Global exception filters in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a global exception filter class in NestJS.

NestJS

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

@Catch()
export class GlobalExceptionFilter implements [1] {
  catch(exception: any, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse();
    response.status(500).json({ message: 'Internal server error' });
  }
}

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

AMiddleware

BPipeTransform

CInterceptor

DExceptionFilter

Attempts:

3 left

💡 Hint

Common Mistakes

Using PipeTransform or Interceptor instead of ExceptionFilter

Not implementing any interface

The class must implement the ExceptionFilter interface to act as an exception filter in NestJS.

2fill in blank

medium

Complete the code to apply the global exception filter in the main bootstrap function.

NestJS

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';
import { GlobalExceptionFilter } from './filters/global-exception.filter';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.[1](new GlobalExceptionFilter());
  await app.listen(3000);
}
bootstrap();

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AuseGlobalFilters

BuseGlobalPipes

CuseGlobalInterceptors

DuseMiddleware

Attempts:

3 left

💡 Hint

Common Mistakes

Using useGlobalPipes or useGlobalInterceptors instead

Trying to use useMiddleware which is not a method on the app instance

To apply a global exception filter, use the useGlobalFilters method on the app instance.

3fill in blank

hard

Fix the error in the global exception filter to correctly get the HTTP response object.

NestJS

catch(exception: any, host: ArgumentsHost) {
  const ctx = host.[1]();
  const response = ctx.getResponse();
  response.status(500).json({ message: 'Internal server error' });
}

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AswitchToHttp

BswitchToRpc

CswitchToWs

DgetHttpContext

Attempts:

3 left

💡 Hint

Common Mistakes

Using switchToRpc or switchToWs which are for other contexts

Using a non-existent method like getHttpContext

To get the HTTP context from ArgumentsHost, use the switchToHttp() method.

4fill in blank

hard

Fill both blanks to create a global exception filter that returns the exception message and status code.

NestJS

catch(exception: any, host: ArgumentsHost) {
  const ctx = host.[1]();
  const response = ctx.getResponse();
  const status = exception.[2] || 500;
  response.status(status).json({
    statusCode: status,
    message: exception.message || 'Internal server error',
  });
}

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AswitchToHttp

BgetStatus

Cstatus

DgetResponse

Attempts:

3 left

💡 Hint

Common Mistakes

Using getStatus() method which does not exist on exception

Using getResponse() instead of switchToHttp()

Use switchToHttp() to get the HTTP context and access the status property from the exception for the HTTP status code.

5fill in blank

hard

Fill all three blanks to create a global exception filter that logs the error and sends a JSON response with status and message.

NestJS

catch(exception: any, host: ArgumentsHost) {
  const ctx = host.[1]();
  const response = ctx.getResponse();
  const request = ctx.getRequest();
  const status = exception.[2] || 500;
  console.error('Error:', exception.message);
  response.status(status).json({
    statusCode: status,
    timestamp: new Date().toISOString(),
    path: request.[3],
    message: exception.message || 'Internal server error',
  });
}

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

AswitchToHttp

Bstatus

Curl

Dmessage

Attempts:

3 left

💡 Hint

Common Mistakes

Using getResponse() instead of switchToHttp()

Using message instead of status for HTTP status code

Using path instead of url for request path

Use switchToHttp() to get HTTP context, status property from exception for status code, and url property from request for the request path.

Practice

1. What is the main purpose of a global exception filter in a NestJS application?

easy

A. To catch and handle exceptions across the entire application in one place

B. To define routes for the application

C. To manage database connections

D. To serve static files

Solution

1. Step 1: Understand the role of exception filters

   Exception filters in NestJS are designed to catch errors and exceptions that occur during request processing.

2. Step 2: Identify the scope of global exception filters

   Global exception filters apply this error handling across the entire app, centralizing error management.

