Catch decorator in NestJS - Mini Project: Build & Apply

Create a Custom Catch Decorator in NestJS

📖 Scenario: You are building a NestJS backend service. You want to create a custom decorator that catches errors from a method and handles them gracefully.

🎯 Goal: Build a custom @CatchError decorator that wraps a method and catches any errors thrown, returning a friendly error message instead.

📋 What You'll Learn

Create a custom decorator function called CatchError

Use the decorator to wrap a method in a class

Catch any errors thrown inside the method

Return a string 'Error caught' when an error occurs

💡 Why This Matters

🌍 Real World

Custom decorators help add reusable behavior like error handling to methods in NestJS services, making code cleaner and more maintainable.

💼 Career

Understanding decorators is essential for backend developers working with NestJS, as they are widely used for middleware, validation, and error handling.

1

Create a simple service class with a method that throws an error

Create a class called TestService with a method dangerousMethod that throws an error with the message 'Something went wrong'.

NestJS

class TestService {
  // Your code here
}

Hint

Define a class and inside it a method that throws an error using throw new Error('Something went wrong').

2

Create the CatchError decorator function

Create a function called CatchError that returns a method decorator function. The decorator should accept target, propertyKey, and descriptor parameters.

NestJS

class TestService {
  dangerousMethod() {
    throw new Error('Something went wrong');
  }
}

// Your code here

Hint

Create a function that returns another function with the parameters target, propertyKey, and descriptor.

3

Implement error catching inside the decorator

Inside the CatchError decorator, save the original method from descriptor.value. Replace descriptor.value with a new function that calls the original method inside a try-catch block. If an error occurs, return the string 'Error caught'.

NestJS

class TestService {
  dangerousMethod() {
    throw new Error('Something went wrong');
  }
}

function CatchError() {
  return function(target: any, propertyKey: string, descriptor: PropertyDescriptor) {
    // Your code here
  };
}

Hint

Save the original method, then replace it with a function that uses try-catch to handle errors and return 'Error caught'.

4

Apply the @CatchError decorator to the method

Add the @CatchError() decorator above the dangerousMethod in the TestService class.

NestJS

class TestService {
  // Add the decorator here
  dangerousMethod() {
    throw new Error('Something went wrong');
  }
}

function CatchError() {
  return function(target: any, propertyKey: string, descriptor: PropertyDescriptor) {
    const originalMethod = descriptor.value;
    descriptor.value = function(...args: any[]) {
      try {
        return originalMethod.apply(this, args);
      } catch (error) {
        return 'Error caught';
      }
    };
  };
}

Hint

Place @CatchError() directly above the method you want to decorate.

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