Custom exception classes in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when a custom exception is thrown in NestJS?

Consider this custom exception class in NestJS:

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

export class MyCustomException extends HttpException {
  constructor() {
    super('Custom error occurred', HttpStatus.BAD_REQUEST);
  }
}

If this exception is thrown inside a controller method, what HTTP status code and message will the client receive?

A Status code 400 with message 'Custom error occurred'

B Status code 500 with message 'Internal server error'

C Status code 404 with message 'Not Found'

D Status code 200 with message 'OK'

💡 Hint

Remember that HttpException allows setting both message and status code.

The custom exception extends HttpException with status BAD_REQUEST (400) and a custom message. NestJS sends this status and message to the client.

📝 Syntax

intermediate

Which option correctly defines a custom exception class in NestJS?

You want to create a custom exception class named `UnauthorizedException` that returns status 401 with message 'Access denied'. Which code snippet is correct?

A

export class UnauthorizedException extends Error {
  constructor() {
    super('Access denied');
    this.status = 401;
  }
}

B

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

export class UnauthorizedException extends HttpException {
  constructor() {
    super('Access denied', HttpStatus.UNAUTHORIZED);
  }
}

C

export class UnauthorizedException extends HttpException {
  constructor() {
    super(HttpStatus.UNAUTHORIZED, 'Access denied');
  }
}

D

export class UnauthorizedException {
  constructor() {
    this.message = 'Access denied';
    this.status = 401;
  }
}

💡 Hint

HttpException constructor takes message first, then status code.

Option B correctly extends HttpException and calls super with message then status code. Other options either misuse parameters or don't extend HttpException.

🔧 Debug

advanced

Why does this custom exception not return the expected status code?

Given this code:

export class MyException extends HttpException {
  constructor() {
    super(HttpStatus.FORBIDDEN, 'Forbidden access');
  }
}

When thrown, the client receives status 500 instead of 403. Why?

A The parameters to super() are reversed; message should come first, then status code.

B HttpStatus.FORBIDDEN is not a valid status code.

C The class must implement an interface to set status code.

D NestJS does not support custom exceptions.

💡 Hint

Check the order of arguments in HttpException constructor.

HttpException expects message first, then status code. Passing status code first causes unexpected behavior and defaults to 500.

❓ state_output

advanced

What is the response body when throwing a custom exception with a response object?

Consider this custom exception:

export class ValidationException extends HttpException {
  constructor(errors: string[]) {
    super({ message: 'Validation failed', errors }, HttpStatus.UNPROCESSABLE_ENTITY);
  }
}

What will the client receive as the JSON response body when this exception is thrown?

A "Validation failed"

B {"error":"Unprocessable Entity"}

C {"statusCode":422,"message":"Validation failed"}

D {"message":"Validation failed","errors":["error1","error2"]}

💡 Hint

Passing an object as the first argument to HttpException sets the response body.

When an object is passed as the first argument, NestJS uses it as the JSON response body exactly.

🧠 Conceptual

expert

Which statement about custom exceptions in NestJS is TRUE?

Select the correct statement about creating and using custom exception classes in NestJS.

A NestJS requires custom exceptions to be registered in a module before use.

B Custom exceptions can extend any class and NestJS will automatically detect the status code from the class name.

C Custom exceptions must extend HttpException and call super with message then status code to set HTTP response correctly.

D Custom exceptions do not need to call super() if they implement the IException interface.

💡 Hint

Think about how NestJS handles exceptions and status codes.

Only exceptions extending HttpException and calling super properly set the HTTP response status and message. Other options are incorrect.