A given software system is a series of components that need each other to work. To test it you can either test all of it or you can treat it like a chain and test each link.
The current system I work on has a database, a Java backend, JSP views and uses AngularJS in the front-end
In order to comprehensivly test it you need a range of components. Featupent re flags can be tested by asserting that the page has certain elements present or absent. You can test for the presence of a given angular tag.
The angular components can be tested in a unit test framework in isolation.
I spent some of last year looking for these assertiobs in React only to find them in an angular library.