NestJS Jest error: TypeError: Cannot read properties of undefined (reading '[any variable from required config]') - jestjs

Question

While trying to cover out project in unit tests using nest's jest I've bumped into a problem of a testing module not being able to pull variables from config.
Basically, I have an EmailService, I want to test it, I use it as a Provider in my testing module. Naturally, as EmailService takes ConfigService in its constructor to pull some variables from config (that initially come from env) I put ConfigService into the providers array as well... well, then upon initialization testing module drops
NestJS Jest error: TypeError: Cannot read properties of undefined (reading 'region')
note: region variable is taken from env in a registered config module
code example of my test that throws
describe('EmailService', () => {
let emailService: EmailService;
let configService: ConfigService;
beforeEach(async () => {
const module: TestingModule = await Test.createTestingModule({
providers: [EmailService, ConfigService],
}).compile();
emailService = module.get<EmailService>(EmailService);
configService = module.get<ConfigService>(ConfigService);
});
it('should be defined', () => {
expect(emailService).toBeDefined();
});
});
I have came to the conclusion that it throws an error specifically because EmailService takes ConfigService in it's constructor in this way:
export class EmailService {
private readonly config: IAwsConfig;
private readonly region: IRegion;
constructor(private readonly configService: ConfigService) {
this.config = this.configService.get('aws');
this.region = this.config.region;
}
aditional info: both EmailService and ConfigService work just fine during a normal runtime, it only fails during jest testing
seems like this.configService.get method returns 'undefined' during a test run and i'm, not sure why or how to fix it. Any ideas?

Answer 1

In case you don't want to import the entire ConfigService but just the config values themselves, then you use them in the test as follows :)
// my-config.ts
import { registerAs } from '#nestjs/config';
export default registerAs('myConfig', () => ({ propA: 'aa', propB: 123 }));
import { Inject } from '#nestjs/common';
import { ConfigType } from '#nestjs/config';
import myConfig from './my-config.ts';
export class EmailService {
private propA: string;
private propB: number;
constructor(
#Inject(myConfig.KEY) config: ConfigType<typeof myConfig>
) {
this.propA = config.propA;
this.propB = config.propB;
}
}
import { ConfigModule, registerAs } from '#nestjs/config';
import { Test, TestingModule } from '#nestjs/testing';
describe('Test', () => {
const configValues = { propA: 'aa', proprB: 123 };
const config = registerAs('testConfig', () => configValues);
beforeEach(async () => {
const app: TestingModule = await Test.createTestingModule({
imports: [ConfigModule.forFeature(config)],
providers: [EmailService],
}).compile();
});
});

Answer 2

Was not able to find an answer for 2 hours straight, but then, 10 minutes after asking a question, there you go, an answer.
Seems like ConfigService doesn't provide configs during jest testing so you have to provide it in the testing module with replaced get method, something like such:
providers: [
EmailService,
{
provide: ConfigService,
useValue: {
get: jest.fn((key: string) => {
return hardcodedConfigFromWithinTheTestFile;
}),
},
},
],