Software Engineering · Architecture
The process of attaching JavaScript event listeners and client-side state to HTML that was rendered on the server, making a static page interactive.
Example: A server-rendered React page arrives as HTML, then React 'hydrates' it to take over interactivity in the browser.
In practice: Hydration cost is real and often overlooked — on a slow device, a page can look complete but be unresponsive while JS hydrates, which is why 'Time to Interactive' and 'First Contentful Paint' can diverge sharply.
Debated: Partial hydration and islands architecture (hydrating only interactive components) are active areas of development precisely because full-page hydration is expensive — Astro and Qwik are built around avoiding it.