Mocking providers in NestJS - Mini Project: Build & Apply

Mocking Providers in NestJS

📖 Scenario: You are building a simple NestJS service that fetches user data. To test this service without calling the real database, you will create a mock provider.

🎯 Goal: Build a NestJS test setup that mocks a provider called UserService using a mock object. This will help you test your service without real dependencies.

📋 What You'll Learn

Create a mock object for UserService with a method findUser returning a fixed user

Create a testing module that provides the mock UserService

Inject the mock UserService into the test

Verify the mock method is called and returns the expected user

💡 Why This Matters

🌍 Real World

Mocking providers is essential for testing NestJS services without relying on real databases or external APIs. It helps isolate the code under test and speeds up testing.

💼 Career

Understanding how to mock providers is a key skill for backend developers working with NestJS to write reliable unit tests and maintain code quality.

1

Create a mock object for UserService

Create a constant called mockUserService that is an object with a method findUser which returns the object { id: 1, name: 'Alice' }.

NestJS

# Create mockUserService with findUser method returning { id: 1, name: 'Alice' }
# Your code here

Need a hint?

Use an object with a method that returns the fixed user object.

2

Create a testing module with the mock provider

Use Test.createTestingModule to create a module that provides UserService using the value mockUserService. Assign the created module to a constant called moduleRef.

NestJS

const mockUserService = {
  findUser: () => ({ id: 1, name: 'Alice' })
};

// Create testing module with UserService provider using mockUserService
// Your code here

Need a hint?

Use provide and useValue to replace UserService with the mock.

3

Inject the mock UserService from the testing module

Use moduleRef.get to get the UserService and assign it to a constant called userService.

NestJS

const mockUserService = {
  findUser: () => ({ id: 1, name: 'Alice' })
};

const moduleRef = await Test.createTestingModule({
  providers: [
    {
      provide: 'UserService',
      useValue: mockUserService
    }
  ]
}).compile();

// Get UserService from moduleRef
// Your code here

Need a hint?

Use moduleRef.get('UserService') to retrieve the mock.

4

Call the mock method and verify the returned user

Call userService.findUser() and assign the result to a constant called user. Then add a comment that this user should be { id: 1, name: 'Alice' }.

NestJS

const mockUserService = {
  findUser: () => ({ id: 1, name: 'Alice' })
};

const moduleRef = await Test.createTestingModule({
  providers: [
    {
      provide: 'UserService',
      useValue: mockUserService
    }
  ]
}).compile();

const userService = moduleRef.get('UserService');

// Call findUser and assign to user
// Your code here

Need a hint?

Call the mock method and store the returned user object.