Mastering React Server-Side Rendering (SSR): A Deep Dive into SSR, CSR, and SSG

When building modern web applications with React, one of the most important architectural decisions revolves around how to render content. React offers three primary methods of rendering: Server-Side Rendering (SSR), Client-Side Rendering (CSR), and Static Site Generation (SSG). Each method has its advantages and trade-offs, depending on the use case and performance requirements.

In this blog, we’ll explain React Server-Side Rendering (SSR) in detail, explore how it compares with CSR and SSG, and provide use cases and scenarios where each method is most beneficial. We’ll also cover performance considerations and interview questions often asked about these rendering methods.

What is React Server-Side Rendering (SSR)?

Server-Side Rendering (SSR) is a technique where a web page is rendered on the server instead of in the browser. In SSR, the server generates the full HTML content for the requested page, which is then sent to the browser. This allows the user to see the content almost immediately, while JavaScript takes over to make the page interactive.

SSR provides several benefits, including faster initial load times, improved SEO, and better performance on slower devices or networks.

How SSR Works in React

When using SSR with React, the server renders the initial HTML content for a page based on the React components. Once the page is loaded in the browser, React takes over (hydration) and adds the interactivity.

React has built-in support for SSR using ReactDOMServer:

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

const express = require('express');
const app = express();

app.get('*', (req, res) => {
    const content = ReactDOMServer.renderToString(<App />);
    const html = `
        <!DOCTYPE html>
        <html>
        <head><title>React SSR</title></head>
        <body>
            <div id="root">${content}</div>
            <script src="/bundle.js"></script>
        </body>
        </html>
    `;
    res.send(html);
});

app.listen(3000, () => console.log('Server is running on port 3000'));

In this example, the React component is rendered to a string using ReactDOMServer.renderToString, which generates the HTML to be sent to the client.

Client-Side Rendering (CSR)

In Client-Side Rendering (CSR), the browser is responsible for rendering the entire application. The server sends a minimal HTML file, usually with a root <div> where the React app is mounted, and the actual rendering is done on the client after the JavaScript bundle is downloaded and executed.

CSR has been the default approach for many React applications because it provides flexibility in how content is rendered after the initial page load. However, CSR can suffer from slower initial load times, especially on slower networks or devices, because the JavaScript bundle must be downloaded and executed before any meaningful content is displayed.

CSR Workflow

The browser sends a request to the server.
The server responds with a minimal HTML file and a link to the JavaScript bundle.
The JavaScript bundle is loaded, and React takes over to render the content.

Here’s a simple example of CSR:

import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';

ReactDOM.render(<App />, document.getElementById('root'));

CSR is well-suited for applications where SEO isn’t a primary concern, such as internal dashboards or apps that require a high degree of interactivity after the initial load.

Static Site Generation (SSG)

Static Site Generation (SSG) is another approach where HTML pages are pre-rendered at build time, rather than on the server or client. These pre-rendered pages are served to users immediately, resulting in fast load times.

SSG works exceptionally well for sites with a lot of static content, like blogs, landing pages, or documentation sites. In React, tools like Next.js support SSG out of the box, allowing developers to pre-render pages at build time.

SSG Workflow

At build time, the pages are pre-rendered and saved as static HTML files.
When a user requests a page, the server serves the pre-rendered HTML directly, with no need for JavaScript execution for initial content.

Here’s a simple example using Next.js for SSG:

// pages/index.js
import React from 'react';

export async function getStaticProps() {
    const data = await fetchData();
    return {
        props: { data }
    };
}

const HomePage = ({ data }) => {
    return <div>{data}</div>;
};

export default HomePage;

With SSG, you get the benefits of fast load times, but there’s a trade-off: the content is static, meaning if the content changes frequently, it may require a rebuild to reflect the changes.

