The Router Class

Router lets you build self-contained route modules and mount them under a path prefix.

4 min read Level 1/5 #express#router#routing

What you'll learn

Create a Router
Mount under a prefix
Know the differences from `app`

express.Router() is a mini app you can compose. Same methods as app — get, post, use, param — but it doesn’t bind to a port. You mount it under a URL prefix.

A Router

// users.js
import { Router } from "express";

const router = Router();

router.get("/",        listUsers);
router.post("/",       createUser);
router.get("/:id",     getUser);
router.delete("/:id",  deleteUser);

export default router;

Mounting It

// app.js
import express from "express";
import users from "./users.js";

const app = express();
app.use("/api/users", users);

Inside the router, paths are relative to the mount point:

router.get("/") → GET /api/users
router.get("/:id") → GET /api/users/:id

Per-Router Middleware

const router = Router();

router.use(requireAuth);           // applies to every route in router
router.use(express.json());

router.get("/", listUsers);

This middleware only runs for requests that hit this router. Great for “auth-required” route groups.

Routers Within Routers

const apiRouter = Router();
const usersRouter = Router();

usersRouter.get("/", listUsers);
apiRouter.use("/users", usersRouter);

app.use("/api/v1", apiRouter);
// → /api/v1/users

Compose as deep as you want. Each level can have its own middleware.

Router vs App

	`app`	`Router`
Can `.listen()`	yes	no
Has `app.set()` settings	yes	inherits from parent app
`req.app` available	yes	yes (points to parent app)
Useful in tests	yes	yes (mount in a test app)

In practice you write almost everything as routers, and the root app.js is just glue.

`mergeParams`

If a parent router has :userId, an inner router can’t see it unless you opt in:

const userPosts = Router({ mergeParams: true });

userPosts.get("/", (req, res) => {
  console.log(req.params.userId);   // available
});

apiRouter.use("/users/:userId/posts", userPosts);

Without mergeParams: true, req.params only shows params captured inside the inner router.

Per-Resource Route Files →