Write JavaScript in Ruby with Opal: Read The Friendly Source Code

Reading the Source Code of Opal – Ruby to JavaScript Magic 🧙‍♂️

In this video, I dive into the source code of Opal (https://opalrb.com) — the Ruby-to-JavaScript source-to-source compiler that lets you write Ruby and run it in the browser (or in Node) by compiling down to JavaScript.

What is Opal?
Opal is a compiler that turns Ruby code into JavaScript code, so you can write Ruby and ship it to the front-end or other JS runtime contexts.
It includes the Ruby core library you already know and love, and is designed to be fast and lightweight.
Use-cases:
Rubyists who want to stay in the Ruby ecosystem even for front-end code.
Applications that want to reuse Ruby logic across server & client.
Prototyping UI logic in Ruby rather than switching languages.
Embedding Ruby workflows inside JavaScript-based projects.

Why read the source code?
To see how Opal transforms Ruby constructs into JavaScript under the hood — class definitions, modules, method dispatch, etc.
To understand the architecture of a real compiler/transpiler built in Ruby.
To pick up techniques for writing high-quality meta-programming, performance-sensitive Ruby, and bridging between language runtimes.
If you’re working with Rails + front-end or looking for ways to unify backend & frontend logic, this is a great deep dive.

What you’ll see in this video:
The structure of the Opal repository: how it’s organized, what the core modules do.
Key sections of source code: how Ruby classes are compiled, how method calls map to JS, how Opal handles corelib, etc.
Commentary on why the code is written this way, what design trade-offs exist, and what lessons you can take away.
Live walkthroughs: navigating files, reading code in real time, and highlighting interesting bits.
Thoughts on applying similar patterns when building tooling, language bridges, or full-stack Ruby apps.

Who is this for?
Ruby and Rails developers who want to explore full-stack Ruby.
Engineers curious about how language-to-language compilation works.
Anyone building apps that span server & client and want to reduce context switching.
Tooling and infrastructure engineers interested in compilers, transpilers, or meta-programming in Ruby.

What you need to know (and what you don’t):
You should be comfortable reading Ruby code.
You don’t need to be a compiler expert.
No prior Opal experience required, but familiarity with Ruby and JavaScript helps.

Links & resources:
Opal official site: https://opalrb.com
GitHub repo: linked on the site — explore the source yourself.
Getting started guide: https://opalrb.com/docs/

If you enjoyed this deep dive, hit Like, subscribe for more code-walkthroughs, and leave a comment: what part of the source surprised you the most? What tools or languages should I explore next?