applyDiscounts

applyDiscounts is a function or process used in e-commerce systems to reduce prices in a shopping cart by applying eligible discount rules. It takes a cart and a collection of discount definitions (such as coupons, promotions, volume discounts) and returns an updated cart along with a record of the discounts applied.

The typical workflow includes validating discount applicability, resolving customer eligibility, checking active dates, and honoring stacking policies. The function computes discount amounts according to rule type (percentage off, fixed amount off, or buy-one-get-one) and applies them to line-item prices or to the cart total, adjusting tax bases as required.

Important constraints include stacking rules (whether discounts can be combined), exclusions (items that are non-discountable), minimum purchase or quantity requirements, and maximum discount limits. Some discounts apply only to specific products or categories, or only to shipping or tax, which must be enforced during calculation and final total generation.

Implementation considerations cover performance, idempotence, and auditability. It is common to maintain an immutable record of the original cart and a discount ledger or receipt showing how each discount was calculated, along with timestamps and rule identifiers, to support refunds and reporting.

In practice, applyDiscounts enables flexible pricing strategies and consistent customer experiences by ensuring discounts are evaluated and applied deterministically during checkout.