Unit testing services in NestJS - Mini Project: Build & Apply

Unit Testing Services in NestJS

📖 Scenario: You are building a simple NestJS service that calculates discounts for customers in an online store. To ensure your service works correctly, you will write unit tests for it.

🎯 Goal: Create a NestJS service called DiscountService with a method to calculate discounts. Then write unit tests to verify the service's behavior.

📋 What You'll Learn

Create a DiscountService class with a method calculateDiscount that takes a price number and returns a discounted price.

Add a configuration variable discountRate set to 0.1 (10%).

Implement the calculateDiscount method to apply the discount rate to the price.

Write a unit test suite for DiscountService using Jest to test the calculateDiscount method.

💡 Why This Matters

🌍 Real World

Unit testing services ensures your business logic works correctly before deploying your NestJS applications.

💼 Career

Writing unit tests for services is a key skill for backend developers working with NestJS to maintain code quality and reliability.

1

Create the DiscountService class

Create a NestJS service class called DiscountService with an empty method calculateDiscount that takes a parameter price.

NestJS

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

Need a hint?

Define a class with the exact name DiscountService and a method calculateDiscount that accepts price as a number.

2

Add discountRate configuration

Inside the DiscountService class, add a property discountRate and set it to 0.1.

NestJS

export class DiscountService {
  discountRate = 0.1

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

Need a hint?

Add a class property named discountRate and assign it the value 0.1.

3

Implement calculateDiscount method

In the calculateDiscount method, return the price after applying the discount rate. Use the formula price * (1 - discountRate).

NestJS

export class DiscountService {
  discountRate = 0.1

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

Need a hint?

Use this.discountRate inside the method to calculate the discounted price.

4

Write unit tests for DiscountService

Create a Jest test suite for DiscountService. Import the service, create an instance, and write a test named 'should calculate discounted price correctly' that checks calculateDiscount(100) returns 90.

NestJS

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

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

  it('should calculate discounted price correctly', () => {
    // Your code here
  })
})

Need a hint?

Use Jest's describe, it, and expect functions to write the test.