Middleware factory pattern in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a middleware factory that returns a middleware function.

Express

function logger(level) {
  return function(req, res, next) {
    console.log(level + ': Request received');
    [1]();
  };
}

Drag options to blanks, or click blank then click option'

Ares

Bnext

Creq

Dconsole

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to call next() causes the request to hang.

The middleware factory returns a function that calls next() to pass control to the next middleware.

2fill in blank

medium

Complete the code to use the middleware factory with a specific log level.

Express

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

app.use(logger([1]));

Drag options to blanks, or click blank then click option'

A'info'

Binfo

Clevel

Dconsole

Attempts:

3 left

💡 Hint

Common Mistakes

Passing an undefined variable instead of a string.

The log level should be passed as a string to the middleware factory.

3fill in blank

hard

Fix the error in the middleware factory to correctly handle the next function.

Express

function logger(level) {
  return function(req, res, next) {
    console.log(level + ': Request received');
    [1];
  };
}

Drag options to blanks, or click blank then click option'

Anext

Bnext.call

Cnext.call()

Dnext()

Attempts:

3 left

💡 Hint

Common Mistakes

Writing next without parentheses, causing the middleware to not proceed.

The next function must be called with parentheses to invoke it.

4fill in blank

hard

Fill both blanks to create a middleware factory that logs the method and URL of requests.

Express

function requestLogger([1]) {
  return function(req, res, [2]) {
    console.log(req.method + ' ' + req.url);
    next();
  };
}

Drag options to blanks, or click blank then click option'

Alevel

Bnext

Cres

Drequest

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter names or forgetting to call next().

The factory takes a parameter level and the middleware function uses next to continue.

5fill in blank

hard

Fill all three blanks to create a middleware factory that adds a custom header with the given name and value.

Express

function headerSetter([1], [2]) {
  return function(req, res, [3]) {
    res.setHeader([1], [2]);
    next();
  };
}

Drag options to blanks, or click blank then click option'

AheaderName

BheaderValue

Cnext

Dres

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing parameter names or forgetting to call next().

The factory takes headerName and headerValue. The middleware function uses next to continue.

Practice

1. What is the main purpose of the middleware factory pattern in Express?

easy

A. To automatically handle errors in middleware

B. To create middleware functions that can be customized with different settings

C. To replace the need for routing in Express

D. To speed up server response time by caching

Solution

1. Step 1: Understand middleware factory pattern concept

   The middleware factory pattern is used to create middleware functions that accept parameters to customize their behavior.

2. Step 2: Compare options with the concept

   Only To create middleware functions that can be customized with different settings correctly describes creating customizable middleware functions. Other options describe unrelated features.

3. Final Answer:

   To create middleware functions that can be customized with different settings -> Option B

4. Quick Check:

   Middleware factory pattern = customizable middleware [OK]

Hint: Middleware factory = function returning middleware with parameters [OK]

Common Mistakes:

• Thinking it automatically handles errors
• Confusing with routing or caching
• Believing it speeds response time directly

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

easy

A. function logger() { /* middleware code */ }

B. function logger(req, res, next) { return function(level) { /* middleware code */ } }

C. function logger(level) { return function(req, res, next) { /* middleware code */ } }

D. function logger(level, req, res, next) { /* middleware code */ }

Solution

1. Step 1: Identify middleware factory structure

   A middleware factory is a function that takes parameters and returns a middleware function with signature (req, res, next).

2. Step 2: Match syntax to this pattern

   function logger(level) { return function(req, res, next) { /* middleware code */ } } matches this: it takes 'level' and returns a function with (req, res, next). Others do not follow this pattern.

