One defect per page
Every exercise is a single realistic mistake, in code short enough to hold in your head.
TypeScript exercises for judgment, not syntax
Each page is one realistic defect. Predict what the code does, check your answer, then read why the compiler stayed quiet.
Start with impossible statesPredict before you read
You commit to an answer first. The explanation only lands once you have been wrong on purpose.
No account needed
Read, predict and check without signing in. The workout behind each page is where practice continues.
THE EXERCISES
Start anywhere. They compound in order.
- 01
Discriminated unions: stop modelling impossible states
Optional properties describe which fields might exist. A discriminated union describes which combinations are allowed — a much stronger claim.
- 02
Exhaustive switches: the default clause that hides new states
A
defaultclause is a promise that you have thought about every future case. Almost nobody means to make that promise. - 03
Missing, empty, zero: three states one falsy check collapses
if (value)asks whether a value is truthy. Most of the time you meant to ask whether it was provided. - 04
Stale async results: when the older request wins
Requests start in order. Responses do not. Any state update that ignores the difference is a race waiting for a slow network.
- 05
Retries: the loop that hammers a server it can never satisfy
A retry is a bet that the same request will behave differently next time. Most failures are not that kind of failure.
- 06
Cancellation that stops at the first await
Accepting an
AbortSignalis a promise to stop. Every await you forget to thread it through is a place where you quietly keep going. - 07
Idempotency: the check-then-act that charges twice
At-least-once delivery is the normal case, not the edge case. "Have I done this already?" is only a useful question if nobody can answer it at the same time as you.
- 08
Async tests that pass for the wrong reason
A green async test proves the assertions ran and passed. Plenty of async tests only prove the first half — and some prove neither.
BEYOND THE READING
Reading about a race condition is not practice.
Each exercise ends in a CodeStamina workout: a specification, a failing test suite and a repair you have to make yourself. One complete guided workout is open without an account.
Open the guided workout