It is not necesssary to execute JavaScript on the server-side, or to use React.js at all (although it is a pretty neat tool.) For instance in Backbone, you can call setElement() on any view to replace its on-screen element with one that you've pre-rendered. If you're clever about templates this means that you can have views that both know how to build themselves up from zero, and can also attach to existing views on screen so you can save yourself that rendering time when you visit the page.
This could result in code duplication, but the end result is the same: a complete state of the application can be rendered server-side and then can evolve independently on the client side.
This could result in code duplication, but the end result is the same: a complete state of the application can be rendered server-side and then can evolve independently on the client side.