Exception filters in NestJS - Mini Project: Build & Apply

Handling Errors with Exception Filters in NestJS

📖 Scenario: You are building a simple NestJS API that returns user data. You want to handle errors gracefully by using exception filters to catch and respond to errors in a clean way.

🎯 Goal: Create a custom exception filter in NestJS that catches HTTP exceptions and returns a JSON response with status code and error message.

📋 What You'll Learn

Create a custom exception filter class named HttpExceptionFilter

Use the ExceptionFilter interface and @Catch(HttpException) decorator

Implement the catch method to handle exceptions

Apply the exception filter globally in the main application bootstrap

💡 Why This Matters

🌍 Real World

Exception filters help APIs respond with clear error messages and status codes, improving user experience and debugging.

💼 Career

Understanding exception filters is essential for backend developers working with NestJS to build robust and maintainable APIs.

1

Create the custom exception filter class

Create a class called HttpExceptionFilter that implements ExceptionFilter and use the @Catch(HttpException) decorator.

NestJS

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

// Your code here

Need a hint?

Use @Catch(HttpException) above the class and implement the ExceptionFilter interface.

2

Add a response object parameter to the catch method

Modify the catch method to accept a second parameter called host of type ArgumentsHost.

NestJS

import { ArgumentsHost } from '@nestjs/common';

@Catch(HttpException)
export class HttpExceptionFilter implements ExceptionFilter {
  catch(exception: HttpException, host: ArgumentsHost) {
    // Your code here
  }
}

Need a hint?

Add host: ArgumentsHost as the second parameter to the catch method.

3

Implement the catch method to send JSON error response

Inside the catch method, get the response object from host.switchToHttp().getResponse(). Then get the status from exception.getStatus() and the message from exception.message. Use response.status(status).json({ statusCode: status, message: message }) to send the error response.

NestJS

catch(exception: HttpException, host: ArgumentsHost) {
  // Your code here
}

Need a hint?

Use host.switchToHttp().getResponse() to get the response object and send JSON with status and message.

4

Apply the exception filter globally in main.ts

In main.ts, import HttpExceptionFilter and use app.useGlobalFilters(new HttpExceptionFilter()) before calling app.listen().

NestJS

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  // Add global exception filter here
  await app.listen(3000);
}

Need a hint?

Call app.useGlobalFilters(new HttpExceptionFilter()) before app.listen().