Return a variable from a function to be used in a different function?

I’d like to return the object id created for a guest based on my if statement below. I’m new-ish to javascript and can’t figure out how to do this from the MDN function docs so wondering if there’s a way to return a variable from the below function so it can be used in another function below

const guest = await Guest.findOne({ email: req.body.email, user: event.user._id }).populate('event');

if (!guest) {          // see if guest already exists
    const newGuest = new Guest(req.body);  // create new guest
    newGuest.user = event.user._id;
    newGuest.event = event._id;
    await newGuest.save(); // save new guest to guest model
    event.guests.push(newGuest);
    await event.save(); // save guest to event
} else {
    event.guests.push(guest);
    await event.save(); // 
}