How is exception and error handling implemented within NestJS?

In NestJS, exception handling is implemented through the exception filters mechanism. By default, if an error occurs in a controller or service, NestJS returns a standard HTTP response with an error code.

To customize error handling, you can create a class that implements the ExceptionFilter interface and override the catch(exception, host) method. In this method, you can handle the error, for example, log it and return a custom response.

Example of a simple exception filter:

import { ExceptionFilter, Catch, ArgumentsHost, HttpException } from '@nestjs/common';
import { Request, Response } from 'express';

@Catch(HttpException)
export class HttpErrorFilter implements ExceptionFilter {
  catch(exception: HttpException, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse<Response>();
    const request = ctx.getRequest<Request>();
    const status = exception.getStatus();

    response.status(status).json({
      statusCode: status,
      timestamp: new Date().toISOString(),
      path: request.url,
      message: exception.message,
    });
  }
}

This filter can be applied globally or at the controller/method level.

Additionally, for handling unexpected errors, you can use a global filter that catches all exceptions (@Catch() without parameters).

NestJS also supports built-in error handling through built-in exceptions (HttpException and its subclasses), which are automatically converted into HTTP responses with the appropriate status.