elsa.magic.ts

type Freezable = {
  unfreezable: never;
};

type Frozen<T extends Freezable> = T & {
  frozen: true;
};

function assertFreezable(thing: unknown): asserts thing is Freezable {
  if (!thing || (thing as Freezable).unfreezable) {
    throw new Error(`Can't freeze that! ❄️`);
  }
}

function freeze<T extends Freezable>(toFreeze: T): Frozen<T> {
  assertFreezable(toFreeze);

  (toFreeze as Frozen<T>).frozen = true;
  return toFreeze as Frozen<T>;
}


const thing = {};
const frozenThing = freeze(thing); //Argument of type '{}' is not assignable to parameter of type 'Freezable'.
                                   //Property 'unfreezable' is missing in type '{}' but required in type 'Freezable'.ts(2345)