Scala.JS & SBT

copy.sbt

// Assumptions:
// 1. You have one or more Scala.JS modules named jsModule1, jsModule2, ...
// 2. You have a web-serving (or otherwise JVM) module that will serve the JS assets

// The only "webby"/JS plugin I use apart from Scala.JS itself, is xsbt-web-plugin,
// and even that, only on my pre-http4s projects.
// xsbt-web-plugin allows me to quickly stop/start a servlet container from SBT.
// The snippet below is for "webappPostProcess" which is an xsbt-web-plugin task but
// if you aren't using it, you can just create a task with the same content and make
// "package" depend on it.

webappPostProcess := {
  implicit val log      = streams.value.log
  val outputOfJsModule1 = (scalaJSLinkedFile in Compile in jsModule1).value
  val outputOfJsModule2 = (scalaJSLinkedFile in Compile in jsModule2).value
  // ...

  (target: File) => {

    // Copy Scala.JS assets
    def copyScalaJs(jsf: VirtualJSFile, to: String): Unit =
      jsf match {
        case f: FileVirtualJSFile => fileSync(f.file, target / to, mandatory = true)
        case other                => sys.error("Unsupported virtual file type: " + other)
      }
    copyScalaJs(outputOfJsModule1, targetForJsModule1)
    copyScalaJs(outputOfJsModule2, targetForJsModule2)
    // ...

    // <copy other assets here too>
  }
}

// Little helper. Is it still needed in SBT 1.3.x? Who knows?? It was definately helpful in the 0.13.x days.
def fileSync(from: File, to: File, mandatory: Boolean)(implicit log: Logger): Unit =
  if (from.exists()) {
    log.info(s"Copying $from → $to")
    IO.copyFile(from, to, preserveLastModified = true)
  } else if (mandatory)
    sys.error("File not found: " + from.absolutePath)
  else if (to.exists()) {
    log.info(s"Deleting $to")
    IO.delete(to)
  }

dev-prod-settings.sbt

// SBT doesn't have a standard concept of dev/prod builds so however you decide to
// specify that, you can use settings similar to below.
// The `scalaJSLinkedFile in Compile` setting referenced in the snippet above will
// choose fast/full opt via the `scalaJSStage` below.

  def jsDevSettings: Project => Project =
    _.settings(emitSourceMaps := true)

  def jsProdSettings: Project => Project =
    _.settings(
      emitSourceMaps := false,
      scalaJSStage := FullOptStage,
      scalaJSSemantics in fullOptJS ~= (_
        .withProductionMode(true)
        .withRuntimeClassNameMapper(Semantics.RuntimeClassNameMapper.discardAll())
        .withArrayIndexOutOfBounds(CheckedBehavior.Unchecked)
        .withAsInstanceOfs(CheckedBehavior.Unchecked)))
      scalaJSOptimizerOptions ~= (_
        .withBatchMode(true)
        .withCheckScalaJSIR(true)))