Q8- What are middlewares in Node.js? How many types of middleware are there?

In Node.js (especially with ), middleware functions are functions that run between the request and the response cycle.

They have access to:

• req (request)
• res (response)
• next() (function to pass control)

What middleware actually does

Middleware can:

• Modify request/response
• Execute logic (logging, validation, auth)
• End the request
• Pass control to the next middleware

Basic Example

app.use((req, res, next) => {
  console.log("Request received");
  next();
});

Here:

• Middleware runs on every request
• next() moves execution forward

Types of Middleware in Node.js

There are 5 main types of middleware in Express:

1. Application-level Middleware

Applied globally using app.use() or app.METHOD()

app.use((req, res, next) => {
  console.log("App-level middleware");
  next();
});

Used for:

• Logging
• Authentication
• Common logic

2. Router-level Middleware

Applied to specific routes using router

const router = express.Router();

router.use((req, res, next) => {
  console.log("Router middleware");
  next();
});

Used when you want logic only for certain routes.

3. Built-in Middleware

Provided by Express itself

Examples:

• express.json() → parse JSON
• express.urlencoded() → parse form data

app.use(express.json());

4. Third-party Middleware

External packages to add functionality

Examples:

• cors → enable cross-origin requests
• morgan → logging
• helmet → security

const cors = require("cors");
app.use(cors());

5. Error-handling Middleware

Special middleware used to handle errors

👉 It has 4 parameters

app.use((err, req, res, next) => {
  res.status(500).send("Something went wrong");
});

Middleware Execution Flow

Middleware runs in sequence (top to bottom)

app.use(middleware1);
app.use(middleware2);
app.use(middleware3);

Execution:

middleware1 → middleware2 → middleware3 → response

Real-world Use Cases

Middleware is used for:

• Authentication (JWT verification)
• Authorization (role check)
• Logging requests
• Input validation
• Error handling
• API rate limiting

Key Takeaway

Middleware in Node.js acts as a pipeline between request and response, allowing you to control, modify, and manage request flow efficiently. There are mainly 5 types of middleware, each serving a different purpose in building scalable backend applications.