Kotlin/JS vs SWC

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.

SWC is a free, extensible Rust-based platform for tools that compile and bundle JavaScript and TypeScript. It transforms modern JavaScript and TypeScript into code intended for major browsers, and supports minification, WebAssembly transforms, and custom plugins. Its core API parses ECMAScript, TypeScript, and Flow syntax; the documentation describes that API as mainly useful to build-tool authors. Integrations include webpack, Rspack, and Jest, while Next.js, Parcel, and Deno are among the tools named as users. A WebAssembly module can transform code synchronously inside a browser. SWC can be installed as prebuilt binaries or built from source, with packages documented for npm, Yarn, pnpm, Bun, and Deno. The site states that it is 20x faster than Babel on one thread and 70x faster on four cores. The bundling documentation says that feature will be dropped in v2 and recommends SWC-based bundlers instead. It also warns that its polyfill usage mode is less efficient than Babel for certain dynamically computed property expressions.