Express Routing

Defining Routes in Express

Express registers routes with method-named functions: app.get(), app.post(), app.put(), app.delete(), and more, each taking a path pattern and one or more handler functions. Dynamic segments in a path are written with a leading colon, /users/:id, and Express automatically extracts them into req.params.

As an app grows, express.Router() lets you group related routes into their own file, then mount that whole group under a path prefix in your main app file, keeping route definitions organized by feature.

app.get('/users/:id', (req, res) => {
  const userId = req.params.id;
  res.json({ id: userId, name: 'Example User' });
});

app.get('/search', (req, res) => {
  const query = req.query.q;
  res.json({ query, results: [] });
});

req.params.id comes from the :id segment in the path; req.query.q comes from the URL's ?q=... query string, Express parses both automatically.

Route Definition Syntax

app.get('/path', handler);
app.get('/path/:param', handler);
app.post('/path', handler);
const router = express.Router();
router.get('/sub-path', handler);
app.use('/prefix', router);

• Route parameters (:id) are always strings in req.params, even if they look numeric.
• req.query holds parsed query string parameters as an object.
• express.Router() creates a mini, mountable Express app for grouping related routes.
• app.use('/api', router) mounts every route in router under the /api prefix.

Express Routing Cheatsheet

The routing patterns you'll use in nearly every Express app.

Organizing Routes With express.Router()

In any Express app beyond a handful of routes, defining everything directly on app becomes unwieldy. express.Router() lets you split routes by resource (users, posts, orders) into their own files, then combine them in your main entry file.

// routes/users.js
import express from 'express';
const router = express.Router();

router.get('/', (req, res) => res.json({ users: [] }));
router.get('/:id', (req, res) => res.json({ id: req.params.id }));

export default router;

// app.js
import usersRouter from './routes/users.js';
app.use('/api/users', usersRouter);

Route Specificity and Order

Just like the hand-rolled router from earlier lessons, Express checks routes in registration order. A dynamic route registered before a more specific static one can shadow it unintentionally.

// Wrong order: /users/:id will match "/users/new" too
app.get('/users/:id', getUserById);
app.get('/users/new', showNewUserForm);

// Correct order: specific routes first
app.get('/users/new', showNewUserForm);
app.get('/users/:id', getUserById);

Common Mistakes

• Registering a dynamic route before a more specific static route that it accidentally shadows.
• Forgetting that req.params values are always strings, even for numeric-looking IDs.
• Defining every route directly on app instead of splitting them with express.Router() as the app grows.
• Mismatching the router's mount path with the paths defined inside the router itself.

Key Takeaways

• Express routes are defined with method-named functions taking a path pattern and handler.
• Dynamic path segments (:id) populate req.params; query strings populate req.query.
• express.Router() groups related routes into a separate, mountable module.
• Route registration order determines which route wins when patterns overlap.