Hydration

Hydration connects client JS to server-rendered HTML. In App Router, only Client Components hydrate.

Initial hydration

• After SSR/SSG, the browser gets HTML plus Client Component bundles.
• React binds events and reconciles the DOM.
• Server Components do not hydrate.

Re-hydration

• Happens when a new RSC payload arrives after navigation or Suspense updates.
• Only affected parts update; layouts keep state.

How App Router behaves

• Server sends serialized RSC fragments.
• Client updates hydrated and non-hydrated UI parts.
• Minimal JS; SPA-like transitions with server rendering.

Key differences

• Hydration: first client run of Client Components.
• Re-hydration: partial updates after new RSC payload.