承诺倍数值

Question

我是新来的，对我而言，如果一个函数只能有一个参数，那感觉就像一个承诺。在许多情况下，这让我非常沮丧。

我应该如何使用promises（简化示例）来处理以下案例

/**
 * @api {post} /payments Create a new payment
 *
 * @apiParam {String} uuid uuid of the user.
 * @apiParam {String} status the payment status
 * @apiParam {String} transactionId Id of the transaction
 * @apiParam {String} creationDate The payment date
 * @apiParam {Number} productId The product Id
 */
router.post('/', function(req, res, next) {
  const userUUID = req.body.userUUID;
  const productId = req.body.productId;

  models.account.findOne({ where: { userUUID }})
    .then(account => {
        //Here I check if the accountId is correct
        if (!account) throw new errors.NotFound('unknown userUUID')
        return account;
    }).then(existingAccount)
        //Here I check if the productId is correct
        return models.product.findOne({ where: { productId }}).then(product => {
            if (!product) throw new errors.NotFound('unknown productId')
            return product;
        })
    })
    .then(existingProduct => {
        //Here I insert the payment in database
        return models.payment.create({
            productId,
            userUUID,
            existingProduct.price
        })
    })
    .then(existingPayment => {
        //My Problem :
        //Here, for example, I need : 
        //   existingProduct
        //   existingAccount
        //   existingPayment
        //How can I access them ???
    })
    .then(s => res.json(s))
    .catch(error => {
      if (error instanceof errors.NotFound)
        return res.status(404).send(error.message);
      return next(error);
    });
});