Free currency API with over 150 currencies.
Amdoren API is a free Currency Exchange API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.

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Yes. The Amdoren API is listed in the Currency Exchange category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The Amdoren API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the Amdoren API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Amdoren API is served over HTTPS, so it's safe to use in production.