Kotlin/Wasm & Compose for Web in 2025: Complete Guide to Browser-First KMP Apps

From mobile-first to web-everywhere: The KMP platform gap is closing

The 2025 breakthrough: Why everything changed this year

• Safari adopted WasmGC in December 2024: The last major browser holdout finally supports WebAssembly Garbage Collection, meaning Kotlin/Wasm apps now run on 100% of modern browsers
• Compose for Web hit Beta in September 2025: No longer experimental, it's production-ready for early adopters
• Performance benchmarks prove it: Kotlin/Wasm is nearly 3x faster than JavaScript in UI-heavy workloads
• Real production apps exist: Kotlin Playground, KotlinConf app, and Rijksmuseum Demo all run on Kotlin/Wasm in production

Understanding the landscape: Kotlin/JS vs Kotlin/Wasm vs Compose for Web

Kotlin/JS: The JavaScript compiler

Kotlin/Wasm: The performance champion

Compose for Web: Shared UI for the browser

Decision matrix: Which technology should you choose?

Performance: Why Kotlin/Wasm is 3x faster than JavaScript

Getting started: Build your first Kotlin/Wasm + Compose for Web app

Prerequisites

• IntelliJ IDEA (2025.3+ recommended) or Android Studio with Kotlin Multiplatform plugin
• JDK 11+ (JDK 17 recommended for Compose projects)
• Modern browser - Chrome, Firefox, Safari, or Edge

Step 1: Create your project

1. Open IntelliJ IDEA
2. Select File → New → Project
3. Choose Kotlin Multiplatform from the left panel
4. Select Web target
5. Enable Share UI tab (this adds Compose for Web)
6. Set project name (e.g., "WasmDemo") and click Create

bashCopy
# Clone the official Compose Wasm template
git clone https://github.com/Kotlin/kotlin-wasm-compose-template.git
cd kotlin-wasm-compose-template

Step 2: Project structure overview

WasmDemo/
├── composeApp/
│   ├── src/
│   │   ├── commonMain/       # Shared code (Android, iOS, Web, Desktop)
│   │   │   └── kotlin/
│   │   │       └── App.kt    # Your main Compose UI
│   │   ├── wasmJsMain/       # Web-specific code (Kotlin/Wasm)
│   │   │   └── kotlin/
│   │   │       └── main.kt   # Web entry point
│   │   └── androidMain/      # (if targeting Android)
│   └── build.gradle.kts
├── gradle/
└── settings.gradle.kts

Step 3: Your first Compose for Web UI

kotlinCopy
import androidx.compose.foundation.layout.*
import androidx.compose.material3.*
import androidx.compose.runtime.*
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.unit.dp

@Composable
fun App() {
    var count by remember { mutableStateOf(0) }

    MaterialTheme {
        Surface(
            modifier = Modifier.fillMaxSize(),
            color = MaterialTheme.colorScheme.background
        ) {
            Column(
                modifier = Modifier
                    .fillMaxSize()
                    .padding(32.dp),
                horizontalAlignment = Alignment.CenterHorizontally,
                verticalArrangement = Arrangement.Center
            ) {
                Text(
                    text = "Welcome to Kotlin/Wasm + Compose for Web!",
                    style = MaterialTheme.typography.headlineMedium
                )

                Spacer(modifier = Modifier.height(24.dp))

                Text(
                    text = "You've clicked $count times",
                    style = MaterialTheme.typography.bodyLarge
                )

                Spacer(modifier = Modifier.height(16.dp))

                Button(onClick = { count++ }) {
                    Text("Click me!")
                }

                Spacer(modifier = Modifier.height(16.dp))

                Button(
                    onClick = { count = 0 },
                    colors = ButtonDefaults.buttonColors(
                        containerColor = MaterialTheme.colorScheme.secondary
                    )
                ) {
                    Text("Reset")
                }
            }
        }
    }
}

Step 4: Web entry point

kotlinCopy
import androidx.compose.ui.ExperimentalComposeUiApi
import androidx.compose.ui.window.CanvasBasedWindow

