105 道问答题，一道一卡105 questions, one card each

In one line: all five are utility types — they build a new type from an existing one by a rule: Partial makes everything optional, Required makes everything required, Pick keeps a whitelist, Omit drops a blacklist, Record builds a dictionary from a key set. The real question is not the API — it is knowing which one a scenario calls for.

Three scenarios that keep coming up:

• The patch parameter of an update function → Partial<User>. Callers pass only the fields they change, but every field they do pass is still checked — a typo still fails to compile (Object literal may only specify known properties). That is what makes it stronger than Record<string, unknown>: optional, without giving up per-field checking.
• Trimming props from a big type → Pick<User, "name" | "role">. The component declares only the fields it actually uses, so adding fields to User later cannot ripple into it. The opposite direction — dropping fields that must not leak — is Omit<User, "email">.
• A dictionary with a known key set → Record<Theme, string>, not an index signature. With a literal union as the key, Record is a closed key set: one key missing or one key extra is a compile error. An index signature waves any string through, then claims the value definitely exists.

Choosing between Pick and Omit: whitelist or blacklist. When the type will grow, Omit silently carries every new field along — a feature when forwarding config, a hole when redacting. So for public-facing types, prefer Pick: a newly added sensitive field stays private by default.

Follow-up: “Is Partial deep or shallow?” — shallow. It only touches the first level; properties inside nested objects stay required. A deep version means writing a recursive mapped type yourself. “What is the difference between Record<string, T> and an index signature?” — almost none, except Record keys can be literal unions and index signatures cannot. Answer both, and the next question is usually “so how is Partial itself implemented?” — which is exactly the next card.

In one line: a mapped type is one loop over a union of property names. The single line { [K in keyof T]?: T[K] } is the whole implementation of Partial: take each property name of T, copy its original type, add the optional modifier.

Reading it piece by piece:

• keyof T — the union of property names. For User that is "id" | "name" | "email".
• [K in keyof T] — the mapping itself: generate one property per member of the union, with K as the loop variable.
• ?: — add the optional modifier to the generated property. The same slot also takes readonly; and -? goes the other way — it removes optionality, which is exactly how Required is implemented.
• T[K] — an indexed access type: whatever type the original property had, copy it.

MyPick needs one extra generic constraint: K extends keyof T. Without it, callers may pass anything as K, and T[P] cannot be computed. With it, MyPick<User, "age"> fails right at the call site with Type '"age"' does not satisfy the constraint, instead of quietly producing a wrong type.

Follow-up: “How do you build Omit out of Pick?” — Pick<T, Exclude<keyof T, K>>: Exclude removes K from the full key set, Pick keeps the rest. Add that the official Omit constrains K to keyof any — looser than keyof T, so the official one lets you omit a key that does not exist — and it shows you have actually read lib.es5.d.ts. People who memorized the API and people who read the implementation part ways on this one question.

In one line: all three are conditional types — a ternary at the type level, T extends U ? X : Y. Exclude and Extract filter unions through its distributive behavior; ReturnType adds infer to pull the return type out of a function type.

Distributivity: when T is a bare type parameter and the argument is a union, the conditional does not judge the union as a whole — it substitutes each member separately and unions the results. Exclude<"a" | "b" | "c", "a"> therefore expands into three checks: matching members become never, and never is the empty union, so it vanishes when merged back in. Filtering really means turning the unwanted members into never.

infer: it digs a hole in the pattern to the right of extends, and the compiler fills the hole when the match succeeds. In T extends (...args: any[]) => infer R ? R : never, R is that hole — when T matches a function type, R gets its return type. The same trick unpacks array elements, the value inside a Promise, or function parameters (Parameters is the same hole dug at the parameter position).

Follow-up: “How do you switch distributivity off?” — wrap both sides in a tuple: [T] extends [U], T is no longer bare, so the union is judged as one piece. “What is Exclude<T, T>?” — never: every member is matched by itself. “And feeding never in?” — still never: the empty union loops zero times. All three are corollaries of one rule — master distributivity and the whole set falls out.