Piggybacking promises

piggyback.js

piggyback(fn) => () => {
        if (!fn["@@piggyback"]) {
            const promise = fn();
            fn["@@piggyback"] = promise
            promise.then(() => {
                delete fn["@@piggyback"];
            });
            return promise
        } else {
            return fn["@@piggyback"].then(res => {
                res["@@piggybacked"] = true;
                return res;
            })
        }
    }
}