DTO pattern for data transfer in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the main purpose of a DTO in Express applications?

In Express, why do developers use a Data Transfer Object (DTO)?

A To store session data on the server for each user automatically.

B To define a strict structure for data sent between client and server, improving validation and security.

C To directly connect the database models with the user interface without any changes.

D To replace middleware functions for routing requests.

💡 Hint

Think about how data is shaped and controlled when moving between parts of an app.

A DTO defines a clear data structure for transferring data, helping validate and secure the data flow. It does not replace routing or store sessions.

❓ component_behavior

intermediate

What output does this Express DTO code produce?

Given this DTO and Express route, what will the server respond with when sending {"name": "Alice", "age": 30, "extra": "ignore"}?

Express

class UserDTO {
  constructor(data) {
    this.name = data.name;
    this.age = data.age;
  }
}

app.post('/user', (req, res) => {
  const userDto = new UserDTO(req.body);
  res.json(userDto);
});

A {"name":"Alice","age":30}

B {"name":"Alice","age":30,"extra":"ignore"}

C {"name":"Alice"}

D {}

💡 Hint

Look at which properties the DTO constructor copies from the input.

The DTO only copies 'name' and 'age' properties, ignoring 'extra'. So the response JSON includes only those two.

📝 Syntax

advanced

Which option correctly defines a DTO class in Express to accept only 'title' and 'content' fields?

Choose the correct DTO class that safely extracts only 'title' and 'content' from input data.

A

class PostDTO {
  constructor(data) {
    this.title = data.title;
    this.content = data.content;
  }
}

B

class PostDTO {
  constructor(data) {
    this.title = data.title;
    this.content = data.content;
    this.author = data.author;
  }
}

C

class PostDTO {
  constructor(data) {
    this.title = data.title;
    this.content = data.content;
    this.date = new Date();
  }
}

D

class PostDTO {
  constructor(data) {
    this.title = data.title;
    this.content = data.content;
    this.tags = data.tags;
  }
}

💡 Hint

Only include the fields 'title' and 'content' exactly as requested.

Option A includes only 'title' and 'content'. Others add extra fields not requested.

🔧 Debug

advanced

What error does this DTO code cause in Express?

Identify the error when this DTO is used in an Express route:

class ProductDTO {
  constructor(data) {
    this.name = data.name;
    this.price = data.price;
  }
}

app.post('/product', (req, res) => {
  const product = new ProductDTO();
  res.json(product);
});

A No error, returns empty object

B SyntaxError: Unexpected token in class definition

C TypeError: Cannot read property 'name' of undefined

D ReferenceError: ProductDTO is not defined

💡 Hint

Check how the constructor is called and what arguments it expects.

The constructor expects a data object but is called without arguments, so 'data' is undefined and accessing 'data.name' causes a TypeError.

❓ state_output

expert

What is the final value of 'filteredData' after applying this DTO function?

Consider this function that uses a DTO pattern to filter input data:

function filterUserData(input) {
  const allowedFields = ['username', 'email'];
  const filteredData = {};
  for (const key in input) {
    if (allowedFields.includes(key)) {
      filteredData[key] = input[key];
    }
  }
  return filteredData;
}

const input = { username: 'bob', email: '[e-mail]', password: 'secret' };
const filteredData = filterUserData(input);

What is the value of filteredData?

A {}

B {"username":"bob","email":"[e-mail]","password":"secret"}

C {"password":"secret"}

D {"username":"bob","email":"[e-mail]"}

💡 Hint

Only keys in allowedFields should be included in filteredData.

The function copies only 'username' and 'email' from input, ignoring 'password'. So filteredData has only those two keys.

Practice

1. What is the main purpose of using a DTO (Data Transfer Object) in an Express application?

easy

A. To clearly define and control the data sent between different parts of the app

B. To handle database connections directly

C. To style the user interface components

D. To manage server configuration settings

Solution

1. Step 1: Understand the role of DTOs

   DTOs are simple objects designed to carry data between parts of an application clearly and safely.

2. Step 2: Identify the correct purpose in Express context

   In Express, DTOs help control what data is sent or received, improving code clarity and security.

3. Final Answer:

   To clearly define and control the data sent between different parts of the app -> Option A

4. Quick Check:

   DTO purpose = data control and clarity [OK]

Hint: DTOs move data cleanly between app parts [OK]

Common Mistakes:

• Confusing DTOs with database or UI code
• Thinking DTOs handle server setup
• Assuming DTOs manage styling

2. Which of the following is the correct way to define a simple DTO class in Express using JavaScript?

easy

A. function UserDTO(name, email) { return { name, email }; }

B. let UserDTO = (name, email) => { this.name = name; this.email = email; };

C. const UserDTO = { name: '', email: '' };

D. class UserDTO { constructor(name, email) { this.name = name; this.email = email; } }

Solution

1. Step 1: Review class syntax for DTO

   A DTO is often a simple class with a constructor to assign properties.

