Modular route organization
Group endpoints for a feature or resource in a dedicated module.
Express.js · Class
Router
express.Router() creates a modular, mountable router that combines middleware and route handlers. Add routes to it and mount it with app.use() or another router.
- Since
- v4.0.0
How it works
express.Router() creates a self-contained routing and middleware object. Define middleware and HTTP-method handlers on it, then mount it with app.use() so its paths are resolved relative to the mount point.
Syntax
express.Router([options])Parameters
| Name | Type | Description |
|---|---|---|
options | Object | Options object controlling router behavior, including caseSensitive, mergeParams, and strict. |
Returns
A new Router object.
Key points
- A router combines middleware and route handling in one reusable object.
- It is a focused mini-application rather than a complete standalone Express application.
- Mount it with
app.use()to integrate its middleware and routes into the application pipeline. - Call
express.Router()to construct a new router. - After construction, add router-level middleware and HTTP-method handlers using application-like APIs.
- Use lowercase HTTP-method methods such as
get,post, andputfor method-specific routing.
Common mistakes
- Use
next('router')when a condition should leave the current router and return control to the outer middleware chain. - Multiple callbacks can be supplied, but a callback that calls
next('route')skips the remaining callbacks for that route.
Example: Mount a modular router
const express = require('express');
const app = express();
const router = express.Router({ mergeParams: true });
router.use((req, res, next) => {
res.locals.source = 'calendar';
next();
});
router.get('/events', (req, res) => {
res.json({ calendarId: req.params.calendarId, source: res.locals.source });
});
app.use('/calendars/:calendarId', router);
app.listen(3000);Creates a mountable router, adds router-level middleware and a GET route, then mounts it beneath a parameterized application path.
When to use it
Scoped middleware
Attach authentication, logging, or other middleware only to one route group.
Mountable route modules
Reuse the same route bundle beneath different application mount paths.
Interview questions
QWhat is an Express Router, and why use one?
express.Router() returns a router object that combines middleware and routing behavior. It can be passed to app.use() or mounted on another router, making it suitable for modular route groups.
Follow-up: How does mounting affect a route path? A route such as /events on a router mounted at /calendar is reached through the combined path /calendar/events.
QWhat does the mergeParams option do?
mergeParams preserves parameter values from the parent router; its default is false. If parent and child routers use the same parameter name, the child value wins.
Follow-up: When should it be enabled? Enable it when handlers in the child need parameters captured by the parent mount path.
QHow are routes and middleware added to a router?
router.METHOD() methods, such as router.get() and router.post(), define handlers for particular HTTP methods. router.route(path) returns a single route object that can then receive multiple HTTP-verb handlers.
Follow-up: How can middleware be applied across methods? Use router.use() for router-level middleware, or pass middleware callbacks to a method-specific route.
Related
express()router.use()router.METHOD()router.route()app.use()