【NestJS】How to common exception handling.

Use NestJs exceptions layer to create a filter that handles all exceptions that are not handled throughout the application.

The result should be empty and the data returned.

{
  "status": "false",
  "errorCode": "E001",
  "result": []
}

Define error codes

Define constants.ts.
Add the required error codes.

export default class ErrorCode {
  /** 内部サーバーエラー */
  static readonly E001 = 'E001';
  /** 認証エラー */
  static readonly E002 = 'E002';
}

Implement exception filters

The exception filter must implement the ExceptionFilter interface, and the @Catch(HttpException) decorator will bind the necessary data to the exception filter.

If HttpStatus is 500 or higher, set E001 in constants.ts as internal server error, otherwise set thrown exception.message.

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

const responseInterface = {
  status: false,
  errorCode: '',
  result: [],
};

@Catch()
export class HttpExceptionFilter implements ExceptionFilter {
  catch(exception: HttpException, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse<Response>();
    const statusCode =
      exception instanceof HttpException
        ? exception.getStatus()
        : HttpStatus.INTERNAL_SERVER_ERROR;

    responseInterface.status = false;
    responseInterface.errorCode = this.setErrorCode(
      statusCode,
      exception.message,
    );

    response.status(statusCode).json(responseInterface);
  }

  private setErrorCode(statusCode: number, message): string {
    if (statusCode >= 500) {
      return 'E001';
    }
    return message;
  }
}

If you want to return E002 as an authentication error, set the error code to UnauthorisedException and throw it.

throw new UnauthorizedException(ErrorCode.E002);

Commonisation of Responses

For a common Response, see this article.

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【NestJS】How to common exception handling.

Define error codes

Implement exception filters

Commonisation of Responses

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