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