Kotlin/JS reflection

The ::class syntax returns a reference to the class of an instance, or the class corresponding to the given type. In Kotlin/JS, the value of a ::class expression is a stripped-down KClass implementation that supports only:

• simpleName and isInstance() members.

• cast() and safeCast() extension functions.

In addition to that, you can use KClass.js to access the JsClass instance corresponding to the class. The JsClass instance itself is a reference to the constructor function. This can be used to interoperate with JS functions that expect a reference to a constructor.

The typeof() function constructs an instance of KType for a given type. The KType API is fully supported in Kotlin/JS except for Java-specific parts.

The createInstance() function from the KClass interface creates a new instance of the specified class, which is useful for getting the runtime reference to a Kotlin class.

Here is an example of the reflection usage in Kotlin/JS.

open class Shape
class Rectangle : Shape()

inline fun <reified T> accessReifiedTypeArg() =
    println(typeOf<T>().toString())

fun main() {
    val s = Shape()
    val r = Rectangle()

    println(r::class.simpleName) // Prints "Rectangle"
    println(Shape::class.simpleName) // Prints "Shape"
    println(Shape::class.js.name) // Prints "Shape"

    println(Shape::class.isInstance(r)) // Prints "true"
    println(Rectangle::class.isInstance(s)) // Prints "false"
    val rShape = Shape::class.cast(r) // Casts a Rectangle "r" to Shape

    accessReifiedTypeArg<Rectangle>() // Accesses the type via typeOf(). Prints "Rectangle"
}