Exception filters in NestJS - Mini Project: Build & Apply

Handling Errors with Exception Filters in NestJS

📖 Scenario: You are building a simple NestJS API that returns user data. You want to handle errors gracefully by using exception filters to catch and respond to errors in a clean way.

🎯 Goal: Create a custom exception filter in NestJS that catches HTTP exceptions and returns a JSON response with status code and error message.

📋 What You'll Learn

Create a custom exception filter class named HttpExceptionFilter

Use the ExceptionFilter interface and @Catch(HttpException) decorator

Implement the catch method to handle exceptions

Apply the exception filter globally in the main application bootstrap

💡 Why This Matters

🌍 Real World

Exception filters help APIs respond with clear error messages and status codes, improving user experience and debugging.

💼 Career

Understanding exception filters is essential for backend developers working with NestJS to build robust and maintainable APIs.

1

Create the custom exception filter class

Create a class called HttpExceptionFilter that implements ExceptionFilter and use the @Catch(HttpException) decorator.

NestJS

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

// Your code here

Hint

Use @Catch(HttpException) above the class and implement the ExceptionFilter interface.

2

Add a response object parameter to the catch method

Modify the catch method to accept a second parameter called host of type ArgumentsHost.

NestJS

import { ArgumentsHost } from '@nestjs/common';

@Catch(HttpException)
export class HttpExceptionFilter implements ExceptionFilter {
  catch(exception: HttpException, host: ArgumentsHost) {
    // Your code here
  }
}

Hint

Add host: ArgumentsHost as the second parameter to the catch method.

3

Implement the catch method to send JSON error response

Inside the catch method, get the response object from host.switchToHttp().getResponse(). Then get the status from exception.getStatus() and the message from exception.message. Use response.status(status).json({ statusCode: status, message: message }) to send the error response.

NestJS

catch(exception: HttpException, host: ArgumentsHost) {
  // Your code here
}

Hint

Use host.switchToHttp().getResponse() to get the response object and send JSON with status and message.

4

Apply the exception filter globally in main.ts

In main.ts, import HttpExceptionFilter and use app.useGlobalFilters(new HttpExceptionFilter()) before calling app.listen().

NestJS

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  // Add global exception filter here
  await app.listen(3000);
}

Hint

Call app.useGlobalFilters(new HttpExceptionFilter()) before app.listen().

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