How to use the node-bitstamp.CURRENCY.BTC_EUR function in node-bitstamp

//apply to current price
        currentPrice = currentPrice + diff;
        prices.push(currentPrice);
    }

    return prices;
};

/**
 * functions that we apply before training as well as before and after prediction
 * map dataset trades [{x,y}] into price floats [{x,y}]
 * ships predict, main and alter functions for every currency
 */
const ETLS = {

    [CURRENCY.BTC_EUR]: {
        predict: trades => { //before prediction (is called)
            return TRADES_TO_ROW(trades, BTC_EUR_FACTOR);
        },
        main: row => { //before training (is mapped)
           // const trades = row.x.slice(0);
            row.x = TRADES_TO_ROW(row.x, BTC_EUR_FACTOR);
            row.y = TRADES_TO_ROW_OUTPUT(row.y, ETL_FACTOR);
            return row;
        },
        alter: (outputs, tradeForPrediction) => { //after prediction (is called)
            return OUTPUT_TO_TRADE_PRICES(outputs, ETL_FACTOR, tradeForPrediction);
        }
    },

    [CURRENCY.ETH_EUR]: {
        predict: trades => { //before prediction (is called)

startForBitcoin(){
        return this.start(CURRENCY.BTC_EUR, 3333);
    }