What are the best client-side JavaScript application frameworks?

Angular is a very big codebase, and while it does handle most of what you'd need to do out of the box, if you do find the need to extend the base functionality, or look into the internals, or debug deep into the framework, it is much harder to comprehend than other frameworks.

Because the internals are so integrated with each other, debugging requires you to have a good grasp on many layers of abstraction at once.