How to create webworker code in Scala.js

Question

How do I create the code for a web worker using Scala.js? Looking at the documentation everything seems to produce JavaScript functions rather than top level JavaScript. The new Worker constructor seems to require top level Javascript, rather than a function within a file. So in my Scala.js code I have:

var myWorker = new Worker("worker.js");
myWorker.onmessage = (e: scala.scalajs.js.Any) =>
{
  val e1 = e.asInstanceOf[MessageEvent]
  println("Worker message recieved:" -- e1.data.toString)
}
myWorker.postMessage("")

And in the file worker.js I have

self.postMessage("from webworker");

The message is picked up and displayed in the web console as intended.

Answer 1

Scala.js will indeed only ever generate non-top-level code, by design. However, you can add a bit of arbitrary JavaScript code at the beginning and/or end of the generated .js file with the scalaJSOutputWrapper sbt setting:

scalaJSOutputWrapper := ("", "example.WorkerMainObject().main();")

This would cause the generated .js file to have the specified top-level statement.

See also https://stackoverflow.com/a/34764513/1829647