NestJS Jest Testing

NestJS Jest Testing

Overview

This skill covers the full test setup for NestJS TypeScript projects using Jest + ts-jest, with separate configs for unit and e2e tests, Stryker for mutation coverage (80% break threshold), jest-it-up for auto-bumping coverage thresholds.

Key Concepts

• ts-jest: Transforms TypeScript for Jest without a separate compile step; uses the project's tsconfig.json
• Unit tests: Use constructor injection mocks — no import mocking, dependencies passed via DI
• E2e tests: Boot the full NestJS module with Test.createTestingModule({ imports: [AppModule] }) + supertest
• Stryker: Runs Jest on mutated code; break: 80 exits with code 1 if mutation score drops below 80%
• jest-it-up: Reads coverage/coverage-summary.json after tests and bumps jest.config thresholds upward

Documentation & References

Recommended Libraries & Tools

Recommended Stack

Use jest + ts-jest (CommonJS preset) for both unit and e2e tests, with separate config files. Use @nestjs/testing for all test modules. Add jest-it-up as a posttest script for unit tests only. Use @stryker-mutator with jest runner and TypeScript checker for mutation testing.

Patterns & Best Practices

Pattern 1: Unit Test with Constructor Injection Mocks

When to use: Testing any NestJS service with injected dependencies. No import mocking — pass fake implementations via constructor.

Trade-offs: Requires services to be refactored to inject external clients (Pool, GraphQLClient) if they create them internally.

// src/consent/__tests__/queue-processor.service.spec.ts
import { QueueProcessorService } from '../queue-processor.service'

const mockPgmqService = {
  readMessage: jest.fn(),
  deleteMessage: jest.fn(),
}

const mockConsentClient = {
  saveConsent: jest.fn(),
}

describe('QueueProcessorService', () => {
  let service: QueueProcessorService

  beforeEach(() => {
    jest.clearAllMocks()
    service = new QueueProcessorService(
      mockPgmqService as any,
      mockConsentClient as any,
    )
  })

  it('processes a message successfully', async () => {
    mockPgmqService.readMessage.mockResolvedValueOnce({ msg_id: '1', message: {} })
    // ...assert saveConsent and deleteMessage called
  })
})

Pattern 2: E2e Test with AppModule and supertest

When to use: Integration testing the full NestJS application over HTTP.

Critical: Set process.env vars in a setupFiles script before the module boots — configify validates at init time, before overrideProvider() takes effect. See E2E Testing with Configify in the configify skill.

// tests/app.e2e.spec.ts
import { Test, TestingModule } from '@nestjs/testing'
import { INestApplication } from '@nestjs/common'
import * as request from 'supertest'
import { AppModule } from '../src/app.module'

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

  beforeAll(async () => {
    const moduleFixture: TestingModule = await Test.createTestingModule({
      imports: [AppModule],
    })
      .overrideProvider(SomeService)
      .useValue(mockSomeService)
      .compile()

    app = moduleFixture.createNestApplication()
    await app.init()
  })

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

  it('/health (GET)', () =>
    request(app.getHttpServer()).get('/health').expect(200))
})

Pattern 4: Separate Jest Configs (Unit vs E2e)

When to use: Always — unit and e2e tests have different roots, patterns, setup files, and timeouts.

// jest.config.ts (unit)
import type { Config } from 'jest'

const config: Config = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  testMatch: ['**/__tests__/**/*.spec.ts'],
  collectCoverage: true,
  collectCoverageFrom: [
    'src/**/*.ts',
    '!src/**/__tests__/**',
    '!src/generated/**',
    '!src/main.ts',
    '!src/**/*.module.ts',
  ],
  coverageDirectory: 'coverage',
  coverageReporters: ['text', 'lcov', 'html', 'json-summary'],
  coverageProvider: 'v8',
  coverageThreshold: {
    global: { branches: 80, functions: 80, lines: 80, statements: 80 },
  },
}

export default config

// jest.e2e.config.ts (e2e)
import type { Config } from 'jest'

const config: Config = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  testMatch: ['**/tests/**/*.spec.ts'],
  setupFiles: ['./tests/e2e-env-setup.ts'],
  testTimeout: 30000,
}

export default config

Stryker Configuration

// stryker.config.json
{
  "testRunner": "jest",
  "coverageAnalysis": "perTest",
  "jest": {
    "projectType": "custom",
    "configFile": "jest.config.ts"
  },
  "checkers": ["typescript"],
  "tsconfigFile": "tsconfig.json",
  "mutate": [
    "src/**/*.ts",
    "!src/**/__tests__/**/*.ts",
    "!src/generated/**",
    "!src/main.ts",
    "!src/**/*.module.ts"
  ],
  "reporters": ["html", "text", "progress"],
  "thresholds": { "high": 80, "low": 60, "break": 80 }
}

break: 80 means Stryker exits with code 1 if mutation score drops below 80%, blocking CI.

Package.json Scripts

{
  "scripts": {
    "test:unit": "jest --config jest.config.ts",
    "posttest:unit": "jest-it-up",
    "test:e2e": "jest --config jest.e2e.config.ts",
    "test:mutation": "stryker run",
    "test:smoke-local": "bash smoke-tests/run-smoke-tests-local.sh",
    "test": "npm run test:unit && npm run test:e2e && npm run test:smoke-local"
  }
}

Note: jest-it-up runs as posttest:unit (not posttest) to avoid running after e2e/smoke.

Similar Implementations

NestJS official CLI generated test setup

• Source: NestJS CLI nest new scaffold
• Approach: Separate jest (unit) and jest-e2e.json (e2e) configs, test/ folder for e2e
• Applicability: Direct match — this skill follows the same conventions

Stryker Node.js guide

• Source: https://stryker-mutator.io/docs/stryker-js/guides/nodejs/
• Approach: npx stryker init generates config, jest runner with ts-jest preset
• Applicability: High — this project is a Node.js/NestJS service

Sources & Verification

Changelog