Delegate Controls in SharePoint 2019 Modern Experience

Question

Are delegate controls supported in the modern experience of SharePoint 2019? If so, how are they used?

I have created a delegate control to place a user control in the AdditionalPageHead area with a simple alert in JavaScript. This alert will work in the classic SharePoint view, but in the modern experience it does not. Is there any documentation around this change?

Answer 1

Problem is that modern experience does not utilize master page as we know it from previous versions of SharePoint and all customizations (like custom actions in ribbon or ECB, Client Side Rendering etc.) are not available in modern experience and different approach needs to be used.

If you want to implement something that is available at all pages, to mimic registering some Javascript that runs everywhere, you have to implement SharePoint Framework Extension.