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Kotlin/JS

kotlinlang.org

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Spec score 7.8 #15 of 28 in Code Transpilers

What it is

Kotlin/JS is a Kotlin compiler target that converts Kotlin code, the Kotlin standard library, and compatible dependencies into JavaScript. Projects can target browsers or Node.js, including server-side and serverless applications. It supports ES5, ES2015, and ES2020 output, as well as ESM, CommonJS, UMD, and AMD modules. Projects use the Kotlin Multiplatform Gradle plugin for configuration, bundling, and JavaScript dependencies from npm. Kotlin/JS can share logic between web frontends and JVM backends, or across Android, iOS, and web clients. Kotlin wrappers are available for React and other JavaScript frameworks, including React Redux, React Router, and styled-components. Browser tests can use Karma or an experimental browser testing DSL; Node.js projects use Mocha. The documentation notes that Karma is deprecated and is not expected to receive new features or bug fixes. The Gradle plugin generates JavaScript source maps for builds. Kotlin/JS is open source and free at free, and is listed for Linux, macOS, web, and Windows.

Spec sheet

Purpose
Kotlin/JS transpiles Kotlin code, the Kotlin standard library, and compatible dependencies to JavaScript.Read 29 Sep 2026 · kotlinlang.org
Intended users
The documentation describes Kotlin/JS use cases including web developers sharing business logic with JavaScript or TypeScript codebases and developers building web frontends in Kotlin.Read 29 Sep 2026 · kotlinlang.org
Integrations
JetBrains provides Kotlin wrappers for React and other JavaScript frameworks, including support for React Redux, React Router, and styled-components.Read 29 Sep 2026 · kotlinlang.org
Support
The Kotlin documentation directs users to the #javascript channel in Kotlin Slack to discuss Kotlin/JS with the community and team.Read 29 Sep 2026 · kotlinlang.org
Founded
2000Read 28 Sep 2026 · kotlinlang.org
Code sharing
Kotlin/JS can share common logic between web frontends and JVM backends, and between Android, iOS, and web clients.Read 29 Sep 2026 · kotlinlang.org
Debugging
The Kotlin Multiplatform Gradle plugin automatically generates JavaScript source maps for project builds.Read 29 Sep 2026 · kotlinlang.org
ECMAScript targets
The current Kotlin/JS implementation targets ES5, ES2015, and ES2020.Read 29 Sep 2026 · kotlinlang.org
Environments
Kotlin/JS projects can target browsers or Node.js.Read 29 Sep 2026 · kotlinlang.org
Gradle setup
Kotlin/JS is configured through the Kotlin Multiplatform Gradle plugin, which supports bundling configuration and JavaScript dependencies from npm.Read 29 Sep 2026 · kotlinlang.org
JavaScript modules
Kotlin/JS supports ESM, CommonJS, UMD, and AMD module systems.Read 29 Sep 2026 · kotlinlang.org
Server-side use
The Node.js target supports server-side or serverless applications, and kotlinx-nodejs provides type-safe access to the Node.js API.Read 29 Sep 2026 · kotlinlang.org
Testing
Kotlin/JS supports browser tests through Karma or a new experimental browser testing DSL, and uses Mocha for Node.js projects.Read 29 Sep 2026 · kotlinlang.org
Testing limitation
The Kotlin documentation says Karma is deprecated and is not expected to receive new features or bug fixes.Read 29 Sep 2026 · kotlinlang.org

Plans and prices

Kotlin/JS Free Kotlin compiler target for JavaScript; configured with the Kotlin Multiplatform Gradle plugin Read 29 Sep 2026 · kotlinlang.org

Every Kotlin/JS plan and price →

Strong points and limits

Strong points

  • Targets browsers and Node.js
  • Supports four JavaScript module systems
  • Can share logic across web and JVM projects
  • Gradle plugin generates JavaScript source maps
  • Free and open source

Limits

  • Karma is deprecated
  • Karma is not expected to receive new features or bug fixes
  • Browser testing DSL is experimental

Who it suits

Kotlin/JS suits web developers who want to write frontends in Kotlin or share logic with JavaScript or TypeScript codebases. It also suits developers targeting Node.js with Kotlin code.

Verdict

Kotlin/JS provides a free Kotlin-to-JavaScript path with browser and Node.js targets. Teams relying on Karma should note its deprecated status.

Where Specifiction places it

Alternatives to Kotlin/JSAll alternatives →