mocks.ts

So this is the general pattern I use:

const getMockPaymentPostPaymentRequest = (overrides?: Partial<PostPaymentsRequestV3>): PostPaymentsRequestV3 => {
  return {
    CardAccountId: '1234567890123456',
    Amount: 100,
    Source: 'Web',
    AccountStatus: 'Normal',
    LastName: 'Doe',
    DateOfBirth: '1980-01-01',
    PayingCardDetails: {
      Cvv: '123',
      Token: 'token',
    },
    AddressDetails: {
      HouseNumber: '123',
      HouseName: 'Test House',
      AddressLine1: 'Test Address Line 1',
      AddressLine2: 'Test Address Line 2',
      City: 'Test City',
    },
    Brand: 'Visa',
    ...overrides,
  };
};

Notice how we enforce the idea that we return the full object, but the function itself accepts an optional partial override.

This also scales well because say if we needed to do the same for AddressDetails later, I make changes incrementally as I need them.
  
So then you could do:
  
const getMockAddressDetails = (overrides?: Partial<AddressDetails>): AddressDetails => {
  return {
    HouseNumber: '123',
    HouseName: 'Test House',
    AddressLine1: 'Test Address Line 1',
    AddressLine2: 'Test Address Line 2',
    City: 'Test City',
    ...overrides,
  };
};

const getMockPaymentPostPaymentRequest = (overrides?: Partial<PostPaymentsRequestV3>): PostPaymentsRequestV3 => {
  return {
    CardAccountId: '1234567890123456',
    Amount: 100,
    Source: 'Web',
    AccountStatus: 'Normal',
    LastName: 'Doe',
    DateOfBirth: '1980-01-01',
    PayingCardDetails: {
      Cvv: '123',
      Token: 'token',
    },
    AddressDetails: getMockAddressDetails(),
    Brand: 'Visa',
    ...overrides,
  };
};

// Use cases:

// Address in isolation
const mockAddress = getMockAddressDetails({ City: 'London' });

// Payment with custom address
const mockPayment = getMockPaymentPostPaymentRequest({
  AddressDetails: getMockAddressDetails({ City: 'Paris', HouseNumber: '456' })
});

// Payment with partial address override (merges with existing address)
const mockPaymentPartial = getMockPaymentPostPaymentRequest({
  Amount: 500,
  AddressDetails: getMockAddressDetails({ City: 'Berlin' })
});
  
// Calling getMockPaymentPostPaymentRequest in isolation still returns the default address
  

I like this a lot - thank you for posting it!