There are several useful widgets available for the web appbuilder that are not available, out-of-the-box in the ArcGIS JS API. It would be great if it was possible to reuse these widgets in an ArcGIS JS application. So my question is:
Is it possible to use web appbuilder widgets within a classic ArcGIS JS API application?
It could possibly be done by a very experienced developer, but there are many dependencies that WAB widget have to are to the core WAB jimu libraries so that will make it very difficult. So the short answer is no, not without a major re-write.