```ts
function mergeIntervals(intervals: number[][]): number[][] {
  if (intervals.some(([start, end]) => start > end)) {
    throw new Error('Interval must be start <= end');
  }
  intervals.sort((a, b) => a[0] - b[0]);
  const merged = [intervals[0]];
  for (let i = 1; i < intervals.length; i++) {
    const current = intervals[i];
    if (current[0] > merged[merged.length - 1][1]) {
      merged.push(current);
    } else {
      merged[merged.length - 1][1] = Math.max(merged[merged.length - 1][1], current[1]);
    }
  }
  return merged;
}
```