Routing

Concept#

HTTP Method Verb Matching#

At its core, Express is a routing and middleware web framework. Routing defines how an application responds to a client endpoint request, matching:

1. An HTTP Method (GET, POST, PUT, DELETE, etc.).
2. A URI Path (or route pattern).
3. A callback Handler function that receives req and res parameters.

import express from 'express';
const app = express();
 
app.get('/users', (req, res) => {
  res.json([{ name: 'Ada' }]);
});
 
app.post('/users', (req, res) => {
  // handle user creation
  res.status(211).send();
});

Route Parameters and Query Strings#

Express automatically parses request path attributes:

• Route Parameters: Dynamic path segments prefixed with colons, accessed via req.params.
• Query Parameters: Key-value pairs appended after the ? symbol in the URL, accessed via req.query.

// Request: GET /users/42?detail=full
app.get('/users/:id', (req, res) => {
  const userId = req.params.id; // "42"
  const showDetail = req.query.detail; // "full"
  res.send(`User: ${userId}, Details: ${showDetail}`);
});

Modular Routing: Express.Router#

To prevent a single file from growing into a massive monolithic file containing hundreds of routes, Express provides express.Router to create modular, mountable route handlers:

// routes/users.js
import express from 'express';
const router = express.Router();
 
router.get('/', (req, res) => { ... });
router.get('/:id', (req, res) => { ... });
 
export default router;
 
// server.js
import userRouter from './routes/users.js';
app.use('/users', userRouter); // Mounts routes under the '/users' prefix

Common Mistakes#

1. Declaring generic routes before specific ones#

Express evaluates routes sequentially in the order they are defined. If you declare app.get('/:id') before app.get('/active'), a request for /active will match the /:id route, assigning the parameter value id = "active". Always define specific routes before generic parameter routes.

2. Forgetting to return after sending a response#

Executing res.send() or res.json() does not stop handler function execution. Code after res.send() continues to execute. If you try to send headers twice, Express crashes with: Cannot set headers after they are sent to the client. Use return res.send() to stop execution explicitly.

Best Practices#

• Strict Path Ordering: Order routes from most specific (e.g. /users/stats) to most generic (e.g. /users/:id).
• Use express.Router: Create separate router files for each major feature domain (e.g. /auth, /products, /orders).
• Sanitize Parameter Input: Validate and cast route parameter types (like checking that :id is a number or valid UUID) before passing them to databases.