KotlinJS

KotlinJS, or Kotlin/JS, is a compiler target that enables Kotlin source code to be compiled to JavaScript. It allows developers to write code in Kotlin and run it in JavaScript environments such as web browsers and Node.js, supporting both frontend and server-side use cases within a single language ecosystem.

KotlinJS is part of the Kotlin Multiplatform project and is integrated through the Kotlin/JS Gradle plugin. It provides interoperability with JavaScript and existing JS libraries through external declarations, dynamic types, and wrappers around JavaScript packages. Developers can consume npm modules and publish results as npm packages. The ecosystem includes wrappers for popular libraries (for example, React) and Kotlin-specific libraries for HTML building, serialization, and other tasks.

The compiler supports two backends for Kotlin/JS: the legacy backend and the IR (intermediate representation) backend. The IR backend is the modern path and is designed to improve compilation speed, code size, and interop performance. Kotlin/JS outputs can target various module systems used by JavaScript environments, such as browser-based bundles or Node.js modules, and can be integrated with existing JavaScript tooling and build pipelines.

Key features include seamless interop with JavaScript code and libraries, a strong type system for Kotlin code operating alongside dynamic JS types, and tooling through Gradle for project configuration, dependency management, and packaging. While KotlinJS enables Kotlin-centric frontend development and code sharing with other Kotlin targets, it may introduce trade-offs in bundle size and JavaScript runtime characteristics compared to pure JavaScript or TypeScript.

See also: Kotlin Multiplatform, Kotlin/JVM, Kotlin/Native.