While working on a customers proof of concept, we determined that we needed to know who the current user opening the add-in is. In most scenarios where there is a Store Add-in, you have the user log in. But we are in a enterprise environment, have an embedded taskpane and did not want to nag the user every single time they opened the document.
Outside of the BETA API set, there actually is not a way to do this in Word, Excel and PowerPoint. In the current BETA API (soon to be released), is the new Single sign-on (SSO) API. Detailed here:
I know, I know… you do not want to have to have the user sign-on and neither did we. But actually you do not need to. You make a call to getAccessToken() – after you have gone through the rather complex process of setting up your application in Azure AD – and in the returned token is the user information. Here is all you need for code to get to the username: