Why Exception filters in NestJS? - Purpose & Use Cases

The Big Idea

What if you could catch all errors in one place and never repeat error code again?

The Scenario

Imagine building a NestJS app where every time an error happens, you manually check and handle it in each controller method.

You write repetitive try-catch blocks everywhere to catch errors and send responses.

The Problem

This manual error handling is tiring and easy to forget.

It clutters your code and makes it hard to maintain or change error responses consistently.

Also, you risk missing some errors or sending inconsistent messages to users.

The Solution

Exception filters in NestJS let you centralize error handling.

You write one filter to catch specific errors and format responses uniformly.

This keeps your controller code clean and your app's error responses consistent and easy to update.

Before vs After

✗ Before

try {
  // controller logic
} catch (error) {
  if (error instanceof NotFoundError) {
    response.status(404).send('Not found');
  }
}

✓ After

@Catch(NotFoundError)
export class NotFoundFilter implements ExceptionFilter {
  catch(exception: NotFoundError, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse();
    response.status(404).json({ message: 'Not found' });
  }
}

What It Enables

You can handle errors in one place and keep your app code clean and consistent.

Real Life Example

When building an API, you want all 'not found' errors to return the same message and status code without repeating code in every route.

Key Takeaways

Manual error handling clutters code and risks inconsistency.

Exception filters centralize and simplify error responses.

This leads to cleaner, easier-to-maintain NestJS applications.