Convenient Transaction API の使用

const txnResult = await client.withSession(async (session) =>
  session
    .withTransaction(async (session) => {
      const invColl = client.db("testdb").collection("inventory");
      const recColl = client.db("testdb").collection("orders");
      let total = 0;
      for (const item of order) {
        /* Update the inventory for the purchased items. End the
        transaction if the quantity of an item in the inventory is
        insufficient to complete the purchase. */
        const inStock = await invColl.findOneAndUpdate(
          {
            item: item.item,
            qty: { $gte: item.qty },
          },
          { $inc: { qty: -item.qty } },
          { session }
        );
        if (inStock === null) {
          await session.abortTransaction();
          return "Item not found or insufficient quantity.";
        }
        const subTotal = item.qty * inStock.price;
        total = total + subTotal;
      }
      // Create a record of the purchase
      const receipt = {
        date: new Date(),
        items: order,
        total: total,
      };
      await recColl.insertOne(receipt, { session });
      return (
        "Order successfully completed and recorded!\nReceipt:\n" +
        JSON.stringify(receipt, null, 1)
      );
    }, null)
    .finally(async () => await client.close())
);
console.log(txnResult);