What is Hydration in Next.js?

Next.js is a popular and powerful framework for building server-side rendered (SSR) and statically generated (SSG) React applications. One of the core concepts in Next.js that plays a crucial role in optimizing performance and providing a seamless user experience is “hydration.” In this article, we will dive deep into understanding what hydration is in the context of Next.js and how it works to enhance the rendering process of React applications.

What is Hydration?

Hydration is the process of turning pre-rendered server-side content into an interactive user interface on the client side. When a user navigates to a page that has been pre-rendered on the server, Next.js sends the fully rendered HTML content to the client’s browser. This HTML content already contains the initial data and UI rendered by the server. The client-side JavaScript then takes over and “hydrates” this static content, attaching interactivity and turning it into a fully interactive React application.

In traditional client-side rendered (CSR) React applications, the browser downloads an empty HTML page and fetches JavaScript bundles that are responsible for rendering the complete UI and fetching data. This approach may lead to a noticeable delay before the user sees any content on the page, especially if the JavaScript bundle is large or the user has a slow internet connection. Next.js’s hydration process aims to improve this initial loading experience significantly.

How Hydration Works

The hydration process in Next.js can be broken down into the following steps:

1. Server-Side Rendering (SSR): When a user requests a page, Next.js renders the React components on the server, using the data available at that moment. The server generates a fully rendered HTML page that includes the initial state and data from the server.
2. Sending the HTML to the Client: Next.js sends this fully rendered HTML page to the client’s browser as the initial response to the request. This HTML content represents the complete static representation of the page.
3. Running Client-Side JavaScript: The client’s browser receives the HTML and starts executing the JavaScript bundles included in the page. The JavaScript bundles re-create the React components on the client side, using the same virtual DOM that was used on the server.
4. Comparison and Reconciliation: During the rehydration process, React on the client compares the server-rendered virtual DOM with the virtual DOM it creates on the client. This process is known as “reconciliation.” React efficiently updates the client-side virtual DOM to match the server-side virtual DOM.
5. Event Listeners and Interactivity: Once the rehydration is complete, the React components on the client side become fully interactive, and any event listeners or state changes work as expected. The user can now interact with the application just as if it were a CSR React app.

Benefits of Hydration

Hydration in Next.js provides several key benefits:

1. Faster Time to Interactive (TTI)

Traditional CSR React applications can suffer from a slow Time to Interactive (TTI), which is the time it takes for a page to become fully interactive and responsive. By using hydration, Next.js can reduce the TTI significantly. When the user receives the pre-rendered HTML from the server, they can see and interact with the content immediately, even before the JavaScript bundles are fully loaded and executed.

2. Improved SEO and Accessibility

Since Next.js supports SSR and SSG, search engines and social media crawlers can access the fully rendered HTML content directly, improving SEO and shareability. Additionally, users with disabilities who rely on assistive technologies can benefit from faster access to content and better accessibility.

3. Better Performance and User Experience

Hydration enhances performance by reducing the time it takes to load and interact with a page. Faster loading times and smooth interactivity lead to a better user experience, higher user engagement, and ultimately better conversions for web applications.

4. Graceful Degradation

In scenarios where JavaScript fails to load or execute properly due to user browser settings or network issues, Next.js gracefully degrades to the server-rendered content. This means users can still access the content and navigate through the application, even in suboptimal conditions.

5. Client-Side Optimizations

Even though the initial HTML content is fully rendered on the server, Next.js still includes client-side JavaScript bundles for further optimizations. These bundles allow for dynamic updates, code splitting, and other client-side rendering benefits once the page is hydrated.

Hydration and Client-Side Rendering (CSR)

It’s important to note that Next.js doesn’t strictly choose one approach over the other. Instead, it smartly combines the benefits of both server-side rendering and client-side rendering.

In a CSR-only approach, the server sends an empty HTML file, and the client-side JavaScript is solely responsible for rendering the complete application. This approach can result in a slower initial loading time as the browser needs to fetch and execute the JavaScript bundles before the user sees any content.

Next.js, however, offers the best of both worlds by providing SSR and SSG capabilities along with hydration. The initial HTML content generated by the server makes the application visible and interactive right away, while the client-side JavaScript handles further updates and dynamic changes.

How to Use Hydration in Next.js

Using hydration in Next.js is almost automatic, thanks to the framework’s default behavior. To enable hydration, you need to follow these standard steps:

1. Create Next.js Pages: Build your Next.js application by creating pages inside the pages directory. These pages can be simple React components or functional pages that fetch data from an API or a database.
2. Enable Server-Side Rendering (Optional): If you need server-side rendering for certain pages, you can use Next.js’s getServerSideProps function or getServerSideProps in combination with getStaticProps. This will ensure that the page content is pre-rendered on the server before being sent to the client.
3. Optimize for Static Generation (Optional): For pages that can be statically generated, use Next.js’s getStaticProps function. Static generation generates the HTML content at build time and can significantly improve performance.
4. Deploy Your Next.js App: Deploy your Next.js application using a hosting provider that supports server-side rendering, like Vercel or Netlify.

Once your Next.js app is deployed, hydration will automatically take place for the SSR and SSG pages. When a user visits your site, they will receive pre-rendered HTML, which will be quickly hydrated on the client-side, resulting in a smooth and responsive user experience.

Conclusion

Hydration is a crucial concept in Next.js that enhances the performance and user experience of React applications. By pre-rendering HTML on the server and then efficiently hydrating it on the client-side, Next.js achieves faster loading times, better SEO, and improved accessibility. The combination of server-side rendering, static generation, and hydration provides a powerful solution for building modern web applications that are both efficient and user-friendly.