Spot the bug - #72

programming web dev
1

There is one subtle logic bug.

function hasDuplicate(nums) {
  const seen = new Set();
  for (const n of nums) {
    if (seen.has(n)) {
      return false;
    }
    seen.add(n);
  }
  return true;
}

console.log(hasDuplicate([2, 7, 4, 7]));

Reply with what is broken and how you would fix it.

2

Your return values are flipped. Right now, when you find a duplicate (seen. has(n)), you return false, and when you finish the loop with no duplicates you return true—so hasDuplicate([2, 7, 4, 7]) incorrectly prints false. The fix is just to swap those booleans:

function hasDuplicate(nums) {
  const seen = new Set();
  for (const n of nums) {
    if (seen.has(n)) return true;
    seen.add(n);
  }
  return false;
}

Mechanism-wise, Set is doing O(1)-ish membership checks, so the whole point is “bail out early” the moment you see a repeat—your code bails out early, just with the wrong answer.