As server-side rendering becomes increasingly common in Angular applications, so does the need for smarter hydration strategies. Hydrating the entire application on the client side can introduce performance bottlenecks, especially in large or highly interactive apps. Angular’s latest experimental feature aims to address this growing concern, offering a more flexible and efficient approach to client-side hydration. In this article, we’ll explore what Incremental Hydration in Angular 19 is and why it matters.
Challenge
Modern web applications demand both high performance and interactivity, especially for server-rendered content. Hydrating an entire application on the client side can be resource-intensive, leading to slower load times and delayed interactivity. Developers need a way to selectively hydrate parts of the application as needed to optimize resource usage and improve the user experience.
Solution
Angular introduces Incremental Hydration as an experimental feature, building on the foundations of defer blocks, deferrable views (introduced in v17), and event replay (v18). This feature hydrates server-rendered sections selectively, depending on predefined triggers. This gives developers control over component activation, enhancing both speed and user experience.
To enable Incremental Hydration, update the application configuration:
export const appConfig: ApplicationConfig = {
providers: [
provideClientHydration(
withIncrementalHydration()
)
// other providers...
]
};Code language: JavaScript (javascript)Incremental Hydration works in tandem with defer blocks. To use it, developers can add new hydration triggers to specific components:
@defer (hydrate on hover) {
<app-hydrated-cmp />
}Code language: HTML, XML (xml)Supported hydration triggers include:
- idle: Hydrate during idle time.
- interaction: Trigger hydration upon user interaction.
- immediate: Hydrate immediately.
- timer(ms): Hydrate after a specified delay.
- hover: Trigger hydration on hover.
- viewport: Hydrate when the component enters the viewport.
- never: Keep the component dehydrated indefinitely.
- when {{ condition }}: Conditional hydration based on a specified condition.
These triggers give developers fine-grained control over hydration timing and behavior, optimizing resources based on app needs and user actions.
Benefits
Incremental Hydration improves load times and interactivity by activating components only when needed, reducing the initial payload and resource consumption. This allows large applications to run more efficiently by hydrating only the necessary components at startup.
In the same way, the flexibility provided by different triggers allows developers to tailor hydration strategies to specific use cases, making applications more efficient and responsive.
Fanis Prodromou, Google Developer Expert
Incremental hydration is an amazing technique that can significantly improve application performance while ensuring a smooth user experience (UX). Every feature has strengths and weaknesses, and it’s important to use them effectively.
While Server-Side Rendering (SSR) and Incremental Hydration offer significant performance gains, they are not meant to replace Single-Page Applications (SPAs) completely. These techniques benefit client-facing applications, especially when it’s essential to minimize the time it takes for users to see and interact with the page for the first time.
Furthermore, incremental hydration can improve performance without negatively impacting the SEO.
Luca Del Puppo, Google Developer Expert
Incremental hydration has been missing for a while. Its predecessor, Server Side Render, was a nightmare to implement, and it has impacted the decisions between React and Angular for many teams in the past. Unfortunately, many chose React because it was already “ready.”
But now it is here and straightforward to use.
Not all projects need this feature, so it will probably impact only a tiny part of the projects followed by the Angular community. But it can help open the doors for new meta frameworks based on Angular. “
Final thoughts
Incremental Hydration is a performance-focused experimental feature in Angular that allows developers to hydrate only the components that matter, when they matter. With flexible triggers, it enables faster initial loads, reduces resource usage, and makes server-rendered apps feel snappy and modern. Whether you’re optimizing for performance or interactivity, incremental hydration gives you the tools to build more innovative Angular applications.
Curious about more cutting-edge features in Angular? Download our free ebook to explore new capabilities like signals, deferrable views, server-side rendering upgrades, and more, complete with examples, expert commentary, and real-world tips.
