NestJS Exception filter messes up error array if it comes from ValidationPipe - node.js

Question

So I use the ValidationPipe to validate my DTOs in NestJS, like this:
// auth.dto.ts
export class AuthDto {
#IsEmail()
#IsNotEmpty()
email: string;
}
Without the Exception filter the error message works as intended. I leave the email field empty and I receive an array of error messages:
// Response - Message array, but no wrapper
{
"statusCode": 400,
"message": [
"email should not be empty",
"email must be an email"
],
"error": "Bad Request"
}
Perfect. Now I want to implement a wrapper for the error messages, so I create a new filter and add it to to bootstrap:
// main.ts
async function bootstrap() {
// ...
app.useGlobalFilters(new GlobalExceptionFilter());
}
bootstrap();
// global-exception.filter.ts
import {
ArgumentsHost,
Catch,
ExceptionFilter,
HttpException,
HttpStatus,
} from '#nestjs/common';
import { Response } from 'express';
import { IncomingMessage } from 'http';
export const getStatusCode = <T>(exception: T): number => {
return exception instanceof HttpException
? exception.getStatus()
: HttpStatus.INTERNAL_SERVER_ERROR;
};
export const getErrorMessage = <T>(exception: T): string => {
return exception instanceof HttpException
? exception.message
: String(exception);
};
#Catch()
export class GlobalExceptionFilter<T> implements ExceptionFilter {
catch(exception: T, host: ArgumentsHost) {
const ctx = host.switchToHttp();
const response = ctx.getResponse<Response>();
const request = ctx.getRequest<IncomingMessage>();
const statusCode = getStatusCode<T>(exception);
const message = getErrorMessage<T>(exception);
response.status(statusCode).json({
error: {
timestamp: new Date().toISOString(),
path: request.url,
statusCode,
message,
},
});
}
}
It works great for most of my errors:
// Response - Good format (wrapped), single message expected
{
"error": {
"timestamp": "2022-05-11T19:54:59.093Z",
"path": "/auth/signup",
"statusCode": 400,
"message": "Email already in use"
}
}
But when I get a ValidationError from the ValidationPipe it should give me an array or messages like before, but it gives this message instead:
// Response - Wrapper: check, single message instead of array
{
"error": {
"timestamp": "2022-05-11T19:59:17.282Z",
"path": "/auth/signup",
"statusCode": 400,
"message": "Bad Request Exception" // it should be "message": ["not empty", "must be email"]
}
}
The exception object in my exception filter has a response field which contains the message array:
// HttpException object inside the filter class
{
response: {
statusCode: 400,
message: [ 'email should not be empty', 'email must be an email' ],
error: 'Bad Request'
},
status: 400
}
But exception.response.message doesn't work, because the field is private and TypeScript throws an error:Property 'response' is private and only accessible within class 'HttpException'.
Does any of you know how could I reach the message array, so I could format my error response properly?
EDIT: Sorry for the long post!

Answer 1

As #tobias-s commented, there's a workaround, which solved the problem:
Try exception["response"]["message"]. This bypasses the private restriction

Answer 2

As you are using #Catch decorator, you could get a HttpException or not, so we need to evaluate it.
Let's create an interface to "parse" Nest.js built-in HttpException class response:
export interface HttpExceptionResponse {
statusCode: number;
message: any;
error: string;
}
Now we can process it:
export const getErrorMessage = <T>(exception: T): any => {
if(exception instanceof HttpException) {
const errorResponse = exception.getResponse();
const errorMessage = (errorResponse as HttpExceptionResponse).message || exception.message;
return errorMessage;
} else {
return String(exception);
}
};
exception.getResponse() can be a string or an object, that's because we handle it as message: any of course.