Middleware factory pattern in Express

Introduction

The middleware factory pattern helps create reusable middleware functions that can be customized with different settings.

When you want to create middleware that behaves differently based on input parameters.

When you need to reuse similar middleware logic with small variations.

When you want to keep your code clean by avoiding repeated middleware code.

When building APIs that require configurable request handling.

When you want to add logging or validation with different rules per route.

Syntax

Express

function middlewareFactory(options) {
  return function middleware(req, res, next) {
    // middleware logic using options
    next();
  };
}

The factory function returns the actual middleware function.

You can pass any configuration to customize middleware behavior.

Examples

This factory creates middleware that logs a greeting message.

Express

function greetFactory(greeting) {
  return function (req, res, next) {
    console.log(`${greeting}, user!`);
    next();
  };
}

This factory creates middleware to check if the user has a specific role.

Express

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

Sample Program

This Express app uses a middleware factory to create logging middleware with different messages. The first middleware logs a message for all requests. The second logs a message only for GET / requests.

Express

import express from 'express';

const app = express();

// Middleware factory that logs a custom message
function loggerFactory(message) {
  return function (req, res, next) {
    console.log(message);
    next();
  };
}

// Use middleware with different messages
app.use(loggerFactory('First middleware running'));
app.get('/', loggerFactory('Handling GET /'), (req, res) => {
  res.send('Hello from middleware factory!');
});

app.listen(3000, () => {
  console.log('Server started on http://localhost:3000');
});

Important Notes

Middleware factories help keep your code DRY (Don't Repeat Yourself).

Always call next() inside middleware to continue the request cycle.

You can pass any data to the factory to customize middleware behavior.

Summary

Middleware factory pattern creates customizable middleware functions.

It helps reuse similar middleware logic with different settings.

Use it to keep your Express code clean and flexible.

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