Comment by columnarx3

I think this is the wrong pattern in this instance. You parse an email or phone number because validating leaves it as a plain string, and you lose the context to know for sure if that string is actually an email or phone number.

In your instance, you could have:

  type User = {
    // ... rest of fields
    email: {
      verified: boolean,
      // branded type here ensures that this string is a proper email address
      value: EmailAddress,
    },
    birthday: Date | null,
  };

In this instance, your logic with a method that accepts birthday and email has all the information it needs to make its choice.