Express.js is a minimal and flexible Node.js web application framework that provides a robust set of features for web and mobile applications. It simplifies server-side development by offering tools for routing, middleware integration, and HTTP utility methods.

Debugging can be enabled by setting the DEBUG environment variable. For example, on Linux: DEBUG=express:* node app.js, and on Windows: set DEBUG=express:* node app.js.

Express.js offers features like middleware support, routing, template engines, and easy integration with databases, making it suitable for building RESTful APIs, single-page applications, and real-time applications.

Template engines in Express.js allow the rendering of dynamic HTML pages. Commonly used template engines include Pug, EJS, and Handlebars.

Authentication in Express.js can be implemented using middleware like passport, which supports various authentication strategies, including OAuth and JWT.

• Middleware functions in Express.js have access to the request and response objects and the next function in the application’s request-response cycle.
• They can execute code, modify request and response objects, and end the request-response cycle or call the next middleware function.

Session management in Express.js can be handled using the express-session middleware, which stores session data on the server and sets a session ID cookie on the client.

• Express.js provides the express.static middleware to serve static files like images, CSS files, and JavaScript files.
• For example, app.use(express.static('public')) serves files from the 'public' directory.

The Express.js Router is a mini-application capable of performing middleware and routing functions. It helps in organizing routes by grouping them into modular, mountable route handlers.

Error handling in Express.js involves creating middleware functions that take four arguments: err, req, res, and next. These functions can log the error and send appropriate responses to the client.

File uploads in Express.js can be managed using middleware like multer, which handles multipart/form-data and allows for file validation and storage configuration.

Express.js uses the app.METHOD(path, handler) syntax to define routes, where METHOD is an HTTP request method, path is the route path, and handler is the function executed when the route is matched.

Common security practices include validating and sanitizing user input, implementing authentication and authorization, enabling CORS appropriately, and using security-focused middleware like helmet.

A large Express.js application can be modularized by separating concerns into different folders like routes, controllers, middlewares, and services. Using the Express Router allows logical separation and better scalability.

app.use() is used to mount middleware functions without a specific HTTP method, while app.all() is used to handle all HTTP methods for a specific route.

CORS (Cross-Origin Resource Sharing) is a security feature that allows or restricts resources on a web server to be requested from another domain. In Express.js, it can be enabled using the cors middleware.

Rate limiting can be implemented using middleware like express-rate-limit to limit repeated requests to public APIs and prevent abuse.

You can connect MongoDB to your Express.js application using Mongoose, a popular ODM. First install it with npm install mongoose, then connect with mongoose.connect() inside your main server file.

GET is used to request data from a server, while POST is used to submit data to be processed to a specified resource. In Express.js, they are handled using app.get() and app.post() respectively.

A catch-all middleware should be placed at the end of all route definitions:

app.use((req, res) => {
  res.status(404).send('404 Not Found');
});

The app.use() method is used to mount middleware functions at a specified path. It is essential for adding middleware like body parsers, static file servers, and custom middleware functions.

The next() function is used to pass control to the next middleware function in the stack. If not called, the request will be left hanging.

Express.js supports both synchronous and asynchronous functions (using async/await or Promises). Asynchronous functions are useful when performing non-blocking operations like API calls or database queries.

To install Express.js, ensure Node.js is installed, then run npm install express --save in your project directory. This command installs Express.js and adds it as a dependency in your package.json file.