Catch decorator in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the @Catch decorator in NestJS?

The @Catch decorator is used to define an exception filter that handles specific exceptions thrown in the application, allowing centralized error handling.

beginner

How do you specify which exceptions a filter should catch using the @Catch decorator?

You pass one or more exception classes as arguments to the @Catch decorator. The filter will only handle those exceptions.

intermediate

What method must be implemented in a class decorated with <code>@Catch</code> to handle exceptions?

The class must implement the <code>catch(exception: any, host: ArgumentsHost)</code> method, which contains the logic to handle the caught exception.

intermediate

Can the @Catch decorator be used without arguments? What does that mean?

Yes, if used without arguments, the filter will catch all exceptions, acting as a global catch-all error handler.

intermediate

How do you apply a class with the <code>@Catch</code> decorator globally in a NestJS application?

You provide the filter class in the <code>app.useGlobalFilters(new YourFilter())</code> method inside the main bootstrap function.

What does the @Catch(HttpException) decorator do?

A Catches all exceptions

B Does nothing without a catch method

C Catches exceptions except HttpException

D Catches only HttpException errors

The decorator catches only exceptions of type HttpException.

Which method handles the exception inside a class decorated with @Catch?

A catch()

B handle()

C process()

D execute()

The catch() method is required to handle exceptions.

If you want to catch all exceptions, how do you use the @Catch decorator?

A <code>@Catch(null)</code>

B <code>@Catch(AllExceptions)</code>

C <code>@Catch()</code> with no arguments

D <code>@Catch(Error)</code>

Using @Catch() without arguments catches all exceptions.

How do you register a catch filter globally in NestJS?

A Use <code>app.useGlobalFilters()</code>

B Add it to <code>providers</code> in a module

C Use <code>app.use()</code>

D Add it to <code>imports</code> in a module

Global filters are registered with app.useGlobalFilters().

What argument does the catch() method receive besides the exception?

A Request object

B ArgumentsHost object

C Response object

D Next function

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

Explain how the @Catch decorator works in NestJS and how you create a custom exception filter.

Think about how you tell NestJS which errors to handle and where you write the handling code.

Describe the steps to apply a custom exception filter globally in a NestJS application.

Focus on how to make the filter work for the whole app, not just one controller.

Practice

1. What is the primary purpose of the @Catch() decorator in NestJS?

easy

A. To mark a class as an exception filter that handles errors

B. To define a route handler for HTTP requests

C. To inject dependencies into a service

D. To create a middleware function

Solution

1. Step 1: Understand the role of @Catch()

   The @Catch() decorator is used to mark a class as an exception filter in NestJS, which means it handles errors thrown in the application.

2. Step 2: Differentiate from other decorators

   Other decorators like route handlers or dependency injections serve different purposes, so @Catch() specifically relates to error handling.

3. Final Answer:

   To mark a class as an exception filter that handles errors -> Option A

4. Quick Check:

   Error handling = @Catch() [OK]

Hint: Catch decorator marks error handlers in NestJS classes [OK]

Common Mistakes:

• Confusing @Catch() with route or middleware decorators
• Thinking @Catch() injects dependencies
• Assuming @Catch() handles normal requests

2. Which of the following is the correct syntax to create an exception filter that catches all exceptions using the @Catch() decorator?

easy

A. @Catch()\nexport class AllExceptionsFilter implements ExceptionFilter { ... }

B. @Catch(Error)\nexport class AllExceptionsFilter implements ExceptionFilter { ... }

C. @Catch(HttpException)\nexport class AllExceptionsFilter implements ExceptionFilter { ... }

D. @Catch(Exception)\nexport class AllExceptionsFilter implements ExceptionFilter { ... }

Solution

1. Step 1: Understand @Catch() usage without parameters

   Using @Catch() with no arguments means the filter will catch all exceptions.

2. Step 2: Check other options

   Options A, C, and D specify particular exception types, so they do not catch all exceptions.

