scalajs_async.scala

import scala.scalajs.js
import scala.scalajs.js.Dynamic.{global, literal}
import js.annotation.JSExport
import scala.concurrent.{Promise, Future}
import scala.util.{Failure, Success}
import scala.scalajs.concurrent.JSExecutionContext.Implicits.runNow

@JSExport
object Main {
  def defer:Future[Int] = {
    val promise = Promise[Int]
    global.setTimeout(() => {promise.success(3)}, 100)
    return promise.future
  }

  @JSExport
  def main(): Unit = {
    defer onSuccess   {
      case num => println(num)
    }
  }
}