Middleware factory pattern in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this middleware factory produce?

Consider this Express middleware factory function. What will be the output when a request hits the route using this middleware?

Express

function greetMiddleware(greeting) {
  return function(req, res, next) {
    res.locals.message = `${greeting}, user!`;
    next();
  };
}

app.use(greetMiddleware('Hello'));
app.get('/', (req, res) => {
  res.send(res.locals.message);
});

A The response will be 'Hello, user!'

B The response will be 'greeting, user!'

C The server will crash with an error

D The response will be undefined

💡 Hint

Think about how the factory function uses the greeting parameter inside the returned middleware.

The factory function greetMiddleware takes a greeting string and returns a middleware function. This middleware sets res.locals.message to the greeting plus ', user!'. When the route handler sends res.locals.message, it outputs 'Hello, user!'.

📝 Syntax

intermediate

Identify the syntax error in this middleware factory

Which option correctly fixes the syntax error in this middleware factory code?

Express

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

A No syntax error, code is correct

B Add curly braces around the else block

C Add a missing closing parenthesis after next()

D Add a semicolon after next() call

💡 Hint

Check if arrow functions require braces or semicolons in this context.

The arrow function syntax is valid without semicolons after next() and the else block is correctly formed. No syntax error exists.

🔧 Debug

advanced

Why does this middleware factory not work as expected?

This middleware factory is supposed to log a custom message. Why does it always log 'undefined'?

Express

function logMessage(msg) {
  return function(req, res, next) {
    console.log(message);
    next();
  };
}

app.use(logMessage('Request received'));

A The middleware does not call next(), so it hangs

B The variable 'message' is not defined; should use 'msg' instead

C The factory function does not return a function

D The console.log syntax is incorrect

💡 Hint

Check variable names inside the returned middleware function.

The middleware tries to log 'message' which is undefined. The parameter is named 'msg', so it should log 'msg'.

❓ state_output

advanced

What is the value of res.locals after this middleware runs?

Given this middleware factory and usage, what will be the value of res.locals.user after the middleware runs?

Express

function userMiddleware(name) {
  return (req, res, next) => {
    res.locals.user = { name };
    next();
  };
}

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

app.get('/', (req, res) => {
  res.json(res.locals.user);
});

A {"user":"Alice"}

B {}

C {"name":"Alice"}

D undefined

💡 Hint

Look at how res.locals.user is assigned inside the middleware.

The middleware sets res.locals.user to an object with a name property equal to the passed name. So res.locals.user is { name: 'Alice' }.

🧠 Conceptual

expert

Why use a middleware factory instead of a simple middleware?

Which reason best explains why middleware factories are useful in Express?

A They replace the need for route handlers entirely

B They improve server performance by caching middleware

C They automatically handle errors without extra code

D They allow creating middleware with custom parameters for reuse

💡 Hint

Think about how you can customize middleware behavior.

Middleware factories let you create middleware functions that can be customized with parameters, making them reusable with different settings.