union type in Kotlin/JS

generated_sample.js

  function sample$lambda() {
    return 'Bar';
  }
  function sample() {
    var v1 = 'Foo';
    var v2 = sample$lambda;
  }

sample.kt

external interface ObjectOrFactory<T>

inline fun <T> ObjectOrFactory(value: T): ObjectOrFactory<T> = value.asDynamic()
inline fun <T> ObjectOrFactory(value: () -> T): ObjectOrFactory<T> = value.asDynamic()

inline fun <T> ObjectOrFactory.toObject(): T = this.asDynamic()
inline fun <T> ObjectOrFactory.toFactory(): () -> T = this.asDynamic()

fun sample() {
    val v1: ObjectOrFactory<String> = ObjectOrFactory("Foo")
    val v2: ObjectOrFactory<String> = ObjectOrFactory { "Bar" }
//    val v3: ObjectOrFactory<String> = "Buz" // error
//    val v4: ObjectOrFactory<String> = ObjectOrFactory(100) // error
}