Simple Outlook Online Calendar Plugin

I need to port a simple existing Outlook VSTO plugin to Outlook Online plugin (node.js). The plugin reads/shows and sets custom user properties in the appointment item. Source code to the current VSTO plugin can be provided. The new plugin needs to be able to run in all modern Outlook implementations (iOS, MacOS, Web, native Windows) and should be packaged and ready to deploy to our Office 365 tenant.