[рони]

TomWalbridge,

function cart(basketAmount, countGoods, promo = null) {
            let discountAmount = 0;
            let rules = [
                () => {
                    if (promo === "ДАРИМ300") discountAmount += 300;
                },
                () => {
                    if (countGoods >= 10) discountAmount += basketAmount * .05;
                },
                () => {
                    if (basketAmount > 50000) discountAmount += (basketAmount - 50000) * 0.2;
                },
                () => {
                    if (promo === "СКИДКА15" && basketAmount >= 20000) discountAmount += basketAmount * 0.15;
                }
            ];
            for (let f of rules) f();
            let result = Math.max(0, basketAmount - discountAmount);
            return result;
        }

        console.log(cart(80000, 20, "СКИДКА15"));