Understanding Middleware in Express.js
Understanding Middleware in Express.js
In Express.js—a popular Node.js framework—middleware is one of the core concepts that gives it flexibility and power. Middleware functions act as building blocks in the request-response cycle, helping you handle everything from authentication to logging to error handling.
Let’s break it down.
✅ What Is Middleware?
Middleware in Express.js is a function that has access to the request (req), response (res), and the next() function in the application’s request-response cycle.
Basic Signature:
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function middleware(req, res, next) {
// Do something
next(); // Pass control to the next middleware
}
🔁 How Middleware Works in Express
Middleware functions can:
Execute any code
Modify the req and res objects
End the request-response cycle
Call the next middleware in the stack using next()
These functions are executed in the order they're added using app.use() or route-specific methods like app.get().
🧱 Types of Middleware in Express.js
Application-Level Middleware
Bound to an instance of express()
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app.use((req, res, next) => {
console.log('Time:', Date.now());
next();
});
Router-Level Middleware
Works like application-level, but bound to an express.Router() instance.
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const router = express.Router();
router.use((req, res, next) => {
console.log('Router Middleware');
next();
});
Built-In Middleware
Included with Express, like:
express.static() – serves static files
express.json() – parses JSON bodies
express.urlencoded() – parses URL-encoded bodies
Third-Party Middleware
Installed via npm
morgan for logging
cors for handling Cross-Origin Resource Sharing
body-parser (now mostly replaced by express.json())
bash
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npm install morgan
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const morgan = require('morgan');
app.use(morgan('dev'));
Error-Handling Middleware
Recognized by Express by its four arguments:
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app.use((err, req, res, next) => {
console.error(err.stack);
res.status(500).send('Something broke!');
});
🚦 Example Flow: Middleware Stack
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app.use((req, res, next) => {
console.log('Middleware 1');
next();
});
app.use((req, res, next) => {
console.log('Middleware 2');
next();
});
app.get('/', (req, res) => {
res.send('Hello World');
});
Output when visiting /:
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Middleware 1
Middleware 2
Hello World
🧠 Why Middleware Matters
Separation of concerns (e.g., auth, logging, validation)
Reusability across routes or apps
Readability and better request flow management
Control over the entire lifecycle of an HTTP request
⚠️ Gotchas to Avoid
Forgetting to call next() will hang your request.
Improper ordering can lead to bugs or security issues.
Error-handling middleware must come last in the stack.
✅ TL;DR
Middleware in Express.js is:
“A function that can inspect, modify, or terminate the request-response cycle—or pass control to the next handler.”
It’s what gives Express its modular, composable structure.
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