Middleware:

Middleware functions in Express.js are functions that have access to the request object (req), the response object (res), and the next function in the application's request-response cycle. They can execute any code, make changes to the request and response objects, end the request-response cycle, or call the next middleware function in the stack. Express

Types of Middleware

1. Application-level Middleware

   • Bound to an instance of the Express app using app.use() or app.METHOD().
   • Executed every time the app receives a request.Express+5Express+5Express+5

   const express = require('express');
   const app = express();
   
   app.use((req, res, next) => {
     console.log('Time:', Date.now());
     next();
   });
   

2. Router-level Middleware

   • Similar to application-level middleware but bound to an instance of express.Router().Express

   const express = require('express');
   const router = express.Router();
   
   router.use((req, res, next) => {
     console.log('Request URL:', req.originalUrl);
     next();
   });
   

3. Error-handling Middleware

   • Defined with four arguments: (err, req, res, next).
   • Used to handle errors in the application.Express

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

4. Built-in Middleware

   • Express comes with some built-in middleware functions like express.static, express.json, and express.urlencoded.

5. Third-party Middleware

   • Middleware functions developed by the community to add functionality to Express apps.
   • Examples include cookie-parser, morgan, cors, etc.Express+1Express+1

   const express = require('express');
   const app = express();
   const cookieParser = require('cookie-parser');
   
   app.use(cookieParser());
   

Writing Custom Middleware

A custom middleware function is a function that takes three arguments: req, res, and next.

const myLogger = function (req, res, next) {
  console.log('LOGGED');
  next();
};

app.use(myLogger);

This middleware logs "LOGGED" to the console every time a request is received. It's important to call next() to pass control to the next middleware function. Express+2Express+2Express+2

Middleware Execution Flow

• Middleware functions are executed in the order they are added to the app.
• If a middleware function does not call next(), the request will be left hanging.
• Error-handling middleware should be defined after all other app.use() and routes calls

Middleware Chaining

• In Express.js, you can use multiple middleware functions to handle a single request.
• These functions are executed sequentially, in the order they are defined.
• Each middleware can either:
  • Pass control to the next middleware using next(), or
  • End the request-response cycle by sending a response.