1. Install Required Packages

npm install -D jest @types/jest ts-jest supertest @types/supertest

If you haven’t:

npm install -D @nestjs/testing

2. Basic Project Structure

src/
  users/
    users.service.ts
    users.controller.ts
test/
  app.e2e-spec.ts
jest.config.ts

3. Jest Configuration (IMPORTANT)

Create jest.config.ts:

import type { Config } from 'jest';

const config: Config = {
  moduleFileExtensions: ['js', 'json', 'ts'],
  rootDir: '.',
  testRegex: '.*\\.spec\\.ts$',
  transform: {
    '^.+\\.(t|j)s$': 'ts-jest',
  },
  collectCoverageFrom: ['src/**/*.(t|j)s'],
  coverageDirectory: './coverage',
  testEnvironment: 'node',
};

export default config;

4. Add Scripts in package.json

{
  "scripts": {
    "test": "jest",
    "test:watch": "jest --watch",
    "test:cov": "jest --coverage",
    "test:e2e": "jest --config ./test/jest-e2e.json"
  }
}

5. Unit Test Example (Service)

users.service.ts

export class UsersService {
  findAll() {
    return [{ id: 1, name: 'Daniel' }];
  }
}

users.service.spec.ts

import { UsersService } from './users.service';

describe('UsersService', () => {
  let service: UsersService;

  beforeEach(() => {
    service = new UsersService();
  });

  it('should return users', () => {
    const result = service.findAll();
    expect(result).toEqual([{ id: 1, name: 'Daniel' }]);
  });
});

6. Controller Test (with Nest Testing Module)

import { Test, TestingModule } from '@nestjs/testing';
import { UsersController } from './users.controller';
import { UsersService } from './users.service';

describe('UsersController', () => {
  let controller: UsersController;

  const mockUsersService = {
    findAll: jest.fn(() => [{ id: 1, name: 'Daniel' }]),
  };

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      controllers: [UsersController],
      providers: [
        {
          provide: UsersService,
          useValue: mockUsersService,
        },
      ],
    }).compile();

    controller = module.get<UsersController>(UsersController);
  });

  it('should return users', () => {
    expect(controller.findAll()).toEqual([
      { id: 1, name: 'Daniel' },
    ]);
  });
});

👉 Key idea: mock service → isolate controller

7. e2e Test Setup

test/jest-e2e.json

{
  "moduleFileExtensions": ["js", "json", "ts"],
  "rootDir": "../",
  "testRegex": ".e2e-spec.ts$",
  "transform": {
    "^.+\\.(t|j)s$": "ts-jest"
  },
  "testEnvironment": "node"
}

8. e2e Test Example

import { Test } from '@nestjs/testing';
import { INestApplication } from '@nestjs/common';
import * as request from 'supertest';
import { AppModule } from '../src/app.module';

describe('App (e2e)', () => {
  let app: INestApplication;

  beforeAll(async () => {
    const moduleRef = await Test.createTestingModule({
      imports: [AppModule],
    }).compile();

    app = moduleRef.createNestApplication();
    await app.init();
  });

  it('/GET users', () => {
    return request(app.getHttpServer())
      .get('/users')
      .expect(200);
  });

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

9. Real-World Improvements (What interviewers expect)

1. Use .env.test

• Separate test database
• Never use production DB

2. Use test database (IMPORTANT)

• SQLite (fast) OR
• Test Postgres schema

3. Add coverage threshold

coverageThreshold: {
  global: {
    branches: 80,
    functions: 80,
    lines: 80,
    statements: 80,
  },
}

4. Folder convention (clean architecture)

users/
  users.service.ts
  users.service.spec.ts
  users.controller.ts
  users.controller.spec.ts

10. Common Mistakes (you should avoid)

• ❌ Testing database in unit tests
• ❌ Not mocking dependencies
• ❌ Writing huge e2e tests only
• ❌ Ignoring test coverage

Final Insight

Think like this:

• Unit test → “Is my logic correct?”
• e2e test → “Does my system actually work?”

If you want next step, I can:

• Review your current NestJS project structure
• Or help you write tests for your real code (service/controller) so you can use it in your interview 💼