Welcome to this lesson on Error Handling in NestJS. So far, we've learned how to integrate MongoDB, transform data store objects to DTOs, and configure middleware in our NestJS application. Today, we'll focus on error handling, a critical aspect of building robust and user-friendly applications. Effective error handling ensures that our application can gracefully manage errors and provide meaningful feedback to users.

By the end of this lesson, you will be able to set up error handling for a NestJS application, create and use exception filters, and test various error scenarios to ensure your application handles errors as expected.

To handle errors effectively, NestJS provides a powerful mechanism called Exception Filters. These filters allow you to manage exceptions and provide custom error responses.

Now, let's move to the heart of our error handling approach: creating an Exception Filter.

Exception Filters in NestJS allow you to catch unhandled exceptions in your application and modify the response that users receive.

We'll walk through the process of creating an HttpExceptionFilter.

Step 1: Import Required Modules Create a new file named http-exception.filter.ts and start by importing the necessary modules:

Step 2: Define the HttpExceptionFilter Class Next, we define the exception filter class:

Explanation:

• @Catch(HttpException) decorator tells NestJS to use this filter for HttpExceptions.
• The catch method handles the exception, extracts the relevant details (status, request URL), and constructs a response.
• Different responses for internal server errors and other types of errors ensure relevant information is sent back to the client.

Next, we'll integrate our HttpExceptionFilter into the application.

Step 1: Modify app.module.ts Edit app.module.ts to add the filter:

Explanation:

• The APP_FILTER token from @nestjs/core registers the HttpExceptionFilter globally.
• Middleware configurations ensure that ValidationMiddleware and TimerMiddleware apply to specified routes.

To ensure our error handling works, we’ll simulate different errors. Create a send_request.ts script with sample code:

Explanation:

• Creating a valid ToDo item: Normal scenario; should return success.
• Creating an invalid ToDo item: Triggers validation error; ensures ValidationMiddleware and HttpExceptionFilter work.
• Simulating internal server error: Triggers server-side error handling; ensures server responses are customized.
• Getting all ToDo items: Verifies CRUD operations continue to function.

Expected output examples from running the script:

In this lesson, we've covered the essentials of error handling in NestJS. You learned how to:

1. Set up error handling using Exception Filters.
2. Create an HttpExceptionFilter to manage exceptions.
3. Integrate the filter into your application.
4. Test various error scenarios to ensure robust error management.

Next, you'll get the chance to solidify your understanding through hands-on practice exercises. These exercises will involve creating error scenarios and verifying that your application handles them gracefully.

Congratulations on completing this course! You've taken significant steps toward mastering NestJS and building robust, enterprise-ready applications. Keep practicing and applying what you've learned, and you'll continue to grow as a developer. Happy coding!