🚀 Node.js Middleware (Express.js Middleware Explained)

Middleware is one of the core features of Express.js.
It allows you to run functions between the request and the response.

✅ What is Middleware?

Middleware functions are functions that have access to:

• req → Request object

• res → Response object

• next → Function to pass control to next middleware

📌 Syntax

🧱 Types of Middleware in Express.js

Express supports many types:

1. Application-Level Middleware

2. Router-Level Middleware

3. Built-In Middleware

4. Third-Party Middleware

5. Error-Handling Middleware

Let’s learn each one.

⭐ 1. Application-Level Middleware

Runs for every request or specific routes.

Example: log every request

Apply to a specific route:

⭐ 2. Router-Level Middleware

Works exactly like application middleware but applied on a Router.

⭐ 3. Built-in Middleware

Express provides some built-in middleware:

✔ Parse JSON

✔ Parse URL-encoded form data

✔ Serve Static Files

⭐ 4. Third-Party Middleware

These are installed from npm.

🔹 Example: morgan (HTTP request logger)

🔹 Example: cors (Enable CORS)

🔹 Example: helmet (Security Headers)

⭐ 5. Error-Handling Middleware

Error middleware has 4 arguments:

Example:

Once thrown → automatically sent to error middleware.

🧬 Middleware Order Matters

Middleware is executed in the order it is defined.

Order =
1️⃣ middleware1
2️⃣ middleware2
3️⃣ route

🔁 Multiple Middleware in One Route

🔐 Example: Authentication Middleware

📁 Organizing Middleware in a Folder

Create /middleware/auth.js:

Use it:

📘 When to Use Middleware?

Use middleware for:

✔ Authentication
✔ Logging
✔ Validation
✔ Parsing request body
✔ Error handling
✔ Security
✔ Rate limiting
✔ Serving static files

🎯 Final Summary

Middleware is essential in Express.js because it:

• Controls request flow

• Adds functionality (logging, auth, security)

• Keeps code modular

• Helps structure large applications