Predict before reveal
Commit to an answer before the trace makes invisible state visible.
JavaScript, understood
See what code
is thinking.
Predict the result, inspect each state change, and replace fragile guesses with a dependable mental model.
Read your first snippetJavaScript reading gym
Read first. Run second.
Challenge1 / 5
Score0
Object referencesFoundation
const original = { score: 4 };const copy = original;copy.score += 3;console.log(original.score);Make a prediction
What is printed?
Curated JavaScript only. Predict before the trace appears. Progress stays in this browser
Reading is a real skill
Follow state,
not syntax.
Name the misconception
Every distractor points to a mental model worth correcting.
Bounded by design
Only reviewed examples run, inside a short-lived browser worker.
A closure keeps the room.
When a function leaves its original scope, it can still retain access to the bindings created there. Follow the binding—not merely the line where the function is called.
The reading habit
Pause at every assignment and ask: what changed, and who can see it?
That question clarifies closures, mutation, loops, callbacks, and asynchronous ordering without relying on memorized trivia.
Try another challengeCode reading questions
Do I need to write code?
No. The gym isolates reading: predict output, trace values, or identify a defect before seeing the explanation.
Does this execute arbitrary code?
No. Only reviewed, built-in output examples have matching worker executors. User-provided code is never evaluated.
Which language is covered?
The first release focuses on modern JavaScript so every explanation and runtime assumption can stay precise.
Where is progress saved?
Only in this browser's localStorage. Challenge answers and progress are not sent to a backend.