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.