Fix the merge without sorting — find the bug

const intervals = [[8, 10], [1, 3], [2, 6]];
const merged = [intervals[0].slice()];
for (let i = 1; i < intervals.length; i++) {
  const [from, to] = intervals[i];
  const last = merged[merged.length - 1];
  if (from <= last[1]) last[1] = Math.max(last[1], to);
  else merged.push([from, to]);
}
document.querySelector('#out').textContent = "merged into " + merged.length;