Unit testing NestJS controller with injection

Question

I need to make an unit test for a controller who use injection with NestJS.

I don't know how to mock and spy this service (MyEmitter). I need to declare it in test.controller.spec.ts inside beforeEach() but how?

test.controller.ts

import {
  Controller,
  Body,
  Post,
} from '@nestjs/common';
import {
  WebhookDto,
} from './dto/webhook.dto';
import { MyEmitter } from './test.events';
import { InjectEventEmitter } from 'nest-emitter';

@Controller()
export class TestController {
  constructor(
    @InjectEventEmitter() private readonly myEmitter: MyEmitter,
  ) {}

  @Post('webhook')
  public async postWebhook(
    @Body() webhookDto: WebhookDto,
  ): Promise<void> {
    ...
    this.myEmitter.emit('webhook', webhookDto);
  }
}

test.controller.spec.ts

import { Test, TestingModule } from '@nestjs/testing';
import { TestController } from './test.controller';
import EventEmitter = require('events');
import { EVENT_EMITTER_TOKEN } from 'nest-emitter';
import { MyEmitter } from './test.events';

describe('Test Controller', () => {
  let testController: TestController;

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      imports: [],
      providers: [
        {
          provide: EVENT_EMITTER_TOKEN,
          useValue: {
            emit: jest.fn(),
          },
        },
      ],
      controllers: [TestController],
    }).compile();

    testController = module.get<TestController>(TetsController);
  });

  describe('postWebhook', () => {
    it('should send the event', async () => {
      const myEmitterSpy = jest.spyOn(myEmitter, 'emit');
      const result = await testController.postWebhook({...});
      expect(myEmitterSpy).toBeCalledTimes(1);
    });
  });
});

Thank you really much for your help.

Answer 1

The easiest way to go about it, with the setup you currently have is to use module.get() like you already are for the controller and pass in the EVENT_EMITTER_TOKEN constant, then save that to a value declared in the describe block, just like how the let testController: TestController works. Something like this should suffice:

import { Test, TestingModule } from '@nestjs/testing';
import { TestController } from './test.controller';
import EventEmitter = require('events');
import { EVENT_EMITTER_TOKEN } from 'nest-emitter';
import { MyEmitter } from './test.events';

describe('Test Controller', () => {
  let testController: TestController;
  let myEmitter: MyEmitter;

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      imports: [],
      providers: [
        {
          provide: EVENT_EMITTER_TOKEN,
          useValue: {
            emit: jest.fn(),
          },
        },
      ],
      controllers: [TestController],
    }).compile();

    testController = module.get<TestController>(TetsController);
    myEmitter = module.get<MyEmitter>(EVENT_EMITTER_TOKEN);
  });

  describe('postWebhook', () => {
    it('should send the event', async () => {
      const myEmitterSpy = jest.spyOn(myEmitter, 'emit'); // you can also add on mockResponse type functions here like mockReturnValue and mockResolvedValue
      const result = await testController.postWebhook({...});
      expect(myEmitterSpy).toBeCalledTimes(1);
    });
  });
});

Answer 2

Rather than injecting every dependencies (which should be tested separately), it is better to use jest.spyOn because controller has a service dependency or dependencies which might have other dependencies.

We should mock the method that will be called in the current test.

Here is the sample controller test.

import { SampleController } from './sample.controller';
import { SampleService } from './sample.service';

describe('SampleController', () => {
  let sampleController: SampleController;
  let sampleService: SampleService;

  beforeEach(() => {
    // SampleService depends on a repository class
    // Passing null becasue SampleService will be mocked
    // So it does not need any dependencies.
    sampleService = new SampleService(null);
    // SampleController has one dependency SampleService
    sampleController = new SampleController(sampleService);
  });

  it('should be defined', async () => {
    expect(sampleController).toBeDefined();
  });

  describe('findAll', () => {
    it('should return array of samples', async () => {
      // Response of findAllByQuery Method
      // findAllByQUeryParams is a method of SampleService class.
      // I want the method to return an array containing 'test' value'.
      const expectedResult = ['test'];

      // Creating the mock method
      // The method structure is the same as the actual method structure.
      const findAllByQueryParamsMock = async (query: any) => expectedResult;

      // I am telling jest to spy on the findAllByQueryParams method
      // and run the mock method when the findAllByQueryParams method is called in the controller.
      jest
        .spyOn(sampleService, 'findAllByQueryParams')
        .mockImplementation(findAllByQueryParamsMock);

      const actualResult = await sampleController.findAll({});

      expect(actualResult).toBe(expectedResult);
    });
  });
});