How to return a custom response from the class-validator in NestJS

Question

Is it possible to return a custom error response from class-validator inside of NestJs.

NestJS currently returns an error message like this:

{
    "statusCode": 400,
    "error": "Bad Request",
    "message": [
        {
            "target": {},
            "property": "username",
            "children": [],
            "constraints": {
                "maxLength": "username must be shorter than or equal to 20 characters",
                "minLength": "username must be longer than or equal to 4 characters",
                "isString": "username must be a string"
            }
        },
    ]
}

However the service that consumes my API needs something more akin to:

{
    "status": 400,
    "message": "Bad Request",
    "success": false,
    "meta": {
        "details": {
            "maxLength": "username must be shorter than or equal to 20 characters",
            "minLength": "username must be longer than or equal to 4 characters",
            "isString": "username must be a string"
        }
    }
}

Soracco

Answer 1

Nestjs has built-in compoments named Exception filters, if you want to decorate your response in case of exceptions. You can find the relevant documentations here.

The following snippet could be helpful for writing your own filter.

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

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

    response
      .status(status)
      // you can manipulate the response here
      .json({
        statusCode: status,
        timestamp: new Date().toISOString(),
        path: request.url,
      });
  }
}

Couldst

Answer 2

When you define ValidationPipe you can provide exceptionFactory.

new ValidationPipe({
      exceptionFactory: (errors) =>  new HttpException({ error: "BAD_REQUEST" }, 400)
})

The first argument type is string, but you can provide object. According to the NestJS documentation:
To override just the message portion of the JSON response body, supply a string in the response argument. To override the entire JSON response body, pass an object in the response argument. Nest will serialize the object and return it as the JSON response body.

Parodic