3. Final Answer:

   @Catch()\nexport class AllExceptionsFilter implements ExceptionFilter { ... } -> Option A

4. Quick Check:

   No argument = catch all exceptions [OK]

Hint: Empty @Catch() catches all exceptions [OK]

Common Mistakes:

• Passing incorrect exception types to @Catch()
• Using @Catch() with a non-error class
• Confusing @Catch() with middleware syntax

3. Given the following exception filter code, what will be the output when a NotFoundException is thrown?

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

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

medium

A. HTTP response with status 404 and default NestJS error message

B. HTTP response with status 500 and default error message

C. No response sent, request hangs

D. HTTP response with status 404 and JSON { message: 'Resource not found' }

Solution

1. Step 1: Analyze the exception filter behavior

   The filter catches only NotFoundException and sends a 404 status with a custom JSON message.

2. Step 2: Understand the response sent

   The response is explicitly set to status 404 with JSON { message: 'Resource not found' }, overriding default behavior.

3. Final Answer:

   HTTP response with status 404 and JSON { message: 'Resource not found' } -> Option D

4. Quick Check:

   Custom 404 JSON response = HTTP response with status 404 and JSON { message: 'Resource not found' } [OK]

Hint: Filter sends custom response for caught exception type [OK]

Common Mistakes:

• Assuming default error message is sent
• Thinking status 500 is returned
• Missing the JSON response in output

4. Identify the error in the following exception filter code:

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

@Catch()
export class MyFilter implements ExceptionFilter {
  catch(exception, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse();
    response.status(500).send('Internal Server Error');
  }
}

medium

A. The response object is not correctly retrieved

B. The exception parameter lacks a type annotation

C. The catch method should return a value

D. The @Catch() decorator must specify an exception type

Solution

1. Step 1: Check method signature

   The catch method's first parameter exception should have a type annotation, typically unknown or any.

2. Step 2: Verify other parts

   The @Catch() decorator can be empty to catch all exceptions, returning void is allowed, and response retrieval is correct.

3. Final Answer:

   The exception parameter lacks a type annotation -> Option B

4. Quick Check:

   Missing parameter type = The exception parameter lacks a type annotation [OK]

Hint: Always type the exception parameter in catch method [OK]

Common Mistakes:

• Forgetting to type the exception parameter
• Thinking @Catch() requires arguments
• Expecting catch to return a value

5. You want to create a single exception filter that handles both BadRequestException and UnauthorizedException with different custom messages. Which is the correct way to use the @Catch() decorator to achieve this?

hard

A. @Catch()\nexport class MultiExceptionFilter implements ExceptionFilter { ... }

B. @Catch([BadRequestException, UnauthorizedException])\nexport class MultiExceptionFilter implements ExceptionFilter { ... }

C. @Catch(BadRequestException, UnauthorizedException)\nexport class MultiExceptionFilter implements ExceptionFilter { ... }

D. Create two separate filters each with @Catch() for one exception

Solution

1. Step 1: Understand how to catch multiple exceptions

   The @Catch() decorator accepts multiple exception classes as separate arguments to catch them in one filter.

2. Step 2: Evaluate options

   @Catch(BadRequestException, UnauthorizedException)\nexport class MultiExceptionFilter implements ExceptionFilter { ... } correctly passes multiple exceptions as separate arguments. @Catch([BadRequestException, UnauthorizedException])\nexport class MultiExceptionFilter implements ExceptionFilter { ... } incorrectly passes an array. @Catch()\nexport class MultiExceptionFilter implements ExceptionFilter { ... } catches all exceptions. Create two separate filters each with @Catch() for one exception uses multiple filters, not one.

3. Final Answer:

   @Catch(BadRequestException, UnauthorizedException)\nexport class MultiExceptionFilter implements ExceptionFilter { ... } -> Option C

4. Quick Check:

   Multiple exceptions as arguments = @Catch(BadRequestException, UnauthorizedException)\nexport class MultiExceptionFilter implements ExceptionFilter { ... } [OK]

Hint: Pass multiple exceptions as separate arguments to @Catch() [OK]

Common Mistakes:

• Passing exceptions as an array to @Catch()
• Using multiple filters instead of one
• Using empty @Catch() to catch specific exceptions