The sheet is sharable, and publicly editable, I know it can be edited without credentials using API. But I couldn't find examples for the same.
gapi.client.init({
apiKey: API_KEY,
But I don't want to provide any key, etc. I just need to update the sheet anonymously. How to initiate the edit, without logging in or providing credentails.
All Google APIs require's some sort of credentials in order to provide authorization, even if if you are trying to access public data, creating an API key will allow you to modify the sheet as you mentioned and you will identify yourself through the API key.
Now there is also this IssueTracker that explains that you need authentication in order to modify a Public Editable Spreadsheet.
You could as well use a service account to perform the changes if the sheet is publicly available, there is a great guide in this article.