PrependFile.scala

 
   val customFullOpt = Def.taskKey[File]("Generate the file ..")
   
  artifactPath in Compile in customFullOpt :=
        baseDirectory.value / "final-bundle.js",
      customFullOpt in Compile := {
        val outFile = (artifactPath in Compile in customFullOpt).value
        val loaderFile = (resourceDirectory in Compile).value / "loader.js" // file to prepended
        IO.copyFile(loaderFile, outFile)
        val fullOutputCode = IO.read((fullOptJS in Compile).value.data)
        IO.append(outFile, fullOutputCode)  // add fullopt output
        val launcher = (scalaJSLauncher in Compile).value.data.content
        IO.append(outFile, launcher) // add launcher code too
        outFile
      }