Unlike Google, Microsoft actually has APIs, so you can use those. Depends on the project, though.

Since Google's fans ironically can't Google, docs below:

https://docs.microsoft.com/en-us/office/dev/add-ins/

is the modern typescript/web API. It's not totally feature-complete (yet), so you may need to develop a VSTO add-in in C# or VB, which will unfortunately not work with the web client. Docs for that:

https://docs.microsoft.com/en-us/visualstudio/vsto/create-vs...