Exception filters in NestJS

Introduction

Exception filters help you catch and handle errors in your NestJS app in one place. They make your app respond nicely when something goes wrong.

You want to send custom error messages to users instead of default ones.

You need to log errors in a consistent way across your app.

You want to handle specific types of errors differently.

You want to keep your controller code clean by separating error handling.

You want to control the HTTP status codes sent when errors happen.

Syntax

NestJS

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

@Catch(HttpException)
export class MyExceptionFilter 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: exception.message,
      custom: 'This is a custom error message',
    });
  }
}

The @Catch() decorator tells NestJS which errors this filter handles.

The catch() method runs when an error happens and lets you customize the response.

Examples

This filter catches all exceptions and sends a generic 500 error message.

NestJS

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

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

    response.status(500).json({
      statusCode: 500,
      message: 'Something went wrong',
    });
  }
}

This filter only catches 404 errors and sends a custom message.

NestJS

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();
    const status = exception.getStatus();

    response.status(status).json({
      statusCode: status,
      message: 'Resource not found',
    });
  }
}

Sample Program

This example shows a controller with a route that throws an error. The custom exception filter catches it and sends a JSON response with the error details and a timestamp.

NestJS

import { Controller, Get, UseFilters, HttpException, HttpStatus } from '@nestjs/common';
import { ExceptionFilter, Catch, ArgumentsHost } from '@nestjs/common';

@Catch(HttpException)
class CustomHttpExceptionFilter 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: exception.message || 'Default error message',
      timestamp: new Date().toISOString(),
    });
  }
}

@Controller('test')
@UseFilters(CustomHttpExceptionFilter)
export class TestController {
  @Get('error')
  throwError() {
    throw new HttpException('Custom error happened', HttpStatus.BAD_REQUEST);
  }
}

Important Notes

Exception filters only catch errors thrown inside the route handlers or services called by them.

You can create filters for specific error types or catch all errors by leaving @Catch() empty.

Use ArgumentsHost to access request and response objects to customize responses.

Summary

Exception filters help handle errors in one place and send custom responses.

Use @Catch() to specify which errors to catch.

Filters keep your code clean and improve user experience by controlling error messages.

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