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Home/Express.js/Routing
beginner20 min read·Updated Jul 2026Expert reviewed

Routing

Routing maps incoming client requests to specific server execution paths. By utilizing dynamic parameters, HTTP method verbs, and isolated Express Routers, developers build highly organized API structures.

expressroutingparamsqueryexpress-routerapi-design

Knowledge Check

2 questions · pass at 70%

0/2
easymcq

1. How does Express evaluate route matching when multiple routes match the same incoming request path?

Interview Questions

1 questions

Cheatsheet

Download-ready reference
Cheatsheet
Route Param    Dynamic path placeholder (/:id) -> req.params.id
Query Param    URL search string (?key=val) -> req.query.key
express.Router Modular path mounting helper.

Basic Setup:
  const app = express();
  app.get('/items', (req, res) => res.json(items));

Router Module:
  // routes/items.js
  const router = express.Router();
  router.get('/', (req, res) => res.send());
  export default router;

Mounting Router:
  app.use('/items', itemsRouter); // matches GET /items

Related topics

MiddlewarePro

On this page

20m
Knowledge CheckInterview QuestionsCheatsheet

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) => { ... });

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.
export
default
router;
// server.js
import userRouter from './routes/users.js';
app.use('/users', userRouter); // Mounts routes under the '/users' prefix