```ts
export function mergeIntervals(intervals: number[][]): number[][] {
  if (!intervals || intervals.length === 0) return [];

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

  let merged: number[][] = [];
  for (const interval of intervals) {
    if (!merged.length || merged[merged.length - 1][1] < interval[0]) {
      merged.push(interval);
    } else {
      const lastInterval = merged.pop()!;
      merged.push([lastInterval[0], Math.max(lastInterval[1], interval[1])]);
    }
  }

  return merged;
}
```