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

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

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

Production deployment checklist

Real-world production examples

What's coming in 2025-2026

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

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

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