<init>

When placed on a function parameter, requires the type of the passed argument to be external.

The compiler mangles identifiers of properties from non-external interfaces and classes, and doesn't mangle from external ones. Requiring a type of the passing argument being external is necessary to avoid non-obvious bugs when identifier has an unstable and unpredictable name in generated JS code.

Example:

@OptIn(ExperimentalStdlibApi::class)
fun extractUuid(@JsExternalArgument x: dynamic) = x.uuid as String

external interface User {
    val uuid: String
}

interface Owner {
    val uuid: String
}

fun checkUser(user: User, owner: Owner): Boolean {
    val userUuid = extractUuid(user) // OK
    val ownerUuid = extractUuid(owner) // Compilation error! Possible bug
    return userUuid == ownerUuid
}

This annotation is experimental, meaning that the restrictions mentioned above are subject to change.