Testing module setup in NestJS - Mini Project: Build & Apply

Testing Module Setup in NestJS

📖 Scenario: You are building a simple NestJS service that manages a list of books. To ensure your service works correctly, you want to set up a testing module using NestJS's testing utilities.This project guides you step-by-step to create a testing module, configure it, and write a basic test setup.

🎯 Goal: Build a NestJS testing module setup for a BooksService that can be used to write unit tests.

📋 What You'll Learn

Create a testing module with Test.createTestingModule

Provide the BooksService in the testing module

Compile the testing module

Retrieve the BooksService instance from the testing module

💡 Why This Matters

🌍 Real World

Setting up a testing module is essential in NestJS projects to isolate and test services and controllers without running the full application.

💼 Career

Understanding how to configure and use NestJS testing modules is a key skill for backend developers working with NestJS to ensure code quality and reliability.

1

Create a Testing Module

Import Test from @nestjs/testing and create a testing module using Test.createTestingModule with a providers array containing BooksService.

NestJS

import { BooksService } from './books.service';
// Create a testing module with BooksService as provider
// Your code here

Hint

Use Test.createTestingModule and pass an object with providers array including BooksService. Then call .compile() to build the module.

2

Add a Variable for BooksService

Declare a variable called booksService to hold the instance of BooksService retrieved from the testing module.

NestJS

import { Test } from '@nestjs/testing';
import { BooksService } from './books.service';

const moduleRef = await Test.createTestingModule({
  providers: [BooksService],
}).compile();

// Declare booksService variable
// Your code here

Hint

Declare booksService with type BooksService using let so you can assign it later.

3

Retrieve BooksService Instance

Assign the booksService variable by getting the BooksService instance from moduleRef using moduleRef.get(BooksService).

NestJS

import { Test } from '@nestjs/testing';
import { BooksService } from './books.service';

const moduleRef = await Test.createTestingModule({
  providers: [BooksService],
}).compile();

let booksService: BooksService;

// Assign booksService from moduleRef
// Your code here

Hint

Use moduleRef.get(BooksService) to get the service instance and assign it to booksService.

4

Export the Testing Setup

Export the booksService variable so it can be used in test files.

NestJS

import { Test } from '@nestjs/testing';
import { BooksService } from './books.service';

const moduleRef = await Test.createTestingModule({
  providers: [BooksService],
}).compile();

let booksService: BooksService;

booksService = moduleRef.get(BooksService);

// Export booksService
// Your code here

Hint

Use export { booksService } to make the variable available outside this module.

Practice

1. What is the main purpose of using Test.createTestingModule() in NestJS testing?

easy

A. To create a small test version of your app module with needed parts

B. To start the full application server for integration testing

C. To generate database seed data automatically

D. To deploy the app to production environment

Solution

1. Step 1: Understand the role of Test.createTestingModule()

   This method creates a lightweight module for testing, including only what you add (controllers, providers, imports).

2. Step 2: Compare with other options

   Starting the full app server or deploying is not the purpose of this method. It is only for isolated testing setup.

3. Final Answer:

   To create a small test version of your app module with needed parts -> Option A

4. Quick Check:

   Testing module setup = small test module [OK]

Hint: Testing module creates mini app for safe isolated tests [OK]

Common Mistakes:

• Thinking it starts the full app server
• Confusing with deployment or seeding
• Assuming it runs tests automatically

2. Which of the following is the correct way to compile a testing module in NestJS?

easy

A. await compileTestingModule(Test.createTestingModule({}));

B. Test.createTestingModule({}).compile();

C. Test.compileTestingModule({});

D. await Test.createTestingModule({}).compile();

Solution

1. Step 1: Recall the async nature of compile()

   The compile() method returns a Promise, so it must be awaited.

2. Step 2: Check syntax correctness

   Only await Test.createTestingModule({}).compile(); correctly uses await with Test.createTestingModule({}).compile(). Others are either missing await or use invalid methods.

3. Final Answer:

   await Test.createTestingModule({}).compile(); -> Option D

4. Quick Check:

   Compile returns Promise, so use await [OK]

Hint: Always await compile() on testing module [OK]

Common Mistakes:

• Forgetting to await compile()
• Using non-existent compileTestingModule method
• Calling compile without parentheses

3. Given this test setup code snippet, what will service.getData() return?

const moduleRef = await Test.createTestingModule({
  providers: [{
    provide: 'DataService',
    useValue: { getData: () => 'mocked data' },
  }],
}).compile();
const service = moduleRef.get('DataService');
return service.getData();

medium

A. Throws runtime error

B. undefined

C. 'mocked data'

D. 'real data'

Solution

1. Step 1: Identify the provider mock

   The provider 'DataService' is mocked with an object whose getData method returns 'mocked data'.

2. Step 2: Understand the get call

   Calling moduleRef.get('DataService') returns the mocked object, so service.getData() returns 'mocked data'.

3. Final Answer:

   'mocked data' -> Option C

4. Quick Check:

   Mock provider method returns 'mocked data' [OK]

Hint: Mocked provider returns its defined value [OK]

Common Mistakes:

• Expecting real data instead of mock
• Assuming get returns undefined without provider
• Confusing provider token with class

4. What is wrong with this NestJS testing module setup?

const moduleRef = await Test.createTestingModule({
  controllers: [AppController],
  providers: [AppService],
});
const controller = moduleRef.get(AppController);

medium

A. Controllers should not be added in testing modules

B. Missing call to compile() before getting controller

C. Providers array should be empty

D. Cannot get controller by class reference

Solution

1. Step 1: Check testing module lifecycle

   After creating the testing module, you must call compile() to finalize it before retrieving instances.

2. Step 2: Identify missing compile call

   The code does not call await moduleRef.compile(), so moduleRef.get() will fail or behave unexpectedly.

3. Final Answer:

   Missing call to compile() before getting controller -> Option B

4. Quick Check:

   Always compile testing module before get() [OK]

Hint: Always await compile() before accessing module instances [OK]

Common Mistakes:

• Forgetting to compile testing module
• Thinking controllers can't be added
• Trying to get by string token instead of class

5. You want to test a service that depends on another service. How do you set up the testing module to replace the dependency with a mock implementation?

hard

A. Add the service and provide a mock using useValue for the dependency in providers array

B. Only add the main service, NestJS auto-mocks dependencies

C. Add both services without mocks, real implementations are always used

D. Use imports to load the dependency module instead of mocking

Solution

1. Step 1: Understand dependency injection in testing

   To replace a dependency, you provide a mock object using useValue in the providers array.

2. Step 2: Setup providers correctly

   Add the main service normally and add the dependency with provide and useValue to inject the mock.

3. Final Answer:

   Add the service and provide a mock using useValue for the dependency in providers array -> Option A

4. Quick Check:

   Mock dependencies with useValue in providers [OK]

Hint: Mock dependencies by providing useValue in providers [OK]

Common Mistakes:

• Assuming NestJS auto-mocks dependencies
• Not providing mock for dependency
• Using imports instead of mocking for isolated tests