In Scala:

@js.native
  trait Props extends js.Object {
    val value: String = js.native
    val items: scala.collection.mutable.Seq[String] = js.native
  }

  @JSExport
  val TestCompB: ReqProps[Props, Unit, Unit, TopNode] =
    ReactComponentB[Props]("HelloMessage")
      .render($ => {
        // val h=$.props.items.head //this does not work
        val h = $.props.value //this works
        <.div("Hello " + h)
      }
      )
      .build


  @JSExport
  val wrapTestCompB: js.Function1[Props, ReactComponentU[Props, Unit, Unit, TopNode]] =
    (x: Props) => TestCompB(x)

  //val TestCompInst: ReactComponentU[String, Unit, Unit, TopNode] = TestCompB("vazzeg")
  
  
IN JS:
  
  
// Using Component defined in scalajs
const state6 = { items: ['Item 1 jelleig', 'Item 2', 'Item 3', 'Item 4', 'Item 5', 'Item 6'] }
const TestComponentS = example.Experiment().wrapTestCompB       // Scala.js version
var target6 = document.getElementById("playground6")
render(<TestComponentS value="shiraly43 - using scalajs component from js" items={state6}/>, target6);