```typescript

export function mergeIntervals(intervals: number[][]): number[][] {
  if (intervals.some(([start, end]) => start > end)) {
    throw new Error("Invalid interval");
  }

  intervals.sort((a, b) => a[0] - b[0]);

  const merged = [intervals[0]];

  for (let i = 1; i < intervals.length; i++) {
    const current = intervals[i];
    const lastMerged = merged[merged.length - 1];

    if (lastMerged[1] >= current[0]) {
      // Merge intervals
      lastMerged[1] = Math.max(lastMerged[1], current[1]);
    } else {
      merged.push(current);
    }
  }

  return merged;
}

```