[KotlinConf2019] Exploring the Power of Kotlin/JS
Sebastian Aigner, a developer advocate at JetBrains, captivated KotlinConf2019 with his deep dive into Kotlin/JS, the JavaScript target for Kotlin. With a passion for web development, Sebastian showcased how recent advancements make Kotlin/JS a compelling choice for building web applications. From streamlined tooling to seamless JavaScript interoperability, he outlined the current state and future potential of Kotlin/JS, inspiring both newcomers and seasoned developers to leverage Kotlin’s paradigms in the browser.
Simplifying Development with the New Gradle Plugin
Kotlin/JS has evolved significantly, with the new Kotlin/JS Gradle plugin emerging as the cornerstone for browser and Node.js development. Sebastian explained that this plugin unifies previously fragmented approaches, replacing deprecated plugins like kotlin2js and kotlin-frontend. Its uniform Gradle DSL simplifies project setup, offering sensible defaults for Webpack bundling without requiring extensive configuration. For developers transitioning to multi-platform projects, the plugin’s compatibility with the Kotlin multi-platform DSL minimizes changes, enabling seamless integration of additional targets. By automating JavaScript environment setup, including yarn and package.json, the plugin empowers Kotlin developers to focus on coding rather than managing complex JavaScript tooling.
Mastering Dependency Management with npm
The JavaScript ecosystem, with over a million npm packages, offers unparalleled flexibility, and Kotlin/JS integrates effortlessly with this vast library. Sebastian highlighted how the Gradle plugin manages npm dependencies directly, automatically updating package.json when dependencies like React or styled-components are added. This eliminates the need for separate JavaScript environment setup, saving time, especially on non-standard platforms like Windows. Developers can import Kotlin libraries (e.g., coroutines, serialization) alongside JavaScript packages, with Gradle handling the JavaScript-specific versions. This unified approach bridges the gap between Kotlin’s structured ecosystem and JavaScript’s dynamic world, making dependency management intuitive even for those new to JavaScript.
Bridging Kotlin and TypeScript with Dukat
Interoperating with JavaScript’s dynamic typing can be challenging, but Sebastian introduced Dukat, an experimental tool that converts TypeScript declaration files into Kotlin external declarations. By leveraging TypeScript’s de facto standard for type definitions, Dukat enables type-safe access to npm packages, such as left-pad or react-minimal-pie-chart. While manual external declarations require tedious annotation, Dukat automates this process, generating headers for packages with TypeScript support or community-contributed definitions. Sebastian encouraged early adoption to provide feedback, noting that Dukat already powers browser and Node.js API wrappers. This tool promises to simplify integration with JavaScript libraries, reducing the friction of crossing the static-dynamic typing divide.
Enhancing Testing and Debugging with Source Maps
Testing and debugging are critical for robust applications, and Kotlin/JS delivers with integrated tools. Sebastian demonstrated how the Gradle plugin supports platform-specific test runners like Karma, allowing tests to run across browsers (e.g., Firefox, headless Chrome). Source maps, automatically generated since Kotlin 1.3.60, provide detailed stack traces for Node.js and interactive debugging in browser DevTools. Developers can set breakpoints in Kotlin code, inspect variables, and trace errors directly in Chrome’s console, as shown in Sebastian’s pong game demo. Gradle test reports further enhance diagnostics, offering HTML-based insights into test failures, making Kotlin/JS development as robust as its JVM counterpart.
Optimizing with the IR Backend
The upcoming Intermediate Representation (IR) backend marks a significant leap for Kotlin/JS. Sebastian outlined its benefits, including aggressive code size optimizations through dead code elimination. Unlike the current backend, which may ship the entire standard library, the IR backend, combined with Google Closure Compiler, reduces zipped file sizes dramatically—down to 30 KB from 3.9 MB in some cases. Faster compilation speeds, especially for incremental builds, enhance developer productivity, particularly in continuous build scenarios with Webpack’s dev server. The IR backend also supports platform-agnostic compiler plugins, simplifying multi-platform development. Sebastian noted that pre-alpha IR support in Kotlin 1.3.70 requires manual exports due to its closed-world assumption, urging developers to explore early releases.
Looking Ahead: WebAssembly and Framework Support
Sebastian concluded with a glimpse into Kotlin/JS’s future, highlighting potential support for ECMAScript 6 modules and frameworks like Angular and Vue.js. While JetBrains provides React wrappers, extending first-class support to other frameworks requires addressing their unique tooling and compilers. The IR backend also opens doors to WebAssembly, enabling Kotlin to target browsers more efficiently. Though no timelines were promised, these explorations reflect JetBrains’ commitment to aligning Kotlin/JS with modern web trends. Sebastian’s call to action—trying the Code Quiz app at the Kotlin booth and contributing to Dukat—emphasized community involvement in shaping Kotlin/JS’s evolution.