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

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

  for (let i = 1; i < intervals.length; i++) {
    const currentStart = intervals[i][0];
    const previousEnd = mergedIntervals[mergedIntervals.length - 1][1];

    if (currentStart <= previousEnd) {
      // Merge the interval
      mergedIntervals[mergedIntervals.length - 1][1] = Math.max(previousEnd, intervals[i][1]);
    } else {
      // Add the new interval
      mergedIntervals.push(intervals[i]);
    }
  }

  return mergedIntervals;
}
```