elsa.magic.ts

type Freezable = {
  unfreezable?: never; // Need to make the unfreezable property optional
};

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

type Thawed<T> = T extends Frozen<infer R>
  ? R
  : never;

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 = Freezable>(toFreeze: T): Frozen<T> {
  assertFreezable(toFreeze);

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

function assertFrozen(thing: unknown): asserts thing is Frozen {
  if (!thing || !(thing as Frozen).frozen) {
    throw new Error(`Can't thaw that! ❄️`);
  }
}

function thaw<T extends Frozen>(toThaw: T): Thawed<T> {
  assertFrozen(toThaw);

  delete toThaw.frozen;
  return toThaw as Thawed<T>;
}

type Thing = Freezable & { thing: boolean }; // Need a type for the thing you're trying to freeze

freeze<Thing>({ thing: false, unfreezable: true }); // Not allowed
const frozenThing = freeze<Thing>({ thing: true });
const thawed = thaw(frozenThing);