Debugging challenges

Multiple-choice questions test whether you can read code. These test whether you can fix it. Every challenge is a short program that compiles and runs, produces the wrong result, and tells you exactly what it should have done instead. Your job is to find the line that lies.

The bugs are the ones that actually cost people afternoons: dangling references, off-by-one loops, floating-point equality, stale closures, missing awaits, and the mutation you did not realise was shared.

Reading a challenge brief is free. Opening the editor, submitting a fix, and seeing the explanation and hints needs a free account, because a fix only counts if you wrote it.