Interoperability with JavaScript

Kotlin/Wasm allows you to use both JavaScript code in Kotlin and Kotlin code in JavaScript.

As with Kotlin/JS, the Kotlin/Wasm compiler also has interoperability with JavaScript. If you are familiar with Kotlin/JS interoperability, you can notice that Kotlin/Wasm interoperability is similar. However, there are key differences to consider.

Use JavaScript code in Kotlin

Learn how to use JavaScript code in Kotlin by using external declarations, functions with JavaScript code snippets, and the @JsModule annotation.

External declarations

External JavaScript code is not visible in Kotlin by default. To use JavaScript code in Kotlin, you can describe its API with external declarations.

JavaScript functions

Consider this JavaScript function:

function greet (name) {
    console.log("Hello, " + name + "!");
}

You can declare it in Kotlin as an external function:

external fun greet(name: String)

External functions don't have bodies, and you can call it as a regular Kotlin function:

fun main() {
    greet("Alice")
}

JavaScript properties

Consider this global JavaScript variable:

let globalCounter = 0;

You can declare it in Kotlin using external var or val properties:

external var globalCounter: Int

These properties are initialized externally. The properties can't have = value initializers in Kotlin code.

JavaScript classes

Consider this JavaScript class:

class Rectangle {
    constructor (height, width) {
        this.height = height;
        this.width = width;
    }

    area () {
        return this.height * this.width;
    }
}

You can use it in Kotlin as an external class:

external class Rectangle(height: Double, width: Double) : JsAny {
    val height: Double
    val width: Double
    fun area(): Double
}

All declarations inside the external class are implicitly considered external.

External interfaces

You can describe the shape of a JavaScript object in Kotlin. Consider this JavaScript function and what it returns:

function createUser (name, age) {
    return { name: name, age: age };
}

See how its shape can be described in Kotlin with an external interface User type:

external interface User : JsAny {
    val name: String
    val age: Int
}

external fun createUser(name: String, age: Int): User

External interfaces don't have runtime type information and are a compile-time-only concept. Therefore, external interfaces have some restrictions compared to regular interfaces:

You can't use them on the right-hand side of is checks.
You can't use them in class literal expressions (such as User::class).
You can't pass them as reified type arguments.
Casting with as to external interfaces always succeed.

External objects

Consider these JavaScript variables holding an object:

let Counter = {
    value: 0,
    step: 1,
    increment () {
        this.value += this.step;
    }
};

You can use them in Kotlin as an external object:

external object Counter : JsAny {
    fun increment()
    val value: Int
    var step: Int
}

External type hierarchy

Similar to regular classes and interfaces, you can declare external declarations to extend other external classes and implement external interfaces. However, you can't mix external and non-external declarations in the same type hierarchy.

Kotlin functions with JavaScript code

You can add a JavaScript snippet to Kotlin/Wasm code by defining a function with = js("code") body:

fun getCurrentURL(): String =
    js("window.location.href")

If you want to run a block of JavaScript statements, surround your code inside the string with curly brackets {}:

fun setLocalSettings(value: String): Unit = js(
    """{
        localStorage.setItem('settings', value);
}"""
)

If you want to return an object, surround the curly brackets {} with parentheses ():

fun createJsUser(name: String, age: Int): JsAny =
    js("({ name: name, age: age })")

Kotlin/Wasm treats calls to the js() function in a special way, and the implementation has some restrictions:

A js() function call must be provided with a string literal argument.
A js() function call must be the only expression in the function body.
The js() function is only allowed to be called from package-level functions.
The function return type must be provided explicitly.
Types are restricted, similar to external fun.

The Kotlin compiler puts the code string into a function in the generated JavaScript file and imports it into WebAssembly format. The Kotlin compiler doesn't verify these JavaScript snippets. If there are JavaScript syntax errors, they are reported when you run your JavaScript code.

JavaScript modules

By default, external declarations correspond to the JavaScript global scope. If you annotate a Kotlin file with the @JsModule annotation, then all external declarations within it are imported from the specified module.

Consider this JavaScript code sample:

// users.mjs
export let maxUsers = 10;

export class User {
    constructor (username) {
        this.username = username;
    }
}

Use this JavaScript code in Kotlin with the @JsModule annotation:

// Kotlin
@file:JsModule("./users.mjs")

external val maxUsers: Int

external class User : JsAny {
    constructor(username: String)

    val username: String
}

Array interoperability

You can copy JavaScript's JsArray<T> into Kotlin's native Array or List types; likewise, you can copy these Kotlin types to JsArray<T>.

