How to use NestJS configurations outside managed class - nestjs

Question

I'm trying to create some custom exceptions for my application. This mostly means i'm extending the HttpException class which is pretty simple.
However, as part of the exception message, I want to pass some of the configurations for the application.
The problem is that Exceptions are not part of the module or service. They're not managed classes, so I cannot use the ConfigService as described by the NestJS documentation.
I could use the process.env.<my_config> approach, but it seems dirty to have to use that when I'm using ConfigService everywhere in my services, specially when i'm also using .env files to load some other variables. My last alternative would be to use dotenv directly in the configuration. However all of them suffer from the same: I could be missing some important data updates/added during the app bootstrapping portion.
How can I access app level configurations from outside the managed classes?
Sample of what i'm trying to do:
import { HttpException, HttpStatus } from '#nestjs/common';
export class MyCustomException extends HttpException {
constructor(message) {
const serviceName = // Get the configuration value
const configA = // Get other configuration value
const payload = {
serviceName,
configA,
message,
}
super(payload, HttpStatus.BAD_REQUEST)
}
}

Answer 1

You can create an exception filter to catch your custom exception or the built-in BadRequestException, then inject ConfigService into the filter when registering the filter in main.ts.
create custom exception filter to catch your custom exception.
my-custom-exception.filter.ts
import { ExceptionFilter, Catch, ArgumentsHost, HttpStatus} from '#nestjs/common';
import { Response } from 'express';
import { ConfigService } from '#nestjs/config';
import { MyCustomException } from './my-custom.exception'; // replace this to your exception file path
#Catch(MyCustomException)
export class MyCustomExceptionFilter implements ExceptionFilter {
constructor(private readonly _configService: ConfigService) {}
catch(exception: MyCustomException, host: ArgumentsHost) {
const ctx = host.switchToHttp();
const response = ctx.getResponse<Response>();
// get configuration from .env
const serviceName = this._configService.get<string>(`SERVICE_NAME`); // replace to your key in .env
const configA = this._configService.get<string>(`CONFIG_A`); // replace to your key in .env
response
.status(HttpStatus.BAD_REQUEST)
.json({
serviceName,
configA,
message: exception.message
});
}
}
Pass configService to your custom exception filter when initializing app.
main.ts
...
const app = await NestFactory.create(AppModule);
// get configService instance, pass into filter
const configService = app.get<ConfigService>(ConfigService);
app.useGlobalFilters(new MyCustomExceptionFilter (configService));
...