Discount Chain

Applying multiple discounts and surcharges in sequence — and understanding when order matters.

Single discount

The straightforward case: one percentage off the original price.

import { from } from '@eriveltondasilva/currency'

const price = from(200, 'US')

price.applyDiscount(20).format()  // => '$160.00'  (20% off)

Compound vs simple discounts

When two discounts are applied in sequence, each discount is applied to the already-reduced amount — not the original. This is not equivalent to adding the percentages together.

const price = from(100, 'US')

// Two 10% discounts applied in sequence
price.applyDiscount(10).applyDiscount(10).amount() // => 81 ⚠

// A single 20% discount
price.applyDiscount(20).amount() // => 80

// Compound formula: 100 × (1 - 0.10) × (1 - 0.10) = 81

When to use compound discounts: loyalty tiers, stacked promotions, or any context where each discount is applied independently to the running total.

When to combine into a single discount: when the intent is a flat total reduction and the compounding effect is undesirable.

Coupon + loyalty discount

A common pattern: a coupon discount applied first, then a membership discount on the result.

interface DiscountConfig {
  couponPercent: number     // e.g. 15 for a 15% coupon
  membershipPercent: number // e.g. 5 for a 5% loyalty tier
}

function applyDiscounts(
  price: MoneyContract,
  config: DiscountConfig,
): MoneyContract {
  return price
    .applyDiscount(config.couponPercent)
    .applyDiscount(config.membershipPercent)
}

const price = from(200, 'US')
const config = { couponPercent: 15, membershipPercent: 5 }

applyDiscounts(price, config).format() // => '$161.50'
// 200 × 0.85 = 170.00 → 170.00 × 0.95 = 161.50

Discount then tax

Tax is typically applied after the discount, to the reduced amount:

const price = from(300, 'BR')

price
  .applyDiscount(10)   // R$ 270,00  (10% off)
  .applySurcharge(12)  // R$ 302,40  (12% tax on discounted amount)
  .format()            // => 'R$ 302,40'

Compare with tax applied before discount — a different result:

price
  .applySurcharge(12)  // R$ 336,00  (tax first)
  .applyDiscount(10)   // R$ 302,40  (then discount)
  .format()            // => 'R$ 302,40'

// In this case the result is the same — mathematically equivalent
// when both operations use the same base. This is not always true.

Order matters when discounts and surcharges use different bases. Always clarify which amount each operation is applied to.

Coupon validation and application

interface Coupon {
  code: string
  type: 'percent' | 'fixed'
  value: number          // percent: 0–100 | fixed: amount in major units
  minOrderValue?: number // minimum subtotal to activate the coupon
}

function applyCoupon(
  price: MoneyContract,
  coupon: Coupon,
  country: 'BR' | 'US',
): MoneyContract {
  // Check minimum order value
  if (coupon.minOrderValue && price.lessThan(coupon.minOrderValue)) {
    return price  // coupon not applicable — return price unchanged
  }

  if (coupon.type === 'percent') {
    return price.applyDiscount(coupon.value)
  }

  // Fixed discount — clamp to zero so price never goes negative
  const discount = from(coupon.value, country)
  return price.minus(discount).max(0)
}

const price = from(150, 'US')

const percentCoupon: Coupon = {
  code: 'SAVE15',
  type: 'percent',
  value: 15,
  minOrderValue: 100,
}

const fixedCoupon: Coupon = {
  code: 'FLAT20',
  type: 'fixed',
  value: 20,
}

applyCoupon(price, percentCoupon, 'US').format()  // => '$127.50'
applyCoupon(price, fixedCoupon, 'US').format()    // => '$130.00'

// Minimum order not met
const smallOrder = from(50, 'US')
applyCoupon(smallOrder, percentCoupon, 'US').format()  // => '$50.00'  (unchanged)

Stacking multiple coupons

When your business rules allow stacking, collect all applicable discounts and apply them in order:

type DiscountSource = 'coupon' | 'membership' | 'volume' | 'seasonal'

interface AppliedDiscount {
  source: DiscountSource
  percent: number
}

function applyAllDiscounts(
  price: MoneyContract,
  discounts: AppliedDiscount[],
): { final: MoneyContract; applied: AppliedDiscount[] } {
  const final = discounts.reduce(
    (current, discount) => current.applyDiscount(discount.percent),
    price,
  )

  return { final, applied: discounts }
}

const price = from(500, 'US')

const discounts: AppliedDiscount[] = [
  { source: 'seasonal',   percent: 10 },  // summer sale
  { source: 'membership', percent: 5  },  // gold member
  { source: 'volume',     percent: 3  },  // 10+ items
]

const { final } = applyAllDiscounts(price, discounts)

final.format()
// 50000 → ×0.90 → 45000 → ×0.95 → 42750 → ×0.97 → 41467
// => '$414.67'

Showing savings

function formatSavings(original: MoneyContract, final: MoneyContract): string {
  const saved = original.minus(final)
  const percent = saved.amount() / original.amount() * 100

  return `Save ${saved.format()} (${percent.toFixed(0)}% off)`
}

const original = from(200, 'US')
const final = from(161, 'US')

formatSavings(original, final)  // => 'Save $39.00 (20% off)'