4.2 introduced a new widget framework that you could use if you wanted to.
Widget development | ArcGIS API for JavaScript 4.2
Besides that, UI development is completely up to you. We have a sample using Angular 2 and React.
The use of ViewModels and Accessor make it possible for you to build a UI in just about anything.
Here are some links off the top of my head.
Couple of Esri videos:
Choosing Frameworks - Home | Esri Video
Using Frameworks - Home | Esri Video
Other resources:
View Models in ArcGIS JS API - odoenet
Custom Widgets in the ArcGIS API for JavaScript - odoenet
GitHub - Esri/angular-esri-map: A collection of directives to help you use Esri maps and services in...
GitHub - Esri/esri-system-js: Load ArcGIS API for JavaScript modules using SystemJS
GitHub - Esri/calcite-maps: A theme for Bootstrap for designing, styling and creating modern map app...
To summarize, there is a new widget framework that is part of 4.2 that you can use and I'm pretty excited about it! But the ability to build a UI in just about any other Framework of your choice is fairly straightforward as well.
