External declarations

external keyword allows to declare existing JavaScript API in a type-safe way.

external fun alert(msg: String)   // 1

fun main() {
  alert("Hi!")                    // 2
}

1. Declares an existing JavaScript function alert which takes a single String argument.
2. Uses alert as if it were regular Kotlin.

Note that Kotlin checks during compilation, that a single argument of type String is passed. Such a check prevents some bugs even when using pure JavaScript API.

Please refer to the docs in order to learn more about describing existing JavaScript API.