Sorted numbers in the wrong order

A k-th smallest helper and a "meeting rooms" interval merge both sort their input first. Tests with single-digit numbers pass, but kthSmallest([10, 2, 5, 33, 4], 1) returns 10 and the interval merge produces overlapping output when start times exceed 9. Find the bug.

1function kthSmallest(nums, k) {
2  const sorted = [...nums].sort();
3  return sorted[k - 1];
4}
5 
6function mergeIntervals(intervals) {
7  intervals.sort();
8  const out = [];
9  for (const [s, e] of intervals) {
10    if (out.length && out[out.length - 1][1] >= s) {
11      out[out.length - 1][1] = Math.max(out[out.length - 1][1], e);
12    } else {
13      out.push([s, e]);
14    }
15  }
16  return out;
17}
18 
19console.log(kthSmallest([10, 2, 5, 33, 4], 1));         // 10, expected 2
20console.log(mergeIntervals([[10, 12], [2, 11], [1, 3]]));  // [[1,3],[10,12],[2,11]], expected [[1,3],[2,12]] merged → [[1,12]]

1. What does Array.prototype.sort() do when called with no comparator? Show what [10, 2, 5, 33, 4].sort() actually returns.
2. Explain why the interval merge is still wrong even though the arrays are "sorted".
3. Write correct comparators for both functions. What must a comparator return?
4. What happens with negative numbers, NaN, undefined and mixed-type arrays?
5. State the complexity and whether sort is stable.