In my programming, I have typically found that the more I do on the client-side, the better my application performs. This is due, in part, because we do not have to go across the network of the internet. This does not de-value the need of a server-side language component. In fact, all of my applications have a server-side component that provides the business logic.
I hope this high-level look at JS and the role it plays in the web space helps clarify things for you. If you have any questions, let me know.