This isn't what it sounds like. Based on the post title and the repo name I thought this was a library that could read and write data from Google Sheets and MS Excel Online. It's not that. It's a library to use sheet2api.com, which is a proxy to read and write JSON data from Google Sheets and MS Excel Online. All of your data goes through a 3rd party service. That is significantly different.

The title really should be "Show HN: Single JavaScript API Client for sheet2api.com"

From a JS front end dev perspective it's not actually very good as a library. There's literally no error handling at all (it just passes you the error from XMLHttpRequest; it could do so much more), there's no auth, and it's based on the old XMLHttpRequest API rather than the more modern, and much better in my opinion, fetch() API.