React 18: Partial Hydration

Understanding Hydration

Hydration is the process of rendering components and attaching event handlers. It "waters" the static HTML produced by the server with interactivity. React frameworks, like Next.js, perform hydration to turn static pages into interactive apps.

However, traditional hydration can come with performance drawbacks. JavaScript needs to be loaded and executed, which can cause delays, especially on slower devices and networks. This is where Partial Hydration and Islands Architecture come into play.

What is Partial Hydration?

Partial Hydration is a strategy that allows developers to only hydrate the parts of a page that need interactivity. This minimizes the amount of JavaScript needed and improves load times. Instead of hydrating the entire page, developers define interactive components as "islands," reducing JavaScript overhead and improving First Input Delay (FID).

How Does SSR Work?

In Server-Side Rendering (SSR), data is fetched and HTML is generated on the server. The browser then loads the JavaScript to make the page interactive. This process is known as Hydration.

React 18 introduces two new features for SSR: Streaming HTML and Selective Hydration.

Streaming HTML

Streaming HTML allows the server to start sending HTML as soon as possible, without waiting for all data to load. It streams the rest of the content as it becomes available.

Selective Hydration

Selective Hydration enables early interaction by prioritizing the components users are engaging with. This creates the illusion of instant interactivity, even if some parts of the page are still loading.

Methods to Implement Partial Hydration

• Static Routes (No Hydration): Deliver purely static HTML with no client-side interactivity.
• Lazy-loading JavaScript (Progressive Hydration): Load JavaScript progressively, hydrating components as needed.
• Islands Architecture: Define parts of your page as static and only hydrate necessary components.
• Out of Order Hydration: Hydrate components as they arrive, regardless of their order in the document.

Benefits of Islands Architecture

Islands Architecture reduces duplication and allows for fine-grained control over which components are hydrated. It results in smaller JavaScript bundles, improved load times, and better performance, especially on mobile devices.