Testing: what's worth testing, and what isn't
By the end of today you can decide what deserves a test and what does not, write tests that survive refactoring, and explain why a high coverage number can mean almost nothing.
YesterdayDay 46 ended with you defending a design. Tests are how a design keeps being correct after you stop looking at it.
TomorrowTomorrow you test across boundaries, where the interesting failures actually live.
Why this matters
Most people either write no tests or write hundreds that break every time they rename something. The skill is knowing which few tests carry real information.
- What deserves a test
- Unit tests
- Test doubles
- What coverage does not measure
Learn it
75 minCopy this into Claude or ChatGPT. It quizzes you before it explains anything, which is deliberate. The resources under it are how you check what it told you.
Today's Master Prompt
Free · sign inA prompt written for this day alone: your level, the exact scope, what to leave out, and an instruction to quiz you before it explains anything. Paste it into Claude or ChatGPT and it teaches you today's material.
Check it against something that is not a model
An assistant can be fluent and wrong, and on a topic you met today you will not catch it. These cover the same ground and were made by people who do this for a living, so they are what you hold the explanation up against. They are other people's work and we only link to them, so judge them for yourself.
3 hand-picked resources
Free · sign inVideos, official docs and articles covering the same ground, each opened and annotated by hand. They are what you check the assistant against on a day you cannot yet catch it being wrong.
Build it
45 minPick the most logic-heavy function in your API and write three tests: the normal case, an edge case, and an error case. Then deliberately invert a comparison in the function and confirm at least one test fails. Then refactor the function's internals without changing behaviour and confirm no test fails.
Recall it
20 minAnswer out loud, reveal, then mark honestly whether you had it. That score is the only thing on this page you do not get to choose.
5 recall questions
Free · sign inQuestions you answer from memory, then grade yourself against the real answer. The score is carried into the mastery rating below it, so an honest miss cannot quietly become a tick.
Rate it
Completion and mastery are tracked separately. Be honest, because an inflated rating only means the concept resurfaces sooner.
Mastery tracking
Free · sign inRate yourself against five named criteria per concept. Completion and mastery are tracked separately, and anything you rate shakily comes back automatically on a spaced schedule.
Recap
- 01A test earns its place by failing when behaviour breaks, and only then
- 02Test behaviour, not internals, or every refactor costs you
- 03Coverage measures execution, not correctness
- 04Mocking everything means testing your mocks
Your progress
Free · sign inMark days complete, pick up where you left off across devices, and watch completion and mastery diverge. Free, and the account exists only so ninety days of work cannot vanish with a cleared browser.