Use Kotlin code from JavaScript

Depending on the selected JavaScript Module system, the Kotlin/JS compiler generates different output. But in general, the Kotlin compiler generates normal JavaScript classes, functions and properties, which you can freely use from JavaScript code. There are some subtle things you should remember, though.

Isolating declarations in a separate JavaScript object in plain mode

If you have explicitly set your module kind to be plain, Kotlin creates an object that contains all Kotlin declarations from the current module. This is done to prevent spoiling the global object. This means that for a module myModule, all declarations are available to JavaScript via the myModule object. For example:

fun foo() = "Hello"

Can be called from JavaScript like this:

alert(myModule.foo());

This is not applicable when you compile your Kotlin module to JavaScript modules like UMD (which is the default setting for both browser and nodejs targets), CommonJS or AMD. In this case, your declarations will be exposed in the format specified by your chosen JavaScript module system. When using UMD or CommonJS, for example, your call site could look like this:

alert(require('myModule').foo());

Check the article on JavaScript Modules for more information on the topic of JavaScript module systems.

Package structure

Kotlin exposes its package structure to JavaScript, so unless you define your declarations in the root package, you have to use fully qualified names in JavaScript. For example:

package my.qualified.packagename

fun foo() = "Hello"

When using UMD or CommonJS, for example, your callsite could look like this:

alert(require('myModule').my.qualified.packagename.foo())

Or, in the case of using plain as a module system setting:

alert(myModule.my.qualified.packagename.foo());

@JsName annotation

In some cases (for example, to support overloads), the Kotlin compiler mangles the names of generated functions and attributes in JavaScript code. To control the generated names, you can use the @JsName annotation:

// Module 'kjs'
class Person(val name: String) {
    fun hello() {
        println("Hello $name!")
    }

    @JsName("helloWithGreeting")
    fun hello(greeting: String) {
        println("$greeting $name!")
    }
}

Now you can use this class from JavaScript in the following way:

// If necessary, import 'kjs' according to chosen module system
var person = new kjs.Person("Dmitry");   // refers to module 'kjs'
person.hello();                          // prints "Hello Dmitry!"
person.helloWithGreeting("Servus");      // prints "Servus Dmitry!"

If we didn't specify the @JsName annotation, the name of the corresponding function would contain a suffix calculated from the function signature, for example hello_61zpoe$.

Note that there are some cases in which the Kotlin compiler does not apply mangling:

external declarations are not mangled.
Any overridden functions in non-external classes inheriting from external classes are not mangled.

The parameter of @JsName is required to be a constant string literal which is a valid identifier. The compiler will report an error on any attempt to pass non-identifier string to @JsName. The following example produces a compile-time error:

@JsName("new C()")   // error here
external fun newC()

@JsExport annotation

By applying the @JsExport annotation to a top-level declaration (like a class or function), you make the Kotlin declaration available from JavaScript. The annotation exports all nested declarations with the name given in Kotlin. It can also be applied on file-level using @file:JsExport.

To resolve ambiguities in exports (like overloads for functions with the same name), you can use the @JsExport annotation together with @JsName to specify the names for the generated and exported functions.

The @JsExport annotation is available in the current default compiler backend and the new IR compiler backend. If you are targeting the IR compiler backend, you must use the @JsExport annotation to make your functions visible from Kotlin in the first place.

For multiplatform projects, @JsExport is available in common code as well. It only has an effect when compiling for the JavaScript target, and allows you to also export Kotlin declarations that are not platform specific.

Kotlin types in JavaScript

See how Kotlin types are mapped to JavaScript ones:

Kotlin	JavaScript	Comments
`Byte`, `Short`, `Int`, `Float`, `Double`	`Number`
`Char`	`Number`	The number represents the character's code.
`Long`	Not supported	There is no 64-bit integer number type in JavaScript, so it is emulated by a Kotlin class.
`Boolean`	`Boolean`
`String`	`String`
`Array`	`Array`
`ByteArray`	`Int8Array`
`ShortArray`	`Int16Array`
`IntArray`	`Int32Array`
`CharArray`	`UInt16Array`	Carries the property `$type$ == "CharArray"`.
`FloatArray`	`Float32Array`
`DoubleArray`	`Float64Array`
`LongArray`	`Array<kotlin.Long>`	Carries the property `$type$ == "LongArray"`. Also see Kotlin's Long type comment.
`BooleanArray`	`Int8Array`	Carries the property `$type$ == "BooleanArray"`.
`Unit`	Undefined
`Any`	`Object`
`Throwable`	`Error`
Nullable `Type?`	`Type \\| null \\| undefined`
All other Kotlin types (except for those marked with `JsExport` annotation)	Not supported	Includes Kotlin's collections (`List`, `Set`, `Map`, etc.), and unsigned variants.

Additionaly, it is important to know that:

Kotlin preserves overflow semantics for kotlin.Int, kotlin.Byte, kotlin.Short, kotlin.Char and kotlin.Long.
Kotlin cannot distinguish between numeric types at runtime (except for kotlin.Long), so the following code works:
fun f() { val x: Int = 23 val y: Any = x println(y as Float) }
Kotlin preserves lazy object initialization in JavaScript.
Kotlin does not implement lazy initialization of top-level properties in JavaScript.

Last modified: 17 November 2023