Refactoring

// primitive obsession + data clump → Introduce Parameter Object → a value object
// before: amount and currency travel together through twelve functions
function fee(amountKobo: number, currency: string, country: string) { /* … */ }
function format(amountKobo: number, currency: string) { /* … */ }

// after: the clump becomes a type that owns its rules (and its bugs go with it)
class Money {
  private constructor(readonly minor: bigint, readonly currency: 'NGN' | 'KES' | 'GHS') {}
  static of(minor: bigint, currency: Money['currency']) { return new Money(minor, currency); }
  plus(o: Money) { this.same(o); return new Money(this.minor + o.minor, this.currency); }
  private same(o: Money) { if (o.currency !== this.currency) throw new Error('currency mismatch'); }
}
function fee(amount: Money, country: Country) { /* … */ }
function format(amount: Money) { /* … */ }