Node.js can be used as the backend for server-side rendering instead of execJS. Before you try this, consider the tradeoff of extra complexity with your deployments versus potential performance gains. While often ExecJS with mini_racer is "fast enough", we've heard of other large websites using Node.js for better server rendering performance.
If you want to use a node server for server rendering, get in touch. ShakaCode has built a premium Node rendering server for React on Rails.