Controller pattern for route handlers in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when calling the Express route with the controller pattern?

Consider this Express route setup using a controller function:

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

const userController = {
  getUser: (req, res) => {
    res.send(`User ID is ${req.params.id}`);
  }
};

app.get('/user/:id', userController.getUser);

// Simulate a GET request to /user/42

What will the response body be?

Express

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

const userController = {
  getUser: (req, res) => {
    res.send(`User ID is ${req.params.id}`);
  }
};

app.get('/user/:id', userController.getUser);

// Simulate a GET request to /user/42

A User ID is 42

B User ID is undefined

C Error: req.params is not defined

D User ID is :id

💡 Hint

Think about how Express passes route parameters to the controller function.

The controller function receives the request object with params parsed from the URL. The id parameter is extracted correctly, so the response sends 'User ID is 42'.

📝 Syntax

intermediate

Which option correctly exports a controller function for Express?

You want to create a controller file userController.js that exports a function getUser to handle requests.

Which code snippet correctly exports the function for use in Express routes?

A exports.getUser() { res.send('Hello'); }

B module.exports = { getUser: (req, res) => { res.send('Hello'); } };

C export getUser = (req, res) => { res.send('Hello'); };

D module.exports.getUser = function(req, res) { res.send('Hello'); }

💡 Hint

Remember the Node.js module export syntax.

Option B correctly assigns an object with a function to module.exports.

🔧 Debug

advanced

Why does this controller function cause an error when used as a route handler?

Look at this controller code:

const userController = {
  getUser(req, res) {
    this.findUser(req.params.id).then(user => {
      res.json(user);
    });
  },
  findUser(id) {
    return Promise.resolve({ id, name: 'Alice' });
  }
};

app.get('/user/:id', userController.getUser);

When calling /user/1, it throws an error. Why?

Express

const userController = {
  getUser(req, res) {
    this.findUser(req.params.id).then(user => {
      res.json(user);
    });
  },
  findUser(id) {
    return Promise.resolve({ id, name: 'Alice' });
  }
};

app.get('/user/:id', userController.getUser);

A findUser is not a function error because it is not defined

B res.json is not a function error because res is undefined

C Promise is not defined error due to missing import

D 'this' is undefined inside getUser when called by Express, causing a TypeError

💡 Hint

Think about how this behaves in JavaScript when a method is passed as a callback.

When Express calls the handler, this is not bound to userController. So this.findUser is undefined, causing a TypeError.

❓ state_output

advanced

What is the output after calling this Express controller with async/await?

Given this controller function:

const userController = {
  async getUser(req, res) {
    const user = await this.findUser(req.params.id);
    res.json(user);
  },
  findUser(id) {
    return Promise.resolve({ id, name: 'Bob' });
  }
};

app.get('/user/:id', userController.getUser.bind(userController));

// Simulate GET /user/7

What will the response JSON be?

Express

const userController = {
  async getUser(req, res) {
    const user = await this.findUser(req.params.id);
    res.json(user);
  },
  findUser(id) {
    return Promise.resolve({ id, name: 'Bob' });
  }
};

app.get('/user/:id', userController.getUser.bind(userController));

// Simulate GET /user/7

A {"id":"7","name":"Bob"}

B {"id":7,"name":"Bob"}

C {"id":"req.params.id","name":"Bob"}

D Error: Cannot read property 'findUser' of undefined

💡 Hint

Remember that route parameters are strings by default.

The id from req.params.id is a string '7'. The object returned has id as string. JSON.stringify keeps it as string, so option A is correct if quotes are included. But option A shows id as number 7, which is incorrect because params are strings.

🧠 Conceptual

expert

Which statement best explains the benefit of using the controller pattern in Express route handlers?

Why do developers use the controller pattern to organize route handlers in Express applications?

A It forces all routes to be synchronous, preventing async bugs

B It automatically improves server performance by caching responses

C It separates route definitions from business logic, making code easier to maintain and test

D It eliminates the need for middleware functions in Express

💡 Hint

Think about code organization and clarity.

The controller pattern helps keep route definitions clean and moves the logic to separate controller files or objects. This improves maintainability and testing. The other options are incorrect or unrelated.