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
}

Need a 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
  }
}

Need a 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
});

Need a 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);
  });
});

Need a hint?

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