Debugging guides, one bug at a time

Each guide takes one common bug, explains it in a minimal example, shows the fix and the wrong fixes, and then opens a matching free challenge so you can fix it yourself.

Browse the current guides

JavaScript The missing await bug in JavaScript async functions

A missing await makes JavaScript read a Promise as if it were the resolved value. See the failure, the one-line fix, and the wrong fixes to avoid.

Python Why a mutable default argument leaks state in Python

A list used as a Python default argument is created once and shared across calls. See why state leaks between calls and the None-guard fix that stops it.

SQL Why a JOIN duplicates rows, and how to fix cardinality

A one-to-many JOIN multiplies rows, so a customer with several orders appears many times. Learn to diagnose cardinality and aggregate to the right grain.

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Features

See how DoesItHold generates bug-hunting exercises, checks your fix, and adapts difficulty. Nine languages, guided hints, and progress you can see.

Docs

A plain-language walkthrough of the DoesItHold training loop: generate a bug, propose a fix, read the feedback, and repeat at an adapting difficulty.