Utility Types — Series 2

Preview — 3 of 10 questions

javascript

class Car {
  constructor(public brand: string) {}
}
type CarInstance = InstanceType<typeof Car>;
const c: CarInstance = new Car("Toyota");
const bad: CarInstance = { brand: "Toyota" };

ACarInstance is the constructor function's own type, so neither c nor bad compiles

BCarInstance is Car — the instance type produced when Car is constructed with new — so c compiles fine; bad also compiles fine, since it structurally matches Car's instance shape ({ brand: string })

CCarInstance is typeof Car, so only values actually produced via new Car(...) are accepted, meaning bad fails to compile

DCompile-time error — InstanceType requires an explicit class name argument, not typeof Car

javascript

type AllStatus = "pending" | "active" | "archived" | "deleted";
type ActiveStatus = Exclude<AllStatus, "archived" | "deleted">;
type ArchivedOrDeleted = Extract<AllStatus, "archived" | "deleted">;

AActiveStatus is "archived" | "deleted"; ArchivedOrDeleted is "pending" | "active" — the two utility types are swapped from what their names suggest

BBoth ActiveStatus and ArchivedOrDeleted resolve to the full AllStatus union, unchanged

CActiveStatus is "pending" | "active" — Exclude removes the listed members from the union; ArchivedOrDeleted is "archived" | "deleted" — Extract keeps only the members that match

DBoth resolve to never, since neither utility type works on string-literal unions

javascript

interface Paginated<T extends { id: number } = { id: number }> {
  items: T[];
  page: number;
}
const p1: Paginated = { items: [{ id: 1 }], page: 1 };
const p2: Paginated<{ id: number; name: string }> = {
  items: [{ id: 1, name: "Ana" }],
  page: 1,
};

ABoth compile fine — p1 uses the default type argument { id: number }; p2 explicitly supplies a more specific type that still satisfies the constraint T extends { id: number }

BCompile-time error — a generic type parameter can't have both a constraint (extends) and a default value (=) at the same time

Cp1 compiles fine; p2 is a compile-time error, because a default type argument, once declared, is always used and can't be overridden

DBoth are compile-time errors — interfaces don't support default generic type parameters

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