Next.js Rendering: SSR vs SSG Explained

Overview

Next.js is a powerful React framework that solves the SEO and performance issues of traditional Single Page Applications (SPAs) by offering multiple pre-rendering techniques. The two most prominent are Server-Side Rendering (SSR) and Static Site Generation (SSG).

The Problem

Standard React apps (Create React App/Vite) send a nearly empty HTML file to the browser, relying entirely on the client's CPU to execute JavaScript, fetch data, and paint the UI (Client-Side Rendering - CSR). For a heavily content-driven site like a blog or an e-commerce store, this causes a blank white screen on initial load and prevents search engine crawlers from indexing the text, destroying the site's SEO rankings.

Solution and Configuration

Next.js solves this by generating the HTML before sending it to the client, but it offers two different timings for when this HTML is built.

• Static Site Generation (SSG): The HTML is generated exactly once, during the build process (next build). When a user requests the page, the server instantly sends the pre-built HTML file.
• Server-Side Rendering (SSR): The HTML is generated on-the-fly, on every single request. The server fetches fresh data, builds the HTML, and sends it to the user.

Technical Details

In the Next.js Pages Router, SSG is achieved using getStaticProps. Because the HTML is pre-rendered at build time, it can be cached on CDNs (Content Delivery Networks) worldwide, resulting in blazing-fast load times (TTFB - Time to First Byte). It is perfect for marketing pages and blogs.

SSR uses getServerSideProps. It is necessary for pages containing highly dynamic, user-specific data that changes constantly (like a shopping cart or a live stock market dashboard). Because the server must do the work on every request, SSR is inherently slower and more expensive to host than SSG. To bridge the gap, Next.js introduced Incremental Static Regeneration (ISR), which allows SSG pages to be rebuilt in the background at specific time intervals, offering the speed of static files with the freshness of SSR.

Conclusion

Choosing the right rendering strategy is the most crucial architectural decision in a Next.js application. The golden rule is to default to Static Site Generation (SSG) for maximum performance, and only fall back to Server-Side Rendering (SSR) when the data is absolutely dependent on the specific user's request context at that exact millisecond.