Building Secure RESTful APIs with NestJS: A Practical Guide/

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Create a Unit Test in NestJS with Mocks

Create a Unit Test in NestJS with Mocks

Follow step-by-step instructions to create a unit test for UserService.

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Implement a unit test for UserService

Now we’ve learned the basics of unit tests and Jest. In this lesson, we’ll explore how to implement unit tests for UserService in our application using Jest.

Install the dependencies

Because we use NestJS CLI to generate the application, the Jest-related dependencies have been provisioned automatically. But when starting a new application without using the CLI generation, we need to have the following dependencies installed:

npm install --save-dev jest @nestjs/testing ts-jest
Command to install the dependencies

The above command installs Jest and related packages, including the ts-jest library, for testing NestJS applications. The ts-jest library is a Jest preprocessor that supports the TypeScript code by transpiling it into JavaScript during the testing process.

Default unit test for UserService

A default unit test is already created when we create UserService using NestJS CLI.

The generated test file is as follows. It defines the skeleton of the service test suite, test setup, and a basic test case. A test suite serves as a container for organizing a group of related test cases in Jest. It provides a structured way to manage and execute tests for a specific module or functionality, promoting organized testing practices.

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// user.service.spec.ts
import { Test, TestingModule } from '@nestjs/testing';
import { UserService } from './user.service';
describe('UserService', () => {
  let service: UserService;
  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      providers: [UserService],
    }).compile();
    service = module.get<UserService>(UserService);
  });
  it('should be defined', () => {
    expect(service).toBeDefined();
  });
});

  • Line 5: This line declares a Jest test suite named 'UserService'. Test suites are used to group related test cases.

  • Lines 8–14: These lines set up the test environment before each test. It creates an instance of TestingModule using Test.createTestingModule() and provides UserService as a provider. NestJS supplies TestingModule. It allows us to configure a module specifically for testing purposes. The compile() method is used to compile the testing module, making it ready for use in the tests. Then, it retrieves an instance of UserService from the module and assigns it to the service variable.

  • Lines 16–18: These lines define the actual test case. It checks whether the service variable, representing an instance of UserService, is defined. It also ensures the service instantiation is successful. This fundamental check is crucial because if the service is not initialized correctly, subsequent tests might encounter errors or produce inaccurate results. It acts as an essential baseline test, confirming that the service is available for further testing.

NestJS’s TestingModule

In the example above, TestingModule sets up UserService as a provider. By ...