5.ts

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

type SantasList<BL extends readonly any[], GL extends readonly any[]> = [
  ...BL,
  ...GL,
];

const bads = ["tommy", "trash"] as const;
const goods = ["bash", "tru"] as const;

type test_0_actual = SantasList<typeof bads, typeof goods>;
//   ^?
type test_0_expected = ["tommy", "trash", "bash", "tru"];
type test_0 = Expect<Equal<test_0_actual, test_0_expected>>;

type test_1_actual = SantasList<[], []>;
//   ^?
type test_1_expected = [];
type test_1 = Expect<Equal<test_1_actual, test_1_expected>>;

type test_2_actual = SantasList<[], ["trash"]>;
//   ^?
type test_2_expected = ["trash"];
type test_2 = Expect<Equal<test_2_actual, test_2_expected>>;

type test_3_actual = SantasList<["john"], ["ashley", "elliot", "ziltoid"]>;
//   ^?
type test_3_expected = ["john", "ashley", "elliot", "ziltoid"];
type test_3 = Expect<Equal<test_3_actual, test_3_expected>>;

type test_4_actual = SantasList<
  ["1", 2, "3"],
  [false, boolean, "4", ["nested"]]
>;
//   ^?
type test_4_expected = ["1", 2, "3", false, boolean, "4", ["nested"]];
type test_4 = Expect<Equal<test_4_actual, test_4_expected>>;

// @ts-expect-error
type error_0 = SantasList<null, undefined>;