interface Person {
  name: string;
  age: number;
  email?: string;
}

function describePerson(person: Person): string {
  const emailPart = person.email ? `, reachable at ${person.email}` : '';
  return `${person.name} is ${person.age} years old${emailPart}.`;
}

console.log(describePerson({ name: 'Philip', age: 35, email: 'philip@example.com' }));
console.log(describePerson({ name: 'Sam', age: 28 }));

/*
  Notes:
  - email being marked optional (email?: string) means an object
    missing it entirely (the second call) is still valid according
    to the Person interface — no error from the type checker.
  - The ternary checks person.email truthiness before building the
    email part of the sentence, since it might be undefined.
*/