Server-Side Rendering refers to the process of rendering a web application on the server, instead of in the browser. The server generates HTML content based on the initial state of the React components, and sends this HTML to the client. The client's browser then displays the HTML while loading the JavaScript bundles in the background to take over and enable a dynamic experience.

SSR is particularly useful for improving initial page load times, which can enhance the perception of performance for end-users and contribute positively to SEO efforts.

React 18 introduces several enhancements that make SSR more powerful and easier to implement. Here’s a breakdown of the key SSR features:

Automatic Batching

React 18 introduces automatic batching for updates, regardless of the origin of the update. This means that updates from fetch requests, timeouts, native event handlers, or any other event will now be batched together, reducing the number of re-renders required. While primarily a client-side improvement, automatic batching also benefits SSR by creating a more efficient hydration process.

Streaming SSR

One of the most exciting features in React 18 is Streaming SSR. This allows React components to be streamed to the client as they are rendered, rather than waiting for the entire application to render before sending it. This incremental approach means that users can start seeing and interacting with content faster than ever before.

Concurrent Features

Concurrent React allows components to prepare multiple versions of the UI at the same time. This is particularly beneficial in an SSR context as it enables the server to render more useful content quicker, even under heavy load or when waiting for data fetching to resolve.

Server Components

React 18 moves forward with the experimental concept of Server Components, which allow developers to build components that only run on the server. These components can do server-side only work (like querying a database directly), and send minimal JavaScript to the client. This reduces the overall size of your JavaScript bundle and decreases load times.