Kotlin/JS reflection

Kotlin/JS provides a limited support for the Kotlin reflection API. The only supported parts of the API are:

Class references (::class)
KType and typeof()
KClass and createInstance()

Class references

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.

KType and typeOf()

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.

KClass and createInstance()

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.

Example

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"
}

Last modified: 21 May 2024