package foo import kotlinx.js.JsPlainObject @JsPlainObject external interface User { var name: String val age: Int } fun box(): String { val user = User(name = "Name", age = 10) if (user.name != "Name") return "Fail: problem with `name` property" if (user.age != 10) return "Fail: problem with `age` property" val json = js("JSON.stringify(user)") if (json != "{\"age\":10,\"name\":\"Name\"}") return "Fail: got the next json: $json" return "OK" }