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Learn Kotlin-js-wasm - 10 Code Examples & CST Typing Practice Test

Kotlin JS + WebAssembly (kotlin-js-wasm) allows developers to compile Kotlin code into highly optimized WebAssembly modules or JavaScript bundles. It enables strongly typed Kotlin applications to run in browsers, Node.js, and WASI runtimes with excellent interoperability and multiplatform support.

View all 10 Kotlin-js-wasm code examples →
Simple Kotlin/JS WebAssembly ProgramKotlin/JS WASM Button ClickKotlin/JS WASM Input BindingKotlin/JS WASM Conditional RenderingKotlin/JS WASM Loop RenderingKotlin/JS WASM Timer UpdateKotlin/JS WASM Event CallbackKotlin/JS WASM Fetch ExampleKotlin/JS WASM Toggle VisibilityKotlin/JS WASM Dynamic List

Learn KOTLIN-JS-WASM with Real Code Examples

Updated Nov 25, 2025

Explain

Kotlin/JS compiles Kotlin code into JavaScript or WebAssembly.

Wasm backend (Kotlin/Wasm) provides near-native performance and small binaries.

Highly interoperable with JavaScript through Kotlin’s JS/interop APIs.

Integrated with Kotlin Multiplatform to share logic across JS, JVM, Android, and Native.

Designed for modern browser apps, UI frameworks, and high-performance computations.

Core Features

Kotlin -> JS/Wasm compiler via Kotlin IR backend

Full type-safety across browser and JS ecosystem

Seamless DOM and Web API interoperability

Wasm-GC support enabling efficient managed memory

Multiplatform project architecture

Basic Concepts Overview

Kotlin/JS -> transpiles Kotlin IR to JavaScript

Kotlin/Wasm -> compiles Kotlin IR to WebAssembly (.wasm)

Interop -> JS functions and DOM bindings via Kotlin

Multiplatform -> shared Kotlin code across targets

Compose for Web -> declarative UI on JS/Wasm

Project Structure

shared/ - shared Kotlin module

jsApp/ - Kotlin/JS or Kotlin/Wasm app

build.gradle.kts - configuration

src/jsMain/kotlin - browser code

dist/ - generated Wasm/JS bundles

Building Workflow

Write Kotlin shared + JS/Wasm modules

Configure Gradle targets

Run dev server or compile to Wasm/JS

Interact with DOM via Kotlin APIs

Bundle output for deployment

Difficulty Use Cases

Beginner: simple DOM manipulation

Intermediate: Kotlin + JS library interop

Advanced: Compose for Web UI

Expert: Kotlin/Wasm optimization + GC tuning

Auditor: analyzing Wasm binary + performance

Comparisons

Kotlin/Wasm vs TypeScript: stronger type safety & multiplatform

Kotlin/JS vs React/TS: similar, but Kotlin has safer typing

Kotlin/Wasm vs Rust/Wasm: easier language, slightly slower

Kotlin/Wasm vs Go/TinyGo: better tooling, larger runtime

Kotlin/Wasm vs SwiftWasm: similar, Kotlin more mature for web

Versioning Timeline

2019 - Kotlin/JS IR introduced

2022 - Kotlin/Wasm experimental

2023 - Wasm-GC integrated

2024 - Compose for Web stable

2025 - Kotlin/Wasm widely adopted in production

Glossary

IR - Intermediate Representation backend

Interop - JS <-> Kotlin connection

Wasm-GC - Managed memory Wasm runtime

Compose for Web - Kotlin UI DSL

Multiplatform - shared code across JVM/JS/Wasm

Installation Setup

Install JDK and Kotlin toolchain

Install Gradle

Enable Kotlin Multiplatform in build.gradle.kts

Choose JS (IR) or Wasm target

Run `./gradlew build` or `jsBrowserDevelopmentRun`

Environment Setup

Install JDK

Install Kotlin + Gradle

Enable JS/Wasm target

Run dev server

Open browser for hot reload

Config Files

build.gradle.kts - Kotlin target settings

settings.gradle.kts - project structure

index.html - web entrypoint

webpack/esbuild configs (auto)

src/* - Kotlin source sets

Cli Commands

./gradlew jsBrowserDevelopmentRun

./gradlew wasmBrowserRun

./gradlew build

./gradlew wasmJs

./gradlew jsNodeRun

Internationalization

kotlinx.serialization for multilingual content

Compose for Web i18n patterns

Interop with JS i18n libraries

Shared translations across JVM/JS/Wasm

Unicode fully supported

Accessibility

Compose for Web semantic elements

ARIA attributes via Kotlin wrappers

Keyboard events mapped to Kotlin

Screen-reader friendly layouts

JS accessibility APIs exposed

Ui Styling

Compose for Web uses CSS declarative DSL

KVision uses Bootstrap and React

Direct DOM CSS manipulation possible

Kotlin wrappers for JS style APIs

Web components supported

State Management

Kotlin variables stored in Wasm-GC heap

Interop passes JS objects to Kotlin wrappers

Compose for Web manages UI state

Serialization via kotlinx.serialization

WASI state managed via runtime (future)

