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

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

  const merged: number[][] = [];
  let currentInterval = sortedIntervals[0];

  for (let i = 1; i < sortedIntervals.length; i++) {
    const nextInterval = sortedIntervals[i];
    if (currentInterval[1] >= nextInterval[0]) {
      currentInterval = [currentInterval[0], Math.max(currentInterval[1], nextInterval[1])];
    } else {
      merged.push([currentInterval[0], currentInterval[1]]);
      currentInterval = nextInterval;
    }
  }

  merged.push(currentInterval);
  return merged;
}
```