ThisType -- https://www.typescriptlang.org/docs/handbook/utility-types.html#thistypet

constructor-challenge--solved.ts

type Constructor<T, Args extends any[]> = {
  new (...args: Args): T;
  (...args: Args): T;
};

function makeConstructor<T, Args extends any[], P = {}>(
  create: (...args: Args) => T,
  prototype?: P & ThisType<T & P>
) {
  const constructor = function(this: T | undefined, ...args: Args) {
    if (this instanceof constructor) {
      return Object.assign(this, create(...args));
    }
    return new constructor(...args);
  } as Constructor<T & P, Args>;

  constructor.prototype = prototype || {};
  return constructor;
}

type CharacterStats = {
  strength: number;
  wisdom: number;
  charisma: number;
};

const Character = makeConstructor(
  (characterName: string, stats: CharacterStats) => ({
    // instance
    characterName,
    stats,
    level: 1,
  }),
  {
    // prototype
    __brand: "Character" as const,
    sayHello() {
      console.log(
        "Hi, I'm",
        this.characterName,
        this.level,
        this.stats,
        this.sayHello,
        this
      );
    },
  }
);

type Character = ReturnType<typeof Character>;

const merlin = Character("Merlin", { charisma: 6, wisdom: 10, strength: 2 });
const kingArthur = new Character("King Arthur", {
  charisma: 7,
  strength: 9,
  wisdom: 5,
});

merlin.sayHello();
kingArthur.sayHello();