javascript 46 lines · 8 steps

How a cart total is calculated with discounts

A single function turns line items and an optional promo code into a rounded, itemized total.

Explained by highlit

Intermediate reduce lookup-table options-object validation money-rounding

1const DISCOUNT_CODES = {
SAVE10: { type: 'percent', value: 0.1 },
WELCOME5: { type: 'fixed', value: 5 },
FREESHIP: { type: 'shipping', value: Infinity },
5};
6 
7function calculateCart(items, { taxRate = 0.08, shipping = 0, code = null } = {}) {
const subtotal = items.reduce(
  (sum, item) => sum + item.price * item.quantity,
  0
);
12 
let discount = 0;
let shippingCost = shipping;
const promo = code ? DISCOUNT_CODES[code.trim().toUpperCase()] : null;
16 
if (code && !promo) {
  throw new Error(`Invalid discount code: ${code}`);
}
20 
if (promo) {
  if (promo.type === 'percent') {
    discount = subtotal * promo.value;
  } else if (promo.type === 'fixed') {
    discount = Math.min(promo.value, subtotal);
  } else if (promo.type === 'shipping') {
    shippingCost = Math.max(0, shipping - promo.value);
  }
}
30 
const discountedSubtotal = subtotal - discount;
const tax = discountedSubtotal * taxRate;
const total = discountedSubtotal + tax + shippingCost;
34 
const round = (n) => Math.round(n * 100) / 100;
36 
return {
  subtotal: round(subtotal),
  discount: round(discount),
  tax: round(tax),
  shipping: round(shippingCost),
  total: round(total),
};
44}
45 
46export { calculateCart, DISCOUNT_CODES };

01 / 01

STEP 01

Walkthrough

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Three takeaways

1A lookup table keyed by code lets you add promo types without touching the calculation logic.
2Validating unknown codes early with a thrown error keeps invalid state out of the math.
3Rounding only at the output boundary avoids compounding floating-point drift across intermediate sums.

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