For the Medium article on Object.defineProperty

financial-year.js

class FinancialYear {
  constructor(turnover, costOfGoods) {
    this.turnover = turnover;
    this.costOfGoods = costOfGoods;
    
    Object.defineProperty(this, 'grossProfitMargin', {
      get () {
        const gpm = ((this.turnover - this.costOfGoods) / this.turnover) * 100;
        return `${gpm}%`;
      }
    });
  }
}