The Negative-Price Flash Sale — JavaScript Bug Hunt

Briefing

Inspired by the seasonal-sale pricing bugs that turn storefronts into charities: stacking a 50% banner discount with a 75% coupon by adding the percentages takes 125% off. The store briefly paid customers to take games.

Discounts compound multiplicatively — fix discounts.js.

Bug report

BUG-125PCT · Priority: Critical · Reported by: storefront

applyDiscounts(priceCents, percents):

• apply each percentage to the RUNNING price (integer floor each step)
• 1000 with [50, 75] -> 500 -> 125

Observed: percentages are summed first — [50, 75] takes 125% off and the checkout total goes negative.

Logs

[cart] total: -250 cents ("you earn $2.50 buying this")

The code as shipped

src/store/discounts.js (editable)

// Applies stacked percentage discounts to a price in cents.
exports.applyDiscounts = function (priceCents, percents) {
  var totalPercent = 0;
  for (var i = 0; i < percents.length; i++) {
    totalPercent += percents[i];
  }
  return Math.floor(priceCents * (100 - totalPercent) / 100);
};