I get the error method "buy is not defined.". What is wrong in the below code? Please can you let me know where I am going wrong?

var Portfolio = function() {

  let stockHoldings = new Map();
  
   function buy(trade) {
    let tradesForTicker = stockHoldings.get(trade.ticker);
    if(tradesForTicker == null){
      stockHoldings.set(trade.ticker,[trade]);
    }else{
      tradesForTicker.push(trade);
    }
  }

   function sell(trade) {
      let ticker = trade.ticker;  
      let tradesForTicker = stockHoldings.get(ticker);
      let precision = 5;
      if(tradesForTicker != null){
          let quantityToSell = Number(Number(trade.quantity).toFixed(precision));
          while(quantityToSell > 0) {
            quantityToSell = Number(Number(quantityToSell).toFixed(precision));          
            if(tradesForTicker.length > 0){
              let nextTradeToSell =  tradesForTicker[0];

              if(nextTradeToSell.quantity == quantityToSell){
                quantityToSell = 0;
                tradesForTicker.splice(0,1);
              } 
              else if (nextTradeToSell.quantity < quantityToSell){
                  quantityToSell = quantityToSell - nextTradeToSell.quantity;
                  tradesForTicker.splice(0,1);
              }else {
                nextTradeToSell.quantity = nextTradeToSell.quantity - quantityToSell;
                quantityToSell = 0;
              }
            }
          }
          if(tradesForTicker.length == 0){
            stockHoldings.delete(ticker)
          }
      }
  }

  this.stockHoldings = function() {
    return stockHoldings();
  }
};

function generetePortfolio(tickers, actions, quantities, prices){
  let portfolio = new Portfolio();
  for(var i=0;i< tickers.length; i++){
    let ticker = tickers[i].toString();
    let action = actions[i].toString();
    let quantity = quantities[i].toString();
    let price = prices[i].toString();

    let transactionTrade = new Trade(ticker,quantity,price,action);

    if(transactionTrade.isBuyOrDrip()){
        portfolio.buy(transactionTrade);
    }
    if(transactionTrade.isSell()){
      portfolio.sell(transactionTrade);
    }
  }
  return portfolio();
}

Further EDIT:

//Added method to generate total qty and avg price, BUT THIS DOES NOT WORK

function myPositions(tickers, actions, quantities, prices){
  let portfolio = generetePortfolio(tickers, actions, quantities, prices);  
  let returnArray = [];
  portfolio.stockHoldings.forEach((value, key) => {  
      let totalQuantity = 0;
      let avgPrice = 0;
      let totalCost = 0;
      value.map( trade => {
        totalQuantity += trade.quantity ;
        totalCost += trade.quantity * trade.price;
      });

      avgPrice = totalCost/totalQuantity;      
      returnArray.push([key, totalQuantity,avgPrice]);
  } )
  return returnArray;
}