Utility Types — Series 3

Preview — 3 of 10 questions

javascript

declare function setProp<T, K extends keyof T>(obj: T, key: K, value: T[K]): void;

const user = { id: 1, name: "Ada" };

setProp(user, "name", "Grace");
setProp(user, "name", 42);

ABoth compile — T[K] widens to the union of all the object's value types

BBoth fail — T and K cannot both be inferred from the same call

CThe first compiles; the second is a compile-time error — T["name"] is string, so 42 isn't assignable

DThe first fails — "Grace" isn't the literal type "Ada" that was inferred for the property

javascript

type Stringify<T> = { [K in keyof T]: string };

type R = Stringify<{ a: number; b: boolean }>;

A{ a: string; b: string }

B{ a: number; b: boolean } — the value type in a mapped type is ignored

C{ [x: string]: string } — mapping erases the specific keys

DCompile-time error — a mapped type's value must reference T[K]

javascript

type Config = Readonly<{ db: { host: string } }>;

declare const cfg: Config;

cfg.db = { host: "a" };
cfg.db.host = "b";

ABoth assignments fail — Readonly applies recursively

BBoth compile — Readonly is erased at compile time and enforces nothing

CThe first compiles, the second fails

DThe first fails, the second compiles — Readonly<T> marks only the top-level properties

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