Middleware in Express

Middleware in Express

Middleware functions are a key part of Express. They provide a powerful way to handle various tasks in your application, such as logging, authentication, error handling, and more. In this section, we’ll explore how to create custom middleware, use third-party middleware, and handle errors.

Creating Custom Middleware

Custom 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.

Example: Logging requests

	// Importing the Express module
	const express = require('express');
	const app = express();
	const port = 3000;
	
	// Custom middleware to log request details
	const requestLogger = (req, res, next) => {
	  console.log(`${req.method} ${req.url}`);
	  next(); // Passing control to the next middleware function
	};
	
	// Using the custom middleware
	app.use(requestLogger);
	
	// Handling GET request at the root URL
	app.get('/', (req, res) => {
	  res.send('Hello, World!');
	});
	
	app.listen(port, () => {
	  console.log(`Server running at http://localhost:${port}/`);
	});

Using Third-Party Middleware

Express has a vast ecosystem of third-party middleware that you can use to add functionality to your applications.

Example: Using morgan for logging

Add the morgan package to your project:

	npm install morgan

Then, use the morgan middleware to log requests:

	// Importing the Express module
	const express = require('express');
	const morgan = require('morgan');
	const app = express();
	const port = 3000;
	
	// Using 'morgan' middleware for logging
	app.use(morgan('dev'));
	
	// Handling GET request at the root URL
	app.get('/', (req, res) => {
	  res.send('Hello, World!');
	});
	
	app.listen(port, () => {
	  console.log(`Server running at http://localhost:${port}/`);
	});

Error-Handling Middleware

Error-handling middleware is used to catch and handle errors in your application. It has the same signature as regular middleware, but with an additional err parameter.

Example: Basic error-handling middleware

	// Importing the Express module
	const express = require('express');
	const app = express();
	const port = 3000;
	
	// Middleware to simulate an error
	app.use((req, res, next) => {
	  const error = new Error('Something went wrong!');
	  next(error); // Passing the error to the error-handling middleware
	});
	
	// Error-handling middleware
	app.use((err, req, res, next) => {
	  console.error(err.message); // Logging the error message
	  res.status(500).send('Internal Server Error'); // Sending a generic error response
	});
	
	// Handling GET request at the root URL
	app.get('/', (req, res) => {
	  res.send('Hello, World!');
	});
	
	app.listen(port, () => {
	  console.log(`Server running at http://localhost:${port}/`);
	});

Built-in Middleware

Express also comes with some built-in middleware functions that you can use.

1. express.static: Serves static files (e.g., images, CSS, JavaScript)

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   	// Serving static files from the 'public' directory
   	app.use(express.static('public'));
   	
   	// Handling GET request at the root URL
   	app.get('/', (req, res) => {
   	  res.send('Hello, World!');
   	});
   	
   	app.listen(port, () => {
   	  console.log(`Server running at http://localhost:${port}/`);
   	});

2. express.json and express.urlencoded: Parses incoming request bodies

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   	// Middleware to parse JSON request bodies
   	app.use(express.json());
   	
   	// Middleware to parse URL-encoded request bodies
   	app.use(express.urlencoded({ extended: true }));
   	
   	// Handling POST request at the '/submit' URL
   	app.post('/submit', (req, res) => {
   	  const { name, age } = req.body;
   	  res.send(`Name: ${name}, Age: ${age}`);
   	});
   	
   	app.listen(port, () => {
   	  console.log(`Server running at http://localhost:${port}/`);
   	});

Summary

In this section, we explored the concept of middleware in Express. You learned how to create custom middleware, use third-party middleware, and handle errors. Middleware is a powerful tool in Express that allows you to manage the flow of requests and responses in your application efficiently.