support plus account balance which account currency does not equal to default currency

src/lib/exchangeRates.js

@@ -1,3 +1,5 @@
+import utils from './utils.js'
+
 const exchangeRatesLocalStorageKey = 'lab_exchange_rates';
 
 function getExchangeRates() {
@@ -14,8 +16,39 @@ function clearExchangeRates() {
     localStorage.removeItem(exchangeRatesLocalStorageKey);
 }
 
+function getExchangeRate(fromCurrency, toCurrency) {
+    const exchangeRates = getExchangeRates().exchangeRates;
+    const exchangeRateMap = {};
+
+    for (let i = 0; i < exchangeRates.length; i++) {
+        const exchangeRate = exchangeRates[i];
+        exchangeRateMap[exchangeRate.currency] = exchangeRate;
+    }
+
+    const fromCurrencyExchangeRate = exchangeRateMap[fromCurrency];
+    const toCurrencyExchangeRate = exchangeRateMap[toCurrency];
+
+    if (!fromCurrencyExchangeRate || !toCurrencyExchangeRate) {
+        return null;
+    }
+
+    return parseFloat(toCurrencyExchangeRate.rate) / parseFloat(fromCurrencyExchangeRate.rate);
+}
+
+function getOtherCurrencyAmount(amount, fromCurrency, toCurrency) {
+    const exchangeRate = getExchangeRate(fromCurrency, toCurrency);
+
+    if (!utils.isNumber(exchangeRate)) {
+        return null;
+    }
+
+    return amount * exchangeRate;
+}
+
 export default {
     getExchangeRates,
     setExchangeRates,
     clearExchangeRates,
+    getExchangeRate,
+    getOtherCurrencyAmount,
 };