One Reason to Change, and Extension Without Edits

// Before: finance asks for wording changes, compliance asks for rate changes,
// a tax-law change breaks the printed layout's tests, and both teams edit here.
export class Payout {
  constructor(
    private readonly grossMinor: number,
    private readonly country: string,
  ) {}

  taxMinor(): number {
    // Compliance owns this. It changes when law changes.
    const rate = this.country === 'DE' ? 0.19 : this.country === 'GB' ? 0.2 : 0;
    return Math.round(this.grossMinor * rate);
  }

  toRemittanceAdvice(): string {
    // Finance owns this. It changes when the template changes.
    return `Gross ${(this.grossMinor / 100).toFixed(2)} — tax ${(this.taxMinor() / 100).toFixed(2)}`;
  }
}

// Compliance's rules, testable against published rates and nothing else.
export interface TaxPolicy {
  taxMinorFor(grossMinor: number, country: string): number;
}

// Finance's presentation, testable with a fixed pair of numbers.
export interface RemittanceFormatter {
  format(payout: PayoutView): string;
}

export interface PayoutView {
  readonly grossMinor: number;
  readonly taxMinor: number;
  readonly currency: string;
}

// The domain object now only changes when what a payout *is* changes.
export class Payout {
  constructor(
    readonly grossMinor: number,
    readonly country: string,
    readonly currency: string,
  ) {}

  view(policy: TaxPolicy): PayoutView {
    return {
      grossMinor: this.grossMinor,
      taxMinor: policy.taxMinorFor(this.grossMinor, this.country),
      currency: this.currency,
    };
  }
}