Exception filters in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of an Exception Filter in NestJS?

An Exception Filter in NestJS catches errors thrown during request handling and allows you to control the response sent back to the client, improving error management.

beginner

How do you create a custom Exception Filter in NestJS?

You create a class that implements the <code>ExceptionFilter</code> interface and define a <code>catch()</code> method to handle exceptions and send custom responses.

beginner

Which decorator is used to bind an Exception Filter to a controller or route in NestJS?

The @UseFilters() decorator is used to apply an Exception Filter to a controller or specific route handler.

intermediate

What is the role of the ArgumentsHost parameter in the catch() method of an Exception Filter?

ArgumentsHost provides access to the underlying platform's request and response objects, allowing you to customize the error response based on the context (HTTP, WebSocket, etc.).

intermediate

Can Exception Filters in NestJS handle both synchronous and asynchronous exceptions?

Yes, Exception Filters can catch exceptions thrown synchronously or asynchronously during request processing, making error handling consistent.

Which interface must a custom Exception Filter implement in NestJS?

A FilterInterface

B ErrorHandler

C Catchable

D ExceptionFilter

Custom Exception Filters must implement the ExceptionFilter interface to define the catch() method.

What decorator applies an Exception Filter to a specific route or controller?

A @UseFilters()

B @HandleError()

C @Exception()

D @Catch()

@UseFilters() binds Exception Filters to controllers or routes.

What does the catch() method receive as arguments?

A Request and Response objects

B Exception object and ArgumentsHost

C Only the Exception object

D Only the ArgumentsHost

The catch() method receives the thrown exception and an ArgumentsHost to access request/response.

Which of these is NOT a typical use of Exception Filters?

A Handling database migrations

B Logging errors

C Transforming error responses

D Sending custom HTTP status codes

Exception Filters do not handle database migrations; they focus on error handling.

Can Exception Filters handle exceptions from asynchronous code in NestJS?

A Only exceptions from Promises

B No, only synchronous exceptions

C Yes, they handle both sync and async exceptions

D Only exceptions from Observables

Exception Filters catch exceptions thrown synchronously or asynchronously.

Explain how to create and apply a custom Exception Filter in NestJS.

Think about the class, method, and decorator involved.

Describe the benefits of using Exception Filters in a NestJS application.

Consider how filters improve error management and user experience.

Practice

1. What is the main purpose of an exception filter in NestJS?

easy

A. To catch and handle errors in a centralized way

B. To create database connections automatically

C. To define routes for HTTP requests

D. To manage user authentication

Solution

1. Step 1: Understand what exception filters do

   Exception filters catch errors thrown during request handling and allow custom responses.

2. Step 2: Compare with other options

   Options B, C, and D relate to other NestJS features, not error handling.

3. Final Answer:

   To catch and handle errors in a centralized way -> Option A

4. Quick Check:

   Exception filters = centralized error handling [OK]

Hint: Exception filters handle errors centrally, not routes or auth [OK]

Common Mistakes:

• Confusing exception filters with middleware
• Thinking filters manage routes or database
• Assuming filters handle authentication

2. Which decorator is used to define an exception filter class in NestJS?

easy

A. @Exception()

B. @Catch()

C. @Filter()

D. @ErrorHandler()

Solution

1. Step 1: Recall the decorator for exception filters

   The correct decorator to mark a class as an exception filter is @Catch().

2. Step 2: Verify other decorators

   @Exception(), @Filter(), and @ErrorHandler() are not valid NestJS decorators for filters.

3. Final Answer:

   @Catch() -> Option B

4. Quick Check:

   @Catch() marks exception filters [OK]

Hint: Remember: @Catch() catches exceptions in filters [OK]

Common Mistakes:

• Using @Exception() instead of @Catch()
• Confusing with @Filter() which is not a decorator
• Assuming @ErrorHandler() exists in NestJS

3. Given this filter code, what will be the HTTP status code sent when a NotFoundException is thrown?

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

@Catch(NotFoundException)
export class NotFoundFilter implements ExceptionFilter {
  catch(exception: NotFoundException, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse();
    response.status(404).json({ message: 'Resource not found' });
  }
}

medium

A. 404

B. 200

C. 500

D. 400

Solution

1. Step 1: Analyze the response status set in the filter

   The filter explicitly sets the status code to 404 using response.status(404).

2. Step 2: Understand the exception type caught

   The filter catches NotFoundException, which corresponds to HTTP 404 errors.

3. Final Answer:

   404 -> Option A

4. Quick Check:

   NotFoundException triggers 404 status [OK]

Hint: Check response.status() call inside catch method [OK]

Common Mistakes:

• Assuming default 500 status without custom filter
• Confusing 404 with 400 or 200
• Ignoring the status set in the filter

4. Identify the error in this exception filter code:

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

@Catch(HttpException)
export class AllExceptionsFilter implements ExceptionFilter {
  catch(exception, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse();
    response.status(500).json({ message: exception.message });
  }
}

medium

A. Missing exception type in @Catch() decorator

B. catch method missing return type

C. Using exception.message instead of exception.getMessage()

D. Not importing HttpException class

Solution

1. Step 1: Check the @Catch() decorator usage

   The @Catch(HttpException) references HttpException, but it is not imported from '@nestjs/common'.

2. Step 2: Validate other parts

   catch method can omit return type and exception type in loose TS, exception.message is valid.

3. Final Answer:

   Not importing HttpException class -> Option D

4. Quick Check:

   Import HttpException for @Catch(HttpException) [OK]

Hint: Import exception classes used in @Catch() [OK]

Common Mistakes:

• Not importing referenced exception types like HttpException
• Assuming catch method needs strict return type
• Thinking exception needs getMessage() method

5. You want to create a global exception filter that catches all exceptions and logs the error before sending a generic message. Which of these is the correct way to apply it globally in NestJS?

hard

A. Register the filter in a module's providers array only

B. Add @Global() decorator to the filter class

C. Use app.useGlobalFilters(new YourFilter()) in main.ts after creating the filter class

D. Call app.setGlobalExceptionFilter(YourFilter) in main.ts

Solution

1. Step 1: Recall how to apply global filters in NestJS

   Global filters are applied using app.useGlobalFilters() method in the main bootstrap file.

2. Step 2: Evaluate other options

   @Global() is for modules, not filters; registering in providers is local; app.setGlobalExceptionFilter() does not exist.

3. Final Answer:

   Use app.useGlobalFilters(new YourFilter()) in main.ts after creating the filter class -> Option C

4. Quick Check:

   Global filters use app.useGlobalFilters() [OK]

Hint: Global filters need app.useGlobalFilters() call [OK]

Common Mistakes:

• Using @Global() decorator on filter class
• Registering filter only in providers without global call
• Assuming app.setGlobalExceptionFilter() exists