Testing module setup in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this NestJS testing module setup?

Consider this NestJS test setup code. What will be the value of service.getHello() after the test module is compiled and the service is retrieved?

NestJS

import { Test, TestingModule } from '@nestjs/testing';
import { AppService } from './app.service';

describe('AppService', () => {
  let service: AppService;

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      providers: [AppService],
    }).compile();

    service = module.get<AppService>(AppService);
  });

  it('should return greeting', () => {
    expect(service.getHello()).toBe('Hello World!');
  });
});

A 'Hello NestJS!'

B 'Hello World!'

C undefined

D Throws an error because service is not defined

💡 Hint

Look at how the service is provided and retrieved from the testing module.

The testing module provides AppService, so when we get it, calling getHello() returns the default string 'Hello World!'.

📝 Syntax

intermediate

Which option causes a syntax error in NestJS testing module setup?

Identify which code snippet will cause a syntax error when setting up a NestJS testing module.

NestJS

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

async function setup() {
  const module = await Test.createTestingModule({
    providers: [
      {
        provide: 'MyService',
        useClass: MyService,
      }
    ]
  }).compile();
  return module;
}

A Missing comma after useClass: MyService

B Using useValue with an object literal

C Providing a string token without useClass or useValue

D Using useClass without quotes around MyService

💡 Hint

Check the commas between object properties carefully.

In JavaScript/TypeScript object literals, missing commas between properties cause syntax errors.

🔧 Debug

advanced

Why does this NestJS test fail with 'Nest can't resolve dependencies' error?

Given this testing module setup, why does the test fail with a dependency resolution error?

NestJS

import { Test, TestingModule } from '@nestjs/testing';
import { AppService } from './app.service';
import { AppController } from './app.controller';

describe('AppController', () => {
  let controller: AppController;

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      controllers: [AppController],
    }).compile();

    controller = module.get<AppController>(AppController);
  });

  it('should return greeting', () => {
    expect(controller.getHello()).toBe('Hello World!');
  });
});

A AppService is not provided in the testing module providers array

B AppController is not declared in controllers array

C Missing async keyword in beforeEach

D Incorrect import path for AppController

💡 Hint

Check what dependencies AppController needs and if they are provided.

AppController depends on AppService, but AppService is not included in the providers array, so Nest cannot resolve it.

❓ state_output

advanced

What is the value of 'result' after this NestJS test runs?

In this test, what will be the value of the variable result after calling service.getData()?

NestJS

import { Test, TestingModule } from '@nestjs/testing';
import { DataService } from './data.service';

class MockDataService {
  getData() {
    return ['mock1', 'mock2'];
  }
}

describe('DataService', () => {
  let service: DataService;

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      providers: [
        DataService,
        { provide: DataService, useClass: MockDataService },
      ],
    }).compile();

    service = module.get<DataService>(DataService);
  });

  it('should return mocked data', () => {
    const result = service.getData();
    expect(result).toEqual(['mock1', 'mock2']);
  });
});

A ['real1', 'real2']

B []

C Throws error because of duplicate providers

D ['mock1', 'mock2']

💡 Hint

Check which provider is used when multiple providers for the same token exist.

The last provider for DataService uses MockDataService, so service.getData() returns the mock data array.

🧠 Conceptual

expert

Which option best describes the purpose of NestJS TestingModule's compile() method?

What does the compile() method do when called on a NestJS TestingModule builder?

A It compiles TypeScript code to JavaScript for the module

B It runs all tests defined in the module automatically

C It creates an instance of the module and resolves all dependencies for testing

D It resets the module to its initial state before each test

💡 Hint

Think about what is needed before you can use the module's providers and controllers in tests.

compile() builds the testing module instance and resolves dependencies so you can retrieve providers/controllers for testing.