TypeScript, and why static types exist
By the end of today you can explain what a type checker does and when, read type annotations fluently, and say honestly what TypeScript does not protect you from at runtime.
YesterdayOn Day 28 you wrote async code where the shape of every awaited value was a guess. Today those shapes become checkable.
TomorrowTomorrow you use the parts of the type system that do real work: unions, narrowing and generics.
Why this matters
Types are documentation the compiler enforces. The same discipline shows up as request validation on Day 37 and as the schema constraining an LLM's output on Day 71.
- Static typing
- Compile time and runtime
- Inference
- Structural typing
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.
5 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 minTake the JavaScript you wrote on Day 26 and add types until it compiles under strict mode. Then write a function that fetches JSON, annotate the result as a specific shape, and deliberately have the source return a different shape. Watch it type-check cleanly and fail at runtime, and write two sentences on what that proves.
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 type checker proves certain mistakes impossible without running the code
- 02Types are erased, so nothing is validated at runtime
- 03Data crossing a boundary needs real validation, not an annotation
- 04any disables checking and spreads; unknown keeps you honest
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.