How to unit test this prisma.service - unit-testing

Question

I'm having trouble unit testing a prisma.service.ts file:
import { INestApplication, Injectable } from '#nestjs/common';
import { PrismaClient } from '#prisma/client';
#Injectable()
export class PrismaService extends PrismaClient {
async enableShutdownHooks(app: INestApplication) {
this.$on('beforeExit', async () => {
await app.close();
});
}
}
The prisma.service.spec.ts I have currently looks like this:
import { INestApplication } from '#nestjs/common';
import { NestFastifyApplication } from '#nestjs/platform-fastify';
import { Test, TestingModule } from '#nestjs/testing';
import { PrismaService } from './prisma.service';
const MockApp = jest.fn<Partial<INestApplication>, []>(() => ({
close: jest.fn(),
}));
describe('PrismaService', () => {
let service: PrismaService;
let app: NestFastifyApplication;
beforeEach(async () => {
app = MockApp() as NestFastifyApplication;
const module: TestingModule = await Test.createTestingModule({
providers: [PrismaService],
}).compile();
service = module.get<PrismaService>(PrismaService);
});
it('should be defined', () => {
expect(service).toBeDefined();
});
describe('enableShutdownHooks', () => {
it('should call $on and successfully close the app', async () => {
const spy = jest.spyOn(PrismaService.prototype, '$on')
.mockImplementation(async () => {
await app.close();
});
await service.enableShutdownHooks(app);
expect(spy).toBeCalledTimes(1);
expect(app.close).toBeCalledTimes(1);
spy.mockRestore();
});
});
});
However, this does not test line 8 of prisma.service.ts:
await app.close();
because I am mocking the implementation of this.$on('beforeExit', callback), with a copy of its original implementation.
Even if I don't mock it, app.close() never gets called.
Is there a way to test this line?

Answer 1

Could you try using a callback:
jest
.spyOn(service, '$on')
.mockImplementation(async (eventType, cb) => cb(() => Promise.resolve()))
await service.enableShutdownHooks(app);
expect(service.$on).toBeCalledTimes(1);
That allows you to use the callback to invoke the function where await app.close() is located.