Wait, what?

The author sees this code and realizes it's unnecessarily serial:

    save('userData', userData)
      .then(() => save('session', sessionPrefences))
      .then(() => ({ userData, sessionPrefences })

But doesn't see that the async/await version is serial, even though it's arguably even _more_ obvious?

    await save('userData', userData);
    await save('session', sessionPrefences);
    return { userData, sessionPrefences }

The transform to being parallel is pretty easy to read too:

    await Promise.all([
      save('userData', userData),
      save('session', sessionPrefences)
    ]);
    return { userData, sessionPrefences }

On top of that, the try/catch section is out of date. try/catch does not deoptimize the block in modern engines, and async/await absolutely integrates better with try/catch for much more bulletproof error handling.