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

#angular #webdev #javascript #typescript

Angular 18 introduces Partial Hydration, 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:

Server-side Rendering: The server renders the essential content of the application along with the @defer block containing the component.
Client-side Hydration: When the application runs on the client, Angular downloads the necessary JavaScript for the deferred component.
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 straightforward. Here's an 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 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.