Edge-case bugs interview questions

Question 1

last_n() powers the "recent activity" widget. A user set their widget size to 0. What do they see?

def last_n(items: list, n: int) -> list:
    """Return the last n items (n >= 0)."""
    if n < 0:
        raise ValueError("n must be non-negative")
    return items[-n:]

Accepted Answer

Line 5: When n is 0, `-n` is also 0, and `items[0:]` is the whole list. Asking for zero items returns everything.

Question 2

formatStock() renders the stock badge on product cards. What does a sold-out product show?

function formatStock(count) {
  if (!count) return "Stock unknown";
  if (count < 5) return `Only ${count} left`;
  return "In stock";
}

formatStock(0); // expected a "Sold out" badge

Accepted Answer

Line 2: `!count` is true for 0 as well as for null and undefined, so a sold-out product shows "Stock unknown" instead of being treated as zero stock.

Question 3

Average() is used on the latency dashboard. What happens for a service with no requests yet?

// Average returns the mean latency in milliseconds.
func Average(latencies []int) int {
	sum := 0
	for _, l := range latencies {
		sum += l
	}
	return sum / len(latencies)
}

Accepted Answer

Line 7: With an empty slice, len is 0 and integer division by zero panics at runtime ("integer divide by zero"), crashing the dashboard handler.

Edge-case bugs: interview questions

How to spot it in code review

Why AI-generated code gets this wrong

Edge-case bugs: 3 practice questions

Last N items with a slice

Falsy zero in a stock label

Average of an empty slice

Attempts distribution