Scala 2.12.0-RC1's new inference improves JS interop!

ImprovedInference.scala

package helloworld

import scala.scalajs.js
import js.|
import js.annotation._

@ScalaJSDefined
trait Foo extends js.Object {
  val bar: Int | String
  val baz: js.UndefOr[Int]
  val f: js.Function1[Int, Int]
}

@ScalaJSDefined
class Foobar extends Foo {
  val bar = 5
  val baz = 6
  val f = (x: Int) => x + 1
}

object HelloWorld extends js.JSApp {
  def main() {
    val foobar: Foo = new Foobar
    println(foobar.bar)
    println(foobar.baz)
    println(foobar.f)
    println(foobar.f(3))

    val obj = new Foo {
      val bar = "hello"
      val baz = js.undefined
      val f = x => x * 2
    }
    println(obj.bar)
    println(obj.baz)
    println(obj.f)
    println(obj.f(3))
  }
}