NCC-js

index.js

// Product data: fetched from a database/any storage
const products = [
  { code: "VOUCHER", name: "Voucher", price: 5 },
  { code: "TSHIRT", name: "T-shirt", price: 20 },
  { code: "MUG", name: "Coffee mug", price: 7.5 },
];
// Rules. This is where business logic lives.
const rules = {
  secondFree: (total, amount) => {
    if (amount < 2) return total;
    if (amount % 2 === 0) {
      // if even: discount 50%
      return (total * 50) / 100;
    } else {
      // if odd: discount 50% and sum rest (1)
      return ((total - total / amount) * 50) / 100 + total / amount;
    }
  },
  bulk: (total, amount) => {
    if (amount >= 3) {
      return total - 1 * amount;
    } else {
      return total;
    }
  },
};
// Price rules. Configurable: it could be fetched from a campaign (CMS, etc)
// and is agnostic to business logic -> it requires validation.
// Each product could contain an array of rules to apply if + than 1 rule is an option
const price_rules = {
  VOUCHER: "secondFree",
  TSHIRT: "bulk",
};

// Provides an interface to scan (add) products.
// In charge of mapping products to price rules/campaigns, apply them and
// return the total price.
class Checkout {
  _total = 0;
  products = new Map(); // hashmap to add "scanned" products

  constructor(rules) {
    this.rules = rules;
  }

  // Getter that returns the checkout total after applying discounts
  get total() {
    // Loop over the scanned products
    for (const [code, amount] of this.products) {
      // Get missing product information (price, etc) previously fetched from DB
      const prod = products.find((p) => p.code === code);
      // guard in case non-existant product is added (could also throw?)
      if (!prod) continue;

      const productTotal = prod.price * amount;
      const productRules = this.rules[code]; // configured rules for current product

      // If current product has price rules, apply them and add to total
      if (productRules) {
        this._total += rules[productRules](productTotal, amount);
      } else {
        this._total += productTotal;
      }
    }

    return this._total;
  }

  // Saves the scanned products into a hashmap to calculate subtotals easier.
  scan(product) {
    const prod = this.products.get(product);
    if (prod) {
      this.products.set(product, prod + 1);
    } else {
      this.products.set(product, 1);
    }
  }
}

const checkout1 = new Checkout(price_rules);
checkout1.scan("VOUCHER");
checkout1.scan("TSHIRT");
checkout1.scan("MUG");
console.log(checkout1.total); // 32.5

const checkout2 = new Checkout(price_rules);
checkout2.scan("VOUCHER");
checkout2.scan("TSHIRT");
checkout2.scan("VOUCHER");
console.log(checkout2.total); // 25

const checkout3 = new Checkout(price_rules);
checkout3.scan("TSHIRT");
checkout3.scan("TSHIRT");
checkout3.scan("TSHIRT");
checkout3.scan("VOUCHER");
checkout3.scan("TSHIRT");
console.log(checkout3.total); // 81

const checkout4 = new Checkout(price_rules);
checkout4.scan("VOUCHER");
checkout4.scan("TSHIRT");
checkout4.scan("VOUCHER");
checkout4.scan("VOUCHER");
checkout4.scan("MUG");
checkout4.scan("TSHIRT");
checkout4.scan("TSHIRT");
console.log(checkout4.total); // 74.5