Microsoft technologies keep changing, if you want to develop addins for Microsoft outlook, you can choose VSTO api or nodejs api which is javascript based. VSTO api just like traditional outlook addins development, everything done in visual studio and output a Dll file for you to deploy the addins. Nodejs api is different, when you develop addins with it, both outlook client and outlook web base can run your addins, I think I should go for nodejs.
VSTO tutorial https://msdn.microsoft.com/en-us/library/cc668191.aspx
Nodejs tutorial https://docs.microsoft.com/en-us/outlook/add-ins/addin-tutorial
JavaScript API for Office reference https://dev.office.com/reference/add-ins/javascript-api-for-office