Creating extension property with backing field in Kotlin JS.

dynamic_extension_property.kt

//In Kotlin, we can't create 'var' extension property that requires backing field. (https://kotlinlang.org/docs/reference/extensions.html#extension-properties)

//However, in Kotlin JS, we can can 'create' backing fields, due to the JS Object's dynamic nature.
var HTMLElement.myProp: String
  get() = this.asDynamic().myProp
  set(value) { this.asDynamic().myProp = value }

//We can also create custom property delegate to remove boilerplate:
class DynamicProperty<T> {
  operator fun getValue(thisRef: Any, prop: KProperty<*>) = thisRef.asDynamic()[prop.name]
  operator fun setValue(thisRef: Any, prop: KProperty<*>, value: T) { thisRef.asDynamic()[prop.name] = value }
}

var HTMLElement.myProp: String by DynamicProperty()