It&#x27;s not because of reflection. Most Scala libraries are reflection free. Exceptions are Scala libraries wrapping Java ones, or Scala libraries that are doing Java serialization &#x2F; deserialization, which many times depends on reflection. For example JSON parsing in Scala many times piggybacks on Jackson, THE Java library for JSON parsing. However there are JSON parsing libraries that are pure Scala and that do not require reflection like Jackson does.<p>But reflection isn&#x27;t usually an issue for Scala as Scala code does most of the same tricks at compile-time. Scala libraries that have problems are those libraries dealing with multi-threading. Do you block threads anywhere, waiting for a Future, or on a CountDownLatch? Any await&#x2F;notify anywhere? Sorry, that won&#x27;t work on top of Scala.js. That doesn&#x27;t mean that you can&#x27;t work around it though and it does take effort on the part of library authors. My own library (sorry for the shameless plug :)) is completely cross-platform: <a href="https:&#x2F;&#x2F;monix.io" rel="nofollow">https:&#x2F;&#x2F;monix.io</a><p>BTW, Scala.js is very new, but the whole ecosystem wants to support it, every major library is being ported if not ported already and everybody is talking about it ;-)