1async function calculatePortfolioValue(transactions) {
2  let totalUSD = 0;
3  const exchangeRates = new Map();
4
5  async function getExchangeRate(currency) {
6    if (exchangeRates.has(currency)) {
7      return exchangeRates.get(currency);
8    }
9    const response = await fetch(`/api/rates/${currency}`);
10    const rate = await response.json();
11    exchangeRates.set(currency, rate.value);
12    return rate.value;
13  }
14
15  transactions.forEach(async (tx) => {
16    const rate = await getExchangeRate(tx.currency);
17    const valueUSD = tx.amount * rate;
18    totalUSD += valueUSD;
19  });
20
21  return {
22    total: totalUSD,
23    timestamp: Date.now()
24  };
25}
26
27// Usage
28const dashboard = {
29  async updateTotals(transactions) {
30    const result = await calculatePortfolioValue(transactions);
31    this.setState({ portfolioValue: result.total });
32  }
33};