react-router

React Router is a library that handles navigation in React web applications. Think of it as a traffic director for your app — when a user visits a URL like /users/john, React Router figures out which screen to show and displays the right components on the page.

At its core, React Router lets you declare your entire site structure upfront using nested routes. You write out all your pages and how they connect to each other — a home page at the root URL, an about page at /about, a users section at /users with sub-pages like /users/john — and the library takes care of the plumbing. When the URL changes, React Router automatically loads the right component and renders it. You can also add behaviors like links that highlight when you're on their page, redirect rules, and 404 pages for broken links.

The main benefit is speed and clarity. Instead of scattered navigation logic throughout your code, all your app's structure lives in one place, almost like a sitemap. New developers can open that file and immediately understand what pages your app has and how they fit together. Adding new screens becomes fast because you don't have to manually wire up transitions and state updates — React Router handles that automatically. The library also supports different URL styles: the modern clean URLs that most sites use today, or hash-based URLs if you need to support older browsers.

You'd use this if you're building a single-page app in React with multiple screens or pages — anything from a simple site with a few sections to a complex dashboard with nested menus and detail views. Developers appreciate it because it reduces boilerplate code and makes the app structure obvious. The README shows that the library has solid features like handling dynamic URL segments (like the user ID in /user/:userId), query parameters, and transition hooks so you can run custom logic when navigating between pages.