day7.ts

/*Solution*/
type AppendGood<T> = {
    [K in keyof T as `good_${K extends string ? K : never}`]: T[K];
};

/*Tests*/
import { Expect, Equal } from "type-testing";

type WellBehavedList = {
    tom: { address: "1 candy cane lane" };
    timmy: { address: "43 chocolate dr" };
    trash: { address: "637 starlight way" };
    candace: { address: "12 aurora" };
};
type test_wellBehaved_actual = AppendGood<WellBehavedList>;
type test_wellBehaved_expected = {
    good_tom: { address: "1 candy cane lane" };
    good_timmy: { address: "43 chocolate dr" };
    good_trash: { address: "637 starlight way" };
    good_candace: { address: "12 aurora" };
};
type test_wellBehaved = Expect<Equal<test_wellBehaved_expected, test_wellBehaved_actual>>;

type Unrelated = {
    dont: "cheat";
    play: "fair";
};
type test_Unrelated_actual = AppendGood<Unrelated>;
type test_Unrelated_expected = {
    good_dont: "cheat";
    good_play: "fair";
};
type test_Unrelated = Expect<Equal<test_Unrelated_expected, test_Unrelated_actual>>;