Interfaces & objects — quiz

You're modeling a signup form for a test data factory. Required fields: name, email. Optional: referralCode (a user may or may not have one). Which interface is correct?

• interface User { name: string; email: string; referralCode: string }
• interface User { name: string; email: string; referralCode?: string }
• interface User { name?: string; email?: string; referralCode?: string }
• interface User { name: string; email: string; referralCode: null }