one of react's differentiating qualities is that it can render the entire page on the back-end, so there's no reason for the page to fail if the user does not run javascript.
Whilst this is often true, not all React code will render server side. You need to take care to make sure your React code stays isomorphic, which usually lasts right up to the point you discover some third party React component you reused would require significant rewriting to support server side rendering. That's how my personal projects usually end up anyway!