Google API:
https://marketbusinessnews.com/how-to-use-google-api-to-get-currency-exchange-rates/321453/

Exchange rates API:
https://exchangeratesapi.io/
Note: not free when api calls are more than 1000 per month?

fixer:
https://fixer.io/
Note: not free when api calls are more than 1000 per month?

currencylayer:
https://currencylayer.com/real-time-exchange
Note: not free when api calls are more than 1000 per month?

currencyapi:
https://currencyapi.com/
Note: not freen when api calls are more than 300 Requests per month?

IEX Cloud Data Bundles:
https://iexcloud.io/forex/
Note: not freen when needs more than 5 Requests Per Second

openexchangerates:
https://openexchangerates.org/
Note: not free when api calls are more than 1000 per month

exchangerate:
https://exchangerate.host/
Note: not free when api calls are more than 1000 per month

European Central Bank (ECB):
https://www.ecb.europa.eu/stats/policy_and_exchange_rates/euro_reference_exchange_rates/html/index.en.html
https://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml
Note 1:
It is free but only update every working day(ex: Mon. to Fri.) except on TARGET closing days.(Saturday, Sunday, and public holidays: closed)
參考利率通常在每個工作日的中歐時間約16:00更新，除了TARGET休市日。
它們是基於歐洲各中央銀行之間的每日協調程序，通常在中歐時間約14:10進行。參考利率僅供資訊參考。強烈建議不要使用這些利率進行交易。
Note 2:
歐洲中部時間（英文：Central European Time，簡寫 CET，又簡稱中歐時間），其定義為UTC+1

比較各種Currency Exchange Rates API:
https://nordicapis.com/10-apis-for-currency-exchange-rates/
https://rapidapi.com/blog/currency-converter-api/

其他參考網站:
https://www.geoplugin.com/

https://www.mycurrency.net/
https://www.mycurrency.net/US.json

比較匯率api重點:

1. 精確度
2. 更新頻率(小時or天or其他)
3. 支援貨幣種類
4. rate limit(ex: 5 api calls/second)
5. count per month
6. pricing plan
7. 匯率換算是否是直接換算或需要間接換算
   (ex: 只有美金兌各幣種，所以台幣兌美金可以直接得到，但台幣兌日幣需要台幣對美金，美金再換算成日幣。)