```ts
export function mergeIntervals(intervals: number[][]): number[][] {
  if (!intervals.length) return [];
  
  intervals.sort((a, b) => a[0] - b[0]);
  const merged = [[intervals[0][0], intervals[0][1]]];

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

    if (currentStart <= lastEnd) {
      // Merge interval
      merged[merged.length - 1][1] = Math.max(lastEnd, currentEnd);
    } else {
      // No merge, add new interval
      merged.push([currentStart, currentEnd]);
    }
  }

  return merged;
}
```