As I understand it it's basically how Pytorch works. A clever trick but also super confusing because while it seems like normal code, as soon as you try and do something that you could totally do in normal code it doesn't work:

  let friendsWithPhotos = friendsPromise.map(friend => {
    return {friend, photo: friend.has_photo ? api.getUserPhoto(friend.id) : default_photo};
  }

Looks totally reasonable, but it's not going to work properly. You might not even realise until it's deployed.