Why testing ensures application reliability in NestJS - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the testing module in a NestJS test file.

NestJS

import { Test, TestingModule } from '[1]';

Drag options to blanks, or click blank then click option'

A@nestjs/testing

Bnestjs/core

C@nestjs/common

Dnestjs/platform-express

Attempts:

3 left

💡 Hint

Common Mistakes

Importing from '@nestjs/common' instead of '@nestjs/testing'.

Using 'nestjs/core' which is not for testing.

The '@nestjs/testing' package provides utilities to create testing modules in NestJS.

2fill in blank

medium

Complete the code to create a testing module for the AppService.

NestJS

const module: TestingModule = await Test.createTestingModule({ providers: [[1]] }).compile();

Drag options to blanks, or click blank then click option'

AAppService

BAppController

CHttpService

DConfigService

Attempts:

3 left

💡 Hint

Common Mistakes

Providing the controller instead of the service.

Providing unrelated services.

The testing module should provide the service you want to test, here 'AppService'.

3fill in blank

hard

Fix the error in the test by completing the code to get the service instance.

NestJS

const service = module.[1](AppService);

Drag options to blanks, or click blank then click option'

Afetch

Bfind

Cget

Dretrieve

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'find' which is not a method on TestingModule.

Using 'fetch' or 'retrieve' which do not exist here.

The 'get' method retrieves an instance of the provider from the testing module.

4fill in blank

hard

Fill both blanks to write a simple test that checks if the service is defined.

NestJS

it('should be defined', () => { expect(service).[1](); });

Drag options to blanks, or click blank then click option'

AtoBeFalsy

BtoBeTruthy

CtoBeNull

DtoBeDefined

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'toBeNull' which checks for null, not existence.

Using 'toBeFalsy' which checks for false-like values.

The 'toBeDefined' matcher checks that the value is not undefined.

5fill in blank

hard

Fill all three blanks to write a test that checks the service returns the expected string.

NestJS

it('should return "Hello World!"', () => { expect(service.[1]()).[2]([3]); });

Drag options to blanks, or click blank then click option'

AgetHello

BtoEqual

C"Hello World!"

DtoBe

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'toBe' which is stricter and may fail for strings.

Using wrong method names or string values.

The test calls 'getHello', expects the result to equal the string 'Hello World!'.

Practice

1. Why is testing important in a NestJS application?

easy

A. It reduces the size of the application

B. It makes the app run faster

C. It automatically writes code for you

D. It helps find bugs before users encounter them

Solution

1. Step 1: Understand the purpose of testing

   Testing is used to check if the application works correctly and to find errors early.

2. Step 2: Identify the benefit of testing in NestJS

   Testing helps catch bugs before users see them, improving reliability.

3. Final Answer:

   It helps find bugs before users encounter them -> Option D

4. Quick Check:

   Testing finds bugs early = C [OK]

Hint: Testing finds bugs early to keep apps reliable [OK]

Common Mistakes:

• Thinking testing makes app faster
• Confusing testing with code generation
• Believing testing reduces app size

2. Which of the following is the correct way to define a simple test in NestJS using Jest?

easy

A. run('should return true', () => { expect(true).toBe(true); });

B. check('should return true', () => { expect(true).toBe(true); });

C. test('should return true', () => { expect(true).toBe(true); });

D. assert('should return true', () => { expect(true).toBe(true); });

Solution

1. Step 1: Recall Jest test syntax in NestJS

   Jest uses the test function to define tests.

2. Step 2: Match the correct function name

   Only test is the valid Jest function; others are invalid.

3. Final Answer:

   test('should return true', () => { expect(true).toBe(true); }); -> Option C

4. Quick Check:

   Jest uses test() = A [OK]

Hint: Remember Jest uses test() to define tests [OK]

Common Mistakes:

• Using check(), run(), or assert() instead of test()
• Forgetting arrow function syntax
• Misplacing expect() inside test()

3. Given this NestJS test snippet, what will be the output when the test runs?

describe('MathService', () => {
  it('adds two numbers', () => {
    const result = 2 + 3;
    expect(result).toBe(5);
  });
});

medium

A. Test passes successfully

B. Test fails with error

C. Syntax error in test code

D. Test is skipped automatically

Solution

1. Step 1: Analyze the test logic

   The test adds 2 + 3 and expects the result to be 5, which is correct.

2. Step 2: Determine test outcome

   Since the expectation matches the actual result, the test will pass.

3. Final Answer:

   Test passes successfully -> Option A

4. Quick Check:

   2 + 3 = 5 passes test = A [OK]

Hint: Check if expected value matches actual result [OK]

Common Mistakes:

• Assuming test fails without checking values
• Confusing syntax errors with logic errors
• Thinking tests skip without skip keyword

4. Identify the error in this NestJS test code:

describe('AppService', () => {
  it('should return hello', () => {
    const message = appService.getHello();
    expect(message).toBe('Hello World');
  });
});

Assuming appService is not defined anywhere.

medium

A. The test will pass without errors

B. appService is not defined, causing a ReferenceError

C. The test description is missing

D. The expect statement is incorrect syntax

Solution

1. Step 1: Check variable definitions

   The variable appService is used but not declared or imported anywhere.

2. Step 2: Understand error caused by undefined variable

   Using an undefined variable causes a ReferenceError at runtime.

3. Final Answer:

   appService is not defined, causing a ReferenceError -> Option B

4. Quick Check:

   Undefined variable error = D [OK]

Hint: Always define variables before use in tests [OK]

Common Mistakes:

• Ignoring missing variable declarations
• Thinking expect syntax is wrong
• Assuming test passes without errors

5. How does writing tests in NestJS improve application reliability over time?

hard

A. By ensuring new changes do not break existing features

B. By automatically fixing bugs without developer input

C. By reducing the need for documentation

D. By making the app run faster in production

Solution

1. Step 1: Understand the role of tests in maintenance

   Tests check that new code does not break what already works.

2. Step 2: Connect testing with reliability

   By catching regressions early, tests keep the app stable and reliable over time.

3. Final Answer:

   By ensuring new changes do not break existing features -> Option A

4. Quick Check:

   Tests prevent regressions = B [OK]

Hint: Tests catch regressions to keep app stable [OK]

Common Mistakes:

• Thinking tests fix bugs automatically
• Assuming tests replace documentation
• Believing tests improve runtime speed