Why testing ensures application reliability in NestJS - See It in Action

Why testing ensures application reliability

📖 Scenario: You are building a simple NestJS service that calculates discounts for customers. To make sure your service works correctly every time, you will write tests. This helps catch mistakes early and keeps your app reliable.

🎯 Goal: Create a NestJS service with a method to calculate discounts and write a test to verify it works as expected.

📋 What You'll Learn

Create a NestJS service class named DiscountService

Add a method calculateDiscount that takes a price number and returns a 10% discount

Create a test file with a test case that checks if calculateDiscount returns the correct discounted price

💡 Why This Matters

🌍 Real World

In real apps, testing services ensures that business logic works correctly before users see it, preventing bugs and improving trust.

💼 Career

Writing tests and using NestJS testing tools are key skills for backend developers to deliver reliable and maintainable applications.

1

Create the DiscountService class

Create a NestJS service class called DiscountService with a method calculateDiscount that takes a parameter price and returns price without any changes.

NestJS

export class DiscountService {
  // Your code here
}

Hint

Start by defining a class and a method that returns the input price.

2

Add discount calculation logic

Modify the calculateDiscount method in DiscountService to return 90% of the price (apply a 10% discount).

NestJS

export class DiscountService {
  calculateDiscount(price: number): number {
    // Your code here
  }
}

Hint

Multiply the price by 0.9 to apply a 10% discount.

3

Set up a test for DiscountService

Create a test file that imports DiscountService and write a test case named 'should return 90% of the price' that checks if calculateDiscount(100) returns 90.

NestJS

import { DiscountService } from './discount.service';

describe('DiscountService', () => {
  const service = new DiscountService();

  // Your test here
});

Hint

Use describe and it blocks with expect to write the test.

4

Complete the test setup with NestJS testing module

Modify the test to use NestJS Test.createTestingModule to provide DiscountService and get the service instance from the module before running the test.

NestJS

import { Test, TestingModule } from '@nestjs/testing';
import { DiscountService } from './discount.service';

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

  beforeEach(async () => {
    // Your code here
  });

  it('should return 90% of the price', () => {
    expect(service.calculateDiscount(100)).toBe(90);
  });
});

Hint

Use NestJS testing utilities to create a testing module and get the service instance.

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