Understanding Hydration in Next.js

Technology makes what was once impossible possible. The design makes it real.

[— Michael Gagliano]

Next.js is renowned for its robust capabilities in constructing server-side rendered (SSR) and statically generated (SSG) React applications. Central to Next.js's optimization strategies and seamless user experiences is a concept called a "hydration."

What is Hydration?

Hydration is the process of transforming pre-rendered server-side content into an interactive user interface on the client side. When a user accesses a pre-rendered page, Next.js promptly sends the fully rendered HTML content to the client's browser, complete with initial data and UI from the server. Then, client-side JavaScript "hydrates" this static content, imbuing it with interactivity to form a fully functional React application.

In contrast to traditional client-side rendered (CSR) React apps, where the browser retrieves an empty HTML page followed by fetching JavaScript bundles for rendering and data acquisition, Next.js's hydration process significantly mitigates initial loading delays.

How Hydration Works in Next.js

To understand hydration in Next.js, it’s helpful to break down its process step by step:

1. Server-Side Rendering (SSR): When a user requests a page, Next.js first renders the React components on the server, including any available data at that time. This results in a fully-rendered HTML page that contains the state and data provided by the server.

2. Sending the HTML to the Browser: Next.js then sends this fully-rendered HTML page as the initial response to the client’s browser. This page contains the static HTML representation of the page content.

3. Running Client-Side JavaScript: After the HTML is loaded, the browser begins executing the JavaScript bundles included in the page. These bundles re-build the React components on the client side using the same virtual DOM that was used on the server.

4. Comparison and Syncing: During the hydration process, React compares the virtual DOM generated on the server with the virtual DOM on the client. This step, called "reconciliation," ensures the client-side DOM matches the pre-rendered HTML, allowing React to synchronize the two versions.

5. Event Handling and Full Interactivity: Once hydration is complete, the React components on the client are fully interactive, enabling event handling and state transitions, just like a traditional client-side React app.

Why Hydration is Beneficial in Next.js

Hydration in Next.js brings several key benefits:

1. Faster Time to Interactive (TTI): By reducing the Time to Interactive, hydration improves the responsiveness of the page. Since the HTML is already rendered on the server, users can start interacting with the page quickly, without waiting for JavaScript bundles to load.

2. Better SEO and Accessibility: With SSR and SSG, Next.js enhances SEO and accessibility by delivering fully rendered HTML content. This makes it easier for search engines to crawl and index the page and ensures the content is accessible to users with disabilities.

3. Improved Performance and User Experience: Hydration contributes to faster page loads and smoother interactions, resulting in a more enjoyable user experience and higher engagement.

4. Graceful Fallback: In cases where JavaScript fails to load or run, Next.js can fall back to the server-rendered HTML, ensuring that the content remains accessible and users can still navigate the site.

5. Client-Side Enhancements: Although the initial page load is server-rendered, Next.js uses client-side JavaScript to enable dynamic updates, code splitting, and other client-side performance optimizations after hydration.