What is hydration?
Hydration is the process where a web page that was first rendered on the server (as plain HTML) becomes interactive in the browser by attaching JavaScript functionality to the existing HTML elements.
Let's break it down
- The server builds the page’s HTML and sends it to the browser.
- The browser shows the page instantly because the HTML is already there.
- Then a JavaScript bundle is downloaded and runs.
- This script “hydrates” the static HTML by adding event listeners, state management, and other dynamic behavior, turning the page into a full‑featured app.
Why does it matter?
Hydration gives you the best of both worlds: fast initial load times (thanks to server‑rendered HTML) and rich interactivity (thanks to client‑side JavaScript). It improves user experience, SEO, and perceived performance.
Where is it used?
- Modern React frameworks (Next.js, Remix)
- Vue’s Nuxt.js and SvelteKit
- Any site that uses server‑side rendering (SSR) followed by client‑side activation, such as e‑commerce pages, blogs, and dashboards.
Good things about it
- Faster first paint and contentful paint, because HTML arrives ready to display.
- Better SEO, since crawlers can read the pre‑rendered content.
- Reduced time‑to‑interactive compared to pure client‑side rendering.
- Allows developers to share code between server and client.
Not-so-good things
- Adds complexity: you need to manage both server‑rendered markup and client‑side JavaScript.
- Larger initial JavaScript bundle can still affect load time if not optimized.
- Hydration errors can occur if the server and client render different output, leading to mismatches.
- Debugging can be trickier because you’re dealing with two rendering phases.