Middleware in Express.js

Introduction

Middleware in Express.js is a fundamental concept that allows developers to handle requests and responses in a flexible way. Middleware functions can execute any code, make changes to the request and response objects, end the request-response cycle, and call the next middleware function in the stack.

What is Middleware?

Middleware is a series of functions that are invoked by the Express.js framework during the request-response cycle. They can perform a variety of tasks, such as:

• Executing code
• Modifying the request and response objects
• Ending the request-response cycle
• Calling the next middleware function

Middleware can be applied globally to all routes or locally to specific routes.

Types of Middleware

In Express.js, middleware can be categorized into several types:

1. Application-level middleware: Defined at the application level and can be used with all routes.
2. Router-level middleware: Defined for specific routers and used with routes defined in that router.
3. Error-handling middleware: Defined to handle errors that occur during request processing.
4. Built-in middleware: Provided by Express.js for common tasks, such as parsing request bodies.
5. Third-party middleware: Middleware developed by the community that can be added to Express applications.

Creating Middleware

Creating a middleware function in Express.js is straightforward. Here’s a simple example:

const express = require('express');
const app = express();

// Custom middleware function
const myMiddleware = (req, res, next) => {
    console.log('Middleware executed!');
    next(); // Pass control to the next middleware
};

app.use(myMiddleware); // Apply middleware globally

app.get('/', (req, res) => {
    res.send('Hello, World!');
});

app.listen(3000, () => {
    console.log('Server is running on port 3000');
});

Using Middleware

Middleware can be used in different ways:

• Globally with app.use().
• Locally with app.get(), app.post(), etc.:

app.get('/user', myMiddleware, (req, res) => {
    res.send('User Profile');
});

• Error-handling middleware should be defined after all other middleware and routes:

app.use((err, req, res, next) => {
    console.error(err.stack);
    res.status(500).send('Something went wrong!');
});

Best Practices

When working with middleware in Express.js, consider the following best practices:

• Keep middleware functions small and focused.
• Order middleware functions thoughtfully, as the order of execution matters.
• Use error-handling middleware to capture errors consistently.
• Document middleware clearly, especially when used across multiple routes.

FAQ