[voraa]

Так можно сделать

function cart(basketAmount, countGoods, promo = null) {
            let discountAmount = 0;
            let rules = [
                (result) => {
                    return (promo === "ДАРИМ300")? result - 300 : result;
                },
                (result) => {
                    return (countGoods >= 10)? result * 0.95 : result;
                },
                (result) => {
                    return (result > 50000)? result - (50000 - result) * 0.2 : result
                },
                (result) => {
                    return (promo === "СКИДКА15" && result >= 20000)?  result * 0.85 : result;
                }
            ];
            let result = basketAmount;
            for (let f of rules) {
				result = f(result);
				if (result <= 0) {
					result = 0;
					break;
				}
            }
            return result;
        }
 
        console.log(cart(80000, 20, "СКИДКА15")); // 69020 А сколько должно быть?