How can I use Override Provider multiple times for different scenarios when using Nest.js to test?

user_9789567 0
Viewed 616

I am trying to Unit Test a file using Nest.js (and Jest), like so:

import { Test, TestingModule } from '@nestjs/testing';

import { MyService } from 'myService';
import { DependencyService } from 'dependencyService';

describe('#mainMethod', () => {
  let myService, dependencyService;

  beforeAll(async () => {
    let mockDependencyService = {
      method1: jest.fn(() => true)
    };

    const module: TestingModule = await Test.createTestingModule({
        providers: [
            MyService,
            DependencyService,
        ]
      })
        .overrideProvider(DependencyService).useValue(mockDependencyService)
        .compile();
  
      myService = module.get<MyService>(MyService);
      dependencyService = module.get<DependencyService>(DependencyService);
  });

  it('should be ok', () => {
    // any test
  })
});

When writing it like that, I am able to test a scenario where method1 from DependencyService returns true.

What is the best way of testing a different response from method1? Should I wrap it all in a BeforeEach, create another describe block, change the mockDependencyService.method1's return, and create the module all over again? Is there a better way to do it?

Thanks!

nestjs jestjs typescript mocking unit-testing
1 Answers

I would probably mock the module like so as a normal function:

let mockDependencyService = {
  method1: () => true
};

And then spy on the instance from each test:

jest.spyOn(myService, 'method1').mockReturnValue(false)
myService.method1()

// useless expectation for demo purposes
expect(myService.method1).toHaveBeenCalled()

This lets you prep the starting circumstances for each test as each test needs.

Squeegy_620760
Related