code: function (currencyA, currencyB, callback) {
        if (typeof currencyB === 'function') {
          callback = currencyB;
          currencyB = null;
        }
  
        const parameters = {
          currencyPair: joinCurrencies(currencyA, currencyB)
        };
  
        return this._private('returnOpenOrders', parameters, callback);
      }
docs: can be called with `returnOpenOrders('all', callback)`