Routing in Express.js

Routing in Express.js

Routing refers to how an application’s endpoints (URIs) respond to client requests. In Express, routing is managed primarily through the app object or via the express.Router class.

Basic Routing

A route definition takes the following structure:

app.METHOD(PATH, HANDLER)

• app is an instance of Express.
• METHOD is an HTTP request method (e.g., get, post, put, delete), in lowercase.
• PATH is the path on the server.
• HANDLER is the function executed when the route is matched.

Common Route Methods

Here are examples of handling the most common HTTP methods:

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

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

// PUT method route (usually to update existing data)
app.put('/user', (req, res) => {
  res.send('Got a PUT request at /user');
});

// DELETE method route
app.delete('/user', (req, res) => {
  res.send('Got a DELETE request at /user');
});

[!TIP] Express supports methods that correspond to all HTTP request methods: get, post, put, delete, patch, options, head, and more. There is also a special routing method, app.all(), which handles all HTTP methods for a path.

Route Parameters

Route parameters are named URL segments that are used to capture the values specified at their position in the URL. The captured values are populated in the req.params object.

app.get('/users/:userId/books/:bookId', (req, res) => {
  // Access the parameters via req.params
  const { userId, bookId } = req.params;

  res.send(`User ID is: ${userId} and Book ID is: ${bookId}`);
});

For a URL like http://localhost:3000/users/34/books/8989, the output will be: User ID is: 34 and Book ID is: 8989.

Query Strings

Query strings are another way to pass data via the URL. They appear after a question mark (?) in the URL. In Express, you can access these using req.query.

// URL: http://localhost:3000/search?keyword=express&page=2
app.get('/search', (req, res) => {
  const keyword = req.query.keyword;
  const page = req.query.page;

  res.send(`Searching for "${keyword}" on page ${page}`);
});

Express Router

As your application grows, defining all your routes in a single file becomes unmanageable. To solve this, Express provides the Router class to create modular, mountable route handlers.

Create a router file (e.g., birds.js):

import express from 'express';
const router = express.Router();

// middleware that is specific to this router
router.use((req, res, next) => {
  console.log('Time: ', Date.now());
  next();
});

// define the home page route
router.get('/', (req, res) => {
  res.send('Birds home page');
});

// define the about route
router.get('/about', (req, res) => {
  res.send('About birds');
});

export default router;

Then, load the router module in your main app file (index.js):

import express from 'express';
import birdsRouter from './birds.js'; // Import your router

const app = express();

// Mount the router at a specific path
app.use('/birds', birdsRouter);

Now, the application will handle requests to /birds and /birds/about, and call the time-logging middleware specific to that route.

[!IMPORTANT] Using express.Router is a best practice for separating your API routes into distinct files (e.g., one router for users, one for products), making your code significantly cleaner and easier to maintain.