Optional promise via chaining.js

const wait = () => new Promise((r) => setTimeout(r, 10));

const fakeDbOp = async () => {
  console.log("fakeDbOp");
  await wait(10);
  return 1;
};

const fakeTrxDbOp = async () => {
  console.log("fakeTrxDbOp");
  await wait(10);
  return 2;
};

const f = () => {
  let catchHandler;
  let finallyHandler;
  const then = (fn) => {
    fakeDbOp()
      .then((x) => fn(x))
      .catch((e) => {
        if (catchHandler) catchHandler(e);
      })
      .finally(() => {
        if (finallyHandler) finallyHandler(e);
      });
  };
  const trx = () => fakeTrxDbOp();
  return {
    then,
    catch: (fn) => {
      catchHandler = fn;
    },
    finally: (fn) => {
      finallyHandler = fn;
    },
    trx,
  };
};

const main = async () => {
  const a = await f();
  console.log("a", a);

  const b = await f().trx();
  console.log("b", b);
};
main();