Find the bug · Easy

Fix the Median (JavaScript Sort)

Easy Find the bug Most asked

median(nums) returns the median of a non-empty array of numbers: the middle value after sorting, or the average of the two middle values when the length is even.

It works in every test with single-digit numbers and gives nonsense for real data. Find the bug and fix it.

Example 1

Input: nums = [3, 1, 2]
Output: 2

Example 2

Input: nums = [10, 9, 2]
Output: 9
Explanation: The buggy version returns 2.

The buggy code

function median(nums) {
  const sorted = [...nums].sort();
  const mid = Math.floor(sorted.length / 2);
  return sorted.length % 2 ? sorted[mid] : (sorted[mid - 1] + sorted[mid]) / 2;
}

function median(nums: number[]): number {
  const sorted = [...nums].sort();
  const mid = Math.floor(sorted.length / 2);
  return sorted.length % 2 ? sorted[mid] : (sorted[mid - 1] + sorted[mid]) / 2;
}

Hints

Hint 1

What does [10, 9, 2].sort() return in JavaScript?

Hint 2

How does Array.prototype.sort compare elements when you pass no comparator?

Find and fix the bug

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Solution and explanation

Show the bug and the fix

Time O(n log n) · Space O(n)

The bug: without a comparator, sort() converts the elements to strings and sorts them in UTF-16 order, so "10" < "2" < "9" and "-1" < "-5". Small single-digit tests happen to sort correctly, which is why the bug slips through.

The fix: pass a numeric comparator: sort((a, b) => a - b). Sort a copy ([...nums] or toSorted) so the caller's array is not reordered.

Fixed version

function median(nums) {
  const sorted = [...nums].sort((a, b) => a - b); // numeric, not string, order
  const mid = Math.floor(sorted.length / 2);
  return sorted.length % 2 ? sorted[mid] : (sorted[mid - 1] + sorted[mid]) / 2;
}

function median(nums: number[]): number {
  const sorted = [...nums].sort((a, b) => a - b); // numeric, not string, order
  const mid = Math.floor(sorted.length / 2);
  return sorted.length % 2 ? sorted[mid] : (sorted[mid - 1] + sorted[mid]) / 2;
}

Common mistakes

sort((a, b) => a > b) returns a boolean, not a number: inconsistent results across engines.

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