Build to understand

Our modules are built around solving problems and discovering the concepts along the way. There are no videos to watch or badges to earn.

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eval.js

function evaluate(node, env) {
  let left = evaluate(node.left, env);
  let right = evaluate(node.right, env);
  // Apply the operator
  return applyOp(node.op, left, right);
}

Build it yourself

Each problem extends what you've already built. By the end of a module, you've made something complete that you understand from the inside out.

No hidden magic

We don't just introduce iteration as "using a loop." We help you understand what makes a process iterative, how its state evolves, and how to reason about it.

Code and curiosity

No sign-up, no video lectures, no gamification. Open the editor, write code, and watch the tests turn green.

Featured Module

Build an Interpreter

End up with a working language

Build a small language you can actually run, one that handles variables, functions, and recursion. You grow it from scratch, one capability at a time, starting with evaluating a single number.

JavaScript Environments Closures Recursion

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Who this is for

If you'd rather build something than watch a video explaining it, this is for you. Maybe you want to understand a subject deeply instead of just getting through it, or maybe you just enjoy taking things apart to see how they work. Both belong here.

How it works

1

Open a module

Each module builds one complete system from scratch.
2

Solve problems in order

Every problem extends your previous code, guided by tests.
3

Understand from the inside

No black boxes. You wrote every line, so you know how it works.

No sign-up. Open the module and start building.

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