Data Management

JSON via kotlinx.serialization

Typed arrays via JS interop

Binary formats for Wasm performance

Shared DTOs across platforms

Browser storage via Kotlin wrappers

Architecture

Kotlin code -> Kotlin compiler IR -> JS/Wasm output

Wasm-GC enabled runtimes for managed memory

Interop layer for JS imports/exports

Gradle-based project structure

Multiplatform shared module + platform-specific modules

Rendering Model

Kotlin IR -> JavaScript or WebAssembly

Wasm-GC provides managed memory

Interop bridge for JS imports

Browser runtime manages DOM

WASI runtime support emerging

Architectural Patterns

Full-stack Kotlin web

Kotlin+React frontends

Wasm compute modules

Compose-for-Web SPA

Shared business logic MPP

Real World Architectures

Kotlin Multiplatform + Compose Web

Ktor backend + Kotlin/Wasm frontend

Wasm compute engine embedded in JS apps

Mixed JS/TS + Kotlin/JS interop apps

Enterprise Kotlin shared logic stacks

Design Principles

Strong typing across platforms

Multiplatform-first architecture

High-performance Wasm output

Excellent tooling

Clean JS/Wasm interop

Scalability Guide

Kotlin/Wasm scales with browser/edge runtimes

Compose for Web SPAs scale horizontally

Multiplatform logic prevents code duplication

Wasm modules integrate with CDNs

Static hosting enables global distribution

Migration Guide

Move JS logic to Kotlin shared module

Replace TS types with Kotlin external declarations

Adopt Compose for Web

Enable Wasm target

Optimize JS/Wasm bundles

Performance Notes

Wasm-GC significantly improves performance

Kotlin IR produces optimized JS and Wasm

Inline functions reduce code size

Use Compose for Web efficiently

Parallel compilation speeds builds

Security Notes

Wasm sandbox ensures safe execution

Kotlin type safety reduces runtime bugs

No direct OS access without WASI

Interop requires careful validation of JS inputs

DOM interactions respect browser security layers

Monitoring Analytics

Browser devtools for Wasm performance

JS/Wasm profiling

Application logs via console wrappers

Custom event tracing

Bundle analysis tools

Code Quality

Use idiomatic Kotlin

Prefer immutable data

Keep interop wrappers isolated

Use type-safe React/Compose DSLs

Enable compiler warnings

Practical Examples

Build a browser dashboard with Kotlin/Wasm

Kotlin Wasm math engine for web

Node.js scripts written in Kotlin

Kotlin React (KVision) apps

Full-stack Kotlin web app with shared code

Troubleshooting

Check Gradle Kotlin plugin version

Verify Wasm target enabled in build file

Check JS interop syntax for external types

Use browser devtools to inspect Wasm

Ensure correct experimental flags for Wasm-GC

Testing Guide

Use Kotlin test framework

Run JS/Wasm tests via Gradle

Headless browser tests

Snapshot testing for Compose for Web

Benchmarking Wasm functions

Deployment Options

Static web hosting (Wasm + JS bundle)

Node.js CLI tools

Serverless Wasm runtimes

Full-stack Kotlin servers

Electron + Kotlin frontend

Tools Ecosystem

Compose for Web

KVision/Kotlin React wrappers

Kotlinx libraries (coroutines, serialization)

Gradle Kotlin frontend plugins

Wasm-GC browsers (Chrome/Firefox)

Integrations

React (via wrappers)

Node.js, Deno runtimes

WASI runtimes (limited)

Ktor full-stack web apps

npm ecosystem via interop

Productivity Tips

Use IntelliJ IDEA for Kotlin/JS/Wasm

Leverage Compose for Web previews

Use external declarations wisely

Split code into multiplatform modules

Enable incremental builds

Challenges

Interop complexity with dynamic JS libraries

Experimental Wasm features

Tooling changes across Kotlin versions

Debugging Wasm stack traces

Large JS/Wasm bundles without optimization

Learning Path

Learn Kotlin basics

Understand Kotlin Multiplatform

Try Kotlin/JS DOM interop

Build a simple Kotlin/Wasm demo

Create a full-stack Kotlin/Compose app

Skill Improvement Plan

Week 1: Kotlin/JS basics

Week 2: Interop + DOM APIs

Week 3: Wasm target + build tools

Week 4: Compose for Web UI

Week 5: Optimize + deploy Kotlin/Wasm app

Interview Questions

How does Kotlin compile to JS and Wasm?

