Middleware vs decorator pattern in Node.js - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Middleware Purpose

In Node.js web frameworks, what is the primary role of middleware functions?

A To handle HTTP requests and responses in a sequence, allowing modification or termination of the request flow.

B To manage database connections and queries automatically.

C To compile JavaScript code into machine code for faster execution.

D To decorate classes by adding new methods or properties at runtime.

💡 Hint

Think about how requests are processed step-by-step in a server.

Middleware functions in Node.js frameworks like Express act as layers that process requests and responses in order. They can modify data, end the request, or pass control to the next middleware.

❓ component_behavior

intermediate

Decorator Pattern Behavior

What does a decorator function typically do in Node.js when applied to another function?

A Deletes the original function from memory to save space.

B Automatically logs all errors thrown by the function without changing it.

C Replaces the original function with a completely unrelated function.

D Wraps the original function to extend or modify its behavior without changing its code.

💡 Hint

Think about how you might add extra features to a function without rewriting it.

A decorator wraps a function to add new behavior before or after the original function runs, keeping the original intact.

📝 Syntax

advanced

Middleware Function Syntax

Which of the following is the correct syntax for a middleware function in Express.js?

A function(req, res, next) { console.log('Middleware'); next(); }

B function(req, res) { console.log('Middleware'); next(); }

C function(err, req, res, next) { console.log('Middleware'); next(); }

D function(req, res, next) { console.log('Middleware'); }

💡 Hint

Middleware needs to call next() to continue the chain.

Middleware functions take three parameters: request, response, and next. Calling next() passes control to the next middleware.

🔧 Debug

advanced

Decorator Pattern Bug Identification

Given this decorator function, which option will cause a runtime error when used to decorate a function?

Node.js

function decorator(fn) {
  return function(...args) {
    console.log('Before');
    const result = fn(...args);
    console.log('After');
    return result;
  };
}

A const decorated = decorator(function() { return 42; }); decorated();

B const decorated = decorator(null); decorated();

C const decorated = decorator(() => 'ok'); decorated();

D const decorated = decorator(() => { throw new Error('Fail'); }); decorated();

💡 Hint

Consider what happens if fn is not a function.

Passing null instead of a function causes a TypeError when trying to call fn(...args).

❓ state_output

expert

Middleware vs Decorator Output Difference

Consider this Express middleware and decorator usage. What will be logged when a request hits the server?

Node.js

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

function logMiddleware(req, res, next) {
  console.log('Middleware start');
  next();
  console.log('Middleware end');
}

function logDecorator(fn) {
  return function(req, res) {
    console.log('Decorator start');
    fn(req, res);
    console.log('Decorator end');
  };
}

app.use(logMiddleware);

app.get('/', logDecorator((req, res) => {
  console.log('Handler');
  res.send('Hello');
}));

A

Decorator start
Middleware start
Handler
Decorator end
Middleware end

B

Middleware start
Handler
Decorator start
Decorator end
Middleware end

C

Middleware start
Decorator start
Handler
Decorator end
Middleware end

D

Middleware start
Decorator start
Decorator end
Handler
Middleware end

💡 Hint

Think about the order middleware and route handlers run in Express.

Middleware runs first, then the decorated handler logs 'Decorator start', then 'Handler', then 'Decorator end', then middleware logs 'Middleware end' after next() completes.