Modular route organization
Split a large application into independently maintained endpoint groups.
Express.js · Function
express.Router
express.Router([options]) creates a modular router that combines middleware and HTTP-method routes. Configure case sensitivity, parent-parameter inheritance, or strict trailing-slash matching with the optional options object.
- Since
- v4.0.0
How it works
express.Router([options]) creates a reusable router object that combines middleware and route handlers. Configure matching behavior when needed, add routes with methods such as get or post, and mount the router with app.use() or another router's use() method.
Syntax
express.Router([options])Parameters
| Name | Type | Description |
|---|---|---|
options | Object | Optional configuration object. caseSensitive defaults to false; mergeParams defaults to false; strict defaults to false. |
Returns
Router object
Key points
- Calling
express.Router()produces a new router instance. - A router accepts middleware and HTTP-method handlers through an application-like API.
- The router is an independent, modular unit for organizing middleware and routes.
mergeParamslets a nested router retain parameters supplied by its parent.- Set
caseSensitive: truewhen route paths must distinguish letter case. - Set
strict: truewhen trailing slashes should affect route matching.
Common mistakes
- Case matching is insensitive unless
caseSensitiveis enabled. - Trailing-slash differences are ignored unless
strictrouting is enabled. - With
mergeParams, child parameters override parent parameters when their names collide.
Example: Create and mount a router
const express = require('express');
const app = express();
const users = express.Router();
users.get('/:id', (req, res) => {
res.json({ id: req.params.id });
});
app.use('/users', users);
app.listen(3000);Creates a router for user endpoints and mounts it under /users, so the route responds at /users/:id.
When to use it
Scoped middleware
Attach authentication, logging, or validation only to a group of routes.
Mountable route modules
Reuse a route module beneath different application mount paths.
Interview questions
QWhat does express.Router() create, and how is it used?
express.Router() returns a router object that acts as middleware and a routing system. It can be passed to app.use() or another router's use() method, then configured with middleware and HTTP-method routes.
Follow-up: How would you preserve parameters captured by a parent router? Pass { mergeParams: true } when creating the child router.
QWhat is the purpose and default of caseSensitive?
The caseSensitive option controls whether route matching distinguishes uppercase and lowercase paths. Its default is false, so paths such as /Foo and /foo match equivalently.
Follow-up: How do you make /Foo and /foo distinct? Create the router with { caseSensitive: true }.
QWhat does the strict option change?
The strict option controls trailing-slash matching. It defaults to false, making /foo and /foo/ equivalent for that router.
Follow-up: How do you require the slash form to matter? Create the router with { strict: true }.
Related
app.use()router.use()router.METHOD()router.route()req.params