const foo

for engineers with 5+ years on the clock

Read like a book.
Practice without a keyboard.

No “what is a variable”. const foo goes where senior careers grow: engine internals, production backend patterns, security that survives contact with attackers, Rust, and guided reading of real open-source code — with exercises you can solve with your thumb on the train.

Start with JavaScript InternalsBrowse all tracks

Code with your thumb, not your keyboard

Reading about code isn't the same as reasoning about it. Every lesson embeds exercises designed for touch screens — deep enough to be worth a senior engineer's time.

Reorder the lines

A working solution, shuffled. Drag the lines back into an order that runs. Tests whether you understand structure and data flow — not whether you can type.

Spot the difference

Two or three variants of the same code. One leaks memory, races, or breaks under load. Pick it — then read exactly why. Judgment over syntax recall.

Predict the output

Read a snippet, commit to what it prints, then watch it actually execute in a sandbox in your browser. The engine settles the argument.

Tracks

Each track is a book you can finish: ordered lessons that build on each other, from mental model to production judgment.

012 lessons

JavaScript Internals

How engines actually run your code: hidden classes, inline caches, the event loop, garbage collection, and the optimizations your code triggers — or kills.

Start reading

02coming soon

Professional Backend

Beyond CRUD tutorials: idempotency, backpressure, connection pooling, graceful shutdown, observability, and the architecture decisions that survive production.

03coming soon

Security for Developers

The attacks that actually happen and the code that stops them: injection beyond SQL, SSRF, prototype pollution, JWT pitfalls, and threat modeling for features.

04coming soon

Rust in Practice

Rust for engineers who already ship software: ownership as an API design tool, fearless concurrency, async runtimes, and when the borrow checker is telling you something.

05coming soon

Source Dives

Guided reading of real open-source code: how Express routes a request, how Vite transforms a module, how Redis stores a string. Learn architecture from code that won.

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