CSR vs SSR: Key Differences Explained

Overview

React (CSR) Flow

Initial Server Response

<!DOCTYPE html>
<html>
  <head>...</head>
  <body>
    <div id="root"></div>
    <script src="/static/js/bundle.js"></script>
  </body>
</html>

Subsequent Requests

Browser requests JavaScript bundles (usually split into chunks):
- main.chunk.js (application code)
- vendor.chunk.js (dependencies)
- runtime.js (webpack runtime)
All static files served from /build directory
Client-side rendering begins only after JS loads
Data fetching starts after JS execution

Next.js (SSR) Flow

Initial Server Response

<!DOCTYPE html>
<html>
  <head>
    <script src="/_next/static/chunks/main.js" defer></script>
  </head>
  <body>
    <div id="__next">
      <!-- Fully populated HTML content -->
      <!-- Server Component Payload -->
    </div>
  </body>
</html>

File Structure After Build

.next/
  ├── server/
  │   ├── pages/
  │   │   └── [page].html
  │   └── chunks/
  ├── static/
  │   ├── chunks/
  │   ├── css/
  │   └── media/

Key Differences in File Serving

React (CSR)
- Serves static files from single build directory
- Initial HTML is empty
- Requires complete JS load for any content display
- All routing handled client-side
Next.js (SSR)
- Serves from .next directory with specialized structure
- Initial HTML contains full content
- JS files load for hydration but content visible first
- Server handles routing and generates specific HTML per route

Performance Implications

React: Larger initial JavaScript bundle, slower first contentful paint
Next.js: Faster first contentful paint, smaller initial JS payload, better SEO

Browser Processing Order

React:

Empty HTML → 2. JS Bundles → 3. Render Content → 4. Data Fetching

Next.js:

Full HTML → 2. Display Content → 3. JS for Hydration → 4. Interactive

This comprehensive approach shows why Next.js typically provides better initial load performance and SEO capabilities, while React's CSR approach might be more suitable for highly interactive applications where initial load time is less critical.

CSR vs SSR

Table of contents