@OptIn(ExperimentalComposeUiApi::class)
fun main() {
    CanvasBasedWindow(canvasElementId = "ComposeTarget") {
        App()
    }
}

Step 5: Run in development mode

1. Look for the run configuration dropdown (top-right)
2. Select composeApp [wasmJs]
3. Click the green play button
4. Your browser will open at http://localhost:8080/

bashCopy
# Run the Wasm development server
./gradlew wasmJsBrowserDevelopmentRun

# Opens browser at http://localhost:8080/ (or similar port)

Step 6: Build for production

bashCopy
# Build optimized production bundle
./gradlew wasmJsBrowserDistribution

# Output location:
# composeApp/build/dist/wasmJs/productionExecutable/

• index.html - Entry HTML file
• *.wasm - Compiled WebAssembly binary
• *.js - JavaScript bootstrap code
• Other assets (CSS, resources)

• GitHub Pages - Free, easy setup
• Cloudflare Workers - Edge deployment
• Netlify / Vercel - One-command deployment
• AWS S3 + CloudFront - Enterprise-grade

Real-world patterns: Building production Compose for Web apps

Type-safe navigation with deep linking

kotlinCopy
@Composable
fun App() {
    val navController = rememberNavController()

    NavHost(navController, startDestination = "home") {
        composable("home") { HomeScreen(navController) }
        composable("profile/{userId}") { backStackEntry ->
            val userId = backStackEntry.arguments?.getString("userId")
            ProfileScreen(userId = userId)
        }
    }
}

HTML interoperability

kotlinCopy
@Composable
fun HybridComponent() {
    Column {
        Text("This is Compose", style = MaterialTheme.typography.headlineMedium)

        Div(attrs = { style { property("padding", "16px") } }) {
            Text("This is native HTML/CSS")
        }

        Button(onClick = { }) { Text("Compose Button") }
    }
}

Library ecosystem: What works with Kotlin/Wasm today

Example: Building a REST API client with Ktor

kotlinCopy
import io.ktor.client.*
import io.ktor.client.call.*
import io.ktor.client.request.*
import io.ktor.client.plugins.contentnegotiation.*
import io.ktor.serialization.kotlinx.json.*
import kotlinx.serialization.Serializable
import kotlinx.coroutines.flow.MutableStateFlow

@Serializable
data class User(val id: Int, val name: String, val email: String)

class UserRepository {
    private val client = HttpClient {
        install(ContentNegotiation) {
            json()
        }
    }

    suspend fun fetchUsers(): List<User> {
        return client.get("https://api.example.com/users").body()
    }

    suspend fun createUser(name: String, email: String): User {
        return client.post("https://api.example.com/users") {
            setBody(mapOf("name" to name, "email" to email))
        }.body()
    }
}

// Use in your Compose UI
@Composable
fun UserListScreen() {
    var users by remember { mutableStateOf<List<User>>(emptyList()) }
    var loading by remember { mutableStateOf(true) }
    val repository = remember { UserRepository() }

    LaunchedEffect(Unit) {
        users = repository.fetchUsers()
        loading = false
    }

    if (loading) {
        CircularProgressIndicator()
    } else {
        LazyColumn {
            items(users) { user ->
                ListItem(
                    headlineContent = { Text(user.name) },
                    supportingContent = { Text(user.email) }
                )
            }
        }
    }
}

Compose for Web vs traditional web frameworks

Use Compose for Web when:

• You're already using Kotlin Multiplatform for mobile (easy to add web target)
• You want one UI codebase for Android, iOS, desktop, and web
• You're building internal tools, dashboards, or data-heavy apps
• Performance is critical (data viz, real-time updates)
• Your team knows Kotlin, not JavaScript

Use React/Vue/Svelte when:

• Building marketing websites or blogs (need SEO and SSR)
• Need the fastest time-to-market (larger ecosystem)
• Your team is 100% web-focused (not targeting mobile/desktop)
• Must support very old browsers (IE11, ancient Safari)

