Router

A router is like the directory signs in a mall. The address /orders goes to the orders page, and /orders/123 goes to the details for order 123. The changing part, like 123, is called a route param, and a rule like that is called a dynamic route.

In a single page application, routing is handled by frontend code, and switching pages doesn't reload the whole page. React Router and Vue Router are common. Frameworks like Next.js and Nuxt use file-based routing: create a folder or file in the app or pages directory, and the matching URL is created automatically.

Common needs: sending logged-out users to the login page (route guards), showing a 404 page for URLs that don't exist, and nested routes so the sidebar stays put while only the right side changes. If refreshing a sub-page after deployment gives a 404, the server usually isn't set up with the single page app fallback rule.