Routing & Middleware in Express
Master Express routing to define API endpoints and implement middleware for request processing.
Routing & Middleware in Express is a free Node.js Backend Development Bootcamp lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Node.js Backend Development Bootcamp learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Routing Your Express API
Welcome back! In this lesson, we'll dive into routing and middleware in Express. These are core concepts for building powerful and organized web applications.
Routing helps your server understand what to do when it receives different requests, like showing a user profile or saving new data.
Defining Basic Routes
A route tells your Express app how to respond to a specific type of request to a particular URL path. You define routes using methods like app.get(), app.post(), app.put(), and app.delete().
Each route method takes a path and a handler function that executes when the route is matched.
First GET Route Example
Let's create a simple GET route that responds to requests at the root URL (/). This is often your homepage or a basic API status check.
The handler function receives req (request) and res (response) objects.
const express = require('express');
const app = express();
const port = 3000;
app.get('/', (req, res) => {
res.send('Hello from our Express API!');
});
app.listen(port, () => {
console.log(`Server running on http://localhost:${port}`);
});Handling Dynamic Route Parameters
Often, you need to capture dynamic values from the URL, like a user's ID. This is done with route parameters, denoted by a colon (:) in the path.
You can access these parameters through the req.params object in your route handler.
const express = require('express');
const app = express();
const port = 3000;
app.get('/users/:userId', (req, res) => {
const userId = req.params.userId;
res.send(`You requested user ID: ${userId}`);
});
app.listen(port, () => {
console.log(`Server running on http://localhost:${port}`);
});What is Middleware?
Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle.
They can:
- Execute any code.
- Make changes to the request and the response objects.
- End the request-response cycle.
- Call the next middleware function in the stack.
The 'next()' Function
The key to middleware is the next() function. If a middleware function does not end the request-response cycle (e.g., by calling res.send()), it must call next() to pass control to the next middleware function or route handler.
If next() is not called, the request will be left hanging.
Application-level Middleware
You can apply middleware globally to all requests using app.use(). This is great for tasks like logging, parsing JSON bodies, or setting common headers.
Let's create a simple logger that runs for every incoming request.
const express = require('express');
const app = express();
const port = 3000;
// Application-level middleware
app.use((req, res, next) => {
console.log(`[${new Date().toISOString()}] ${req.method} ${req.url}`);
next(); // Don't forget to call next()
});
app.get('/', (req, res) => {
res.send('Check your server console for the log!');
});
app.listen(port, () => {
console.log(`Server running on http://localhost:${port}`);
});Route-level Middleware
Sometimes, you only want middleware to run for specific routes. You can apply route-level middleware by passing it as an argument before the route handler function.
You can even chain multiple middleware functions for a single route.
const express = require('express');
const app = express();
const port = 3000;
// A simple authentication middleware
const isAuthenticated = (req, res, next) => {
if (req.headers.authorization === 'Bearer mysecrettoken') {
next(); // User is authenticated, proceed
} else {
res.status(401).send('Unauthorized: Invalid token');
}
};
// Apply isAuthenticated middleware only to this route
app.get('/dashboard', isAuthenticated, (req, res) => {
res.send('Welcome to the protected dashboard!');
});
app.get('/', (req, res) => {
res.send('Public content here.');
});
app.listen(port, () => {
console.log(`Server running on http://localhost:${port}`);
});Common Middleware Uses
Middleware is incredibly versatile. Here are some common use cases:
- Authentication/Authorization: Checking user credentials or permissions.
- Logging: Recording request details for debugging or monitoring.
- Body Parsing: Extracting JSON or URL-encoded data from request bodies (e.g.,
express.json()). - Error Handling: Catching and responding to errors (special middleware type).
- Input Validation: Ensuring request data meets specific criteria.
Middleware & Routing Check
Let's test your understanding of Express routing and middleware!
Recap: Routing & Middleware
Great job! You've learned how Express uses routing to direct requests to specific handlers based on URL paths and HTTP methods.
You also mastered middleware, powerful functions that can intercept and process requests at various stages, either globally with app.use() or for specific routes. Understanding these concepts is fundamental to building robust Express applications!
Frequently asked questions
Is the “Routing & Middleware in Express” lesson free?
Yes — the full text of “Routing & Middleware in Express” is free to read here on the web, and the Node.js Backend Development Bootcamp course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Node.js Backend Development Bootcamp course, upgrade to CoddyKit PRO.
What will I learn in “Routing & Middleware in Express”?
Master Express routing to define API endpoints and implement middleware for request processing. You practise Node.js Backend Development Bootcamp with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Node.js Backend Development Bootcamp?
No prior experience is required. Node.js Backend Development Bootcamp on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Routing & Middleware in Express” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Node.js Backend Development Bootcamp lesson?
Yes. Every Node.js Backend Development Bootcamp lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Express.js Framework Fundamentals
- Routing & Middleware in Express
- Designing RESTful API Endpoints
- Handling Request Data: Body, Query & Params