interface FullName {
  firstName?: string,
  lastName?: string
}

function sayName(params: FullName = {}) {
  const { firstName = 'Andrew', lastName = 'Smith' } = params;

  const name = firstName + ' ' + lastName;
  console.log(name);
}

sayName();
sayName({ });
sayName({ firstName: 'Shaun' });
sayName({ firstName: 'Shaun', lastName: 'Greene' });