What is SSR and SPA?

If you're already familiar with SSR and SPA and you only want to decide which one to choose, see Guides > SSR vs SPA instead.

If you're new to modern web development with JavaScript and UI frameworks like React or Vue, you might have heard the terms SSR (Server-Side Rendering) and SPA (Single Page Application) but aren't sure what they mean.

This guide explains both concepts and their differences.

JavaScript has a unique capability: it can run both in the browser (browsers support only two languages: JavaScript and WebAssembly) and on the server (using a JavaScript runtime such as Node.js).

For example:

// Welcome.jsx
 
function Welcome() {
  return (
    <>
      <h1>Welcome to my blog</h1>
      <p>This is an article about web development...</p>
    </>
  )
}

Vite compiles this JSX component to JavaScript that can execute on both the client and server.

SPA refers to rendering components exclusively on the client-side (in the browser):

// render.js
// Environment: client
 
import { Welcome } from './Welcome'
 
// Render <Welcome> on the client-side
import { createRoot } from 'react-dom/client'
const root = createRoot(document.getElementById('root'))
root.render(<Welcome />)

The HTML sent to the browser is just an empty shell that delivers the client-side JavaScript:

<html>
  <body>
    <!-- Empty container: content will be added dynamically by render.js -->
    <div id="root"></div>
    <script src="/render.js"></script>
  </body>
</html>

SSR (Server-Side Rendering) refers to rendering components to HTML on the server-side and then hydrating them (making them interactive) on the client-side.

// ssr.js
// Environment: server
 
import { Welcome } from './Welcome'
 
// Render <Welcome> to HTML on the server-side
import { renderToString } from 'react-dom/server'
const html = renderToString(<Welcome />)

// hydrate.js
// Environment: client
 
import { Welcome } from './Welcome'
 
// Hydrate <Welcome> on the client-side
import { hydrateRoot } from 'react-dom/client'
hydrateRoot(document.getElementById('root'), <Welcome />)

In this example, hydration is unnecessary since <Welcome> isn't interactive. However, interactive components (e.g. <Counter>) require hydration to function properly.

<html>
  <body>
    <!-- Content is rendered to HTML -->
    <div id="root">
      <h1>Welcome to my blog</h1>
      <p>This is an article about web development...</p>
    </div>
    <script src="/hydrate.js"></script>
  </body>
</html>

Notice that with SSR, the content of <Welcome> is included in the HTML, while with SPA it's missing.

This is the key difference between SSR and SPA: with SSR, the page's content is included in the HTML, whereas with SPA, the HTML is just an empty shell.

This difference has important implications for:

Crawlers (search engines, social sites, AI)
Performance (particularly on mobile devices or slow connections)

Crawlers

The crawlers of search engines (e.g. Google, Bing), social sites (e.g. Instagram, Twitter), and AI (e.g. ChatGPT) navigate your website by reading the HTML of your pages and following <a href="/some-other-page"> links.

With SPA, crawlers only see an empty HTML:

<!-- ❌ Crawlers don't see the page's content -->
<div id="root"></div>

With SSR:

<!-- ✅ Crawlers see the page's content -->
<div id="root">
  <h1>Welcome to my blog</h1>
  <p>This is an article about web development...</p>
</div>

Google's crawler can execute client-side JavaScript, but we still recommend using SSR even if you only target Google.

Why SSR is still recommended for Google:
Google's crawler is unique because it can execute client-side JavaScript. This means Google can index the content of SPA apps, but there are several limitations:

Unreliable execution: JavaScript execution may fail or time out.

Delayed indexing: Google processes JavaScript content in a second wave, which can delay indexing.

Performance impact: Heavy JavaScript can cause crawling issues.

Future uncertainty: Google may discontinue JavaScript execution as SSR has become ubiquitous (Google started JavaScript execution at a time when SSR wasn't as widespread as it is today).

For these reasons, SSR remains the recommended approach even if you only target Google.

Performance

SSR provides faster initial page load, especially on mobile devices where JavaScript loading and execution is significantly slower. With SPA, users may see a blank page for several seconds while JavaScript loads and executes.

Subsequent page navigation (also known as client-side navigation) is equally fast between SSR and SPA.

Initial page load comparison (mobile device)

Time	SPA	SSR
0s	User visits page	User visits page
1s	HTML loaded (empty content)	HTML loaded (with content) ✅ User sees content
3s	JavaScript downloaded	JavaScript downloaded
5s	JavaScript executed (content is rendered) ✅ User sees content	JavaScript executed (content is hydrated)

See also: Guides > SSR vs SPA

SPA is a misnomer

The term SPA (Single Page Application) has become a misnomer in modern web development.

Historically, SPA referred to applications that truly had only one page — like Gmail — where you never navigate to different URLs and everything happens within a single interface.

Today, SPA is commonly used to mean not SSR — referring to any client-side rendered application, even those with multiple pages and different URLs. While technically incorrect, this usage has become widely accepted.

A more accurate term would be CSR (Client-Side Rendering), but "SPA" is the established terminology in the web development community. That's why we use the term SPA in Vike's documentation.