3. Final Answer:

   To catch and handle exceptions across the entire application in one place -> Option A

4. Quick Check:

   Global exception filters = app-wide error handling [OK]

Hint: Global filters catch errors everywhere in the app [OK]

Common Mistakes:

• Confusing filters with routing or database management
• Thinking filters serve static files
• Assuming filters only handle specific routes

2. Which of the following is the correct way to apply a global exception filter in the main.ts file of a NestJS app?

easy

A. app.useGlobalFilters(new MyExceptionFilter())

B. app.setGlobalFilters(new MyExceptionFilter())

C. app.applyGlobalFilters(MyExceptionFilter)

D. app.registerGlobalFilter(MyExceptionFilter)

Solution

1. Step 1: Recall the NestJS method for global filters

   The official method to apply global exception filters is useGlobalFilters() on the app instance.

2. Step 2: Check the syntax for passing the filter

   The filter must be instantiated with new before passing it to useGlobalFilters().

3. Final Answer:

   app.useGlobalFilters(new MyExceptionFilter()) -> Option A

4. Quick Check:

   Global filter method = useGlobalFilters() [OK]

Hint: Use app.useGlobalFilters(new Filter()) in main.ts [OK]

Common Mistakes:

• Using incorrect method names like setGlobalFilters
• Passing class without instantiation
• Confusing with middleware registration methods

3. Given this global exception filter code snippet, what will be the HTTP status code returned when an exception is thrown?

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

@Catch()
export class AllExceptionsFilter implements ExceptionFilter {
  catch(exception: unknown, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse();

    if (exception instanceof HttpException) {
      const status = exception.getStatus();
      response.status(status).json({ message: exception.message });
    } else {
      response.status(500).json({ message: 'Internal server error' });
    }
  }
}

medium

A. 200

B. 500

C. Depends on the exception type

D. 404

Solution

1. Step 1: Analyze the filter's catch method

   The filter checks if the exception is an instance of HttpException and uses its status code.

2. Step 2: Determine status code for other exceptions

   If not an HttpException, it returns status 500 with a generic error message.

3. Final Answer:

   Depends on the exception type -> Option C

4. Quick Check:

   HttpException status or 500 for others [OK]

Hint: Check if exception is HttpException for status code [OK]

Common Mistakes:

• Assuming always 500 status
• Ignoring HttpException status extraction
• Confusing 404 with 500 errors

4. Identify the error in this global exception filter implementation:

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

@Catch()
export class MyFilter implements ExceptionFilter {
  catch(exception: any, host: ArgumentsHost) {
    const response = host.getResponse();
    response.status(400).json({ error: 'Bad Request' });
  }
}

medium

A. Missing @Injectable() decorator on the filter class

B. The @Catch() decorator must specify exception types

C. The catch method must return a value

D. host.getResponse() is incorrect; should use host.switchToHttp().getResponse()

Solution

1. Step 1: Check how to get response object

   In NestJS, to get the HTTP response, use host.switchToHttp().getResponse(), not host.getResponse().

2. Step 2: Verify other parts

   The @Catch() decorator can be empty to catch all exceptions, and the catch method does not need to return a value. @Injectable() is not required for filters.

3. Final Answer:

   host.getResponse() is incorrect; should use host.switchToHttp().getResponse() -> Option D

4. Quick Check:

   Use switchToHttp() to access response [OK]

Hint: Use host.switchToHttp().getResponse() to get response [OK]

Common Mistakes:

• Calling host.getResponse() directly
• Thinking @Injectable() is mandatory for filters
• Expecting catch to return a value

5. You want to create a global exception filter that logs all exceptions and returns a JSON response with a custom message and status code 418 (I'm a teapot) for all errors except HttpExceptions, which should keep their original status. How should you implement the catch method?

hard

A. Always respond with status 418 and ignore HttpException status

B. Check if exception is HttpException, use its status; else log and respond with 418 and custom message

C. Only log exceptions but do not modify the response

D. Throw the exception again to let NestJS handle it

Solution

1. Step 1: Handle HttpException separately

   Check if the exception is an instance of HttpException to preserve its status and message.

2. Step 2: For other exceptions, log and respond with status 418

   Log the error details, then send a JSON response with status 418 and a custom message.

3. Final Answer:

   Check if exception is HttpException, use its status; else log and respond with 418 and custom message -> Option B

4. Quick Check:

   Preserve HttpException status, else respond 418 with log [OK]

Hint: Preserve HttpException status; else respond 418 with log [OK]

Common Mistakes:

• Ignoring HttpException status and always sending 418
• Not logging exceptions
• Re-throwing exceptions instead of handling