Comparing SSR, CSR, and SSG

Feature	SSR	CSR	SSG
Rendering	Server-side rendering of initial HTML	Client-side rendering using JavaScript	Pre-rendered static HTML at build time
SEO	Good SEO (content is available immediately)	Weaker SEO (initial page is often blank)	Excellent SEO (pre-rendered HTML is served)
Initial Load Time	Fast initial load (server-rendered HTML)	Slower (JavaScript needs to be executed)	Fast (pre-rendered content)
Performance	Great for SEO and initial page load	Great for highly interactive apps	Great for static content or blogs
Use Cases	E-commerce, content-heavy sites	Single-page applications, dashboards	Blogs, marketing sites, documentation
Interactivity	Hydration after the initial render	Full interactivity after page load	Requires client-side JavaScript for interactivity

When to Use SSR, CSR, and SSG

Use SSR if your application requires fast initial load times and good SEO, such as content-heavy websites, e-commerce platforms, or pages with dynamic content that needs to be indexed by search engines.
Use CSR if your app is heavily interactive, like a dashboard or an internal tool, where SEO isn’t a concern. CSR allows for greater control over the user interface after the page is loaded.
Use SSG when you have static content that doesn’t change often, like a blog or marketing site. SSG ensures blazing-fast load times and excellent SEO out of the box.

React SSR vs CSR vs SSG: Pros and Cons

Server-Side Rendering (SSR)

Pros:
Improved SEO and faster time-to-content.
Great for dynamic applications where content changes frequently.
Works well with slower networks and devices by reducing the amount of JavaScript required on initial load.
Cons:
Increased server load (every request requires rendering on the server).
Can be more complex to set up compared to CSR or SSG.

Client-Side Rendering (CSR)

Pros:
Great for highly interactive applications.
Offloads rendering from the server, reducing server costs.
More flexible UI updates once the JavaScript bundle is loaded.
Cons:
Slower initial load times as JavaScript has to be downloaded and executed.
Poor SEO unless additional techniques (e.g., SSR or pre-rendering) are applied.

Static Site Generation (SSG)

Pros:
Extremely fast load times since the content is pre-rendered.
Perfect for websites with mostly static content, like blogs or marketing pages.
Less server load because the same HTML is served to every user.
Cons:
Dynamic content can be harder to manage (rebuilding pages for new content).
Requires build-time processes to update content, which can be slow for large sites.

Interview Questions on React SSR, CSR, and SSG

What is the difference between Server-Side Rendering (SSR) and Client-Side Rendering (CSR)?

Answer: SSR renders the initial HTML on the server and sends it to the client, improving SEO and initial load times. CSR renders content in the browser using JavaScript, which can lead to slower initial load times but allows for more interactive applications.

When would you choose SSR over CSR in React?

Answer: SSR is preferred when you need better SEO and faster initial content rendering, such as for blogs, news sites, or e-commerce platforms where content needs to be indexed by search engines.

What are the key benefits of using Static Site Generation (SSG) in React?

Answer: SSG provides faster load times and excellent SEO by serving pre-rendered HTML at build time. It’s ideal for sites with static content, like blogs or marketing pages.

Can you explain how ReactDOMServer.renderToString() works in SSR?

Answer: ReactDOMServer.renderToString() is used to render React components to HTML on the server. It generates a string of HTML which can then be sent to the client, allowing for fast initial rendering and improved SEO.

How does Next.js handle SSR and SSG?

Answer: Next

.js provides out-of-the-box support for both SSR and SSG. SSR is achieved by rendering pages on the server per request, while SSG pre-renders pages at build time, making them static.

Mastering React Server-Side Rendering (SSR): A Deep Dive into SSR, CSR, and SSG

What is React Server-Side Rendering (SSR)?

How SSR Works in React

Client-Side Rendering (CSR)

CSR Workflow

Static Site Generation (SSG)

SSG Workflow

Comparing SSR, CSR, and SSG

When to Use SSR, CSR, and SSG

React SSR vs CSR vs SSG: Pros and Cons

Server-Side Rendering (SSR)

Client-Side Rendering (CSR)

Static Site Generation (SSG)

Interview Questions on React SSR, CSR, and SSG

Additional Essential JavaScript Interview Questions on Various Topics

React Js Interview questions:

Top Javascript Books to Read

Conclusion

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