index_create_order_endpoint.js

app.post('/customers/:customerId/orders/', (req, res) => {
  if (req.body.items.length === 0) {
    res.status(400).end()
  } else {
    MongoClient.connect(url, function (err, client) {
      if (err) {
        console.error(err)
      }
      const db = client.db(dbName)
      const collection = db.collection('orders')
      const order = req.body
      order.customerId = req.params.customerId
      collection.insertOne(order, function (err, result) {
        if (err) {
          console.error(err)
        }
        res.send(result.ops[0])
      })
      client.close()
    })
  }
})