Why Custom exception classes in NestJS? - Purpose & Use Cases

The Big Idea

Discover how custom exceptions can save you hours of debugging and make your API shine!

The Scenario

Imagine building a NestJS app where you handle errors by throwing generic exceptions everywhere.

You want to show specific error messages for different problems, but all errors look the same.

The Problem

Using only generic exceptions makes it hard to tell what went wrong.

You end up writing repetitive code to check error messages and status codes manually.

This slows development and causes bugs when errors are not handled properly.

The Solution

Custom exception classes let you define clear, reusable error types with their own messages and HTTP status codes.

They make your error handling clean, consistent, and easy to maintain.

Before vs After

✗ Before

throw new HttpException('Bad request', 400);

✓ After

throw new BadRequestException('Invalid user input');

What It Enables

Custom exceptions enable precise error handling and better communication with API clients.

Real Life Example

When a user submits a form with wrong data, a BadRequestException clearly tells the client what went wrong, improving user experience.

Key Takeaways

Generic exceptions make error handling confusing and repetitive.

Custom exception classes provide clear, reusable error types.

This leads to cleaner code and better API responses.

Practice

1. What is the main purpose of creating a custom exception class in NestJS?

easy

A. To create new HTTP methods like GET or POST

B. To replace the built-in JavaScript Error object globally

C. To automatically log errors to a database

D. To define specific error messages and HTTP status codes for your app

Solution

1. Step 1: Understand the role of custom exceptions

   Custom exceptions in NestJS help you send clear, specific error messages with HTTP status codes.

2. Step 2: Compare with other options

   Options B, C, and D describe unrelated features not handled by custom exceptions.

3. Final Answer:

   To define specific error messages and HTTP status codes for your app -> Option D

4. Quick Check:

   Custom exceptions = specific messages + status [OK]

Hint: Custom exceptions customize error messages and HTTP codes [OK]

Common Mistakes:

• Thinking custom exceptions replace JavaScript Error globally
• Assuming they log errors automatically
• Confusing exceptions with HTTP methods

2. Which of the following is the correct way to define a custom exception class in NestJS?

easy

A. const MyException = () => { return 'Error'; }

B. class MyException extends HttpException { constructor() { super('Error', 400); } }

C. class MyException { constructor() { this.message = 'Error'; } }

D. function MyException() { throw new Error('Error'); }

Solution

1. Step 1: Check syntax for custom exception

   In NestJS, custom exceptions extend HttpException and call super with message and status.

2. Step 2: Validate options

   class MyException extends HttpException { constructor() { super('Error', 400); } } correctly extends HttpException and calls super with message and status code. Others do not follow NestJS pattern.

3. Final Answer:

   class MyException extends HttpException { constructor() { super('Error', 400); } } -> Option B

4. Quick Check:

   Extend HttpException + super(message, status) [OK]

Hint: Custom exceptions must extend HttpException and call super() [OK]

Common Mistakes:

• Not extending HttpException
• Not calling super() with message and status
• Using plain functions instead of classes

3. Given this custom exception class, what will be the HTTP status code when it is thrown?

class ForbiddenException extends HttpException {
  constructor() {
    super('Access denied', 403);
  }
}

medium

A. 200

B. 500

C. 403

D. 404

Solution

1. Step 1: Read the constructor call

   The constructor calls super with message 'Access denied' and status 403.

2. Step 2: Understand HttpException behavior

   The second argument to super is the HTTP status code returned when exception is thrown.

3. Final Answer:

   403 -> Option C

4. Quick Check:

   Status code = 403 from super() call [OK]

Hint: Status code is second argument in super() call [OK]

Common Mistakes:

• Confusing message with status code
• Assuming default 500 status
• Ignoring the status argument in super()

4. What is wrong with this custom exception class?

class BadRequestException extends HttpException {
  constructor() {
    super('Bad request');
  }
}

medium

A. Missing HTTP status code in super() call

B. Class should not extend HttpException

C. Constructor should not call super()

D. Message should be a number, not a string

Solution

1. Step 1: Check super() arguments

   The super() call only has a message string, but HttpException requires message and status code.

2. Step 2: Understand HttpException constructor

   HttpException constructor needs both message and HTTP status code to work correctly.

3. Final Answer:

   Missing HTTP status code in super() call -> Option A

4. Quick Check:

   super() needs message + status [OK]

Hint: Always pass status code as second argument to super() [OK]

Common Mistakes:

• Forgetting the status code argument
• Thinking message alone is enough
• Misunderstanding HttpException constructor

5. You want to create a custom exception that returns a JSON response with a custom error code and message. Which approach correctly extends HttpException to achieve this?

class CustomErrorException extends HttpException {
  constructor() {
    super(
      { errorCode: 1234, message: 'Custom error' },
      400
    );
  }
}

hard

A. This code correctly sends a JSON object with errorCode and message and HTTP 400 status

B. HttpException cannot send JSON objects, only strings

C. The status code should be inside the JSON object, not a separate argument

D. Custom exceptions must not use objects in super(), only strings

Solution

1. Step 1: Review HttpException constructor usage

   HttpException accepts either a string or an object as the first argument for the response body.

2. Step 2: Confirm JSON response with status code

   Passing an object with errorCode and message as first argument and 400 as second is valid and sends JSON response with status 400.

3. Final Answer:

   This code correctly sends a JSON object with errorCode and message and HTTP 400 status -> Option A

4. Quick Check:

   HttpException supports object response + status [OK]

Hint: Pass object as first argument, status code as second in super() [OK]

Common Mistakes:

• Thinking only strings allowed in super()
• Putting status code inside JSON object
• Believing custom exceptions can't send JSON