Hello! the API is in preview now! Make sure to have the latest insiders fast and use our Beta CDN, more info on this here https://aka.ms/excelapiopenspec
Here is a sample code snippet using the API
key line of code context.workbook.worksheets.addFromBase64(mybase64)
send an xlsx (base64 encoded)
please use it and send us feedback!Cory Isaacson commented
This will be a very important feature. It is also important to be able to load a sheet into a workbook using Open XML so that the application can provide this on the fly.