React Hydration Explained: CSR vs SSR vs SSG, Errors & Best Practices

Hydration is a powerful feature in React that lets server-rendered or static HTML become a fully interactive React app in the user's browser. Let's break down what it means, why it's useful, and how you can use it effectively!

What is Hydration in React?

Hydration is the process which React takes HTML generated by the server (SSR) or during build time (SSG), and attaches event listeners, state, and interactivity with client-side JavaScript. The user sees a page instantly, then React “wakes up” and makes it interactive without rebuilding the whole DOM.

Three steps in hydration:

1. Server/Static Rendering: HTML is generated for the view and sent to the browser.
2. Hydration in Browser: React attaches JavaScript, event handlers, and internal state to the existing HTML.
3. Interactivity: App behaves just like a client-rendered React app.

SSR vs SSG vs CSR: Key Differences

Strategy

How It Works

Pros

Cons

CSR (Client Side Rendering)

Builds all HTML in the browser from JS on a blank page

Simple, fast updates

Slow first load, poor SEO

SSR (Server Side Rendering)

Server sends HTML per request; React hydrates on client

Instant paint, better SEO

More server load, slower for heavy content

SSG (Static Site Generation)

HTML pre-built at deploy, served as static files, hydrated after load

Lightning-fast, great SEO, scales well

Static content unless rebuilt

Hydration is only relevant for SSR & SSG, because HTML already exists when JS loads!

How Hydration Works: Example

SSR Example:

• Server: Generates <button>Click Me</button> as HTML.
• Browser: React hydrates with <button onClick={...}>Click Me</button>.
• Click now works, state is tracked.

Progressive Hydration:

Hydrate only critical components first, defer non-essential ones for faster interactivity. Use Suspense and dynamic imports:

Common Hydration Errors and Mismatches

Hydration errors happen when the server-rendered HTML does not match what's rendered by React on the client.

Causes:

• Content depends on window, localStorage, random values, timers, etc.
• HTML tags incorrectly nested.
• CSS-in-JS/JS-modified styles that differ.
• Browser extensions or CDNs modifying HTML .

Fixes:

• Separate code that runs only on the client into useEffect.
• Use suppressHydrationWarning={true} for expected mismatches (use sparingly).
• Disable SSR for specific components using dynamic imports (ssr: false in Next.js).
• Always produce identical HTML server- and client-side.

Best Practices and Performance Considerations

• Keep SSR/SSG outputs deterministic: Don’t use random data or browser-only APIs outside useEffect.
• Selective Hydration: Only hydrate what's needed first; use Suspense for lower-priority content.
• Streaming SSR: Send critical HTML first, hydrate remaining UI as resources load.
• Error Handling: Identify mismatched components using error traces and debugging tools.
• Test hydration locally: Check consistency between server and client output in development.

Real-World Example: Next.js SSG and SSR

Static Site Generation:

SSR Example:

React hydrates these after initial load, making dynamic interactions possible.

Conclusion

By understanding React hydration and recognizing common errors, you can build apps that load fast, stay interactive, and provide a seamless user experience, whether you’re using SSR, SSG, or a mix. Hydration bridges the world of static HTML and dynamic React!