever wondered what is the secret sauce of Scala.js? What defines Scala.js, above all else, is the overarching will to make it cross-platform. A cross-platform language is both portable-most source code cross-compiles and behaves the same way on multiple platforms-and interoperable-code written in that language can interoperate with other languages on the target platforms. Most multi-platform languages are designed with one of those two properties in mind, and only address the other one as an afterthought. This results in poor integration, difficulty to write portable code, or even the impossibility to use some platform-specific libraries.

This talk shows how Scala.js was designed as a cross-platform language. We show how portability and interoperability drive every design decision, from which language feature to include to performance considerations. We state a criteria of completeness for interoperability, and show how Scala.js satisfies it (with the exception of one JavaScript language feature, still work-in-progress). We give insights about what features are critical for portability, and which ones can be relaxed in the name of performance. In particular, we stress the importance of the ecosystem of libraries: existing assumptions in the ecosystem can force the language designer into reproducing behaviors of Scala/JVM, even when they are accidental.

As a practical exercise, we study the main language changes that made it into Scala.js 1.x wrt. 0.6.x-as well as possible future developments-and how they fit into the cross-platform language design methodology.