Production deployment checklist

Real-world production examples

• Kotlin Playground: Full IDE in the browser with real-time compilation at near-native speeds
• KotlinConf App: Conference app with shared UI across Android, iOS, and web
• Rijksmuseum Demo: Data-heavy museum collection browser handling thousands of items smoothly
• Jetsnack Wasm Demo: E-commerce app with complex Material 3 UI patterns

What's coming in 2025-2026

• Stable Compose for Web (late 2025 / early 2026)
• Compatibility mode: Automatic Wasm/JS fallback for all browsers
• Stable @JsExport: Better JavaScript interoperability
• Modern JavaScript output (ES2015+): Smaller bundles, faster execution
• Multithreading support: Web Workers for parallel execution
• Enhanced debugging: Better source maps and IDE integration
• TypeScript wrapper generation: Auto-generated definitions for seamless JS integration

Should you adopt Kotlin/Wasm + Compose for Web in 2025?

Adopt now if:

• You're building a new cross-platform app or already have a KMP mobile app
• You're comfortable with Beta software (stable enough for production)
• Performance matters (data viz, real-time updates)
• Your team knows Kotlin

Wait if:

• Company policy forbids Beta software (Stable coming late 2025 / early 2026)
• Building a pure web app with no mobile/desktop plans
• SEO is critical (need server-side rendering)
• Must support IE11 or ancient browsers

Getting started: Your next steps

1. Try the examples - Visit Kotlin Playground and the KotlinConf app to see Kotlin/Wasm in action
2. Clone the template - Start with kotlin-wasm-compose-template or use the IntelliJ wizard
3. Build something small - Create a simple dashboard or CRUD app to learn the patterns
4. Measure performance - Run your own benchmarks comparing Kotlin/Wasm vs your current web tech
5. Join the community - Kotlin Slack (#webassembly channel) is active and helpful

• 🎨 Compose Multiplatform UI (all platforms)
• 🔐 Authentication (Google, Apple, Email)
• 💳 In-app purchases + subscriptions
• 📡 REST API client with Ktor

• 🚀 CI/CD for Android, iOS, and web
• 📱 Complete sample app (not just hello world)
• 📚 Comprehensive documentation
• 🏗️ Clean architecture (testable, scalable)

Sources and references

Official documentation and guides:

• Get started with Kotlin/Wasm and Compose Multiplatform - Official Kotlin Documentation
• Kotlin/Wasm Overview - Kotlin Documentation
• Compose Multiplatform 1.9.0 Released: Compose for Web Goes Beta - JetBrains Blog
• Present and Future of Kotlin for Web - JetBrains Blog
• What's Next for Kotlin Multiplatform and Compose Multiplatform – August 2025 Update - JetBrains Blog

Performance benchmarks and analysis:

• Kotlin/Wasm Benchmarks Collection - Official Kotlin GitHub
• Kotlin VS Javascript benchmarks - Programming Language Benchmarks
• Compose for Web (WASM) – What and Why? - Handstand Sam

Templates and examples:

• kotlin-wasm-compose-template - Official Kotlin Template Repository
• kotlin-wasm-examples - Official Examples Repository
• Kotlin Playground - Production Kotlin/Wasm App

Community and tutorials:

• Getting Started with Web Assembly and Kotlin Multiplatform - Touchlab
• klibs.io - Kotlin Multiplatform Library Directory
• Kotlin Slack Community - #webassembly channel

Continue your Kotlin Multiplatform journey

• Compose Multiplatform for iOS Stable in 2025: What Developers Need to Know
• Kotlin Multiplatform vs Flutter vs React Native: The 2025 Developer Guide
• How to Set Up Kotlin Multiplatform: Complete Development Guide 2025
• Big Tech's Secret Weapon: How Netflix, McDonald's & Cash App Ship Faster with KMP

Quick Questions About KMP Web Development

• Do I need a Mac for iOS development? - Platform requirements explained
• How long does setup actually take? - Real timeline expectations
• What's the total cost comparison? - KMP vs traditional development costs

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