**Course Title:** Mastering NestJS: Building Scalable Server-Side Applications **Section Title:** Error Handling and Validation **Topic:** Using built-in exception filters and custom exception handling In this topic, we will explore how to handle exceptions in NestJS applications using built-in exception filters and custom exception handling mechanisms. Proper exception handling is crucial for building robust and scalable server-side applications. ### Built-in Exception Filters NestJS provides several built-in exception filters that can be used to handle exceptions in a centralized manner. These filters can be applied globally or at the module level. #### 1. AllExceptionsFilter The `AllExceptionsFilter` is a built-in exception filter that catches all exceptions and returns a 500 Internal Server Error response. ```typescript import { AllExceptionsFilter } from '@nestjs/common'; @Module({ imports: [AppModule], controllers: [AppController], providers: [AppService], exceptions: [AllExceptionsFilter], }) export class AppModule {} ``` #### 2. HttpExceptionFilter The `HttpExceptionFilter` is a built-in exception filter that catches HTTP exceptions and returns a response with the corresponding HTTP status code. ```typescript import { HttpExceptionFilter } from '@nestjs/common'; @Module({ imports: [AppModule], controllers: [AppController], providers: [AppService], exceptions: [HttpExceptionFilter], }) export class AppModule {} ``` ### Custom Exception Handling While built-in exception filters provide a convenient way to handle exceptions, custom exception handling mechanisms offer more flexibility and control. #### 1. Creating a Custom Exception Filter To create a custom exception filter, you can extend the `ExceptionFilter` class and implement the `catch(exception: any, response: Response, next: CallHandler) => void` method. ```typescript import { ExceptionFilter, Catch, ArgumentsHost, HttpException } from '@nestjs/common'; @Catch(HttpException) export class CustomHttpExceptionFilter implements ExceptionFilter { catch(exception: HttpException, host: ArgumentsHost) { const ctx = host.switchToHttp(); const response = ctx.getResponse(); const status = exception.getStatus(exception); response.status(status).json({ statusCode: status, message: exception.message, }); } } ``` #### 2. Applying the Custom Exception Filter To apply the custom exception filter, you can add it to the `exceptions` array in the module decorator. ```typescript import { Module } from '@nestjs/common'; import { AppController } from './app.controller'; import { AppService } from './app.service'; import { CustomHttpExceptionFilter } from './custom-http-exception.filter'; @Module({ imports: [AppModule], controllers: [AppController], providers: [AppService], exceptions: [CustomHttpExceptionFilter], }) export class AppModule {} ``` ### Best Practices When implementing exception handling in NestJS applications, keep the following best practices in mind: * Use built-in exception filters to handle common exceptions. * Create custom exception filters for specific use cases. * Implement logging and error tracking mechanisms to monitor and debug exceptions. * Use try-catch blocks to catch and handle exceptions in controllers and services. By following these best practices and using built-in exception filters and custom exception handling mechanisms, you can build robust and scalable server-side applications that handle exceptions effectively. **Leave a comment below if you have any questions or need further clarification on this topic.** **Next topic:** Implementing validation pipes for data validation. **External Resources:** * [NestJS Documentation: Exception Filters](https://docs.nestjs.com/exception-filters) * [NestJS Documentation: Custom Exception Filters](https://docs.nestjs.com/exception-filters#custom-exception-filters)