Why Exception filters in NestJS? - Purpose & Use Cases

The Big Idea

What if you could catch all errors in one place and never repeat error code again?

The Scenario

Imagine building a NestJS app where every time an error happens, you manually check and handle it in each controller method.

You write repetitive try-catch blocks everywhere to catch errors and send responses.

The Problem

This manual error handling is tiring and easy to forget.

It clutters your code and makes it hard to maintain or change error responses consistently.

Also, you risk missing some errors or sending inconsistent messages to users.

The Solution

Exception filters in NestJS let you centralize error handling.

You write one filter to catch specific errors and format responses uniformly.

This keeps your controller code clean and your app's error responses consistent and easy to update.

Before vs After

✗ Before

try {
  // controller logic
} catch (error) {
  if (error instanceof NotFoundError) {
    response.status(404).send('Not found');
  }
}

✓ After

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

What It Enables

You can handle errors in one place and keep your app code clean and consistent.

Real Life Example

When building an API, you want all 'not found' errors to return the same message and status code without repeating code in every route.

Key Takeaways

Manual error handling clutters code and risks inconsistency.

Exception filters centralize and simplify error responses.

This leads to cleaner, easier-to-maintain NestJS applications.

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