3. Final Answer:

   function logger(level) { return function(req, res, next) { /* middleware code */ } } -> Option C

4. Quick Check:

   Factory returns middleware function = function logger(level) { return function(req, res, next) { /* middleware code */ } } [OK]

Hint: Factory takes params, returns (req, res, next) function [OK]

Common Mistakes:

• Placing req, res, next in factory parameters
• Not returning a middleware function
• Confusing middleware and factory signatures

3. Given the middleware factory below, what will be logged when a request is made?

function greet(name) {
  return function(req, res, next) {
    console.log(`Hello, ${name}!`);
    next();
  };
}

app.use(greet('Alice'));

medium

A. Hello, Alice!

B. Hello, undefined!

C. Hello, name!

D. No output, middleware does not log

Solution

1. Step 1: Understand middleware factory usage

   The factory greet('Alice') returns a middleware that logs 'Hello, Alice!'.

2. Step 2: Predict middleware behavior on request

   When a request comes, the middleware logs the greeting with the name 'Alice'.

3. Final Answer:

   Hello, Alice! -> Option A

4. Quick Check:

   Factory param used in log = Hello, Alice! [OK]

Hint: Factory param appears in log output [OK]

Common Mistakes:

• Expecting 'undefined' if factory param missing
• Thinking middleware does not log
• Confusing variable name with string literal

4. Identify the error in this middleware factory code:

function checkRole(role) {
  return function(req, res, next) {
    if(req.user.role === role) {
      next;
    } else {
      res.status(403).send('Forbidden');
    }
  };
}

medium

A. res.status should be res.send

B. Incorrect comparison operator in if statement

C. Middleware factory should not return a function

D. Missing parentheses when calling next()

Solution

1. Step 1: Review middleware function call

   Inside middleware, next must be called as a function: next().

2. Step 2: Identify the error

   The code uses 'next;' instead of 'next();', so middleware will not proceed correctly.

3. Final Answer:

   Missing parentheses when calling next() -> Option D

4. Quick Check:

   Call next() with parentheses [OK]

Hint: Always call next() as a function, not just next [OK]

Common Mistakes:

• Writing next without parentheses
• Confusing status and send methods
• Thinking factory should not return a function

5. You want to create a middleware factory that limits requests based on a max count per user. Which approach correctly applies the middleware factory pattern to achieve this?

hard

A. function rateLimiter(maxRequests) {
const counts = new Map();
return function(req, res, next) {
const user = req.user.id;
counts.set(user, (counts.get(user) || 0) + 1);
if(counts.get(user) > maxRequests) {
res.status(429).send('Too many requests');
} else {
next();
}
};
}

B. function rateLimiter(req, res, next, maxRequests) {
// logic here
}

C. function rateLimiter(maxRequests) {
return function(req, res) {
// missing next call
};
}

D. function rateLimiter() {
// no parameters, fixed limit
}

Solution

1. Step 1: Understand the requirement for customizable limit

   The middleware must accept maxRequests as a parameter to customize the limit.

2. Step 2: Check which option correctly returns middleware with (req, res, next) and uses maxRequests

   function rateLimiter(maxRequests) { const counts = new Map(); return function(req, res, next) { const user = req.user.id; counts.set(user, (counts.get(user) || 0) + 1); if(counts.get(user) > maxRequests) { res.status(429).send('Too many requests'); } else { next(); } }; } correctly implements a factory that returns middleware tracking counts and enforcing maxRequests, calling next() properly.

3. Final Answer:

   function rateLimiter(maxRequests) { const counts = new Map(); return function(req, res, next) { const user = req.user.id; counts.set(user, (counts.get(user) || 0) + 1); if(counts.get(user) > maxRequests) { res.status(429).send('Too many requests'); } else { next(); } }; } -> Option A

4. Quick Check:

   Factory with param returns middleware calling next() = function rateLimiter(maxRequests) { const counts = new Map(); return function(req, res, next) { const user = req.user.id; counts.set(user, (counts.get(user) || 0) + 1); if(counts.get(user) > maxRequests) { res.status(429).send('Too many requests'); } else { next(); } }; } [OK]

Hint: Factory takes limit param, returns middleware calling next() [OK]

Common Mistakes:

• Putting next as a factory parameter
• Not calling next() in middleware
• Not using maxRequests parameter