Routing in Express.js: A Complete Guide to Application Endpoints

1. Introduction to Routing in Express

Routing determines how an application responds to HTTP requests. Each route consists of:

• A route path
• An HTTP method (GET, POST, PUT, DELETE, etc.)

Express uses app.get(), app.post(), app.all(), and app.use() to define routes and middleware.

2. Route Methods

Route methods correspond to HTTP methods. Example:

// GET route
app.get('/', (req, res) => {
  res.send('GET request to homepage')
})

// POST route
app.post('/', (req, res) => {
  res.send('POST request to homepage')
})

app.all() handles all HTTP methods:

app.all('/secret', (req, res, next) => {
  console.log('Accessing secret section...')
  next()
})

3. Route Paths

Route paths can be defined using strings, patterns, or regular expressions.

3.1 String-Based Paths

app.get('/about', (req, res) => res.send('about'))
app.get('/random.text', (req, res) => res.send('random.text'))

3.2 String Pattern Paths

(Note: These patterns behave differently in Express 5.)

app.get('/ab?cd', (req, res) => res.send('ab?cd'))
app.get('/ab+cd', (req, res) => res.send('ab+cd'))
app.get('/ab*cd', (req, res) => res.send('ab*cd'))
app.get('/ab(cd)?e', (req, res) => res.send('ab(cd)?e'))

3.3 Regular Expression Paths

app.get(/a/, (req, res) => res.send('/a/'))
app.get(/.*fly$/, (req, res) => res.send('/.*fly$/'))

4. Route Parameters

Route parameters capture dynamic values from the URL and store them in req.params.

app.get('/users/:userId/books/:bookId', (req, res) => {
  res.send(req.params)
})

Example:

URL: /users/34/books/8989
req.params = { userId: "34", bookId: "8989" }

Parameters can include hyphens or dots:

/flights/:from-:to
/plantae/:genus.:species

Restricting parameters with regex:

app.get('/user/:id(\\d+)', ...)

5. Route Handlers

Express allows multiple callback functions for a single route.

5.1 Single Handler

app.get('/example/a', (req, res) => res.send('Hello A'))

5.2 Multiple Handlers

app.get('/example/b', (req, res, next) => {
  console.log('Next handler...')
  next()
}, (req, res) => res.send('Hello B'))

5.3 Array of Handlers

app.get('/example/c', [cb0, cb1, cb2])

5.4 Combination of Arrays and Functions

app.get('/example/d', [cb0, cb1], (req, res, next) => {
  next()
}, (req, res) => res.send('Hello D'))

6. Response Methods

Key res methods that end the request-response cycle:

Method	Description
res.send()	Send a response
res.json()	Send JSON
res.redirect()	Redirect
res.render()	Render a template
res.sendFile()	Send a file
res.download()	Download a file
res.sendStatus()	Send status code

7. app.route(): Chained Route Definitions

Use app.route() to define multiple handlers for the same path without repeating the path.

app.route('/book')
  .get((req, res) => res.send('Get a book'))
  .post((req, res) => res.send('Add a book'))
  .put((req, res) => res.send('Update a book'))

8. express.Router: Modular Route Handlers

express.Router creates modular, mountable route handlers—essentially a “mini-app”.

Example birds.js

const router = express.Router()

router.use((req, res, next) => {
  console.log('Time:', Date.now())
  next()
})

router.get('/', (req, res) => res.send('Birds home'))
router.get('/about', (req, res) => res.send('About birds'))

module.exports = router

Mounting the Router

app.use('/birds', require('./birds'))

To access parent route parameters:

const router = express.Router({ mergeParams: true })

Conclusion

Express routing is one of the most powerful and flexible routing systems in the Node.js ecosystem. By combining simple paths, pattern-based routes, dynamic parameters, middleware-style handlers, app.route(), and express.Router, you can build modular, readable, and maintainable applications.