Master Low-Level Design by Building Real-World Systems.

interface DiscountStrategy {
  apply(total: number): number;
}


class PercentOff implements DiscountStrategy {
  constructor(private percent: number) {}
  apply(total: number) {
    return Math.floor((total * (100 - this.percent)) / 100);
  }
}


class Checkout {
  constructor(private strategy: DiscountStrategy) {}
  total(prices: number[]) {
    return this.strategy.apply(prices.reduce((a, b) => a + b, 0));
  }
}