Testing Split Payment Logic Safely

// commission.ts
export function calculateSplit(totalAmount: number, commissionRate: number) {
  const vendorShare = Math.round(totalAmount * (1 - commissionRate) * 100); // kobo
  const platformShare = Math.round(totalAmount * commissionRate * 100); // kobo
  return { vendorShare, platformShare };
}

// commission.test.ts
describe('calculateSplit', () => {
  it('splits 10% commission correctly', () => {
    var result = calculateSplit(10000, 0.10);
    expect(result.vendorShare).toBe(900000); // NGN 9,000 in kobo
    expect(result.platformShare).toBe(100000); // NGN 1,000 in kobo
  });

  it('handles non-round amounts without floating point errors', () => {
    var result = calculateSplit(999.99, 0.10);
    // Should not produce fractional kobo
    expect(result.vendorShare % 1).toBe(0);
    expect(result.platformShare % 1).toBe(0);
  });

  it('returns full amount to vendor at 0% commission', () => {
    var result = calculateSplit(5000, 0);
    expect(result.vendorShare).toBe(500000);
    expect(result.platformShare).toBe(0);
  });
});

// Mock Paystack API in Jest
jest.mock('../lib/paystack', () => ({
  initializeTransaction: jest.fn().mockResolvedValue({
    status: true,
    data: {
      authorization_url: 'https://checkout.paystack.com/test',
      access_code: 'test_access',
      reference: 'test_ref_001',
    }
  }),
}));

describe('Checkout with vendor split', () => {
  it('passes correct percentage_charge to Paystack', async () => {
    var paystack = require('../lib/paystack');
    var vendor = { subaccount_code: 'SUB_test123', commission_rate: 0.15 };

    await checkoutService.initializeSplitPayment({
      email: 'buyer@test.com',
      amount: 10000,
      vendor,
    });

    expect(paystack.initializeTransaction).toHaveBeenCalledWith(
      expect.objectContaining({
        subaccount: 'SUB_test123',
        percentage_charge: 85, // 1 - 0.15 = 0.85 = 85%
        bearer: 'account',
      })
    );
  });
});

This guide is part of the Paystack split payments guide.

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