NestJs e2e Testing issue overriding provider

Question

I tried following the documentation here as best i could, but im getting an error where the DI seems broken. What am i missing

Here is the error, with some logs:

stdout | src/resources/cats/cat.spec.ts > Cats
beforeAll

stdout | src/resources/cats/cat.spec.ts > Cats
app initialized

stdout | src/resources/cats/cat.spec.ts > Cats > /GET cats
catsService undefined

[Nest] 34132  - 09/29/2025, 5:41:37 PM   ERROR [ExceptionsHandler] Cannot read properties of undefined (reading 'findOne')
TypeError: Cannot read properties of undefined (reading 'findOne')
    at CatsController.findOne (/Users/user/code/dvmonorepo/apps/backend/src/resources/cats/cat.controller.ts:16:29)

module:

import { Module } from '@nestjs/common';
import { CatsService } from './cat.service';
import { CatsController } from './cat.controller';

@Module({
  controllers: [CatsController],
  providers: [CatsService],
  exports: [CatsService],
})
export class CatsModule {}

controller:

import { Controller, Get } from '@nestjs/common';
import { CatsService } from './cat.service';

@Controller('cats')
export class CatsController {
  constructor(
    private readonly catsService: CatsService,
  ) {}

  @Get(':catId')
  findOne() {
    console.log('catsService', this.catsService);
    return this.catsService.findOne('1');
  }
}

service:

import { Injectable } from '@nestjs/common';

@Injectable()
export class CatsService {
  constructor() {}

  async findOne(id: string) {
    return { id, email: '[email protected]', name: 'Test Cat' }
  }
}

spec:


import request from 'supertest';
import { Test } from '@nestjs/testing';
import { CatsModule } from './cat.module';
import { CatsService } from './cat.service';
import { INestApplication } from '@nestjs/common';

describe('Cats', () => {
  let app: INestApplication;
  let catsService = { findOne: () => ['test'] };

  beforeAll(async () => {
    console.log('beforeAll');
    const moduleRef = await Test.createTestingModule({
      imports: [CatsModule],
    })
      .overrideProvider(CatsService)
      .useValue(catsService)
      .compile();

    app = moduleRef.createNestApplication();
    await app.init();
    console.log('app initialized');
  });

  it(`/GET cats`, () => {
    return request(app.getHttpServer())
      .get('/cats/1')
      .expect(200)
      .expect({
        data: catsService.findOne(),
      });
  });

  afterAll(async () => {
    await app.close();
  });
});

some extra words down here because stack overflow thinks i posted too much code lol. blah blah blah. Please let me know if you need more information.

user3666653 · Accepted Answer · 2025-09-30 03:20:39Z

Although i have not determined why yet, this issue can be fixed by using string tokens. Ive double checked that i dont have a conflicting name, so it may be caused by somthing in my monorepo setup or caching

import { Module } from '@nestjs/common';
import { CatsService } from './cat.service';
import { CatsController } from './cat.controller';

@Module({
  controllers: [CatsController],
  providers: [
    {
      provide: 'CatsService', // Use string token
      useClass: CatsService,
    },
  ],
  exports: ['CatsService'],
})
export class CatsModule {
  constructor() {
    console.log('CatsModule: Initialized with providers =', CatsService);
  }
}
// --- controller ----
import { Controller, Get, Inject } from '@nestjs/common';
import { CatsService } from './cat.service';

@Controller('cats')
export class CatsController {
  constructor(@Inject('CatsService') private readonly catsService: CatsService) {
    if (!this.catsService) {
      console.error('CatsController constructor: WARNING - CatsService is undefined!');
    }
  }

  @Get(':catId')
  findOne() {
    console.log('findOne: catsService =', this.catsService);
    return this.catsService.findOne('1');
  }
}

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