How we can implement Server Side Rendering or SSR in react.js easily?

Answer

React is already equipped to handle rendering on Node servers.
A special version of the DOM renderer is available, which follows the same pattern as on the client side.

import ReactDOMServer from 'react-dom/server'
import App from './App'

ReactDOMServer.renderToString(<App />)
This method will output the regular HTML as a string, which can be then placed inside a page body as part of the server response.
On the client side, React detects the pre-rendered content and seamlessly picks up where it left off.