Null & undefined bugs interview questions

Question 1

loadTheme() runs on page load. It works for returning visitors. What happens on someone's first visit?

interface Prefs {
  theme: "light" | "dark";
}

export function loadTheme(): Prefs["theme"] {
  const prefs: Prefs = JSON.parse(localStorage.getItem("prefs")!);
  return prefs.theme;
}

Accepted Answer

Line 6: On a first visit getItem returns null. The `!` hides that from TypeScript, `JSON.parse(null)` returns null, and `prefs.theme` throws a TypeError that breaks page load.

Question 2

greetingFor() builds the email greeting for the password-reset flow. What happens for an email that isn't registered?

public String greetingFor(String email) {
    Optional<User> user = userRepository.findByEmail(email);
    return "Hi " + user.get().getFirstName() + ",";
}

Accepted Answer

Line 3: findByEmail returns Optional.empty() for unknown emails, and get() on an empty Optional throws NoSuchElementException. The reset endpoint returns a 500, which also tells attackers which emails exist.

Question 3

This should list users who haven't verified their email (verified_at is NULL). Why does it always return zero rows?

SELECT id, email
FROM users
WHERE verified_at = NULL
ORDER BY created_at;

Accepted Answer

Line 3: Any comparison with NULL using = yields NULL (unknown), never TRUE, so no row passes the filter. NULL checks need IS NULL.

Null & undefined bugs: interview questions

How to spot it in code review

Why AI-generated code gets this wrong

Null & undefined bugs: 3 practice questions

Reading preferences from localStorage

Calling Optional.get()

Filtering with = NULL

Attempts distribution