汇率计算器显示NaN问题求助：附JS代码与API响应数据

我正在开发一款货币转换计算器，但计算结果始终显示NaN，尝试多种方法仍未解决。以下是我的JavaScript代码、页面截图及API返回的响应数据，恳请帮忙排查问题：

JavaScript代码 #

const dropList = document.querySelectorAll("form select"),
  fromCurrency = document.querySelector(".from select"),
  toCurrency = document.querySelector(".to select"),
  getButton = document.querySelector("form button");

for (let i = 0; i < dropList.length; i++) {
  for (let currency_code in country_list) {
    let selected =
      i == 0
        ? currency_code == "USD"
          ? "selected"
          : ""
        : currency_code == "NPR"
        ? "selected"
        : "";
    let optionTag = `<option value="${currency_code}" ${selected}>${currency_code}</option>`;
    dropList[i].insertAdjacentHTML("beforeend", optionTag);
  }
  dropList[i].addEventListener("change", (e) => {
    loadFlag(e.target);
  });
}

function loadFlag(element) {
  for (let code in country_list) {
    if (code == element.value) {
      let imgTag = element.parentElement.querySelector("img");
      imgTag.src = `https://flagcdn.com/48x36/${country_list[
        code
      ].toLowerCase()}.png`;
    }
  }
}

window.addEventListener("load", () => {
  getExchangeRate();
});

getButton.addEventListener("click", (e) => {
  e.preventDefault();
  getExchangeRate();
});

const exchangeIcon = document.querySelector("form .icon");
exchangeIcon.addEventListener("click", () => {
  let tempCode = fromCurrency.value;
  fromCurrency.value = toCurrency.value;
  toCurrency.value = tempCode;
  loadFlag(fromCurrency);
  loadFlag(toCurrency);
  getExchangeRate();
});

function getExchangeRate() {
  const amount = document.querySelector("form input");
  const exchangeRateTxt = document.querySelector("form .exchange-rate");
  let amountVal = amount.value;
  amountVal = parseFloat(amountVal);
  if (amountVal == "" || amountVal == "0") {
    amount.value = "1";
    amountVal = 1;
  }
  exchangeRateTxt.innerText = "Getting exchange rate...";
  let url = `https://api.fastforex.io/convert?from=${fromCurrency.value}&to=${toCurrency.value}&amount=${amountVal}&api_key=xxx-xxx-xxx`; // api key on p
  fetch(url)
    .then((response) => response.json())
    .then((result) => {
      let exchangeRate = result.rate;
      let totalExRate = (amountVal * exchangeRate).toFixed(2);
      exchangeRateTxt.innerText = `${amountVal} ${fromCurrency.value} = ${totalExRate} ${toCurrency.value}`;
    })
    .catch(() => {
      exchangeRateTxt.innerText = "Something went wrong";
    });
}

API响应数据 #

{
  "base": "USD",
  "amount": 1,
  "result": {
    "AED": 3.67,
    "rate": 3.67189
  },
  "ms": 5
}

页面截图 #

内容的提问来源于stack exchange，提问作者John