Angular 19 Incremental Hydration Guide with Examples

Let's explore Angular's new Incremental Hydration, a feature that rehydrates parts of your app incrementally for faster and smoother performance.

Angular 19 introduced Incremental Hydration, a powerful feature aimed at optimizing Server-Side Rendering (SSR) workflows. This enhancement ensures that your applications are not just faster to render but also provide a smoother user experience by focusing on what's most important—hydrating only the parts of your app that need it.

In this post, I'll walk you through what Incremental Hydration is, how to use it and where it fits in real-world scenarios.

What Is Incremental Hydration?

In simple terms, hydration is the process of connecting the static HTML rendered on the server with the JavaScript on the client. In traditional SSR, the entire page is rehydrated all at once, which can lead to performance issues, especially for large applications.

Incremental Hydration solves this by rehydrating only parts of the page as they come into view or when they are needed. This approach improves performance by reducing the amount of JavaScript executed on page load.

Setting Up Incremental Hydration in Angular 19

Incremental Hydration is built into Angular 19's SSR system. Here's how you can enable and use it:

1. Install SSR packages: If you don't already have SSR in your Angular app, start by adding the SSR packages:

   ng add @angular/ssr
   

2. Enable Hydration: Angular 19 automatically supports hydration when using the @angular/ssr library. Simply ensure hydration is enabled in your server.ts:

   import "zone.js/node"; // Required for Angular SSR
   import { ngExpressEngine } from "@angular/ssr/node";
   import * as express from "express";
   import { AppServerModule } from "./src/main.server";
   
   const app = express();
   app.engine("html", ngExpressEngine({ bootstrap: AppServerModule }));
   app.set("view engine", "html");
   app.set("views", "dist/your-app/browser");
   

3. Mark Components for Incremental Hydration: You can use Angular's directive ngSkipHydration to exclude certain components from hydration or prioritize others:

   <app-header></app-header>
   <!-- Hydrated immediately -->
   <app-sidebar ngSkipHydration></app-sidebar>
   <!-- Skipped -->
   <app-main-content></app-main-content>
   <!-- Incrementally hydrated -->
   

Example: Incremental Hydration in Action

Let's for example take a simple e-commerce application. The main layout includes:

1. A header (logo, navigation, and search bar).
2. A sidebar (categories and filters).
3. A product list (items fetched dynamically).

Here's how Incremental Hydration improves the user experience:

<app-header></app-header>
<app-sidebar ngSkipHydration></app-sidebar>
<div>
  <app-product-list></app-product-list>
</div>

What Happens?

• The header is hydrated immediately since it contains critical user interactions like search.
• The sidebar is skipped because its content is static and doesn't require interactivity at first.
• The product list is hydrated as the user scrolls, minimizing JavaScript execution on load.

Another Example: News Websites

Incremental Hydration is a perfect fit for optimizing a large news websites with SSR. Here is why:

1. The top stories section needs to be interactive and rendered immediately for engagement.
2. The sidebar ads don't require hydration, as they are simple image links.
3. The comments section at the bottom of articles can wait until a user scrolls down.

Incremental Hydration ensures:

• Faster initial load times for better SEO and user retention.
• Reduced strain on client-side JavaScript.
• A smoother user experience as interactive elements are gradually hydrated.

Why Incremental Hydration Matters

Incremental Hydration is not just a performance optimization; it's a philosophy shift in how Angular handles SSR. By focusing on critical elements first and deferring non-essential hydration, Angular apps can now achieve a few things that were hard to implement in previous versions:

• Faster load times: Your users see meaningful content faster.
• Better resource management: Reduced JavaScript overhead on load.
• Improved scalability: Large apps with complex UIs perform better.

Wrapping Up

Angular 19's Incremental Hydration is a game-changer for building performant, user-friendly SSR applications. Whether you're creating a large-scale e-commerce platform or a news site, this feature can help you deliver a faster and smoother experience.

If you haven't already, try enabling Incremental Hydration in your Angular 19 projects—it's a small step that makes a big difference!