Why structured errors improve API quality in NestJS - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to throw a structured error with a message.

NestJS

throw new HttpException({ message: [1] }, HttpStatus.BAD_REQUEST);

Drag options to blanks, or click blank then click option'

A"Invalid input"

BInvalid input

C400

DHttpStatus.BAD_REQUEST

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the message string

Using a number instead of a string for the message

The error message must be a string inside quotes to be valid JSON in the error body.

2fill in blank

medium

Complete the code to create a structured error with a custom error code.

NestJS

throw new HttpException({ message: 'Not found', code: [1] }, HttpStatus.NOT_FOUND);

Drag options to blanks, or click blank then click option'

A"404_NOT_FOUND"

B404_NOT_FOUND

CHttpStatus.NOT_FOUND

D"NOT_FOUND"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name instead of a string

Omitting quotes around the code

The error code should be a string inside quotes to be part of the structured error object.

3fill in blank

hard

Fix the error in the code to properly return a structured error response.

NestJS

return response.status(HttpStatus.BAD_REQUEST).json({ error: [1] });

Drag options to blanks, or click blank then click option'

A"Bad Request"

BHttpStatus.BAD_REQUEST

C{ message: 'Bad Request' }

DBad Request

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a plain string instead of an object

Using a variable without quotes

The error field should be an object with a message property for structured errors.

4fill in blank

hard

Fill both blanks to create a structured error with message and details.

NestJS

throw new HttpException({ message: [1], details: [2] }, HttpStatus.CONFLICT);

Drag options to blanks, or click blank then click option'

A"Conflict occurred"

B"Resource already exists"

C{ resource: 'User' }

D[ 'Duplicate entry' ]

Attempts:

3 left

💡 Hint

Common Mistakes

Using array instead of object for details

Omitting quotes around message

The message is a string describing the error, and details is an object with extra info.

5fill in blank

hard

Fill all three blanks to create a structured error with message, code, and timestamp.

NestJS

throw new HttpException({ message: [1], code: [2], timestamp: [3] }, HttpStatus.INTERNAL_SERVER_ERROR);

Drag options to blanks, or click blank then click option'

A"Server error"

B"500_INTERNAL_ERROR"

Cnew Date().toISOString()

DDate.now()

Attempts:

3 left

💡 Hint

Common Mistakes

Using Date.now() which returns a number instead of string

Omitting quotes around strings

The message and code are strings, and timestamp is an ISO string from the current date.

Practice

1. Why is it important to use structured errors in a NestJS API?

easy

A. They provide consistent and clear error messages for clients.

B. They make the API run faster by skipping error checks.

C. They hide error details to prevent debugging.

D. They automatically fix bugs in the code.

Solution

1. Step 1: Understand the role of structured errors

   Structured errors give clear, consistent feedback about what went wrong in the API.

2. Step 2: Identify the benefit for clients

   Clients get meaningful messages that help them understand and fix issues faster.

3. Final Answer:

   They provide consistent and clear error messages for clients. -> Option A

4. Quick Check:

   Clear error messages = A [OK]

Hint: Structured errors give clear feedback, not speed or auto-fixes [OK]

Common Mistakes:

• Thinking errors speed up the API
• Believing errors hide details always
• Assuming errors fix bugs automatically

2. Which of the following is the correct way to throw a structured HTTP error in NestJS?

easy

A. throw new HttpException({ message: 'Error occurred', status: 400 }, 400);

B. throw HttpException('Error occurred', 400);

C. throw new HttpException('Error occurred', 400);

D. throw new Error('HttpException', 400);

Solution

1. Step 1: Recall NestJS HttpException syntax

   HttpException constructor accepts a response object and a status code.

2. Step 2: Identify the correct structured error format

   Passing an object with message and status as the first argument is the proper way.

3. Final Answer:

   throw new HttpException({ message: 'Error occurred', status: 400 }, 400); -> Option A

4. Quick Check:

   HttpException with object and status = A [OK]

Hint: Use new HttpException({message, status}, code) syntax [OK]

Common Mistakes:

• Omitting 'new' keyword
• Passing string only without object
• Using generic Error instead of HttpException

3. Given this NestJS controller snippet, what will the client receive when getUser is called with an invalid ID?

import { Controller, Get, Param, HttpException, HttpStatus } from '@nestjs/common';

@Controller('users')
export class UserController {
  @Get(':id')
  getUser(@Param('id') id: string) {
    if (id !== '123') {
      throw new HttpException({ error: 'User not found', code: 'USER_404' }, HttpStatus.NOT_FOUND);
    }
    return { id, name: 'Alice' };
  }
}

medium

A. "User not found" string with 404 status

B. { "error": "User not found", "code": "USER_404" } with 404 status

C. { "message": "User not found" } with 200 status

D. Empty response with 404 status

Solution

1. Step 1: Analyze the error thrown for invalid ID

   The code throws HttpException with an object containing error and code, and status 404.

2. Step 2: Understand the client response

   The client receives the JSON object with error details and HTTP 404 status.

3. Final Answer:

   { "error": "User not found", "code": "USER_404" } with 404 status -> Option B

4. Quick Check:

   Structured error object with 404 = B [OK]

Hint: HttpException sends object and status as response [OK]

Common Mistakes:

• Expecting plain string instead of object
• Assuming 200 status on error
• Missing error code in response

4. Identify the problem in this NestJS error handling code:

throw new HttpException('Invalid input', 400);

Why might this be less helpful for API clients?

medium

A. It should use a custom error class instead of HttpException.

B. It uses the wrong status code for invalid input.

C. It does not log the error to the console.

D. It throws a string instead of a structured object, limiting error details.

Solution

1. Step 1: Examine the error thrown

   The error is a simple string message without structure.

2. Step 2: Understand impact on API clients

   Clients get only a string, missing extra info like error codes or fields.

3. Final Answer:

   It throws a string instead of a structured object, limiting error details. -> Option D

4. Quick Check:

   String error limits details = C [OK]

Hint: Use objects for detailed errors, not plain strings [OK]

Common Mistakes:

• Confusing status code correctness
• Thinking custom classes are mandatory
• Ignoring logging importance here

5. You want to improve your NestJS API error responses by adding a timestamp and a unique error ID to every error. Which approach best uses structured errors to achieve this?

hard

A. Use try-catch blocks to catch errors and ignore timestamp and errorId.

B. Add console.log statements with timestamp and errorId before throwing errors.

C. Extend HttpException to include timestamp and errorId in the response object.

D. Throw plain strings with timestamp and errorId concatenated.

Solution

1. Step 1: Understand how to add extra info to errors

   Extending HttpException allows customizing the error response object with extra fields.

2. Step 2: Evaluate options for structured error enhancement

   Adding timestamp and errorId inside the error object keeps responses consistent and useful for clients.

3. Final Answer:

   Extend HttpException to include timestamp and errorId in the response object. -> Option C

4. Quick Check:

   Custom HttpException with extra fields = D [OK]

Hint: Customize HttpException to add fields, not just logs or strings [OK]

Common Mistakes:

• Thinking console logs affect API response
• Using plain strings loses structure
• Ignoring error details in catch blocks