Improving Core Web Vitals including LCP and CLS with Partial Hydration in Angular 18

Angular 18 introduced Partial Hydration in ng-conf 2024, a powerful technique that significantly improves application performance in conjunction with Server-Side Rendering (SSR). This article dives into the concept of partial hydration, its benefits, and how it leverages deferrable views introduced in Angular 17.

According to Angular Roadmap:

We’re already seeing significant improvements to Core Web Vitals, including LCP and CLS. In lab tests, we consistently observed 45% better LCP of a real-world app.

Why Partial Hydration and SSR Matter

Traditional Angular applications often suffer from a performance bottleneck when loading all JavaScript upfront. This can significantly impact the initial load time, especially for large and performance-critical applications. By strategically reducing the amount of JavaScript loaded at the start, we can drastically enhance user experience.

Partial Hydration: A Smarter Approach to SSR

Partial hydration builds upon the foundation of deferrable views, introduced in Angular 17. Instead of rendering a simple placeholder on the server, Angular can now render the main content of a designated block marked with @defer. Here's how it works:

1. Server-side Rendering: The server renders the essential content of the application along with the @defer block containing the component.
2. Client-side Hydration: When the application runs on the client, Angular downloads the necessary JavaScript for the deferred component.
3. Selective Activation: The deferred component only becomes interactive when it meets specific conditions, like entering the user's viewport.

This approach offers several advantages:

• Faster Initial Load Times: By deferring unnecessary JavaScript, users experience a quicker initial page load.
• Improved Perception: The application feels more responsive as core functionalities are available instantly.
• Reduced Data Consumption: Smaller initial JavaScript payloads translate to lower data usage.

Enabling Partial Hydration

Utilizing partial hydration is going to be straightforward as mentioned in the Angular blog. Here's a potential example:

{
  @defer (render on server; on viewport) {
    <my-deferrable-component></my-deferrable-component>
  }
}

In this example:

• my-deferrable-component is rendered on the server.
• Client-side, Angular downloads the required JavaScript for the component.
• Interaction with my-deferrable-component only occurs when it enters the viewport, optimizing rendering and performance.

Conclusion

Partial hydration is one of the most requested features of Angular that empowers Angular developers to create performant and user-friendly applications. By strategically deferring component hydration based on user interaction or visibility, Angular 18 ensures a smooth and responsive user experience, especially for complex and data-heavy applications.