Fix #26: Cart total ignores line item quantities

src/business.js

@@ -12,8 +12,7 @@ function calculateDiscountedPrice(price, discountPercent) {
 }
 
 function totalCart(items) {
-  // BUG: ignores quantity, so multi-quantity line items are undercounted.
-  return roundToCents(items.reduce((total, item) => total + item.price, 0));
+  return roundToCents(items.reduce((total, item) => total + item.price * item.quantity, 0));
 }
 
 function normalizeUsername(username) {

test/business.test.js

@@ -22,7 +22,10 @@ test('cart total includes item quantities', () => {
     { name: 'Notebook', price: 2.5, quantity: 2 },
   ];
 
-  assert.equal(totalCart(items), 17);
+  const total = totalCart(items);
+
+  assert.notEqual(total, 5.5);
+  assert.equal(total, 17);
 });
 
 test('username normalization trims, lowercases, and collapses whitespace', () => {