Response transformation in NestJS - Mini Project: Build & Apply

Response Transformation in NestJS

📖 Scenario: You are building a simple NestJS API that returns user data. You want to transform the response so that only specific fields are sent back to the client, hiding sensitive information.

🎯 Goal: Create a NestJS controller that returns user data. Use response transformation to send only the id and name fields in the response, excluding password and email.

📋 What You'll Learn

Create a user data object with id, name, email, and password fields

Add a configuration variable to specify which fields to include in the response

Use a method to transform the user data to include only the specified fields

Return the transformed user data from a NestJS controller method

💡 Why This Matters

🌍 Real World

APIs often need to hide sensitive data like passwords or emails before sending responses to clients. Response transformation helps control what data is exposed.

💼 Career

Backend developers use response transformation to secure APIs and improve data privacy, which is critical in real-world applications.

1

Create the user data object

Create a constant called user with these exact fields and values: id: 1, name: 'Alice', email: '[e-mail]', password: 'secret'.

NestJS

# Create the user data object
# Your code here

Hint

Use a const object with the exact keys and values.

2

Add a configuration for included fields

Create a constant called fieldsToInclude and set it to an array containing the strings 'id' and 'name'.

NestJS

const user = {
  id: 1,
  name: 'Alice',
  email: '[e-mail]',
  password: 'secret'
};

# Create the fieldsToInclude array
# Your code here

Hint

Use an array with the exact strings 'id' and 'name'.

3

Create a function to transform the user data

Create a function called transformUser that takes a user object and returns a new object containing only the keys in fieldsToInclude. Use fieldsToInclude.forEach to copy fields.

NestJS

const user = {
  id: 1,
  name: 'Alice',
  email: '[e-mail]',
  password: 'secret'
};

const fieldsToInclude = ['id', 'name'];

# Create transformUser function
# Your code here

Hint

Use a new object and copy only the allowed fields using forEach.

4

Return the transformed user from a NestJS controller method

Create a NestJS controller class called UserController with a method getUser that returns the result of transformUser(user). Use the @Controller('user') and @Get() decorators.

NestJS

const user = {
  id: 1,
  name: 'Alice',
  email: '[e-mail]',
  password: 'secret'
};

const fieldsToInclude = ['id', 'name'];

function transformUser(user) {
  const transformed = {};
  fieldsToInclude.forEach(field => {
    transformed[field] = user[field];
  });
  return transformed;
}

# Create UserController with getUser method
# Your code here

Hint

Use NestJS decorators @Controller and @Get and return the transformed user.

Practice

1. What is the main purpose of response transformation in NestJS?

easy

A. To modify data before sending it to the client

B. To handle database connections

C. To manage user authentication

D. To log server errors

Solution

1. Step 1: Understand response transformation concept

   Response transformation means changing the data before it is sent to the user.

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

   In NestJS, response transformation is used to modify or format the response data.

3. Final Answer:

   To modify data before sending it to the client -> Option A

4. Quick Check:

   Response transformation = modify data before sending [OK]

Hint: Response transformation changes output data before sending [OK]

Common Mistakes:

• Confusing response transformation with authentication
• Thinking it manages database connections
• Assuming it logs errors

2. Which of the following is the correct way to apply a response transformation interceptor in NestJS?

easy

A. controller.transform(TransformInterceptor)

B. @UseInterceptors(new TransformInterceptor())

C. app.use(TransformInterceptor)

D. @Injectable(TransformInterceptor)

Solution

1. Step 1: Recall NestJS interceptor usage

   Interceptors are applied using the @UseInterceptors decorator on controllers or methods.

2. Step 2: Match correct syntax

   The correct syntax is @UseInterceptors(new InterceptorClass()).

3. Final Answer:

   @UseInterceptors(new TransformInterceptor()) -> Option B

4. Quick Check:

   Use @UseInterceptors with new instance [OK]

Hint: Use @UseInterceptors with new interceptor instance [OK]

Common Mistakes:

• Using @Injectable instead of @UseInterceptors
• Calling interceptor as a method on controller
• Trying to use app.use for interceptors

3. Given this interceptor code snippet, what will be the transformed response?

intercept(context, next) {
  return next.handle().pipe(map(data => ({ result: data }))); 
}

medium

A. { data: originalData }

B. originalData

C. { result: originalData }

D. An error will occur

Solution

1. Step 1: Understand the interceptor's map function

   The map operator wraps the original data inside an object with key 'result'.

2. Step 2: Predict the transformed output

   The response will be an object: { result: originalData }.

3. Final Answer:

   { result: originalData } -> Option C

4. Quick Check:

   map wraps data in { result: ... } [OK]

Hint: map operator wraps data in new object key [OK]

Common Mistakes:

• Assuming original data is unchanged
• Confusing key name as 'data' instead of 'result'
• Expecting an error from this code

4. What is wrong with this interceptor code that tries to transform the response?

intercept(context, next) {
  next.handle().pipe(map(data => data.trim()));
}

medium

A. next.handle() should not be called

B. map operator cannot be used inside intercept

C. trim() is not a valid method on data

D. Missing return statement for the observable

Solution

1. Step 1: Check interceptor return value

   Interceptors must return the observable from next.handle().pipe(...).

2. Step 2: Identify missing return

   The code calls next.handle().pipe(...) but does not return it, so the response is undefined.

3. Final Answer:

   Missing return statement for the observable -> Option D

4. Quick Check:

   Always return next.handle().pipe(...) [OK]

Hint: Always return the observable from intercept method [OK]

Common Mistakes:

• Forgetting to return the observable
• Assuming map cannot be used in intercept
• Misusing trim() on non-string data

5. You want to hide the user's password field from all API responses using response transformation. Which approach is best in NestJS?

hard

A. Create an interceptor that maps the response and deletes the password field

B. Modify the database schema to exclude password

C. Use a middleware to filter out password from requests

D. Send password as null in the controller methods

Solution

1. Step 1: Understand response transformation goal

   We want to remove sensitive info from outgoing responses, not database or requests.

2. Step 2: Choose best NestJS method

   Interceptors can transform responses globally, ideal for removing password fields before sending.

3. Step 3: Evaluate other options

   Modifying DB schema or middleware won't affect response data; setting password null in controllers is repetitive.

4. Final Answer:

   Create an interceptor that maps the response and deletes the password field -> Option A

5. Quick Check:

   Use interceptor to hide sensitive response data [OK]

Hint: Use interceptor to remove sensitive fields from responses [OK]

Common Mistakes:

• Trying to filter password in middleware (affects requests)
• Changing database schema instead of response
• Manually nulling password in every controller