7.ts

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

type DoAppend<T extends string> = `good_${T}`;
type AppendGood<L extends { [key: string]: any }> = {
  [key in keyof L as DoAppend<string & key>]: L[key];
};

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>
>;