

Gender from a first name, full name, email or username, with probability.
NameGender API is a free Development API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
NameGender REST API reference: gender from names, email addresses and usernames, batch requests, response fields, error codes and rate limits.
Details from namegender.com.

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The NameGender API is provided by NameGender (namegender.com). NameGender REST API reference: gender from names, email addresses and usernames, batch requests, response fields, error codes and rate limits.
Yes. The NameGender API is listed in the Development 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 NameGender API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The NameGender API supports CORS, so front-end JavaScript can call it directly.
Yes. The NameGender API is served over HTTPS, so it's safe to use in production.