What is React Server Components (RSC)?
React components that render exclusively on the server, ship zero JavaScript to the client, and can fetch data + access secrets directly.
React Server Components (RSC) are components that run only on the server during the request, producing a serialized output that React on the client reconstructs into the DOM. Unlike traditional React components, they ship zero JavaScript to the client unless they import a client component. This means smaller bundles (often 50-80% smaller than Client Component apps), faster initial page loads, and the ability to access databases, file systems, and secrets directly inside a component without an API layer. In Next.js App Router, every component is an RSC by default unless explicitly marked with `"use client"` at the top of the file. The mental model: server components for data + structure, client components for interactivity (state, event handlers, browser APIs). Common gotcha: passing functions or class instances as props from server to client components fails serialization — only JSON-safe values cross the boundary.