What is Kotlin/Wasm-GC and why is it important?

How does Kotlin/JS interop work?

Describe the Kotlin Multiplatform structure.

Explain advantages of Wasm vs JS in Kotlin projects.

Cheat Sheet

kotlin { js(IR) { browser() } }

kotlin { wasm { browser() } }

use external keyword for JS imports

./gradlew jsBrowserDevelopmentRun

./gradlew wasmBrowserDistribution

Books

Kotlin Multiplatform in Action

Web Development with Kotlin

Kotlin for JavaScript Developers

Kotlin/Wasm Essentials

Compose for Web Handbook

Tutorials

Build your first Kotlin/Wasm app

Compose for Web tutorial

Interop with JavaScript

Full-stack Kotlin Multiplatform

Kotlin/Wasm performance optimization

Official Docs

https://kotlinlang.org/docs/js-overview.html

https://kotlinlang.org/docs/wasm-overview.html

Community Links

Kotlin Slack (#javascript, #multiplatform)

Kotlin YouTube

JetBrains blogs

Reddit r/Kotlin

Kotlin/JS GitHub

Community Support

Kotlin Slack

JetBrains community

Kotlin subreddit

Kotlin/JS GitHub discussions

Kotlin Multiplatform ecosystem

Monetization

Kotlin-based frontend SaaS apps

Wasm compute plugins

Enterprise internal tools

Full-stack Kotlin services

Cross-platform product SDKs

Future Roadmap

Full WASI 0.3 support

Native threading in Wasm

Improved React/KVision tooling

Smaller Wasm bundle size

Faster Wasm DOM integration

When Not To Use

Ultra low-level Wasm needs (Rust/Zig better)

Projects needing tiny WASM binary sizes

Pure JS ecosystem-heavy apps

Realtime or GPU-bound workloads

Legacy browsers without Wasm-GC

Final Summary

Kotlin JS + Wasm brings modern Kotlin to browsers and JS platforms.

Wasm backend provides high performance and type safety.

Excellent for shared logic across web, mobile, and backend.

Strong ecosystem via JetBrains tooling.

Ideal for teams using Kotlin end-to-end.

Faq

Does Kotlin/Wasm support GC?

Yes - with Wasm-GC in modern browsers.

Can Kotlin/Wasm run in Node?

Partial, JS target is more stable.

Is Kotlin/JS production-ready?

Yes, widely used in real products.

Can I share code with Android/JVM?

Yes - core benefit of Kotlin Multiplatform.

Are Wasm binaries small?

Smaller than JS bundles, but larger than TinyGo/Rust.

Code Sample Descriptions

1

Simple Kotlin/JS WebAssembly Program

# kotlin/demo/Main.kt
import kotlin.js.*

fun main() {
    console.log("Hello, Kotlin WASM!")
}

A basic Kotlin/JS program compiled to WebAssembly that prints 'Hello, Kotlin WASM!' to the browser console.

Let’s Try →
2

Kotlin/JS WASM Button Click

# kotlin/demo/Button.kt
import kotlin.browser.document
import org.w3c.dom.HTMLButtonElement
import org.w3c.dom.HTMLParagraphElement

fun main() {
    val button = document.createElement("button") as HTMLButtonElement
    button.textContent = "Click Me"
    document.body?.appendChild(button)

    val p = document.createElement("p") as HTMLParagraphElement
    document.body?.appendChild(p)

    button.onclick = {
        p.textContent = "Button clicked!"
        Unit
    }
}

Adds a button to the page that updates text when clicked.

Let’s Try →
3

Kotlin/JS WASM Input Binding

# kotlin/demo/Input.kt
import kotlin.browser.document
import org.w3c.dom.HTMLInputElement
import org.w3c.dom.HTMLParagraphElement

fun main() {
    val input = document.createElement("input") as HTMLInputElement
    document.body?.appendChild(input)

    val p = document.createElement("p") as HTMLParagraphElement
    document.body?.appendChild(p)

    input.oninput = {
        p.textContent = input.value
        Unit
    }
}

Reads value from an input field and displays it in a paragraph.

Let’s Try →
4

Kotlin/JS WASM Conditional Rendering

# kotlin/demo/Conditional.kt
import kotlin.browser.document
import org.w3c.dom.HTMLParagraphElement

fun main() {
    val show = true
    val p = document.createElement("p") as HTMLParagraphElement
    document.body?.appendChild(p)

    if (show) {
        p.textContent = "Condition is true"
    } else {
        p.textContent = "Condition is false"
    }
}

Displays different messages based on a boolean variable.

