Custom exception classes in NestJS - Mini Project: Build & Apply

Custom Exception Classes in NestJS

📖 Scenario: You are building a NestJS backend for a bookstore. You want to handle errors clearly by creating custom exception classes for specific error cases like when a book is not found or when a user tries to buy a book that is out of stock.

🎯 Goal: Create custom exception classes in NestJS by extending the built-in HttpException class. Use these classes to throw meaningful errors in your application.

📋 What You'll Learn

Create a custom exception class called BookNotFoundException that extends HttpException.

Create a custom exception class called OutOfStockException that extends HttpException.

Use the HttpStatus.NOT_FOUND status code for BookNotFoundException.

Use the HttpStatus.BAD_REQUEST status code for OutOfStockException.

Each exception class should have a constructor that calls the parent HttpException constructor with a custom message and status code.

💡 Why This Matters

🌍 Real World

Custom exceptions help backend developers give clear error messages and HTTP status codes. This improves API usability and debugging.

💼 Career

Understanding how to create and use custom exceptions is important for backend developers working with NestJS or similar frameworks to build robust and maintainable APIs.

1

Create the BookNotFoundException class

Create a class called BookNotFoundException that extends HttpException. Inside the constructor, call super with the message 'Book not found' and the status code HttpStatus.NOT_FOUND. Import HttpException and HttpStatus from @nestjs/common.

NestJS

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

// Your code here

Need a hint?

Remember to import HttpException and HttpStatus from @nestjs/common. Your class should extend HttpException and call super with the message and status code.

2

Create the OutOfStockException class

Create a class called OutOfStockException that extends HttpException. Inside the constructor, call super with the message 'Book is out of stock' and the status code HttpStatus.BAD_REQUEST. Make sure to export the class.

NestJS

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

export class BookNotFoundException extends HttpException {
  constructor() {
    super('Book not found', HttpStatus.NOT_FOUND);
  }
}

// Create OutOfStockException class here

Need a hint?

Follow the same pattern as BookNotFoundException. Use the message 'Book is out of stock' and status code HttpStatus.BAD_REQUEST.

3

Throw BookNotFoundException in a service method

Inside a class called BookService, create a method called findBookById that takes a parameter id. Inside the method, throw a new BookNotFoundException. Import BookNotFoundException from the current file.

NestJS

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

export class BookNotFoundException extends HttpException {
  constructor() {
    super('Book not found', HttpStatus.NOT_FOUND);
  }
}

export class OutOfStockException extends HttpException {
  constructor() {
    super('Book is out of stock', HttpStatus.BAD_REQUEST);
  }
}

export class BookService {
  // Add findBookById method here
  // Your code here
}

Need a hint?

Create the BookService class and add the findBookById method. Inside it, throw new BookNotFoundException().

4

Throw OutOfStockException in a service method

Inside the BookService class, create a method called buyBook that takes a parameter id. Inside the method, throw a new OutOfStockException. Make sure to keep the previous code intact.

NestJS

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

export class BookNotFoundException extends HttpException {
  constructor() {
    super('Book not found', HttpStatus.NOT_FOUND);
  }
}

export class OutOfStockException extends HttpException {
  constructor() {
    super('Book is out of stock', HttpStatus.BAD_REQUEST);
  }
}

export class BookService {
  findBookById(id: number) {
    throw new BookNotFoundException();
  }

  // Add buyBook method here
  // Your code here
}

Need a hint?

Add the buyBook method inside BookService and throw new OutOfStockException() inside it.