**Course Title:** Mastering NestJS: Building Scalable Server-Side Applications **Section Title:** Error Handling and Validation **Topic:** Best practices for error handling in NestJS applications **Overview** Error handling is a crucial aspect of building robust and scalable applications with NestJS. Proper error handling ensures that your application remains stable and provides a good user experience even when unexpected errors occur. In this topic, we will explore best practices for error handling in NestJS applications, including how to handle errors at different levels, create custom error classes, and use built-in exception filters. **Why Error Handling is Important** Error handling is essential for several reasons: 1. **User Experience**: Errors can be frustrating for users. Proper error handling ensures that users receive helpful error messages and can recover from errors quickly. 2. **Application Stability**: Unhandled errors can cause your application to crash or become unstable, leading to downtime and lost revenue. 3. **Debugging**: Error handling makes it easier to debug your application by providing valuable information about errors. **Best Practices for Error Handling in NestJS** Here are some best practices for error handling in NestJS: ### 1. Handle Errors at Different Levels NestJS provides several levels at which you can handle errors: * **Controller**: Handle errors at the controller level using try-catch blocks or exception filters. * **Service**: Handle errors at the service level using try-catch blocks or custom error classes. * **Module**: Handle errors at the module level using global exception filters. ### 2. Create Custom Error Classes Create custom error classes to handle specific error scenarios. This makes it easier to handle errors and provides more informative error messages. ```typescript // Custom error class export class CustomError extends Error { constructor(message: string) { super(message); this.name = 'CustomError'; } } ``` ### 3. Use Built-in Exception Filters NestJS provides several built-in exception filters that you can use to handle errors: * **AllExceptionsFilter**: Handles all exceptions. * **HttpExceptionFilter**: Handles HTTP exceptions. * **MethodNotAllowedExceptionFilter**: Handles method not allowed exceptions. ```typescript // Use built-in exception filter import { AllExceptionsFilter } from '@nestjs/common'; @Module({ controllers: [AppController], providers: [ { provide: APP_FILTER, useClass: AllExceptionsFilter, }, ], }) export class AppModule {} ``` ### 4. Log Errors Log errors to track and debug your application. You can use a logging library like Winston or Morgan. ```typescript // Log errors import { Logger } from '@nestjs/common'; @Controller('app') export class AppController { constructor(private readonly logger: Logger) {} @Get() getHello(): string { try { // Code that may throw an error } catch (error) { this.logger.error(error); throw error; } } } ``` **Conclusion** Error handling is a crucial aspect of building robust and scalable applications with NestJS. By following best practices for error handling, you can ensure that your application remains stable and provides a good user experience even when unexpected errors occur. Remember to handle errors at different levels, create custom error classes, use built-in exception filters, and log errors to track and debug your application. **What's Next?** In the next topic, we will explore how to use built-in exception filters and custom exception handling in NestJS applications. **Leave a comment or ask for help if you have any questions or need further clarification on this topic.** **External Resources:** * [NestJS Documentation: Error Handling](https://docs.nestjs.com/exception-filters) * [NestJS Documentation: Exception Filters](https://docs.nestjs.com/exception-filters) * [Winston Logging Library](https://www.npmjs.com/package/winston) * [Morgan Logging Library](https://www.npmjs.com/package/morgan)