To convert JsArray<T> to Array<T> or the other way around, use one of the available adapter functions.

Here's an example of conversion between generic types:

val list: List<JsString> =
    listOf("Kotlin", "Wasm").map { it.toJsString() }

// Uses .toJsArray() to convert List or Array to JsArray
val jsArray: JsArray<JsString> = list.toJsArray()

// Uses .toArray() and .toList() to convert it back to Kotlin types 
val kotlinArray: Array<JsString> = jsArray.toArray()
val kotlinList: List<JsString> = jsArray.toList()

Similar adapter functions are available for converting typed arrays to their Kotlin equivalents (for example, IntArray and Int32Array). For detailed information and implementation, see the kotlinx-browser repository.

Here's an example of conversion between typed arrays:

import org.khronos.webgl.*

    // ...

    val intArray: IntArray = intArrayOf(1, 2, 3)
    
    // Uses .toInt32Array() to convert Kotlin IntArray to JavaScript Int32Array
    val jsInt32Array: Int32Array = intArray.toInt32Array()
    
    // Uses toIntArray() to convert JavaScript Int32Array back to Kotlin IntArray
    val kotlnIntArray: IntArray = jsInt32Array.toIntArray()

Use Kotlin code in JavaScript

Learn how to use your Kotlin code in JavaScript by using the @JsExport annotation.

Functions with the @JsExport annotation

To make a Kotlin/Wasm function available to JavaScript code, use the @JsExport annotation:

// Kotlin/Wasm

@JsExport
fun addOne(x: Int): Int = x + 1

Kotlin/Wasm functions marked with the @JsExport annotation are visible as properties on a default export of the generated .mjs module. You can then use this function in JavaScript:

// JavaScript

import exports from "./module.mjs"

exports.addOne(10)

The Kotlin/Wasm compiler is capable of generating TypeScript definitions from any @JsExport declarations in your Kotlin code. These definitions can be used by IDEs and JavaScript tools to provide code autocompletion, help with type-checks, and make it easier to consume Kotlin code from JavaScript and TypeScript.

The Kotlin/Wasm compiler collects any top-level functions marked with the @JsExport annotation and automatically generates TypeScript definitions in a .d.ts file.

To generate TypeScript definitions, in your build.gradle.kts file in the wasmJs{} block, add the generateTypeScriptDefinitions() function:

kotlin {
    wasmJs {
        binaries.executable()
        browser {
        }
        generateTypeScriptDefinitions()
    }
}

Type correspondence

Kotlin/Wasm allows only certain types in signatures of JavaScript interop declarations. These limitations apply uniformly to declarations with external, = js("code") or @JsExport.

See how Kotlin types correspond to Javascript types:

Kotlin	JavaScript
`Byte`, `Short`, `Int`, `Char`, `UByte`, `UShort`, `UInt`,	`Number`
`Float`, `Double`,	`Number`
`Long`, `ULong`,	`BigInt`
`Boolean`,	`Boolean`
`String`,	`String`
`Unit` in return position	`undefined`
Function type, for example `(String) -> Int`	Function
`JsAny` and subtypes	Any JavaScript value
`JsReference`	Opaque reference to Kotlin object
Other types	Not supported

You can use nullable versions of these types as well.

JsAny type

JavaScript values are represented in Kotlin using the JsAny type and its subtypes.

The Kotlin/Wasm standard library provides representation for some of these types:

Package kotlin.js:
- JsAny
- JsBoolean, JsNumber, JsString
- JsArray
- Promise

You can also create custom JsAny subtypes by declaring an external interface or class.

JsReference type

Kotlin values can be passed to JavaScript as opaque references using the JsReference type.

For example, if you want to expose this Kotlin class User to JavaScript:

class User(var name: String)

You can use the toJsReference() function to create JsReference<User> and return it to JavaScript:

@JsExport
fun createUser(name: String): JsReference<User> {
    return User(name).toJsReference()
}

These references aren't directly available in JavaScript and behave like empty frozen JavaScript objects. To operate on these objects, you need to export more functions to JavaScript using the get() method where you unwrap the reference value:

@JsExport
fun setUserName(user: JsReference<User>, name: String) {
    user.get().name = name
}

You can create a class and change its name from JavaScript:

import UserLib from "./userlib.mjs"

let user = UserLib.createUser("Bob");
UserLib.setUserName(user, "Alice");

Type parameters

JavaScript interop declarations can have type parameters if they have an upper bound of JsAny or its subtypes. For example:

external fun <T : JsAny> processData(data: JsArray<T>): T

Exception handling

You can use Kotlin try-catch expression to catch JavaScript exceptions. However, accessing specific details about the thrown value in Kotlin/Wasm isn’t possible by default.

