Middleware Pattern in Node.js

Middleware is a design pattern used in Node.js, most notably in Express.js, to handle requests and responses in a pipeline.

What is Middleware?

Middleware functions 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.

How it Works

When a request is received, it passes through a series of middleware functions. Each function can:

1. Execute any code.
2. Make changes to the request and the response objects.
3. End the request-response cycle.
4. Call the next middleware function in the stack.

Basic Express Middleware

const express = require('express');
const app = express();

// Application-level middleware
app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next(); // Don't forget to call next()!
});

app.get('/', (req, res) => {
  res.send('Hello World');
});

app.listen(3000);

Types of Middleware

• Application-level: Bound to an instance of app.
• Router-level: Bound to an instance of express.Router().
• Error-handling: Middleware specifically for catching errors (defined with 4 arguments: err, req, res, next).
• Built-in: Middleware provided by Express (e.g., express.json()).
• Third-party: Middleware created by the community (e.g., cookie-parser, helmet).

Creating custom middleware

const checkAuth = (req, res, next) => {
  if (req.headers.authorization) {
    next();
  } else {
    res.status(401).send('Unauthorized');
  }
};

app.get('/admin', checkAuth, (req, res) => {
  res.send('Welcome Admin');
});