code: function (currency, callback) {
        // Call an asynchronous function, often a save() to DB
        self.getPricesForSingleCurrency(from, to, currency, function (err, result) {
          if (err) {
            callback(err)
          } else {
            ratesWithTimes[currency] = result
            callback()
          }
        })
      }
docs: 2nd param is the function that each item is passed to