COM isn't for headless, it is for interacting with the application programatically. VBA communicates with Office via COM. You can interact from within an excel

COM isn't for headless, it is for interacting with the application programatically. VBA communicates with Office via COM. You can interact from within an excel session (for instance through VBA or an Excel-DNA addin) or from outside of an excel session (by launching an excel session from your script). It's not headless in the sense that it will open a normal excel sessions with the UI (and potential for modal dialog boxes blocking your code). COM however does not allow you to create your own user defined functions in excel, so for that you need to create an xll (for instance via Excel-DNA). I am less familiar with the javascript API but I understand it is newer, and meant to create components for office 365, it does not offer the full access to the office API.