```typescript
export function mergeIntervals(intervals: number[][]): number[][] {
  if (intervals.some(i => i[0] > i[1])) throw new Error('Invalid interval');
  const sortedIntervals = intervals.slice().sort((a, b) => a[0] - b[0]);
  const merged: number[][] = [];

  for (const interval of sortedIntervals) {
    if (!merged.length || interval[0] > merged[merged.length - 1][1]) {
      merged.push(interval);
    } else {
      merged[merged.length - 1][1] = Math.max(merged[merged.length - 1][1], interval[1]);
    }
  }

  return merged;
}
```