The Counter That Forgot Debugging Lab

The Counter That ForgotData IntegrityEasyPreparing workspaceAutosaved

00:00

Incident Dossier

The Counter That Forgot

Two callers race to bump the same inventory counter at the same moment. The sum of their increments never appears in the final value, even though no exception is thrown and both calls report success.

Initial failing signalconcurrent increments lose one updateExpected widget == 18 · received widget == 13

Progress

1Reproduce the bugRun checks

2Edit the fixOpen

3Pass checksOpen

4Submit fixOpen

Reproduction Steps

Run npm test.
Trace the increment path; both calls report success but the sum is wrong.
Identify the place where the read can race the write.
Serialize increments per key so each read observes the previous write.

Focus Mode

Start

src/counter.js

Likely area

src/counter.js

Progressive hints

Hints reveal broad direction first, then narrow toward the suspect path without showing hidden tests or exact solution code.

TopicData Integrity

DifficultyEasy

Est. time12 min

Workspacecreating

Explorer

Preparing workspace files…

Evidence Map

Failing signal loaded from challenge API
Workspace files loaded from workspace API
Results rendered only after API run/submit

Autosaved

workspace

Outputread-onlyready$ workspace boot

Workspace booting...