Middleware vs decorator pattern in Node.js

Introduction

Middleware and decorator patterns help add extra features to code without changing the main parts. They keep code clean and easy to manage.

When you want to add logging or security checks to web requests.

When you need to modify or extend behavior of functions or objects without rewriting them.

When you want to handle tasks like authentication or error handling in a chain of steps.

When you want to add features like caching or timing to existing functions.

When you want to keep your code organized by separating extra tasks from main logic.

Syntax

Node.js

Middleware example (Express.js):
app.use(function(req, res, next) {
  // do something
  next();
});

Decorator example:
function decorator(originalFunction) {
  return function(...args) {
    // extra work
    return originalFunction(...args);
  };
}

Middleware functions usually take three arguments: request, response, and next.

Decorators wrap a function to add behavior before or after calling the original function.

Examples

Middleware logs each request before moving to the next step.

Node.js

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

Decorator adds a log message every time the wrapped function runs.

Node.js

function logDecorator(fn) {
  return function(...args) {
    console.log('Function called');
    return fn(...args);
  };
}

Middleware checks if user is logged in before continuing.

Node.js

app.use((req, res, next) => {
  if (!req.user) {
    res.status(401).send('Unauthorized');
  } else {
    next();
  }
});

Decorator measures how long a function takes to run.

Node.js

function timingDecorator(fn) {
  return function(...args) {
    const start = Date.now();
    const result = fn(...args);
    console.log(`Time: ${Date.now() - start}ms`);
    return result;
  };
}

Sample Program

This example shows middleware logging every request to the server. It also shows a decorator that adds extra text to a greeting function.

When you run this, the server logs requests and prints a decorated greeting once at start.

Node.js

import express from 'express';
const app = express();

// Middleware to log requests
app.use((req, res, next) => {
  console.log(`Received ${req.method} request for ${req.url}`);
  next();
});

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

// Decorator to add greeting to a function
function greetDecorator(fn) {
  return function(name) {
    const original = fn(name);
    return original + ' Have a nice day!';
  };
}

function greet(name) {
  return `Hello, ${name}!`;
}

const decoratedGreet = greetDecorator(greet);

// Start server
app.listen(3000, () => {
  console.log('Server running on port 3000');
  console.log(decoratedGreet('Alice'));
});

Important Notes

Middleware is mainly used in web frameworks to process requests step-by-step.

Decorators are more general and can wrap any function to add features.

Middleware uses a 'next' function to pass control; decorators return a new function.

Summary

Middleware runs in a chain to handle requests or responses.

Decorator wraps a function to add or change behavior.

Both help keep code clean by separating extra tasks from main logic.

Practice

1. What is the main difference between middleware and decorator patterns in Node.js?

easy

A. Middleware wraps functions; decorators process requests in a chain.

B. Middleware is only for error handling; decorators only for logging.

C. Middleware and decorators are the same and used interchangeably.

D. Middleware processes requests in a chain; decorators wrap functions to add behavior.

Solution

1. Step 1: Understand middleware role

   Middleware functions run one after another to handle requests or responses in a chain.

2. Step 2: Understand decorator role

   Decorators wrap a function to add or modify its behavior without changing the original function.

3. Final Answer:

   Middleware processes requests in a chain; decorators wrap functions to add behavior. -> Option D

4. Quick Check:

   Middleware = chain, Decorator = wrap [OK]

Hint: Middleware chains; decorators wrap functions [OK]

Common Mistakes:

• Confusing middleware with decorators
• Thinking middleware wraps functions
• Assuming both are identical

2. Which of the following is the correct way to define a middleware function in Express.js?

easy

A. function middleware(req, res, next) { next(); }

B. function middleware(req, res) { return next(); }

C. function middleware(next, req, res) { next(); }

D. function middleware(res, req, next) { next(); }

Solution

1. Step 1: Recall Express middleware signature

   Middleware functions take three parameters in order: request, response, and next function.

2. Step 2: Check parameter order and usage

   function middleware(req, res, next) { next(); } correctly uses (req, res, next) and calls next() to pass control.

3. Final Answer:

   function middleware(req, res, next) { next(); } -> Option A

4. Quick Check:

   Middleware signature = (req, res, next) [OK]

Hint: Middleware params order: req, res, next [OK]

Common Mistakes:

• Mixing parameter order
• Forgetting to call next()
• Returning next() instead of calling

3. Consider this decorator function in Node.js:

function logDecorator(fn) {
  return function(...args) {
    console.log('Calling function');
    return fn(...args);
  };
}

function greet(name) {
  return `Hello, ${name}!`;
}

const decoratedGreet = logDecorator(greet);
console.log(decoratedGreet('Alice'));

What will be printed to the console?

medium

A. Calling function\nHello, Alice!

B. Hello, Alice!\nCalling function

C. Calling function

D. Hello, Alice!

Solution

1. Step 1: Understand decorator behavior

   The decorator wraps greet, logging 'Calling function' before calling greet.

2. Step 2: Trace function calls and output

   Calling decoratedGreet('Alice') logs 'Calling function' then returns 'Hello, Alice!'. Both are printed by console.log.

3. Final Answer:

   Calling function\nHello, Alice! -> Option A

4. Quick Check:

   Decorator logs then calls function [OK]

Hint: Decorator logs before calling wrapped function [OK]

Common Mistakes:

• Assuming only greeting prints
• Confusing order of console logs
• Missing decorator wrapping effect

4. This Express middleware is intended to log request URLs but has a bug:

function logger(req, res, next) {
  console.log('Request URL:', req.url);
  next;
}

What is the bug and how to fix it?

medium

A. Middleware must return a value; fix by adding return next()

B. req.url is undefined; fix by using req.path

C. next is not called as a function; fix by changing to next()

D. console.log syntax is wrong; fix by adding parentheses

Solution

1. Step 1: Identify middleware flow control

   Middleware must call next() as a function to continue the chain.

2. Step 2: Spot the bug in next usage

   Code uses next without parentheses, so next middleware is never called.

3. Final Answer:

   next is not called as a function; fix by changing to next() -> Option C

4. Quick Check:

   Call next() to continue middleware chain [OK]

Hint: Always call next() as a function in middleware [OK]

Common Mistakes:

• Writing next instead of next()
• Assuming req.url is invalid
• Thinking middleware must return a value

5. You want to add authentication to an existing function without changing it. Which pattern fits best and why?

function fetchData(userId) {
  return `Data for user ${userId}`;
}

hard

A. Modify fetchData directly to add authentication logic inside.

B. Use a decorator to wrap fetchData and check authentication before calling it.

C. Use middleware to process fetchData since it handles requests in a chain.

D. Use middleware to wrap fetchData because middleware wraps functions.

Solution

1. Step 1: Understand the goal

   You want to add authentication without changing fetchData itself.

2. Step 2: Choose the right pattern

   Decorators wrap functions to add behavior like authentication without modifying original code.

3. Final Answer:

   Use a decorator to wrap fetchData and check authentication before calling it. -> Option B

4. Quick Check:

   Decorator = add behavior without changing function [OK]

Hint: Decorator wraps function to add features without changes [OK]

Common Mistakes:

• Confusing middleware with decorators
• Modifying original function directly
• Thinking middleware wraps functions