
API to read, write, and format Google Sheets data.
Google Sheets API is a free Development API that requires OAuth authentication, is served over HTTPS and has unconfirmed CORS support.
Details from developers.google.com.

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The Google Sheets API is provided by Google Sheets API (developers.google.com).
Yes. The Google Sheets 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 Google Sheets API uses OAuth authentication, so you need to register for credentials before making requests.
CORS support for the Google Sheets API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Google Sheets API is served over HTTPS, so it's safe to use in production.