Catch decorator in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when an exception is caught by a Catch decorator?

Consider a NestJS filter using the @Catch() decorator that catches HttpException. What will be the response status code if the filter catches an exception with status 404?

NestJS

import { ExceptionFilter, Catch, ArgumentsHost, HttpException } from '@nestjs/common';

@Catch(HttpException)
export class NotFoundFilter implements ExceptionFilter {
  catch(exception: HttpException, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse();
    const status = exception.getStatus();
    response.status(status).json({
      statusCode: status,
      message: 'Resource not found',
    });
  }
}

A The response status code will be 404 with message 'Resource not found'.

B The response status code will be 200 with message 'OK'.

C The response status code will be 500 with message 'Internal server error'.

D The response status code will be 400 with message 'Bad request'.

💡 Hint

Think about what exception.getStatus() returns for an HttpException.

The @Catch(HttpException) decorator catches exceptions of type HttpException. The getStatus() method returns the HTTP status code of the exception, which in this case is 404. The filter sets the response status to this code and sends a JSON message.

📝 Syntax

intermediate

Which option correctly applies the Catch decorator to catch multiple exception types?

In NestJS, how do you correctly use the @Catch() decorator to catch both HttpException and TypeError exceptions?

A

@Catch(HttpException, TypeError)
export class MultiCatchFilter implements ExceptionFilter { ... }

B

@Catch(HttpException || TypeError)
export class MultiCatchFilter implements ExceptionFilter { ... }

C

@Catch({HttpException, TypeError})
export class MultiCatchFilter implements ExceptionFilter { ... }

D

@Catch([HttpException, TypeError])
export class MultiCatchFilter implements ExceptionFilter { ... }

💡 Hint

Remember the decorator expects an array for multiple exception types.

The @Catch() decorator accepts either a single exception class or an array of exception classes. To catch multiple exceptions, you must pass an array like @Catch([HttpException, TypeError]). Other syntaxes are invalid.

🔧 Debug

advanced

Why does this Catch decorator not catch exceptions as expected?

Given this filter, why does it fail to catch HttpException errors?

NestJS

import { ExceptionFilter, Catch, ArgumentsHost } from '@nestjs/common';

@Catch()
export class EmptyCatchFilter implements ExceptionFilter {
  catch(exception: any, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse();
    response.status(500).json({ message: 'Caught an error' });
  }
}

A Because the catch method is missing the exception type parameter.

B Because @Catch() with no arguments does not catch any exceptions.

C Because the filter does not call next() to continue the pipeline.

D Because the response object is not properly retrieved from ArgumentsHost.

💡 Hint

Check what happens when @Catch() has no parameters.

The @Catch() decorator requires at least one exception type to catch. If no arguments are provided, the filter will not catch any exceptions. To catch all exceptions, you must use @Catch() with no parameters only if you implement a global filter, but in this context, it does not catch anything.

❓ state_output

advanced

What is the JSON response output of this Catch filter?

What JSON response will be sent when this filter catches a BadRequestException?

NestJS

import { ExceptionFilter, Catch, ArgumentsHost, BadRequestException } from '@nestjs/common';

@Catch(BadRequestException)
export class BadRequestFilter implements ExceptionFilter {
  catch(exception: BadRequestException, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse();
    const status = exception.getStatus();
    response.status(status).json({
      error: 'Bad Request',
      details: exception.message,
    });
  }
}

A {"statusCode":400,"message":"Bad Request Exception"}

B {"error":"Bad Request","details":["Bad Request Exception"]}

C {"error":"Bad Request","details":"Bad Request Exception"}

D {"error":"Bad Request","details":null}

💡 Hint

Check how exception.message is used in the JSON response.

The filter sends a JSON with keys error and details. The details field contains the exception's message string. So the output is a JSON string with those keys and the message as a string.

🧠 Conceptual

expert

Which statement about the Catch decorator's behavior is true?

Choose the correct statement about how the @Catch() decorator works in NestJS exception filters.

A The <code>@Catch()</code> decorator can catch exceptions thrown both synchronously and asynchronously in the request lifecycle.

B The <code>@Catch()</code> decorator can only catch exceptions thrown synchronously inside controllers.

C The <code>@Catch()</code> decorator automatically logs all caught exceptions to the console.

D The <code>@Catch()</code> decorator must be used on every controller method to catch exceptions.

💡 Hint

Think about how NestJS handles exceptions in async code.

The @Catch() decorator applies to exception filters that catch exceptions thrown anywhere in the request lifecycle, including asynchronous operations. It does not automatically log exceptions, nor must it be applied on every method. It works globally or scoped to controllers/providers.