Scala's Futures work unchanged, it just has a single-threaded threadpool instead of a multithreaded one. Anything built on Futures (e.g. my Castor actor library

Scala's Futures work unchanged, it just has a single-threaded threadpool instead of a multithreaded one. Anything built on Futures (e.g. my Castor actor library) is unchanged as well. Scala Futures are more or less a 1:1 mapping to Javascript Promises, and work identically. There isn't long running IO or sleeps, same JS, but this isn't a big problem in practice. As long as you're not juggling threads directly - and most Scala code doesn't - your code should run unchanged concurrency-wise in Scala.js just with no parallelism. Though of course you can't use OS interfaces like java.nio.file.Files in the browser!