Middleware in Express.js

Middleware in Express.js is a fundamental concept that allows you to perform actions on the request and response objects, as well as modify them, before they reach their final destinations (routes or handlers). This makes it a powerful tool for tasks like authentication, error handling, data parsing, and more.

Here's an overview of how middleware works in Express.js:

Middleware Functions:

A middleware function is a JavaScript function that takes three arguments: req (request object), res (response object), and next (a callback function). It can perform operations on the request and response, and either terminate the request-response cycle or pass control to the next middleware or route handler.

const myMiddleware = (req, res, next) => {
  // Perform operations on req and res
  // ...
  
  // Call next to pass control to the next middleware or route handler
  next();
};

Using Middleware:

Middleware can be used at the application level or at the route level.

1. Application-Level Middleware:

Application-level middleware is defined using app.use() and is executed for every request made to the server.

app.use(myMiddleware);

const logRequest = (req, res, next) => {
  console.log(`Received ${req.method} request for ${req.url} at ${new Date()}`);
  next();
};

app.use(logRequest);

In this example, logRequest is a middleware function that logs information about each incoming request. It is then applied at the application level using app.use().