Non-leaky implementation of Crockford's make_sealer

makeSealer.ts

type Box<A> = () => A | null;
type Sealer<A> = {
  seal(value: A): Box<A>;
  unseal(box: Box<A>): A | null;
}
const makeSealer = <A>(): Sealer<A> => {
  let open: Box<A> | null = null;
  return {
    seal(value: A): Box<A> {
      const box = (): A | null => (open === box ? value : null);
      return box;
    },
    unseal(box: Box<A>): A | null {
      open = box;
      const result = box();
      open = null;
      return result;
    },
  };
};

const s = makeSealer();
const o = { a: 1 };
const box = s.seal(o);
console.log(s.unseal(box));

http://www.crockford.com/javascript/encyclopedia/I.html#indexOf