Opal vs SWC

Opal is a Ruby-to-JavaScript source-to-source compiler that converts Ruby ahead of time into plain JavaScript, without a runtime interpreter. It includes implementations of Ruby core and standard libraries. Ruby code can call JavaScript, JavaScript can call Ruby, and native JavaScript classes can be treated as Ruby classes. The project describes sharing a gem, objects, and specs between server and client code.

Opal can run in browsers and JavaScript environments including Node.js, Deno, Bun, and QuickJS. For its upcoming 2.0 release, the project lists current and previous stable Firefox, Chrome, Safari, and Edge as supported, while Internet Explorer is unsupported. That release targets ES2021 and has no transpilation fallback for older JavaScript engines. Listed integrations include Rails, Sinatra, Roda, and static apps; the Rails example uses opal-rails and requires opal_ujs and turbolinks. Installation is through `gem install opal` or by adding the gem to a Gemfile. The gemspec declares Ruby >= 2.3, and CI exercises Ruby 3.0 through 4.0 and JRuby. Opal is MIT-licensed and free, with CLI and API access. Support channels include Slack, Stack Overflow’s #opalrb tag, and GitHub issues.

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.