Description

An API to calculate the total price of products in a checkout list considering current promotions.

Core features

The system should have the following promotions:

• Buy 3 Google Homes for the price of 2

• Each sale of a MacBook Pro comes with a free Raspberry Pi

• Buying more than 3 Alexa Speakers will have a 10% discount on all Alexa speakers

Example Scenarios:

• Scanned Items: MacBook Pro ($5,400), Raspberry Pi B ($30)

  Total: $5,400

• Scanned Items: Google Home, Google Home, Google Home ($50)

  Total: $100

• Scanned Items: Alexa Speaker, Alexa Speaker, Alexa Speaker ($109.50)

  Total: $295.65

Prerequisite

• Node.js with NVM
• Docker
• Visual Studio Code with the REST Client extension

More on REST Client extension

REST Client extension by Huachao Mao

What extension?

How?

• Simply open the client.http file inside the root folder on VS Code

• Click on "Send Request" next to the API call you want to execute

  

Installation

# Clone the repo
$ git clone https://github.com/avpalmeira/checkout-api
$ cd checkout-api

# Install the dependencies
$ npm install

Running the app

# Run a PostgreSQL Database inside a container on port 5434
$ docker compose up -d

# Run the migrations
$ npm run migration:run

# Run the API in watch mode
$ npm run start:dev

# Run tests
$ npm run test

Test the API

1. Make sure all prerequisites are satisfied

2. Open the client.http file

3. Run the existing calls inside file or run new calls to the API

Thought process

Deciding on how to handle the business logic for having different types of promotions in the system and also making it extensible to new promotions

Promotion structure

The promotion is divided into 2 pieces:

1. A promotion activation rule which includes the products and how many of them must be included in the checkout so the promotion is active
2. A promotion discount rule which includes all the products that would be applied the discount in case the promotion is active, how many of them would apply the discount and the discount amount from 0 to 100%

There were also some rules and edge cases that should be dealt with regarding some described scenario, for instance, applying the discount for every item after a certain number of it is met. For this specific case a discount rule with the quantity set to 0 (zero) was an indicator that this specific rule should be applied to the products in the checkout.

Examples of promotions based on the scenarios described above

Example 1

ProductActivation: [
  {macbook pro, qty: 1},
  {raspberry pi, qty: 1}
],
ProductDiscount: [
  {raspberry pi, qty: 1, discount: 1}
]

Example 2

ProductActivation: [
  {google home, qty: 3}
],
ProductDiscount: [
  {google home, qty: 1, discount: 1}
]

Example 3

ProductActivation: [
  {alexa, qty: 3}
],
ProductDiscount: [
  {alexa, qty: 0, discount: 0.1}
]

Calculation

This is the logic for how to calculate the price for every product on the checkout list based on saved promotions:

> Based on the products’ SKUs as input and promotions saved on the database...
> Check each promotion if it applies to a group of items in the list of products
    > If found a group that matches:
        > Substitute items in the list to a single new "product with calculated price" (using special SKU with all repeated characters)
        > If product discount quantity is 0, find all the products that are in the checkout, apply discount and substitute the products with a single "product with calculated price"
    > If no group was found that matches the promotion, check the next promotion

Improvements

On this file I’m listing how I would invest further time in making the solution better