Mastering Progressive Hydration for Enhanced Web Performance

Every day, thousands of modern web applications are developed and launched, and with this proliferation comes increasing complexity. Over time, many developers encounter challenges related to optimizing application performance.

Progressive hydration is one technique to address these issues. By gradually loading interactive components, developers can enhance the user experience, especially in large applications. Whether you’re creating an e-commerce site or a dashboard, progressive hydration can reduce load times, improve performance and enable a seamless experience. If you’re using frameworks like React or Next.js, you already have the necessary tools to get started.

Progressive Hydration vs. Lazy Loading: What’s the Difference?

While lazy loading and progressive hydration are related concepts, they are not the same. Lazy loading delays loading resources (such as images, JavaScript code or components) until they are needed. Instead of loading everything upfront, only the parts of the application that are visible or necessary at the moment are loaded. This approach is often used to improve initial page-load performance by breaking large code or asset bundles into smaller chunks and loading them on demand, as the user scrolls.

Progressive hydration is a more specialized technique used primarily in server-side rendering (SSR) applications. When a webpage is initially rendered on the server, the HTML is sent to the client, and JavaScript is used to “hydrate” the static HTML, turning it into an interactive React (or similar framework) application. Instead of hydrating the entire application simultaneously, progressive hydration focuses on only hydrating interactive components and other critical parts first, deferring the rest until needed. This improves the time-to-interactive (TTI) metric and is particularly beneficial for large SSR applications where you don’t want to block user input while waiting for the entire app to be hydrated.

While progressive hydration can incorporate lazy loading to progressively hydrate and load components, its main goal is optimizing the partial hydration process after server-side rendering, whereas lazy loading is used to optimize resource loading in both client-side and server-side contexts.

How Progressive Hydration Improves Performance

Traditional hydration methods can lead to significant performance challenges, such as users having to wait for the entire page to hydrate before they can interact with it, negatively impacting TTI. Large JavaScript bundles are sent all at once, slowing the page and increasing load times — particularly in large applications like complex e-commerce platforms or data-heavy dashboards.

However, by hydrating only the essential parts of the page initially, users can interact with critical features (such as navigation and buttons) while secondary, non-urgent components (like footers and less-used widgets) hydrate later. Progressive hydration’s pros include reducing blocking time and enhancing metrics like first contentful paint (FCP) and first input delay(FID).

Imagine an e-commerce site where a user is primarily interested in product details and checkout options. With traditional hydration, the entire page — including the footer, product reviews and sidebar — must be hydrated before the user can engage. In contrast, progressive or partial hydration ensures that product information and checkout buttons become interactive almost immediately, allowing less critical elements to hydrate in the background.

Progressive hydration is selectively applicable in certain frontend frameworks, such as React and Next.js, that support SSR.

How Progressive Hydration Works

Progressive hydration involves three key steps:

1. Initial load: The application sends a minimal amount of HTML and JavaScript code to the client. This typically includes the critical UI components necessary for initial user interaction.
2. Loading priority: The hydration process is divided into two phases:
   • Critical hydration: Core elements essential for immediate user input are hydrated first. This may include navigation bars, important forms or other interactive components.
   • Deferred hydration: Components deemed less critical (such as images, modals or secondary content) are hydrated after the critical components are fully interactive. This can encompass off-screen or not-yet-visible components.
3. ‍Background loading: As users interact with the application, the remaining non-urgent components are hydrated in the background. This can be efficiently managed through techniques like lazy loading, where components are loaded only when they enter the viewport or as needed.

Implementing Progressive Hydration

To use progressive hydration, you need to initialize a React application. If you require assistance creating a React app, I recommend consulting how to create a React application using Vite.

Once you have scaffolded your base application, navigate to app.js if you use JavaScript, or app.tsx if you opt for TypeScript.

For your application’s initial rendering, you must prioritize the most important components users will interact with right away. In a simple React app, this could include key elements such as the header and the main content area. By focusing on these critical components, users can engage with the core functionality of your app almost immediately, while other components can be progressively hydrated as needed.

Replace the existing code in app.js with:

import React from 'react';
import DeferredComponent from './DeferredComponent'; // Importing the DeferredComponent

const Header = () => <header><h1>My App</h1></header>;
const MainContent = () => <main><p>Welcome to the app!</p></main>;

const App = () => {
 return (
   <div>
     <Header />
     <MainContent />
     {/* Defer loading of this component */}
     <DeferredComponent />
   </div>
 );
};

export default App; // Don't forget to export the App component

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Now, create another JavaScript file in the same directory as app.js named DeferredComponent.js (or a name of your choice). Place the JavaScript code below in it; it will export your second component in the progressive hydration implementation.

import React, { lazy, Suspense } from 'react';

// Lazy load the NonCriticalComponent
const NonCriticalComponent = lazy(() => import('./NonCriticalComponent'));

const DeferredComponent = () => (
 <Suspense fallback={<div>Loading...</div>}>
   <NonCriticalComponent />
 </Suspense>
);

export default DeferredComponent; // Export the DeferredComponent

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Then, create a third file called noncriticalcomponent.js, which will host the NonCriticalComponent that will be imported in the DeferredComponentabove.

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import React from 'react';

const NonCriticalComponent = () => {
 return <div>This is non-critical content that was deferred.</div>;
};

export default NonCriticalComponent;

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If you wish, you can implement background hydration using an event listener or visibility API to ensure that non-critical components are hydrated when the user is likely to see them. To achieve this, you must make changes to the NonCriticalComponent.js file.