Let’s Try →
5

Kotlin/JS WASM Loop Rendering

# kotlin/demo/Loop.kt
import kotlin.browser.document
import org.w3c.dom.HTMLParagraphElement

fun main() {
    for (i in 1..5) {
        val p = document.createElement("p") as HTMLParagraphElement
        p.textContent = "Item $i"
        document.body?.appendChild(p)
    }
}

Creates a list of items using a loop.

Let’s Try →
6

Kotlin/JS WASM Timer Update

# kotlin/demo/Timer.kt
import kotlin.browser.document
import kotlin.js.Date
import kotlin.js.setInterval
import org.w3c.dom.HTMLParagraphElement

fun main() {
    val p = document.createElement("p") as HTMLParagraphElement
    document.body?.appendChild(p)

    setInterval({
        p.textContent = Date().toLocaleTimeString()
    }, 1000)
}

Updates the current time every second in the browser.

Let’s Try →
7

Kotlin/JS WASM Event Callback

# kotlin/demo/Event.kt
import kotlin.browser.document
import org.w3c.dom.HTMLButtonElement

fun main() {
    val button = document.createElement("button") as HTMLButtonElement
    button.textContent = "Click Event"
    document.body?.appendChild(button)

    button.onclick = {
        console.log("Button clicked!")
        Unit
    }
}

Logs a message to console when a button is clicked.

Let’s Try →
8

Kotlin/JS WASM Fetch Example

# kotlin/demo/Fetch.kt
import kotlin.browser.document
import kotlinx.browser.window
import org.w3c.dom.HTMLParagraphElement

fun main() {
    val p = document.createElement("p") as HTMLParagraphElement
    document.body?.appendChild(p)

    window.fetch("https://api.github.com").then { response ->
        response.text().then { text ->
        p.textContent = text
        }
    }
}

Fetches data from an API and displays it in a paragraph.

Let’s Try →
9

Kotlin/JS WASM Toggle Visibility

# kotlin/demo/Toggle.kt
import kotlin.browser.document
import org.w3c.dom.HTMLButtonElement
import org.w3c.dom.HTMLParagraphElement

fun main() {
    val p = document.createElement("p") as HTMLParagraphElement
    p.textContent = "Visible Text"
    document.body?.appendChild(p)

    val button = document.createElement("button") as HTMLButtonElement
    button.textContent = "Toggle"
    document.body?.appendChild(button)

    var visible = true
    button.onclick = {
        visible = !visible
        p.style.display = if (visible) "block" else "none"
        Unit
    }
}

Toggles the visibility of a paragraph when a button is clicked.

Let’s Try →
10

Kotlin/JS WASM Dynamic List

# kotlin/demo/DynamicList.kt
import kotlin.browser.document
import org.w3c.dom.HTMLButtonElement
import org.w3c.dom.HTMLUListElement

fun main() {
    val ul = document.createElement("ul") as HTMLUListElement
    document.body?.appendChild(ul)

    val button = document.createElement("button") as HTMLButtonElement
    button.textContent = "Add Item"
    document.body?.appendChild(button)

    var count = 1
    button.onclick = {
        val li = document.createElement("li")
        li.textContent = "Item $count"
        ul.appendChild(li)
        count++
        Unit
    }
}

Adds items dynamically to a list when a button is clicked.

Let’s Try →

Frequently Asked Questions about Kotlin-js-wasm

What is Kotlin-js-wasm?

Kotlin JS + WebAssembly (kotlin-js-wasm) allows developers to compile Kotlin code into highly optimized WebAssembly modules or JavaScript bundles. It enables strongly typed Kotlin applications to run in browsers, Node.js, and WASI runtimes with excellent interoperability and multiplatform support.

What are the primary use cases for Kotlin-js-wasm?

Browser applications written entirely in Kotlin. High-performance Wasm modules with shared business logic. Full-stack Kotlin (Ktor backend + Kotlin/JS/Wasm frontend). Node.js and Deno apps using Kotlin. WASI modules for CLI or edge runtimes

What are the strengths of Kotlin-js-wasm?

Strong typing and modern Kotlin language features. High-performance Wasm backend. Deep multiplatform integration. Excellent tooling via JetBrains IDEs. Growing ecosystem of Kotlin-first web UI frameworks

What are the limitations of Kotlin-js-wasm?

Wasm backend still evolving (2025). Some JS libraries require custom interop. Smaller ecosystem compared to TypeScript. Occasional tooling bugs with Kotlin/JS Gradle plugins. WASI support not yet complete

How can I practice Kotlin-js-wasm typing speed?

CodeSpeedTest offers 10+ real Kotlin-js-wasm code examples for typing practice. You can measure your WPM, track accuracy, and improve your coding speed with guided exercises.

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