Routing

Learning objectives#

• Define routes for each HTTP method
• Read route params and query strings
• Group routes with express.Router
• Understand route order and add a 404 handler

Explanation#

app.get, app.post, app.put, app.patch and app.delete register a handler for a method and path. app.route("/products").get(...).post(...) groups handlers for one path.

Route parameters capture parts of the path: /products/:id → req.params.id (a string). Query strings (?page=2&q=pen) arrive in req.query.

express.Router() creates a mini-app with its own routes. Mount it with app.use("/api/products", productsRouter) — paths inside the router are relative to the mount point.

Express checks routes in the order they were added and uses the first match, so put specific paths (/products/featured) before generic ones (/products/:id).

A final app.use((req, res) => res.status(404)...) catches everything no route handled. In Express 5, wildcards must be named (/*splat), and async handlers that throw are sent to the error handler automatically.

Syntax#

1const router = express.Router();
2router.get('/', listProducts);          // GET  /api/products
3router.get('/:id', getProduct);         // GET  /api/products/7
4router.post('/', createProduct);        // POST /api/products
5app.use('/api/products', router);

Code example: A products router mounted under /api#

1import express from 'express';
2

3const products = [
4  { id: 1, name: 'Pen', price: 20, category: 'stationery' },
5  { id: 2, name: 'Backpack', price: 1200, category: 'bags' },
6  { id: 3, name: 'Notebook', price: 60, category: 'stationery' },
7];
8

9// routes/products.js
10const productsRouter = express.Router();
11

12productsRouter.get('/', (req, res) => {
13  const { category, sort } = req.query;
14  let list = category ? products.filter(p => p.category === category) : products;
15  if (sort === 'price') list = [...list].sort((a, b) => a.price - b.price);
16  res.json(list.map(p => p.name));
17});
18

19productsRouter.get('/featured', (req, res) => {          // before /:id on purpose
20  res.json({ featured: products[1].name });
21});
22

23productsRouter.get('/:id', (req, res) => {
24  const product = products.find(p => p.id === Number(req.params.id));
25  if (!product) return res.status(404).json({ error: `Product ${req.params.id} not found` });
26  res.json(product);
27});
28

29// app.js
30const app = express();
31app.use('/api/products', productsRouter);
32app.use((req, res) => res.status(404).json({ error: `Cannot ${req.method} ${req.path}` }));
33

34const server = app.listen(3010, async () => {
35  for (const path of ['/api/products?category=stationery&sort=price', '/api/products/featured', '/api/products/3', '/api/products/9', '/api/orders']) {
36    const res = await fetch('http://localhost:3010' + path);
37    console.log(res.status, path, '→', await res.text());
38  }
39  server.close();
40});

200 /api/products?category=stationery&sort=price → ["Pen","Notebook"]
200 /api/products/featured → {"featured":"Backpack"}
200 /api/products/3 → {"id":3,"name":"Notebook","price":60,"category":"stationery"}
404 /api/products/9 → {"error":"Product 9 not found"}
404 /api/orders → {"error":"Cannot GET /api/orders"}

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

1. productsRouter holds all product routes; paths are relative to where it is mounted.
2. req.query.category and req.query.sort filter and sort the list.
3. /featured is registered before /:id, otherwise “featured” would be treated as an id.
4. req.params.id is a string, so it is converted with Number().
5. The final app.use catches any request no route handled and returns a JSON 404.

Real-world use case#

Common mistakes#

Best practices#

• One Router per resource
• Specific routes before parameter routes
• A JSON 404 handler at the end
• Validate params and query values

Remember this#

Practice exercise#

1import express from 'express';
2

3const ordersRouter = express.Router();
4ordersRouter.get('/', (req, res) => res.json([]));
5ordersRouter.get('/:id', (req, res) => res.json({ id: Number(req.params.id) }));
6

7const app = express();
8app.use('/api/orders', ordersRouter);
9

10const server = app.listen(3011, async () => {
11  console.log(await (await fetch('http://localhost:3011/api/orders')).text());      // []
12  console.log(await (await fetch('http://localhost:3011/api/orders/42')).text());   // {"id":42}
13  server.close();
14});

Mini quiz#

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