2. Step 2: Check each option's correctness

   class UserDTO { constructor(name, email) { this.name = name; this.email = email; } } correctly defines a class with a constructor assigning properties. function UserDTO(name, email) { return { name, email }; } returns an object but is a function, not a class. const UserDTO = { name: '', email: '' }; is a plain object, not a class. let UserDTO = (name, email) => { this.name = name; this.email = email; }; uses arrow function incorrectly with 'this'.

3. Final Answer:

   class UserDTO { constructor(name, email) { this.name = name; this.email = email; } } -> Option D

4. Quick Check:

   DTO class = constructor with properties [OK]

Hint: Use class with constructor for DTOs [OK]

Common Mistakes:

• Using arrow functions with 'this' in DTOs
• Defining DTO as plain object without constructor
• Returning object from function instead of class

3. Given this DTO and Express route, what will be the JSON response when sending {"name":"Alice","email":"[e-mail]","password":"secret"} in the request body?

class UserDTO {
  constructor(name, email) {
    this.name = name;
    this.email = email;
  }
}

app.post('/user', (req, res) => {
  const userDto = new UserDTO(req.body.name, req.body.email);
  res.json(userDto);
});

medium

A. {"name":"Alice","email":"[e-mail]","password":"secret"}

B. {"name":"Alice","email":"[e-mail]"}

C. {"password":"secret"}

D. {}

Solution

1. Step 1: Understand DTO construction

   The UserDTO constructor only assigns name and email properties, ignoring password.

2. Step 2: Analyze response JSON

   The response sends the userDto object, which includes only name and email fields.

3. Final Answer:

   {"name":"Alice","email":"[e-mail]"} -> Option B

4. Quick Check:

   DTO filters out extra fields [OK]

Hint: DTO only includes defined properties [OK]

Common Mistakes:

• Expecting password to be included in response
• Confusing request body with DTO properties
• Assuming all request fields are sent back

4. Identify the error in this DTO usage in an Express route:

class ProductDTO {
  constructor(id, name) {
    this.id = id;
    this.name = name;
  }
}

app.post('/product', (req, res) => {
  const productDto = ProductDTO(req.body.id, req.body.name);
  res.json(productDto);
});

medium

A. Response should use res.send instead of res.json

B. Incorrect property names in constructor

C. Missing 'new' keyword when creating ProductDTO instance

D. DTO class should be a function, not a class

Solution

1. Step 1: Check how DTO instance is created

   The code calls ProductDTO as a function without 'new', which is incorrect for classes.

2. Step 2: Understand class instantiation in JavaScript

   Classes must be instantiated with 'new' keyword to create an object.

3. Final Answer:

   Missing 'new' keyword when creating ProductDTO instance -> Option C

4. Quick Check:

   Class instantiation requires 'new' [OK]

Hint: Always use 'new' with class constructors [OK]

Common Mistakes:

• Calling class like a regular function
• Confusing res.send and res.json usage
• Thinking DTO must be a function, not class

5. You want to create a DTO that only accepts non-empty strings for title and description in an Express app. Which approach correctly enforces this validation inside the DTO class?

hard

A. class TaskDTO {
constructor(title, description) {
if (!title || typeof title !== 'string') throw new Error('Invalid title');
if (!description || typeof description !== 'string') throw new Error('Invalid description');
this.title = title;
this.description = description;
}
}

B. class TaskDTO {
constructor(title, description) {
this.title = title || 'default';
this.description = description || 'default';
}
}

C. class TaskDTO {
constructor(title, description) {
this.title = title;
this.description = description;
}
}

D. function createTaskDTO(title, description) {
return { title, description };
}

Solution

1. Step 1: Understand validation needs

   The DTO must check that title and description are non-empty strings and throw errors if not.

2. Step 2: Evaluate each option's validation

   class TaskDTO { constructor(title, description) { if (!title || typeof title !== 'string') throw new Error('Invalid title'); if (!description || typeof description !== 'string') throw new Error('Invalid description'); this.title = title; this.description = description; } } explicitly checks and throws errors for invalid inputs. class TaskDTO { constructor(title, description) { this.title = title || 'default'; this.description = description || 'default'; } } assigns defaults instead of validating. class TaskDTO { constructor(title, description) { this.title = title; this.description = description; } } assigns without checks. function createTaskDTO(title, description) { return { title, description }; } is a function without validation.

3. Final Answer:

   class TaskDTO { constructor(title, description) { if (!title || typeof title !== 'string') throw new Error('Invalid title'); if (!description || typeof description !== 'string') throw new Error('Invalid description'); this.title = title; this.description = description; } } -> Option A

4. Quick Check:

   Validation inside constructor = class TaskDTO { constructor(title, description) { if (!title || typeof title !== 'string') throw new Error('Invalid title'); if (!description || typeof description !== 'string') throw new Error('Invalid description'); this.title = title; this.description = description; } } [OK]

Hint: Validate inputs inside constructor, throw errors if invalid [OK]

Common Mistakes:

• Skipping validation and assigning defaults silently
• Using functions without validation for DTOs
• Not throwing errors on invalid data