Server-side rendering in ReactJS is done by JavaScript (NodeJS or io.js). This method actually prerenders the initial state of the React components at the server side. Thus, it is helpful in fast rendering of web pages, as the users can see the web pages without having to wait for the entire JavaScript at the client side to finish loading.
However, this kind of rendering should not be used for those applications where a huge amount of data has to be piped from the server to the client side, which may slow the page load. In such cases, we may use pagination or bulk load the data in chunks, which won't slow the page load, but can be fetched from the server side in specific time intervals.
The following two methods from the React API provides the backbone of server-side rendering (https://facebook.github.io/react/docs/top-level-api.html).