You can configure the JsException type to include the original error message and stack trace from JavaScript. To do so, add the following compiler option to your build.gradle.kts file:

kotlin {
    wasmJs {
        compilerOptions {
            freeCompilerArgs.add("-Xwasm-attach-js-exception")
        }
    }
}

This behavior depends on the WebAssembly.JSTag API, which is only available in certain browsers:

Chrome: Supported from version 115
Firefox: Supported from version 129
Safari: Not yet supported

Here’s an example demonstrating this behavior:

external object JSON {
    fun <T: JsAny> parse(json: String): T
}

fun main() {
    try {
        JSON.parse("an invalid JSON")
    } catch (e: JsException) {
        println("Thrown value is: ${e.thrownValue}")
        // SyntaxError: Unexpected token 'a', "an invalid JSON" is not valid JSON

        println("Message: ${e.message}")
        // Message: Unexpected token 'a', "an invalid JSON" is not valid JSON

        println("Stacktrace:")
        // Stacktrace:

        // Prints the full JavaScript stack trace 
        e.printStackTrace()
    }
}

With the -Xwasm-attach-js-exception compiler option enabled, the JsException type provides specific details from the JavaScript error. Without enabling this compiler option, JsException includes only a generic message stating that an exception was thrown while running JavaScript code.

If you try to use a JavaScript try-catch expression to catch Kotlin/Wasm exceptions, it looks like a generic WebAssembly.Exception without directly accessible messages and data.

Kotlin/Wasm and Kotlin/JS interoperability differences

Although Kotlin/Wasm interoperability shares similarities with Kotlin/JS interoperability, there are key differences to consider:

	Kotlin/Wasm	Kotlin/JS
External enums	Doesn't support external enum classes.	Supports external enum classes.
Type extensions	Doesn't support non-external types to extend external types.	Supports non-external types.
`JsName` annotation	Only has an effect when annotating external declarations.	Can be used to change names of regular non-external declarations.
`js()` function	`js("code")` function calls are allowed as a single expression body of package-level functions.	The `js("code")` function can be called in any context and returns a `dynamic` value.
Module systems	Supports ES modules only. There is no analog of the `@JsNonModule` annotation. Provides its exports as properties on the `default` object. Allows exporting package-level functions only.	Supports ES modules and legacy module systems. Provides named ESM exports. Allows exporting classes and objects.
Types	Applies stricter type restrictions uniformly to all interop declarations `external`, `= js("code")`, and `@JsExport`. Allows a select number of built-in Kotlin types and `JsAny` subtypes.	Allows all types in `external` declarations. Restricts types that can be used in `@JsExport`.
Long	Type corresponds to JavaScript `BigInt`.	Visible as a custom class in JavaScript.
Arrays	Not supported in interop directly yet. You can use the new `JsArray` type instead.	Implemented as JavaScript arrays.
Other types	Requires `JsReference<>` to pass Kotlin objects to JavaScript.	Allows the use of non-external Kotlin class types in external declarations.
Exception handling	You can catch any JavaScript exception with the `JsException` and `Throwable` types.	Can catch JavaScript `Error` using the `Throwable` type. It can catch any JavaScript exception using the `dynamic` type.
Dynamic types	Does not support the `dynamic` type. Use `JsAny` instead (see sample code below).	Supports the `dynamic` type.

The kotlinx-browser library is a standalone library that provides JavaScript browser APIs, including:

Package org.khronos.webgl:
- Typed arrays, like Int8Array.
- WebGL types.
Packages org.w3c.dom.*:
- DOM API types.
Package kotlinx.browser:
- DOM API global objects, like window and document.

To use the declarations from the kotlinx-browser library, add it as a dependency in your project's build configuration file:

val wasmJsMain by getting {
    dependencies {
        implementation("org.jetbrains.kotlinx:kotlinx-browser:0.3")
    }
}

Last modified: 24 April 2025