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Express.js Application-Level Middleware

Application-level middleware is a function that is bound to an instance of the Express app. It is executed on every request to the server unless specified otherwise. This type of middleware is often used for global functionality such as logging, body parsing, authentication, or handling errors.

 

Key Features of Application-Level Middleware

  • Global or Route-specific: Can be applied globally to all routes or to specific routes.
  • Request Processing: It modifies the request (req) and response (res) objects or ends the request-response cycle.
  • Executed Sequentially: Middleware functions are executed in the order they are defined.
  • Reusable: Middleware functions can be reused across multiple routes or even multiple applications.

 

Steps to Implement Application-Level Middleware

Set up a Basic Express Application
First, initialize a Node.js project and install Express:

npm init -y
npm install express --save

Define Application-Level Middleware
You can apply middleware globally or to specific routes. Here’s an example of global middleware:

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

// Application-level middleware function
app.use((req, res, next) => {
    console.log('This middleware is applied to all routes');
    next();  // Pass control to the next middleware or route handler
});

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

// Define another route
app.get('/about', (req, res) => {
    res.send('About Us');
});

// Start the server
app.listen(3000, () => {
    console.log('Server running on port 3000');
});

Test Application-Level Middleware
Run the application by executing node app.js and open a browser or use an API testing tool like Postman to visit:

  • http://localhost:3000/
  • http://localhost:3000/about
  • "This middleware is applied to all routes" will be logged in the console every time you visit any route.
  • The respective route response (Hello, World! or About Us) will be sent as a response.

Use Middleware for Specific Routes
You can also apply application-level middleware only to specific routes by specifying the route path:

app.use('/about', (req, res, next) => {
    console.log('Middleware for the /about route');
    next();  // Pass control to the next middleware or route handler
});

Error-Handling Middleware
You can add a custom error-handling middleware at the end of your middleware stack:

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

 

Example of Logging and Authentication Middleware

Logging Middleware:
A middleware function that logs the request method and URL:

app.use((req, res, next) => {
    console.log(`${req.method} request made to: ${req.url}`);
    next();  // Pass control to the next middleware
});

Authentication Middleware:
A middleware that checks if the user is authenticated before allowing access to a protected route:

const isAuthenticated = (req, res, next) => {
    if (req.isAuthenticated()) {
        return next();  // User is authenticated, allow the request
    } else {
        res.status(401).send('Authentication required');
    }
};

app.use('/profile', isAuthenticated);

 

Summary

Application-level middleware in Express is applied globally or to specific routes, enabling you to handle common tasks such as logging, body parsing, authentication, or error handling. This middleware is defined using app.use() and is executed sequentially in the order it’s defined. It is an essential part of handling requests and managing the request-response cycle in Express applications.

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