// Use Intersection Observer to trigger hydration when component is in view
import React, { useEffect, useState } from 'react';

const NonCriticalComponent = () => {
 const [isVisible, setIsVisible] = useState(false);

 useEffect(() => {
   const observer = new IntersectionObserver(([entry]) => {
     if (entry.isIntersecting) {
       setIsVisible(true);
       observer.disconnect();
     }
   });
  
   const element = document.querySelector('#non-critical');
   if (element) observer.observe(element);

   return () => observer.disconnect();
 }, []);

 if (!isVisible) return <div id="non-critical">Loading...</div>;

 return <div>Non-Critical Content Loaded!</div>;
};

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What’s Happening Under the Hood?

I’ll take a moment to explain what’s happening in the app.js file. First, it imports React and the DeferredComponent from another file. The Headercomponent is a simple functional component that renders a header with the app’s title, while the MainContent component displays a welcoming message.

The main App component returns a div that contains the Header, MainContent and DeferredComponent. The DeferredComponent includes a comment indicating that its loading will be deferred to focus on the rendering of the essential components first. Finally, it exports the Appcomponent for use in other parts of the application.

The DeferredComponent file first imports React along with the lazy and Suspense components from the React library. Next, it uses the lazyfunction to dynamically import the NonCriticalComponent. This means the NonCriticalComponent will not be loaded until it is needed, helping to optimize performance by reducing the application’s initial load time.

The DeferredComponent is defined as a functional component that uses Suspense. Within it, a fallback prop provides a loading indicator (in this case, a simple <div>Loading...</div>) that will be displayed while the NonCriticalComponent is being loaded. Once the component finishes loading, it is rendered in place of the fallback content.

Next, exporting the DeferredComponent makes it available for use in other parts of the application. This setup allows it to defer loading non-critical components, ensuring that the main parts of the application are prioritized during initial rendering.

Finally, the initial code in NonCriticalComponent imports React from the React library and defines a functional component called NonCriticalComponent. This component returns a simple JSX element, specifically a <div> that contains the text: “This is non-critical content that was deferred.” This represents content that is not essential for the application’s initial rendering; deferring its loading can improve the app’s performance by allowing users to interact with more critical components first.

The updated NonCriticalComponent introduces background hydration to enhance loading. The useState hook creates a state variable, isVisible, which tracks whether the component is in view. The useEffect hook sets up an IntersectionObserver to monitor the component’s visibility in the viewport. When the component becomes visible, the observer updates isVisible to true and disconnects.

If the component is not visible, it returns a <div> with the text “Loading…” to indicate it is waiting to be hydrated. Once it is in view, the message changes to “Non-Critical Content Loaded!” This approach allows the non-critical component to be hydrated only when it enters the viewport, further minimizing performance impact by deferring loading until the user needs the content.

Upon initial load, the Header and MainContent components are rendered right away, providing users immediate access to essential elements and interaction. Meanwhile, the DeferredComponent, which contains the non-critical component, starts loading in the background through React’s lazy loading mechanism.

During this phase, a fallback user interface (UI), such as “Loading…”, informs users that additional content is being fetched or processed, thus enhancing perceived performance. Once the NonCriticalComponent has fully loaded, it replaces the fallback message, offering additional functionality or information that enhances the overall user experience.

This allows users to interact with the header and main content without loading the entire application, resulting in a smoother experience, especially on slower networks. Overall, the application’s progressive loading makes essential features available while additional content is added as it becomes ready.

Evaluate Page Load Performance

There are several methods available to proper evaluation of progressive hydration performance impacts. One approach is tracking loading time metrics using the Performance API, which allows you to log timestamps at key points, such as initial page load and when critical components are fully rendered.

Tools like Lighthouse can measure important metrics like FCP and TTI. User experience testing, where you gather feedback on perceived loading times and responsiveness, can also offer valuable insights.

Testing the application under varying network conditions, such as 3G or 4G, using developer tools like Chrome DevTools can reveal how progressive hydration impacts performance on slower connections. Performance profiling tools, such as the React Profiler, can analyze the rendering times and interactivity of components. Lastly, real user monitoring (RUM) solutions like Google Analytics or Sentry can provide real-world data on user interactions and performance metrics, offering a clear view of how the app performs under actual usage conditions.

Conclusion

Progressive hydration offers a powerful solution for enhancing the performance and user experience of modern web applications. By deferring loading non-critical components, developers can prioritize essential content, allowing users to interact with key features faster, even on slower networks. This technique not only improves perceived performance but also ensures a smoother, more responsive interface as the rest of the content progressively loads in the background.

By incorporating lazy loading, IntersectionObserver and performance monitoring tools, developers can fine-tune applications for optimal speed and user satisfaction. As web performance becomes increasingly important, a progressive hydration strategy is an effective way to deliver efficient, user-friendly applications.

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About The Author: Zziwa Raymond Ian

Zziwa Raymond Ian is a full-stack engineer and a member of the Andela Talent Network, a private global marketplace for digital talent. Specializing in Next.js, React, JavaScript, TypeScript, NestJs and others, he has developed a deep holistic understanding of both frontend and backend technologies. Zziwa loves to tackle diverse and difficult technology challenges, as they are a driving force in his continuous learning.